Conjugated compounds, reaction with organocuprates

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

As in the case of addition reactions of carbon nucleophiles to activated dienes (Section HA), organocopper compounds are the reagents of choice for regio- and stereoselective Michael additions to acceptor-substituted enynes. Substrates bearing an acceptor-substituted triple bond besides one or more conjugated double bonds react with organocuprates under 1,4-addition exclusively (equation 51)138-140 1,6-addition reactions which would provide allenes after electrophilic capture were not observed (cf. Section IV). 

C-Trapping. Alkylation or hydroxy alkylation (i.e., reaction with RCHO) of enolates derived from conjugate addition of organocuprates affords vicinal dialkylated products. However, the reaction is confined to highly reactive alkylating agents such as methyl, allyl, propargyl, benzyl, and a-halocarbonyl compounds or aldehydes. 

Conjugate addition is also a feature of the reaction of organocuprates with a,p-acetylenic carbonyl compounds. By conducting the reaction at —78 °C, high yields of cis addition compounds can be obtained (1.163). This allows the stereocontrolled... 

There is also a correlation between the reduction potential of the carbonyl compound and the ease of reaction with cuprate reagents. The more easily reduced, the more reactive is the compound toward organocuprate reagents. Compounds such as a,/3-unsaturated esters and nitriles which are not as easily reduced as a,j3-unsaturated ketones do not react readily with simple alkyl cuprates even though they are good acceptors in conjugate addition reactions involving other types of nucleophiles (Michael reactions). 

INT2, Scheme 10.7) undergoes further reaction (Li/Cu transmetalation) and generates a new organocuprate compound. (Note however that this difference could become more subtle since the product of conjugate addition (PD) might behave more like an a-cuprio(I) ketone complexed with a lithium cation [52] than a lithium enolate complexed with copper(I)). In neither reaction was any evidence of radical intermediates (i.e., SET) found by theoretical calculations [79]. 

Enones.—The synthetic utility of organocuprates has been reviewed. House has presented a correlation between the conjugate addition of lithium dimethylcopper to a -unsaturated carbonyl compounds and their polarographic reduction potentials such a correlation is compatible with the first step of the mechanism (Scheme 100) proposed for this reaction. In conjunction with empirical rules for the estimation (with an accuracy of 0.1 V) of the reduction potentials of such unsaturated compounds, this... 

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