[54294-45-0] · C20H43Ta · Tris(2,2-dimethylpropyl)(2,2-dimethylpropylidene)tantalum · (MW 464.58)
(converts a carbonyl group into a neopentylidene group1)
Physical Data: mp 71 °C.
Solubility: sol diethyl ether, hydrocarbon solvents.
Form Supplied in: not commercially available.
Preparative Methods: prepared in a two-step procedure from tantalum pentachloride.2 First, tris(2,2-dimethylpropyl)tantalum dichloride is prepared from the reaction of tantalum pentachloride and 3 equiv of 2,2-dimethylpropyllithium. Treatment of tris(2,2-dimethylpropyl)tantalum dichloride with 2 equiv of 2,2-dimethylpropyllithium affords tris(2,2-dimethylpropyl)(2,2-dimethylpropylidene)tantalum.
Purification: recrystallized from pentane at -30 °C.
Handling, Storage, and Precautions: air and moisture sensitive.
Treatment of carbonyl compounds with the title reagent (1) affords the carbonyl alkenation product.1 In the reaction of ketones and aldehydes with (1), the reagent functions as a Wittig substitute (eq 1). Unlike the Wittig reaction, the carbonyl alkenation reaction also proceeds for esters (eq 2) and amides (eq 3), affording enol ethers and enamines, respectively. Reaction with carbon dioxide affords the symmetrical allene (eq 4). Only the neopentylidene unit can be transferred using the tantalum chemistry; however, the work by Schrock1 has led to the discovery of similar reagents (Schrock carbene complexes) capable of transferring a variety of organic groups.3
Treatment of acid chlorides with (1) provides the tantalum enolate rather than the vinylic chloride (eq 5).2 A similar reaction pathway is observed with phenyl esters. In this case, a similar type of product would be predicted from the analogous Wittig reaction.
James W. Herndon
University of Maryland, College Park, MD, USA