Aldehydes enolate formation

Aldehydes are so electrophilic that, even with LDA at -78°C, the rate at which the deprotonation takes place is not fast enough to outpace reactions between the forming lithium enolate and still-to-be-deprotonated aldehyde remaining in the mixture. Direct addition of the base to the carbonyl group of electrophilic aldehydes can also pose a problem, reactions which compete with aldehyde enolate formation... 

Our experience to this point has been that C—H bonds are not very acidic Com pared with most hydrocarbons however aldehydes and ketones have relatively acidic protons on their a carbon atoms pA s for enolate formation from simple aldehydes and ketones are m the 16 to 20 range... 

Thus, mixed aldol additions can be achieved by the tactic of quantitative enolate formation using LDA followed by addition of a different aldehyde or ketone. 

Properties of Latia luciferin. Latia luciferin is a highly hydrophobic, fat-soluble compound, and volatile under vacuum. It is a colorless liquid, with an absorption maximum at 207nm (s approx. 13,700 Fig. 6.1.2). The chemical structure of Latia luciferin has been determined to be 1 (C15H24O2), an enol formate of a terpene aldehyde 3 (Fig. 6.1.3 Shimomura and Johnson, 1968b). The enol formate group of Latia luciferin is unstable the luciferin is spontaneously hydrolyzed... 

The high selectivity of the catalyst in forming ( )-alkenes can be used in interesting ways (eq. 1). For example, in acetone-iie solution, within 15 min at room temperature allyl alcohol is converted to nearly pure enol (E)-26. Under these mild conditions, the product slowly isomerizes to the more stable aldehyde tautomer. We know of one other report of rapid enol formation from allyl alcohol, using a Rh... 

Among the compounds capable of forming enolates, the alkylation of ketones has been most widely studied and applied synthetically. Similar reactions of esters, amides, and nitriles have also been developed. Alkylation of aldehyde enolates is not very common. One reason is that aldehydes are rapidly converted to aldol addition products by base. (See Chapter 2 for a discussion of this reaction.) Only when the enolate can be rapidly and quantitatively formed is aldol formation avoided. Success has been reported using potassium amide in liquid ammonia67 and potassium hydride in tetrahydrofuran.68 Alkylation via enamines or enamine anions provides a more general method for alkylation of aldehydes. These reactions are discussed in Section 1.3. 

The carbonyl group in a ketone or aldehyde is an extremely versatile vehicle for the introduction of functionality. Reaction can occur at the carbonyl carbon atom using the carbonyl group as an electrophile or through enolate formation upon removal of an acidic proton at the adjacent carbon atom. Although the carbonyl group is an integral part of the nucleophile, a carbonyl compound can also be considered as an enophile when involved in an asymmetric carbonyl-ene reaction or dienophile in an asymmetric hetero Diels-Alder reaction. These two types of reaction are discussed in the next three chapters. 

The [1,4]-Wittig rearrangement is potentially useful not only for the carbon-carbon bond formation but also for enolate formation. However, synthetic applications have been rather limited, because of the low yields and restricted range of substrates. Schlosser s group have developed a practical approach to aldehydes based on a [1,4]-rearrangement/ enolate trapping sequence. In contrast, standard aqueous workup gave poor yield of aldehyde. This protocol was employed as the key step in a synthesis of pheromone (102) from 99 via 100 and 101 (equation 56f. ... 

The ozonolyses of enol ethers has been reviewed <91MI 4l6-0l>. The relative dipolarophilicity of certain species to attack by carbonyl oxides has been investigated and, in general, the order of reactivity is aldehydes > enol ethers > esters ss ketones. It is apparent that enol ethers are very reactive towards carbonyl oxides, so much so that 1,2-dioxolane formation can be a major reaction pathway (especially for formaldehyde-O-oxide) <85JOC3365>. 

The formation of aldehyde enolates is complicated by the disposition of aldehydes to undergo aldol condensation. Therefore, there are very few examples of direct asymmetric alkylations of aldehydes. 

SAMPLE SOLUTION (a) A good way to correctly identify the aldol addition product of any aldehyde is to work through the process mechanistically. Remember that the first step is enolate formation and that this must involve proton abstraction from the a carbon. 

