η3-Diphosphavinylcarbene: a P2 analogue of the Dötz intermediate

Olefin metathesis[1] constitutes a powerful tool for the construction of a plethora of unsaturated building blocks, pharmaceuticals, and advanced materials. The principle steps involve a transition-metal carbene complex (A) that undergoes a [2+2] cycloaddition/cycloreversion protocol with alkenes according to the Chauvin mechanism.[2] For the widely used Grubbs catalysts, however, support for the key intermediate of this process, the four-membered metallacyclobutane, relies on theoretical studies[3] and low-temperature NMR spectroscopy.[4, 5] The unsaturated analogues provide more insight. For example, reaction of alkynes with the second-generation Grubbs catalyst gives stable h3-vinylcarbene complexes (1, Mes=2,4,6-Me3C6H2) that form after rearrangement of the initial ruthenium cyclobutenes (B).[6,7] These puckered metal h3-vinylcarbene complexes[8] (C) are highly potent and represent the key intermediate in enyne metathesis,[9] alkyne polymerization (via D),[10] and the versatile Fischer carbene mediated D tz (benzannulation) reaction (which involves complexes such as 2).[11, 12]