Electronic Control of Frustrated Lewis Pair Behavior: Chemistry of a Geminal Alkylidene-Bridged Per-pentafluorophenylated P/B Pair

“Markovnikov” hydroboration of bis(pentafluorophenyl)(propynyl)phosphine (12) with Piers' borane [HB(C₆F₅)₂] gives the frustrated Lewis pair (FLP) (C₆F₅)₂P–(μ-CCHCH₃)–B(C₆F₅)₂ (13) as the major product in good yield. FLP 13 adds cleanly to 1-alkynes to form the five-membered heterocyclic products 15 (two examples). It adds to the CN double bond of arylisocyanates to yield the five-membered heterocycles 16 (two examples), and it regioselectively adds to aldehydes to yield the B–O/P–C bonded heterocycles 17 (from benzaldehyde) and 18 (from trans-cinnamic aldehyde), respectively. Dynamic ¹⁹F NMR spectroscopy indicates a rapidly occurring reversible P–C bond dissociation process taking place in 17 and 18 [17: ΔG^⧧_diss(328 K) = 14.7 ± 0.3 kcal/mol; 18: ΔG^⧧_diss(368 K) = 11.9 ± 0.2 kcal/mol]. The products 15a, 15b, 17, and 18 were characterized by X-ray diffraction. This chemistry indicates that frustrated Lewis pair behavior can be induced electronically by the choice of suitable substituents at the Lewis base.