Degenerate olefin cross metathesis

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. 

Tebbe and co-workers reported the first olefin metatheses between titanocene-methyli-dene and simple terminal olefins [13]. They showed cross-metathesis between isotopically labeled isobutene and methylenecyclohexane to be catalyzed by titanocene-methylidene. This process is referred to as degenerate olefm metathesis as it does not yield any new olefin (Scheme 14.6). The intermediate titanacyclobutane has been isolated and characterized [14], and its stability [15] and mechanism of rearrangement [16] have been investigated. 

The forward reaction (1) is an example of cross-metathesis between two different olefins and provides a route to styrene. The reverse of (1) is a self-metathesis reaction such reactions may be either productive as in (4), or non-productive (also called degenerate) as in (5). 

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