Terminal and Internal Acyclic Olefins

Molybdenum catalyst are usually efficient for the metathesis of terminal olefins, whereas tungsten complexes are not. However tungsten complexes are active for the metathesis of internal olefins. One possible explanation of this observation is that metallacarbenes combine with terming alkenes selectively according to  

This possibility has been investigated by using a mixture of [1,1-D2] 1-octene and 1-hexene with different catalyst and considering the ratio of the rate constants for formation of 1-octene and [1,1-D2]-1-hexene (- non-productive rate constant) with 7-tetradecene and 5-decene (- kp, productive rate constant). 

Values of kjk were 25 6 for a molybdenum catalyst and much higher, 75—155, for tungsten catalysts [38]. The only reaction observed using [W(=CPh2)(CO)5] and a mixture of CH2==CHMePh and CH2=CMePr was exchange of the methylene groups [39]. 

Another study of CH2 and CD2 exchange between 1-hexene and [l,l-D2]-l-pentene showed that this reaction was approximatively 10 times faster than productive metathesis in WCl —BuLi or WCl —. AlEtCl2 systems [35]. The structural selectivity of metathesis reactions can be summed up as follows Degenerate exchange of =CH2 groups between terminal olefin cross metathesis between terminal and internal alkenes metathesis of internal olefins non-degenerate metathesis of terminal alkenes. 

There is a marked effect on the resulting stereochemistry of the products of metathesis reactions depending on the transition metal. By careful analysis of the cisitrans ratios at very low conversion of 2-butenes, obtained by metathesis of c/5-2-pentene, different behaviour is observed for tungsten and molybdenum complexes [40,41]. 

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