Diastereocontrol in Olefin Metathesis: the Development of Z ‐Selective Ruthenium Catalysts

Abstract

The diastereocontrol of the double bond formed during an olefin metathesis reaction has been a long-standing goal and challenge for the field. Only within the past few years have molybdenum (Mo), tungsten (W), and ruthenium (Ru) catalysts been able to achieve the level of stereocontrol necessary for synthetic applications. This chapter will focus on the development of Ru-based, Z-selective olefin metathesis catalysts, and consists of my personal account of the initial discoveries, as well as our current mechanistic understanding of the catalysts available. Examples of the potential synthetic uses for these catalysts will also be discussed. Due to the relative youth of this subfield of olefin metathesis, discoveries are being made at a rapid pace; thus, it is impractical to include the very latest results in this chapter. Nevertheless, I will attempt to demonstrate the potential applications of these new Ru catalysts, as well as address some of the synthetic limitations that remain. Molybdenum and tungsten Z-selective catalysts are discussed in Chapter 1, and also see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7. The Challenge of Z-Selective Olefin MetathesisIn the quintessential description of an olefin metathesis reaction, two terminal olefins are reacted in the presence of a catalyst to form a new internal olefin and ethylene gas (Scheme 3.1). Importantly, metathesis is a thermodynamically controlled reaction that requires a driving force, such as the release of ring strain or the formation of a volatile small molecule product, in order to reach completion [1]. An additional consequence of thermodynamic control and the reversibility of the metathesis reaction is the formation of the thermodynamically most stable product or product mixture. In the case of internal olefins, the most stable product is typically the trans, or E, olefin [2]. Thus, in reactions like the one presented in