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

Abstract

Intramolecular isomerisation of the pendant allyl unit present in the model compound [MoH(η5-C5H4SiMe2CH2CH=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-C5H4SiMe2CH=CHCH3)(CO)3] relative to the corresponding tricarbonyl molybdenum alkyl metallacycles. A comparison with an analogous zirconium system is included. To contrastthese 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-C5H4SiMe2CH2CH2-η1-CH2)] (2), [MH(η5-C5HMe4)(CO)3] [M = Mo (3), W (4)], and [ZrH(η5-C5H4SiMe2-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-C5HMe4)(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.