Heteroatom-Stabilized Cations

The a-cyclopropylcyclopropylidenmethyl cation 28 is the only vinyl cation experimentally accessible50 in superacid solution which enjoys neither re-bond nor heteroatom stabilization of the positive charge. The 13C NMR spectrum measured at -138 °C is shown in Fig. 4. 

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

Cyclopropylcarbinyl-cyclobutyl ring expansions (Eq. 5) are facilitated by the presence of the heteroatom substituent in the order O > S > Se. In this case, the heteroatom stabilized cyclobutyl cation (see Eq. 39) can suffer hydrolysis to give the... 

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). 

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

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... 

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 (246).17,509... 

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. 

A variety of 1,3-diselenolylium an(j, 3-thiaselenolylium ions has been generated by the reaction of protons with 1,3-diselenoles and 1,3-thiaselenoles, respectively. Protonation of an exocyclic double bond at the 2-position with trifluoroacetic acid has generated 1,3-diselenolylium cations (54) and (55) (79JHC1303). The driving force for such reactions is presumably the heteroatom stabilization of the positive charge or formation of the aromatic sextet. 

Electrophilic substitutions of alkenyl-, aryl-, and alkynylsilanes with heteroatom-stabilized cationic carbon species generated by the action of a Lewis or Brpnsted acid (acyl cation, oxocarbenium ion, etc.) provide powerful methods for carbon-carbon bond formation. Particularly, intramolecular reactions of alkenylsilanes with oxocarbenium and iminium ions are very valuable for stereoselective construction of cyclic ether and amine units.21-23 For example, the BFj OEt -promoted reaction of (E)- and (Z)-alkenylsilanes bearing an acetal moiety in the alkenyl ligand gives 2,6-disubstituted dihydropyrans in a stereospecific manner (Scheme l).23 Arylsilanes also can be utilized for a similar cyclization.24... 

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. 

As mentioned in previous sections, carbocation intermediates are subject to rearrangement to a more stable ion (sec. 2.7.B.iii). If the cation were stabilized in some manner, rearrangement would be much less likely, as when a heteroatom is attached to the electrophilic carbon. An example is the oxygen stabilized cation (3041 generated by reaction of a ketone with an acid catalyst. The electrons on the heteroatom are donated to the positive center leading to resonance stabilization (this is called back donation). Such a cation is usually... 

The cyclopropanation of non-functionalized alkenes requires even stronger electrophilic metal carbenes as provided by non-heteroatom-stabilized group 6 carbene complexes 17 or cationic iron carbene complexes the reaction is highly syn-selective (Scheme 18). Iron carbenes bearing optically active phosphine ligands allowed for an efficient enantioselective cyclopropanation. [34]... 

Low energy (55 eV) electron irradiation of multilayers of methanol on silver has been used in conjunction with temperature-programmed desorption of the products as a method to identify radiolysis products. In this case several products such as methoxymethanol were identified . The primary photochemical process in the methanol system is thought to involve the fission of a C—H bond forming a heteroatom stabilized cation such as 6. In addition to methoxymethanol glycolaldehyde (5) or methyl formate are also detected. Apparently the methanol system is unreactive to UV irradiation either at 240 nm or at 355 nm . 

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. 

The two examples portrayed below, each occurring via different mechanistic pathways, give one a sense of the breadth of the chemistry. In the first (Eq. 2), different nucleophiles, viz., azide and methanol, add across a double bond [5]. The more nucleophilic of the pair adds to the less substituted carbon to afford a heteroatom-stabilized cation that is then intercepted by the second nucleophile, ultimately delivering the product (Eq. 3). The second example portrays the replacement of a C-H bond on anthracene with acetonitrile to afford the acetamide adduct after hydrolysis of the nitrilium ion intermediate (Eq. 4) [6]. 

The utility of heteroatom-stabilized aUylic cation intermediates in 4 -i- 3-cycloaddition... 

A review of the use of heteroatom-stabilized cations in 4 -F 3-cycloaddition reactions has been published. 2,6- and 2,2-dichlorocyclohexanones react with base to produce oxyallylic cations, which react with various dienes to yield 4 -F 3-cycloadducts in good... 

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]. 

In view of the overall increased reactivity of furan compared with thiophene it would be anticipated that furan would be less regioselective in its reactions with electrophiles than thiophene. Possible reasons for the high regioselectivity of furan in electrophilic substitution reactions include complex formation between substrates and reagents and the ability of heteroatoms to assist in the stabilization of cationic intermediates (80CHE1195). 

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 ... 

The classification fragmentation applies to reactions in which a carbon-carbon bond is broken. One structural feature that permits fragmentation to occur readily is the presence of a carbon that can accommodate carbocationic character (3 to a developing electron deficiency. This type of reaction, known as the Grob fragmentation, occurs particularly readily when the y-atom is a heteroatom, such as nitrogen or oxygen, that has an unshared electron pair that can stabilize the new cationic center.96... 

The cation radical at the heteroatom generated by one-electron oxidation is stabilized by a silyl group situated at the jS-position. The C-Si a orbital interacts with the SOMO to stabilize the cation radical when they are in the same plane. 

The presence of a heteroatom adjacent to the cationic centre and bearing an unshared pair of electrons, like nitrogen, oxygen or halogen increases stability... 

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