Reactivity of Cp*Mo(NO)(= CHCMe3) with olefins and dienes: C-H activation reactions of molybdenacyclobutanes

Reactions of a variety of cyclic and acyclic olefins with the title alkylidene complex (formed spontaneously by loss of neopentane from Cp*Mo(NO)(CH2CMe3)(2) under ambient conditions) result in the initial formation of molybdenacyclobutane complexes (Cp* = C5Me5). These molybdenacyclobutane complexes do not react via olefin metathesis or cyclopropanation pathways, but instead via C-H activation. Thus, when cyclopentene is the olefinic substrate, the direct result of C-H activation at the beta-position of the metallacyclobutane affords a thermally stable allyl hydrido complex that can be isolated. Such an allyl hydride intermediate is not isolable for larger cyclic olefins (cyclohexene, cycloheptene, and cyclooctene) or acyclic olefins (allylbenzene and 1-hexene). Instead, those complexes react further, undergoing a second C-H activation at the allylic position to produce eta(4)-trans-diene complexes concomitant with the loss of dihydrogen. Upon heating, these eta(4)-trans-diene complexes liberate diene, thereby enabling the 14e Cp*Mo(NO) metal fragment to catalyze the oligomerization of cyclic olefins and dienes including cyclohexene and 1,4-cyclohexadiene. In the case of the acyclic olefin allylbenzene, the metal fragment catalyzes a dimerization to (E)-(4-methylpent-1-ene-1,5-diyl)dibenzene under ambient conditions.