The answer of a long-standing question about how the alkylidene can be formed thermally from a metallacyclopentane complex is demonstarted here. In an unobservable a,b-disubstituted tantalacyclopentane intermediate formed from TaCp*Cl₂(RCH=CH₂) (Cp* = η₅-C₅Me₅) and RCH=CH₂, there is strong evidence for the thermal cleavage of a secondary Ta-C bond during the process of dimerizing RCH=CH₂ to two dimers. Making an α,β-disubstituted metallacyclopentane from ethylene and a cyclic olefin can drive the formation of a metallacyclopentane with a secondary M—C bond and restrict it from converting to a more stable metallacyclopentane. In literature, there is only one known example of this type of Mo or W metallacyclopentane, Mo(NC₆F₅)(¹³CH₂¹³CH₂/NBE)(DFTO)₂ (DFTO = O-2,6-(C₆F₅)2C₆H₃; NBE = norbornene). It was formed upon reacting NBE with Mo(NC₆F₅)(¹³CH₂¹³CH₂)(DFTO)₂ at -70 °C. As the temperature was raised to -50 °C, norbornene underwent polymerization to polynorbornene containing no labelled ¹³C.

It might be possible to see ring contraction process if ethylene and an unpolymerizable cyclic olefin can form an α,β-disubstituted metallacyclopentane. It has long been known that cyclohexene is virtually inert to metathesis processes. Therefore we wish to explore the possibility to see ring contraction thermally from a α,β-disubstituted tungstacyclopentane made from cyclohexene and ethylene. Upon addition of cyclohexene to W(NAr)(OSiPh₃)₂(C₂H₄) a 7-tungstabicyclo[4.3.0]nonane complex forms slowly at ambient temperature. Either the loss of cyclohexene or ring-contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow at 80 °C. However, above ~80 °C cyclohexene comes out to give W(NAr)(OSiPh₃)₂(C₂H₄). In presence of excess cyclohexene, a considerable amount of the 7-tungstabicyclo[4.3.0]nonane complex can form and ring-contract to yield methylene complex and methylenecyclohexane, or cyclohexylidene and ethylene complex. This is the first experimental evidence that a metallacyclopentane can thermally ring-contract at a substituted Cα site to generate a metallacyclobutane, and therefore, alkylidenes that are metathesis-active.

Veronica Carta

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