A Methylene-Bridged Titanium–Aluminium Complex for Olefinations of Carbonyls: The Tebbe Reagent

Key words

titanium - aluminum - olefination - methylene transfer

Significance

In 1978, Frederick N. Tebbe, George W. Parshall, and G. S. Reddy published a procedure for the generation of methylene-bridged compounds of the general structure Cp₂Ti-CH₂AIXR₂ and their application in insertion reactions into alkenes and olefinations of carbonyl compounds. The authors conducted mechanistic studies, using Cp₂Ti-CH₂AICl(CD₃)₂. When performing olefination reactions with the deuterated Tebbe reagent and cyclohexanone, no deuterium was observed in the resulting olefin product, indicating that solely the bridging methylene group is transferred. Generating similar reagents using ZnMe₂ instead of AlMe₃ was also possible, and those showed similar reactivities in olefination reactions.

Comment

Active species in the reaction is most likely not the Tebbe Reagent itself but a bis(η⁵-­cyclopentadienyl)-(methylene)titanium(IV), which is generated by treatment of the Tebbe Reagent with a Lewis base. The mechanism is likely to proceed via [2+2] cycloaddition, forming an oxatitanacyclo­butane, which fragments irreversibly into the olefin product and titanocene oxide. The procedure allows for instance the methenylation of aldehydes, ketones, esters, amides, and carbonates. Alternatively, the Petasis reagent (J. Am. Chem. Soc. 1990, 112, 6392) can be obtained by treatment of Cp₂TiCl₂ with methyl Grignard or methyllithium. Petasis reagent reacts upon heating with the mentioned carbonyl compounds and in addition allows olefination of acid anhydrides. Furthermore, the reaction conditions are milder since no basic conditions and no Lewis acids are needed.

Publication History

Article published online:
20 July 2021