Bifunctional Behavior of Unsaturated Intramolecular Phosphane-Borane Frustrated Lewis Pairs Derived from Uncatalyzed 1,4-Hydrophosphination of a Dienylborane

Three unsaturated C-4-bridged phospane/borane frustrated Lewis pairs (P/B FLPs) are prepared by uncatalyzed hydrophosphination of a dienylborane. The systems are bifunctional. Consequently, two examples undergo clean hydroboration reactions with HB(C6F5)(2) to yield B/B/P systems. The 1,4-P/B system (C6F5)(2)B-CH2CH=CMeCH(2)PMes(2) reacts with benzaldehyde initially by allylborane addition, followed by internal P/B FLP addition to the pendant C=C double bond, to yield a bicyclic product. The corresponding reaction of (C6F5)(2)B-CH2CH=CMeCH2PtBu2 stops at the allylborane/benzaldehyde addition product. The related system (C6F5)(2)B-CH2CH=CMeCH2PPh2 shows a similar bifunctional reaction pattern, whereby allylborane addition to benzaldehyde is combined with P/B addition to a second aldehyde equivalent to form the eight-membered heterocyclic 1: 2 addition product.