Carbonyl compounds aldol reactions

Kasha s rule, 253, 310 Kelene, 380. 386 acetal. 326-27, 469 Ketoiminoelher, 411 Ketone. See also Carbonyl compounds aldol reaction. 427 excited state basicity, 52 Keiyl radical. 397-98. 467 Kinetics of photophysical processes, 250. 297-301... 

Base- and acid-catalyzed aldol reactions of aldehydes generally lead to useful amounts of aldol product, the p-keto alcohol, or the product of dehydration, the a,P-unsaturated carbonyl compound. Aldol reactions of ketones are less useful because equilibrium does not usually favor product. Special techniques can be used (Soxhlet extractor) to thwart thermodynamics. If dehydration to the a,P-unsaturated carbonyl compound occurs, as it often does in acid-catalyzed aldols, the reaction may be practically successful. 

Besides the aldol reaction in the true sense, there are several other analogous reactions, where some enolate species adds to a carbonyl compound. Such reactions are often called aldol-type reactions the term aldol reaction is reserved for the reaction of aldehydes and ketones. 

Reaction of stabilized carbanions with carbonyl compounds Aldol condensation... 

Aldol reactions Enolates are formed from a-bromo carbonyl compounds on reaction with germanium (prepared in situ from Gel and K). The aldol reaction is jyn-selective. 

I. Aldol reactions, in which no subsequent elimination occurs and which lead to /3-hydroxy carbonyl compounds (Aldol formation). 

An interesting variant of the original reaction discovered by Wittig has been reported in which carbonyl compounds undergo reactions with vinylaminodichloroboranes (17) derived from ketimines to give, upon hydrolytic work-up, the expected aldol adducts (18 Scheme 5). The reactions proceed in fair to good overall yields but with relatively poor diastereoselectivity (Table 3). The extension of this reaction... 

P-Hydroxy carbonyl compounds. Aldol and Reformatsky reactions involving debrominative enolization of a-bromoketones and a-bromo thioesters with TiCl2-Cu-r-BuCN are readily achieved. Acceptors are limited to aliphatic aldehydes because pinacol formation by aromatic aldehydes predominates under the conditions. 

Nitro groups can be added to organic molecules through an aldol reaction between the anion of a nitroalkane and an aldehyde or ketone carbonyl. Intramolecular aldol reactions can be used to create five- or six-membered rings from dicarbonyl compounds (either aldehydes or ketones), which form in preference to smaller or larger rings that may be possible. 

Although both the acid- and base-catalyzed aldol condensations lead to the same initial product, a 3-hydroxy carbonyl compound, the reaction usually goes further in acid (sometimes in base) to give an a,P-unsaturated carbonyl compound. All P-hydroxy carbonyl compounds and all a,P-unsaturated carbonyl compounds can,... 

Crossed (mixed) aldol condensation (Section 19.7) An aldol condensation between two different carbonyl compounds. This reaction is not very useful unless strategies are employed to limit the number of possible products. 

In an aldol reaction, an enolizable carbonyl compound reacts with another carbonyl compound that is either an aldehyde or a ketone. The enolizable carbonyl compound, which must have at least one acidic proton in its a-position, acts as a nucleophile, whereas the carbonyl active component has electrophilic reactivity. In its classical meaning the aldol reaction is restricted to aldehydes and ketones and can occur between identical or nonidentical carbonyl compounds. The term aldol reaction , in a more advanced sense, is applied to any enolizable carbonyl compounds, for example carboxylic esters, amides, and carboxylates, that add to aldehydes or ketones. The primary products are always j5-hydroxycarbonyl compounds, which can undergo an elimination of water to form a,j5-unsaturated carbonyl compounds. The reaction that ends with the j5-hydroxycarbonyl compound is usually termed aldol addition whereas the reaction that includes the elimination process is denoted aldol condensation . The traditional aldol reaction [1] proceeds under thermodynamic control, as a reversible reaction, mediated either by acids or bases. 

The aldol reaction is one of the most efficient carbon-carbon bond forming reactions in organic chemistry. It combines an enol or an enolate with a carbonyl compound to yield a 3-hydroxycarbonyl compound. Aldol reactions of simple... 

Aldol additions and ester condensations have always been and still are the most popular reactions for the formation of carbon-carbon bonds (A.T. Nielsen, 1968). The earbonyl group acts as an a -synthon, the enoi or enolate as a d -synthon. Both reactions will be treated together here, and arguments, which are given for aldol additions, are also valid for ester condensations. Many famous name reactions belong to this category ). The products of aldol additions may be either /J-hydroxy carbonyl compounds or, after dehydration, or, -unsaturated carbonyl compounds. 

The usual base or acid catalyzed aldol addition or ester condensation reactions can only be applied as a useful synthetic reaction, if both carbonyl components are identical. Otherwise complicated mixtures of products are formed. If two different aldehydes or esters are to be combined, it is essential that one of the components is transformed quantitatively into an enol whereas the other component remains as a carbonyl compound in the reaction mixture. 

The Michael reaction is of central importance here. This reaction is a vinylogous aldol addition, and most facts, which have been discussed in section 1.10, also apply here the reaction is catalyzed by acids and by bases, and it may be made regioselective by the choice of appropriate enol derivatives. Stereoselectivity is also observed in reactions with cyclic educts. An important difference to the aldol addition is, that the Michael addition is usually less prone to sterical hindrance. This is evidenced by the two examples given below, in which cyclic 1,3-diketones add to o, -unsaturated carbonyl compounds (K. Hiroi, 1975 H, Smith, 1964). 

Reaction With Carbonyl Compounds. Primary and secondary nitroparaffins undergo aldol-type reactions with a variety of aldehydes and ketones to give nitro alcohols (11). Those derived from the lower nitroparaffins and formaldehyde are available commercially (see Nitro alcohols). Nitro alcohols can be reduced to the corresponding amino alcohols (see Alkanolamines). 

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