Cation 2 + 2-cycloaddition

The total synthesis of cannabicyclol, clusiacyclol A and B, iso-erio-brucinol A and B and eriobrucinol involves an oxa-[3 + 3] annulation for the construction of the chroman nucleus and a stepwise cationic [2 + 2] cycloaddition for the cyclobutane formation (13OL3130).The synthesis of... 

SCHEME 12.28 Cationic [2+2] cycloaddition of daurichromenic ester 118a. 

The gas-phase reactions of the fulvene radical cation with neutral 1,3-butadiene, alkenes and 2-propyl iodide have been investigated by Russell and Gross131a using ICR mass spectrometry. Unlike ionized benzene, ionized fulvene undergoes no C—C coupling with 2-propyl iodide. On the basis of deuterium and 13C labelling, the reaction of ionized fulvene with 1,3-butadiene was suggested to occur by [6 + 4] cycloaddition to yield tetrahydroazulene radical cations. Cycloadditions of neutral fulvene were also studied in this work. 

The same group studied the radical cation cycloaddition of 2-vinylbenzofti-rans with various alkene and diene compounds initiated by photoinduced electron transfer. Depending on the unsaturated compound used, yields up to 60% were feasible. In contrast to 2-vinylindoles, 2-vinylbenzofurans prefer to react as die-nophiles, very similar to styrenes [82]. 

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

A wide range of variously substituted thiochromans has been obtained from the readily accessible ot-(benzotriazo-lyl)methyl thioethers by their Lewis acid-catalyzed reaction with styrenes. Initial loss of the benzotriazole unit generates a thionium cation which undergoes an efficient cationic cycloaddition to the alkene. The reaction, which generally proceeds with high diastereoselectivity, is considered to occur in a stepwise manner rather than as a concerted [4++2] process (Scheme 174) <2001JOC5595>. 

SCHEME 4.1 Thermodynamic representation of radical cation cycloaddition reaction acceleration. 

As a result, the transition state of the radical cation cycloaddition is much more reactant-like and the activation energy will be much smaller than in case of the cycloadditions between two neutral reactants. Finally, the thermodynamic driving force for this radical cation/neutral addition is equal to the large difference in ionization potentials between the bonded and the nonbonded orbitals. 

As shown in Table 4.1, formation of the mixed adduct is favored over homodimerization of 8a with the simple styrene 13a, but this selectivity is inverted for the case of the more bulky dienophile tra 5-[3-methylstyrene 13b, presumably due to steric effects. Although the overall reaction is highly exothermic on the radical cation surface, the reaction is not insensitive to steric effects. Chemoselectivity in the radical cation cycloaddition is largely a consequence of a substrate s ability to stabilize the radical cation of the oxidized species through the formation of a weakly bound ion-molecule complex. Such complexes have been known for a long time in gas-phase... 

TABLE 4.9 Dimerization of 8a SELECTIVITY IN RADICAL CATION CYCLOADDITIONS Ratio 9a/10a in CH3CN Sensitized by 6 ... 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Enaminoesters, -ketones, and -nitriles have also successfully been applied in electro-chemically induced radical cation cycloaddition reactions, especially with 2-vinylindoles or 2-vinylpyrroles. Some examples are given in Eq. (31). Depending on the structures, the reaction starts with the formation of either the vinylindole or vinylpyrrole radical cation or the radical cation of the enamidoester, -ketone, or -nitrile. In Eq. (32), a representative reaction pathway for the cycloaddition between a 2-vinylpyrrole and an acceptor-substituted enamine is formulated via the enamine radical cation [159]. 

Problem 4.13. What do the Woodward-Hoffmann rules suggest about the facial reactivity of the components of the following thermal [6 + 4] cycloaddition and thermal [4 + 3] cationic cycloaddition ... 

Cationic cycloadditions. Ionization of p-benzoquinone monoacetals is induced by MejSiOTf in highly polar media (LiClO /EtOAc) and the carbocations are captured in situ by alkenes to give bicyclo[3.2.1]octene derivatives. A synthesis of 2-(l -phenylethoxy)-8-oxabicyclo[3.2.1]oct-6-en-3-one in good enantiomeric purity from furan and a mixed acetal of 2-(triethylsiloxy)acrolein (prepared from pyruvaldehyde dimethylacetal in three steps) is promoted by MejSiOTf. 

Butadiene Radical Cations. Cycloaddition reactions between the radical cations of... 

Giguere, R.J., Duncan, S.M., Bean, J.M., and Purvis. L.. Intramolecular [3 + 4] allyl cation cycloaddition. Novel route to hydro azulenes. Tetrahedron Lett.. 29. 6071. 1988. 

Kuja, E., and Giguerc, R.J., On the effect of diene geometry in intramolecular allyl cation cycloadditions, Synth. Commun., 25, 2105, 1995. 

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