Four-membered titanium oxide ring

The Petasis reagent (Me2TiCp2, dimethyltitanocene) undergoes similar olefi-nation reactions with ketones and aldehydes. The originally proposed mechanism [3] was very different from that of Tebbe olefmation. However, later experimental data seem to suggest that both Petasis and Tebbe olefmation share the same mechanism, i.e. the carbene mechanism involving a four-membered titanium oxide ring intermediate [6]. 

First, the five-membered ring instead of being a cyclopentadineyl (C H ) may also be an indenyl (C H. ) or a fluorinenyl (CjjH,) ring. Second, in 6.9 and 6.10, M is titanium or zirconium in the oxidation state of four. Third, A is an optional bridging atom, generally a carbon or a silicon atom with Rj usually methyl, and R = H, CHj, etc. Finally, in 6.9, X is usually Cl or CH3. ... 

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