Metal ions carrying more than one charge function as super acids (super

electrophiles) since they can attract electrons more effectively than the proton and

they act as catalysts by bonding to organic reactants, introducing single or multiple

positive charge into them, and often polarizing them. 1 Transition metal ions are

found to be better catalysts than other metal ions as they have vacant d-orbitals

enabling the formation of sigma bonds with ligands which are Lewis-bases. Many

enzymes contain metal ions as essential parts of the catalytic sites. 2,3 These

metal ions play a variety of roles, 2,3 ranging from simple Lewis acids, typified by

Zn 2 , to redox centres involved in one transfers, typified by Fe2 and

Fe3.

A compound capable of adding molecular oxygen and giving it up

reversibly is called an oxygen-carrying compound (eq. 1). There are several well-

Compound + 02 Compound. (0 2 ) ( 1)

known naturally occurring or synthetic 5,6 compounds of this type. A transition

metal is present in all such compounds and plays a major role in the oxygen

carrying properties of these systems. In biological systems, many hemoproteins

have been found to behave as oxygen carriers. All higher animals use hemoglobin

for oxygen transport and myoglobin for oxygen storage. The active site of

hemoglobin contains an Fe(Ill) protoporphyrin IX moiety with a globin molecule

coordinated to the iron on one side of the plane. The sixth coordination site is

available for coordination of an oxygen molecule. A nonheme iron protein,

hemerythrin, 7 is the oxygen carrier for some marine worms.

2

Cytochrome P-450 enzymes are unique class of hemoproteins capable of

hydroxylating unactivated carbon-hydrogen bond. 8 The active site contains an

iron(II) protoporphyrin IX moiety with a cystein thiolate as one prosthetic group.

These enzymes are known to activate molecular oxygen by sequential two-electron

reduction followed by 0-0 and C-H bond scission.9

RH + + NADPH + H ---- ROH + H 20 + NADP (2)

The active species for the oxidizing function has been proposed to be an oxenoid,

i.e., oxene, formed at an iron-porphyrin active centre of the enzyme. 9

Enzymes catalyze reactions that, at least formally, are oxygen atom transfer

processes. These oxygen transfer reactions are of two types: oxidation or

oxygenation, involving the addition of an oxygen atom to the substrate, and

reduction, involving the removal of an oxygen atom from the substrate. Biological

oxygen transfer reactions are essential for the biosynthesis of steroids and

neurotransmitters, the degradation of endogenous substances, 12 and for the

catabolism of drugs and hormones. 9 The monooxygenase enzymes responsible

for these transformations require biological cofactors such as flavin, heme and

nonheme iron, copper, or pterin and typically utilize NADPH for cofactor

regeneration. 13

1.1 Oxygen Atom Transfer Reactions Catalyzed

by Metal Complexes

One approach to an understanding of the fundamental chemistry underlying

enzymatic oxo transfer reactions requires the development of well-characterized

3

systems of synthetic metal complexes capable of executing these or related

reactions. In order for the information obtained from such systems to be most

relevant to the enzyme problem, several additional criteria must be met. 14,15 First,

the ligand environment should approximate those which have been implicated in

enzyme sites. 15,16 Second, the complexes should be mononuclear and not form

biologically irrelevant and potentially unreactive t -oxo dimers during the course

of oxo transfer reactions. Finally, the oxidized and reduced complexes should be

interconvertible in both directions in order that catalytic cycle can be developed.

Oxygenation or oxygen atom transfer reactions are simply signified as oxo

transfer reactions, which can, in general, be represented as in eq. 3 where (i)

LnM70a + TO LMOI + T (3)

reactants TO/T are oxygen atom donors/acceptors; (ii) the metal oxidation state

is increased by two units for each oxygen atom added; (iii) transferable or

transferred oxygen (a = 0 - 3) is oxidic and directly bound in a terminal or

bridging mode to metal.

Mechanistic studies of metal-catalyzed oxidations are of value not only from

the standpoint of understanding enzymatic oxidations, but also for the development

of new synthetic methods. 10,17 Ulirich and others demonstrated that cytochrome

P-450 mediated hydroxylation can occur in the presence of iodosobenzene 8 as

well as organic peroxides 19 without the requirement for molecular oxygen and

NADPH. Since then, the desire to understand the mechanism of meta!loenzyme-

catalyzed biological processes and the need for selective and effective synthetic

4

oxygenation catalysts have motivated numerous investigators to examine a variety

of model oxygen atom transfer reactions. 20-23

These model studies employ various transition metal complexes with

porphyrin, 2447 Schiff base 48-79 or other ligands8094 in the presence of dioxygen

and other oxygen atom donors (Terminal oxidants, TO) such as PhIO, H 202 , or

hypochiorite etc. Among the various transition metal catalysts the metal-porphyrin

and metal-salen (N, N'-ethylenebis(salicylideneaminato)) complexes offer some

attractive advantages and play a major role in model oxygen transfer studies.

They provide a strong four-coordinate ligand, which can be very stable toward

destruction and has a potential for elaboration to provide chemo-, regio-, and

stereoselectivity. 95 It is therefore of some interest to study the mechanisms by

which these complexes transfer an oxygen atom to the substrate (eqs. 4-6).

RE -I + TO Metal Complex > R011 ± T (4)

1-1 0

Rd-I = CHR' +TO Metal Conmlcx ± T (5)

R

R-S-R' ± TO Metal Complex > R-SO-R ± T (6)

1.1.1 Oxygen Atom Transfer Reactions Catalyzed by

Metal-Porphyrin Complexes

In the last two decades, much attention has been devoted to synthetic

metalloporphyrins as they are able to mimic catalytic oxygenation reactions,

epoxidation and hydroxylation, usually performed in living systems by mono-

oxygenases like cytochrome P-450. 9697 High-valent Mn-porphyrin complexes have

been shown to be versatile synthetic oxidation catalysts for the oxidation of a

S

wide variety of organic substrates 27,28,98,99 and also they are capable of oxidizing

water to dioxygen or H 202 in both thermal 100 and photochemical 0 processes

via the agency of oxo species generated by the reaction of terminal oxidants with

manganese(III) complexes. Wiliner et al. 29 isolated [(TPP)Mn'=O(X)], X = Cl - and

Br (TPP = mesotetraphenylporphyrin dianion) from the reaction of (TPP)Mnm(X)

and PhlO. The two types of oxo-manganese complexes isolated by Hill and

others 102,103 from the reaction of (TPP)Mn III X with PhlO, [(TPP)Mn'X(OIPh)2JO,

X = Cl - and Br and [(TPP)MnrX]2O, X = N 3 are capable of oxidizing alkane

substances in good yields at room temperature. Oxo intermediates resulting from

Mn-0-Mn and Mn-O-1 moieties are postulated to be the active species for the

hydroxylation of alkanes. The formation of alkyl radicals by abstraction of hydrogen

atom by the active species has been attributed to the radical character of the

oxo complexes and they probably have high degree of triplet character in their

ground state and are better represented 103 as Mn"-O rather than Mn" =O. In

contrast to the (TPP)Fe"/PhlO systems, 25,26,30 the yields of hydroxylated alkane

products with (TPP)MnUh/PhlO systems 27-29 are high.

