. Can. J. Chem. 54, 193 (1976). The Sommelet rearrangement of (+)-ethylmethyl-p-nitrobenzyls~~lfoniun~ perclllorate, (+)-I, and (+)-etliylniethyl-p-clilorobenzylsulfoniun perchlorate, (+)-2, are described. Elution of (+)-I through an hydroxide exchange resin generated etliylmetl~ylsulfoni~~nip-nitrobenzylide (+)-3 which decomposed in methanol at room teriiperature to ethyl 2-methyl-5-nitrobenzyl sulfide, 6, and (+)-methyl a-(2-methyl-5-nitropheny1)ethyl sulfide, (+)-7, with IS to 20.3(/; asymmetric induction. Deconlposition of (+)-2 in sodium methoxide solution at 70 OC for 2 I1 produced ethyl 2-methyl-5-clilorobenzyl sulfide, 8, and (+)-methyl a-(2-methyl-5-nitrop11enyl)ethyl sulfide, (+)-I), with 21 to 25.5:; asymmetric induction. The lower estimates of asymmetric induction for each sulfide were made by coniparison with specific rotations of authentic san~ples obtained by synthesis and resolution. The higher estimates were obtained by the use of a chiral lanthanide shift reagent Eu(11fbc)~ with the sulfone derivatives of these chiral sulfides. The ylide (+)-3 reacted with aldehydes in high yield to produce oxiranes with no induction of asymmetry.
Treatment of (−)-ethylmethylsulfonium phenacylid, (−)-1, in THF at room temperature with 15 equiv. of benzoic anhydride, acetic anhydride or 1 equiv. phenyl isocyanate produces the corresponding (−)-ethylmethylsulfonium diacylmethylids, (−)-5, (−)-6, and (−)-8. Treatment of (−)-1 with dimethyl sulfate in acetone gives (Z)-(+)-ethylmethyl-α-methoxy-β-styrylsulfonium methyl sulfate, (+)-10. Resolution of (±)-10 via its DBT salt and conversion to perchlorate gives the corresponding vinyl perchlorates (+)- and (−)-13. Under the resolution conditions the ketal perchlorates, (−)- and (+)-ethylmethyl-2,2-dimethoxy-2-phenylethylsulfonium perchlorate, (−)- and (+)-14, are also obtained. The rates of racemization at 50° in benzene of 1, 5, 6, and 8 are 39.7, 55.5, 106, and 79.1 × 10−5 s−1 respectively. The rates of racemization at 50° in methanol of 10, 13, and 14 are 1.93, 2.08, and 5.64 × 10−5 s−1. Racemization is proposed to proceed via pyramidal inversion. The synthetic potential of chiral sulfonium ylids is discussed with respect to their ease of racemization.
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