Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts.

Exposure of a solution of the square pyramidal tungstacyclopentane complex W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ) (Ar = 2,6- i -Pr 2 C 6 H 3 ) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W(NAr)(OSiPh 3 ) 2 (C 3 H 6 ). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λ max ). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W(NAr)(OSiPh 3 ) 2 (αMeC 3 H 5 )), and then from it, a β methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W(NAr)(OSiPh 3 ) 2 (C 3 H 6 ). The W-C α bond in an unobservable TBP W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ) isomer in which the C 4 H 8 ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W(NCPh 3 )(OSiPh 3 ) 2 (C 4 H 8 ) as for W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ), except that the rate of consumption of W(NCPh 3 )(OSiPh 3 ) 2 (C 4 H 8 ) is about half that of W(NAr)(OSiPh 3 ) 2 (C 4 H 8 ). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W(NCPh 3 )(OSiPh 3 ) 2 (C 3 H 6 ) and propylene from W(NCPh 3 )(OSiPh 3 ) 2 (C 4 H 8 ) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.