Beyond Lindlar and without hydrogen: Transfer hydrogenation of internal alkynes catalyzed by a palladium(0) catalyst containing an N‐heterocyclic carbene ligand gives Z alkenes without over‐reduction to alkanes (see scheme). Contrary to most transfer hydrogenations, ketones are not reduced. As such, this is the first catalyst that shows excellent stereo‐ and chemoselectivity for the semihydrogenation of alkynes without the need for hydrogen gas.

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo-and stereoselectivity when using a zero-valent palladium(NHC)(maleic anhydride)-complex as pre-catalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuteriumlabeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations we propose a mechanism in which hydrogen transfer from formate anion to zero-valent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the 2 high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.

The synthesis of an air-stable series of Pd 0 complexes with dissymmetric bidentate N-heterocyclic carbene-amine ligands has been performed. The key step is an unprecedented carbene transfer from Ag I to obtain electron-rich zero-valent palladium precatalysts. The coordination behavior of the ligands was determined with X-ray crystallography. Surprising results were obtained in the catalysis of transfer semi-hydrogenation, where the addition of base appeared not to be necessary to obtain the desired product stereoselectively.

Survival of the fittest? Amplification of the best receptor from a dynamic combinatorial library does not always proceed according to survival of the “fittest”. However, it has recently resulted in the discovery of some unexpected receptors for various substrates, as illustrated by the example of a catenane receptor (see picture; green) for the neurotransmitter acetylcholine (red).