Ketone aspects

An interesting aspect of this reaction is the contrasting stereoselective behaviour of the dimethyisulfonium and dimethyloxosuifonium methylides in reactions with cyclic ketones (E.J. Corey, 1963 B, 1965 A C.E. Cook, 1968). The small, reactive dimethyisulfonium ylide prefers axial attack, but with the larger, less reactive oxosulfonium ylide only the thermodynamically favored equatorial addition is observed. 

The chemistry of the carbonyl group is probably the single most important aspect of organic chemical reactivity Classes of compounds that contain the carbonyl group include many derived from carboxylic acids (acyl chlorides acid anhydrides esters and amides) as well as the two related classes discussed m this chapter aldehydes and ketones... 

Nitriles contain the —C=N functional group We have already discussed the two mam procedures by which they are prepared namely the nucleophilic substitution of alkyl halides by cyanide and the conversion of aldehydes and ketones to cyanohydrins Table 20 6 reviews aspects of these reactions Neither of the reactions m Table 20 6 is suitable for aryl nitriles (ArC=N) these compounds are readily prepared by a reaction to be dis cussed m Chapter 22... 

The physical properties of finish removers vary considerably due to the diverse uses and requirements of the removers. Finish removers can be grouped by the principal ingredient of the formula, method of appHcation, method of removal, chemical base, viscosity, or hazardous classification. Except for method of apphcation, a paint remover formulation usually has one aspect of each group, by which it can be used for one or more appHcations. A Hst of the most common organic solvents used in finish removers has been compiled (3). Many are mentioned throughout this article others include ethyl lactate [97-64-3] propylene carbonate [108-32-7] furfural alcohol [98-01-1/, dimethyl formamide [68-12-2] tetrahydrofuran [109-99-9] methyl amyl ketone [110-43-0] dipropylene glycol methyl ether [34590-94-8] and Exxate 600, a trade name of Exxon Chemicals. 

Raw Material and Energy Aspects to Pyridine Manufacture. The majority of pyridine and pyridine derivatives are based on raw materials like aldehydes or ketones. These are petroleum-derived starting materials and their manufacture entails cracking and distillation of alkanes and alkenes, and oxidation of alkanes, alkenes, or alcohols. Ammonia is usually the source of the nitrogen atom in pyridine compounds. Gas-phase synthesis of pyridines requires high temperatures (350—550°C) and is therefore somewhat energy intensive. 

The alkylation reactions of enolate anions of both ketones and esters have been extensively utilized in synthesis. Both very stable enolates, such as those derived from (i-ketoesters, / -diketones, and malonate esters, as well as less stable enolates of monofunctional ketones, esters, nitriles, etc., are reactive. Many aspects of the relationships between reactivity, stereochemistry, and mechanism have been clarified. A starting point for the discussion of these reactions is the structure of the enolates. Because of the delocalized nature of enolates, an electrophile can attack either at oxygen or at carbon. 

Enolates of aldehydes, ketones, and esters and the carbanions of nitriles and nitro compounds, as well as phosphorus- and sulfur-stabilized carbanions and ylides, undergo the reaction. The synthetic applications of this group of reactions will be discussed in detail in Chapter 2 of Part B. In this section, we will discuss the fundamental mechanistic aspects of the reaction of ketone enolates with aldehydes md ketones. 

The reaction of tnfluoromethyl-substituted A -acyl umnes toward nucleophiles in many aspects parallels that of the parent polyfluoro ketones Heteronucleophiles and carbon nucleophiles, such as enarmnes [37, 38], enol ethers [38, 39, 40], hydrogen cyanide [34], tnmethylsilylcarbomlnle [2,47], alkynes [42], electron-nch heterocycles [43], 1,3-dicarbonyl compounds [44], organolithium compounds [45, 46, 47, 48], and Gngnard compounds [49,50], readily undergo hydroxyalkylation with hexafluoroace-tone and amidoalkylation with acyl imines denved from hexafluoroacetone... 

One of the most actively investigated aspects of enamine chemistry has been the acylation process (i). Initial intensive studies by Hiinig (373-375) showed the ease of preparing a variety of 9-diketones and particularly the synthetic potential of acylated cyclic ketones as intermediates in the preparation of aliphatic keto acids, keto dicarboxylic acids and diketo dicarboxylic acids (376-378). 

