Rhodium(I) Complexes with Hemilabile Phosphines: Rational Design for Efficient Oxidative Amination Catalysts

A series of cationic square-planar rhodium(I) complexes of type [Rh(cod){Ar2P(CH2)nZ}]+, [Rh(cod){Ar2P(CH2)nZ}2]+, and [Rh{Ar2P(CH2)nZ}2]+, which contained diverse functionalized hemilabile phosphine ligands of type Ar2P(CH2)nZ (n=13; Z=OMe, OEt, OnBu, NMe2, SMe), were synthesized and spectroscopically characterized. The crystal structures of representative compounds were determined by X-ray diffraction. Most complexes were active catalysts for the anti-Markovnikov oxidative amination of styrene with piperidine to produce (E)-1-styrylpiperidine. Catalyst screening showed a remarkable relationship between the hemilabile ligand, the precatalyst structure, and the catalytic activity. The more-efficient catalysts were those that had arylphosphine ligands with a 2-alkoxyethyl- or 3-alkoxypropyl hemilabile moiety, Ar2P(CH2)nOR (n=2, 3; R=Me, Et, nBu). This study has revealed the outstanding catalytic activity of bis-phosphine complexes [Rh{(4-R-C6H4)2P(CH2)3OEt}2][PF6] (R=H, Me, OMe), with unprecedented turnover frequencies of up to 80 h-1 (R=Me) and excellent enamine selectivity (96?%).