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

Abstract

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 ¹78 and 25°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 retroallylation 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 alkoxidehalide 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.

Transition‐Metal‐Mediated Cleavage and Activation of C–C Single Bonds

“…For homoallylic alcohols a cleavage of a C–C single bond similar to that of a β‐carbon elimination can occur. In this case, however, the mechanism may be (and in most cases probably is) rather different, proceeding by a retro‐allylation pathway1n,14 (Scheme ).…”

C–C Activation by Retro-Aldol Reaction of Two β-Hydroxy Carbonyl Compounds: Synergy with Pd-Catalyzed Cross-Coupling To Access Mono-α-arylated Ketones and Esters

“…However, in recent years C–C activation reactions, particularly those promoted by transition metal complexes, have begun to attract increasingly more attention of chemists from across the world . Significant progress has been achieved through the wise implementation of effective promoting strategies, including the release of ring strain, chelation assistance, β-C elimination, etc …”

Enantioselective Construction of 3‐Hydroxypiperidine Scaffolds by Sequential Action of Light and Rhodium upon N‐Allylglyoxylamides

“…Initially, the hydroxide ligand on rhodium is exchanged with the hydroxy group of (±)‐ 2 a to generate the rhodium alkoxide B and water. There are two modes available to open the strained four‐membered ring of B through β‐carbon elimination: 1) cleavage of the bond bound to the allylic sp 3 ‐carbon atom9h,l and 2) cleavage of the bond bound to the carbonyl sp 2 ‐carbon atom. The bond bound to the allylic sp 3 ‐carbon atom is preferentially cleaved11 to furnish the ring‐opened allylrhodium species C .…”