oxidation of thiols to dissulfides with molecular bromine on hydrated silica gel support

8/12/2019 Oxidation of Thiols to Dissulfides with Molecular Bromine on Hydrated Silica Gel Support

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OXIDATION OF THIOLS TO

DISULFIDES WITHMOLECULAR BROMINE ON

HYDRATED SILICA GEL

SUPPORT

Mohammed Hashmat Ali*, J. Mario

McDermottChemistry Department

Southeast Missouri State University

Cape Girardeau, Missouri 63701.


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Reaction Scheme

RSH

Hydrated Silica gel,Methylene chlorideBromine

RSSR

SH

SH

S

S

Hydrated Silica gel,Methylene chlorideBromine


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Introduction

We have developed a procedure for oxidation

of thiols to disulfides with moleculer bromine

on hydrated silica gel support. This procedure

is simple, highly efficient, and very selective.

Presence of a base to neutralize the hydrogenbromide formed in the reaction is not

necessary. A variety of thiols can be oxidized

to the corresponding disulfides. Cyclicdisulfides of various ring size can also be

easily prepared from dithiols utilizing this

procedure.


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Oxidation of Thiols to Disulfides with Molecular Bromine

CH3(CH2)6CH2S-SCH2(CH2)6CH3

HOCH2CH

2SH

CH3(CH2)6CH2SH

CH3CH2CH2CH2S-SCH2CH2CH2CH3

HOCH2CH2S-SCH2CH2OH

CH3CH2CHCH3SH CH3CH2CHCH3S-SCH3CHCH2CH3

CH3CH2CH2CH2SH

100%

100%

98%

100%


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(CH3)3CSH (CH3)3CS-SC(CH3)3

CH3CH2SH CH3CH2S-SCH2CH3

(CH3)2CHSH (CH3)2CHS-SCH(CH3)2

97%

98%

98%

PhCH2S-SCH2PhPhCH2SH

97%

PhCH2CH2CH2SH

92%

Ph(CH2)2CH2S-SCH2(CH2)2Ph


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SH3C S CH3

SHCH3O SCH3O S OCH3

SHCl SCl S Cl

SH

SH

S

S

100%

100%

97%

100%

SHCH3


7/18

94%

SHS

S

SH

SH

HO

S

S

HO

HO

O SH

HO

95%

OSO S

SH

SH S

S

86%


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SH

SH

S

S

89%

91%

SH

S

S

SH


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Experimental Procedure

The following procedure is general for the

reported reactions.

2.5 mL of water was added dropwise to a 5 gm of

silica gel with stirring. Stirring continued until afree flowing solid was obtained. 25 mL of

methylene chloride was added to the flask

followed by the addition of 2-3 mmole of thethiol under investigation. A solution of bromine

on methylene chloride was added drop by drop


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until a faint orange color persisted or TLC

indicated the complete disappearance of thestarting thiol. The reaction mixture was

then filtered and the solid was washed with

about 75-80 mL of methylene chloride.Removal of the solvent from the filtrate

produced the disulfide product which was

often pure by NMR and TLC. Impure

products were purified by radial

chromatography.


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Advantages of Hydrated Silica Gel

Support

Organic solvents can be employed

in the reactions which previously

required aqueous media.

Activates reagents by dispersing

active sites.

Simplifies work-up of the reaction.


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Eliminates or reduces exposure to

toxic reagents.

Produces easy-to-handle solidwaste.

Generates smaller amounts of

waste.


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Conclusions:

The procedure described has the

following desirable qualities:

Employs non-aqueous media.

Simple procedure.


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Presence of a base is not requiredto neutralize the hydrogen bromide

formed in order to prevent side

reactions. Hydrogen bromides are

absorbed on the silica gel and are

removed from the reaction.


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Excellent efficiency.

Highly selective.

Work-upis trivial.

Environmentally benign.


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Grants and Research Funding CommitteeChemistry Department


Cape Girardeau, Missouri 63701

Acknowledgment


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oxidation of thiols to dissulfides with molecular bromine on hydrated silica gel support

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