Alkyne aluminum reagents

As shown in Table 15, alkynic products (61) are obtained exclusively in the reactions of the aluminum reagent, while mixtures enriched mainly in the alkynic product are obtained in the reactions of the zinc and lithium reagents. Yields tend to be lower and more variable than those of allyl organometallics (see Table 2). If cyclic six-centered transition states are involved, the dominance of alkynic products (61) could be explained by preference of a transition state in which the metal is coordinated to the more accessible methine instead of methylene carbon. 

The scope of nitriles for the carbocyanation reaction of alkynes can be expanded by cooperative nickel/Lewis acid catalysis. Alkenyl [80, 81] and alkynyl cyanides [83, 84] also participate in the addition reaction to give highly conjugated nitrile products by nickel/BPh3 catalysis (Scheme 27). The use of aluminum-based Lewis acids causes isomerization of the double bond of alkenylcyanation products, whereas alkynylcyanation is sluggish with the aluminum reagents. 

Because neither lithium aluminum hydride nor sodium borohydride reacts with alkenes or alkynes, these reagents selectively reduce a carbonyl group in compounds with carbon—carbon multiple bonds. 

Finally, cross-coupling reactions involving aluminum reagents have primarily been used for alkenyl transfers from the carbo- and hydroalumination products of terminal alkynes. A number of interesting results were published in recent years and will surely broaden the application of alanes and aluminum sesquihalides to these C,C-bond forming reactions. 

The regioselectivity of the addition of terminal alkynes to epoxides is improved, when the reagents prepared from the lithiated alkynes and either trifluoroborane or chlorodiethyl-aluminum arc employed (M. Yamaguchi, 1983 S. Danishefsky, 1976). (Ethoxyethynyl)lithium-trifluoroborane (1 1) is a convenient reagent for converting epoxides to y-lactones (M. Naka-tsuka, 1990 see p. 327f. cf. S. Danishefsky, 1976). 

The conversion of an alkyne to a trans-alkene can be accomplished by heating with lithium aluminum hydride (LAH), by reaction with lithium in liquid ammonia (Li, NH3). Thus all of these reagents (H2/P-2 Ni, LAH, and Li, NH3) are reducing agents for alkynes and give alkenes as the reduced products. 

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