(methylenation reagent of unsaturated carbonyl groups1)
Solubility: sol THF, toluene.
Preparative Methods: the reaction of Diiodomethane with Zinc dust in THF at 45 °C produces CH2(ZnI)2 in ca. 45% yield. Its reaction with Dichlorobis(cyclopentadienyl)titanium (25 °C, 30-60 min) affords a deep red solution of the titanocenemethylene-zinc halide complex (ca. 45-55% yield).2
Handling, Storage, and Precautions: should be handled and stored under an inert atmosphere.
The titanocenemethylene-zinc halide complex reacts rapidly with ketones in good yields (eq 1).2 The alkenation of an a,b-unsaturated ketone gives the corresponding diene (eq 2).2 Whereas no clean reaction is observed with acid chlorides, the addition of the titanocenemethylene complex to a nitrile provides, after a workup with D2O, an a-deuterated ketone (eq 3).2 The reaction with alkynes is less straightforward and moderate regio- and chemoselectivity is observed (eq 4).2
Alternative titanium-based reagents can be used and have found more applications than the titanocenemethylene-zinc halide complex.1 The Tebbe reagent m-Chlorobis(cyclopentadienyl)(dimethylaluminum)-m-methylenetitanium, although involving the use of pyrophoric Trimethylaluminum, has found broader application in synthesis.1,3 A related reagent prepared from zinc, a dihalomethane, and Titanium(IV) Chloride (Takai reagent)4 proved also to be a useful methylenation reagent of aldehydes and ketones. After a modification, this reagent is also able to perform a methylenation (and alkylidenation) of various carboxylic acid derivatives (eq 5).5 The preparation modification reported by Lombardo provides a mild, nonbasic method for the methylenation of ketones. The Lombardo reagent is compatible with a wide variety of functional groups. Compared to the original procedure
Philipps Universität, Marburg, Germany