Functionalized Aldehydes and Ketones

If heteroatoms are present in the vicinity of the carbonyl group, the formation of chelates around an alkali, another cation, or the aluminum or boron atom can influence the course of the reduction by playing the role of a Lewis acid. The Lewis acid-base interaction can be quite strong depending on the nature of the heteroatom and its substituents, on the ligands attached to boron and aluminum, and finally on 

Other additives can induce the formation of chelates. a-Phosphinyloxyketones 3.98 undergo a stereoselective reduction to anti alcohols with NaBH4-CeCl3 in 

In the case of polysubstituted derivatives, such as a,p-dialkoxycarbonyl compounds, Li(5-Bu)3BH and Zn(BH4 2 lead each to a different isomer, but the stereoselectivity is lower than in the former cases because of competition among the different association sites [FK3, lYl, YK3, YK4]. Analogous problems also arise in the chemistry of sugars [MT2]. 

excess catecholborane at -10°C also reduces -hydroxyketones to syn 1,3-diols, most likely via an intermediate borate [EH3], In some cases, catalysis by RblPPbjljCl improves tbe selectivity. Syn 1,3-diols can also be formed by TiCl4 or BClj-mediated reduction of -bydroxyketones [SC2] via chelation control. Depending to tbe substituents, one or tbe other co-reagent is recommended. 

Similarly, the Felkin-Anh products are formed when chelation is prevented by the formation of r-BuMe2Si ethers of -hydroxyketones 3.118 and 3.119. This allows access to syn,anti or antUanti a-alkylated diols, depending on the configura- 

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Framen s procedure. A solution of potassium /-butoxide in DMSO is added by drops to a solution of trimethylsulfonium bromide or perchlorate in DMSO at room temperature. Sodium methoxide and sodium ethoxldc serve also as the base, and DMF is satisfactory as solvent. Reagent generated by this method converts mono-functional aldehydes and ketones into epoxides in good yield. I ranzcn initially... 

Derivatization of drugs containing a carbonyl function aldehydes and ketones... 

Converting aldehydes and ketones to cyanohydrins is of synthetic value for two reasons (1) a new carbon-carbon bond is formed and (2) the cyano group in the prod uct can be converted to a carboxylic acid function (CO2H) by hydrolysis (to be discussed in Section 19 12) or to an amine of the type CH2NH2 by reduction (to be discussed m Section 22 9)... 

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... 

Although carbohydrates exist almost entirely as cyclic hemiacetals m aqueous solution they are m rapid equilibrium with their open chain forms and most of the reagents that react with simple aldehydes and ketones react m an analogous way with the carbonyl functional groups of carbohydrates... 

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

Based on this variety of properties, amorphous polybutadiene has found a niche in the mbber industry. Moreover, it appears that the anionicaHy prepared polymer is the only polymer that can be functionalized by polar groups. The functionalization is done by using aromatic substituted aldehydes and ketones or esters. Functionalization has been reported to dramatically improve polymer-filler interaction and reduce tread hysteresis (70—73). 

An ability to fonn caibon-caibon bonds is fundamental to organic synthesis. The addition of Grignaid reagents to aldehydes and ketones is one of the most frequently used reactions in synthetic organic chemistry. Not only does it pennit the extension of caibon chains, but because the product is an alcohol, a wide variety of subsequent functional group transformations is possible. 

Dihydropyrans 88-90 are deprotonated at the vinylic position adjacent to oxygen by t-BuLi and the resulting anions add readily to alkyl halides, aldehydes, and ketones. Subsequent acid hydrolysis provides the products expected from reaction of an oj-functionalized pentanoyl anion 88 acts as HOCH2(CH2)3CO , 89 as 0CH(CH2)3C0 , and 90 as MeCO(CH2)3CO (77TL4187 81T3997). 

Aldehydes and ketones are similar in their response to hydrogenation catalysis, and an ordering of catalyst activities usually applies to both functions. But the difference between aliphatic and aromatic carbonyls is marked, and preferred catalysts differ. In hydrogenation of aliphatic carbonyls, hydrogenolysis seldom occurs, unless special structural features are present, but with aryl carbonyls either reduction to the alcohol or loss of the hydroxy group can be achieved at will. 

Acetals are useful because they can act as protecting groups for aldehydes and ketones in the same way that trimethylsilyl ethers act as protecting groups for alcohols (Section 17.8). As we saw previously, it sometimes happens that one functional group interferes with intended chemistry elsewhere... 

Figure 18-10 summarizes the successive oxidation products that can be obtained from alcohols. When the hydroxyl group, OH, is attached on an end carbon atom, an aldehyde and a carboxylic acid can be obtained through oxidation. When the hydroxyl group is on a carbon atom attached to two other carbon atoms, oxidation gives a ketone. Huge amounts of aldehydes and ketones are used industrially in a variety of chemical processes. Furthermore, these functional groups are important in chemical syntheses of medicines, dyes, plastics, and fabrics. 

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