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] containing diverse functionalized phosphine ligands of hemilabile character, Ar2P(CH2)nZ (n = 1, 2, or 3; Z = OMe, OEt, OnBu, NMe2, SMe), have been synthesized and fully characterized by spectroscopic means. The crystal structure of representative compounds has been determined by X-ray diffraction methods. Most complexes are active catalysts for the antiMarkovnikov oxidative amination of styrene with piperidine to produce (E)-1-styrylpiperidine. Catalyst screening has shown a remarkable relationship between the hemilabile ligand, the pre-catalyst structure and catalytic activity. The more efficient catalysts are those having arylphosphine ligands with a 2-alkoxoethyl or 3-alkoxopropyl hemilabile moiety, Ar2P(CH2)nOR (n = 2, 3; R = Me, Et, nBu). This study has shown the outstanding catalytic activity of the bisphosphine complexes [Rh{(4-R-C6H4)2P(CH2)3OEt}2][PF6] (R = H, Me, OMe) with unprecedented turnover frequencies up to 80 h (R = Me) and excellent enamine selectivity (96%).