Historical Background and Mechanism of Alkene Metathesis

At present, Mo, W, Re and Ru complexes are known to catalyse alkene metathesis [7]. This unique reaction, catalysed by transition metal complexes, is impossible to achieve by other means. Later, based on studies of the reactivities of Fischer-type carbene complexes, it was discovered that carbene complexes are the intermediates in alkene metatheses. WClg reacts with EtAlCl2 to afford the diethyltungsten complex 3 by transmetallation, and subsequent elimination of a-hydrogen generates ethane and the carbene complex 4 which is the active catalyst. 

Metathesis of alkene 6 to give the new alkenes 11 and 15 is explanined by the following mechanism. The first step is [2+2] cycloaddition between metal carbene 5 and alkene 6 to generate the metallacyclobutane 7 as an intermediate. The real catalyst 8 is generated by retrocycloaddition of the metallacyclobutane 7. Reaction of 8 with alkene 6 generates the metallacyclobutanes 9 and 10 as intermediates. The intermediate 10 is a nonproductive intermediate, which reproduces 6 and 8, while 9 is a productive intermediate and yields the new alkene 11 and the real catalyst 12. Cycloaddition of 12 to alkene 6 produces the productive intermediate 14, from which the new alkene 15 and the active catalytic species 8 are formed. The intermediate 13 is a nonproductive one. 

Synthetic reactions via transition metal carbene complexes 

Although not strictly correct from a mechanistic viewpoint, the course of alkene metathesis can be easily understood and the products predicted by considering the reversible formation and cleavage of the hypothetical cyclobutane 16 from alkene 6 to give the alkenes 11 and 15. The whole process involves the cleavage of double bonds and the formation of new double bonds. Such a process cannot be realized by any other means. The cyclobutane 17 is nonproductive. 

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