Kinetics, Thermodynamics, and Effect of BPh3 on Competitive C−C and C−H Bond Activation Reactions in the Interconversion of Allyl Cyanide by [Ni(dippe)]

Abstract

Reaction of [(dippe)Ni(micro-H)](2) with allyl cyanide at low temperature quantitatively generates the eta(2)-olefin complex (dippe)Ni(CH(2)=CHCH(2)CN) (1). At ambient temperature or above, the olefin complex is converted to a mixture of C-CN cleavage product (dippe)Ni(eta(3)-allyl)(CN) (3) and the olefin-isomerization products (dippe)Ni(eta(2)-crotonitrile) (cis- and trans-2), which form via C-H activation. The latter are the exclusive products at longer reaction times, indicating that C-CN cleavage is reversible and the crotononitrile complexes 2 are more thermodynamically stable than eta(3)-allyl species 3. The kinetics of this reaction have been followed as a function of temperature, and rate constants have been extracted by modeling of the reaction. The rate constants for C-CN bond formation (the reverse of C-CN cleavage) show a stronger temperature dependence than those for C-CN and C-H activation, making the observed distribution of C-H versus C-CN cleavage products strongly temperature-dependent. The activation parameters for the C-CN formation step are also quite distinct from those of the C-CN and C-H cleavage steps (larger DeltaH(++) and positive DeltaS(++)). Addition of the Lewis acid BPh(3) to 1 at low temperature yields exclusively the C-CN activation product (dippe)Ni(eta(3)-allyl)(CNBPh(3)) (4). Independently prepared (dippe)Ni(crotononitrile-BPh(3)) (cis- and trans-7) does not interconvert with 4, indicating that 4 is the kinetic product of the BPh(3)-mediated reaction. On standing in solution at ambient temperature, 4 decomposes slowly to complex 5, with structure [(dippe)Ni(eta(3)-allyl)(N triple bond C-BPh(3)), while addition of a second equivalent of BPh(3) immediately produces [(dippe)Ni(eta(3)-allyl)](+)[Ph(3)BC triple bond NBPh(3)](-) (6). Comparison of the barriers to pi-sigma allyl interconversion (determined via dynamic (1)H NMR spectroscopy) for all of the eta(3)-allyl complexes reveals that axial cyanide ligands facilitate pi-sigma interconversion by moving into the P(2)Ni square plane when the allyl group is sigma-bound.