iodine in organic synthesis

Journal of Scientific & Industrial Research

Vol. 65, April 2006, pp 299-308

Iodine in organic synthesis

Ajoy K Banerjee1,* , Willliam Vera

1, Henry Mora

1, Manuel S Laya

1, Liadis Bedoya

1 and Elvia V Cabrera

2

1IVIC, Centro de Química. Apartado 21827, Caracas 1020-A, Venezuela 2Universidad del Zulia, Facultad de Ciencias, Departamento de Química, Maracaibo, Venezuela

Received 15 September 2005; revised 27 January 2006; accepted 03 February 2006

Commercially available iodine has played an important role in organic synthesis. This review discusses the versatile

uses of iodine in different chemical transformations. Reactions include esterification, cycloaddition reaction, allylation of

aldehydes, acetalization of carbonyl compounds, acylation of alcohols, synthesis of cyclic ethers and aromatization of α,β-

unsaturated ketones.

Keywords: Cycloaddition, Esterification, Iodine, Organic synthesis

IPC Code: C01J1/00

Introduction Selective functional group transformation of com-

plex molecules to target compounds is the most im-

portant requirements of modern organic synthesis.

The lack of selectivity blocks the road for obtaining

the desired compounds. Iodine, commercially avail-

able brown solid, m p 113oC, has found widespread

use not only in conduction of selective transformation

but also shown interesting and varied reactions. The

present review concentrates on the utility of iodine in

certain organic transformations, excluding iodolac-

tonization and iodocyclization studies1-5

.

Synthesis of Benzodiazepine Derivatives

Synthesis of 1,5-benzodiazepine derivatives from

phenyldiamines and acyclic ketones under mild con-

ditions in presence of iodine as catalyst has been re-

ported6. O-Phenyldiamine 1 with acyclic ketone 2 and

cyclic ketone 3 yields benzodiazepines 4 and 5 re-

spectively (Scheme I). Such synthesis can also be ac-

complished in presence of other catalysts (boron

trifluoride etherate, sodium borohydride, polyphos-

phoric acid, silicon dioxide, etc.) but these procedures

suffer from drastic reaction conditions, low to moder-

ate yield and occurrence of several side reactions. Io-

dine helps to carry out reactions under neutral and

mild condition and in high yield. In addition, work-up

procedure is very simple and reaction can be per-

formed at room temperature.

Esterification and Transesterification

Iodine has been utilized as Lewis acid catalyst for

esterification7 of acids (saturated, unsaturated, hy-

droxy and dicarboxylic acids) with alcohols. Thus,

conversion of acids 6-7 to corresponding esters 8-9

has been accomplished in high yield by heating with

methanol at refluxing temperature (Scheme II). Esters

of tertiary alcohols, which are difficult to prepare, can

be obtained by heating the acid with t-butanol but it

requires longer reaction period and increased amount

of catalyst. The carboxylic acid group, directly at-

tached to aromatic ring such as benzoic acid, p-

nitrobenzoic acid, can not be esterified.

Transesterification of esters with alcohols have

been accomplished using molecular iodine. Thus es-

ters 10-11 on heating with alcohols (n-butanol) in

presence of iodine are converted to esters 12-13

——————

*Author for correspondence

Tel: 58-212-5041324; Fax: 58-212-5041324

E-mail: [email protected]

NH2

NH2

NH

N

NH

NMe

Scheme I

1

2

5 (95%)

4 (99%)

Me Me

O

O

3

Me

Me

Me

Me

Ph

Me

+

I2, MeCN

25º C, 5 min

I2, MeCN

25º C, 10 min

J SCI IND RES VOL 65 APRIL 2006

300

H2C=CH-COOHI2, MeOH, 12 hr, reflux

H2C=CH-COOMe

6 8 (95%)

C6H5-CH2-COOMe

7 9 (95%)

C6H5-CH2-COOH

Scheme II

I2, MeOH, 10 hr, reflux

respectively (Scheme III). Vegetable oils (castor, pea-

nut, coconut, jatropha) have been smoothly trans-

esterified with methanol. Esterification and trans-

esterification reactions are highly sensitive to mois-

ture but the reaction catalyzed by iodine does not re-

quire special precaution to exclude moisture or air

from the system. Simultaneous esterification and

transesterification reactions can also be accomplished

using iodine.

