New insight into role of ortho-metallation in rhodium triphenylphosphite complexes. Hydrogen mobility in hydrogenation and isomerization of unsaturated substrates

The hydrogen transfer from two rhodium(I) hydride complexes HRh{P(OPh)(3)}( 4) and HRh(CO){P(OPh)(3)}(3) to methyl acrylate and/or allylbenzene leads t o the formation of ortho-metallated complexes Rh{P(OC6H4)(OPh)(2)} {P(OPh)( 3)}(3) (I) and Rh{P(OC6H4)(OPh)(2)} (CO) {P(OPh)(3)}(2) (II), respectively. During these reactions unsaturated substrates, methyl acrylate or allylben zene undergo stoichiometric hydrogenation. A similar reaction was also obse rved for HRh{P(OR)(3)}(4) complexes (R = 3-CH3C6H4, 4-CH3C6H4). The complex HRh{P(OPh)(3)}(4) catalyses the isomerization of hex-1-ene to hex-2-ene in the absence of H-2; however at 1 atm of H-2 the formation of hexane is obs erved. Hydride complexes of the type HRh{P(OR)(3)}(4) in D-2 atmosphere und ergo H/D exchange at the ortho position of coordinated triarylphosphite. De uteration of the ortho protons in complexes with R = Ph, 3,5-(CH3)(2)C6H3 a nd 4-CH3C6H4 is total, whereas only one ortho hydrogen is replaced in the c ase of R = 3-CH3C6H4. The formation of an ortho-metallated chelating ring c auses a downfield shift in the H-1-NMR signal of one proton from the phenyl ring to delta ca. 8 ppm. (C) 2000 Elsevier Science S.A. All rights reserve d.