Cobalt‐Mediated [2 + 2 + 2]‐Cycloadditions: A Maturing Synthetic Strategy [New Synthetic Methods (43)]

TLDR

Dicarbonyl (η5‑cyclopentadienyl)cobalt serves as a matrix enabling diverse unsaturated substrates to undergo mutual bond formation. The cobalt catalyst mediates [2+2+2] cycloadditions of α,ω‑diynes, monoalkynes, o‑diethynylbenzenes, α,ω‑enynes, nitriles, isocyanates, and CO to generate annelated benzenes, biphenyls, bi‑ and tricyclic dienes, pyridines, 2‑pyridones, and cyclopentadienones. The method delivers remarkable chemo‑, regio‑, and stereoselectivity, partly due to a trimethylsilyl directing group, and has been applied to synthesize novel molecules and natural products such as protoberberines, steroids, vitamin B6, and camptothecin.

Abstract

Abstract Dicarbonyl (η 5 ‐cyclopentadienyl)cobalt functions as a matrix on which a variety of unsaturated organic substrates undergo mutual bond formation. In this way α,ω‐diynes cocyclize with monoalkynes to give annelated benzenes, while o ‐diethynylbenzenes furnish biphenylenes, and α,ω‐enynes lead to the formation of complexed bi‐and tricyclic dienes. Nitriles cocyclize with two alkynyl groups to give pyridines and other heterocycles, isocyanates allow access to annelated 2‐pyridones, and incorporation of carbon monoxide provides complexed cyclopentadienones. In many cases remarkable chemo‐, regio‐, and stereoselectivity are observed, partially facilitated by use of the trimethylsilyl substituent as a controlling group. The scope and level of maturity of the method are demonstrated by the synthesis of a series of hitherto inaccessible, novel, and theoretically interesting molecules, and by its utilization in several unique approaches to a variety of natural products, e.g. protoberberines, steroids, vitamin B 6 , and camptothecin.