Palladium-catalyzed cyclization of 6-aminohex-1-yne

Mechanistic details and limiting factors for the palladium-catalyzed additi on of amines to alkynes (hydroamination) were studied, using the cyclizatio n of 6-aminohex-1-yne to 2-methyl-1,2-dehydropiperidine as a specific examp le. The complex [Pd(Triphos)](CF3SO3)(2) showed the highest catalytic activ ity of a series of structurally and electronically distinct palladium compl exes. Several methods, such as calorimetry and in situ IR and NMR spectrosc opy, were used to obtain evidence for a possible reaction cycle. It seems l ikely that (i) the substrate initially coordinates via the amine group and (ii) an intermediate is formed which is the product of a nucleophilic attac k of the amine on a coordinated alkyne. Addition of an acid to the reaction mixture led to a strong increase in the reaction rate, probably by acceler ating protolytic cleavage of the palladium-carbon bond in the intermediate complex.