Factors Affecting Initiation Rates

This chapter provides an overview of the many factors that affect initiation rates: the rate of formation of an active ruthenium carbene from a precatalyst complex. Initiation rate is an important consideration when choosing a particular Grubbs complex for an alkene metathesis application. Experimental conditions have an effect on the initiation process, but by far, the greatest influence on initiation rate is the precatalyst structure itself. Over the past decade, catalyst development has resulted in several distinct families of ruthenium carbene precatalysts that are suited to different applications and are responsive to reaction conditions in slightly different ways. This chapter is divided into several sections; each section discusses a different precatalyst family. In the first section, the parent Grubbs catalysts are discussed, where incremental structural changes have been made, and their effects on rate have been studied in detail. From there, other classes of precatalysts are covered, including the Hoveyda (Grubbs-Hoveyda) catalysts, the Grubbs pyridine solvates, the Piers catalyst, indenylidene catalysts, Z-selective ruthenium carbene catalysts, and bis(N-heterocyclic carbene) complexes.