Allylation of Aldehydes Several aldehydes (aromatic and aliphatic) can be

converted to the corresponding homoallylic alcohols

with allylic trimethylsilane in presence of iodine in

acetonitrile8. Thus the homoallylic alcohols 16-17

have been obtained in high yield from the correspond-

ing aldehydes 14-15 (Scheme IV). These reactions

proceed smoothly at 0oC and with high selectivity.

The resulting products can be isolated after a short

reaction period.

Cycloaddition Reactions Intramolecular (4+2) cycloaddition

9 of O-

quinonemethanes (generated in situ from O-

hydroxybenzaldehydes) and unsaturated alcohol has

been accomplished in presence of trimethyl orthofor-

mate (TMOF) and elemental iodine to obtain the cor-

responding trans-annelated pyrone 3,2-c benzopyrans

in high yields with high diastereoselectivity

(Scheme V). This method has provided a useful

method for the synthesis of benzopyrans 21-22 re-

spectively from hydroxybenzaldehydes 18-19 and

unsaturated alcohol 20. The remarkable advantages

that have been noted in use of elemental iodine in this

cycloaddition include high yield, mild reaction condi-

tion, high diastereoselectivity, short time period and

simplicity in operation.

Hydrides from Alcohol A variety of substituted benzohydrols 23-24 have

been reduced by a mixture of hypophosphorous acid

(H3PO2) and iodine in acetic acid to the corresponding

methylene derivatives 25-26 in high yield10

(Scheme VI). Acetic acid is the best solvent for these

transformations. The present method would be prefer-

PhCH=CH-COOMe PhCH=CH-COO(CH2)3Me

10 12 (88%)

H2C=CH(CH2)8COO(CH2)3-Me

11 13 (92%)

H2C=CH(CH2)8-COOMe

Scheme III

I2, Me(CH2)3OH, 20 hr, reflux

I2, Me(CH2)3OH, 20 hr, reflux

CHO

Scheme IV

14 16 (96%)

SiMe3 OH

NO2

CHO

15 17 (85%)

SiMe3 OH

NO2

I2, 0ºC, 50 s

I2, 0ºC, 60 s

, MeCN

, MeCN

OH

CHO

21 (85%)

O

O

HO

MeMe20

PhOPhO

Scheme V

18

OH

CHO

19 22 (92%)

O

O

I2, CH2Cl2, TMOF,

2 hr, rt

20

I2, CH2Cl2, TMOF

1.5 hr, rt

MeMe

MeMe

red not only in terms of cost and yield but easy ma-

nipulation in comparison with previous methods10

for

deoxygenation of benzohydrols.

Acylation of Alcohols Acetyl group is widely applied in protection of the

hydroxyl functionality in organic synthesis11

. An ex-

cellent method for acylation of alcohols (primary,

secondary, tertiary) and benzylic alcohols with acetic

anhydride has been developed12

under solvent-free

conditions in presence of iodine at room temperature.

The conversion of many alcohols 27-28 to respective

29-30 acetates has been accomplished in high yield

utilizing this procedure (Scheme VII). The reaction is

very slow in absence of iodine and the functional

groups like chloro, double and triple bonds are not

affected during the reaction. It is necessary to indicate

BANERJEE et al: IODINE IN ORGANIC SYNTHESIS

301

Scheme VI

23 R = H 25 (100%)

C

OHH2C

R R

24 R = Me 26 (100%)

H3PO2, I2 (cat)

HOAc, 60ºC, N2,

26 hr

H

29 (99%)

Scheme VII

27

28 30 (99%)

OAc

OAc

OH

MeOMeO

OH Ac2O (1.05 eq)

I2 (cat), 1 min,

rt

Ac2O (1.05 eq)

I2 (cat), 1 min,

rt

that there exists many catalysts12

, which can be util-

ized for acetylation, but they suffer from many disad-

vantages and therefore iodine appears the most con-

venient and efficient catalyst for acetylation.

