329769-62-2Relevant academic research and scientific papers
Thermal activation of hydrocarbon C - H bonds by tungsten alkylidene complexes
Adams,Legzdins,Tran
, p. 612 - 624 (2001)
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 W = C 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)(CH2 C6H5) (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.
Intermolecular C-H activation of hydrocarbons by tungsten alkylidene complexes: An experimental and computational mechanistic study
Adams, Craig S.,Legzdins, Peter,McNeil, W. Stephen
, p. 4939 - 4955 (2008/10/08)
Cp*W(NO)(CH2CMe3)2 (1) and Cp*W(NO)(CH2CMe3)(CH2C6 H5) (2) under moderate conditions (70 °C, 40 h) generate the reactive complexes Cp*W(NO)(=CHCMe3) (A) and Cp*W(NO)(=CHC6H5) (B), respectively, which activate hydrocarbon solvents via the addition of C-H across the M=C bond. The α-deuterated derivative Cp*W(NO)(CD2CMe3)2 (1-d4) undergoes intramolecular H/D exchange within the neopentyl ligands, consistent with the formation of σ-neopentane complexes prior to neopentane elimination. The thermolysis of 1 in a 1:1 molar mixture of tetramethylsilane-h12 and tetramethylsilane-d12 (70 °C, 40 h) yields an intermolecular KIE of 1.07(4):1. Thermolysis of 1 and 2 in 1:1 benzene/benzene-d6 yields intermolecular KIEs of 1.03(5):1 and 1.17(19):1, respectively. The KIE values are inconsistent with rate-determining C-H bond addition to the M=C linkage and indicate that coordination of the substrate to the metal center is the discriminating factor in alkane and arene intermolecular competitions. The complexes Cp*W(NO)(CH2CMe3)(C6D5) (5-d5) and Cp*W(NO)(CH2C6H5) (C6D5) (6-d5) convert to the respective H/D scrambled products Cp*W(NO)(CHDsynCMe3) (C6D4H1) (5′-d5) and Cp*W(NO)(CHDsynC6H5)(C6 D4H1) (6′-d5) under thermolytic conditions, consistent with the occurrence of reversible aromatic sp2 C-H bond cleavage. The results suggest that the previously reported discrimination between the aryl and benzyl products of toluene activation by A and B originates from coordination of toluene to the metal center in two distinct modes. Supporting DFT calculations on the activation of toluene by CpW(NO)(=CH2) (C) indicate that aromatic sp2 C-H bond activation proceeds through a π-arene complex, while benzylic sp3 C-H bond activation proceeds through a η2(C,H),σ-phenylmethane complex. The principal factor behind the preferential formation of the aryl products appears to be the relative energies of formation of these intermediates.
