Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions.

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a, b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative (Br Eind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b (Br Eind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b. It reacts similarly with the carbon-carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder-Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern.