Syntheses of sterically shielded stable carbenes of the 1,2,4-triazole series and their corresponding palladium complexes: efficient catalysts for chloroarene hydrodechlorination.

The new sterically shielded 1,3,4-trisubstituted 1,2,4-triazol-5-ylidenes 8b­d were synthesized by a three step method starting from 2-phenyl-1,3,4-oxadiazole. The syntheses of palladium complexes 9a­d and 10a­d (including the sterically shielded derivatives 9c,d and 10a­d) were carried out via the reactions of the stable carbenes 8a­d with palladium halogenide salts in THF or toluene solution. Complexes 9c,d and 10a­d were found to be excellent catalysts for the reductive dechlorination (hydrodechlorination) of p-dichlorobenzene. The structures of 8c, 9a,b, and 10a were determined by single-crystal X-ray diffraction.