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Mechanism of rhodium-catalyzed cyclopropanation/cyclization of allenynes.

The rhodium-catalyzed cyclopropanation/cyclization of allenynes was investigated by means of DFT calculations. The results show that the cyclopropanation via the proposed stepwise C(sp(3))-H activation (σ-bond metathesis/C-H reductive elimination) was kinetically unfavorable. Instead, a concerted C(sp(3))-H activation pathway, namely the metal-assisted σ-bond metathesis, in which the hydrogen was directly transferred to the rhodacyclopentane assisted by the Rh center followed by a C-C reductive elimination, was found to explain the experimental results.