Oxidation of Benzylic Alcohols Many methods have been developed for the oxida-

tion of alcohols to aldehydes and to ketones because

this is a common reaction in organic synthesis13

. Re-

cently, Banik et al14

have shown (Scheme VIII) that

iodine can be used in the oxidation of benzylic alco-

hols 31-32 to the corresponding ketones 33-34 respec-

tively in high yield, under microwave irradiated

method.

Protection of Carbonyl and Hydroxyl Groups

Protection of carbonyl group and hydroxyl group

becomes necessary requirement during the synthesis

of multifunctional organic molecules. Blocking of

carbonyl group as thioketals is widely used owing to

its stability toward a wide range of reagents. Thi-

oketalization is usually performed in presence of

acids15

. Recently, the thioketalization of several alde-

hydes and ketones has been carried out in tetrahydro-

furan in presence of catalytic amount of iodine16

.

Probably hydroiodic acid is the actual catalyst in-

volved in this reaction. This method can be applied in

transformation of carbonyl compounds 35-36 to cor-

responding thioketals 37-38 respectively

(Scheme IX).

The present method can also be applied for the se-

lective protection of ketone in presence of another in a

complex molecule. Iodine has proven useful in

Scheme VIII

Ph

Ph PhOH O

Ph

OH O

31 33 (90%)

32 34 (81%)

Power level 5, I2, (cat)

THF, 8 min, rt

Power level 5, I2, (cat)

THF, 8 min, rt

35 37 (97%)

36 38 (98%)

O

OSS

SS

Scheme IX

MeMe

SH(CH2)2SH, I2, (cat)

THF, 4hr, rt

SH(CH2)2SH, I2, (cat)

THF, 3hr, rt

chemoselective17

thioacetalization of carbonyl func-

tions and trans thioacetalization of O, O and S, O-

acetals and acylals. Dithioacetalizaton of aldehydes

and ketones has been performed in high yield in pres-

ence of catalytic amount of iodine supported on alu-

mina surface18

. The reaction can be carried out under

mild, neutral and solvent free conditions. The conver-

sion of carbonyl group to dithioacetal or dithioketal

has been reported employing samarium and iodine in

acetonitrile19

. Not only thioketal, carbonyl group has

also been protected as acetals and ketals. The reaction

is generally performed in presence of acids but this

process suffers from several defects namely long reac-

tion time, reflux temperature, undesired side reactions

and non-selectivity. These difficulties have been

overcome by using iodine20

. Thus aldehydes 39-40

and ketone 41 have been protected as acetals 42-43

and ketal 44 respectively by using catalytic amounts

of iodine and methanol or ethanol (Scheme X).

Iodine catalyzed acetalization is simple, mild, se-

lective and new. The utility of iodine has been in the

conversion of several types of carbonyl compounds to

their 1,3-dioxanes by the use of 1,3-

bis(trimethylsiloxy) propane (BTSP) and a catalytic

amount iodine has recently been reported21

. It is

known that protection of hydroxyl group as tetrahy-


302

41 44 (90%)

OOO

Scheme X

39 42 (98%)

40 43 (95%)

O O

PhCHO CH

OMe

OMe

Ph

CHOMe

OMe

CHO

I2 (10% , MeOH)

1 hr, rt

I2 (10%, MeOH)

1 hr, rt

I2 (10% , EtOH)

4 hr, rt

Scheme XI

45 47 (91%)

46 48 (92%)

CH2OH CH2OTHP

OH OTHP

I2, DHP (1:3 eq)

Power level 3, 7 min

I2, DHP (1:3 eq)

Power level 3, 7 min

dropyranyl ether is very common in schemes of an

organic synthesis strategy. Protection of hydroxyl

group as tetrahydropyranyl ether has been accom-

plished with several reagents22

. Deka & Sharma23

have shown that protection of alcohols as their tetra-

hydropyranyl ethers in high yield can be performed

without any difficulty through a microwave irradiated

reaction catalyzed by iodine. Scheme XI illustrates

the conversion of alcohols 45-46 to the corresponding

tetrahydropyranyl ethers 47-48 using this method.

