Metal-Mediated Retro-Allylation of Homoallyl Alcohols for Highly Selective Organic Synthesis

Metal-mediated or -catalyzed retro-allylation of homoallyl alcohol that proceeds via a six-membered transition state has appeared as a new useful method to prepare allylmetals. A crotylzirconium reagent undergoes stereoselective allylation of aromatic aldehyde at -78 and 25 degrees C to provide threo- and erythro-homoallyl alcohols, respectively. Although the threo isomer is formed as usual via a six-membered chair transition state under kinetic control, the erythro selectivity is rationalized by considering isomerization of the threo adduct through a retro-allylation process. The retro-allylation of a bulky gallium homoallyloxide occurs to generate (Z)- and (E)-crotylgallium reagents stereospecifically, starting from erythro- and threo-homoallyl alcohols, respectively. The (Z)- and (E)-crotylgallium reagents react with aromatic aldehydes in situ to afford the corresponding erythro- and threo-homoallyl alcohols, respectively. Treatment of rationally designed tertiary homoallyl alcohols with aryl halides under palladium catalysis yielded the corresponding allylarenes stereo- and regioselectively, by replacing the conventional transmetalation step in the cross-coupling reaction with alkoxide-halide exchange followed by a retro-allylation reaction. The retro-allylation reaction is also applicable to nickel and rhodium at this stage, and may to any other metals in principle. Reversible additions of pentamethylcyclopentadienyl anion to carbonyl compounds are also described.