Cyclopentylidene carbene

Cyclohexylidene carbene and cyclopentylidene carbene have been generated by the base-induced decomposition of the appropriate 1 -(A-acety 1-jV-nitroso-aminomcthyl)cycloalkan-l-ol in the presence of Aliquat [47-49] and they have been shown to react in high yield with electron rich alkenes (see Section 7.3). Cyclopropylmethylidene, di(cyclopropyl)methylidene, and isopropylidene carbenes have been generated by an analogous route [49]. 

Since the reactive substructure of cA-2-(l,3-butadienyl)cyclopropylidene is contained in the bicyclic carbene 124, there is a possibility that a carbene-carbene rearrangement occurs together with 1,5-carbon migration. Analysis of the probable reaction pathways allows one to conclude that 1,5-C-migration (124 - 132) in the fixed c -l,3-butadienyl fragment of structure 124 is impossible. The 1,3-carbon migration (124 - 130) which takes place instead is mechanistically analogous to the vihylcyclopropylidene-cyclopentylidene... 

Alkylidene carbenes (isopropylidene [150] cyclohexylidene [150-152] cyclopentylidene [150] cyclopropylethylidene and dicyclopropylmethylidene carbenes [153] and alkenylalkylidene carbenes (Me,C=C=C [154-159] adamantylvinylidene carbene [160] MeCH=CH.CH=C [161] react in good yield... 

Metallacyclopent-l-enes have been obtained by formal [2 + 3] cycloaddition of ethylene to Re(CtBu)(CHtBu)(OR)2 (a carbene/carbyne complex) [612]. Nucleophilic tungsten(II) complexes can react directly with cyclopentanone to yield cyclopentylidene complexes [613]. 

The only direct evidence for the presence of metal-carbene-olefin intermediates in catalytic metathesis systems comes from a study of the interaction of the tungsten cyclopentylidene complex 27 with cycloalkenes such as cycloheptene 28 in CD2CI2. When these are mixed at —96 °C and the temperature raised to between —53 and —28 °C, no polymerization occurs but the 13C NMR spectrum contains additional resonances which may be assigned to the metal-carbene-olefin complex 29. The line intensities show that the equilibrium 7 moves to the right as the temperature is lowered120. 

Butin- -diethylester V/2a, 636f. Cyclopentyliden-methan- -dimethylester E19b, 1854 (Carben-Umlagerung)... 

The structural analogy between carbenes and their isoelec-tronic carbocations is also observed for singlet cyclopentylidene (8), which has a C2 symmetric structure (Figure 8) similar to the cyclopentyl cation. " Similarly to 2, two CH bonds stabilize the carbene center in 8. As the system is incorporated into a five-membered ring, the CCC angle at the carbene center is smaller than in 2 (104° vs. 112°). Thus, the. singlet. state... 

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