Selected reactions of some cyclic sulfoxides

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Virginia Tech


The previously unreported 3-methyltetrahydrothiophene I-oxide (1) and 3-methyl-2, 5-dihydrothiophene I-oxide (2) have been prepared by oxidation of the corresponding sulfides. The structure of these unsymmetric sulfoxides was established by chemical and physical methods. Nuclear magnetic resonance analyses have provided significant information concerning some stereochemical aspects of these molecules. In the case of 1. which has four possible configurations, both cis and trans isomers were shown to exist in the synthetic material. A study on the temperature dependence of the nuclear magnetic resonance spectrum of 2 between -34° and 100°, suggested little or no association of a dimeric nature between molecules of this unsaturated sulfoxide.

The rapid and clean reduction of 2 to the corresponding sulfide was carried out using hydriodic acid and detected by gas chromatographic analysis.

The partial base catalyzed isomerization of 2 to 3-methyl-4, 5- dihydrothiophene 1-oxide (13) was carried out in sodium hydroxide solution and detected by gas chromatographic analysis of the reduced products. Maximum isomerization occurred after 96 hours when 2 and 13 were present in a ratio of 1.78:1 respectively. This finding is similar to the previously observed isomerization of the corresponding sulfone, but is in contrast to the lack of isomerization of the olefinic bond of the related 2, 5-dihydrothiophene l-oxide. A plausible mechanism has been proposed for this interesting isomerization of 2.

Sulfoxide 1 was reacted with various Grignard reagents and shown to undergo a substitution reaction at the α-carbon atoms. Predominant reaction occurred at position-5 of 1 and binary sulfide mixtures composed of 2-substituted-4-methyl- and 2-substituted-3-methyltetrahydrothiophenes were obtained in yields ranging from 17-31%. The ratio of isomeric sulfides ranged from 1.50:1, in the case of phenylmagnesium bromide, to 8.09:1 when using neopentylmagnesium chloride. The binary sulfide mixtures were desulfurized and the hydrocarbon products were identified. In this manner, the structure and composition of the components of the sulfide mixtures was established. Preparative-scale gas chromatography was used to separate two binary sulfide mixtures. Desulfurization of the individual sulfides led to the discovery of isomerization reactions which occurred during desulfurization. By-products of the Grignard reaction were identified as 3-methyltetrahydrothiophene, 3-methyl-2, 3- and 3-methyl-4, 5-dihydrothiophene, and, in some cases the alkene and alcohol derived from the Grignard reagent. A reaction pathway, by which these products and by-products may conceivably be formed has been proposed.

Results of the reaction of the unsaturated sulfoxide 2 with Grignard reagents were quite different than with sulfoxide 1. Sulfoxide 2 underwent reaction at the sulfur atom with a subsequent cleavage of the ring yielding single dienyl sulfides. These novel compounds were formed by a stereospecific ring cleavage of 2, a reaction similar to that previously observed with the corresponding sulfone. The dienyl sulfides, 1-phenylthio- and 1-n-propylthio-2-methyl-1, 3-butadiene, were isolated in good yield and characterized by their spectral properties and by conversion to known compounds. The only detectable by-product of this reaction was a trace amount of 3-methyltetrahydrothiophene. A mechanism, consistant with the data obtained, has been suggested.