Groves and Stern 104 have reported that in (TMP)Mn mCl/MCPBA catalytic

systems (TMP = mesotetramesitylporphyrin dianion; MCPBA = m-chloroperbenzoic

acid), two different oxidants, oxomanganese(V) and oxomanganese(IV), are

produced. The mesityl substituents provide steric hindrance to the formation of

i-oxodimer species, {(TMP)Mn 1' X]2O. Both oxo complexes epoxidize cis-

R-methylstyrene but it is the manganese(IV) complex that is responsible for the

nonstereoretentive portion of the epoxidation reactions. An oxo compound,

6

(TPP)Mn(OMe) 2 , has been isolated from the reaction of (TFP)Mnm(OAc),

NaBH4 and iodosobenzene in methanol. 105 A stepwise radical reaction has been

proposed for the epoxidation of olefins by oxo(porphyrinato)manganese(V)

complexes. 28 The mechanism of LiOCI epoxidation 106 of olefin catalyzed by

(TPP)Mn mCl has been shown to involve two concerted processes: the reversible

formation of an intermediate oxo-olefin complex and its subsequent irreversible

breakdown to form epoxide. Oxygen transfer from p-cyano-N,N-dimethylaniline

N-oxide (p-CNDMANO) to cyclohexene as well as intramolecular oxygen transfer

accomplished by demethylation to yield p-cyanomonomethylaniline 107 are strongly

catalyzed by (TPP)Mn'X (X = F, Cl - , B(, [ and OCN) and (CAPTPP)MnmX

(CAPTPP = C 2capporphyrin). Oxygen transfer occurs through the reversible

formation of the hexacoordinated species p-CNDMANO-(TPP)Mn(X). This species

decomposes to p-cyanodimethylaniline and (TPP)MnV=O(X). Reaction of cyclo-

hexene with (TPP)MnV=O(X) yields cyclohexane epoxide and (TPP)MnmX.

In the alkene epoxidation studies with pentafluoroiodosobenzene catalyzed

by (TPP)M 111 C1 (M = Cr, Mn and Fe) complexes, 95 it has been found that the

electron deficiency at the metal centre decreases and the exo/endo ratio of the

epoxide products increases markedly from the iron to manganese to chromium

porphyrins. The iron porphyrins are more sensitive to electronic effects whereas

the manganese porphyrins are more sensitive to steric effects. Based on these

results, it has been proposed that the Fe(lll)-porphyrin catalyzed epoxidations are

limiting electron transfer processes, and the Crlll)-porphyrin catalyzed epoxidations

are limiting electrophilic addition mechanisms involving MV=O oxidizing species. 95

7

The bridged, threitol-strapped Mn(lll)-porphyrin systems 47 are reported as

effective asymmetric epoxidation catalysts with PhIO. When small ligands like

pyridine or imidazole are added to the epoxidation reactions, lower optical yields

are obtained. However, other terminal oxidants such as H 202 , NaOCl, LiOCI,

TBHP (t-butyl hydroperoxide) provide lower enantioselectivities and lower yields

than are obtained with PhlO. This observation is explained by two possibilities.

The first is that these other oxygen donors are modifying the catalysts in some

way, either by reacting with the catalyst or by promoting the scrambling of the

axial ligand. The second is that the active oxidant is not a discrete high-valent

metal-oxo species but is rather a metalloporphyrin-oxidant complex. The stereo-

selectivity obtained with such complexes would then be dependent upon the

structures of both the oxidant and the catalyst as well as the interaction between

the two. 47

The oxidations of benzylic amines to imines 108 and of primary aromatic

amines to nitro compounds 109 have been observed in catalytic systems containing

TBHP and (TPP)M 111 C1 (M = Fe or Mn) in the presence of 1-methylimidazole as

axial Jigand. For all the enzymes having iron protoporphyrin IX as the prosthetic

group, the different activities are modulated not only by the axial ligands, but also

by the 'cage-effect' due to the three-dimensional shape of the protein around the

prosthetic group. A similar effect, designated as 'open-well effect' has been

observed in the catalytic epoxidation reaction with theIII

system. 40 Such an 'open-well effect' is expected to avoid the dimerization of the

catalyst through the formation of a i-oxo bridge. 102,103 In contrast to the

8

(TPP)Mn mX./NaOCI system, the (TMP)MnIHX/NaQCI system, due to 'open-well

effect', shows high selectivity and conversion for the epoxidation of cyclohexene,

1-methylcyclohex-1-ene, norbornene, etc.

A comparative study of hydroxylation of alkanes 33 by cumyl hydroperoxide

(cumOOH) and PhlO catalyzed by (porphyrin)M III (M Cr, Fe, Mn, Co and

Rh) complexes shows that these reactions involve oxo species and mechanisms,

which greatly differ as a function of the oxidant nature. The PhlO-dependent

hydroxylation of alkanes is dramatically dependent on the nature of the metal ion

and on its environment (eq. 7). On the contrary, the cumOOH-dependent reaction

(Porph)M" + PhlO -PhI

(Porph)M2 = 0+RJ-I

(Porph)M°' -OH + ROH + (Porph)M' (7)

is independent of the nature and environment of the metal (eq. 8).

(Porph)M' + curnOOi-I > (Porph)M'" L. 011 + cumO+Rjj

-cumOH

R+ (Porph)M'- OH > ROH + (Porph)M' (8)

The kinetics and mechanism of the oxygenation of (TMP)MnmCl,

(TDCPP)MnCl (TDCPP= 5,10,1 5,20-tetrakis(2',6'-dichloropheny!)porphyrin dianion)

and (Br8TMP)Mn m Cl (Br8TMP = 5,10,1 5,20-tetrakis(2',4',6'-trimethylphenyl)-I-octa-

bromoporphyrin dianion) by Ph 4 PHSO5 (tetraphenyiphosphonium monopersulphate)

has been investigated 110 in CH 2 Cl2 and CH 3CN in the presence of axial ligands

and follows a simple bimolecular mechanism. The donor ligands such as pyridine,

imidazole, 2-methylimidazole and 4-methylimidazole have considerable catalytic

9

effect on the epoxidation of olefins with NaOCl, 45 Bu4NlO4 , ill Na104 112 and

H202 113 catalyzed by (TPP)Mn m Cl complexes and high selectivity and yield have

been obtained under mild conditions. In the oxidation of cis- and trans-stilbenes

with metal porphyrins in presence of 0 2/RCHO, 114 (TPP)Mn m Cl gives higher ratio

of cis to trans products than (TPP)Cr ill CI, (TPP)Fe mCl, (TPP)COH or (TPP)Ni"

complexes.

