Zerovalent [Pd(NHC)(Alkene)1,2] Complexes Bearing Expanded-Ring N-Heterocyclic Carbene Ligands in Transfer Hydrogenation of Alkynes

In search of more active catalysts for the transfer hydrogenation of alkynes, a series of [Pd(NHC)(MA)(1,2)] (8-14) and [Pd(NHC)(dvtms)] complexes (1-7), in which the NHC ancillary ligands are expanded-ring N-heterocyclic carbenes (erNHC's), have been prepared. These very bulky, strong a-donor ligands impart a highly constrained geometry on the complexes and in some cases enable the isolation of coordinatively and electronically unsaturated complexes (10 and 14). Their strong a-donor character is reflected in a decrease in IR stretching frequency for the C=O bond of the maleic anhydride ligands (8-14) in comparison to their five-membered counterparts. Significantly enhanced catalytic activity in the transfer hydrogenation of 1-phenyl-1-propyne is observed using [Pd(erNHC)(dvtms)] complexes (1-7) as precatalysts. The catalysts show high initial selectivity toward (Z)-alkene. However, double-bond isomerization and over-reduction to the corresponding alkane occur when all the alkyne substrate is consumed; this feature reflects the very high efficiency of these catalysts in the transfer hydrogenation of alkynes as well as alkenes.