Tebbe FN,
Parshall GW,
Reddy GS.
E. I. du Pont de Nemours and Company, Wilmington, USA
Olefin Homologation with Titanium Methylene Compounds.
J. Am. Chem. Soc. 1978;
100: 3611-3613
DOI:
10.1021/ja00479a061
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.