In contrast with the above situation, the polymerization of 2-furfurylidene methyl ketone, di-2-fiirfurylidene ketone and their homologues has been the subject of a large volume of (mainly technical) publications because of the useful applications of the final cross-linked products. As pointed out in the introduction, this review does not deal with the technological aspects of furan resins and in this section only the mechanistic aspects of the first phase of these polymerizations will be discussed. 

The tautomeric equilibrium between enols and ketones or aldehydes is not normally a preparative reaction, though for some ketones both forms can be prepared (see p. 75 for a discussion of this and other aspects of tautomerism). For most ketones and aldehydes only the keto form is detectable under ordinary conditions, though the equilibrium must occur, since aldehydes and ketones often react through their enol forms. 

Although the conversion of an aldehyde or a ketone to its enol tautomer is not generally a preparative procedure, the reactions do have their preparative aspects. If a full mole of base per mole of ketone is used, the enolate ion (10) is formed and can be isolated (see, e.g., 10-105). When enol ethers or esters are hydrolyzed, the enols initially formed immediately tautomerize to the aldehydes or ketones. In addition, the overall processes (forward plus reverse reactions) are often used for equilibration purposes. When an optically active compound in which the chirality is due to an asymmetric carbon a to a carbonyl group (as in 11) is treated with acid or base, racemization results. If there is another asymmetric center in the molecule. 

There are other stereochemical aspects to the reduction of aldehydes and ketones. If there is a chiral center to the carbonyl group, even an achiral reducing agent can give more of one diastereomer than of the other. Such diastereoselective reductions have been carried out with considerable success. In most such cases Cram s rule (p. 147) is followed, but exceptions are known. ... 

The structure-reactivity relationship between a 19-Me- and 19-nor-5,10-seco-steroid has been investigated using lOOC and intramolecular nitrone cycloaddition taking into account various stereochemical aspects (Schemes 27 and 28) [67]. The E-19-nor-5,10-seco-ketone 255 a, on treatment with hydroxylamine hydrochloride (R = H), undergoes lOOC via 256a to a single isoxazolidine 257... 

The preparation of ketones and ester from (3-dicarbonyl enolates has largely been supplanted by procedures based on selective enolate formation. These procedures permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of keto ester intermediates. The development of conditions for stoichiometric formation of both kinetically and thermodynamically controlled enolates has permitted the extensive use of enolate alkylation reactions in multistep synthesis of complex molecules. One aspect of the alkylation reaction that is crucial in many cases is the stereoselectivity. The alkylation has a stereoelectronic preference for approach of the electrophile perpendicular to the plane of the enolate, because the tt electrons are involved in bond formation. A major factor in determining the stereoselectivity of ketone enolate alkylations is the difference in steric hindrance on the two faces of the enolate. The electrophile approaches from the less hindered of the two faces and the degree of stereoselectivity depends on the steric differentiation. Numerous examples of such effects have been observed.51 In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a small preference for... 

One of the potentially most useful aspects of the imine anions is that they can be prepared from enantiomerically pure amines. When imines derived from chiral amines are alkylated, the new carbon-carbon bond is formed with a bias for one of the two possible stereochemical configurations. Hydrolysis of the imine then leads to enantiomerically enriched ketone. Table 1.4 lists some examples that have been reported.118... 

Schuster, D. I. Krull, I. S. Photochemistry of unsaturated ketones in solution. XIX. Photochemistry of spiro[2.5]octa-4,7-dien-6-one. 2. Mechanistic aspects and the relationship to the photochemistry of quinone methides. Mol. Photochem. 1969, 1, 107-133. 

Prevost, M., and R. Bugarel, 1981. Theoretical and technical aspects of a chemical heat pump Secondary alcohol-ketone-hydrogen system, Proc. 2nd World Cong. Chem. Eng.,... 

Whereas general activities and selectivities for hydrogenations of ketones are similar to those of aldehydes, one big difference exists between the two. The hydrogenation of prochiral ketone carbonyls produces chiral carbons. Over symmetrical catalysts, racemic alcohols are formed however, over unsymmet-rical surfaces, enantioselectivity may occur. Enantioselective hydrogenations of ketones is an increasingly active research held and is covered in Chapter 3. Here we discuss that aspect of stereoselectivity associated with ring systems. 

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

Exchange of the butyl vinyl ether for 2-hydroxyethyl vinyl ether enabled the facile transformation of vinyl triflates or bromides into protected a,/i-unsaturated methyl ketones (Eq. 11.14) [27]. One interesting aspect of this reaction is that a masked methyl ketone can easily be introduced into a structure even in the presence of other free ketone groups. 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

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