Abstract
This chapter presents a historical development of the metal carbene/ metallacyclobutane mechanism of olefin metathesis. For the hundreds of first-generation catalysts used before and since 1980, the precise nature of the metal carbene or metallacyclobutane intermediates remains a matter for speculation since their concentrations are too small to be detected, except in a few cases. However, the structure of the initiating carbene can often be deduced from the initial products, or by using a carbene trap, or from the polymer end groups. The identity, number, and spatial arrangement of the permanent ligands in the propagating complexes operating in the first-generation systems and their effect on the stereochemistry of the metathesis reaction are generally not well understood. Closer investigation of systems in which well-defined metal carbene complexes act as initiators provide a better understanding of these factors, but at present it is difficult to arrive at generalizations because quite small changes in the structure of the initiator or substrate can have a marked effect on the stereochemistry of the reaction.
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