Vinyl ethers resonance

Tentative assignments of /2-carbon vinyl ether resonances in acetylated kraft lignin preparations have previously been made (S3), Values of 144.3-144.4 ppm (trans) and 141.5-142.0 ppm (cis) in CDCI3 were reported. However, a rather low-field (20 MHz) instrument was used along with the conventional proton noise-decoupled technique, so extreme overlap with quaternary carbons was unavoidable. 

There are also substituents that can act as electron-releasing groups through resonance. Among familiar examples are alkoxy and amino groups in vinyl ethers and enamines, respectively. 

Di-p-chloro-bis(i74-l,5-cyclooctadiene)dirhodium(I) is a yellow-orange, air-stable solid. It can be used directly as obtained for preparative purposes5 or as a precursor for homogeneous catalysts.3,4 It can be recrystallized from dichloro-methane-diethyl ether to give orange prisms. The compound is soluble in dichloro-methane somewhat less soluble in acetone and insoluble in pentane and diethyl ether. Characteristic strong bands occur in the infrared spectrum at 819, 964, and 998 cm 1 (Nujol mull). The cyclooctadiene vinylic protons resonate in the 1H NMR spectrum at t 5.7 and the allylic protons at t 7.4-8.3 (deuteriochloroform solution). Other physical properties are given by Chatt.1... 

Spectra of aryl alkyl ethers display an asymmetrical C—O—C stretching band at 1275-1200 cm-1 with symmetrical stretching near 1075 -1020 cm1. Strong absorption caused by asymmetrical C—O—C stretching in vinyl ethers occurs in the 1225-1200 cm-1 region with a strong symmetrical band at 1075-1020 cm"1. Resonance, which results in strengthening of the C—O bond, is responsible for the shift in the asymmetric absorption band of arylalkyl and vinyl ethers. 

The two bands arising from =C—H wagging in terminal alkenes occur near 1000 and 909 cm1. In the spectra of vinyl ethers, these bands are shifted to longer wavelengths because of resonance. 

The mechanism Favorskii envisioned involved the initial attack of the ethoxy anion on the triple bond to form a vinyl ether. The now accepted carbanionic mechanism assumes the formation of resonance-stabilized anions, allowing the stepwise interconversion of 1- and 2-alkynes, and allenes143 147 (Scheme 4.9). 

The first examples of ortho cycloaddition can be found in a U.S. patent of Ayer and Buchi [1], Benzonitrile and 2-methylbut-2-ene are reported to yield 7,8,8-trimethylbicyclo[4.2.0]octa-2,4-diene-l-carbonitrile upon irradiation under nitrogen with a mercury resonance arc. Similar reactions, all leading to derivatives of bicyclo[4.2.0]octa-2,4-diene-l-carbonitrile occurred when benzonitrile was irradiated in the presence of 2,4,4-trimethylpent-l-ene, ethyl vinyl ether, vinyl acetate, methyl vinyl ketone, and methyl acrylate. The addend pairs para-tolunitrile/oct-l-ene, ort/m-dicyanobenzene/2-methylbut-2-ene, para-dicyanobenzene/but-l-ene, 2,3-dimethylbenzonitrile/propene, and 3,4,5-trimethylbenzonitrile/ethene likewise produced ortho photocycloadducts. 

Atkins et al. [130] reported in 1977 that irradiation of mixtures of benzene and methyl acrylate or methyl methacrylate, both acceptors, yields mixtures of endo and exo adducts. A subsequent report from the same groups [120] describes the results of the irradiations of benzene in the presence of ethyl vinyl ether, //-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene. In all these cases, the major products were exo-ortho photocycloadducts. The orientations of these vinyl ethers with respect to benzene, in their loose ground-state associations, were inferred from NMR spectra. For ethyl vinyl ether, n-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene, the vinyl proton resonances were either unaffected by a solvent change from carbon tetrachloride to hexadeuterobenzene or appeared 4-10 Flz downfield, whereas the methyl and/or methylene signals all moved up-field by 10-25 Hz. This implies an endo arrangement of the molecules in the ground state. Thus, the ortho photocycloadducts of vinyl ethers with benzene show exo stereochemistry, even when the ground-state orientation is endo. 

The preparation involves an oxymercuration (Section 3.5.3) of the C=C double bond of the ethyl vinyl ether. The Hg(OAc) ion is the electrophile as expected, but it forms an open-chain cation A as an intermediate rather than a cyclic mercurinium ion. The open-chain cation A is more stable than the mercurinium ion because it can be stabilized by way of oxocarbe-nium ion resonance. Next, cation A reacts with the allyl alcohol, and a protonated mixed acetal B is formed. Compound B eliminates EtOH and Hg(OAc) in an El process, and the desired enol ether D results. The enol ether D is in equilibrium with the substrate alcohol and ethyl vinyl ether. The equilibrium constant is about 1. However, the use of a large excess of the ethyl vinyl ether shifts the equilibrium to the side of the enol ether D so that the latter can be isolated in high yield. 

In a substituted vinyl ether, the oxygen atom deshields the a-proton by an inductive effect and shields the /3-proton by resonance. 

Polar resonance structures explain these shifts for alkynyl ethers, which are analogous to the shifts for vinyl ethers (Section 3.34). 

It should be noted, in addition, that the resonances of the vinylic protons and the methoxyl group in the vinyl ether 151 are observed33 at considerably lower field as compared with the vinylthio derivatives, and this indicates a shielding effect of the sulfur atom. 

Due to resonance stabilization and their higher nucleophilicity, heteroatoms stabilize the growing carbenium ions better than alkyl and aryl groups do N-vinyl carbazole is more reactive than vinyl ethers because of nitrogen s higher nucleophilicity. However, the reactivity of the growing carbenium ions follows the opposite order shown above, with the most stable... 

Electron density is influenced by resonance (mesomerism), as well as by inductive effects, as seen in unsaturated molecules such as alkenes and aromatics. The donation of electrons through resonance by a methoxy group increases the electron density at the p position of a vinyl ether (3-1) and at the para position of anisole (C6H5OCH3). Thus, the... 

Cationic polymerization shares several of the features discussed above for anionic polymerization, but the requirements in terms of the monomer structure will be opposite, i.e. the vinyl monomers should have an electron-donating side group since the propagating centre carries a positive charge. Alternatively, the presence of an aromatic ring can stabilize the cation by making available resonance structures. Examples of these are alkyl vinyl ethers, isobutylene, isoprene, styrene and a-methyl styrene (Sauvet and Sigwalt, 1989). 

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