Oxenium ion

An unusual cationic domino transformation has been observed by Nicolaou and coworkers during their studies on the total synthesis of the natural product azadirachtin (1-105) [30]. Thus, exposure of the substrate 1-106 to sulfuric acid in CHjClj at 0°C led to the smooth production of diketone 1-109 in 80% yield (Scheme 1.27). The reaction is initiated by proto nation of the olefinic bond in 1-106, affording the tertiary carbocation 1-107, which undergoes a 1,5-hydride shift with concomitant disconnection of the oxygen bridge between the two domains of the molecule. Subsequent hydrolysis of the formed oxenium ion 1-108 yielded the diketone 1-109. 

A fourfold anionic sequence which is not initiated by a Michael but an aldol reaction has been reported by the group of Suginome and Ito (Scheme 2.129) [295]. In this approach, the borylallylsilane 2-573 reacts selectively in the presence of TiCl4 with two different aldehydes which are added sequentially to the reaction mixture. First, a Lewis acid-mediated allylation of the aldehyde with 2-573 takes place to form a homoallylic alcohol which reacts with the second aldehyde under formation of the oxenium ion 2-574. The sequence is terminated by a Prins-type cyclization of 2-574 and an intramolecular Friedel-Crafts alkylation of the intermediate 2-575 with formation of the fraws-1,2-be rizoxadeca lines 2-576 as single diastereomers. 

Quite recently, Mattay and coworkers reported on several related photochemi-cally induced domino processes for the synthesis of steroids [16]. In these cases, the initially formed radical cation is stabilized by formation of an oxenium ion. For in-... 

The addition of zinc organometallics to in situ generated oxenium ions by the reaction of mixed acetals with TMSOTf " allows a highly stereoselective preparation of protected a r/-l,3-diols of type 223 (Scheme 66). The addition of TMSOTf triggers also the allylic substitution of glycal derivatives, providing the substitution products of type 224 with excellent regio- and diastereoselectivity. 

Oxenium Ions. Oxenium ions similar to nitronium ions are, in general, too reactive to be observed. The parent ion—that is, the hydroxyl cation HO+—is elusive, and it is improbable that it can be observed in its free form in the condensed state. However, the incipient hydroxyl cation is involved in acid-catalyzed electrophilic hydroxylation with protonated (or Lewis acid complexed) hydrogen peroxide (HO—OH2+) or ozone (HO—O—0+).125 Nitrous oxide is also apotential precursor for the hydroxyl cation (in its protonated form). The hydroxy diazonium ion HON2+ has not yet been observed. 

Starting with phosphite 59 stannous triflate catalyzes the formation of an oxenium ion 60, which is stabilized by the solvent (61). The complex 61 is more favored than 63, justified by the reverse anomeric effect until now. But as it is shown there is no possibility for 61 to react in an SN2 reaction to 62. The intermediate 63 can be attacked by the alcohol functionality of the glycosyl acceptor, and with MeCN as leaving group you get 64. Ultimately the equilibrium between 61 and 63 shifts completely to 63. This kind of stereocontrol during a glycosylation is known as the nitrile effect.28... 

The entries in rows 3-5 in Table 14.1 refer to one-step eliminations/rearrangements of this type in which oxenium ions are avoided. 

Carbocations occupy a unique place in the world of electron-deficient cations. Electron-deficient nitrogen and oxygen compounds (nitrenium and oxenium ions, respectively) are extremely unstable and are rarely or never seen. Electron-deficient boron and aluminum compounds are quite common, but these species are not cations. Silenium ions (R3Si+) are extremely unstable kinetically still, it is useful to think of the R3Si+ group as a large, kinetically more stable H+. 

In the biosynthesis of the characteristic methylenedioxyphenyl group a decisive role is attributed to the oxenium ion, which is formed from the properly substituted... 

The p-hiphenyloxenium ion, isoelectronic with the nitrenium ion, has been generated by elimination of ammonia from the corresponding 0-aiylhydro q lamine chloridrate and directly spectroscopically characterized. The open-shell triplet and closed shell oxenium ions and the product radical have been related as shown in the Scheme 12. ... 

Sol 5. (i) The initial step involves the formation of an oxenium ion by the coordination of the Lewis acid to the carbonyl group of (R)-citronellal. This then undergoes an intramolecular carbonyl ene reaction via a chair-like transition state having all the substituents in the equatorial positions to give pure 1-isopulegol. 

Calculations indicate that the position of oxidation of substituted phenols by PIDA and phenyliodonium bis(trifluoroacetate) is in accord with the intervention of phen-oxenium ions as intermediates. The lack of induction of chirality observed in the reaction, whether using a preformed chiral iodonium reagent or a homochiral alcohol as the medium, also supports this hypothesis. ... 

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