Benzene ring assembly promoted by a camphor derived palladium complex

Trans-[PdCl2L2] (1, L=3-NNMe2C10H14O), under mild reaction conditions, acts as a catalyst for the cyclic trimerization of alkynes. The best performance is achieved for the reaction with PhCCMe that affords 1,3,5-trimethyl-2,4,6-triphenyl benzene with high activity and selectivity (ca. 99%). As a general trend the catalytic activity is higher for internal (PhCCMe, PhCCPh) than for terminal alkynes (HCCPh, HCCtBu, HCCCO2Me). Under more drastic experimental conditions the reaction of 1 with PhCCPh yields trans-[PdCl2(PhCCPh)2] and no catalytic activity is observed. The molecular structure of 1,3,5-trimethyl-2,4,6-triphenyl benzene was confirmed by X-ray diffraction analysis. The molecules were characterized by 1H- and 13C-NMR spectroscopies, FAB-MS and, in some cases, elemental analyses.