Electron transfer processes catalyzed cycloadditions

For this comparison die [4 + 2] cycloadditions of methyl vinyl ketone to cyclopentadiene and to furan are chosen as reference reactions (Scheme 60, Table 9). Smectile clays, with the interlayer cations exchanged by CH" or Fe" ions, catalyzed the addition of cyclopentadiene in providing the endo product (258 Y = CH2) at a drastically reduced reaction temperature in high yield together with 10% of the exo product (259 Y = CH2) (entries 3,4). These results are comparable with those obtained under aqueous reaction conditions (entry 2), which supports the idea that die presence of water pockets in clays could account for their catalytic activity. The Lewis acidity as well as one-electron-transfer processes, involving the internal Fe " or Ci" cations, have also been invoked as possible explanations. 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

A formal iron-catalyzed [3 + 2]-cycloaddition of styrene derivatives with benzoqui-none was reported by Itoh s group [96]. The process is believed to proceed via electron-transfer reactions mediated by a proposed Fe3+/Fe2+ couple, which generates a styrene radical cation and a semiquinone. These intermediates undergo stepwise addition to yield the benzofuran product 51 (Scheme 9.38). The reaction seems to be limited to electron-rich alkoxy-functionalized styrenes, as the Fe3+/Fe2+ redox couple is otherwise unable to transfer the electrons from the styrene to the quinone. 

Bauld and coworkers studied the [2+2] cycloaddition of A-vinyl carbazoles 86a and electron-rich styrenes 86b catalyzed by iron(III) catalysts A or B in the presence of 2,2 -bipyridine as a ligand, which was reported originally by Ledwith and coworkers (Fig. 21) [142, 143]. Deuterium-labeling studies provided support for the stepwise nature of the process, consisting of reversible SET oxidation of the electron-rich olefin to a radical cation 86 A. Nucleophilic addition of excess 86 leads to distonic radical cation 86B, which cyclizes to cyclobutane radical cation 86C. Back electron transfer affords cyclobutanes 87 and regenerates the catalyst. Photoelectron transfer catalysis gave essentially the same result, thus supporting the pathway. 

Thus the first electron transfer to Pb relates to the reaction (a) in Section 7.4.3.1.1, and the second involves the oxidation of the cyclobutyl radicals either by electron transfer/deprotonation with Cu" in equation (17) or by ligand transfer of chlorine with PlAci in equation (18). When the product of a generic reaction is itself a radical cation (such as in Sections 7.4.3.1.8 and 7.4.3.1.9), an electron-transfer chain or ETC process can ensue, as in the hole-catalyzed cycloadditions and autoxidations of dienes,The electron-transfer propagation sequence for the latter is simply given as in equations (19) and (20). 

Due to the presence of an electron-withdrawing group on the dipolarophile, these processes are classified as type 1 reactions. The process involves the transference of charge from the dipole to the dipolarophile. When catalyzed by metallic compounds, coordination of the dipolarophile is highly desired. Usually, coordination of a nitrone to the Lewis acid is more feasible than coordination of a carbonyl compound. For this reason, alkenes that enable a bidentate coordination to the Lewis acid, such as 3-alkenoyl-oxazolidinones (Scheme 5), have been frequently employed as a model system to smdy the metal-catalyzed 1,3-dipolar cycloaddition... 

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