Specific enol equivalents for aldehydes

The best specific enol equivalents for aldehydes are enamines (75) and these are also very useful for ketones. They are easily made from the carbonyl compound and a secondary amine, are stable isolable compounds, and react... 

Summary specific enol equivalents for aldehydes and ketones ... 

Among the enolates of carboxylic acid derivatives, esters are the most widely used. Ester enolates cannot be used in crossed aldols with aldehydes because the aldehyde is both more enolizable and more electrophilic than the ester. It will just condense with itself and ignore the ester. The same is true for ketones. A specific enol equivalent for the ester will therefore be needed for a successful ester aldol reaction. 

Specific enol equivalents will be needed for both synthons (61) and (66), Since (61) is to give a double bond but (66) is to give an alcohol, the logical choices are a Wittig reagent - actually (67) - for (61) and a Reformatsky reagent for (66). The ester to aldehyde conversion (65 63) Is easiest by over-reduction and re-... 

You met all of these briefly in Chapter 21, and we shall discuss how to use them to alkylate aldehydes shortly. All three types of specific enol equivalent are useful not just with aldehydes, but with ketones as well, and we shall introduce each class with examples for both types of carbonyl compound. 

One specific enol equivalent that works very well for aldehydes is an enamine. Secondary amines such as Me2NH, Et2NH, pyrrolidine, piperidine, or morpholine 34 add cleanly to aldehydes to give enamines 36. These relatively stable nitrogen analogues of enols react well with reactive alkyl halides, such as a-carbonyl halides, and in Michael additions. They were considered in chapter 2 and only a few extra examples will be given here. 

Finally, acid catalysed reaction gives the expected thermodynamic product 40. All three products involve the aldehyde the enamines select for d2 behaviour of the aldehyde while acid catalysis brings it in as an electrophile (a1). You cannot expect to achieve such control in all examples, but it is clear that enamines have something special to offer as aldehyde specific enol equivalents. 

In Chapter 20 we established that enolates can be formed from acid chlorides, but that they decompose to ketenes. Enolates can be formed from amides with difficulty, but with primary or secondary amides one of the NH protons is likely to be removed instead. For the remainder of this section we shall look at how to make specific enol equivalents of acids, esters, aldehydes, and ketones. 

The fact that ketones, aldehydes, and geminal diacetates are readily available from these reactions illustrate their complementarity to reactions with allylsilanes. Specifically, the equivalency of allylic ethers to homoenolates allows for the formation of compounds extended by one carbon unit as compared to the products of couplings with enolate equivalents already discussed. 

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