Selective protection of one hydroxyl group as its tet-

rahydropyranyl ether in 1, n-symmetrical diol (ethane-

1,2-diol, propane-1,3-diol, butane-1,4-diol, hexane-

1,6-diol, cyclohexane-1,4-diol, etc.) has been re-

ported24

by microwave irradiation of the diol with

dihydropyran catalyzed by iodine.

Reduction In reduction of various functional groups, iodine

has proven to be an important reagent in organic syn-

thesis. It has been reported25

that reduction of olefinic

double bond of several α, β -unsaturated carboxylate

acid derivatives can be realized with metallic samar-

ium and iodine in alcohol at room temperature to the

corresponding saturated products in high yield. The 1,

4-reduction is very slow in absence of iodine. It can

be observed that unsaturated esters 49-50 undergo

smooth conversion to the corresponding saturated es-

ters 51-52 (Scheme XII). The reaction is rapid and

carried out in protic solvent under mild condition.

Chinese scientists have observed that aromatic nitro

compounds can be reduced to the corresponding pri-

mary amines and hydrazines in high yield using sa-

marium metal in presence of catalytic amount of io-

dine under aqueous media26

. Banik et al have also

demonstrated27-30

the use of samarium metal and io-

dine in reduction of aromatic nitro compounds and

imines to the amino derivatives. Halogen and amido

substituents on aromatic ring remain unaffected dur-

ing the reaction. Utility of iodine has also been dem-

onstrated31

in the regioselective reduction of the α, β-

double bond of some naturally occurring dienamides

using sodium borohydride and iodine system.

Cyclic Ether

Several cyclic alcohols 53-57 (angular methyl

group) have been converted32-36

by heating with lead

tetraacetate and cyclohexane in presence of iodine to

respective cyclic ethers 58-62, which proved potential

intermediates for the total synthesis of pisiferic acid

63, glutonisone 64, juneno 65, lactone 66(a), frullano-

lide 66(b) and drimenin 67 respectively

(Scheme XIII). Synthesis of several cyclic ethers and

their utility in the synthesis of terpenoid compounds

has been discussed in detail37

.

Aromatization of α,β-Unsaturated Ketones

Iodine in alcohol has been utilized for the aromati-

zation of several α, β-unsaturated ketones and esters.

Kotnis has reported38

aromatization of a wide variety

of Hagemann´s ester 68-69 to the corresponding p-

methoxybenzoate derivatives 70-71 respectively

(Scheme XIV). The precursor in almost all synthesis

to antibiotic milbemycin β3 is highly functionalized p-

methoxy-benzoate derivative. A short and efficient

synthesis of p-methoxybenzoate has been developed

with the aid of aromatization process of unsaturated

ketones. The aromatized products can be transformed

to p-methoxybenzoic acids and p-hydroxybenzoates,

which is a common subunit present in many marine

natural products39

. Study of Kotnis is based on the

observation of Tamura & Yoshimoto40

who reported

aromatization of cyclo-hexenone using iodine and

methanol at reflux.


303

Scheme XII

PhCOOEt

PhCOOEt

EtOOCCOOEt

COOEt

COOEt

50 52 (94%)

49 51 (92%)

I2, Sm (0), MeOH

1 min, rt

I2, Sm (0), MeOH

10 min, rt

Scheme XIII

53

54

55

56

57

58 (45%)

59 (30%)

60 (62%)

61 (40%)

62 (33%)

63

64

65

66a: R = H66b: R = CH2

67

Me Me

MeOTHP

O

OTHPHOOC

Me

Me Me Me

HO MeOMe

OOMe

Me

CH2

O

Me

Me

MeMe OMe

Me Me

Me

Me Me

Me

O

O

O

MeMe

OH

Me

Me Me Me

Me MeOH

Me

Me

Me

Me

CH2

H

OH

H

Me O

Me Me Me

R

O

H

H

Me MeHH H

MeMe Me

Me Me

O

Me

OH

H

H H H

H H H

HH

H

OH

Cyclohexane, Pb(OAc)4

I2, anhyd CaCO3,

hv 250 W, 1 hr

Cyclohexane, benzene,

Pb(OAc)4

I2, anhyd CaCO3,

hv 250 W, 1.5 hr

Cyclohexane,Pb(OAc)4

I2, anhyd CaCO3,

hv 250 W, 1.5 hr


I2, anhyd CaCO3,

hv 250 W, 1 hr


I2, anhyd CaCO3,

hv 250 W, 1 hr


304

COOEt

Me

O

COOEt

Me

OMe

COOEt

Me

O

COOEt

Me

OMe

Scheme XIV

Me Me

71 (87%)

