Thermal Activation of Hydrocarbon C−H Bonds by Tungsten Alkylidene Complexes
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Abstract
Thermal activation of CpW(NO)(CH(2)CMe(3))(2) (1) in neat hydrocarbon solutions transiently generates the neopentylidene complex, CpW(NO)(=CHCMe(3)) (A), which subsequently activates solvent C-H bonds. For example, the thermolysis of 1 in tetramethylsilane and perdeuteriotetramethylsilane results in the clean formation of CpW(NO)(CH(2)CMe(3))(CH(2)SiMe(3)) (2) and CpW(NO)(CHDCMe(3))[CD(2)Si(CD(3))(3)] (2-d(12)), respectively, in virtually quantitative yields. The neopentylidene intermediate A can be trapped by PMe(3) to obtain CpW(NO)(=CHCMe(3))(PMe(3)) in two isomeric forms (4a-b), and in benzene, 1 cleanly forms the phenyl complex CpW(NO)(CH(2)CMe(3))(C(6)H(5)) (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 alpha-H elimination of neopentane from 1 to form A and (2) 1,2-cis addition of a substrate C-H bond across the W=C linkage in A. The thermolysis of 1 in cyclohexane in the presence of PMe(3) yields 4a-b as well as the olefin complex CpW(NO)(eta(2)-cyclohexene)(PMe(3)) (6). In contrast, methylcyclohexane and ethylcyclohexane afford principally the allyl hydride complexes CpW(NO)(eta(3)-C(7)H(11))(H) (7a-b) and CpW(NO)(eta(3)-C(8)H(13))(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, CpW(NO)(CH(2)CMe(3))(C(6)H(4)-3-Me) (9a) and CpW(NO)(CH(2)CMe(3))(C(6)H(4)-4-Me) (9b), in approximately 47 and 33% yields. Of the other four products, one is the aryl isomer of 9a-b, namely, CpW(NO)(CH(2)CMe(3))(C(6)H(4)-2-Me) (9c) ( approximately 1%). The remaining three products all arise from the incorporation of two molecules of toluene; namely, CpW(NO)(CH(2)C(6)H(5))(C(6)H(4)-3-Me) (11a; approximately 12%), CpW(NO)(CH(2)C(6)H(5))(C(6)H(4)-4-Me) (11b; approximately 6%), and CpW(NO)(CH(2)C(6)H(5))(2) (10; approximately 1%). It has been demonstrated that the formation of complexes 10 and 11a-b involves the transient formation of CpW(NO)(CH(2)CMe(3))(CH(2)C(6)H(5)) (12), the product of toluene activation at the methyl position, which reductively eliminates neopentane to generate the C-H activating benzylidene complex CpW(NO)(=CHC(6)H(5)) (B). Consistently, the thermolysis of independently prepared 12 in benzene and benzene-d(6) affords CpW(NO)(CH(2)C(6)H(5))(C(6)H(5)) (13) and CpW(NO)(CHDC(6)H(5))(C(6)D(5)) (13-d(6)), respectively, in addition to free neopentane. Intermediate B can also be trapped by PMe(3) to obtain the adducts CpW(NO)(=CHC(6)H(5))(PMe(3)) (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 sp(2) 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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