Allyl Isomerization Mediated by Cyclopentadienyl Group 6 Metal Compounds

Abstract

Reactions at mild temperatures of the acetonitrile adducts [M(CO) 3 (NCMe) 3 ] (M ) Mo (1), W (2)) with various substituted cyclopentadienes in THF give the hydrido derivatives 17)) in isolated yields of 60-70%. THF solutions of derivatives 5a, 7a, and 8a undergo isomerization of the cyclopentadienyltethered-allyl unit to give the corresponding methyl-vinyl-dimethylsilyl-η 5 -cyclopentadienyl hydrido compounds) in final a:b ratios of ca. 2:1, 2:1, and 1:1, respectively. Solvent polarity seems to be crucial in these transformations. Dehydrogenation and desilylation of 5a in THF solutions to give the dinuclear derivative [Mo{η 5 -C 5 H 4 Si(CH 3 ) 2 (CH 2 CHdCH 2 )}(CO) 3 ] 2 ( 9) and [MoH(η 5 -C 5 H 5 )(CO) 3 ] (15), respectively, are processes competitive with the allyl isomerization. Treatment of 5a and 7a with equimolar amounts of ONMe 3 permits isolation of pure compound 9 and [W{η 5 -C 5 H 4 Si(CH 3 ) 2 (CH 2 CHdCH 2 )}-(CO) 3 ] 2 (10) in isolated yields of 80-90%. Coordination of the olefin pendant unit to the metal was detectable only by reaction of 5a and 7a with [CPh 3 ][B(C 6 F 5 ) 4 ] to give the cationic species [M{η 5 -C 5 H 4 Si-(CH 3 ) 2 (CH 2 CHdCH 2 )}(CO) 3 ][B(C 6 F 5 ) 4 ] (M ) Mo (18), W ( 19)). Molecular structure determinations by X-ray diffraction methods for derivatives 8a and 9 are also reported.

Insight into cis‐to‐trans Olefin Isomerisation Catalysed by Group 4 and 6 Cyclopentadienyl Compounds

“…In summary, theoretical calculations for the intramolecular isomerisation reaction permit us to conclude that although six-membered zirconacycle derivative 2 is expected to be isolated, only the allyl or the methylvinyl cyclopentadienyl molybdenum compound can be synthesised, which agrees with the experimental observations. [7,8] …”

“…[3] Within this context, transition-metal hydrides are of fundamental importance, as they are involved as catalysts or proposed intermediates in alkene isomerisation reactions. [4][5][6][7][8] In particular, when a M-H bond is present in the catalyst, the generally accepted mechanism involves olefin coordination to give a π-olefin metal complex, which by [a] (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.…”

“…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(η 5 -C 5 HMe 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.…”

“…[4] The design of model compounds that contain cyclopentadienyl ligands with a hemilabile binding profile has proved to be a useful strategy to stabilise intermediates that are transient under typical catalytic conditions. [7][8][9] We began our investigations of early transition metal olefin isomerisation reactions with a family of allyldimethylsilyl-η 5 -cyclopentadienyl hydride group 4 and 6 metal model systems. [7,8] Whereas group 6 metal compounds of formula [MH(η 5 -C 5 R 4 SiMe 2 CH 2 CH=CH 2 )(CO) 3 ] can isomerise the intramolecular pendant allyl unit selectively to the transprop-1-en-yl-dimethylsilyl group, [7] …”

“…DFT calculations on these model zirconium systems confirmed [8] that formation of derivative 2 is a consequence of the higher thermodynamic stability of the six-membered zirconacycle compound relative to that of Zr hydride derivative 1 and the analogous trans-prop- 1-en-yl-dimethylsilylcyclopentadienyl ]. This result moved us to perform analogous theoretical calculations on the allyldimethylsilylcyclopentadienyl Mo hydride compound described before [7,8] to compare the be-haviour of both group 4 and 6 compounds and to provide a detailed picture of the favoured pathways followed for these metal hydride species in their isomerisation processes. In addition, the ability of these hydride organometallic derivatives to participate in intramolecular allyl isomerisation [7,8] prompted us to test similar Mo, W and Zr cyclopentadienyl compounds lacking an alkene functionality in the cyclopentadienyl ring as selective olefin isomerisation catalysts.…”