Cyclopentanes ring-opening mechanisms

With various alkyl-substituted cyclopentanes, ring-opening selectivity with Ir/A Os depended on the number of CH2—CH2 bonds, which argues for the dicarbene mechanism, in which the intermediate is a l,l,2,2-a4 species. The chemisorbed state of cyclopentene was reported to be more or less perpendicular to the Ir(l 11) surface by a NEXAFS study, in support of this mechanism. 

The product distributions in the ring opening of substituted cyclopentanes show striking similarities to those in isomerization of the corresponding alkanes (n-hexane, 2- and 3-methylpentane),301 which led to the formulation of the cyclic mechanism involving adsorbed cyclopentane intermediates to interpret alkane isomerization15,251,252 (see Section 4.3.1). 

Thus, an opening of the cyclopentane ring followed by a 1,6-ring closure would lead only to 1,1-dimethylcyclohexane and toluene. A carbonium ion mechanism (see below) would lead to all xylenes, ortho as well as para and meta. Barron... 

RhCl(PPh8)a] catalyses similar cyclizations of 4,4-disubstituted 1,6-dienes to methylene cyclopentanes. The stereochemistry of iron-carbonyl-promoted ring-openings may be predicted by Frontier Molecular Orbital Theory.A mechanism involving initial oxidative-addition to give an acylrhodium(iii) hydride intermediate has been proposed to explain the Rh f-catalysed cyclization reaction (13) - alkyl substituents in either the 2 or 5 positions substantially... 

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