Frustrated Lewis pair olefin addition reactions: P-, N-, C- and H-based nucleophilic additions to an olefin-tethered borane

Abstract

The electrophilic alkoxyborane, B(C₆F₅)₂(OC(CF₃)₂CH₂CHCH₂) 2, was synthesized and the reactivity of the tethered olefinic fragment examined in frustrated Lewis pair (FLP) addition reactions. Treatment of 2 with tBu₃P or Me₃P afforded B(C₆F₅)₂(OC(CF₃)₂CH₂CHCH₂)(PR₃) (R = tBu 3, Me 4) with addition of the nucleophile to the internal carbon of the olefinic group. In contrast, reaction of 2 with bulky nitrogen-based nucleophiles, 2,6-lutidine or 2,2,6,6-tetramethylpiperidine, gave B(C₆F₅)₂(OC(CF₃)₂CH₂CHCH₂)(NR′) (R′ = C₅H₃Me₂N 5, NHC₅H₆Me₄6) where the nucleophile adds to the terminal carbon of the olefinic unit. Treatment with the carbon-based nucleophiles, 1,2,5-trimethylpyrrole, N-tert-butylpyrrole, 1,3-di-tert-butylimidazole-2-ylidene and benzylidene triphenylphosphorane, afforded the zwitterions B(C₆F₅)₂(OC(CF₃)₂CH₂CHCH₂)(R′′) (R′′ = C₄H₂Me₂NMe 7, C₄H₄NtBu 8, ItBu, 1,3-di-tert-butylimidazole-2-ylidene 9, P(CHPh)Ph₃10) which contained a new C–C bond at the internal carbon of the olefin. In the presence of 1,2,2,6,6-pentamethylpiperidine and a catalytic amount of B(C₆F₅)₃, 2 reacts with H₂ to convert to [HPMP][B(C₆F₅)₂(OC(CF₃)₂CH₂CH₂CH₂)] (PMP = 1,2,2,6,6-pentamethylpiperidine) 11. DFT calculations provide evidence that this latter reaction proceeds by heterolytic cleavage of H₂ by B(C₆F₅)₃ and 1,2,2,6,6-pentamethylpiperidine followed by transfer of the hydride from B(C₆F₅)₃ to the internal carbon of the vinyl group of alkoxyborane 2.