1.1.2 Oxygen Atom Transfer Reactions Catalyzed by

Metal-Salen Complexes

The use of metal-salen and -Schiff base complexes to catalyze oxidation

of hydrocarbons by oxygen atom donors has been receiving much attention in

recent years. This area is highlighted by the works of Kochi and others on the

oxidation chemistry of chromium, 48-50 manganese 52 and nickel. 55,61 The isolation 48

of the reactive [(salen)CrV=O]f by these workers indicated that the pathways for

the PhlO epoxidation of olefins catalyzed by metal-salen complexes and the

analogous metal loporphyrins could be similar. Since dianionic saten ligand is

known to stabilize metal complexes in their high oxidation states, 48,115 it is not

surprising that metal-salen complexes with oxidation states higher than Ill should

be easily accessible. Agarwal and others have studied the oxovanadium(IV)-Schiff

base complexes catalyzed epoxidation of olefins with TBHP and proposed that

coordination of hydroperoxide to the metal centre takes place during epoxidation.73

Oxo(salen)chromium(V) complexes 48 are capable of transferring an oxygen

atom to olefins as well as to phosphines (eqs. 9 & 10). The epoxidation of olefins

0

+ [(salen)Lr V ]+

0

11Bu 3 + [ (sa1en)Cr"

> + [(sa1en)r ]__III +

> 0=PBu 3 + [(salen)ur II ]+

IJ

(9)

(10)

with PhIO catalyzed by (salen)CrIII complexes 49 is promoted by pyridine N-oxide

and related donor ligands. The mechanism of oxygen transfer involves the rate-

limiting attack on the olefin by the electrophilic oxochromium(V) cation. In this

catalytic system, addition of 1-cycIodextrin as a phase-transfer agent enhanced

the rates of epoxidation in CH 2 Cl 2-H 20. 56 (Salen)Crm complexes in the presence

of phosphine oxide epoxidize the cis-R-methylstyrene more selectively than the

trans-R- met hyl sty rene. 72 Also, oxo(salen)chromium(V) complexes oxidize alkynes65

to 1,2-diones and phosphorous ylides57 to carbonyl compounds via the inter-

mediacy of metallaoxetene or related species.

Cationic ( salen)Mn Ht complexes are effective catalysts for the epoxidation

of a wide variety of unfunctionalized olefins with various terminal

oxidants. 52,60,62,69,116 The electron-withdrawing groups present at the 5 and 5'-

positions of salen ligand enhance the catalytic activity in measure with the

electron-deficient character of the manganese centre in the epoxidation of olefins

with PhIO catalyzed by (salen)MnIII complexes. 52,60 The donor ligands such as

pyridine and imidazole have a minimum effect on the epoxidation and the reactive

intermediate, oxomanganese(V) species are described to have a radical-like

behaviour. 52 A comparison 71 of the catalytic activity of Mn(lll)-mannich base (the

reduced form of salen) complexes towards epoxidation of olefins using PhIO with

that of the corresponding salen complexes indicates that the former show relatively

lower catalytic activity. A (salen)Mn" complex has been immobilized on a clay

for catalytic epoxidation with TBHP. 117 This supported catalyst was 3 times more

active than its soluble analogue. The (salen)Mn'Y complexes also catalyze the

oxidation of y,6-unsaturated ketones by oxygen, 51 and of enol ethers by PhIO.74

Remarkably high enantioselectivities have been obtained for trisubstituted

and cis-disubstituted alkenes using the Jacobsen and Katsuki type (salen)Mnm

complexes. 118 Jacobsen and others have used chiral salen-based manganese(III)

catalysts for the asymmetric epoxidation of cyclic 1,3-dienes 69 and cis-9-methyl-

styrene 62 with terminal oxidants PhlO and NaOCl. The presence of bulky and

electron-donating substituents at the 5,5-positions of the salen ligand afford slightly

higher selectivity. Both the terminal oxidants produce a common oxo intermediate,

[(salen)Mn"O], as the oxidizing agent. In contrast, the (salen)Ni"- and (salen)CoU

catalyzed epoxidations by NaOCl appear to involve 010 as the oxidizing agent. 61

Recently, Norrby and co-workers 119 and Hamada et al. 1 20 independently have

proposed that manganese-salen catalyzed epoxidation proceeds through a metalla-

oxetane intermediate. The high enantioselectivity observed in this reaction 120 is

attributed to twofold stereodifferentiation, enantioselective metallaoxetane formation

and diastereoselective decomposition of the metallaoxetane.

Pietikainen has studied the epoxidation of unfunctionalized olefins (1,2-di-

hydronaphthalene and trans-1-methylstyrene) using Jacobsen-type chiral Mn(IlI)-

salen catalysts with a nitrogen heterocycle as axial ligand in the presence of 30%

aq. hydrogen peroxide 68 and tetra-n-butylammonium periodates 121 as oxidants and

12

reported reasonable yields and enantioselectivity. Also, it has been noted that N-

alkylimidazoles behave as effective axial ligands in the aerobic asymmetric

epoxidation of unfunctionalized olefins catalyzed by optically active (salen)Mnm

complexes. 66

Chiral Schiff base manganese complexes based on optically active 1,2-

diamines have been used in the epoxidation of cis-disubstituted olefins, 122 cyclic

dienes 123 or polyenes. 124 This family of catalysts was also recently found to be

efficient for the enantioselective epoxidation of styrene 125 and hydroxylation at the

allylic position of 1,2-dihydronaphthalene-1,2-oxide. 126 Katsuki and co-workers 127

reported that the epoxidation of cis-disubstituted olefins or styrene derivatives

could be catalyzed by manganese complexes of chiral Schiff bases containing

four chiral centres. Bernardo et al. 75 have developed a new series of chiral Schiff

base complexes by condensation of 1,1'-binaphthyl-2,2'-diamine with various

substituted salicylaldehydes and by subsequent metalation with Mn, Fe, Co, Ni,

Cu or Zn. The new chiral manganese complexes have been used as catalysts

in the oxygenation of prochiral olefins and sulphides using NaOCl, H 202 and

MCPBA. The complexes bearing chioro- and bromo-substituents are more reactive

than those bearing t-butyl group. The catalytic activity and enantioselectivity of

these binaphthyl complexes are found 75 to be poor compared to those of their

diaminocyclohexyl analogoues synthesized by the condensation of 1,2-diamino-

cyclohexane with 4-(dimethylamino)-2-hydroxybenzaldehyde and subsequent

metalation.

