Takai-Oshima-Lombardo methylenation

Overview

The Takai-Oshima-Lombardo methylenation refers to reactions involving a combination of zinc, a dihalomethane, and titanium tetrachloride to perform methylenation of carbonyl derivatives. This reagent system was originally reported by Kazuhiko Takai and Koichiro Oshima, later elaborated upon by Luciano Lombardo, with a variation which is notable for its increased activity, milder nature, and greater compatibility with acidic protons in substrate molecules compared to the system originally reported. The Lombardo modification involves a longer period for the preparation of active reagent in the absence of lead catalyst which is necessary for the high yields reported in Takai and Oshima's original work. The structure of the active reagent is unknown in both cases, but has been speculated upon and likely contains a titanium alkylidene species similar to those involved in the Tebbe and Petasis methylenations. These methodologies provide an exceptionally mild and selective set of conditions amenable to late-stage methylenation of complex scaffolds with unprotected acidic C-H functionality, thus complementing existing anionic methylenation chemistry such as the Wittig reaction.

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