Alkenes enolates

Like simple alkenes, enols are nucleophilic by virtue of their n electrons. Enols are much more reactive than simple alkenes, however, because the hydroxyl group can participate as an electron donor during the reaction process. The strong C—O bond is re-formed, providing a favorable energy contribution. 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

As shown in Figure 3-2, titanium is coordinated with the oxygen from both the aldehyde and the alkene enol silyl ether. When aldehyde approaches the enol species, intermediate A is favored to B, and anti-aldol is obtained as the major product. Table 3-4 presents some results of these reactions. 

Coupling of two electron-rich components like alkenes, enol acetates, enol ethers, carbanions, or carboxylates ... 

Ab initio molecular orbital calculations, coupled with activation energies and entropies from experimental data, have been employed to determine the nature of the intermediates in the reaction of singlet oxygen with alkenes, enol ethers, and enamines.214 Allylic alkenes probably react via a perepoxide-like conformation, whereas the more likely pathway for enamines involves a zwitterionic cycloaddition mechanism. The reactions of enol ethers are more complex, since the relative stabilities of the possible intermediates (biradical, perepoxide, and zwitterionic) here depend sensitively on the substituents and solvent polarity. 

Step 1 Selective epoxidation of the more electron-rich alkene (enol ether) occurs selectively at the oc-face to avoid unfavorable interactions with the (3-face angular methyl group. 

Two dipoles especially important in organic synthesis are nitrile oxides and nitrones. The frontier orbital picture for a simple nitrile oxide is shown in Fig. 6.37, where we can see that the easy reactions ought to be decisively dipole-LU-controlled, and fast with C- and X-substituted alkenes. This matches well with the reactions of benzonitrile oxide with styrene, terminal alkenes, enol ethers and enamines which all give only the 5-substituted isoxazolines 6.235. 

Annulation of furans via electrochemical oxidation at the anode has become an important process for the synthesis of complex polycycles, and was covered in a review <2000T9527>. Furans tethered at the 3-position to electron-rich alkenes, enol ethers, or vinyl sulfides were converted to [6,5] and [7,5]-fused ring systems <1996JOC1578, 2002OL3763, 2004JOG3726, 2005JA8034>, as illustrated in Scheme 20. Analysis of crude reaction mixtures and side... 

The phthalazide bis(cinchona) derivatives [(DHQD)2-PHAL] are the best ligands for the asymmetric dihydroxyla-tion of trans, 1,1-disubstituted, and trisubstituted alkenes, enol ethers, a,p-unsaturated ketones, and a,p- and p,y-unsaturated esters, whereas the DHQD-IND ligand turns out to be superior for c/j -alkenes (Table 1). The bis(cinchona) alkaloid-substituted pyrimidine ligand was found to be the best for monosubstituted terminal alkenes. The addition of Methanesulfonamide to enhance the rate of osmate(VI) ester hydrolysis is recommended for all nonterminal alkenes. 

Davis oxaziridine oxidation Oxidation of electron-rich substrates (e.g. alkenes, enolates, enol ethers etc.) with oxaziridines. 130... 

Potassium ert-butoxide, sodium hydride, butyllithium have all been used for this purpose. The alkyl(aryl)sulfanylcarbene (carbenoid) thus generated undergoes addition, often effectively, across the double bond of alkenes, enol ethers, ketene acetals and enamines. The use of chloromethyl phenyl sulfide, oxirane, tetraethylammonium bromide as a catalyst and an alkene gave phenylsulfanylcyclopropanes in rather low yield. For the synthesis of l,l-dimethyl-2-phenylsulfanylcyclopropane, see Houben-Weyl, Vol.4/3, p250 and of endoj exo-7-phenylsulfanylbicyclo[4.1.0]heptane, see Vol. E19b, pl691. 

Houk control concerns electrophilic attack on alkenes, enolates and the like. The alkylation of enolate 56 would be an example if it were not held in a ring by chelation. It can in fact be difficult to tell whether chelation is involved or not with many enolates and the outcome of the reaction may tell which. Chamberlin s asymmetric preparation of both pyrrolidine 2,3-dicarboxylic acids 141 and 142 from natural aspartic acid illustrates this perfectly. The key to the stereochemical control is the very large protecting group 9-phenylfluorenyl-143 introduced by Rapoport.22... 

They will not be detailed in a separate section here but the reactivity of individual molecular types, where significant in a general mechanistic sense, will be mentioned where appropriate. Most of the mechanistic discussion has centered around relatively simple n-systems, alkenes, enol ethers, 1,3-dienes and so on, and several reviews have appeared [14, 15, 121-124], Basic aspects of reactivity up to about 1980 are summarized below followed by a discussion of more recent results. 

Coupling reaction. Photoinduced three-component coupling combines an alkene (enol ether, silyl enol ether, 1,3-diene, etc.) and ethyl propiolate to form a carbon chain in which the two PhSe groups from PhSeSePh are separately located. The reaction starts from p-addition of PhSe radical to the ester. 

Fluorination. This new reagent, which that needs no special caution or glassware in handling, can be used for fluorination of aromatic rings, alkenes, enol ethers, and dienol acetates. In the presence of ZnCl2, either mono- or difluoro derivatives of active methylene compounds can be isolated from its reactions. 

Beyond Diels-Alder reactions, a, 3-unsaturated carbene ligands have been applied to [3+2]-cycloadditions with 1,3-dipoles, [27] thermal [2-i-2]-cycloadditions with electron-rich alkenes (enol ethers, ketene acetals) [26] and in intramolecular Pauson-Khand reactions. [28]... 

Like simple alkenes, enols are nucleophilic by virtue of their 7r-electrons. Enols are... 

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