Design considerations for chiral frustrated Lewis pairs: B/N FLPs derived from 3,5-bicyclic aryl piperidines

Abstract

Herein, 3,5-bicyclic aryl piperidines are derivatized to generate chiral B/N FLPs. Initially, the twofold symmetric amine C₆H₂F₂(C₅H₈NiPr) 1 was converted in a series of synthetic steps to the styrene-derivative C₆HF₂(C₅H₈NiPr)(CHCH₂) 4. Efforts to hydroborate the vinyl fragment proved challenging as a result of the strongly basic nitrogen, although the species C₆HF₂(C₅H₈N(H)iPr)(CH₂CH₂B(OH)(C₆F₅)₂) 5 was crystallographically characterized. Modification of the system was achieved by conversion of the amine C₆H₂F₂(C₅H₈NH) 6 to C₆HF₂(C₅H₈NPh)(CHCH₂) 9. Hydroboration of 9 with 9-BBN or HB(C₆F₅)₂ gave C₆HF₂(C₅H₈NPh)(CH₂CH₂BBN) 10 or C₆HF₂(C₅H₈NPh)(CH₂CH₂B(C₆F₅)₂) 11, respectively. The latter species was derivatized by complexation of PPh₃ to give C₆HF₂(C₅H₈NPh)(CH₂CH₂B(C₆F₅)₂)(PPh₃) 12. The Lewis acidities of 10 and 11 were assessed by the Gutman–Beckett test and by computations of the FIA and GEI. While 10 did not effect HD scrambling or hydrogenation of N-phenylbenzylimine, 11 was effective in HD scrambling. Despite this, no reduction of N-t-butylbenzylimine or N-phenylbenzylimine was achieved. These data demonstrate that 10 lacks the threshold combination of Lewis acidity and basicity to activate H₂, while 11 lacks the steric demands about boron to preclude classical Lewis acid–base bond formation with imine substrates.