Aldehydes saturated metal enolates

This chapter will provide coverage of the scope and limitations of alkylations of metal enolates of saturated and unsaturated ketones, aldehydes and carboxylic acid derivatives, together with a discussion of alkylations of various enols and enolate equivalents. Where applicable, the utility of these reactions for the diastereoselective and enantioselective synthesis of a-substituted carbonyl compounds will be described. Inevitably, the coverage of a vast research area such as this will be incomplete and in part will reflect the author s interests. However, it is hoped that most of the useful methods of carbon-carbon o-bond formation by alkylations of enolates and enols will be included. 

Although carboxylic acids and their derivatives are somewhat weaker carbon acids than aldehydes and ketones, it is generally possible to quantitatively convert them to the corresponding metal enolates with dialkylamide bases, the most popular of which is LDA. - - Thus, monoanions of saturated esters, lactones, nitriles, /VA -dialkylamides and V-alkyllactams and dianions of carboxylic acids and V-unsub-stituted amides and lactams are easily prepared in aprotic solvents such as THF and C-alkylated with a variety of simple and functionalized SN2-reactive alkylating agents at room temperature or below. When more-hindered systems are involved, the basicity of the metal dialkylamide and the reactivity of the metal enolate can be enhanced by the addition of HMPA. Of course, many of the indirect methods used for the generation of aldehyde and ketone enolates are also applicable to the preparation of enolates of carboxylic acid derivatives (Section 1.1.2.1). O-Alkylations or dialkylations at carbon generally are of minimal importance with metal enolates of carboxylic acid derivatives. 

Transition metal catalyzed hydrosilylation of a, -unsaturated ketones and aldehydes (78) proceeds in a 1,4- and/or 1,2-fashion to give enol silyl ethers (79) (or the saturated carbonyl compounds 80 after acidic work-up) and/or allyl silyl ethers (81) (or allyl alcohols 82), respectively (equation 54). Monohydrosilanes combined with a Pt or Rh catalyst prefer 1,4-addition. This reaction is an alternative meth for the preparation of enol silyl ethers (79). Diphenylsilane with [RhCl(PPh3)3] catalyst reduces the car-... 

The transformation of saturated ketones into a,/8-unsaturated ketones is often carried out by the introduction of an a-seleno-substituent, followed by oxidation and elimination. An alternative method for selenation of ketones (and esters) has been described wherein the lithium enolate reacts with selenium metal (rather than a selenyl halide) followed by methyl iodide (Scheme 29). Yields are comparable to the classical method. Phenylselenyl chloride reacts with enamines derived from aldehydes to give a-phenylselenoaldehydes, and hence o(,/3-unsaturated aldehydes by oxidative elimination [equation (21)]. ... 

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