Accessing five- and seven-membered phosphorus-based heterocycles via cycloaddition reactions of azophosphines

Heterocycles containing both phosphorus and nitrogen have seen increasing use in recent years in luminescent materials, coordination chemistry and as building blocks for inorganic polymers, yet their chemistry is currently dominated by five- and six-membered derivatives. Seven-membered P/N heterocycles are comparatively scarce and lack general, high yielding syntheses. Here, we explore the synthesis and characterisation of 1,2,5-diazaphosphepines from azophosphines. The mechanism has been probed in detail with both computational and experimental studies supporting a stepwise mechanism to form a five-membered ring, and subsequent ring expansion to the diazaphosphepine. Regioselective synthesis of five- and seven-membered rings is possible using asymmetric alkynes. The Lewis acidic borane B(C₆F₅)₃ could either catalyse the formation of the seven-membered ring (ⁱPr derivative) or trap out a key intermediate via a frustrated Lewis pair (FLP) mechanism (^tBu derivative).