13

In the epoxidation reactions of olefins with NaOCI catalyzed by substituted

(salen)Mn m complexes, 128 it has been observed that electron-donating groups on

the catalyst decrease the rate of epoxidation leading to higher enantioselectivities

in epoxidation. These effects on enantioselectivity result from changes imparted

by the substituents on the reactivity of Mn(V)-oxo intermediate. A milder oxidant

is expected to transfer oxygen to alkene via a more product-like transition state,

resulting in more specific nonbonded interactions. 128 Several mechanisms, both

concerted and nonconcerted, have been proposed for the epoxidation reactions

catalyzed by Mn-Schiff base complexes. 129 Most often a radical intermediate is

invoked where an electrophilic oxomanganese species attacks the electron-rich

double bond. 119

In the reactions of (salpn)Mn" complexes (salpn = N,N'-bis(salicylidene)-

1,3-diaminopropane dianion) with TBHP in the presence of a base 130 it has been

established on the basis of electrochemical and isotope labeling

experiments that the active species responsible for the epoxidation of

cyclohexene is [(salpn)Mn hV(0)]2, excluding species such as [(salpn)Mn 1 "(0)] and

[(salpn)Mn']. However, a similar dioxodimeric species has not been isolated in

the case of (salen)Mn III complexes, 52,130 because the steric demands of salen

ligand are too great. Recently it has been reported 131 that addition of a small

amount of water to the reaction mixture speeds up the rate of epoxidation of

unfunctionalized olefins with MCPBA catalyzed by (salen)Mn III complex bound on

a polystyrene-divinylbenzene polymer.

14

Epoxidation of cyclohexene by 02 plus aldehyde in the presence of metal-

cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and metal-porphyrin

complexes (M = Cr, Mn, Co, Ni, Cu and Zn) has been shown 114 to follow a

general mechanism (eqs. 11-15). Cobalt-saloph complexes (saloph = N,N'-bis(sali-

+ RCHO

+ RCO + H

(11)

RCO +02 --- R003 (12)

RCO3 + RCHO

RCO3H + RCO

(13)

+ RCO3H -----> LM 2 =0 + RCOOH

(14)

LM 2 0 + S -----* Ln M n+ + SO (15)

cylidene-o-phenylenediaminato) containing nitrosyl ligand has been used in the

stoichiometric and catalytic oxidation of PPh 3 to PPh30 in the presence of

molecular oxygen by an oxygen transfer mechanism. 132 This is a peculiar reaction

in which the metal formally remains at a constant oxidation state with the redox

chemistry occurring only at the ligand (eqs. 16 & 17). With TBHP, Schiff baseCoH

(saloph)Co(N0) + py + 02 --> (saloph)Co(py)NO 2(16)

(saloph)Co(py)NO2 + PPh3 - - (saloph)Co(N0) + PPh 30 + py (17)

complexes catalyze the oxidation of anilines 76 to nitro and imine derivatives and

t-butylphenols77 to t-butyl-peroxylated products. Ru(ill)-saloph 58 and -salen59

complexes catalyze epoxidation of olefins by PhlO and molecular oxygen

respectively and involvement of high-valent oxoruthenium(V) species has been

established.

15

1.2 Oxygen Atom Transfer to Organic SulphidesCatalyzed by Metal Complexes

Sulphides, owing to their high reactivity compared with that of other

oxidizable nucleophiles, e.g., alkenes or tertiary amines, are excellent model

compounds for mechanistic studies. 133 The oxidation of dialkyl and alkyl aryl

sulphides with hydroperoxides in the presence of catalytic amount of TiO(acac)2

(acac = 2,4-pentanedione) has been studied in aqueous ethanol at 25 00.84 A

non-linear dependence of rates on substrate concentration is observed for

sulphides which are not sterically hindered, whereas for the t-butyl derivatives, a

kinetic first-order is found. An inhibitive complexation' by the substrate has been

suggested as the sulphide-complexed peroxotitanium compound is less effective

electrophilic than the uncomplexed one. An efficient asymmetric

oxidation of prochiral organic sulphides into optically active sulphoxides has been

reported 134 using a modified Sharpless reagent - Ti(OPr') 4/diethyl (R,R')-tartrate

(DET) with cumOOH and TBHP (eq. 18). The oxidative coupling of an aryl

•'\2°

S )h(Me)C(OOH) Ti(O-P)4/DET (18)

CH202,-20°CM' 'ML

sulphide with oxygen to form the corresponding sulphonium cation by an

electron-transfer has been catalyzed with VO(acac) 2 in presence of tetrabutyl-

ammonium perchlorate in CH2012 92 (eq. 19).

SCH,SCH, Demethylation

C113

U^-S-0- SC H3 (19)

Ir1

The reaction of di-n-butyl sulphide with TBHP in presence of V0(acac)2

proceeds by a mechanism involving rate-determining nucleophilic attack by the

sulphide on the 0-0 bond of a hydroperoxidevanadium(V) complex. 80,85 lodyl-

arenes (Ar102) improve 135 the selectivity and yield of sulphoxide formation in this

system. The asymmetric oxidation of sulphides into the corresponding sulphoxides

has been achieved with organic hydroperoxides in the presence of optically active

Schiff base-oxovanadium(IV) complex. 53

For oxidation reactions performed by cytochrome P-450 dependent mono-

oxygenases, two possible ways viz., a one-electron abstraction (Route A) and a

direct two electron oxidation (Route B) have been proposed for the interaction of

the reactive oxidizing species, Fe 1--0 with sulphur. 13

R–S–R

R–S--RB

o-J 0< 0—Fe

Fe > R–S—R

0

An enzyme model system, consisted of (TPP)Fe tU Cl/imidazole has been

found to catalyze H 202 dependent S-oxygenation and oxidative dealkylation of

organic sulphides. 32 Watanabe et al. 136- 1 38 have suggested that the oxygenation

of divalent sulphur compounds with cytochrome P-450 proceeds through one-

3+electron transfer from the sulphenyl sulphur to the active centre (FeO ) of the

enzyme to give the corresponding suiphenium radicals which eventually afford

17

either suiphoxides, or mixture of suiphoxides and disulphides if the substituent X

is electron-withdrawing 139 (eq. 20). By using a bonafide electron transfer oxidant,

0

Ar-S-CI-1-X + Fe3+.

