Synthesis, coordination to Rh(I), and hydroformylation catalysis of new β-aminophosphines bearing a dangling nitrogen group:: An unusual inversion of a Rh-coordinated P center

Variants of the beta-aminophosphine L-1 [Ph2PCH2CH(Ph)NHPh] containing additional nitrogen donor functions have been prepared. These functions are branched off the C atom adjacent to the P atom, or the P atom itself. Ligand [Ph2PCH(o-C6H4NMe2)CH(Ph)NHPh] has been obtained as a mixture of two diastereomers L-3A and L-3B is by lithiation of L-2 [Ph2PCH2(o-C6H4NMe2)] with n-BuLi followed by PhCH=NPh addition and hydrolysis. The diastereomers have been separated by fractional crystallization from ethanol. Ligand Et2NCH2P(Ph)CH2CH(Ph)NHPh has been obtained as a mixture of two diastereomers L-5A and L-5B starting with P-Ph reductive cleavage of L-1 by lithium and subsequent hydrolysis to give PhP(H)CH2CH(Ph)NHPh (mixture of two diastereomers L-4A and L-4B). The latter reacts with diethylamine and formaldehyde to afford the L-5 diastereomeric mixture. Complexes RhCl(CO)(L) (L = L-3A, 1(A); L-3B, 1(B); L-5A/B, 2(A/B)) were obtained by reaction of [RhCl(CO)(2)](2) and the appropriate ligand or ligand mixture. Complexes 1(A), 1(B), and 2(A) have been isolated in pure form and characterized by classical techniques and by single-crystal X-ray diffraction. All structures exhibit a bidentate kappa-P,kappa-N(NHPh) mode similar to the complex containing L-1. While complexes 1(A) or 1(B) are stable in CDCl3 solution, complex 2(A) slowly converts to its diastereomer 2(B). This unexpected epimerization appears to take place by inversion at the Rh-coordinated P center, an apparently unprecedented phenomenon. A mechanism based on a reversible P-C bond oxidative addition is proposed. The influence of the pendant nitrogen function of the diaminophosphines L-3A and L-5A/B on the rhodium catalytic activity in styrene hydroformylation has been examined and compared to that of the aminophosphines L-1 or L-2. The observed trends are related to the basicity of the dangling amine function and to its proximity to the metal center.