The conjugate addition of unstabilized enolates to various acceptors was conceptually recognized by early researchers however, complications were encountered depending on the enolates and acceptors employed. Reexamination of this strategy was made possible by the development of techniques for kinetic enolate formation. This discussion is divided into three enolate classes (a) aldehyde and ketone enolates, azaenolates or equivalents, (b) ester and amide enolates, dithioenolates and dienolates and (c) a,0-carboxylic dianions and a-nitrile anions, in order to emphasize the differential reactivity of various enolates with various acceptors."7 The a-nitrile anions are included because of their equivalence to the hypothetical a-carboxylic acid anion. 

A Et2Zn-(5, S)-linked-BINOL (21) complex has been found suitable for chemos-elective enolate formation from a hydroxy ketone in the presence of isomerizable aliphatic iV-diphenylphosphinoylimines.103 The reaction proceeded smoothly and /9- alkyl-yS-amino-a-hydroxy ketones were obtained in good yield and high enantioselectivity (up to 99% ee). A titanium complex derived from 3-(3,5-diphenylphenyl)-BINOL (22) has exhibited an enhanced catalytic activity in the asymmetric alkylation of aldehydes, allowing the reduction of the catalyst amount to less than 1 mol% without deterioration in enantioselectivity.104... 

Stoichiometric, irreversible formation of enolates from ketones or aldehydes is usually performed by addition of the carbonyl compound to a cold solution of LDA. Additives and the solvent can strongly influence the rate of enolate formation [23]. The use of organolithium compounds as bases for enolate formation is usually not a good idea, because these reagents will add to ketones quickly, even at low temperatures. Slightly less electrophilic carbonyl compounds, for example some methyl esters [75], can, however, be deprotonated by BuLi if the reactants are mixed at low temperatures (typically -78 °C), at which more metalation than addition is usually observed. A powerful lithiating reagent, which can sometimes be used to deproto-nate ketones at low temperatures, is tBuLi [76],... 

Ketones and aldehydes have been economically a-hydroxylated (to give a-hydroxy-acetals), using iodine in basic methanol.327 Enolate formation and iodination to give cY-iodocarbonyl is then followed by transformation into the hydroxyacetal, a dimethyl acetal under the MeO /MeOH conditions employed. 

Another important contribution is to the regioselectivity of enolate formation from unsym-metrical ketones. As we established in chapter 13, ketones, particularly methyl ketones, form lithium enolates on the less substituted side. These compounds are excellent at aldol reactions even with enolisable aldehydes.15 An application of both thermodynamic and kinetic control is in the synthesis of the-gingerols, the flavouring principles of ginger, by Whiting.16... 

Gas-phase transfers of hydride from methoxide to C02, CS2 and S02 have been observed by the flowing afterglow technique (Bierbaum et al., 1984) and by Fourier transform ion cyclotron resonance spectroscopy (FT-ICR) (Sheldon et al., 1985). With aldehydes and ketones, the normal gas-phase reaction with methoxide is enolate formation, but FT-ICR methods have been used to demonstrate reduction of non-enolizable aldehydes including benzaldehyde, pivalaldehyde, and 1-adamantylaldehyde. 

The data in Table 6 show that an alkyl group at the a-carbon stabilizes the carbonyl isomer much more than either the enol or the enolate. That is, aldehydes are both more acidic and more highly enolized than otherwise similar ketones. This inhibitions of enol and enolate formation on the part of ketones can be somewhat countered by a / -aryl group, especially if coplanarity of the entire 7r-electron system is enforced (see the last... 

This newly developed heteropolymetallic catalyst system was applied to a variety of direct catalytic asymmetric aldol reactions, giving aldol products 48-64 in modest to good ee, as shown in Table 16. It is worthy of note that even 62 can be produced from hexanal 54 in 55% yield and 42% ee without the formation of the corresponding self-aldol product (-50 °C). This result can be imderstood by considering that aldehyde enolates are not usually generated by the catalyst at low temperatme, an assumption which was confirmed by several experimental results. It is also worthy of note that the direct catalytic asymmetric aldol reaction between 46 and cyclopenta-none 55 also proceeded smoothly to afford 64 in 95% yield synlanti = 93 7, syn = 76% ee, anti = 88% ee). 

Which kind of reaction is performed via enolate formation using a ketone, an aldehyde and a base as main reagents ... 

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