Ar-S-C,-X LyiD-4cn'> Ar-S-CH 2X (20)

(FeO")Y2ArSSAr + XCHO + Fe3

namely potassium 12-tungstocobalt(Ill)ate, K5(Co III W12 O40), in the oxidation of

benzyl sulphides and sulphoxides, Baciocchi et al. 140 have established that oxygen

transfer is the most likely mechanism. A single-electron transfer involving the

formation of a cation radical has been postulated in the oxidation of alkyl aryl

sulphides by Fe(lll)-polypyridyl complexes. 141

A chiral iron(lll)-porphyrin (5,10,1 5,20-tetrakis(o-aminopheflyl)POrPhYrifl)

complex has been found 142 to be active as a catalyst for oxygen atom transfer

from PhlO to alkyl phenyl sulphides affording suiphoxides in 60-84% yields. Ferric

porphyrins bearing chiral binaphthalene moieties on their both faces are reported

to catalyze the asymmetric oxygenation of sulphides with PhIO as an oxidant in

moderate to good optical and chemical yields. 143 Total turnover numbers of

sulphoxide formation in the range of 55 to 180 (28-90% yield on the amount of

PhlO used) have been obtained and an electron transfer mechanism has been

envisaged. In this study, donor ligands such as 1-methylimidazole improves the

enantiomeric excesses in 2-3 times. However, in the (TPP)Mn mCl catalyzed

oxidation of sulphides with PhIO addition of imidazole causes a slight decrease

in the reaction rate and this observation has been attributed to the stabilization

of the active species (TPP)MnV O(Cl) by imidazole. 38 The catalyst (TPP)FemCl

18

accelerates the PhlO oxidation of sulphides faster than (TPP)Mn m Cl, however, the

yield of sulphoxide in (TPP)Fe m Cl-catalyzed oxidation is slightly lower than that

in (TPP)Mn"Cl-catalyzed one, because of less stability of oxoiron intermediate. 36

The (TPP)Mn m Cl/NaOCl system also has been reported to be efficient in the

oxidation of sulphides to sulphoxides.37

Optically active Katsuki-type 67 ' 74 and Jacobsen-type 64 (salen)Mnm

complexes are effective catalysts for the oxidation of sulphides to sulphoxides

with modest enantioselectivity. The presence of bulky substituents on the 3,3' and

5,5' positions of the salen ligands has a marked effect on selectivity in sulphide

oxidation with (salen)MnIII catalysts, 64 indicating that these groups improve stereo-

chemical communication in the transition state leading to oxo transfer by inducing

substrate approach near the dissymmetric diimine bridge. 144 An electronic effect

on enantioselectivity is also very pronounced in sulphide oxidation with these

catalysts. Catalysts bearing electron-withdrawing substituents are less enantio-

selective than the electron-rich analogues. This effect has been attributed to the

greater reactivity of the high-valent oxo intermediates bearing electron-withdrawing

groups. 64,128,145 The enantioselective aerobic oxidation of sulphides with the

combined use of molecular oxygen and pivalaldehyde using Jacobsen-type

complexes as catalysts 146 affords optically active sulphoxides in good yields. An

aerobic asymmetric oxidation of sulphides by using optically active 1-oxo-

aldiminatomangaflese(lll) complex catalysts in the coexistence of pivalaldehyde has

been reported by Imagawa et al. 1 47 and they have found that these aldiminato

complexes were more effective than (salen)Mn 1 complexes.

The oxidation of organic sulphides 148 by CrO(02 )2 HMPT (HMPT =

hexamethyiphosphorietriamide) and sulphides and olefins 149 by MoO(02)2HMPT

have been studied. Both the metaldiperoxides appear to act as electrophilic

oxidizers and substrate coordination with the oxidant has been proposed. Recently,

for the oxidation of aryl methyl sulphides by sodium bis(2-ethyl-2-hydroxybutanato)-

oxochromium(V), a mechanism proceeding through a complex formation between

the oxidant and substrate, followed by ligand coupling has been proposed. 91

Acquaye et al. 1 50 have studied the kinetics of oxidation of organic sulphides and

sulphoxides with oxo(phosphine)Ru(IV) complexes. On the basis of electronic-

substrate effect (p = -1.56 for substituted sulphides and -0.42 for substituted

suiphoxides) and electronic-oxidant effect (p 0.49 for substituted Ru(IV)

complexes) studies, single electron transfer (SET) mechanism for sulphide

oxidation and 5N2 mechanism for suiphoxide oxidation have been elucidated.

The transfer of oxygen from sodium perruthenate 151 (Ru0) to sulphide in

an aqueous base has been found to exhibit a first-order dependence on the

concentrations of both the oxidant and the reductant and a p value of -0.66 has

been observed. A mechanism involving an activating expansion of the ruthenium

coordination shell through incorporation of a hydroxide ion is proposed. Oxygen

transfer is then initiated by reaction of a nonbonding pair of sulphur electrons

with either a vacant ruthenium d-orbital or a Ru=O 7t*orbital (eqs. 21 & 22).

!AIJ

011\ I R2SRu -

o o

o 0\ 0H

Ru

o 0

0 01-1

Ru

O 0-SR.

OH

R 2 SO ± 1-JRuO < Ru/\-0

SR,

HRuO ± Ru0 fast ) 2RuO ± H

Similarly, tetra propylammonium perruthenate 52 has

(21)

OH 0-

Ru

0 1 7 0'-IR2S '

(22)

also been used for the

oxidation of sulphides to sulphones in the presence of a cooxidant, N-methyl-

morpholine N-oxide (NMO) (eq.23).

0n-Pr4RuO 4 II

R-S-R2 > R-S-R,NMO,40°C II

0

Methylrhenium trioxide (MTO) has been successfully used as a catalyst in

the oxidation of organic sulphides to the corresponding sulphoxides by hydrogen

peroxide. 153 The kinetics has been carried out at pH 1 in 1:1 acetonitrile-

water(v/v) at 25 °C. Hydrogen peroxide and MTO first react to form 1:1 and 2:1

reactive rhenium peroxides. The mechanism involves the nucleophilic attack of the

sulphur atom on the rhenium peroxide. More recently, a rhenium(V) oxo complex,

Re(0)C1 3 (PPh 3 ) 2 has been reported to catalyze the oxidation of alkyl and aryl

(23)

21

sulphides with diphenyl sulphoxide as the cooxidant. 154 This catalyst is reported

to be mild, efficient, rapid and devoid of sulphone byproducts (eq. 24).

PhS OPh

ReOCI3(PPh3)2RSOR' + Ph 2 S (24)RSR'

CHCI3

1.3 Oxidation of Organic Sulphides by Other Oxidants

Electrophilic oxidants are capable of executing both sulphide to suiphoxide

and sulphoxide to sulphone conversions while nucleophilic oxidants execute only

sulphoxide to suiphone conversion. 155 Mechanism for the oxidation of organic

sulphides by peroxo compounds have been reviewed 156 and studied in detail.

