Insight into cis-to-trans olefin isomerisation catalysed by group 4 and 6 cyclopentadienyl compounds

Intramolecular isomerisation of the pendant allyl unit present in the model compound [MoH(eta;5-C5H4SiMe 2CH2CH=CH2)- (CO)3] reported before was investigated by DFT calculations. The coordination of CO and the splitting of the agostic Mo- H interactions found in metallacyclic transition states stabilise the cis and trans hydride compounds [MoH(η5-C 5H4Si- Me2CH=CHCH3)(CO)3] relative to the corresponding tricarbonyl molybdenum alkyl metallacycles. A comparison with an analogous zirconium system is included. To contrast these results with the behaviour of metal hydride cyclopentadienyl compounds, which have no intramolecular alkene functionality, group 4 and 6 derivatives such as [Zr(η5- C5H4SiMe2- η1-NtBu)(η5-C5H4SiMe 2CH2CH2-η1-CH2)] (2), [MH(η5-C5HMe4)(CO)3] [M = Mo (3), W (4)], and [ZrH(η5- C5H4SiMe 2-η1-NtBu)(η5-C5H4R)] [R = H (5), SiMe3 (6)] were examined as selective catalysts for the intermolecular isomerisation of the terminal olefins allyltrimethylsilane (A) and 4-methyl-1-pentene (B). Zirconium hydride compounds were the most efficient catalysts. Compound 4 catalysed the same reaction but required heating at 140 °C, whereas compound 3 was inactive due to a dehydrogenation process, which produced the dinuclear compound [Mo(η5-C5HMe 4)(CO)3]2 (7). Reaction of 4 and 5 with the internal alkenes trimethyl(1- propenyl)silane (C) and 4,4-dimethyl-2-pentene (D) favoured cis-to-trans isomer conversion with poor production of the corresponding terminal olefins. (© Wiley-VCH Verlag GmbH and Co. KGaA, 69451 Weinheim, Germany, 2009).