Mechanistic Study of Cycloisomerization of 1,7-Allenenes and Guidance to Develop a [2 + 1 + 2] Reaction of 1,7-Allenenes with CO for Accessing <i>cis</i> - and <i>trans</i> -5/5 Skeletons

Rh-catalyzed cycloisomerization of 1,7-allenenes has been developed in the literature to synthesize seven-membered rings, but the mechanism of this reaction has not been elucidated, even though a tentative one has been proposed. We applied DFT calculations to dispute the previous mechanism and proposed a novel one supported computationally: the cycloisomerization reaction actually starts with rarely hypothesized <i>endo</i>-oxidative cyclometalation of allene and alkene (metal is in a bridged position of the formed ring skeleton), followed by β-H elimination and reductive elimination. This new mechanism can also explain the overlooked side reaction of [2 + 2] cycloaddition and inspired us to develop a [2 + 1 + 2] reaction of 1,7-allenenes and CO, which starts from oxidative tetraatomic cyclometalation of allene, CO, and Rh, followed by <i>endo</i>-alkene insertion and transannular reductive elimination. This new carbonylation reaction usually generates a <i>cis</i>-5/5 skeleton. But for 1,7-allenenes with substituted alkenes, the [2 + 1 + 2] reaction can deliver a highly strained and challenging <i>trans</i>-5/5 skeleton, which so far can be accessed by only limited reactions. DFT understanding of the stereochemistry and mechanisms showed that oxidative tetraatomic cyclometalation adopted in this carbonylation reaction is key to building the <i>trans</i>-5/5 skeleton, which allows alkene insertion to form a <i>trans</i>-5/6 bicyclic metalacycle and afford the final <i>trans</i>-5/5 structure via reductive elimination. The present reaction synthesizing both <i>cis</i>- and <i>trans</i>-5/5 skeletons represents a significant advance for Pauson-Khand-type reactions. The proposed <i>endo</i>-oxidative cyclometalation pathway and the reason for choosing this in the present reaction will enrich the textbook of organometallics.