Catalytic hydrocyanation of olefins by nickel(0) phosphite complexes - Effects of Lewis acids

Article abstract of DOI:10.1021/om00079a008

Mechanistic studies on the addition of HCN to olefins using Ni[P(O-o-tolyl)₃]₃ catalyst show that the reactions proceed under mild conditions via (η¹-organo)- or (η³-organo)nickel cyanide intermediates formed by the insertion of monoenes or dienes into the nickel-hydrogen bond of HNiL₃CN. The RNiL_mCN intermediates can be identified spectroscopically by NMR and IR in some cases. In the case of very electronegative R groups (formed for example from reactions of HNiL₃CN with C₂F₄ or acrylonitrile), reductive elimination does not occur, and the catalyst is poisoned for olefin hydrocyanation. Improved rates, catalyst lifetimes, and product linearity can be obtained by the addition of Lewis acid (A) cocatalysts. Spectroscopic studies show that Lewis acids can coordinate strongly to the nitrogen lone-electron pair of hydrido cyanide intermediates to form HNiL₃CN-A complexes. An explanation of the effects of Lewis acids on hydrocyanation is proposed, which involves increasing the concentration of nickel in catalytic loop species [in the P(O-p-tolyl)₃ system], accelerating the rate of carbon-carbon coup ing to form alkanenitriles from alkylnickel cyanide complexes and destabilizing relatively bulky branched alkyl intermediates relative to less crowded linear ones.