Reactions of Organozinc Reagents with a,p-Unsaturated Ketones

The lithium reduction of zinc (II) cyanide using naphthalene or biphenyl as a catalytic electron carrier yields a more reactive form of Rieke zinc. This new form of Rieke zinc is able to undergo direct oxidative addition to alkyl chlorides under mild conditions and tolerates the presence of nitriles and bulky tertiary amides [17]. Table 3.5 shows representative reactions of alkyl zinc chloride reagents with benzoyl chloride. The activation of the zinc surfeice could originate by the adsorption of the Lewis base cyanide ion on the metal surface. The adsorbed cyanide ion can affect the metal s reactivity in two possible ways. One possible mode of activation would be the reduction of the metal s work function in the vicinity of the adsorbed cyanide ion, and the second could be that the cyanide ion is acting as a conduction path for the transfer of the metal s electrons to the alkyl chloride. One or both processes could account for the observed enhanced chemical reactivity. 

4 Reactions of Organozinc Reagents with a,p-Unsaturated Ketones 

Substituting lithium 2-thienylcyanocuprate for the CuCN/LiBr complex and omitting boron trifluoride etherate from the reaction of the organozinc halide species with 2-cyclohexenone indeed gave a reasonable yield (76% GC) of the 

4- adduct (entry 3, Table 3.6). The reaction rate, temperature, and yield were similar to the reactions employing CuCN/LiBr. However, contrary to all previous reports by Lipshutz, a noticeable amount (ca. 9% GC yield) of the product resulting from the 1,4-addition of the 2-thienyl group was observed. This tendency was even more pronounced when the reaction was carried out in the presence of boron trifluoride etherate. In this reaction, 1,4-addition of the 2-thienyl group was the major product (59% GC yield) along with the usual 

A detailed experimental procedure for the copper-mediated conjugate addition of organozinc reagents to a,p-unsaturated ketones is given later in the text. 

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