Nickel retro-allylation

A diorganonickel complex generally undergoes reductive elimination more rapidly than the corresponding palladium complex. Nickel can mediate retro-allylation, which is involved in allylation of allylic carbonate with homoallylic alcohols for efficient synthesis of 1,5-hexadienes [25]. A combination of Ni(cod)2 (cod = 1,5-cyclooctadiene) and triethyl phosphite catalyzes the allylation reaction of Boc-protected cinnamyl alcohol (Boc = f-butoxycarbonyl) with homoallylic alcohols (Scheme 5.34). The reactions with alkyl-substituted homoallylic alcohols (R = alkyl) are not regiospecific but are sterically controlled. The highest linear... 

Some mechanistic experiments support the reaction mechanism shown in Scheme 5.35. Oxidative addition of Boc-protected cinnamyl alcohol is accompanied by decarboxylation to yield the (3t-cinnamyl)(tert-butoxy)nickel intermediate 28. Alkoxide exchange between 28 and homoallylic alcohol and subsequent retro-allylation would initially generate (o-allyl)(it-dnnamyl)nickel 29. The configuration of 29 would be fluxional to yield a mixture of several diallylnickel species and to lose regiospecificity. Reductive elimination would take place under steric control by the substituents or under electronic control by the phenyl group. 

Rhodium catalyzes retro-allylation of homoallylic alcohols to generate nucleophilic allylic rhodium species. A combination of [RhCl(cod)]2, PMcj, and cesium carbonate catalyzes crotyl transfer from homoallylic alcohol to benzaldehyde in dioxane at 100 "C to afford the corresponding homoallylic alcohol (Scheme 5.37) [26]. Like allylic nickel, allylic rhodium experiences rapid and E-Z interconversions under the reaction conditions, which prevents stereoselectivity of the crotylation. Interestingly, a similar crotylation is followed by isomerization into saturated ketone with the aid of a bulkier catalyst [RhCl(cod)]2/PtBu3 at a higher temperature. 

Hydrocyanation of butadiene is more complicated than that of ethene it requires two hydrocyanation steps and several isomers can be observed. The isomers obtained in the first step of the HCN addition to butadiene are shown in Figure 11.3. The addition first leads to compounds 1 and 2, in a 1 2 ratio, but they equilibrate to a favourable 1 9 ratio via the retro-reaction. The retro reaction involves a C-C bond breaking reaction, which is rare, but in this case the intermediate is a Tt-allyl species and a stable, anionic cyanide group. Electron-rich nickel species (Ni-dippe) can cleave aromatic nitrile C-C bonds... 

---