Organochromium Reagents for Highly Selective Carbon-Carbon Bond Formation

Although the reducing power of chromium (II) salts is not so strong as such metals like lithium, magnesium, and zinc, active organic halides are reduced by chromium (II) to give the corresponding organochromium reagents. We describe here a new entry of the organochromium reagents, which add to aldehydes with unique selectivities, especially chemoselective addition to aldehydes in the presence of ketones. The following topics are included : (1) γ-Alkoxy-substituted allylchromium reagents, produced by reduction of acrolein dialkyl acetals with CrCl₂, add to aldehydes in a erythro-selective manner. (2) Alkynyl halides and α-chloro sulfides are reduced by CrCl₂ to afford alkynyl and α-thioalkylchromium reagents, respectively. (3) Under nickel catalysis, alkenyl halides (triflates) are reduced by CrCl₂ to produce alkenylchromium reagents. (4) Selective synthesis of E-olefins from aldehydes are achieved by using geminal dichromium reagents derived by chromium (II)-reduction of geminal dihalo compounds such as RCHI₂, Me₃SiCHBr₂, and CHX₃.