Bonchio et al., 155 more recently, have established, on the basis of HOMO/LUMO

calculations, that in the electrophilic oxidations by peroxides the reactivity of

suiphoxides is considerably less affected by structural effects than that of

sulphides. Modena and others 157,158 have studied the oxidation of organic

sulphides by hydrogen peroxide and other organic peroxides. Modena and

Ma1o1i 157 have obtained p values of -1.13 and -0.98 in the oxidation of substituted

aryl methyl and diaryl sulphides respectively with H 202. Silica gel and basic

alumina mediate the TBHP oxidation of sulphides and sulphoxides. 59 Peroxo

acids 160 have also been used for the oxidation of aryl methyl sulphides. Oxidation

of sulphides carried out with peroxo compounds are electrophilic oxygen transfer

reactions involving a heterolytic splitting of the 0-0 bond, which is promoted by

a concerted inter- or intra-molecular proton transfer. 157,158

22

Srinivasan et al. 161,162 have studied the oxidation of alkyl aryl sulphides

by peroxodisulphate and peroxodi phosphate. Since both the oxidants are iso-

electronic and isostructural they are reported to behave almost in a similar fashion,

the mechanisms involving the nucleophilic displacement of the sulphide sulphur

on the peroxide oxygen of the respective peroxo-salts. Peroxomonosulphate ion 163

(HSO) also oxidizes aryl thiobenzoates to benzoic and sulphonic acids and aryl

methyl sulphides to corresponding suiphoxides in acetonitrile. Oxidation of the

ester is less sensitive to electronic effects (p = -0.6) and are more dependent

on solvent (m = 1 .4, m is the Grunwald-Winstein solvent parameter 164) than those

of the sulphides (p = -1.0 and m = 0.9). A mechanism (eq. 25) which is neither

S N2 nor SET has been proposed.1-I I-I

R I+ 0 - 0SO > [R2S 0 - 0s0] r> R2 SO + H0SO (25)

R

A mechanism involving a one-step electrophilic oxygen transfer from MnO

through a polar product-like transition state has been proposed by Banerji 165 for

the oxidation of aryl methyl, alkyl phenyl, dialkyl and diphenyl sulphides by per-

manganate ion to yield the corresponding sulphoxides. However, Lee and Chen 166

have disproved the above mechanism on the basis that permanganate ion acts

as an powerful electrophile in its reactions with alkenes and alkynes. 167 They

proposed an alternative mechanism (eq. 26) that is initiated by the formation

R +M7> RN N 0 slow R ° 0N/N7

V o NQ V o' NQ V - N07

R2 SO + I-I 2Mn0 (26)

23

of a coordinate covalent bond utilizing an unshared pair of sulphur electrons and

empty manganese d-orbitals. Rearrangement of this intermediate then leads to

the formation of suiphoxide and manganate(IV) ion. The electrophilic nature of

manganese centre is also established in a study of reduction of manganate(Vl)

by mandelic acid. 168 A SET mechanism has been proposed for the Mn 3 oxidation

of alkyl phenyl sulphides. 169 Barium permanganate 170 also oxidizes sulphides in

refluxing acetonitrile to sulphoxides in reasonable yields (54-80%) under mild

conditions. In the perborate oxidation of organic sulphides, 171 different oxidizing

species and rate-determining steps have been proposed for alkyl phenyl, diphenyl

and dialkyl sulphides.

The oxidation of dialkyl and diphenyl sulphides by pyridinium chloro-

chromate (FCC) 172 is found to be catalyzed by organic acids and a Michaelis-

Menten behaviour has been reported. The kinetics of FCC oxidation of several

ortho- and para-substituted phenyl methyl sulphides in binary solvent mixtures of

60% (vlv) aq. acetic acid and 50% (vlv) chlorobenzene-nitrobenzene has been

reported. 173 Fyridinium flourochromate (FF0) also oxidizes several aryl methyl,

alkyl phenyl, dialkyl and diphenyl sulphides to the corresponding sulphoxides and

a rate-determining electrophilic oxygen transfer from PFC to the sulphide has been

proposed. 174

The Cr(Vl) oxidation of alkyl aryl and diphenyl sulphides has been studied

in aqueous acetic acid and aqueous acetonitrile by Srinivasan and others. 175-178

On the basis of p value (-2.07) and the excellent correlations obtained between

log k and oxidation potentials/ionization energies of the sulphides, a SET

24

mechanism has been suggested 175 (eqs. 27-29). A similar mechanism has been

ArN:S + Cr(VI) + + slow> Ar -S - Me + Cr(V)

Me (HCrO4) (I-I2CrOT)

+. Ar

Ar - S - Me + Cr(V)

S- 0- Cr(V)Me

Ar ArS- 0 - Cr(V) solvolysis

= 0 + Cr(IV) + F1

Me

proposed for the Cr(Vl) oxidation of aryl methyl suIphoxides. 79 The effect of

picolinic acid, 2,2'-bipyridyl, 1,10-phenanthroline, pyridine, 2,6-dicarboxylic acid and

EDTA (ethylenediaminetetraacetate) as catalysts in this oxidation has also been

studied. 177 ' 178 The micellar effect on the Cr(Vl) oxidation of dialkyl sulphides has

been investigated 180 and found that anionic micelle enhances and cationic micelle

inhibits the rate of oxidation. A mechanism involving the rate-determining nucleo-

philic attack of sulphide on Cr followed by fast ligand coupling between 0 and

S to form sulphoxide has been formulated.

Cerium(IV) ammonium nitrate-catalyzed autooxidation of sulphides has been

described by Riley et al. 8 A zwitterion R2S00 has been envisaged as the

probable intermediate (eqs. 30-33). But, for the oxidation of dialkyl, alkyl aryl and

R2S + Ce(IV) ------* R2S + Ce(lll)

(30)

R2S + 02 ----*

R2SOO

(31)

R2SOO

+ Ce(lll) ----> R2S00 + Ce(IV)

(32)

R2S0O

+ R2S

2R2S0

(33)

(27)

(28)

(29)

25

diaryl sulphides with Ce(IV) carried out in the absence of oxygen, 82 an electron-

transfer mechanism has been proposed on the basis of observed electronic effect

(p = -3.3) and also supported by the observation that alkyl aryl sulphides are

significantly more reactive than dialkyl sulphides and that the reaction rate is

slowed by added Ce(lll) (eq. 34).

ArSR + CeVONO2 ArS + Cc'0NO 2 ), ArSOR (34)

Lead tetraacetate (LTA) has been found 183 to oxidize various organic

sulphides to sulphoxides. In a detailed kinetic study, Banerji 183 has reported that

this oxidation is catalyzed by H and susceptible to changes in the solvent

composition. A mechanism involving a nucleophilic attack of sulphide on lead in

LTA in the rate-determining step has been postulated (eqs. 35-37).

