ON THE REASON FOR OPPOSITE DIASTEREOSELECTIVITIES OF BENZYLLITHIUM COMPOUNDS CONTAINING LITHIUM AMIDE AND LITHIUM ALKOXIDE FUNCTIONALITIES

The carbolithiation of N-methyl-3-phenyl-prop-2-enylamine with tert-bu tyllithium leads to the monomeric benzyllithium compound 7 in good yie ld. The consecutive reaction with electrophiles exhibits a high anti d iastereoselectivity, opposite to what has been earlier found for the o xygen analogue 3. PM3 semiempirical calculations on the intermediates show a preference of the anti configuration which is confirmed by H-1 NMR. Measurements of the degree of aggregation show the dilithio compo und 3 to exist as a dimer and higher aggregates in THF. PM3 calculatio ns on this dimer can explain the different diastereoselectivity. Copyr ight (C) 1996 Elsevier Science Ltd