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 σ-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 σ-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.