The allenic Pauson-Khand reaction in synthesis

The Pauson-Khand (PK) reaction is a formal [2+2+1] cycloaddition involving an alkene, an alkyne and carbon monoxide. The allene moiety represents a versatile and useful building block in organic synthesis. In place of the usual alkene, allene reagents are fascinating substrates in the Pauson-Khand-type reaction because of their unique reactivity and the synthetic utility of the final products. However, there are significant problems of selectivity. Several studies, most of them recent, have greatly enhanced the synthetic utility of this reaction by the use of allenes instead of alkenes. This paper presents recent advances in the less exploited allenic and dienic variants of the Pauson-Khand-type cycloaddition. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.