Chemoselective semihydrogenation of alkynes to alkenes with E- or Z-stereoselectivity is among the most important transformations in the synthesis of highly functional organic building blocks. Use of hydrogen transfer reagents instead of gaseous hydrogen represents a viable, safe alternative in many cases. This review summarises recent developments in the field using homogeneous, molecularly defined catalyst systems with focus on hydrogen sources and mechanistic aspects of the respective systems.

The present work provides an overview of possible activation and deactivation phenomena in homogeneous catalytic processes promoted by different types of rhodium complexes containing diphosphine ligands. While awaiting more efficient 1st row metal-based catalysts, a more economic and greener approach to the use of expensive catalysts based on precious metals requires an in-depth knowledge of both the reaction mechanism and the activation and deactivation phenomena which may reduce the temporary solution concentration of the metal effectively available for catalysis. The present work provides in particular a quantitative description of activation and deactivation phenomena such as generation of active species from suitable precatalysts (induction periods); catalyst deactivation due to formation of non-reactive, monomeric species with either the solvent, the substrate, the product or additives; and catalyst aggregation to usually non-reactive multinuclear complexes. Some critical remarks concerning the often applied ‘in situ’ procedure for ‘catalyst’ generation are also included.

In the present work, the rich chemistry of rhodium/phosphine complexes, which are applied as homogeneous catalysts to promote a wide range of chemical transformations, has been used to showcase how the in situ generation of precatalysts, the conversion of precatalysts into the actually active species, as well as the reaction of the catalyst itself with other components in the reaction medium (substrates, solvents, additives) can lead to a number of deactivation phenomena and thus impact the efficiency of a catalytic process. Such phenomena may go unnoticed or may be overlooked, thus preventing the full understanding of the catalytic process which is a prerequisite for its optimization. Based on recent findings both from others and the authors’ laboratory concerning the chemistry of rhodium/diphosphine complexes, some guidelines are provided for the optimal generation of the catalytic active species from a suitable rhodium precursor and the diphosphine of interest; for the choice of the best solvent to prevent aggregation of coordinatively unsaturated metal fragments and sequestration of the active metal through too strong metal–solvent interactions; for preventing catalyst poisoning due to irreversible reaction with the product of the catalytic process or impurities present in the substrate.