Stereoelectronic effects in a homologous series of bidentate cyclic phosphines. A clear correlation of hydroformylation catalyst activity with ring size

Abstract

The homologous series of diphosphines (CH₂)_n−1P(CH₂)₃P(CH₂)_n−1 where n = 5 (L₅), 6 (L₆), or 7 (L₇) have been synthesized from the corresponding PhP(CH₂)_n−1. Treatment of [PtCl₂(cod)] with L_5–7 gave the 6-membered chelates cis-[PtCl₂(L_5–7)], the crystal structures for which reveal that L_5–7 have very similar steric bulk and bite angles. Treatment of [Rh₂Cl₂(CO)₄] with L_5–7 gave the binuclear trans-[Rh₂Cl₂(CO)₂(μ-L_5–7)₂] with syn and anti orientations of the CO and Cl ligands suggested by the ³¹P NMR spectra and the crystal structures of syn–trans-[Rh₂Cl₂(CO)₂(μ-L₅)₂] and anti–trans-[Rh₂Cl₂(CO)₂(μ-L₇)₂]. The ν(CO) values for trans-[Rh₂Cl₂(CO)₂(μ-L_5–7)₂] indicate that the donor strength increases in the order L₅ < L₆ < L₇. A study of rhodium-catalysed hydroformylation of 1-octene using diphosphines L_5–7 is described. The catalyst activity decreases with increasing phosphacycle ring size: L₅ > L₆ > L₇.