Metallocarbenes isomerization

Metallocarbene formation by hydrogen shift explains the observed selectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/AljOj (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydro-cylization to gaseous cyclic molecules was determined. Relative rates of 3 2 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-10%) and the temperature (32O°-38O°C). 

However, metallocarbenes may isomerize in different ways to carbene-olefin species and further to adsorbed diolefins (Scheme 70). The latter may readily be desorbed as paraffins, and variations of the experimental con-... 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

The massive synthetic potential of the olefin metathesis reaction has yet to be realized in terms of generality and yields. However, since the metallocarbene addition mechanism has been largely accepted, more useful mechanistic studies have appeared this year. The whole area has been adequately reviewed by the timely publication of the proceedings of the international symposium held at Noordwijkerhout in September 1977. The synthetic utility of this transformation has now been extended by tungsten catalysis to the functionalized olefins of aj-olefinic esters (49), to give reasonable yields (35—40%) of the products as isomeric mixtures (50) in which the trans-isomer predominates. Small amounts of the chlorinated starting materials (51) are also present. The complex tungsten... 

Metallocarbenes have been implicated in the iridium-catalysed isomerization of branched hydrocarbons, such as that of 2-methylpentane (1) to 3-methylpentane (3). Studies with C-labelled (1) support a mechanism which proceeds via (2) as intermediate. Polymer-bound triphenylphosphine-lithium diorganocuprates may offer advantages in the Wurtz-type coupling of alkyl halides, in that work-up is easier and the product is not contaminated with residual tertiary phosphine. However, yields in general are comparable with those from the corresponding homogeneous reagents. ... 

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