Alkenes from carbonyl compounds with

Peterson alkenation of carbonyl compounds with trimethylsilylmethoxymethyl carbanion (14) can also give enol ethers.6 7 An example is the reaction of (14) with adamantanone. Another method of alkene synthesis is the elimination of CO2 and H2O from a (3-hydroxycarboxylic acid. If the carboxylic acid is substituted by an a-alkoxy group, the reaction can be used for the synthesis of enol ethers, as illustrated in Scheme 4.8... 

Introduction. The Peterson-type olefination reaction has emerged as an extremely useful method for the preparation of alkenes from carbonyl compounds. Synthetically useful vinylsilanes can be prepared by using the Peterson-type olefination reaction of bis(silyl)methylmetal with carbonyl compounds. Among the various reported bis(silyl)methyhnetals, [bis(2-pyri-dyldimethylsilyl)methyl]lithium is an extremely efficient reagent for the stereoselective preparation of vinylsilane. ... 

The synthesis of terminal alkenes from carbonyl compounds is a very important reaction in organic synthesis. Najera and co-workers found out that BTFP methyl sulfone is a good reagent to perform methylation reactions of carbonyl compounds through the Julia-Kocienski protocol. The reaction proved to be much more efficient when Barbier-type conditions were used. Example, alkene 90 was obtained in 45% yield by reacting sulfone 88 with diketone 89 in the presence of P4- -Bu in THF. For example, the sulfone 92, prepared from 84 by two-steps reaction in 81% yield, was converted to alkene 94 in 95% yield and 96 in 71% yield respectively. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

Functionalized zinc carbenoids have been prepared from carbonyl compounds by an indirect strategy. The deoxygenation of a carbonyl compound to an organozinc carbenoid can be induced by a reaction with zinc and TMSCl. Therefore, the aldehyde or ketone, when treated with TMSCl or l,2-bis(chlorodimethylsilyl)ethane in the presence of an alkene, generates the cyclopropanation product. This method is quite effective for the production of alkoxy-substituted cyclopropane derivatives. A 55% yield of the... 

Dihydroisoxazoles with a substituent at nitrogen are most conveniently prepared by 1,3-dipolar cycloaddition of nitrones to alkenes or alkynes. Nitrones are usually prepared in situ from carbonyl compounds and /V-(alkyl)hydroxylamines (Figure 15.10). 

In contrast to radical ions generated from alkenes or carbonyl compounds, substantially fewer recent reports have appeared which describe the chemistry of radical ions generated from the >C=N— functional group. This situation likely results from the relative obscurity of the >C=N— group (compared to >C=0 and >C=C<), rather than specific problems with the chemistry, per se. Based upon the limited data available, and as might be anticipated, >C=N— + chemistry appears to be analogous to that of >C=C< +, while >C=N— chemistry is reminiscent of >C=0. ... 

Of special interest is a new type reaction discovered with N20 direct oxidation of alkenes to carbonyl compounds, called carboxidation. Beside various individual alkenes, carboxidation can be applied effectively to unsaturated polymers, opening up a way for the preparation of new materials. Reactions of this type should receive special attention from the catalytic community, since currently they are conducted in a thermally. This oxidation area is waiting for the beneficial arrival of catalysis to provide better control of the activity and selectivity. 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

In this section, we review the development of organocatalytic ylide-based epox-idation methods which allow a one-step route from carbonyl compounds, and therefore compete with the more traditional two-step approach of olefination followed by epoxidation of the resulting alkene (see Scheme 10.1) [13]. Indeed, ylide-based methodologies side-step the construction of a C=C double bond and achieve the whole transformation in one step thus, they are potentially more atom-efficient (see Scheme 10.1). However, there are greater challenges as both the absolute and relative stereochemistries must be controlled in one step [14-22]. 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

In contrast, the related silyl enol ethers are available by mild selective transformations from carbonyl compounds or other precursors 55). Their stability and that of products derived from these alkenes can easily be regulated by choosing suitable substituents at silicon. Selective cleavage of a Si—O-bond is possible with fluoride reagents under very mild conditions, and this is why cyclopropane ring opening can now be performed with high chemoselectivity. 

Addition of anionic nucleophiles to alkenes and to heteronuclear double bond systems (C=0, C=S) also lies within the scope of this Section. Chloride and cyanide ions are effieient initiators of the polymerization and copolymerization of acrylonitrile in dipolar non-HBD solvents, as reported by Parker [6], Even some 1,3-dipolar cycloaddition reactions leading to heterocyclic compounds are often better carried out in dipolar non-HBD solvents in order to increase rates and yields [311], The rate of alkaline hydrolysis of ethyl and 4-nitrophenyl acetate in dimethyl sulfoxide/water mixtures increases with increasing dimethyl sulfoxide concentration due to the increased activity of the hydroxide ion. This is presumably caused by its reduced solvation in the dipolar non-HBD solvent [312, 313]. Dimethyl sulfoxide greatly accelerates the formation of oximes from carbonyl compounds and hydroxylamine, as shown for substituted 9-oxofluorenes [314]. Nucleophilic attack on carbon disulfide by cyanide ion is possible only in A,A-dimethylformamide [315]. The fluoride ion, dissolved as tetraalkylammo-nium fluoride in dipolar difluoromethane, even reacts with carbon dioxide to yield the fluorocarbonate ion, F-C02 [840]. 

A variety of reagents are used for the construction of carbon-carbon double bonds from carbonyl compounds. For example, ylides such as Wittig-type reagents or carbanions stabilized by an a-heteroatom react with carbonyls to give the corresponding alkenes. 

Alkenes may be prepared by elimination reactions with a regio-chemistry (Hofmann or SaytzefO that depends on the structure of the substrate and the reaction conditions. Alkenes may also be obtained from carbonyl compounds by the Witlig reaction and by the hydrogenation of alkynes. 

A useful alternative to phosphorus ylids are the titanium reagents, such as, 71, prepared from dicyclopentadienyltitanium dichloride and trimethylaluminum. Treatment of a carbonyl compound with the titanium cyclopentadienide complex 71 Tebbe s reagent) in toluene-THF containing a small amount of pyridine " leads to the alkene. Dimethyltitanocene (Me2TiCp2), called the Petasis reagent, is a convenient and highly useful alternative to The mechanism of Petasis... 

The pathway in Scheme 3 relates mainly to alkenes activated by keto or aldehyde groups, for reduction in hydroxylic solvents. Under these conditions, radical anions derived from carbonyl compounds are protonated at oxygen, and the resulting enolic radical, HI, is more difficult to reduce than the starting compound. Consequently, fast dimerization of the enol radicals may compete with further reduction. For other substrate types, especially in aprotic solvents containing added acids, proton transfer is to carbon... 

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