The selective N-methylation of BOC-protected valine 1a with MeI and NaH in THF (i.e., in the presence of a free carboxyl group) has been attributed to the protection of the carboxylate by chelation to Na(+). An alternative mechanism, involving the formation of the carbene intermediate generated from MeI and its insertion into the N-H bond, has been ruled out by isotopic labeling.
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Malkov et al. (2009) studied this question.
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