Olefins hydroboration catalytic cycle

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

Even an organolanthanide-catalyzed variant of the (anti-Markownikofl) hydroboration of olefins has been developed. The catalytic cycle is illustrated in Scheme 8. 

In this review we summarized the results of the latest ab initio studies of the elementary reaction such as oxidative addition, metathesis, and olefin insertion into metal-ligand bonds, as well as the multistep full catalytic cycles such as metal-catalyzed hydroboration, hydroformylation, and sila-staimation. In general, it has been demonstrated that quantum chemical calculations can provide very useful information concerning the reaction mechanism that is difficult to obtain from, and often complementary to, experiments. Such information includes the structures and energies of unstable intermediates and transition states, as well as prediction of effects of changing ligands and metals on the reaction rate and mechanism. 

There are two potential side reactions that can derail the catalytic cycle shown in Scheme 10.6, namely, hydroboration of the olefin monomer by borane and ligand exchange reactions between the borane and cocatalysts containing aluminum alkyl groups. Fortunately, borane compounds containing B-H groups usually form stable dimers (Figure 10.1) that are unreactive toward olefins in typical olefin polymerization solvents (e.g., hexane, toluene). To eliminate the second process, however. 

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