Basicity of carbonyl compounds

Table 18 Yukawa-Tsuno correlations for gas-phase basicities of carbonyl compounds to form a-hydroxybenzylic cations. 

Berthelot, M., Gal, J.F., Helbert, M. et al. (1985) Chemometrics of basicity. Part 2. Comparison of the basicity of carbonyl compounds with respect to boron trifluoride and methanol. J. Chim. Phys. Phys.-Chim. Biol., 82, 427-432. 

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

The accessibility of enols and enolates, respectively, in acidic and basic solutions of carbonyl compounds makes possible a wide range of reactions that depend on the nucleophilicity of these species. The reactions will be discussed in Chapter 8 and in Chapters 1 and 2 of Part B. 

The relative basicity of carbonyl oxygen atoms can be measured by studying strength of hydrogen bonding between the carbonyl compound and a hydrogen donor such as phenol. In carbon tetrachloride, values of for 1 1 complex formation for the compounds shown have been measured. Rationalize the observed order of basicity. 

The Basic Mechanisms of Cycloaddition Reactions of Carbonyl Compounds with Conjugated Dienes... 

The cycloaddition reactions of carbonyl compounds with conjugated dienes cannot be discussed in this context without trying to understand the reaction mechanistically. This chapter will give the basic background to the reactions whereas Chapter 8 dealing with theoretical aspects of metal-catalyzed cycloaddition reactions will give a more detailed description of this class of reactions, and others discussed in this book. 

BalLini and coworkers have developed a new strategy for alkenyladon of carbonyl compounds based on the Michael adthdon followed by eliminadon of HNO fsee Secdon 7 3 A variety of 2-alkylidene 1,4-tholes have been conveniently prepared, in two steps, by the Michael adthdon of a nltroalkane to the appropriate enedione derivadves under basic conthdons, followed by chemoselecdve reducdon v/ith LLAlHl CEq 4 123 ... 

Racemic hydantoins result from the reaction of carbonyl compounds with potassium cyanide and ammonium carbonate or the reaction of the corresponding cyanohydrins with ammonium carbonate (Bucherer-Bergs reaction). Hydantoins racemize readily under basic conditions or in the presence of hydantoin racemase, thus allowing DKR (Figure 6.43). Hydantoinases (EC 3.5.2.2), either isolated enzymes or whole microorganisms, catalyze the hydrolysis of five-substituted... 

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

It has been reported that the conversion of carbonyl compounds into their oximes, which is normally acid-catalysed, can be effected under basic conditions in the presence of tetra-n-butylammonium chloride [37]. Good yields are generally obtained with most aldehydes and ketones with the exception of benzophenone. 

Cyanoalkenes are obtained from the catalysed reaction of carbonyl compounds and O-ethyl S-cyanomethyldithiocarbonates, or diethyl S-cyanomethylphosphoro-thionates, under basic conditions [11]. The reaction is analogous to the Darzens reaction (see Section 6.3) and probably proceeds via an intermediate thiirane. 

Two main types of processes which belong to basic synthetic reactions have been investigated recently the allylation of carbonyl compounds and the Refor-matsky reaction. 

Degani et al., 1968). A comparison of the basicity of these compounds with those of the corresponding pyridone analogues (2- and 4-quinolones and 9-acridones) is given in Table 4. The order of basicities for these compounds is NH > S > Se > O. They are all protonated on the carbonyl oxygen. Such is the resonance stabilization of the pyrone cations that they do not accept any further... 

Radical-anions of carbonyl compounds are basic and undergo protonation at the oxygen centre generating highly reactive radical species. 

The present preparation illustrates a general and convenient method for a two-step deoxygenation of carbonyl compounds to olefins. Related procedures comprise the basic decomposition of p-toluenesulfonylhydrazones, the hydride reduction of enol ethers, enol acetates, enamines, the reduction of enol phosphates (and/or enol phosphorodiamidates) by lithium metal in ethylamine (or liquid ammonia),the reduction of enol phosphates by titanium metal... 

Meerwein-Pondorf-Verley reduction, discovered in the 1920s, is the transfer hydrogenation of carbonyl compounds by alcohols, catalyzed by basic metal compounds (e.g., alkoxides) [56-58]. The same reaction viewed as oxidation of alcohols [59] is called Oppenauer oxidation. Suitable catalysts include homogeneous as well as heterogeneous systems, containing a wide variety of metals like Li, Mg, Ca, Al, Ti, 2r and lanthanides. The subject has been reviewed recently [22]. In this review we will concentrate on homogeneous catalysis by aluminium. Most aluminium alkoxides will catalyze MPV reduction. 

Iodine may be employed as a mediator to achieve a-hydroxylation of carbonyl compounds. In basic methanolic solution containing iodide, oxidation was reported to lead to a-iodo ketones, which further reacted to give a-hydroxy ketals [171]. The electrolysis of alkylidenemalonates in the presence of iodide as a mediator has been shown to yield cyclopropane derivatives [172]. 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

Basic Protocol 2 Determination of Carbonyl Compounds in Food Lipids D2.4.7... 

This method describes the quantitative determination of carbonyl compounds in fats and oils. It is based upon the formation of 2,4-dinitrophenylhydrazones of carbonyl compounds in the presence of a trichloroacetic acid (TCA) catalyst, followed by colorimetric determination of the hydrazone compounds (i.e., as quinoidal ions) in an alkanolic basic solution. 

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