The Role of Fluorine Substituents in the Regioselectivity of Intramolecular C-H Bond Functionalization of Benzylamines at Palladium(II)

The effect of fluorine substituents on the regioselectivity of intramolecular reactions of mono- and difluorinated N,N-dimethylbenzylamines (1a-f) at palladium, to form palladacycles di-μ-acetatobis[o-dimethylaminomethyl-n-fluorophenyl-C,N)dipalladium(II) (2a-f) and di-μ-chlorobis[o-dimethylaminomethyl-n-fluorophenyl-C,N)dipalladium(II) (3a-e), has been investigated. When fluorinated substrates with two sites available for the C-H functionalization (1c and 1e) undergo cyclopalladation via a CMD mechanism (acetate-bridged palladacycles), they do not exhibit regioselectivity. In contrast, the same substrates exhibit complete regioselectivity for the C-H functionalization para to fluorine in cyclopalladation reactions that proceed via an S_EAr mechanism (involving chloride-bridged palladacycles). X-ray crystal structures were obtained for all the palladacycles synthesized, and a structural analysis showed that the number and the position of the fluorine atoms on the aromatic ring have a marked effect on the "clamshell" structure of the acetate-bridged palladacycles. By contrast, there is no great variation in the structures of the planar chloride-bridged palladacycles.