Reaction of the coordinatively unsaturated methylene complex Ir=CH2[N(SiMe2CH2PPh2)2] with olefins: Stereoselective formation of allyl hydride derivatives

Article abstract of DOI:10.1021/ja00116a015

The reactivity of the coordinatively unsaturated methylene complex Ir=CH₂[N(SiMe₂CH₂PPh₂)₂] with olefins is described. In all cases, carbon-carbon bond formation is observed with the isolation of Ir(III) allyl hydride derivatives Ir(η³-C₃H₄R)H[N(SiMe₂CH ₂PPh₂)₂] (R = H, CN, Ph, and CO₂Me). With ethylene, exo and endo allyl hydride isomers are formed; with acrylonitrile, anti-exo and syn-endo diastereomers result and with both styrene and methyl acrylate, only the corresponding syn-endo derivative is isolated. The reaction is facilitated by electronwithdrawing olefins since propene, ethyl vinyl ether and norbornene fail to react with the methylene complex. Variable-temperature NMR studies have provided some information on the mechanism of the reaction; it is suggested that the olefin approaches the iridium-methylene complex to generate a perpendicular binding of the olefin which then undergoes C-C bond formation via migratory insertion to generate a highly puckered iridacyclobutane. Rapid β-elimination results in the formation of an isomeric allyl hydride species (detected by NMR spectroscopy) that isomerizes to the observed final products. Molecular orbital calculations (INDO-1 using CAChe Worksystem) support the idea that the olefin binds to the metal perpendicular to the π-system of the iridium-carbon double bond. Suggestions are made regarding the observed dependence on the nature of the metal center to direct the reaction to form allyl hydride derivatives or undergo olefin metathesis. Ir(η³-C₃H₄CN)H[N(SiMe₂CH ₂PPh₂)₂], 2b, crystallizes as a (1:1) toluene solvate, monoclinic, a = 10.884(1) A?, b = 18.672(2) A?, c = 20.169(3) A?, β = 100.32(2)°, Z = 4, space group P2₁/c; and crystals of Ir(η³-C₃H₄Me)H[N(SiMe₂CH ₂PPh₂)₂], 3e, are also monoclinic, a = 13.606(2) A?, b = 14.687(4) A?, c = 17.672(3) A?, β = 100.86(1)°, Z = 4, space group P2₁/n. The structures were solved by Patterson methods and were refined by full-matrix least-squares procedures to R = 0.032 and 0.034 (R_w = 0.028 and 0.033) for 6304 and 4226 reflections with I ≥ 3σ(I), respectively.