R RN

S + Pb(OAc) 3 - OAc slow> N S - Pb(OAc)3 + Ac0 (35)

R'

R R+N fast N

S - Pb(OAc)3 S - OH + Ac20 + Pb(OAc) 2(36)R' R'

0R II

- OH + Ac0 fast R - S - R' + AcOH(37)

Other oxidants known to oxidize organic sulphides are iodosobenzene, 184 phenyl-

iodosodiacetate (PIA) 185,186 selenonic acids 187 and Zn(Mn04)2.188

26

1.4 Electron Transfer Reactions

Redox reactions involving transition metals having more than one stable

oxidation states are of wide importance in chemistry. 189,190 The role of transition

metal ions in life processes depends on their ability to participate selectively in

electron transfer reactions. The electron exchange between an inorganic oxidant

and reductant is often a chemically reversible process owing to the common

occurrence of thermodynamically stable redox pairs differing by a single

electron. 189,190 Electron transfer reactions may involve one, two or more electron

exchange but simultaneous transfer of more than one electron has lower

probability and greater Franck-Condon barrier. Changes of oxidation number

greater than two will not occur in a single step of any redox reaction. As a

consequence unusual valences are to be expected for intermediates in redox

reactions.

The redox reactions of metal complexes are considered to proceed through

two mechanisms. In the inner-sphere mechanism, a bridging ligand is coordinated

to both the oxidant and the reductant and forms part of the first coordination

sphere of each. In the outer-sphere mechanism, the two separate inner

coordination spheres remain intact. However, deciding between inner-sphere and

outer-sphere mechanisms still remains a major problem. When one of the two

reactants are substitution-inert, the mechanism is likely to be inner-sphere and

when both of the reactants are substitution-inert outer-sphere is the probable

mechanism. There are also a number of methods 190 to distinguish between these

two mechanisms.

27

The redox reaction' of organic substances with metal complexes is not as

simple as in the cases of inorganic reactants. Only recently, electron transfer

concept in organic chemistry is being given much attention. The electron

detachment from most electron-rich organic donor (D) generates transient cation

radicals and the analogous electron attachment to electron-poor organic acceptors

(A) generally affords transient anion radicals. 191 This leads to a mechanistic

situation in which the stepwise formation of products via electron transfer (eq. 38)

k, _____D + A ________ - > products (38)k,

is kinetically difficult to distinguish from a concerted single-step process, especially

when back-electron transfer and the follow-up steps are facile. 191,192

1.4.1 Theories of Electron Transfer

Electron transfer from donor to acceptor is in some respects a simple

process, but its theoretical treatment presents considerable difficulty. These

theories take into account factors such as solvent reorganization, free-energy

change and electron transfer distance. In non-adiabatic theories, developed by

Weiss 193 and by Marcus et al., 194 electron is treated as a discrete particle and

it jumps from one potential energy surface to another through a potential energy

barrier. The adiabatic theories of electron transfer, developed by Marcus 195 and

Hush, 196 are based on the idea of a single potential energy surface. The electron

is not considered as a discrete particle; instead the system passes through a

series of intermediate states as in any chemical reaction, as represented in eqs.

39 and 40.

28

D + A [D ---- A] (39)

[D ---- A] kcr > [D A]

(40)

In eq. 39, the diffusion together of reactants forms an precursor complex with

the equilibrium constant K, followed by eq. 40 where electron transfer occurs

within the precursor complex to form a successor complex. The rate constant for

the electron transfer reaction is given by the expression

1kei = Ke i vn exP[T (1

+ AG 2) j (41)

In eq. 41, Kei is the electronic factor (transmission coefficient) which is assumed

to be unity in the classical Marcus theory; 2 is the reorganization energy needed

before the electron can have the same energy in both ions and is made up of

two components, 2, the inner-shell (intramolecular) and, 2, the outer-shell

(solvent) reorganization energies; AG is the free-energy change accompanying

the electron-transfer. The application of this equation involves determination of

AG and 2. This aspect with respect to the present study has been dealt in

detail in Chapter 2.

1.5 Structure-Reactivity Relationships

For justifying and generalizing a particular reaction mechanism for similar

reactions, almost all the kinetic studies invoke structure-reactivity relationships

which depend on the empirical and qualitative rule that like substances react

similarly and that similar changes in structure produce similar changes in

reactivity. 197 The most successful quantitative correlation between structure and

29

reactivity is given by the Hammett equation, 198 a linear free-energy relationship,

log k = log k0 + pc (42)log K = log K0 +Pa (43)

where k or K is the rate or equilibrium constant respectively, for a side-chain

reaction of meta- or para-substituted benzene derivatives. The k0 or K0 denotes

the corresponding quantity for the parent compound. The substituent constant

is independent of the nature of the reaction and gives a measure of the polar

effect of replacing H by a given substituent (in the m- or p-position). The reaction

constant p depends on the nature of the reaction and its conditions (reagent,

catalyst or temperature) and is independent of substituents. For evaluating CY for

a given substituent the ionization of benzoic acid in water at 25 °C is chosen

as the standard process for which p was arbitrarily defined as 1.00.

Hammett equation is applied to a given reaction by getting a best straight

line, by the method of least squares, between log k or log K and and the

slope of that line gives the value of the reaction constant p. The success of the

Hammett equation is commonly assessed 199-203 in terms of correlation coefficient

(r) and the standard deviation (s).

To account for the failure of Hammett equation in reactions where the

conjugation involving substituent and reaction centre is substantially more marked

than in the ionization of benzoic acids (cross-conjugation) 204,205 exalted'

constants (cr and ) are introduced. The f values are used for +R substituents

(e.g. NO2 , ON, 002H, CO2Me and S02Me) and are based on the ionization of

anilinium ions or of phenols in water. The values, introduced by Brown and

30

Okamato206 based on the solvolysis of t-cumyl chloride in 90% acetone-water at

25 00, are-used for -R substituents (e.g. OMe, Me, OH, NH 2, SMe and Hal).

The differences ( - ) and ( - c) give a measure of conjugative ability of

a given acceptor and donor respectively. Both and values are sometimes

used in the same correlation for reactions in which an electron-deficient/electron-

rich reaction centre can directly conjugate with electron-donating/electron-

withdrawing substituent.

On the view that the contribution of the resonance effect of a substituent

must vary continuously as the electron-demanding quality of the reaction centre

is varied, Wepster207 introduced a 'sliding scale' of c (unexalted) values, called

G". Taft 208 also evaluated similar set of unexalted a constants, called cyo , on

the basis of ionization of phenylacetic and phenyipropionic acids.

