Role of β-H Elimination in Rhodium-Mediated Carbene Insertion Polymerization

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The importance of β-H elimination as a possible mechanism to induce chain termination/transfer and/or the formation of stereodefects in the Rh(diene)-mediated oligomerization and polymerization of carbenes has been studied by means of different approaches. As a remarkable feature, this reaction is associated with low initiation efficiencies (∼5%) and a clear incubation time, during which the selectivity of the catalyst drastically changes from predominant dimerization and oligomerization activity to almost fully selective polymerization. In an attempt to shine more light on these observations, we performed a full computational study using a simplified and unmodified Rh(cod) model, wherein we looked at the relative barriers for chain propagation versus chain transfer via β-H elimination. These DFT calculations indicate that the energy of the transition state (TS) for β-H elimination and complete dissociation of the elimination product are considerably lower in free energy than the TS for chain propagation, thus partially explaining the experimentally observed change of the selectivity during the incubation time. Furthermore, we discuss the importance of β-H elimination as a probable mechanism to introduce stereoerrors (thus explaining the experimental formation of atactic oligomers at the beginning of the reaction). We also explored its importance experimentally, by investigating the influence of deuteration of the diazo substrate and addition of competing olefins. All experiments indicate that β-H elimination does not play a major role regarding chain transfer in the case of our catalyst−substrate combination, and probably generally, if α-carbonyl-substituted diazo compounds are used. The most likely explanation for the combined experimental and computational data is that the Rh(diene) precatalyst gets modified under the applied catalytic conditions in order to form new active species for which apparently β-H elimination is slowed down (leading to nonstereospecific oligomerization activity) and eventually even completely absent (leading to stereoselective polymerization activity).