Chloroformates, addition allenes

Terminal allenes.1 A synthesis of 1,2-dienes (3) from an aldehyde or a ketone involves addition of ethynylmagnesium bromide followed by reaction of the adduct with methyl chloroformate. The product, a 3-methoxycarbonyloxy-l-alkyne (2), can be reduced to an allene by transfer hydrogenolysis with ammonium formate catalyzed by a zero-valent palladium complex of 1 and a trialkylphosphine. The choice of solvent is also important. Best results are obtained with THF at 20-30° or with DMF at 70°. 

The addition of a-lithiomethoxyallene 144 [55] to benzaldehyde dimethylhydra-zone 145 (Eq. 13.48) leads to a mixture of pyrroline 146 and dihydroazete 147 [56]. The cydization in this case, which takes place in the same operation as the addition to the hydrazone, follows two distinct pathways, with attack of the nitrogen atom taking place at the inner, in addition to the terminal, carbon atom of the allene. A similar reaction of 144 with SAMP-hydrazone 148 (Eq. 13.49) leads to 3-pyrroline 149 in 88% yield and excellent diastereoselectivity [57]. Cleavage of the chiral auxiliary group from 149 takes place in two steps (1, methyl chloroformate 2, Raney nickel, 50 bar, 50 °C) in 74% overall yield. When the addition of 144 to 148 is conducted in diethyl ether, cydization of the adduct does not take place. Surprisingly, the hydrazones of aliphatic aldehydes react with 144 in poor yield in THF, but react quantitatively and diastereoselectively in diethyl ether to give the (uncyclized) allenyl hydrazone products. 

In a recently reported synthesis of pyridines, lithiated methoxyallenes react with nitriles in the presence of trifluoroacetic acid (Scheme 107) <2004CEJ4283>. The mechanism is postulated to proceed via initial protonation followed by nucleophilic addition of the trifluoroacetate ion with subsequent intramolecular acyl transfer and aldol condensation to give the pyridine. An additional pyridine formation starting from azaenyne allenes forms a-5-didehydro-3-picoline diradicals, which can be trapped by 1,4-cyclohexadiene, chloroform, and methanol to produce various pyridines <20040L2059>. 

In general, 1,1-disubstituted allenes form the adducts from addition of dichlorocarbene to the more highly substituted double bond, and mixtures of monoadducts occasionally result from addition to both double bonds by the chloroform/base/phase-transfer catalyst method. For example, 18 19 and 20 i 21, 22 and 23 ° 24 and 25. ° ... 

A new preparative method for allenes has appeared (Scheme 4). Several alk-2-ynyl carbonates (prepared by nucleophilic addition of magnesium and lithium acetylides to ketones or aldehydes followed by quenching with methyl chloroformate) were selectively hydrogenolysed with ammomium formate under palladium catalysis to give allenes in good yields. 

The 1-allylcycloalkanols (66 n = 0, 1, or 2 R = H) are cyclopropanated to (67 R = H) by dichlorocarbene. Yields are higher using ethyl trichloroacetate and sodium methoxide in hexane than with chloroform-potassium t-butoxide as the carbene source, but the former method gave slightly lower yields on reaction with the trimethyl-silyl derivatives of the alcohols (66 R = SiMe,). This was suggested to beastericeffect and such effects are also seen in the addition of dichlorocarbene to allenes and butatrienes. ... 

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