C-H activation of substituted arenes by tungsten alkylidene complexes: Products, selectivity, and mechanism

Thermolyses (70 degreesC, 40 h) of Cp*W(NO)(CH2CMe3)(2) (1) and Cp*W(NO)(CH2CMe3)(CH2C6H5) (2) in xylenes, mesitylene, and a,a,(x-trifluorotoluene generate mixtures of the corresponding aryl and/or benzyl products derived from aromatic sp(2) and benzylic sp(3) C-H bond activations of the solvent molecules by the intermediate alkylidene complexes Cp*W(NO)(=CHCMe3)(sigma-CMe4) (sigma-A) and Cp*W(NO)(=CHC6H5)(sigma-CMe4) (sigma-B), respectively. For instance, the thermolysis of 1 in p-xylene affords products resulting from the activation of one and two molecules of p-xylene. The two products derived from the activation of one solvent molecule are Cp*W(NO)(CH2CMe3)(C6H3-2,5-Me-2) (10) and CP*W(NO)(CH2CMe3)(CH2C6H4-4-Me)(2) (11). The other two complexes derived from the activation of two solvent molecules are Cp*W(NO)(CH2C6H4-4-Me)(C6H4-2,5-Me-2) (12) and Cp*W(NO)(CH2C6H4-4Me)(2) (13). The ratio of the four products in the final reaction mixture is 10:11:12:13 = 1.81 +/- 0.09:0.44 +/- 0.05:1.0:0.15 +/- 0.02. All new complexes prepared have been characterized as fully as possible by conventional spectroscopic methods, and the solid-state molecular structures of Cp*W(NO)(CH2C6H5)(C6H3-3,5-Me-2) (7) and Cp*W(NO)(CH2C6H5)(CH2C6H3-3,5-Me-2) (9) have been established by X-ray diffraction methods. Analyses of the product distributions resulting from the various thermolyses indicate that the benzyl products are increasingly favored over the aryl products as the number of methyl substituents on the solvent molecule increases. Likewise, the movement of the methyl groups from the ortho to meta to para position in the xylenes shifts the aryl vs benzyl product distribution toward the benzyl products. With respect to the aryl product regioselectivities, only the least sterically congested aryl regioisomers are formed in the activations of o- and m-xylene, while the meta and para aryl products are formed preferentially for (x,(x,(x-trifluorotoluene. Finally, the distributions obtained from sigma-B are more abundant in the aryl products than are those obtained from a-A. However, despite these general trends, it is also apparent that the origin of the observed product selectivities is highly dependent on the nature of the substrate, the nature of the C-H activation products, and the intermediate alkylidene complex.