Z Selectivity: Recent Advances in one of the Current Major Challenges of Olefin Metathesis

The stereoselective formation of carbon-carbon double bonds is a longstanding topic in organic chemistry. Since the early 90s, catalytic olefin metathesis has emerged as a synthetic tool of choice for the creation of carbon-carbon double bonds, and its use is now widespread. However, except for the synthesis of small cycles, the reversible character of olefin metathesis usually results in the formation of the thermodynamic E product, and more generally in mixtures of E and Z products. This issue has been addressed in different ways; these are reviewed herein. Synthetic protocols, which aim at the selective synthesis of Z olefins, for example, sequential alkyne metathesis/cis-hydrogenation, and the use of removable silyl groups, are summarized. Tailor-made molybdenum, tungsten and ruthenium catalysts, which allow for very high stereoselectivity, are also reviewed. The advances achieved with these methods and catalysts are illustrated by the synthesis of various natural products or their precursors.