Carbenium ions vinyl

The following sections will provide an overview of the living carhocationic polymerizations of the three most important monomer families, namely, isobutene, vinyl ethers, and styrenics. The reactivity of the monomers for carhocationic polymerization is directly related to the stabilization of the formed carbenium ion vinyl ethers are the most reactive monomers, followed by styrenics and. Anally, 1,1-disubstituted alkenes (Scheme 8.4). As a direct consequence of this stabilization, the reactivity of the formed carbocations is opposite to that of the monomer. 

The Ritter reaction with unsaturated carbenium ions under either silver-assisted solvolysis or photolytic conditions leads to excellent yields of isoquiaolines (173). The ease of preparation of the requited vinyl bromides makes an attractive route to highly substituted isoquiaolines. 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

The formation of w-donor complexes (IV). This involves stronger forces than the previous two types because the lone pair of the hetero-atom is involved. It is clear that the polymerisations of some of the favourite monomers, such as the alkyl vinyl ethers, 4-MeO-styrene, and N-vinylcarbazole may be dominated by this phenomenon. A corollary of the complexation by an w-donor monomer is that the hetero-atoms in the corresponding polymers will also interact with the growing carbenium ions. The authors who have proposed this include Stannett (alkyl vinyl ethers) [11], Boelke (dimethoxyethene) [12], and Sauvet (4-MeO-St) [7]. 

The second part of the theory, which is a logical consequence of the first, is that monomers that have more than one basic site, e.g., an aromatic ring or an oxygen atom, can form more than one type of complex with the carbenium ion this idea was first proposed by Plesch (1990) in the context of chemically initiated polymerizations. It helps to explain why aryl alkenes and alkyl vinyl ethers polymerize more slowly than isobutene and cyclopentadiene. The reason is that all the complexes formed by the alkyl alkenes are propagators, whereas for the aryl alkenes and vinyl ethers only a fraction of the population of complexes can propagate. 

We report on the measurement of the propagation rate constants kp of styrene, indene, phenyl vinyl ether (PhViE) and 2-chloroethyl vinyl ether (CEViE) in nitrobenzene at (mostly) 298 K with 4-ClC6H4CO+SbF 6 as initiator. The dependence of the conductivity on the [4-ClC6H4CO+SbF"g] = c0 helped to establish that [Pn+] = c0 and thus to validate the foundation of this work. It is shown that most probably the propagating species are the uncomplexed, unpaired, solvated carbenium ions. Some new enthalpies of polymerisation have been found. 

The author s theory which has been used here was developed in detail to explain the polymerisations by ionising radiations of some alkyl vinyl ethers, the polymerisations of which proceed by secondary ions. Although it was shown that the theory is also perfectly serviceable for the tertiary carbenium ions considered here, it must be realised that there is a fundamental difference between these two types of carbenium ions. When one of the bonds of the carbenium ion is a C—H bond, the solvators, especially of course an ion, can get much closer to the positive centre, and they are therefore correspondingly more firmly held to which effect is added that of a smaller steric hindrance. The most researched monomer propagating by secondary cations, apart from the alkyl vinyl ethers, is, of course, styrene. Thus, Mayr s many studies with diaryl methylium cations are directly relevant to the polymerisation of styrene. 

TABLE 6. Effect of the methyl group at the electrophilically attacked vinylic carbon by p-methoxydiphenylmethyl carbenium ion... 

Electrolysis of /J-hydroxy carboxylic acid (90) gave ketone (91) as the main product, which was converted to dl-muscone (Scheme 34) [113]. The migratory aptitude of alkyl substituents at the fi-position in the intermediate -hydroxy carbenium ion was scrutinized to show that vinyl, cyclopropyl, and benzyl groups rearrange predominantly. 

The measured stabilization energies are consistent with the theoretically predicted large hyperconjugative interaction between the Si—C cr-bond and the formally empty 2p(C) orbital. The results for the trivalent carbenium ions show a consistent decrease of 10 kcal mol-1 with each successive methyl group substitution on the carbenium carbon (Table 1, entries 1,4,6). Even the very stable /-butyl cation Me3C+ is stabilized by an additional 24 kcalmol-1 by a /l-MejSi substituent (Table 1, entry 6). The stabilization of vinyl cations due to the presence of a /l-McsSi group is found to be smaller than for... 

Trivalent ( classical ) carbenium ions contain an, vp2-hybridized electron-deficient carbon center that tends to be planar in the absence of constraining skeletal rigidity or steric interference. (It should be noted that ip-hybridized, linear acyl cations and vinyl cations also show substantial electron deficiency on carbon.) The carbenium carbon contains six valence electrons, and thus it is highly electron-deficient. The stmcture of trivalent carbocations can always be adequately described by using two-electron, two-center bonds (Lewis valence bond structures). 

In vinyl cations (Figure 3.13), as in trisubstituted carbenium ions, the positive charge is stabilized by electron donating substituents or by aryl or vinyl groups via jt-conjugation. Further stabilization may be achieved by c participation— that is, by hyperconjugation of a substituents, by complexation to a metal, or by the (3-silyl effect. 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

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