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] (<b>8</b>–<b>14</b>) and [Pd­(NHC)­(dvtms)] complexes (<b>1</b>–<b>7</b>), 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 (<b>10</b> and <b>14</b>). Their strong σ-donor character is reflected in a decrease in IR stretching frequency for the CO bond of the maleic anhydride ligands (<b>8</b>–<b>14</b>) 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 (<b>1</b>–<b>7</b>) as precatalysts. The catalysts show high initial selectivity toward (<i>Z</i>)-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