27 August 2009 (online)
The reagent combination Zn/CH2Br2/TiCl4 gives rise to an organometallic reagent which is known as Lombardo’s reagent. It converts ketones into methylene groups. [¹] The active reagent is presumed to be a dimetalated species (1) which adds to the ketone under the influence of the Lewis acidity of titanium to give a β-oxymetal-substituted organometallic compound (2). Following β-elimination generates the methylene group (3) [²] (Scheme [¹] ).
Ketones are not enolized by the reagent and as an important consequence adjacent enolizable chiral centers are not epimerized. The reagent is compatible with a wide variety of functional groups, e.g., THP and tert-butyldimethylsilyl ethers, acetals, esters, carboxylic acids, alcohols, and lactones. [³] Such selectivity makes it a valuable procedure in organic synthesis and appreciably augments Wittig methodology.
TakaiK. HottaY. OshimaK. NozakiH. Tetrahedron Lett. 1978, 19: 2417
LombardoL. Tetrahedron Lett. 1982, 23: 4293
OkazoeT. TakaiK. UtimotoK. J. Am. Chem. Soc. 1987, 109: 951
LombardoL. Org. Syn. 1993, Coll Vol. 8: 386
LombardoL. Org. Syn. 1987, Coll Vol. 65: 81
LiuB. ZhouW.-S. Tetrahedron 2003, 59: 3379
PiersE. OrellanaA. Synthesis 2001, 2138
ScottME. LautensM. Org. Lett. 2005, 7: 3045
MarsdenPS. DuceptPC. Beilstein J. Org. Chem. 2005, 1: 5