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Differing Reactions of Functionalized Hydrocarbons with Cp*M(NO)(alkyl)(η3-allyl) Complexes of Molybdenum and Tungsten

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journal contribution
posted on 28.02.2011, 00:00 by Tommy Tran, Catherine Chow, Amanda C. Zimmerman, Michelle E. Thibault, W. Stephen McNeil, Peter Legzdins
Cp*W(NO)(CH2CMe3)(η3-CH2CHCHMe) (1) is known to initiate facile and selective aliphatic C−H bond activations of hydrocarbons at ambient temperatures. Its ability to effect C−H activations of unfunctionalized hydrocarbon portions of more complex molecules containing various functional groups has now been investigated in some detail. In addition, molybdenum analogues of 1 have also been examined in order to see how the C−H activation chemistry is affected when the central metal is changed. Thermolyses of 1 in neat 1-chloropropane, 1-chlorobutane, and 1-bromobutane at room temperature result in activation of the terminal C−H bonds at the end opposite the carbon−halogen linkage and the clean formation of the alkyl-allyl complexes Cp*W(NO)(CH2CH2CH2Cl)(η3-CH2CHCHMe) (2), Cp*W(NO)(CH2(CH2)2CH2Cl)(η3-CH2CHCHMe) (3), and Cp*W(NO)(CH2(CH2)2CH2Br)(η3-CH2CHCHMe) (4), respectively. No reaction occurs with the C−Cl or C−Br bonds in the haloalkanes even though they are weaker than the C−H bonds that are activated. Similarly, treatment of 1 with n-Bu2O yields exclusively the terminal sp3 C−H activated product, Cp*W(NO)((CH2)4O(CH2)3CH3)(η3-CH2CHCHMe) (5), whereas the reaction with THF results in the single activation of a secondary sp3 C−H bond α to the oxygen atom in THF and the formation of Cp*W(NO)(C4H7O)(η3-CH2CHCHMe) (6). Consistently, reaction of 1 with ethylcyclohexane results in preferential activation of one of the primary sp3 C−H linkages of the ethyl group and the formation of Cp*W(NO)(CH2CH2C6H11)(η3-CH2CHCHMe) (7). The Cp*Mo(NO)(alkyl)(η3-allyl) complexes analogous to 1 are generally thermally unstable and react at or slightly above room temperature. The first member of this family of complexes to be studied was Cp*Mo(NO)(CH2CMe3)(η3-C3H5) (8), whose thermolysis in the presence of pyridine at 35 °C over 3 days leads to the formation of Cp*Mo(NO)(C5H5N)(η2-CH2CHCH2CH2-t-Bu) (9), an η2-olefin complex in which the allyl and neopentyl ligands have coupled. The related Cp*Mo(NO)(CH2SiMe3)(η3-CH2CHCHMe) complex (11) exists as a 2:1 mixture of isomers distinguishable by the orientation of the endo, syn allyl ligand. In the less sterically congested major isomer, the methyl group on the allyl ligand is adjacent to the NO ligand, but in the minor isomer the methyl group is adjacent to the more sterically demanding CH2SiMe3 ligand. In general, the thermal reaction of 11 is similar to that of 1. Spectroscopic monitoring indicates that the loss of TMS from 11 at room temperature results in the formation of a 16e η2-diene intermediate complex that can be trapped with PMe3 as an 18e adduct, Cp*Mo(NO)(η2-CH2CHCHCH2)(PMe3) (12). However, reactions of 11 with various substrates (e.g., pentane, Et2O, and mesitylene) all lead to a single product, Cp*Mo(NO)(η4-trans-butadiene) (13). Evidently, the formation of the 18e butadiene complex is favored over the activation of a relatively electron-poor C−H bond by this molybdenum system. The results of DFT calculations on the model reaction of CpW(NO)(η2-CH2CHCHCH2) with propane confirm that the rate-determining step is the cleavage of a propane C−H bond and that the activation barrier for terminal activation is 8.6 kJ/mol lower in energy than that for internal activation. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of complexes 3, 5, 6, 8, 9, and 13 have been established by X-ray crystallographic analyses.