Heteroatom-stabilized carbocations

In contrast to hydrocarbon cations, heteroatom-substituted carbocations are strongly stabilized by electron donation from the unshared electron pairs of the heteroatoms adjacent to the carbocation center  

The stabilizing effect is enhanced when two, or even three, electron-donating heteroatoms coordinate with the electron-deficient carbon atom. 

The experimental 13C NMR chemical shift of the simplest member of the acylium ions, the formyl cation [HCO]+ was reported as S = 139.5 (measured under CO pressure of 85 atm) which compares well with the GIAO-MP2 calculated shift of S — 136ppm.116,117 The analogous fluoroformyl cation [FCO]+ and protonated fluoroformic acid [FC(OH)2]+ were characterized by 13C NMR spectroscopy, experimentally118 ((5=117.5 and 157.8 ppm, respectively) as well as computationally (S = 118.6 and 170.6ppm, respectively).119 

The 13C, 15N and 170 NMR chemical shifts of some substituted methyl cations and the corresponding protonated dications were calculated by the GIAO-MP2 method for MP2/6-31G(d) optimized geometries.129 

The o-, m- and /)-phenylenebis(l, 3-dioxolanium) dications 101 and the related tris(l,3-dioxolanium) trication have been prepared and the calculated 13C and 170 NMR chemical shifts (GIAO-DFT) closely match the experimental values.130 

The calculated 13C NMR chemical shift of the carbonyl carbon of monoproton-ated benzaldehyde131,132 for the. E-form 102 (205.5 ppm) and that for the Z-form 103 (207.4ppm) agree well with the experimental shifts of 203.5 and 205.9 ppm, respectively. Protonation of a-substituted cinnamic acids such as 104 was studied by 13C NMR spectroscopy and IGLO-HF calculations.133 Protonated deltic acid (105) and related compounds,134,135 as well as protonated urea 106 (X = O)136 and thiourea 106 (X = S)137 have been investigated by 13C NMR spectroscopy and quantum chemical calculations.138 

GIAO-MP2 calculated NMR chemical shifts for DFT optimized geometries and comparison with experimental data were used to study the site of protonation of dimethyl sulfoxide.139 The calculated 13C NMR chemical shift of O-protonated DMSO 107 (40.0 ppm) matches with the experimental value of 34.3 ppm. The calculated 13C NMR chemical shift of S-protonated DMSO 108 is 3 ppm deshielded compared to that calculated for 107. 

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

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... 

Figure 5.9 Heteroatoms stabilize carbocations better than hyperconjugation effects.

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]. 

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. 

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). 

Halogen as Heteroatom. In 1966 Olah, Cupas, and Comisarow511 reported the first a-fluoromethyl cation. Since then, a large variety of fluorine-substituted carbocations have been prepared. a-Fluorine has a particular ability to stabilize carbocations via back-donation of its unshared electron pairs into the vacant p orbital of the carbocationic carbon atom. 19F NMR spectroscopy is a particularly efficient tool for the structural investigations of these ions.512,513 The 2-fluoro-2-propyl cation 247 (NMR spectra, Figure 3.16) and 1-phenylfluoroethyl cation 248 are representative examples of the many reported similar ions.514... 

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. 

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. 

The normal pathway for rearrangement of the substrates (1) is via loss of the heteroatom substituent with migration of an adjacent alkyl (or aryl) group, and concomitant ketone formation (Scheme 1, path a). An alternative migration (path b), is occasionally observed in protic media, i.e. protonation of the alcohol (1) and migration with loss of water generates the stabilized carbocation (2), which is then hydrolyzed. 

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. 

This reaction is a minor variant of the Ac1e2 in which the electrophile, usually a proton, attacks the lone pair rather than the less available heteroatom-carbon pi bond. The lone pair is protonated, path p.t., to give a highly polarized multiple bond that can be viewed as a stabilized carbocation. The nucleophilic attack on this carbocation could also be viewed as path An, trapping of a cation by a nucleophile, instead of path AdN-... 

Lone-pair-bearing heteroatoms strongly stabilize carbocations via resonance or ar-overlap, as shown below ... 

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). 

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