Section 1.3 Other Organometallic Reagents

Lithium triethylborohydride (Super-Hydride) is a much more powerful reducing agent than lithium aluminium hydride. It is useful for the reductive dehalogenation of alkyl halides, but unlike lithium aluminium hydride does not affect aryl halides. It is available as solution in tetrahydrofuran in sealed containers under nitrogen. The solutions are flammable and moisture sensitive and should be handled with the same precautions as are taken with other organometallic reagents (see Section 4.2.47, p. 442). 

The synthesis of 2-methylbutanoic acid (Expt 5.129) is illustrative of the method. Other organometallic reagents undergo a similar carboxylation reaction, and examples of the use of organolithium and organosodium reagents are included in the section on the synthesis of aromatic carboxylic acids (Section 6.13.3, p. 1069). 

The stereoselectivity of reduction of carbonyl groups is effected by the same combination of steric and stereoelectronic factors which control the addition of other nucleophiles, such as enolates and organometallic reagents to carbonyl groups. A general discussion of these factors on addition of hydride is given in Section 3.10 of Part A. 

Aldehydes and ketones are usually prepared on insoluble supports by the acylation of arenes, C,H-acidic compounds, or organometallic reagents. Alcohols or other substrates can also be converted into carbonyl compounds by oxidation (Figure 12.1). Linkers that enable the generation of aldehydes and ketones upon cleavage from a support are considered in Section 3.14. 

Not only alkenes and arenes but also other types of electron-rich compound can be oxidized by oxygen. Most organometallic reagents react with air, whereby either alkanes are formed by dimerization of the metal-bound alkyl groups (cuprates often react this way [80]) or peroxides or alcohols are formed [81, 82]. The alcohols result from disproportionation or reduction of the peroxides. Similarly, enolates, metalated nitriles, phenolates, enamines, and related compounds with nucleophilic carbon can react with oxygen by intermediate formation of carbon-centered radicals to yield dimers (Section 5.4.6 [83, 84]), peroxides, or alcohols. The oxidation of many organic compounds by air will, therefore, often proceed faster in the presence of bases (Scheme 3.21). 

To stop at the ketone stage, a weaker organometallic reagent is needed one that reacts faster with acid chlorides than with ketones. Lithium dialkylcuprates and other organocuprates (Section 17-13a) react with acid chlorides to give good yields of a wide variety of ketones. 

Enediamine 93 undergoes an easy C-alkylation with a,/ -unsaturated ketones under comparable conditions and no cyclocondensation products are observed36. Similar C-alkylations with other electrophilic olefins such as unsaturated lactones, propenonitrile, vinyl sulfoxide, sulfone and phenylphosphonium bromide have been achieved36. All the adducts 135 exist predominantly as the imine form (see Section II.A). Treatment with organometallic reagents transforms the C-alkylated products into... 

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