Cation Radical Cyclizations

Anodic oxidation reactions have been utilized to reverse the polarity of enol ethers and to initiate radical cation cyclizations. As shown below, the ketene acetal 97 is oxidized on a... 

Fig. 18 Crossings of in-plane and out-of-plane frontier MOs in radical-anionic Bergman and C1C5 cyclizations (crossings for photochemical, dianionic and radical-cationic cyclizations involve the same MOs but differ in the number of electrons).

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

Fig. 22 Iron(ni)-catalyzed oxidative radical cation cyclizations (only major diastereomer 92 shown)...

Demuth et al. recently developed an efficient radical cationic cyclization of functionalized polyalkenes using 1,4-dicyano-tetramethylben-zene (DCTMB) as an acceptor and biphenyl (BP) as co-sensitizer [43]. The transformation is in general highly stereo- and chemoselective and the substitution pattern of the polyalkene allows the construction of either five-or six-membered rings (Sch. 21). So far, this methodology has been applied for the synthesis of several natural products such as stypoldione, hydroxyspongianone and abietanes, respectively [43]. 

Photocyclodimerization of electron-rich alkenes via photoinduced electron transfer is a useful method for preparation of cyclobutanes. A commonly accepted mechanism for this photoreaction is shown in Scheme 19. The radical cation of an alkene reacts with another neutral alkene to give a dimer radical cation. The dimer radical cation (1,4-radical cation) cyclizes, after one-electron reduction, to give a 1,2-disubstituted cyclobutane. 

Similarly, at a carbon anode in 1 1 MeOH-THF, anodic cyclization of allylsilane enol ether (XCIX) proceeded stereoselectively to give (C) [Eq. (63)]. The use of allyl silanes as the unsaturated nucleophilic component in such radical-cation cyclizations proved to be beneficial, though the exact mechanistic details remain somewhat speculative [147]. The method represents an improvement over earlier methods involving anodic cyclization of alkenyl-substituted enol acetates [148]. 

Vinyl ether radical-cation cyclizations using appended electron-rich aryl rings as the nucleophilic component have also been explored [149]. The nature of the products obtained was found in part to be a function of the position of electron donors on the... 

The photooxidation of m-unsaturated a-alkylsilylamines by the singlet excited state of DCN proceeds with selective desilylation to generate a-amino radicals from a-alkylsilylamine radical-cation. Cyclization of the a-amino radicals leads to substituted pyrrolidines and piperidines [42] (Scheme 20). 

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