70 (90%)

69

68

I2, MeOH

reflux, 30 min

I2, MeOH

reflux, 30 min

EtOOC

Me O OH

EtOOC

EtOOC O

EtOOC

EtOOC OH

Scheme XV

75 (77%)73

74 (66%)72

I

I

EtOOC

Me

NaOEt (6 eq), I2 (2 eq)

EtOH, -78ºC, 3hr, rt

NaOEt (6 eq), I2 (2 eq)

EtOH, -78ºC, 3hr, rt

Transformation of a wide variety of easily accessi-

ble 2-cyclohexenone-4-carboxylates 72-73 to the cor-

responding iodophenols 74-75 in high yield has been

accomplished41

with iodine and sodium ethoxide in

ethanol (Scheme XV). 2-Iodophenols are versatile

building blocks for synthesis of a varity of benzohet-

erocyclic systems. Several unsaturated cyclohexenone

derivatives undergo oxidative aromatization42

with

iodine-cerium (IV) ammonium nitrate in alcohol

(methanol, ethanol, 1-propanol, 2-propanol) affording

the corresponding alkyl phenyl ethers in good yield.

Banerjee et al43

recorded aromatization and fragmen-

tation of cyclic diones 76-79 on treatment with iodine

and methanol affording anisole derivatives 80-83 re-

spectively (Scheme XVI). These examples represent

the first report of aromatization and fragmentation of

cyclic diones with iodine and methanol.

Miscellaneous Phukan

44 observed the utility of iodine in

condensation of aldehydes 84, benzyl carbamate and

allyl trimethylsilane for the synthesis of protected

homoallylic amine 85 (Scheme XVII). An efficient

and convenient procedure for Mukaiyama aldol

reaction of silyl enolate 86 and carbonyl compound

87 has been accomplished catalyzed by iodine to

obtain the

MeO

Scheme XVI

80 (70%)76

Me

COOMe

Me

MeO

83 (68%)79

Me

COOMe

Me

MeO

82 (70%)78

Me

COOMe

MeO

81 (65%)77

Me

COOMe

Me

O

MeO

O

O

Me Me

Me

Me

O

O

Me

Me

O

O

MeMe

Dione (6 mmol)

I2 (7 mmol)

MeOH, reflux, 2hr

Dione (7 mmol)

I2 (9 mmol)

MeOH, reflux, 2hr

Dione (6 mmol)

I2 (9.34 mmol)

MeOH, reflux, 2hr

Dione (6mmol)

I2 (8 mmol)

MeOH, reflux, 2hr


305

85 (74-80%)

84

Cl2NH2Ph H ++

O

SiMe3

NHCl2

Scheme XVII

I2 (10%)

MeCN, 25ºC, 10 min

OHO

Ph

88 (87%)86

Ph

87

OTMS

PhCHO+

Scheme XVIII

I2, CH2Cl2

15 hr, rt

91 (84%)89 92 (99%)90S

O

Ph

Ph S

S

Me(H2C)8Me(CH2)8-CHO

Ph

PhOHC

Scheme XIX

I2 (3eq),

AgNO2 (6 eq)

3 hr, rt

I2 (0.6eq),

AgNO2 (1.2 eq)

