Carbocations interactions with anions

Unlike mononuclear carbonylates, reaction of carbocation reagents with anionic polynuclear carbonylates often results in coordination at the bridging carbonyl oxygen rather than at the metal center. This type of interaction was first observed for the reaction of [HFe3(CO)II] with CH3S03F (88) [Eq. (12)], and a higher yield route is indicated in Eqs. (13) and (14). The X-ray crystal structure of this molecule is shown in Fig. 13. 

The utilization of compound 54 in the aldolization showed higher yield of the product (92%) after 30 min, compared to that (73%) of a trityl catalyzed reaction. The similar results were obtained in the glycosylation reaction 85% (o/p ratio 9 91) and 72% (cx/p ratio 10 90) respectively. The application of the highly hindered tetrakis[pentafluorophenyl]borate anion is remarkably advantageous for the stabilization of the positive charge in the carbocation 54 and at the same time promotion of its accessibility to the interaction with a carbonyl species. 

Polar protic solvents like H2O and ROH solvate both cations and anions well, and this characteristic is important for the SnI mechanism, in which two ions (a carbocation and a leaving group) are formed by heterolysis of the C-X bond, The carbocation is solvated by ion-dipole interactions with the polar solvent, and the leaving group is solvated by hydrogen bonding, in much the same way that Na" and Br are solvated in Section 7.8C. These interactions stabilize the reactive intermediate. In fact, a polar protic solvent is generally needed for an SnI reaction. SnI reactions do not occur in the gas phase, because there is no solvent to stabilize the intermediate ions. 

As alluded to above, the major precursors to OH radical induced ssb are the C-4 radical and the C-5 radicd resulting from H-atom abstraction of H-4 and H-5 /H-5 respectively [2]. From irradiation of plasmid DNA in aqueous solution under aerobic conditions, it is estimated that the probability of prompt ssb formation on interaction with the OH radical is -13% [88]. To measure prompt ssb it is essential to irradiate and subsequently maintain the DNA at 4 °C during electrophoresis to avoid the formation of heat-labile sites. The yield of heat labile sites is -30% of the yield of prompt ssb [63]. Further, DNA should be retained at near neutral pH to avoid the formation of alkali-labile sites. For instance, if C-1 radicals are produced in DNA by OH radicals or direct effects, these C-1 radicals lead to abasic sites with the formation of deoxyribonolactone, an alkali-labile site [89,90]. In the presence of oxygen, the C-1 radical forms a peroxyl radical adduct which decomposes into a carbocation at C-1 and the superoxide radical anion. 

Ions termed free may be involved in electrostatic interactions with the coim-teranion, or in similar interactions limiting their freedom, but in relative terms these effects are long-range interactions. Similar to anionic polymerization, both free carbocations and ion pairs are capable of polymerization. However, the difference between their reactivity is significantly smaller than in the case of anionic polymerization. As mentioned before, it was recently suggested that in fact there is no difference between k-g+ and k-p+ (21) - this is currently debated and needs... 

The aza and sulfonium analogs of the linalyl and a-terpinyl carbocation intermediates of the isomerization-cycHzation reaction (Fig. 1) were utilized to determine the nature of active site interactions with several monoterpene synthases [49, 75, 76]. Synergistic inhibition with each analog in the presence of inorganic diphosphate was demonstrated, suggesting ion-pairing of the diphosphate anion with various carbocationic intermediates generated in the course of the reaction sequence. 

The difference in the strength of the cation-anion interaction is also manifested by polymer molecular weights. MX initiators yield high polymers at low temperature, as their counteranions only weakly interact with the propagating carbocation. In contrast, the polymers obtained with non-MX initiators are mostly of low molecular weight, and for this reason polymerization by these initiators has not been studied as extensively as that by MX initiators. 

The living cationic polymerizations discussed above are invariably based on the nucleophilic iodide counteranion (activation of the carbon-iodine terminal bond Eq. 3). It is expected, however, that similar living processes are equally possible with other counteranions that can exert, as the iodide anion does, a suitably strong nucleophilic interaction with the growing carbocation. We have in fact found the phosphate anions to meet this requirement (10). Similarly to hydrogen iodide, monoacidic phosphate esters [H0P(0)R 2 R alkyl, alkoxyl, etc.] like diphenyl phosphate ( ) form a stable adduct 5) with a vinyl ether (Eq. 5). Zinc chloride or iodide then activates the phosphate bond in 5 by increasing its polarization (as in 6), and living cationic polymerization proceeds via an intermediate (7) where the carbocationic site is stabilized by a phosphate anion coupled with the zinc halide activator. 

It is to be noted that N-vinylcarbazole (NVC) undergoes also living cationic polymerization with hydrogen iodide at —40 °C in toluene or at —78 °C in methylene chloride and that in this case no assistance of iodine as an activator is necessary 10d). NVC forms a more stable carbocation than vinyl ethers, and the living propagation proceeds by insertion between the strongly interacting NVC-cation and the nucleophilic iodide anion. 