To deal with the influence of -R and +R substituents respectively on

reactions that are more or less electron-demanding than the ionization of benzoic

acid, Yukawa and Tsuno209 and Yasioka210 formulated equations 44 and 45,

known as Yukawa-Tsuno equations,

log k = log k° + p( + rA(5+ ) (44)

log = log AcTR (45)

where =- and A = - c. r± gives a measure of the extent to which

cross-conjugation of substituents with reaction centres stabilizes the transition state

or product relative to the initial state. The r in eq. 44 can have values varying

from 0 to unity and values greater than one are also possible for r in eq. 45.

IN

A quantitative separation of substituent effects into inductive and resonance

contributions by Taft 211 led to the possibility of a 'dual substituent parameter'

(DSP) treatment of reaction series, where simple correlations based on Hammett

equation fail, in the form of eq. 46

log (k/k0) 01 cy + R R (46)

where cT, and a are the inductive and resonance substituent constants, and p1

and PRare the corresponding reaction constants.

1.6 Scope of the Present Investigation

Metal complexes containing porphyrin, Schiff base and other ligands have

widely been explored in the past decades as catalysts in the selective and efficient

oxidation of organic substrates by oxygen transfer processes 21-94 as models for

cytochrome P-450 dependent monooxygenase enzymes. These studies differ from

one another mainly on one or other of the four aspects viz., metal, ligand, terminal

oxidant and substrate. The present investigation employs manganese as metal,

salen as ligand, iodosobenzene and hydrogen peroxide as terminal oxidants and

organic sulphides as substrates. The choice of this system may be rationalized

on the following aspects.

Of the many efficient transition-metal complexes based oxidation systems

known, those involving high-valent manganese ions are among the most numerous

and well studied. The preparation of cytochrome 450cam' which acts as a

catalyst for the epoxidation of enzyme-bound alkene substrates in the presence

of PhIO as oxygen donor, by Gelb et al. 212 demonstrates that manganese can

32

provide a reasonable substitute for iron in model studies of cytochrome P-450.

Also, manganese has many biological significances associated with it and plays

an essential role in the metabolism of dioxygen and its reduced forms. Few

examples are: the dismutation of superoxide to hydrogen peroxide and dioxygen

by manganese superoxide dismutase (MnSOD), 213 the conversion of hydrogen

peroxide to dioxygen and water by the manganese catalase (azide-insensitive

catalase), 214 and the four-electron oxidation of two water molecules to form

dioxygen by the photosynthetic oxygen-evolving complex, photosystem II (PSII)

which has a tetranuclear manganese complex in the active centre. 215,216

Manganese clusters are found to be catalytically active in manganese ribo-

nucleotide reductase. 217 In model studies also, manganese complexes 27-29,33-35

are very much reactive than the corresponding iron 25,26,30 and chromium 49,52

complexes towards hydroxylation and epoxidation of hydrocarbons. In view of

these facts, manganese is a suitable metal to be employed in model oxygen

atom transfer studies.

In model studies porphyrins and salen are the mostly used ligands. The

porphyrin catalysts generally give lower yields of oxygenated products and are

more difficult to synthesize than the corresponding salen systems. Dianionic salen

ligand stabilizes metal complexes in high oxidation states. 48,115 The salen ligand

consists of a rigid and kinetically nonlabile template wherein steric and electronic

properties of the metal centre may be tuned in a synthetically straightforward

manner. In particular, substituents on the 5-positions which are para to the pair

of ligating oxygen atoms strongly perturb the redox properties of the metal

W

complexes. 128,218 The steric properties of the complexes are modulated by the

substituents present at the etheno bridge. 50,144 The facile synthesis and ease of

modifying the electronic and steric environment of the metal complexes through

substitution at 5 and 8 positions makes salen as excellent ligand for model oxygen

atom transfer studies.

Among the various extensively used terminal oxidants, iodosobenzene, first

introduced by Groves and others 24.25 as an oxygen donor, has proved a valuable

reagent for formation of oxometal species and thus as an oxygen source in

catalytic oxygenations of organic substrates. Because of its limited solubility in

most of the organic solvents (CH 3CN, CH 2Cl2 , etc.) and its simple mode of

oxygen transfer (giving only PhI as byproduct) it finds immense use in model

oxygen atom transfer studies. Hydrogen peroxide is also a cheap and simple

oxygen donor and water is the only byproduct. 219 Further it has been shown that

PhlO and H 202 , in the presence of same metal complex, oxygenate hydrocarbons

by different mechanisms. 33,54

Oxidation reactions capable of converting sulphides to sulphoxides and then

to sulphones could perhaps be useful in the detoxification of harmful and

poisonous substances like nerve agents and mustard gas. 220,221 Insecticides

which are sulphoxides are commercially manufactured by oxidizing sulphides with

hydrogen peroxide and the conversion of penicillin to their S-oxides is

commercially important for cephalosporin derivatives. 222,223 Although a large

number of reports on the reactivity of oxometal complexes have appeared, most

of them describe hydroxylation and epoxidation of hydrocarbons; those dealing

34

with the oxidation of compounds of hetero atoms are very limited. In fact, sulphur

compounds are more reactive than the other oxidizable nucleophiles, e.g., alkenes

and tertiary amines. Many sulphur containing compounds are present in biological

systems and some play key roles in the activity of some enzymes. 224 These

enzymes are often deactivated by active oxygen species. Further, the reactions

of sulphur with oxygen are complex because of the number of oxidation states

of sulphur, which can lead to variety of intermediates and products. 225 The fact

that cytochrome P-450 can readily catalyze the oxygenations of nitrogen and

sulphur compounds 11 makes the study of reactivity of oxometal complexes towards

organosulphur compounds interesting and useful in elucidating the mechanism of

biologically important oxygen atom transfer processes.

The lack of a systematic kinetic investigation in almost all of the studies

reported so far on the oxidation of organic sulphur compounds catalyzed by

manganese- and other metal-salen complexes in the presence of terminal oxidants

prompted the author to undertake the present kinetic study. Here are presented

the results obtained in the kinetic studies on the PhlO and H 202 oxidations of

aryl methyl, alkyl phenyl, dialkyl and diphenyl sulphides catalyzed by cationic

(salen)Mn' complexes. The experiments have been focussed to explore the

following aspects:

(i) The mechanism by which (salen)Mn III complexes catalyze oxygen atom

transfer from PhlO and H 202 to aryl alkyl, dialkyl and diaryl sulphides.

(ii) The effect of donor ligands such as pyridine N-oxide, pyridine and 2-

methylimidazole.

(iii) The substituent effects of para-substituted phenyl methyl sulphides.

35

(iv) The effect of variation in electronic and steric environment of the salenligand. p

(v) The steric effects in alkyl phenyl sulphides.

(vi) The substituent effects of 4-substituted diphenyl sulphides.

(vii) The applicability of Reactivity-Selectivity Principle (RSP) in (salen)Mnm/PhlO - sulphide system.