Alkene metathesis associative mechanism

Computational studies have shown that alkane metathesis cannot occur via the o-bond metathesis between the C-C a-bonds and the M-C a-bonds originally proposed [101]. Experimental evidence has also suggested that the reaction mechanism must involve alkene metathesis as the key step and alkylidene hydrido metal complexes as associated intermediates [92, 93, 102]. To date, only few computational studies on alkane metathesis have been reported [103-106]. 

In 1970, Chauvin and Herisson presented a study of the co-metathesis of cycloalkene/alkene mixtures using a WOCLj/SnBuj pro-catalyst mixture [12]. Whilst the fully quantitative analysis of the product mixtures was made complicated by the range of techniques that were required for the low, medium and high molecular weight products (mono alkenes, telomers and polymers), it became clear that product ratios were not consistent with what would be predicted by either mechanism in Scheme 12.14. The analysis and associated mechanistic interpretation were seminal and worthy of consideration in some detail here. The key point is that both mechanisms in Scheme 12.14 are pairwise, i.e. each turnover of the catalyst cycle involves two alkenes that undergo concerted alkylidene exchange. When a single alkene, e.g. pent-2-ene (C5), is considered, the products of alkylidene metathesis... 

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