Carbonyl compound , acidity from alcohols

Trimethylsilylation of enolizable carbonyl compounds and alcohols has also been accomplished by the fluoride ion promoted reaction with hexamethyldisilane and ethyl trimethylsilylacetate [48, 49], with high stereospecificity giving Z-enol ethers from ketones [50]. l-Trimethylsilyl-(l-trimethylsilyloxy)alkanes, produced from the reaction of aldehydes with hexamethyldisilane, undergo acid-catalysed hydrolysis during work up to yield the trimethylsilylcarbinols [51]. In the case of aryl aldehydes, the initially formed trimethylsiloxy carbanion produces the pinacol (Scheme 3.1). 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Fig. 9.2. Base-catalyzed (top) and acid-catalyzed (bottom) hemiacetal formation from carbonyl compounds and alcohols.

When one follows the reaction arrows in Figure 9.12 from the bottom upward, the following important information can be noted In an acidic water-containing solution 0,0-acetals are hydrolyzed to give carbonyl compounds and alcohols. Such a hydrolysis consists of seven elementary reactions. First, the hemiacetal A (Nu = OR3) and one equivalent of alcohol are produced from the 0,0-acetal and water in an exact reversal of the latter s formation reaction, i.e., through a proton-catalyzed SN1 substitution (in four steps). What follows is a three-step decomposition of this hemiacetal to the carbonyl compound and a second equivalent of the alcohol. 

The formation of 0,0-acetals from carbonyl compounds and alcohols takes place according to the mechanism of Figure 9.12. As acidic catalysts one uses a mineral acid (HC1 or H2S04) or a sulfonic acid (p-toluenesulfonic acid, camphorsulfonic acid, or a strongly acidic cation exchange resin) ... 

The carbonyl compound, an alcohol, and an acid are dissolved in benzene. As the mixture is heated, the carbonyl compound is converted to the acetal with water as a by-product. Benzene and water co-distill from the reaction mixture. When the hot vapors reach the cold condenser, they condense, forming a liquid that then collects in the glass tube below. Water, the more dense liquid, forms the lower layer, so that as it collects, it can be drained through the stopcock into a flask. In this way, water can be removed from a reaction mixture, driving the equilibrium. 

Dialkyl acetals and ketals can easily be formed from carbonyl compounds with alcohols under acidic conditions. Some representative examples for the great variety of methods available for this transformation are given in Scheme 77. As is demonstrated, both simple alcohols themselves or formic acid ortho esters can be used for acetal formation in the presence of hydrochloric acid, toluenesulfonic acid °° or activated alumina (Montmorrilonite clay K-10). ° Owing to different carbonyl reactivities, regio- and chemo-selective differentiation is often realizable, as has been shown, for example, on androstane-3,17-dione (78). Acid-catalyzed acetalization selectively delivers the 3-ketal, whereas the sterically hindered 17-carbonyl function remains unaffected. Under neutral conditions the reactions are promoted by cata-... 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

From the carbonyl compound in alcohol or water with p-nitrophenylhydrazme tn aqueous acetic-hydrochloric acids... 

Ketals and acetals can also be prepared from the carbonyl compound and alcohol in the presence of an acidic catalyst. Provision of a means for removal of the water formed ensures that the reaction will go to completion. 

Draw the structural formulas for the products of the hydrolysis of each in aqueous acid (i.e., provide the carbonyl compound and alcohol(s) from which each acetal was derived). 

Suspend 0 25 g. of 2 4-dinitrophenylhydrazine in 5 ml. of methanol and add 0-4 0-5 ml. of concentrated sulphuric acid cautiously. FUter the warm solution and add a solution of 0 1-0-2 g. of the carbonyl compound in a small volume of methanol or of ether. If no sohd separate within 10 minutes, dUute the solution carefuUy with 2N sulphuric acid. CoUect the solid by suction filtration and wash it with a little methanol. RecrystaUise the derivative from alcohol, dUute alcohol, alcohol with ethyl acetate or chloroform or acetone, acetic acid, dioxan, nitromethane, nitrobenzene or xylene. 

Alcohols can be prepared from carbonyl compounds by reduction of car boxy he acids and esters See Table 15 3... 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

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