Allenyl halides, carbonylation

In a similar process, nickel compounds also catalyze the carbonylation of allenyl halides, under phase-transfer conditions (PTC), to give allenyl acids in poor to reasonably good yields (equation 94)368. 

The coupling reaction of a-keto esters with allyl, propargyl, and allenyl halides using indium metal in aqueous solvents affords a-hydroxy-y,<5-unsaturated esters (Equation (28)).197,198 1,3-Dicarbonyl compounds undergo efficient carbonyl allylation reactions in an aqueous medium through a Barbier-type reaction (Equation (29)). The reaction is general and a variety of 1,3-dicarbonyls has been alkylated using allyl bromide or allyl chloride in conjunction with indium.199... 

The catalytic pair Ni(CN)2/Bu4N Br readily catalyzes the biphasic carbonyla-tion of a-haloalkynes [144, 145] and allenyl halides [145]. The first reaction results in a mixture of allenic monoacids and unsaturated diacids, whereas allenyl halides transform to allenic acids with high regioselectivity. The carbonylation of gem-dibromocyclopropanes under PTC conditions is catalyzed by Ni" and Co" salts. The reaction gives the corresponding cyclopropanecarboxylic acids in fair yields [146]. 

The anti stereoisomer was thought to arise from the allenylzinc reagent through a transition state in which the allenyl and aldehyde substituents adopt an anti arrangement (equation 6). The alternative transition state leading to the minor syn adducts would be disfavored by eclipsing interactions between these two substituents. In the proposed transition states, complexation between the zinc halide and the carbonyl oxygen was not considered. 

As expected according to the HSAB principle, hard electrophiles such as silyl halides and triflates react at the enolate oxygen atom to form allenyl ketene acetals, while soft electrophiles such as carbonyl compounds attack at C2. Only allylic and propargylic halides react regioselectively at G4 of the allenyl enolate to give substituted conjugated dienes. 

The mechanism of the condensation between allenyl organometallics and carbonyl groups is thought to be a cyclic Se< (or 5e2 ) process. In the reaction of aldehydes with 1,2-butadienylmagnesium halides, the anti syn product ratio depends on the size of substitutent group of the aldehyde and may be rationalized as shown in (1). ... 

The TT-allyl complex thus generated is of prime interest since its reactivity toward nucleophiles can provide a catalytic route to a whole class of molecules. This feature was essential for the discovery of catalytic reactions involving the acylpalladation of allenes as a key step. First, y-allenyl alcohols react with aryl halides under carbonylative conditions at 65 °C to form furans in a 24-89% yield range, typically 50-89% yields (Scheme... 

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