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)(2)P - (mu-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 F-19 NMR spectroscopy indicates a rapidly occurring reversible P C bond dissociation process taking place in 17 and 18 [17: Delta G(diss)(double dagger)(328 K) = 14.7 +/- 0.3 kcal/mol; 18:Delta G(diss)(double dagger)(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.