Annulation reactions of chromium carbene complexes: scope, selectivity and recent developments

Over the past three decades Fischer-type carbene complexes have received increasing interest as selective reagents in organic synthesis. Apart from its electron-acceptor properties exploited in carbene ligand centered reactions, the metal carbonyl fragment provides a template for non-classical cycloaddition reactions. The most useful among them is the chromium-mediated benzannulation which allows a one-pot access to densely functionalized oxygenated arenes coordinated to a Cr(CO)3 fragment. It is compatible with a variety of functional groups, occurs under mild conditions with remarkable chemo-, regio- and diastereoselectivity, and thus has considerable potential in the synthesis of complex targets and natural products.