Cations with double bond

Coupling reactions of silyl enol ether radical cations with double bonds other than silyl enol ethers have been investigated as well. Reactions with butadiene, ethyl vinyl ether, and allylic silanes have been reported. 

In chlorination, loss of a proton can be a competitive reaction of the cationic intermediate. This process leads to formation of products resulting from net substitution with double-bond migration ... 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

Yet another development which is worth mentioning in this context is the 5n substitution of acetals of unsaturated carbonyl compounds. The phenomenon that the reaction of an allylic acetal with a Grignard reagent in the presence of CuBr may occur as a vinylogous substitution with double bond shift has been long known.The reaction can be utilized in an efficient synthesis of 3-substituted propionaldehydes using the acrolein acetal as a homoenolate cation equivalent (Scheme 38). ... 

In Section 7.4.1 we have been concerned with double bonds predisposed for symmetrical (homoaromatic) participation whereas Section 7.4.2 was devoted to systems where only one end of the double bond is accessible to the incipient carboca-tion. 3-Buten-1-yl substrates (455) cannot approach a symmetrical transition state, but two homoallylic interactions are possible (387), leading toward a cyclopropylcarbinyl cation (388), and (456), leading toward a cyclobutyl cation (458). Structure (387) implies strong interaction between C-l and C-3 with little or no overlap be-... 

The chemistry of aromatic alkylation is more complicated than implied by equations (15, 16, 17). Polymerization, CP production, and the isomerization of heavier olefins also occur. Olson (32) has reported many details of the positional isomerization of 1-aIkenes in the Ce—Cm range. Using 1-dodecene as an example, the alkylated product is a mixture of 2- through 6-dodecyl benzenes. In the absence of isomerization, only 2-dodecyl benzene is produced. Attachment at the first carbon atom is not expected when propylene or heavier olefins are employed since primary cations would then be obtained. Secondary cations are however more stable (or preferred) and lead to the attachment at the second or higher carbon atom of the cation. Olson suggests that positional isomerization involves the formation of dodecyl acid sulfates or dodecyl fluorides when sulfuric acid or HF are used as catalysts reverse reactions then lead to the formation of olefins with double bonds in a new position on the chain. In one example reported, at least 80% of the dodecene isomerized before alkylation (by reactions similar to eqs. 16 and 17). Olson also found that some of the initial dodecylbenzene produced were isomerized. The 2-dodecyl benzene that was initially produced isomerized in the presence of AICI3 catalysts to give from 3-dodecyl to 6-dodecyl benzenes. 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

A.H. Cowley - Stable Compounds with Double Bonds Between the Heavier Main-Group Elements, Acc. Chem. Res. 17,386,1984 From Multiple Bonds to Materials Chemistry, J. Organomet. 400, 71,1990 Some Past Vignettes and Future Prospects for Main Group Chemistry, J. Organomet. Chem. 600,168,2000 From Group 13-Donor-acceptor Bonds to Triple-decker Cations, Chem. Commun. (Feature Article), 2369, 2004. 

Divinylbenzene is easily radically polymerized or cationically polymerized thus it is necessary to store or ship it in small quantities and add a large amount of polymerization inhibitor. Homopolymerized or copolymerized with vinyl polymers. Forms a reactive microgel with double bonds by emulsion polymerization. 

Both give TM 31 on treatment with HBr as the cation A reacts preferentially with Br" at the less substituted carbon atom to give the more substituted double bond. Think again. 

The resulting macrocyclic ligand was then metallated with nickel(II) acetate. Hydride abstraction by the strongly electrophilic trityl cation and proton elimination resulted in the formation of carbon-carbon double bonds (T.J. Truex, 1972). 

Cyclopentene derivatives with carboxylic acid side-chains can be stereoselectively hydroxy-lated by the iodolactonization procedure (E.J. Corey, 1969, 1970). To the trisubstituted cyclopentene described on p. 210 a large iodine cation is added stereoselectively to the less hindered -side of the 9,10 double bond. Lactone formation occurs on the intermediate iod-onium ion specifically at C-9ot. Later the iodine is reductively removed with tri-n-butyltin hydride. The cyclopentane ring now bears all oxygen and carbon substituents in the right stereochemistry, and the carbon chains can be built starting from the C-8 and C-12 substit""" ... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

The kinds of vinyl monomers which undergo anionic polymerization are those with electron-withdrawing substituents such as the nitrile, carboxyl, and phenyl groups. We represent the catalysts as AB in this discussion these are substances which break into a cation (A ) and an anion (B ) under the conditions of the reaction. In anionic polymerization it is the basic anion which adds across the double bond of the monomer to form the active center for polymerization ... 

When aiomatics aie present, they can capture the intermediate vinyl cation to give P-aryl-a,P-unsatutated ketones (182). Thus acylation of alkyl or aryl acetylenes with acyhum salts in the presence of aromatics gives a,P-unsaturated ketones with a trisubstituted double bond. The mild reaction conditions employed do not cause direct acylation of aromatics. 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

These oxazolines have cationic surface-active properties and are emulsifying agents of the water-in-oil type. They ate acid acceptors and, in some cases, corrosion inhibitors (see Corrosion). Reaction to oxazoline also is useful as a tool for determination of double-bond location in fatty acids (2), or for use as a protective group in synthesis (3). The oxazolines from AEPD and TRIS AMINO contain hydroxyl groups that can be esterified easily, giving waxes (qv) with saturated acids and drying oils (qv) with unsaturated acids. 

The electron-rich carbon—carbon double bond reacts with reagents that are deficient in electrons, eg, with electrophilic reagents in electrophilic addition (6,7), free radicals in free-radical addition (8,9), and under acidic conditions with another butylene (cation) in dimerization. 

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