Catalytic activity and selectivity of group vib metal carbonyl complexes in dehydrohalogenation reactions

The homogeneous catalytic activities and selectivities of a series of complexes ArM(CO)₃, where M = Cr,Mo,W, and their derivatives in dehydrohalogenation reactions have been investigated. In addition to dehydrohalogenation, double-bond migrations also occurred. For Mo-based catalysts, their activity decreased and the induction time increased in the order: (Tol)Mo(CO)₃, (Mes)Mo(CO)₃, Mo(CO)₆ and (CH₃CN)₃Mo(CO)₃. Phosphine ligands attached to the metal poisoned the catalytic activity of the system. For W derivatives, (Mes)W(CO)₃ > W(CO)₆. The ArCr(CO)₃ catalysts were mild and their catalytic activity was not greatly affected by the nature of the arene ligand attached on the metal. Their approximate catalytic activity decreased in the order: anisole, methyl benzoate, benzene, toluene, p-xylene, mesitylene and tetralin. The catalytic activity with respect to the metal was: Mo > W > Cr. A coordinative unsaturation mechanism, probably of the redox type, is proposed. Preliminary ESR study shows that free radicals formed in a homogeneous solution of 2-chloro-2-methylbutane and Mo(CO)₆ can be trapped using a phenyl N-butylnitrone spin trap, and the resulting six-line ESR spectrum has a g-factor of 2.0045.