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

Article abstract of DOI:10.1021/ic011035i

Variants of the β-aminophosphine L¹ [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 by lithiation of L² [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 L¹ 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 L⁵ 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 κ-P,κ-N(NHPh) mode similar to the complex containing L¹. 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 L¹ or L². The observed trends are related to the basicity of the dangling amine function and to its proximity to the metal center.