Phosphinoboration of Diazobenzene: Intramolecular FLP Synthon for PN2 B-Derived Heterocycles.

Phosphinoboration of diazobenzene with Ph2 PBR'2 cleanly affords products of the form Ph2 P(PhNNPh)BR'2 (2: R'2 =catechol, cat; 4: R'2 =phenanthrenediol, quin) and shows evidence of Ph2 P(PhNNPh)Bpin 7 (pin: pinacol). The mechanism of these reactions was probed computationally and shown to proceed via intermediates involving a diazobenzene-adduct of the boron center of the PB reagent. The resulting PNNB species 2 and 4 are shown to be frustrated Lewis pairs (FLPs). Despite the presence of weakly Lewis acidic boron centers, these species react with diazobenzene. Additionally, the FLP reactivity of 2 was further probed with 4-phenyl-1,2,4-triazole-3,5-dione, 1,10-phenanthroline-5,6-dione, and benzyl azide to give unique five-, six-, and eight-membered heterocyclic rings. Thus, phosphinoboration provides a new avenue to FLPs in which donor and acceptor sites are linked by nitrogen atoms.