Thermal Activation of Hydrocarbon C−H Bonds by Tungsten Alkylidene Complexes

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Abstract
Thermal activation of Cp*W(NO)(CH2CMe3)2 (1) in neat hydrocarbon solutions transiently generates the neopentylidene complex, Cp*W(NO)(CHCMe3) (A), which subsequently activates solvent C−H bonds. For example, the thermolysis of 1 in tetramethylsilane and perdeuteriotetramethylsilane results in the clean formation of Cp*W(NO)(CH2CMe3)(CH2SiMe3) (2) and Cp*W(NO)(CHDCMe3)[CD2Si(CD3)3] (2-d12), respectively, in virtually quantitative yields. The neopentylidene intermediate A can be trapped by PMe3 to obtain Cp*W(NO)(CHCMe3)(PMe3) in two isomeric forms (4a−b), and in benzene, 1 cleanly forms the phenyl complex Cp*W(NO)(CH2CMe3)(C6H5) (5). Kinetic and mechanistic studies indicate that the C−H activation chemistry derived from 1 proceeds through two distinct steps, namely, (1) rate-determining intramolecular α-H elimination of neopentane from 1 to form A and (2) 1,2-cis addition of a substrate C−H bond across the WC linkage in A. The thermolysis of 1 in cyclohexane in the presence of PMe3 yields 4a−b as well as the olefin complex Cp*W(NO)(η2-cyclohexene)(PMe3) (6). In contrast, methylcyclohexane and ethylcyclohexane afford principally the allyl hydride complexes Cp*W(NO)(η3-C7H11)(H) (7a−b) and Cp*W(NO)(η3-C8H13)(H) (8a−b), respectively, under identical experimental conditions. The thermolysis of 1 in toluene affords a surprisingly complex mixture of six products. The two major products are the neopentyl aryl complexes, Cp*W(NO)(CH2CMe3)(C6H4-3-Me) (9a) and Cp*W(NO)(CH2CMe3)(C6H4-4-Me) (9b), in approximately 47 and 33% yields. Of the other four products, one is the aryl isomer of 9a−b, namely, Cp*W(NO)(CH2CMe3)(C6H4-2-Me) (9c) (∼1%). The remaining three products all arise from the incorporation of two molecules of toluene; namely, Cp*W(NO)(CH2C6H5)(C6H4-3-Me) (11a; ∼12%), Cp*W(NO)(CH2C6H5)(C6H4-4-Me) (11b; ∼6%), and Cp*W(NO)(CH2C6H5)2 (10; ∼1%). It has been demonstrated that the formation of complexes 10 and 11a−b involves the transient formation of Cp*W(NO)(CH2CMe3)(CH2C6H5) (12), the product of toluene activation at the methyl position, which reductively eliminates neopentane to generate the C−H activating benzylidene complex Cp*W(NO)(CHC6H5) (B). Consistently, the thermolysis of independently prepared 12 in benzene and benzene-d6 affords Cp*W(NO)(CH2C6H5)(C6H5) (13) and Cp*W(NO)(CHDC6H5)(C6D5) (13-d6), respectively, in addition to free neopentane. Intermediate B can also be trapped by PMe3 to obtain the adducts Cp*W(NO)(CHC6H5)(PMe3) (14a−b) in two rotameric forms. From their reactions with toluene, it can be deduced that both alkylidene intermediates A and B exhibit a preference for activating the stronger aryl sp2 C−H bonds. The C−H activating ability of B also encompasses aliphatic substrates as well as it reacts with tetramethylsilane and cyclohexanes in a manner similar to that summarized above for A. All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 4a, 6, 7a, 8a, and 14a have been established by X-ray diffraction methods.

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