Classification of Alkene Metathesis

Alkene metatheses useful for organic synthesis can be classified into four types. 

The symmetric alkenes 11 and 15 are formed by homometathesis of the unsymmetric alkene 6. Alkene metathesis is an equilibrium reaction, and the homometathesis of internal alkene 6 may be a useful one only when separation of the products 11 and 15 from the starting alkene 6 is easy, namely when R] and R are clearly different functional groups. 

Cross-metathesis of two different alkenes 11 and 42 usually produces a mixture of products 6 and 15. However, depending on the functional groups R1 and R2, the cross-product 6 is obtained with high selectivity rather than the homoproduct 15 from 11 and 42. Some terminal alkenes, such as allylstannane [16], acrylonitrile [17,18] and allylsilane [19], undergo clean cross-metathesis to give cross-products 6 as the main product, rather than homoproducts 15. Cross-metathesis of the cyclic alkenes 43 with terminal alkenes 42 can be used for the synthesis of dienes 44. 

A very useful cross-metathesis is the reaction involving ethylene, which is called ethenolysis. Reaction of ethylene with internal alkenes produces the more useful terminal alkenes. Two terminal alkenes 45 and 42 are formed from the unsymmetric alkene 6 and ethylene. The symmetric alkenes 11 are converted to single terminal alkenes 45. The terminal dienes 46 are formed by ethenolysis of the cyclic alkenes 43. 

Synthetic reactions via transition metal carbene complexes 

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