Sulfide, ethyl methyl regioselectivity

By far most of the reports on addition reactions of hetero-nucleophiles to activated dienes deal with sulfur-nucleophiles17,48,80,120-137, in particular in the synthesis of 7/3-sulfur-substituted steroids which, like their carbon-substituted counterparts (Section n.A), are of interest because of their ability to inhibit the biosynthesis of estrogens80,129-137. Early investigations17,120-122 concentrated on simple acyclic Michael acceptors like methyl sorbate and 2,4-pentadienenitrile. Bravo and coworkers120 observed the formation of a 3 1 mixture of the 1,6- and 1,4-adduct in the reaction of methyl sorbate with methanethiol in basic medium (equation 39). In contrast to this, 2,4-pentadienenitrile adds various thiols regioselectively at C-5, i.e. in a 1,6-fashion (equation 40)17,121,122, and the same is true for reactions of this substrate with hydrogen sulfide (equation 41), sodium bisulfite and ethyl thioglycolate17. 

The woik of Tuleen and Stevens on the regioselectivity of chlorination of a series of unsymmetrically substituted dialkyl sulfides with NCS provides clues to Ae directive effects implicit in the mechanisms encompassed by Scheme 3. These observations are collected in Scheme 6 where the preferred site of chlorination in each case is indicated with an arrow, the number over the arrow indicating the majortminor product ratio (minor = 1). In the first example, chlorination of benzyl methyl sulfide (2) produces chlorobenzyl methyl sulfide exclusively. Secondly, chlorination of benzyl ethyl sulfide (3) and benzyl isopropyl sulfide (4) also shows a marked, though not exclusive, preference for the benzylic position. In the latter case the extent of benzylic chlorination can be modulated by ring substitution. The directive effects in these internal competitions for p-methyl and p-chloro substituents are correlated by the Hammett relationship with a value of p = 1.0, which is consistent with a mechanism involving abstraction of the more acidic proton in the chlorosulfonium ion intermediate. Further indications of the im-... 

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