The C-4-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 schemes leading to these products are discussed. Essential examples were characterized by X-ray diffraction.