Mo(0) carbonyl complexes bearing the bis(3,5-dimethylpyrazole) ligand: catalysis of the regioselective [2 + 2 + 2] cycloaddition of terminal alkynes to synthesize 1,3,5-isomers

The reaction between 1,3-bis(3,5-dimethylpyrazolylmethyl)hexahydropyrimidine L and Mo(CO)₆ in CH₃CN at 130 °C afforded a binuclear Mo(0) complex 1 containing a new macrocycle formed upon C-N bond cleavage in L in good yield. Conversely, a clean reaction takes place between L and [Mo(CO)₄(COD)] in THF at 60 °C to give a new metalloligand complex [Mo(CO)₄(κ²-<i>N</i>,<i>N</i>-L)] 2 containing a spectator pyrazole arm in 83% yield. Their structures were determined by X-ray diffraction methods, and a plausible mechanism is proposed for the C-N bond cleavage leading to complex 1. In view of a limited number of reports of catalysts for the synthesis of 1,3,5-trisubstituted benzene from the [2 + 2 + 2] cycloaddition of terminal alkynes, the catalytic application of complex 2 was explored. Interestingly, 14 terminal alkynes including those containing polar functional groups such as OH, NH₂, and CHO among others were converted into their 1,3,5-isomers as the major products with a high regioselectivity ratio of >90 : 10 (1,3,5-/1,2,4-isomer) for most cases.