Carbonyl compounds direct

Lewis acid-mediated addition of allylsilanes to carbon nucleophiles. Also known as the Hosomi-Sakurai reaction. The allylsilane will add to the carbonyl compound directly if it is not part of an a,P-unsaturated system (Example 2), giving rise to an alcohol. 

Even alcohols can be converted to a,/i-unsaturated carbonyl compounds directly by using an excess of IBX (1), as shown in the oxidation of the phenylalanine derivative 12 to 13. 

Tlie use of Hs S04 or HCI results in the isolation of the a.fJ-Unsaturoted carbonyl compound directly. 

The first step of the Wittig reaction proper is just like an aldol reaction as it consists of an enolate attacking an electrophilic carbonyl compound. But, instead of forming an aldol product, this adduct goes on to form an unsaturated carbonyl compound directly. 

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

A number of reagents are available for transforming carbonyl compounds directly to a-oxygenated products without the preformation of a metal enolate and are discussed in Section 4.1.5. 

Although the reduction of carbonyls to alcohols or hydrocarbons is well documented, no general method was available for the reductive coupling of carbonyl compounds directly to alkenes prior to the use of low-valent transition metals. The reductive coupling of carbonyl compounds by low-valent transition metals is now an important method for C=C bond formation, and has been widely reviewed." ... 

Fig. 17 Preparation of gem-dichloroalcanes from carbonyl compounds directly by reaction with PCI5 or more selective preparation via conversion to oximes with subsequent chlorination [14]...

Aldol condensation. Silyl enol ethers react with carbonyl acceptors to give )3-siloxy ketones and esters. As BiX, is also a catalyst for the replacement of /3-siloxy groups, prolonged reaction times can be applied to prepare /3-halo carbonyl compounds directly. The reaction is enhanced by ultrasound. 

CH3)2. Jacobson reports a modified Nef reaction in which a solution of the primary nitroalkane in methanolic sodium methoxide is added slowly to a solution of sulfuric acid in methanol cooled to -35°. The acetal of the aldehyde is formed, usually in good yield. Yields are considerably lower in the conventional (aqueous) Nef reaction, resulting in carbonyl compounds directly. 

As in the alcohol activation step, like the TM-catalyzed direct hydrogen transfer route (Schemes 5, 14), the MPV-O process cannot provide carbonyl compounds directly according to the reaction mechanism (Scheme 39). This may make the alcohol activation reaction the rate-limiting step of the whole process. Most probably for this reason, the early TM-free iV-alkylation reactions [4—9] were performed under harsh conditions to facilitate the initial formation of aldehydes by high temperature dehydrogenation of the alcohols. Otherwise, alternative ways for alcohol activation should be adopted (vide infra). 

Direct Silylation of Heteroarylcarbonyl Compounds. Under ruthenium catalysis vinyltrimethylsilane reacts to ortho silyl-ate heteroaryl carbonyl compounds directly in good yields (eqs 32 and 33). The reaction only works with heteroaromatic systems. The resulting aryltrimethylsilanes can be used to introduce electrophiles regioselectively through electrophilic desilyla-tion. The reaction also works with vinyltriethoxysilane, opening the possibility of silicon-based cross-coupling reactions. 

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