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 = CH2CMe3). 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 β-chloride elimination of the 2-chloropentyl activation product. Activation of chloromethane gives Tp′Rh(CNR)(CH2Cl)H, with no activation of the C−Cl bond. Dichloromethane, however, gives only C−Cl cleavage product Tp′Rh(CNR)(CH2Cl)Cl. By comparing the kinetic stabilities of a series of 1-chloroalkane activation products (C1−C5), 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 α to the chloro substituent. Reactions of neopentyl chloride also showed evidence for small quantities of α-chloro C−H activation product. Reactions with the cyclic substrates 1-chlorocyclopentane and 1,1-dichlorocyclopentane yielded a mixture of diastereomeric activation products.