Selectivity in the Transfer of Specified Diastereotopic Methyl Groups from a Dimethylsulfonium Ion.

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Abstract
By specific 14C-labeling it is demonstrated that the prochiral methyl groups present in the (RS)-dimethyl(1-methylpropyl)sulfonium ion are transferred to an acceptor molecule, the 4-methylbenzenethiolate ion, with greatly different rates. Only to a minute extent, however, is the observed rate difference attributable to the diastereotopic character of the 2 methyl groups, whereas a surprisingly large composite isotope effect of 1.16 .+-. 0.02, slightly surpassing the highest previously reported values for primary 12C/14C-isotope effects, accounts almost entirely for the observed rate difference. This observation, pertaining to transmethylation from a dimethylalkylsulfonium ion, may, in certain cases, be of consequence for biochemical methodology.