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Tebbe's reagent

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 Systematic name
 μ-Chlorobis(cyclopentadienyl)-
(dimethylaluminum)-μ-methylenetitanium	 


 Other names
 Tebbe Reagent 


 Molecular formula
 C13H18AlClTi    


 Molar mass
 284.60 g/mol


 CAS number


 Solubility 
 toluene, benzene, dichloromethane, 
THF (low temperatures only)


[Chemical infoboxDisclaimer and references]






The Tebbe reagent is (C5H5)2TiCH2ClAl(CH3)2.  This classic organometallic compound finds use in organic synthesis.  It is a red solid that is pyrophoric in the air, and thus is typically used under nitrogen (N2) or argon (Ar) gas.  
This compound contains two cyclopentadienyl (C5H5-,  Cp) rings bonded to titanium.  The titanium and aluminum atoms are bridged by both CH2 and chloride ligands.  Aluminum is also bonded to two methyl groups.  This compound shares the same general structure as Al2X6 compounds, which exhibit a nearly square (Ti-CH2-Al-Cl) bridge.Wells, A.F., Structural Inorganic Chemistry Fifth Ed., Oxford University Press, New York, p. 976.


The Tebbe reagent was the first reported compound where a methylene group bridges a transition metal (Ti) and a main group metal (Al).Herrmann, W.A., "The Methylene Bridge" Advances in Organometallic Chemistry 1982, 20, 195-197.





Contents





Preparation

The Tebbe reagent is synthesized from titanocene dichloride in toluene solution.Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. "Olefin Homologation with Titanium Methylene Compounds" J. Am. Chem. Soc. 1978, 100, 3611-3613.Straus, D. A., "μ-Chlorobis(cyclopentadienyl)(dimethylaluminum)-μ-methylenetitanium": Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd, 2000.Herrmann, W.A., "The Methylene Bridge" Advances in Organometallic Chemistry 1982, 20, 195-197.



:Cp2TiCl2 + 2 Al(CH3)3 → CH4 + Cp2TiCH2AlCl(CH3)2 + Al(CH3)2Cl



After stirring for about 3 days at at 25 °C, the product is recrystalized to remove Al(CH3)2Cl.  This procedure gives 80-90% yield of the Tebbe reagent.Herrmann, W.A., "The Methylene Bridge" Advances in Organometallic Chemistry 1982, 20, 195-197.  Although syntheses using the isolated Tebbe reagent give a cleaner product, successful procedures using the reagent "in situ" have been reported.Organic Syntheses, Coll. Vol. 8, p.512, Vol. 69, p.72.
Cannizzo, L.F.; Grubbs, R.H., "In situ preparation of (μ-chloro)(μ-methylene)bis(cyclopentadienyl)(dimethylaluminum)titanium (Tebbe's reagent)" J. Org. Chem.  1985, 50, 2386-2387. Instead of isolating the Tebbe reagent, the solution is merely cooled in an ice bath or dry ice bath before adding the starting material.  


An alternative but less convenient synthesis entails the use of dimethyltitanocene: 



:Cp2Ti(CH3)2 + Al(CH3)2Cl → Cp2TiCH2AlCl(CH3)2 + CH4



One drawback to this method, aside from requiring Cp2Ti(CH3)2, is the difficulty of separating product from unreacted Cp2Ti(CH3)2.  



Synthetic applications

The Tebbe reagent is used to convert carbonyl groups, commonly ketones, to olefin groups.  This conversion can also be effected using the Wittig reaction, although the Tebbe reagent is more efficient especially for sterically encumbered carbonyls.  Furthermore, the Tebbe reagent is less basic than the Wittig reagent and does not give the β-elimination products.  


The reaction mechanism involves formation of a cyclic TiC2O intermediate (see reaction (3)), analogous to the mechanism for the Wittig reagent.  Reactions proceed most easily in the presence of  a Lewis base, such as pyridine or even THF.




(1)


Also analogous to the Wittig reagent, the reactivity appears to be driven by the high oxophilicity of Ti(IV):Straus, D. A., "μ-Chlorobis(cyclopentadienyl)(dimethylaluminum)-μ-methylenetitanium": Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd, 2000.





(2)




(3)





(4)


Methylenation reactions also occur for aldehydes as well as esters, lactones and amides.  The Tebbe reagent converts esters and lactones to enol ethers and amides to enamines.  In compounds containing both ketone and ester groups, the ketone selectively reacts in the presence of one equivalent of the Tebbe reagent.  










The Tebbe reagent methylenates carbonyls without racemizing a chiral α carbon.  For this reason, the Tebbe reagent has found applications in reactions of sugars where maintenance of stereochemistry can be critical.Marra, A.; Esnault, J.; Veyrieres, A.; Sinay, P. "Isopropenyl glycosides and congeners as novel classes of glycosyl donors: theme and variations" J. Am. Chem. Soc. 1992, 114, 6354-6360.


The Tebbe reagent reacts with acid chlorides or other compounds with the same functionality.  As in the above reactions, the Tebbe reagent converts the carbonyl group to a methylene group, but the intermediate Cp2TiO reacts further to form a titanium enolate by replacing Cl-.





References

 



See also



External links


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