Some reagents, such as metal hydrides and organometallic reagents, add to aldehydes, ketones and esters in an irreversible fashion, and it is likely that similar reactions of vinylogous functions will also be irreversible. Since 1,2-additions to the carbonyl group are fast, we would expect to find a predominance of 1,2-products from these reactions.
For the hydride reductions shown in the first three equations below, this is the case. However, not all compounds of this kind give clean 1,2-reduction. Lithium aluminum hydride often reacts further with allylic alcohols, reducing the carbon-carbon double bond as well. It must therefore be used with care. Sodium borohydride may also give conjugate addition products in some cases. Fortunately, this can be prevented by adding cerium trichloride (CeCl3) to the reaction mixture. If the 1,4-reduction product is desired it is best obtained by using a dissolving metal reduction.
The remaining five equations displayed here describe the use of various organometallic reagents. Alkyl lithium compounds usually give 1,2-addition products, as shown in equation # 4. Grignard reagents, on the other hand, may add in both a 1,2- and 1,4-manner, depending on the substitution at the electrophilic sites. Unsaturated aldehydes usually give 1,2-addition, as in equation # 5. An equivalent ketone having a large carbonyl substituent, as in equation # 6, gives 1,4-addition, and if the isopropyl group is replaced by a smaller methyl group a nearly 50:50 mixture of 1,2- and 1,4-addition products is obtained. Grignard reactions may be shifted to a 1,4-addition mode by adding copper salts, but a better strategy is to use a Gilman reagent, as shown in the last two equations. The metal enolate that results from this conjugate addition may be quenched by hydrolysis, as in equation # 7, trapped as a silyl enol ether, as in equation # 8, or alkylated by a suitable alkyl halide.