6 hr, rt

94 (90%)93

OPnB

OTBDMS

OBn

OH

OH

OBn

96 (76%)95

OPnB OH

Scheme XX

I2 - MeOH (1%),

12 hr, reflux

I2 - MeOH (1%),

TLC controled

reflux

98 (77%)97

PreO OAc HO OAcI2 (3 eq), CH2Cl2

1 hr, rt

100 (85%)99

I

OAc

102 (75%)101

HPh

Ph

HH

EtO

H

H

I

Scheme XXI

I2 , Pb(OAc)2

5 - 6 hr, rt

I2 ( 2 eq), EtOH

rt

OMe

104(89%)103

OMe

OMe

OMe

I

106 (87%)105

COOH COOH

I

Scheme XXII

I2, NO2 (excess)

AcOH, H2O, CHCl3(3:1:1)

I2, MnO2 (activated)

AcOH, Ac2O/H2SO4

2hr, r.t then 2 hr, 45 - 55ºC


306

aldol product 88 in high yield (Scheme XVIII). Iodine

has played an important role in deprotection46

of

monothioacetal 89-91 and dithioacetal 90-92

(Scheme XIX), in cleavage of t- butyldimethyl-

silylether47

(OTBDMS) 93-94, p-methoxybenzyl

ether48

(OPMB) 95-96 and prenylether49

(OPre) 97-98

(Scheme XX). Iodination of double bond50-52

99-100,

101-102 (Scheme XXI) and aromatic compounds53-56

103-104, 105-106 (Scheme XXII) has been realized in

high yield. The importance of iodine has been re-

corded in process of deoxygenation57,58

of sulphone

107 to 108, in conversion of various hydroxyphos-

phonate to trimethylsilyloxyphosphonate 109 to 110

under neutral conditions using HMDS59

(Scheme

XXIII), to promote O-glycosidation60

111-112 of gly-

cal, C-glycosidation61

113-114 (Scheme XXIV) of

glycal with allyltrimethylsilane and trimethylsilyla-

tion62

of a variety of alcohols 115-116 and 117-118

(Scheme XXV).

Conjugate addition63

of α, β-unsaturated ketone

119 with allyltrimethylsilane yields adduct 120

(Scheme XXVI) with high selectivity in presence of

iodine. Synthesis of substituted pyrole 123 from

amine 121 and hexanedione 122 using iodine-

catalyzed modified Paul-Knorr method has recently

been published64

(Scheme XVII). 3-Pyroles have also

116 (98%)115

118 (97%)117

PhCH2OH PhCH2OSiMe3

OHHO OSiMe3Me3SiO

Scheme XXV

I2, HMDS

CH2Cl2, 2 min, rt

I2, HMDS

CH2Cl2, 4 min, rt

120 (87%)119

Ph

O O Ph

Scheme XXVI

I2, Allyl trimetyl silane

CH2Cl2, 4 hr, rt

123 (90%)121 122

PhNH2 +Me

MeN

Me

Ph Me

O

O

Scheme XXVII

Iodine (cat)

THF, rt

been synthesized by reductive coupling of diaryl-2-2-

dicyano ethylenes and aromatic nitrile induced by

samarium and iodine65

. Other uses include molecular

iodine published by Wang66

and use of iodine in or-

ganic synthesis67-69

.

Conclusions

This review focusses the importance of iodine as an

effective catalyst for various organic transformations.

Although many observations have not received appli-

cations in synthesis of natural products or complex

structures in details, it is believed that in near future

these observations will be useful in synthesis of these

compounds. The present review would serve the need

of organic chemists engaged in searching new appli-

cations of iodine for organic synthesis.

Acknowledgements

The senior author thanks Fondo Nacional de Cien-

cias Tecnológicas e Innovaciones (FONACIT) and

Instituto Venezolano de Investigaciones Científicas

(IVIC) for financial support.

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108 (100%)107

110 (98%)109

MeS

Ph

O

MeS

Ph

OH

Ph P(OEt)2

O

OTMS

Ph P(OEt)2

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Scheme XXIII

I2 (1.2 eq), NaBH4 (1 eq)

THF, 5 min, rt

I2 (0.01 eq), HMDS (0.7 eq)

CH2Cl2, rt

112 (75%)111

114 (82%)113

O

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OR

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Scheme XXIV

I2, PhCH2OH

SnCl4, -18ºC

I2, Me3SiCN,

CH2Cl2, 12 hr, rt


307

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