Base-catalyzed self-condensation

The base-catalyzed self-condensation of diketene is a similar type of reaction and yields dehydroacetic acid together with the tripyranone (322) (49JOC460, B-68MI22400). Ring opening of diketene and attack of the oxyanion on a second molecule of diketene followed by an intramolecular condensation accounts for the formation of the pyranone. The selfcondensation of malonyl dichloride in the presence of a ketone, which acts as a weak base, is a similar reaction (52JCS4109). 

The use of acidic conditions avoids base-catalyzed self-condensation and retro-Michael reactions and tolerates base-sensitive groups, as exemplified by the triflic-acid (CF3S03H)-catalyzed Michael addition shown below. ... 

Early attempts to utilize acetaldehyde and monosubstituted acetaldehyde derivatives in the Darzens reaction were unsuccessful, presumably due to competitive base-catalyzed self-condensations of the aldehydes. It was subsequently reported Aat generation of the requisite bromo ester enolates with LHMDS at -78 °C provides a species which reacts with a variety of aldehydes and ketones to afford glycidic esters in good to excellent isolated yields (equation 25). ... 

The aldol condensation reaction is the acid- or base-catalyzed self-condensation of a ketone or aldehyde.Under certain conditions, the reaction product may undergo further transformations, especially dehydration. The reaction may also occur between two different carbonyl compounds, in which case the term mixed aldol condensation is applied. The mechanistic pattern of the reaction, involving attack by... 

H. Baron, etal, J. Chem. Soc. 85, 1726 (1904) K. Ziegler er a/., Ann. 504,94 (1933). Base-catalyzed self-condensation of nitriles to yield imines which tautomerize to enamines ... 

Ketones are chemically reactive and undergo acid- and base-catalyzed selfcondensation reactions. For acetone the base-catalyzed self-condensation reaction yields a product of diacetone alcohol [854] ... 

In Chapter 1 we mentioned that efficient alkylation of aldehydes and ketones requires essentially quantitative formation of their enolates. When a low concentration of an enolate ion is generated, it may react more competitively with the parent aldehyde or ketone present in the reaction mixture than it does with an alkyl halide. The aldol condensation reaction is this acid- or base-catalyzed self-condensation of... 

The most generally used carbonyl compounds for the acylation of enolate ions are esters. Here, nucleophilic addition of the anion to the carbonyl group is followed by loss of alkoxide from the tetrahedral intermediate. A classic example of this type of reaction is the Claisen condensation—the base-catalyzed self-condensation... 

In recent years, there has been a considerable effort to expand the range of iodonium salts that can be employed as photoinitiators for cationic polymerizations. Stable iodonium salts can be obtained when one of the aryl groups in 51 is replaced by a moiety that can provide resonance stabilization to the positively charged iodine atom. For example, Koser et prepared stable aryl (phenyl ethynyl)iodonium salts, 55, that were shown by Kitamura et al. and by Hofer and Liska to exhibit good activity as cationic photoinitiators. Similarly, iodonium salts, 56, in which the positively charged iodine atom is attached to the central carbon of a resonance-stabilized 1,3-diketone moiety such as dimedone are also isolable compounds that display the ability to serve as photoinitiators for cationic polymerization. Finally, it has been reported that diaryl(oxo)iodonium salts, 57, can be readily prepared by the base-catalyzed self-condensation of diaryliodyl compounds. Replacement of the initial... 

The aldol condensation reaction is the acid- or base-catalyzed self-condensation of a ketone or aldehyde. Under certain conditions, the reaction product may undergo further transformations, especially dehydration. 

Reactions with Aldehydes and Ketones. The base-catalyzed self-addition of acetaldehyde leads to formation of the dimer, acetaldol [107-89-1/, which can be hydrogenated to form 1,3-butanediol [107-88-0] or dehydrated to form crotonaldehyde [4170-30-3]. Crotonaldehyde can also be made directiy by the vapor-phase condensation of acetaldehyde over a catalyst (53). 

A further frequent complication is that acetylenic esters undergo base and acid-catalyzed self-condensation on standing under normal laboratory conditions, and they will add methanol or similar nucleophiles in Michael-type addition reactions (cf. 4 and 6). Such products are almost invariably present in reaction mixtures obtained from heterocycles. A short account of these products is included as their early recognition in a new investigation can save much time. 

A -Alkylated thiazolium and benzothiazolium salts also experience base-promoted deprotonation at the 2-position to form ylides. Such compounds, often referred to as TV-heterocyclic carbene (NHC), are nucleophilic catalysts in benzoin condensation. In 1943, Ugai and co-workers reported that thiazolium salts catalyze self-condensation of benzaldehyde to generate benzoin via an umpoulong process. Breslow at Columbia University in 1958 proposed thiazolium ylide as the actual catalyst for this transformation. In this mechanism, the catalytically active species was represented as a thiazolium zwitterion, the resonance structure of an NHC, and the reaction was postulated to ensue via the enaminol or the Breslow intermediate. ... 

Siegrist Reaction. Double bonds can be synthesized by a variety of base-catalyzed condensations between an active methylene compound and a carbonyl group. Meier et al. [895, 896] used a base-catalyzed self-coupling of the Schiff bases of various p-methylbenzaldehydes to prepare PPV. 

Ba.se Catalyzed. Depending on the nature of the hydrocarbon groups attached to the carbonyl, ketones can either undergo self-condensation, or condense with other activated reagents, in the presence of base. Name reactions which describe these conditions include the aldol reaction, the Darzens-Claisen condensation, the Claisen-Schmidt condensation, and the Michael reaction. 

The earliest reported reference describing the synthesis of phenylene sulfide stmctures is that of Friedel and Crafts in 1888 (6). The electrophilic reactions studied were based on reactions of benzene and various sulfur sources. These electrophilic substitution reactions were characterized by low yields (50—80%) of rather poorly characterized products by the standards of 1990s. Products contained many by-products, such as thianthrene. Results of self-condensation of thiophenol, catalyzed by aluminum chloride and sulfuric acid (7), were analogous to those of Friedel and Crafts. 

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

Diels-Alder reactions are thermal reactions requiring no catalysts (120). However, over the years both acid- and metal-based homogeneous or heterogeneous catalysts have been developed (121—127). Some catalysts used in Diels-Alder catalyzed reactions of butadiene are Fe(NO)2Cl—(CH3CH2)2A1C1, Pd[P(C H5)3]4, Cu(I) exchanged silica—alumina (128,129), large pore zeolites (130), and carbon molecular sieves. An electrochemical process has also been used to catalyze the self-condensation to vinylcyclohexene (131). When the asymmetric Ni catalyst (4) was used, specificity to the enantomeric (5)-4-vinylcyclohexene (132,133) was observed (26% enantiomeric excess). 

The self-condensation of 1,3-dicarbonyl compounds provides a useful route to 4-hydroxy-pyran-2-ones and is catalyzed by acids or bases. Alcohol is continuously removed during the reaction. Amongst a number of examples, mention can be made of the detailed procedure for the synthesis of 3-acetyl-4-hydroxy-6-methylpyran-2-one (dehydroacetic acid) (550SC(3)23l) and the formation of the cyclopentyl derivative (321) (64RTC39). Deacylation at C-3 can generally be achieved on heating with acid. 

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