The Intermolecular Pauson-Khand Reaction: Applications, Challenges, and Opportunities

The rapid assembly of complex organic molecules from simple and structurally diverse building blocks is a prevalent challenge in organic synthesis. The Pauson-Khand reaction (PKR) is a method of choice for the construction of five membered rings that has historically been popularized as a late-stage intramolecular cyclization method. The intermolecular version of the PKR, on the other hand, constitutes a powerful approach for the rapid assembly of densely functionalized cyclopentanic cores at early stages of a synthetic sequence. Despite its potential, the intermolecular PKR is much less prevalent in the organic synthesis literature due to several historical limitations, most importantly a reduced scope with respect to the alkene component of the reaction. The last decade has witnessed important developments in the area including a) experimental and theoretical studies that provide a good mechanistic understanding of the reaction and its selectivity, b) methodological developments that have broadened the scope of potential alkene partners, and c) the development of catalytic enantioselective versions that provide useful levels of enantioselectivity. In parallel, remarkable synthetic applications of the intermolecular PKR have emerged, including (enantioselective) total syntheses of complex natural products (polycyclic terpenes, alkaloids, prostanes) as well as examples of industrial relevance. A fundamental limitation of the PKR that needs to be addressed in the future is the current lack of a ligand-accelerated version of the reaction, which would be a promising advance towards developing more efficient and general catalytic (enantioselective) reactions. image