Cyclopalladation of N-( p-thiotoluoyl)pyrrolidine and -piperidine

Abstract

Reaction of N-( p-thiotoluoyl)pyrrolidine (Hpr) with lithium tetrachloropalladate(II) in methanol at room temperature resulted in an ortho C—H activation of the benzene ring and a benzene ring- orthopalladated complex PdCl (bpr) was obtained. Another reaction in hexamethylphosphoric triamide at 80°C resulted in an α-CH 2 activation of the pyrrolidine ring to form PdCl (ppr) with an aliphatic α-C—Pd bond. Under the latter reaction conditions N-( p-thiotoluoyl)piperidine (Hpi) was similarly cyclopalladated at α-CH 2 of the piperidine ring to give PdCl (ppi) but under the former experimental conditions no cyclopalladation occurred. These complexes and some of their derivatives were characterized spectroscopically. The results suggested that the steric bulk of the thioamide-N substituents was of prime importance to cyclopalladation-reactivity of thioamides. To verify the facts, the structures of N-(2-thionaphthoyl)pyrrolidine, Pd(acac) (bpr) (acac = acetylacetonate ion), and {l,3-bis(1-pyrrolidinothiocarbonyl)-phenyl-C 2,S,S′} chloropalladium(II) were determined by X-ray analysis. A pyrrolidino group is a sterically more favorable thioamide-substituent than a dimethylamino group to fulfil the requirements of aromatic ring cyclopalladation of tertiary thioamides with palladium(II).