Cp2TiCl2-Catalyzed Reductive Methylidenation of Alkyl Ketones with CH2Br2

Abstract

Direct methylidenation of carbonyl compounds represents an appealing route to terminal alkenes. Existing methods for this transformation typically rely on stoichiometric phosphorus ylides (methylene Wittig) or organotitanium complexes (Tebbe or Petasis reagents). Here, we report a Cp2TiCl2-catalyzed method that uses CH2Br2 as a readily available carbene precursor and Zn as the reductant for methylidenation of alkyl ketones. The reaction is proposed to proceed via the in situ generation of an alkyl–Zn species, which then generates a Ti–carbene intermediate. Alkali metal salts, such as KBF4 and LiPF6, improve the reaction efficiency. Cyclic voltammetry studies implicate the alkali metal ions in promoting dissociation of chloride from Ti and enhancing reactivity with the alkyl–Zn reagent. The resulting protocol is operationally simple and offers a practical alternative to the use of stoichiometric methylidenation reagents.