In the hydrogenation of alkenes over a metal catalyst, the two hydrogen atoms are added with syn stereochemistry.

In the metal-catalysed dehydrogenation of alkanes, is the removal of hydrogen likewise concerted with syn stereochemistry? And if so, why?


The answer you seek can be found here uwyo.edu/roddick/site/alkane_dehydrogenation.html , the catalytic cycle of dehydrogenation of alkanes by Iridium complexes does not follow a simple syn addition such as in the hydrogenation of alkenes. In the hydrogenation of alkenes, the $\ce {H2}$ dissociates and is added syn to the substrate in a well understood mechanism.

In the dehydrogenation though, the coordination of the alkane to the Ir metal center occur first, then a migratory insertion forming a 5-coordinated trigonal bipyramidal complex, then a β-elimination occur forming an unstable 6-coordinated complex which readily loses the alkene (final product). The catalyst is then regenerated by a hydrogen acceptor which can be a bulky alkene (so its product can't participate in the cycle due to steric hindrance). If no regio inducing group is present in the catalyst, the formation of the Saytzeff product (most substituted alkene) is favored.

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