Cations oxocarbenium ions

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... 

Prakash GKS, Rasul G, Liang G, Olah GA (1996) 13C NMR spectroscopic and Density Functional Theory (DPT), ab initio, and IGLO theoretical study of protonated cycloalkyl-carboxylic acids (carboxonium ions) and their acyl cations (oxocarbenium ions). J Phys Chem 100 15805-15809... 

Finally, a chiral 2-phenylpyrrolidine-derived thiourea was demonstrated to be an efficient organocatalyst to promote the clean substitution of 1-chloro-isochromans by silyl ketene acetals to provide the corresponding chiral substituted isochromans with high yields and enantioselectivities of up to 90% ee. Actually, the 1-chloroisochromans were prepared from the corresponding more stable methyl acetals, which were directly used without purification in the protocol evolving through a cationic oxocarbenium ion (Scheme 5.13). 

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

A semiquantitative procedure used to estimate the lifetimes of carbocations and oxocarbenium ions by using diffusion-controlled trapping of the cations by nucleophiles . Ions of intermediate stability react with azide ions at a constant, diffusion-controlled rate and react with water by an activated process. The ratio of the products obtained from the azide path and the water path is dependent on the electronic characteristics of the cation. 

Quantum mechanical calculations show that the silyl cation (19) has a twisted structure, and that the a-C02 group provides substantial electrostatic stabilization.58 Isotope effects for its formation reaction are also reported.58 Evidence is provided for the stabilization of incipient oxocarbenium ions by axial electronegative substituents, as in (20) the presence of the most electronegative substituent results in the fastest reaction.59 Lewis acid-promoted cleavage of spirocyclic dioxanes such as (21) involves oxonium ions, and high axial vs equatorial product selectivities are possible with the correct choice of Lewis acid and nucleophile.60 Reactions which lead to 1,3-dioxenium salts have been reviewed.61... 

The facile synthesis of (24) from (22) has the oxonium cation (23) as an intermediate.62 Vinyl oxocarbenium ions are reported to take part in intermolecular... 

Carboxylic acid anhydrides cleave immediately when protonated even in HSC F-SbFs solution at low temperature (—80°C).541 569 Monoprotonated anhydrides 291, however can easily be generated reacting oxocarbenium ions (acyl cations) with carboxylic acids in SO2 solution. Rapid intramolecular proton transfer between the carbonyl groups was observed by NMR. 

If alkenes are used instead of silanes, the intermediate oxocarbenium ion undergoes an acetal -ene reaction. An example is the reaction of acetal 11 with methylenecy-clohexane to give the cyclohexenylmethyl-substituted product 12 (Scheme 8.4) [20]. Silenol ethers are electron-rich alkenes particularly suited for addition to cationic species. Pinacolone-derived enol ether 13, for example, adds to an thioxocarbenium ion generated in situ from S,S-acetal 14 to give thioether 15 (Scheme 8.4) [21]. 

Using a binary sensitizing system (phenanthrene P/DCNB /7-dicyanobenzene) in acetonitrile solution, O-aryl glycosides are transacetalized with alcohols after generation of aromatic radical cations [23], According to kinetic anomeric effects, the a-side attack of nucleophiles to cyclic oxocarbenium ions follows scheme 9. 

The best result so far available through a non-EG-acid approach has been obtained with 100% formic acid leading to the desired 69 in 32% yield after alkaline hydrolysis. In contrast, the electrolysis of 67 in an acetone—LiC104—(Pt) system at constant current (3.3 mA/cm2 for 1 h, 0.36 F/mol) in an undivided cell affords the desired alcohol 69 in 52 % yield together with 68 (25 %) (Scheme 3-25). The effect of the solvent is remarkable. Acetone is suitable for the selective preparation of 69, presumably because it traps the cationic intermediate, leading to an oxocarbenium ion, which... 

The putative cation that would be generated by C—N bond cleavage in D would contain an O atom with a very weak pi electron donating ability (+M effect). This is because the z BUjAl group, which binds to this O atom, acts as a Lewis acid. Consequently, the cation in question would be an acceptor-substituted carbenium ion (with a C—O single bond) rather than an oxocarbenium ion (with a C=0 double bond). 

The Au-catalysed [3,3] sigmatropic rearrangement of propargyl propynoates 29 leads to 5-vinylpyran-2-ones probably by way of a cationic intermediate 30 (Scheme 23). This oxocarbenium ion can be intercepted by electron-rich arenes and heteroarenes with attack occurring at the vinylic double bond leading to more complex pyranones <07AG(E)8250>. 

Arnold s demonstration" that oxocarbenium ion intermediates can be formed through homobenzylic ether radical cation fragmentation reactions shows that mild oxidizing conditions can be used to prepare important reactive intermediates. Scheme 3.2 illustrates a critical observation in the development of an explanatory model that allows for the application of radical cation fragmentation reactions in complex molecule synthesis. In Arnold s seminal work, cleavage of the benzylic carbon-carbon bond in substrate 1 is promoted by 1,4-dicyanobenzene (DCB) with photoirradiation by a medium-pressure mercury vapor lamp. With methanol as the solvent, the resulting products were diphenylmethane (2) and formaldehyde dimethyl acetal (3). 

The situation is completely different in the presence of 150 mol % LiNTf2- Indeed, whether isomer 110 or 111 is used, the formation of a anomer 110 predominates (a /i = >99 <1). It has been proposed that the intermediate oxocarbenium ion 1-3 is involved in the anomerization mechanism. In the presence of the trityl cation, the equilibrium between anomers 110 and 111 might shift to the more thermodynamically stable anomer 111. The significant role of perchlorate and bis(sulfonimide) anions has been consistently claimed. Thus, intermediate 1-3 would be efficiently stabilized by these anions located at the sterically less-encumbered / side of anomeric centers, achieving nucleophilic attack from the a side. It is also suggested that the Li cation is likely to effect the selective stabilization of product a anomer 110 by forming coordination complexes as exemplified by putative chelated structures C-1 and C-2 (Fig. 10). 

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