Alkenes from aldehydes and ketones

The polymer-bound phosphinate 10 has been prepared by successive treatments of 2% cross-linked brominated polystyrene with n-butyllithium, diethylchlorophosphite, and ethyl bromoacetate. It was used to form alkenes from aldehydes and ketones, but the yields were not consistently high, and the reagent was contaminated with an unidentified polymer-bound phosphorus species (52). 

Formation of l,3-Bis(phenylthio)alkenes from Aldehydes and Ketones. a nsaturated aldehydes and ketones form 1,3-bis(phenylthio)alkenes on treatment with phenyl thioborate (eq 3). The (phenylthio)alkenes are ) -acyl vinyl anion equivalents (eq 4). 

Finally, although not shown in the table, the material that is shown there can be combined with other reactions. The Slrecker amino acid synthesis, which is discussed again in Chapter 12, is among the most valuable, while the Shapiro Olefination and the related Bamford-Sfevens reaction have proved useful in producing alkenes from aldehydes and ketones. 

Wittig reactions, and reactions related to it, are used for the regiospecific synthesis of alkenes from aldehydes and ketones. Their retrosynthetic analysis begins with disconnecting the double bond as shown and introduces a novel stmctural type called an yiide. 

Another useful reagent in nucleophilic additions contains a carbanion that is stabilized by an adjacent, positively charged phosphorus group. Such a species is called a phosphorus ylide, and its attack on aldehydes and ketones is called the Wittig reaction. The Wittig reaction is a powerful method for the selective synthesis of alkenes from aldehydes and ketones. 

Examples of name reactions can be found by first considering the nature of the starting material and product. The Wittig reaction, for instance, is in Section 199 (Alkenes from Aldehydes) and Section 207 (Alkenes frorm Ketones). The aldol condensation can be found in the chapters on difunctional compounds in Section 324 (Alcohol, Thiol-Aldehyde) and in Section 330 (Alcohol, Thiol-Ketone). 

Oxidations. Cleavage of alkenes to aldehydes and ketones is promoted by Wilkinson s catalyst under pressures of air or oxygen, but these reactions are inferior to ozonolysis because they tend to form a mixture of products. More useful are the oxidations of anthracene derivatives to anthraquinones in the presence of oxyg a/tert-Butyl Hydroperoxide and catalytic RhCl(PPh3)3 (eq 54). Wilkinson s catalyst reacts with oxygen to form an adduct so RhCl(PPh3)3 is clearly quite different from the true catalyst in all the reactions mentioned in this section. 

The hydroboration of enamine is remarkable regio- and chemoselective as the boron adds to the electron-rich enamine double bond. Consequently, hydroboration of enamines and the treatment of the resulting trialkylboranes with methanol afford the corresponding alkenes in excellent yield [13]. The process is a general procedure for the synthesis of terminal alkenes from aldehydes and internal or cyclic alkenes from ketones (Eq. 24.5). 

The Wittig reaction is an important carbon-carbon bond-forming reaction that produces alkenes directly from aldehydes and ketones. 

The Wittig reaction provides a path from aldehydes and ketones to alkenes and, consequently, is a valuable tool in organic synthesis. For example, the Wittig reaction will convert an a,p-unsaturated ketone to a conjugated alkene. 

The final step can involve introduction of the amino group or of the carbonyl group. o-Nitrobenzyl aldehydes and ketones are useful intermediates which undergo cyclization and aromatization upon reduction. The carbonyl group can also be introduced by oxidation of alcohols or alkenes or by ozonolysis. There are also examples of preparing indoles from o-aminophcnyl-acetonitriles by partial reduction of the cyano group. 

Many aldehydes and ketones are made m the laboratory from alkenes alkynes arenes and alcohols by reactions that you already know about and are summarized m Table 17 1... 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

By-Products. Almost all commercial manufacture of pyridine compounds involves the concomitant manufacture of various side products. Liquid- and vapor-phase synthesis of pyridines from ammonia and aldehydes or ketones produces pyridine or an alkylated pyridine as a primary product, as well as isomeric aLkylpyridines and higher substituted aLkylpyridines, along with their isomers. Furthermore, self-condensation of aldehydes and ketones can produce substituted ben2enes. Condensation of ammonia with the aldehydes can produce certain alkyl or unsaturated nitrile side products. Lasdy, self-condensation of the aldehydes and ketones, perhaps with reduction, can lead to alkanes and alkenes. 

Vinyllithium [917-57-7] can be formed direcdy from vinyl chloride by means of a lithium [7439-93-2] dispersion containing 2 wt % sodium [7440-23-5] at 0—10°C. This compound is a reactive intermediate for the formation of vinyl alcohols from aldehydes, vinyl ketones from organic acids, vinyl sulfides from disulfides, and monosubstituted alkenes from organic halides. It can also be converted to vinylcopper [37616-22-1] or divinylcopper lithium [22903-99-7], which can then be used to introduce a vinyl group stereoselectively into a variety of a, P-unsaturated systems (26), or simply add a vinyl group to other a, P-unsaturated compounds to give y, 5-unsaturated compounds. Vinyllithium reagents can also be converted to secondary alcohols with trialkylb o r ane s. 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

There are actually three reactions called by the name Schmidt reaction, involving the addition of hydrazoic acid to carboxylic acids, aldehydes and ketones, and alcohols and alkenes. The most common is the reaction with carboxylic acids, illustrated above.Sulfuric acid is the most common catalyst, but Lewis acids have also been used. Good results are obtained for aliphatic R, especially for long chains. When R is aryl, the yields are variable, being best for sterically hindered compounds like mesi-toic acid. This method has the advantage over 18-13 and 18-14 that it is just one laboratory step from the acid to the amine, but conditions are more drastic. Under the acid conditions employed, the isocyanate is virtually never isolated. 

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