Double bond electron rich character

Trimethylsilyloxy-substituted alkenes are by far the most widely used enol ethers because of their straightforward preparation from the corresponding ketones (equation 20)78-82 -pjjg electron-rich character of silyl enol ethers allows for highly chemoselective cyclopropanations in the presence of additional double bonds (eqnation 21). ... 

Due to the highly electron-rich character of their double bonds, vinyl ethers are susceptible to cationic polymerization using a variety of Bronsted and Lewis acids as initiators. Bronsted acids as weak as H2SO3 (SO2-I-H2O) and H3PO4 effect the cationic polymerization of... 

As described in equation 78, the relatively electron-rich ring double bond of a- or fi-ionone was regioselectively epoxidized, whereas the electron-deficient double bond was not oxidized at all. This result also supports the radical rather than the anionic character of 52. 

It has also been shown that die electronic character of the olefin to which the radical adds has little influence on die efficiency of the intramolecular cyclizadon. Intramolecular competition between addition to an electron-rich enol ether or a simple double bond gives a 1 1 ratio of products, demonstrating that free-radical cyclizations have a remarkable insensitivity to inductive effects. 

The electron-deficient double bond in 2-phenylsulfonyI-l,3-dienes underwent regioselective cyclopropanation with sulfur ylides in very good yield, e.g. cyclopropanation of 13, 14 and 15 with sulfur ylides.On the other hand, the use of zinc-copper couple and diiodomethane afforded a 83 17 mixture of vinylcyclopropanes 16 and 17 from 2-(phenylsulfonyl)cyclohexa-1,3-diene (15) in moderate yield. The reaction at the electron-deficient double bond rather than at the expected electron-rich double bond shows that the Simmons-Smith carbenoid is fairly nucleophilic in character. 

The energy-rich nature of thioesters, as compared with ordinary esters, is related primarily to resonance stabilization (Figure 14.9). Most esters can resonate between two forms (Figure 14,9). Stabilization involves Pi-electron overlap, giving partial double-bond character to the C-0 link. In thioesters, the larger atomic size of S (as compared with O) reduces the Pi-electron overlap between C and S, so that the C-S structure does not contribute significantly to resonance stabilization. Thus, the thioester is destabilized relative to an ester, so that its iG of hydrolysis is increased. 

In Figure 5, the values for the addition reaction of PhS to various olefins are plotted against Alfrey-Price s e values, which are a measure of the polar character of the double bonds. The more negative the e value, the more electron-rich the double bond [40]. The plots are divided into two groups according to conjugation ability. The activation energy of the forward reaction (EJ) depends... 

Alkenes are converted to epoxides using a range of peracids, RCO3H. The reaction is essentially electrophilic in character, and electron-rich double bonds react most rapidly. 

Not only oxidants with nucleophilic character but also intermediates of electrophilic nature can be used in the enantioselective epoxidation of a,p-unsaturated carbonyl compounds. Among the possible candidates, dioxirane reagents have been successfully used for this purpose. Contrary to the usual nucleophilic oxidants, dioxiranes add to double bonds in a concerted manner. These dioxiranes could be easily prepared in situ by reaction of oxone (2KHS05-KHS0 K3S0 ) with chiral ketones. Chiral ketones derived from quinic acid such as compounds 61 [67], which have been successfully used in the enantioselective epoxidation of electron rich olefins, have been also applied to the epoxidation of electron poor olefins such as chalcone 7 or a,P-unsaturated esters 59 (Scheme 4.10) to give the corresponding... 

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