Tertiary Amines from methiodides and lithium aluminum hydride

Suitable conditions for a little-used general synthesis of NN-dimethyl tertiary amines from primary amines are described. The primary amine is treated with methyl iodide and a base, and the methiodide so obtained is reduced with lithium aluminum hydride in boiling tetrahydrofuran, forming a tertiary amine and methane. This method is compared with a standard procedure for methylating primary amines, the Clarke-Eschweiler reaction (treatment with formaldehyde and formic acid) in the preparation of N,N-dimethyl-(+)-neomenthylamine, N,N-dimethyl-(-)-menthylamine and benzylamine. In each case the tertiary amines obtained by the two methods had identical optical rotations, indicating that the Clarke-Eschweiler method proceeds without epimerization of the carbon atom to which the amino group is attached.