Rhodium(i)-catalysed cycloisomerisation/6π electrocyclisation of 5-(ethynylamino)pent-2-yn-1-yl esters to 2,3-dihydrobenzo[f]indoles

A synthetic method to prepare 2,3-dihydro-1H-benzo[f]indoles that relies on the rhodium(i)-catalysed cycloisomerisation/6 pi electrocyclisation of 5-(ethylnylamino)pent-2-yn-1-yl esters is described. The reaction mechanism was delineated to involve a rare instance in rhodium catalysis of an in situ formed rhodacycle that is susceptible to a formal 1,8-acyloxy migration-initiated reductive elimination. This is followed by the proposed ensuing 1,4-allenene undergoing a 6 pi electrocyclisation to give the N-heterocyclic product.

Automated summary

This study describes a synthetic method for preparing 2,3-dihydro-1H-benzo[f]indoles using a rhodium(i)-catalysed cycloisomerisation/6π electrocyclisation of 5-(ethylnylamino)pent-2-yn-1-yl esters. The reaction mechanism involves the formation of a rare in situ rhodacycle in rhodium catalysis, which is susceptible to a formal 1,8-acyloxy migration-initiated reductive elimination. The proposed 1,4-allenene then undergoes a 6π electrocyclisation to give the N-heterocyclic product.