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.