A systematic carbocation concentration dependency study on NMR chemical shifts was performed for the C-l-protonated 477-cyclopenta[fi e/ phenanthrenium cation 7H+ and the C-l-protonated pyrenium cation 2H+ (Fig. 11). Shielding of the PAH arenium ion protons and carbons was observed with decreasing FSO3H PAH ratios without noticeable line-broadening. This was attributed to cation-anion interactions in the low FSO3H PAH domain and possible formation of contact ion... 

The interaction of an acid with an alkenyl monomer can generate ionic chain carriers, but also covalent products with varying degrees of polarity. It has been shown that in certain systems these ester molecules can propagate the growth of a polymer chain, while in others they are inactive. Another source of covalent species in cationic polymerisation is the collapse (recombination) of the ionic pair or the X displacement from the anion to the carbocation discussed in the previous section. 

This hypothesis could also account for the observed higher elimination tendency with strong Lewis acids. Thus, if the interaction between the cation and counteranion is weak (weakly nucleophilic counter anion), another nucleophile, the olefin in the system, has a better chance to approach the electrophilic site than with the tighter ion-counteranion pairs (strongly nucleophilic counter anion). However, in the system under investigation, steric hindrance prevents propagation, the ultimate union between the cation and olefin. In this sense, tte Cg olefin behaves as a nucleophile that is able to abstract irreversibly a proton from the carbocation. Counteranions are also able to abstract a proton, however, in this case a reversible equilibrium exists ... 

Carbenes bear a resemblance to carbocations in that there is an empty p orbital that can behave as an electron sink. However, a full orbital that can serve as an electron source is on the same atom. Trichloromethyl anion loses chloride, forming the reactive dichlorocarbene, a neutral, electron-deficient, electrophilic intermediate. Stabilization in dichlorocarbene results from the interaction of the full lone pair orbitals of chlorine with the empty p orbital of the carbene (Fig. 8.11). If the donors on the carbene are good enough, the carbene becomes nucleophilic. With few exceptions, carbenes react stereospecifically with double bonds to produce three-membered rings. 

The relative strengths of weakly basic solvents are evaluated from the extent of protonation of hexamethylbenzene by trifluoro-methanesulfonic acid (TFMSA) in those solvents or from the effect of added base on the same protonation in solution in trifluoroacetic acid (TFA), the weakest base investigated. The basicity TFA < di-fluoroacetic acid < dichloroacetic acid (DCA) < chloroacetic acid < acetic acid parallels the nucleophilicity. 2-Nitropropane appears to be a significantly stronger base than DC A by the first approach, although in the second type of measurement, the two have essentially equal basicity. The discrepancy is due to an interaction, possible for hydroxylic solvents such as DC A, with the anion of TFMSA. This anion stabilization is a determining factor of carbocationic reactivity in chemical reactions, including solvolysis. A distinction is made between carbocation stability, determined by structure, and persistence (existence at equilibrium, e.g., in superacids), determined by environment, that is, by anion stabilization. 

Any molecule that could specifically stabilize the cation II will be a base and combine with the proton therefore, the equilibrium is displaced away from II. For other carbocations (such as the alkyl cations), the solvents interacting specifically with the positively charged species will be nucleophiles and combine with the carbocations. This relationship does not mean that carbocations cannot be nucleophilically solvated as discussed recently (34, 35). Interestingly, though, apparently n donors interact preferentially with hydrogen atoms rather than the cationic carbon (36), thus reacting as bases rather than nucleophiles (37). Also, Sharma et al. (34) concluded that nucleophilic stabilization is not the dominant solvation for carbocations in solution. In any event, nucleophilic solvation decreases rather than enhances carbocationic character. On the contrary, any specific interaction between the solvent (SOH) and the anion, such as formation of hydrogen bonds (equation 5), displaces the equilibrium of equation 4 toward formation of carbocations ... 

In superacid systems consisting of a composite of a Brpnsted acid and a Lewis acid, the anion interacts chemically with one or more molecules of the latter. Thus, up to 4 equiv of SbF5 has been recommended for conversion of alkyl halides to the corresponding carbocations (38). For those systems, the anion stabilization is accomplished by chemical bonding between the Lewis acid and the anion. The existence of carbocations in such solutions has been demonstrated by various physical methods. 

The propagation step is fast, aided by the coulombic interaction between the carbo-cation and the negatively polarized ether oxygen, and the strain in the three-membered ring. Termination may occur by the reaction of the carbocation with an adventitious nucleophile (a base, for example) or, very occasionally, with the anion of the initiator. ... 

Corey and co-workers have shown that solvolysis of optically active 2-exo-norbornyl m-carboxybenzenesulphonate yields a racemic product. If the reaction proceeded via a classical ion, then since the interaction between the carbocation and the carboxylate-anion must be high compared with the Wagner-Meerwein rearrangement (this is not always true, see ), some optical activity would be retained. But the solvolysis via a classical ion implies such a fast interconversion of classical ions that Corey, for chemical purposes, takes the ion as symmetrical. 

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