Thermal Activation of Hydrocarbon C-H Bonds by Tungsten Alkylidene Complexes
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- 作者：Craig S. Adams ; Peter Legzdins ; and Elizabeth Tran
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：January 31, 2001
- 年：2001
- 卷：123
- 期：4
- 页码：612 - 624
- 全文大小：206K
- 年卷期：v.123,no.4(January 31, 2001)
- ISSN：1520-5126
文摘
Thermal activation of Cp*W(NO)(CH2CMe3)2 (1) in neat hydrocarbon solutions transiently generatesthe 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 cleanformation 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 toobtain Cp*W(NO)(=CHCMe3)(PMe3) in two isomeric forms (4a-b), and in benzene, 1 cleanly forms thephenyl complex Cp*W(NO)(CH2CMe3)(C6H5) (5). Kinetic and mechanistic studies indicate that the C-Hactivation chemistry derived from 1 proceeds through two distinct steps, namely, (1) rate-determiningintramolecular 
-H elimination of neopentane from 1 to form A and (2) 1,2-cis addition of a substrate C-Hbond across the W=C linkage in A. The thermolysis of 1 in cyclohexane in the presence of PMe3 yields 4a-bas well as the olefin complex Cp*W(NO)(
2-cyclohexene)(PMe3) (6). In contrast, methylcyclohexane andethylcyclohexane afford principally the allyl hydride complexes Cp*W(NO)(3-C7H11)(H) (7a-b) andCp*W(NO)(3-C8H13)(H) (8a-b), respectively, under identical experimental conditions. The thermolysis of 1in toluene affords a surprisingly complex mixture of six products. The two major products are the neopentylaryl complexes, Cp*W(NO)(CH2CMe3)(C6H4-3-Me) (9a) and Cp*W(NO)(CH2CMe3)(C6H4-4-Me) (9b), inapproximately 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 incorporationof 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 formationof complexes 10 and 11a-b involves the transient formation of Cp*W(NO)(CH2CMe3)(CH2C6H5) (12), theproduct of toluene activation at the methyl position, which reductively eliminates neopentane to generate theC-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 byPMe3 to obtain the adducts Cp*W(NO)(=CHC6H5)(PMe3) (14a-b) in two rotameric forms. From their reactionswith toluene, it can be deduced that both alkylidene intermediates A and B exhibit a preference for activatingthe stronger aryl sp2 C-H bonds. The C-H activating ability of B also encompasses aliphatic substrates aswell 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 molecularstructures of 4a, 6, 7a, 8a, and 14a have been established by X-ray diffraction methods.
-H elimination of neopentane from 1 to form A and (2) 1,2-cis addition of a substrate C-Hbond across the W=C linkage in A. The thermolysis of 1 in cyclohexane in the presence of PMe3 yields 4a-bas well as the olefin complex Cp*W(NO)(2-cyclohexene)(PMe3) (6). In contrast, methylcyclohexane andethylcyclohexane afford principally the allyl hydride complexes Cp*W(NO)(3-C7H11)(H) (7a-b) andCp*W(NO)(3-C8H13)(H) (8a-b), respectively, under identical experimental conditions. The thermolysis of 1in toluene affords a surprisingly complex mixture of six products. The two major products are the neopentylaryl complexes, Cp*W(NO)(CH2CMe3)(C6H4-3-Me) (9a) and Cp*W(NO)(CH2CMe3)(C6H4-4-Me) (9b), inapproximately 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 incorporationof 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 formationof complexes 10 and 11a-b involves the transient formation of Cp*W(NO)(CH2CMe3)(CH2C6H5) (12), theproduct of toluene activation at the methyl position, which reductively eliminates neopentane to generate theC-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 byPMe3 to obtain the adducts Cp*W(NO)(=CHC6H5)(PMe3) (14a-b) in two rotameric forms. From their reactionswith toluene, it can be deduced that both alkylidene intermediates A and B exhibit a preference for activatingthe stronger aryl sp2 C-H bonds. The C-H activating ability of B also encompasses aliphatic substrates aswell 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 molecularstructures of 4a, 6, 7a, 8a, and 14a have been established by X-ray diffraction methods.