Liberation of low-coordinate phosphorus species from anthracene-based molecular precursors

Abstract

The study of heavier p-block analogs to unsaturated organic small molecules is of fundamental importance, allowing insight into the differing bonding and reactivity patterns that control their synthetic utility. A major complication in this pursuit is the instability of these unsaturated compounds containing heavier elements, as they can rarely be isolated without sterically encumbering substituents to provide kinetic protection. Herein, a series of anthracene-based molecular precursors are leveraged for the generation of low-coordinate phosphorus species that have been generally too unstable to isolate and use under ambient conditions. These precursors exhibit a dibenzo-7-phosphanorbornadiene framework that has allowed access to free phosphinidenes, phosphinidene sulfides, and phosphaalkynes. The thermal release of anthracene as a relatively innocuous coproduct allows for studies of these species' reactivities toward new substrates. Where possible, the intermediacy of the unstable phosphorus compound is demonstrated by direct spectroscopic detection, particularly using nuclear magnetic resonance (NMR) and microwave (rotational) spectroscopy. The mechanism and scope of thermal fragmentation of these molecular precursors is described from experimental kinetic studies bolstered by density functional theory (DFT) calculations. Initial forays into further uses of dibenzo-7-phosphanorbornadiene compounds have included the preparation of a new material of composition P2S and the transition metal-catalyzed transfer of tert-butylphosphinidene to styrene to generate phosphiranes.