HOMOGENEOUS HYDROGENATION OF OLEFINS CATALYZED BY RHODIUM(I) COMPLEXES OF NEW HETERODIFUNCTIONAL PHOSPHORUS-NITROGEN CHELATING LIGANDS

A range of rhodium-phosphine catalytic systems have been prepared from Rh(I) complexed or bound to newly developed heterodifunctional phosph oranimine Ph2PQP(Ph2) = NR ligands. The ligands consist of one exposed phosphine (herein Ph2P) connected, via a selection of backbones (Q) s uch as CH2, CH2CH2, CH(CH3) and 1,2-C6H4, to another Ph2P unit which i s oxidized to an imine. The imine substituents R are generally fluorin ated aromatics although one case of the benzyl substituent and a few w ith nitroaromatic substituents were evaluated. With R = SiMe3 eliminat ion of Me3SiCl gives access to compounds in which the ligand is bound to the metal by means of a M-N sigma bond. Also investigated was the p hosphine imine Ph2P-N=C(H)Ph. Cationic complexes of the type [Rh(cod) (Ph2PQP(Ph2)=NR)]+PF6- (Q = C(H)CH3, 1,2-C6H4; R = substituted aromati c) gave unusually poor results for the homogeneous hydrogenation of 1- hexene. Freshly prepared mixtures of [Rh(cod)Cl]2 + 2Ph2PCH2P(Ph2) = N R (R=Me3Si) in ethanol yield the complex [ (cod)RhPPh2CH2P(Ph2) = N] i n situ which gave an effective catalyst system with good turnover rate s for the hydrogenation of hexene, cyclohexene and styrene. An analogo us catalytic system assembled with the ligand Ph2PN(Pr)P(Ph2) = NR (i. e. Q = N(Pr) and R = Me3Si) and the same Rh (cod) precursor also showe d good turnover rates for the hydrogenation of hex-1-ene, but also gav e high rates of isomerization to hex-2-enes. The effects of solvent me dia, ligand to metal ratio, amine and varying ligand substituents have been investigated to optimize reaction rates. Our results are compare d with measurements under our own conditions of other well known Rh(I) hydrogenation catalysts. Despite the good hydrogenation rates shown b y these catalysts, their use may be limited by their tendency to promo te isomerization. The inclusion of heterodifunctionality, especially t hrough the contrasting effects of nitrogen (hard) and phosphorus (soft ) ligation, and the additional contrasts of reactivity introduced by t he possibility of nitrogen sigma bonding versus the more common nitrog en Lewis base coordination opens new pathways to ligand design and con sequent control of catalyst activity.