An approach to 1,3,4-dioxaphospholane complexes through an acid-induced ring expansion of an oxaphosphirane complex: the problem of construction and deconstruction of O,P-heterocycles.

Treatment of oxaphosphirane complex 1, triflic acid (TfOH), and various aldehydes yielded 1,3,4-dioxaphospholane complexes 5a,b-7a,b after deprotonation with NEt(3). In addition to NMR spectroscopy, IR spectroscopy, and MS data, the X-ray structures of complexes 5a and 7a were determined. (31)P NMR spectroscopic monitoring and DFT calculations provided insight into the reaction course and revealed the transient TfOH 1,3,4-dioxaphospholanium association complex TfOH-5a,b and/or TfOH-5a,b' as key reactive intermediates. Furthermore, it was observed that the five-membered ring system was cleaved upon warming and yielded side-on (E,Z)-methylenephosphonium complexes 8a,b if deprotonation did not occur at low temperature. Overall, a novel temperature- and acid-dependent construction and deconstruction process of the 1,3,4-dioxaphospholane ring system is described.