Open cation

Structures with a cyclic character (70° a 110°) are less solvated than open cation structures (a < 70° a > 110°) due to a larger charge delocalization in the former. Thus, the alterations of the potential energy surface described above are plausible. There are two possible structures for activated complexes in solution. They... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

A similar result (open cations) was obtained with carbocations Ar2CH as electrophiles Mayr, H. Pock, R. Chem. Ben, 1986, 119, 2473. 

When the kinetics indicate that there is competition between bridged and open cations, neither the solvent-dependent stereoselectivity, nor the... 

H receptor activation induces depolarizing responses in many brain areas, notably hypothalamus, thalamus and cerebral cortex. In vertebrate brain, many of these effects are mediated by opening cation channels. H,-induced excitation can also occur by blockade of KUak conductances [ 1 ]. In other cases, however, H, receptors can attenuate neuronal excitation by activating certain voltagegated potassium channels. Most of the H, receptor-induced conductance changes are mediated by the IP3-Ca2+ cascade. 

Inaki (1992) synthesized a wide range of nucleobase-functionalized random and homopolymers. In addition, Inaki et al. (1980) synthesized block copolymers containing thymine and uracil groups in the main chain through ring-opening cationic and anionic polymerization of cychc derivatives of the nucleobases. 

Attack by Cl, 111 I, 19 and RS + 2il is similar to that by Br there is a spectrum of mechanisms between cyclic intermediates and open cations. As might be expected from our discussion in Chapter 10 (p. 312), iodonium ions compete with open carbocations more effectively than bromonium ions, while chloronium ions compete less effectively. There is... 

Bromination of tetrafluorobenzobarrelene with Br2 has been reported to produce stereoisomers of the annulated tricyclic dibromide. However, when pyridine, 15-crown-5, or Me3S were present, the /ranv-dibromide was obtained as the main product. MNDO calculations suggested that different cationic intermediates are responsible for the two pathways cyclic bromonium ions lead to the fonner, whereas open cations give rise to the latter product.26... 

Ab initio theoretical calculations for the 4-methylpent-2-yl cation by Farca iu et al.115 have shown both a distorted 1 -protonated 1,1,2-trimethylcyclopropane and the open cation to be energy minima along the reaction coordinate at the B3LYP/ 6-31G level. In contrast, only the protonated cyclopropane was found to be an energy minimum in the MP2/6-31G optimization. Whereas the open cation was a transition structure at this level, a coupled cluster geometry optimization (CCSG/6-31G ) showed that the open ion is also a true energy minimum. 

Electrophilic Attack on a C=C Double Bond by Nonbridging Electrophiles. Electrophilic attack by a proton or a cationic carbon on a C=C double bond may give an open cation 5.34 or a bridged cation 5.36. We have met the problem of hyperconjugating and bridged cations before [see (Section 2.2.1.2) page 72], and the same problem arises here. 

On the other hand, however, these two areas of cationic polymerization are not completely separated fields. In spite of the differences, both processes proceed on electron-deficient active species cations or species with a partial positive charge. Thus, propagation in both cases involves attack of the nucleophile (double bond or heteroatom) on electrophilic active centers. Several basic principles will therefore hold for both vinyl and ring-opening cationic polymerization. 

It is clear from the present survey that a significant number of commercially available polymers and copolymers are produced by alkene and ring-opening cationic processes. Thus even if cationic polymerization has the reputation of an unextinguishable source of brain-storming parties for academic scientists, it is the only route for elaboration of some major structural or technical polymeric materials and there is no doubt concerning its present and future commercial importance. 

Several alternatives were proposed one, that participation by phenyl in expulsion of tosylate occurs, but is weak another, that bridging occurs, not in the rate-determining step, but rapidly, following formation of an open cation. H. C. Brown (p. 507) suggested that—for unsubstituted phenyl, at least—the intermediate is not a bridged ion at all, but a pair of rapidly equilibrating open car-bonium ions phenyl, now on one carbon and now on the other, blocks back-side attack by the solvent and thus gives rise to the observed stereochemistry. 

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