Phospha-Claisen Type Reactions at Frustrated Lewis Pair Frameworks

The C4-bridged unsaturated phosphane/borane frustrated
Lewis pairs (P/B FLPs) 4 undergo borane induced phosphane
addition to a variety of acetylenic esters or ketones to generate
heterocyclic 10-membered intermediates that contain pairs of allyl
phosphonium/allenic enolate functionalities. These subsequently undergo
phospha-Claisen type rearrangement reactions to give the respective
substituted phosphanyl pentadiene products. In two exceptional cases
subsequent reactions leading to anomalous phospha-Claisen products
were found. One example involved cyclopropane ring formation, and
the other carbon−carbon bond activation. Potential mechanistic
schemes leading to these products are discussed. Essential examples
were characterized by X-ray diffraction.