Alkenes are versatile building blocks for a broad range of applications in organic chemistry that can be synthesised by semi-hydrogenation of alkynes. The selective formation of E- or Z-alkenes however still represents a major challenge, especially when considering functional or aliphatic substrates. While Z-selective hydrogenation can be conveniently achieved using a multitude of catalyst systems, including the Lindlar catalyst as the most prominent example, E-selectivity is more difficult to obtain and one of the most common routes still is Birch-reduction using alkali metals in ammonia, producing stoichiometric amounts of waste. The use of suitable hydrogen transfer reagents instead of gaseous hydrogen is a practical and safe alternative. We present two Co(II) PNN complexes as active catalysts in the dehydrogenation of ammonia borane and the semihydrogenation of different substituted internal alkynes in methanol at exceptionally low catalyst concentration in less than 30 minutes. This mild system shows an excellent E-selectivity due the isomerization of the initially formed Z-alkene. Furthermore, labeling experiments with deuterated ammonia borane and methanol showed that H-transfer to the substrate occurs from the B-H bond of H3B-NH3 and the O-H of MeOH. A reliable method to determine the end point of the reaction is the monitoring of the H2 evolution, which takes place due to the dehydrogenation of the ammonia borane mediated by the catalyst during the transfer semihydrogenation, using a gas burette.

Highly E-selective transfer-semihydrogenation of internal alkynes with ammonia borane and a Co(II) PNN pincer catalyst

ACS Spring Meeting 2021

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