Study of the reaction of bulky aryllithium reagents with 3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine-2-borane derived from ephedrine

The ring opening of enantiomerically pure 3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine-2-borane (1) with a variety of bulky aryllithium reagents was studied. Our results are not in total agreement with those obtained by others. In fact, several 2,6-disubstituted aryl groups were successfully appended to the phosphorus atom, furnishing the corresponding (N-ephedrino)phosphine boranes 2a-g with dr>99:1. However, when the attack on the phosphorus atom is hindered, deprotonation on the benzylic position occurs, leading to the formation of enantiomerically pure trans-(N-methylamino)(phenyl)(1-phenyl-1-propenyloxy)phosphine-P-borane (3). While the attack of 2,2'-dilithio-1,1'-biarenes leads to the corresponding (P-phenyl)phosphole derivatives (4i,j) and to [bis(N-ephedrino)](phenyl)phosphine-P-borane (5), the attack of 1,1'-dilithiometallocenes (M=Fe, Ru) leads to a separable diastereomeric mixture of 1,1'-bis[(N-ephedrino)(phenyl)phosphino-P-borane]metallocenes (2k,l/2k',l') with dr approximately 80:20.