Reactivity of Alkynyl Phosphines with Lewis Acids for the Synthesis of Allenic Phosphonium Borate Zwitterions

The reactivity of alkynylphosphines with Lewis acidic boranes has been well established in recent years thanks to the advent of frustrated Lewis pairs (FLPs), often leading to a 1,1-carboboration reaction and the formation of an intramolecular phosphinoborane FLP. Our group recently discovered that introduction of an ionizable group in the gamma-position of an alkynylphosphine result in an unexpected rearrangement reaction to yield zwitterionic allenic-phosphonium borate products which are produced via a transient 3-coordinate phosphonium allenylidene. Herein, we describe how substituent effects on the gamma-carbon impact this reactivity and explore how bis(alkynyl)phosphines behave under similar reaction conditions.