Organometallic compounds with conjugated carbonyl

Most main group organometallic compounds undergo nucleophilic reactions with carbonyl groups, whereas 1,4-conjugate addition to enones... 

Figure 12.13 shows that the iso-A enols of the /3-diketones A react with an a,/3-unsaturated carboxonium ion C that acts as a C electrophile. This oxocarbenium ion is formed by reversible protonation of the oc,/3-unsaturated methyl vinyl ketone in acetic acid. However, the oxocarbenium ion C in this figure does not react with the iso-A enols at its carbonyl carbon atom—as the protonated acetone in Figure 12.12 does with the enol of acetone—but at the center C-/3 of the conjugated C=C double bond. Accordingly, an addition reaction takes place whose regioselectivity resembles that of a 1,4-addition of an organometallic compound to an 0C,/3-unsaturated carbonyl compound (see Section 10.6). 1,4-additions of enols (like in this case) or enolates (as in Section 13.6) to a,/3-unsaturated carbonyl and carboxyl compounds are referred to as Michael additions. 

The reduction of unsaturated carbonyl compounds by metal hydrides, and the reaction of organometallic nucleophiles with them, is a complicated story.87 It is more common than not, in each case, to get direct attack at the carbonyl group, but reaction in the conjugate position is well known. Conjugate reduction of a/i-unsaturated ketones by metal hydrides increases88 in the sequences Bu2iAlH < LiAlH4 < LiAlH(OMe)3 < LiAlH(OBu )3 and... 

In the last chapter we introduced the Heck reaction as a way to add nucleophiles to electrophilic alkenes. The details of the mechanism are discussed there. The reaction also occurs with allylic alcohols 218 and is a good way to make carbonyl compounds, particularly aldehydes, 219 as the alternative conjugate addition of an organometallic compound to an enal often shows poor regioselectivity (chapter 9). 

The /S-bromodihydropyran (652) on halogen-metal exchange with f-butyllithium affords a useful organometallic (653) that can function as the equivalent of a 5-hydroxy aldehyde enolate (Scheme 151) (79TL67). The vinyllithium species adds to ketones and aldehydes at -110 °C in good yield and also reacts in a conjugate mode with a,/S-unsaturated carbonyl compounds in the presence of copper(I) ions. On hydrolysis and oxidation, the dihydropyran component (654) can be transformed into a substituted 5-lactone (655). 

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