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 β-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 η4-trans-diene complexes concomitant with the loss of dihydrogen. Upon heating, these η4-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.多种环状与非环状烯烃可与本文标题所述的亚烷基配合物发生反应，该配合物由Cp*Mo(NO)(CH₂CMe₃)₂在常温条件下自发失去新戊烷生成，其中Cp*为五甲基环戊二烯基（Cp* = C₅Me₅），反应的初始产物为钼杂环丁烷配合物。此类钼杂环丁烷配合物并非通过烯烃复分解或环丙烷化路径发生反应，而是经由碳氢键活化过程转化：当以环戊烯为烯烃底物时，在金属杂环丁烷的β位发生碳氢键活化，可直接得到可分离的热稳定烯丙基氢化物配合物；对于环尺寸更大的环状烯烃（环己烯、环庚烯与环辛烯）或非环状烯烃（烯丙基苯、1-己烯），该类烯丙基氢化物中间体无法被分离，此类配合物会进一步发生反应，在烯丙基位发生第二次碳氢键活化，生成η⁴-反式二烯配合物，同时释放氢气。经加热后，这些η⁴-反式二烯配合物会释放二烯烃，使得14电子的Cp*Mo(NO)金属片段得以催化环状烯烃与二烯烃（包括环己烯与1,4-环己二烯）的低聚反应；当以非环状烯烃烯丙基苯为底物时，该金属片段可在常温条件下催化其发生二聚反应，生成(E)-(4-甲基戊-1-烯-1,5-二基)二苯。