Condensation reactions, carbonyl compounds

Difunctional compounds are generally made by carbonyl compound condensation reactions. 

I. Condensation of N-Monosubstituted Hydroxylamines with Carbonyl Compounds Condensation of N -monosubstituted hydroxylamines with carbonyl compounds is used as a direct synthesis of many acyclic nitrones. The synthesis of hydroxylamines is being carried out in situ via reduction of nitro compounds with zinc powder in the presence of weak acids (NH4CI or AcOH) (14, 18, 132). The reaction kinetics of benzaldehyde with phenylhydroxylamine and the subsequent reaction sequence are shown in Scheme 2.21 (133). 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

Various nitro compounds have been condensed with carbonyl compounds in reactions catalyzed by alkaline earth metal oxides and hydroxides 145). It was found that the reactivities of the nitro compounds were in the order nitro-ethane > nitromethane > 2-nitropropane, and those of carbonyl compounds were propionaldehyde > isobutyraldehyde > pivalaldehyde > acetone > benzaldehyde > methyl propionate. Among the catalysts examined, MgO, CaO, Ba(OH)2, and Sr(OH)2, exhibited high activity for nitroaldol reaction of nitromethane with propionaldehyde. In reactions with these catalysts, the yields were between 60% (for MgO) and 26% (for Sr(OH)2) at 313 K after 1 h in a batch reactor. On Mg(OH)2, Ca(OH)2, and BaO, the yields were in the range of 3.8% (for BaO) and 17.5% (for Mg(OH)2). Investigation of the influence of the pre-treatment... 

A variety of macrocyclic complexes which have adjacent nitrogen atoms (cyclic hydrazines, hydrazones or diazines) are formed by condensations of hydrazine, substituted hydrazines or hydrazones with carbonyl compounds. The reactions parallel in diversity those of amines, but are often more facile since the reacting NH2 groups is generally not coordinated and the electrophile is thus not in competition with the metal ion. The resulting macrocycles may be capable of coordination isomerism, since either of the adjacent nitrogen atoms can act as donor atom. 

In summary, the SMS reaction is a truly efficient process, possessing a broad scope and applicable to a number of carbonyls, allylsilanes, alcohols (silyl ethers) or amines. Its usefulness has been validated in several total syntheses and demonstrated by the preparation of chiral homoallylic alcohols. During the development of the SMS reaction, Melkafia and Marko [48] realized that the homoallylic alcohol (ether), if connected to an allylsilane, would form novel annelating agents that would lead to tetrahydropyran derivatives via condensation with carbonyl compounds. This reaction was called IMSC for intramolecular Sakurai cydization and will be discussed in the next section. 

Condensation of Phosphonium Ylides with Carbonyl Compounds Wittig Reaction... 

As mentioned above (Section 4.31.2.1), 3-alkylidene-l,2-dithioles (3d) may possess considerable ionic character consistent with contributions from structures (62), and condense with suitable carbonyl or other compounds. Condensation reactions of 3-aIkyI-l,2-dithiol-ylium salts (105) (66ZC321, 64CI(L)461, 80AHC(27)15l> are readily explained in terms of initial proton transfer from the salt to the substrate, as in the formation of salts of type... 

Condensations are some of the most important enolate reactions of carbonyl compounds. Condensations combine two or more molecules, often with the loss of a small molecule such as water or an alcohol. Under basic conditions, the aldol condensation involves the nucleophilic addition of an enolate ion to another carbonyl group. The product, a /3-hydroxy ketone or aldehyde, is called an aldol because it contains both an aldehyde group and the hydroxy group of an alcoho/. The aldol product may dehydrate to an a,/3-unsaturated carbonyl compound. 

Aromatic and aliphatic carbonyl compounds condense with glycols, such as ethylene, propylene, and trimethylene glycols, to form cyclic acetals p-toluenesulfonic acid has proved to be an excellent catalyst. As before, the water formed in these reactions is conveniently removed by an azeotropic distillation with benzene. Representative aldehydes and ketones that undergo this acetalization include acetone, cyclohexanone, pinacolone, acetophenone, benzophenone, n-heptaldehyde. 

Reductive alkylation of amines proceeds by the hydrogenation of the imine or enamine formed, in situ, by the condensation of the amine with a carbonyl compound. This reaction can give a mixture of products if the amine produced initially competes with the reactant amine in the carbonyl condensation step. The proper selection of reagent concentrations avoids this difficulty and leads to the formation of good yields of the desired product. 50 jhe use of a large excess of ammonia gives the primary amine as the predominant product (Eqn. 19.50). 51 An excess of a primary amine as the reactant leads to the preferential formation of the secondary amine product. An excess of the carbonyl compound gives the symmetrical secondary or tertiary amines (Eqn. 19.51). 50... 

Diels-Alder reactions. Activated carbonyl compounds condense with dienes to furnish dihydropyrans at room temperature when catalyzed by BiCl,. Conventional Diels-Alder reactions catalyzed by BiClj or Bi(OTf), have also been scrutinized. ... 

A more traditional reaction is the condensation of nitroalkanes with carbonyl compounds (Henry reaction). Nitroaldol products can be isolated but are more commonly oxidised or dehydrated. The nitroaldol products formed from methyl-8-nitrooctanoate and aldehydes, in the presence of Amberlyst A21 resin, were oxidised with... 

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,... 

In acid, a second reaction, the dehydration of the p-hydroxy carbonyl compound to the a,P unsaturated aldehyde or ketone is very common, and it is generally the dehydrated products that are isolated (Fig. 19.69). In the overall reaction, two carbonyl compounds condense to produce a larger carbonyl compound and give off a small molecule— water in this case. Thus, the aldol reaction is often called the aldol condensation. 

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. 

General Discussion. (Z)-(Trimethylsilyloxy)vinyllithium (1) has essentially been used for a one-pot vinylogation of carbonyl compounds. Condensation of (1) with aldehydes and ketones (2) occurs readily at low temperature (—70 °C) then treatment of the reaction mixture with an acidic solution in mild conditions (IN hydrochloric acid, 0 °C to rt) produces the Q ,/3-unsaturated aldehydes (3), without double bond migration. The ( 0 isomers are obtained alone (from aldehydic compounds) or very predominantly (fromketonic compounds). The intermediate adducts, y-hydroxy enol ethers (4), can be isolated by slightly basic mild hydrolysis (eq l).l... 

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