Selective C-H activation of haloalkanes using a rhodiumtrispyrazolylborate complex.

Several halogenated substrates are found to participate in C-H bond cleavage reactions with the photochemically generated fragment [Tp'Rh(CNR)] (Tp' = hydrotris(3,5-dimethylpyrazolyl)borate; R = CH(2)CMe(3)). Reaction with 1- or 3-chloropentane gives only terminal C-H activation products. Reaction with 2-chloropentane gives a mixture of 4-chloropentyl activation product and Tp'Rh(CNR)HCl, arising from beta-chloride elimination of the 2-chloropentyl activation product. Activation of chloromethane gives Tp'Rh(CNR)(CH(2)Cl)H, with no activation of the C-Cl bond. Dichloromethane, however, gives only C-Cl cleavage product Tp'Rh(CNR)(CH(2)Cl)Cl. By comparing the kinetic stabilities of a series of 1-chloroalkane activation products (C(1)-C(5)), it was found that the chlorine substituent dramatically decreases reductive elimination rates as the substitution is closer to the metal center. With 1-chloroalkanes, there is evidence for the formation of small quantities of C-H cleavage products alpha to the chloro substituent. Reactions of neopentyl chloride also showed evidence for small quantities of alpha-chloro C-H activation product. Reactions with the cyclic substrates 1-chlorocyclopentane and 1,1-dichlorocyclopentane yielded a mixture of diastereomeric activation products.