Carbocation heteroatom-stabilized

Heteroatom stabilized carbocations, NMR spectroscopy, 156-158 halomethyl cation, 156... 

Some of the most important reaction intermediates in organic chemistry are the carbocations. Neglecting some heteroatom-stabilized cations, most carbocations are divided into two groups trivalent carbenium ions and five-coordinate or higher coordinate carbonium ions. The parent carbenium ion is CHJ, and the parent carbonium ion is CHJ. Carbonium ions have been proposed as reactive intermediates in superacid-catalyzed reactions however, they have never been directly observed in condensed media. In contrast, a variety of carbenium ions have already been prepared in superacidic media and been characterized by various physical methods, mainly 13C NMR spectroscopy (5). 

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabibzed carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetrakis(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

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. 

Figure 5.9 Heteroatoms stabilize carbocations better than hyperconjugation effects.

The stability of the carbocation is also increased due to the presence of heteroatom having an unshared pair of electrons, e.g. oxygen, nitrogen or halogen, adjacent to the cationic centre. Such carbocations are stabilized by resonance. The methoxymethyl cation is obtained as a stable solid, MeOCH2 SbF4. ... 

Whether the nucleophile attacks the carbon or the heteroatom attacks the electrophilic species, the rate-determining step is usually the one involving nucleophihc attack. It may be observed that many of these reactions can be catalyzed by both acids and bases.Bases catalyze the reaction by converting a reagent of the form YH to the more powerful nucleophile (see p. 490). Acids catalyze it by converting the substrate to an heteroatom-stabilized cation (formation of 3), thus making it more attractive to nucleophilic attack. Similar catalysis can also be found with metallic ions (e.g., Ag ) which act here as Lewis acids. We have mentioned before (p. 242) that ions of type 3 are comparatively stable carbocations because the positive charge is spread by resonance. 

A nonbonding lone pair on a heteroatom stabilizes a carbocation more than any other interaction. A resonance structure can be drawn in which there is a ir bond between the positively charged heteroatom and the adjacent cationic center. The heteroatom must be directly attached to the electron-deficient C for stabilization to occur if not, destabilizing inductive effects take over. Even if the heteroatom is directly attached, if geometric constraints prevent overlap between the two orbitals, as in the bicyclic carbocation shown, then no stabilization occurs, and inductive effects take over. 

Common error alert When a heteroatom stabilizes a carbocation by sharing its lone pair, it still has its octet and hence is neither electron-deficient nor electrophilic (Remember that when an electronegative atom has a formal positive charge and its full octet, the atoms adjacent to it are electrophilic.) Electronegative atoms like N, O, and F are capable of stabilizing carbocations by resonance exactly because they do not surrender their octet when they participate in resonance. 

Both alkyl radicals and carbocations are electron-deficient species, and the structural features that stabilize carbocations also stabilize radicals. Alkyl radicals are stabilized by adjacent lone-pair-bearing heteroatoms and it bonds, just as carbocations are, and the order of stability of alkyl radicals is 3° > 2° > 1°. However, there are two major differences between the energy trends in carbocations and alkyl radicals. 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

These secondary interactions are present even in enzymes such as oxidos-qualene cyclase, which can stabilize the cationic intermediates by cation-rc interactions. However, in the design of these catalysts a critical element needs to be present. In many of the catalysts designed by Jacobsen, the presence of aromatic group is useful to direct the stereochemical results, in particular an increase in the efficiency of the reaction was observed when large arenes were employed [86]. Most of the work described by Jacobsen was directed to the generation of heteroatom-stabilized carbocations, such as N-acyliminium and oxocarbenium ions in addition, the alkylation of branched aldehydes with bromo diaryl derivatives was also described (Scheme 26.13) [87]. 

A wide variety of carbocations and carbodications, including those that are aromatically stabilized as well those as stabilized by heteroatoms, were reported in the nearly 200 publications on the topic during my Cleveland years. 

Another structural feature that increases carbocation stability is the presence, adjacent to the cationic center, of a heteroatom bearing an unshared pair," for example, oxygen," nitrogen," or halogen. Such ions are stabilized by resonance ... 

On the contrary sp2-hybride phenyl and vinyl carbocations are less stable in comparison with their sp3-hybride analogues. The resonance notably increases ion stability in the case of participation of heteroatomic n electrons (Scheme 5.4). 

Changing the emphasis to synthetic chemistry in superacid media, in Chapter 8, D. Klumpp examines the chemistry of dicationic electrophiles, demonstrating their enhanced reactivity via heteroatom protonation. In Chapter 9, S. Ito et al. explore the potential utility of stabilized carbocations for designing redox-active chromophores. 

The results so far suggest that stabilization of the intermediate carbocation by an adjacent heteroatom, aromatic ring or by steric factors stimulates fluorination of a CLH bond by sulfur tetrafluoride. 

Other stable carbocations are those with an adjacent heteroatom, e.g. oxygen, which can stabilize the cation through resonance (Scheme 5.8). 

Significantly slower rates are found only for compounds that do not exhibit any aromatic ring or carbon-carbon double bond, and for aliphatic compounds with no easily abstractable H-atoms. Such H-atoms include those that are bound to carbon atoms carrying one or several electronegative heteroatoms or groups. (Note that the stabilization of a carbon radical (R ) is similar to that of a carbocation.) We will come back to such structure-reactivity considerations in Section 16.3, when discussing reaction of HO" with organic pollutants in the gas phase (i.e., in the atmosphere). 

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