Conversions of Tungsten(IV) Cycloalkene Complexes to Metathesis‐Active Cycloalkylidene Complexes Are Catalyzed by Cycloalkene and Can be Inhibited by Cycloalkene

Olefin metathesis reactions catalyzed by molybdenum or tungsten alkylidene (M═CRR’) complexes are robust methods of forming carbon‐carbon double bonds, but how catalytically active alkylidene complexes are formed from olefins alone has not been determined. Here we show that two electron reductions of W(NR)(OR') 2 Cl 2 complexes in the presence of cyclopentene, cyclohexene, cycloheptene, or cyclooctene yield W(NR)(OR') 2 (cycloalkene) and/or W(NR)(OR') 2 (cycloalkylidene) complexes (R = 2,6‐diisopropylphenyl (Ar), 2,6‐dichlorophenyl (Ar Cl ); OR' = OSiPh 3 , OCMe 2 (CF 3 ) (OR F3 ), OCMe(CF 3 ) 2 (OR F6 ), OC(CF 3 ) 3 (OR F9 )). Mechanistic studies suggest that alkylidenes are formed from unobserved bis ‐alkene complexes through the transfer of an allylic proton from one bound olefin to the second to give an alkyl/allyl intermediate followed by the return of an α proton from the alkyl to form an alkylidene (the AA mechanism). Tungstacyclopentane complexes formed from two equivalents of cyclopentene or cyclohexene can be isolated, but there is no compelling evidence that these complexes can form alkylidenes through a thermal ring‐contraction mechanism. In fact, these metallacyclopentanes appear to inhibit alkylidene formation.