An intramolecular alkyne insertion/carbonylation/cyclization sequence of chromium aminocarbene complexes: A novel access to indole and indenoindole skeletons

Article abstract of DOI:10.1002/cber.19961290606

2-Alkynylanilinocarbene chromium complexes 1-7 bearing a rigid arene C₂ spacer between the aminocarbene and alkyne units were prepared from pentacarbonylfaroyljchromates(-I), acetyl bromide, and 2-alkynylanilines. They undergo intramolecular cyclization the course of which depends on the substitution pattern at the alkyne terminus. A tandem alkyne insertion into the metal-carbene bond/carbonylation sequence affords Cr(CO)₃-coordinated 3-indolylketenes 8, 9, 12-14 by using a bulky substituent; the rate of the reaction increases with N-alkylation. Less bulky n-alkynylanilinocarbene complexes 4, 5 exhibit two competing carbene annulation sequences: Benzannulation leads to benzofajcarbazoles 15, 16, whereas cyclopentannulation without prior carbonylation furnishes indeno[l,2-b]indoles 17, 18. VCH Verlagsgesellschaft mbH, 1996.