Cerium -mediated radical reaction

More recently, cerium(IV)-mediated radical reactions were extended to additions to alkenes [3]. One of the many examples from the pioneering work of Nair et al. is the addition of dimedone Ic to the cyclohexene derivative 8 [7]. In the first step, adduct radicals 9 are formed, which are rapidly oxidized to the cation 10 by CAN. Intramolecular trapping of this intermediate by the carbonyl group affords after deprotonation the dihydrofuran 11 in excellent yield (Scheme 3). 

Compared to iron(III), copper(ll), and especially manganese(III) and cerium(IV) other metals have found less application for the oxidative generation of radicals [1]. An exception is cobalt(III)-mediated radical reactions, based on the pioneering work of Iqbal et ah, which was recently reviewed [20] (see also Volume 1, Chapter 1.8). Some examples of oxidative couplings of silyl enol ethers 44 in the presence of silver(I) oxide were developed [21]. However, there is no advantage over copper(II)-mediated radical reactions, since the reagent is more expensive and the 1,4-diketones 45 are isolated in only moderate yield (Scheme 15). 

Braslau and coworkers [83] synthesized cyclic PSTY through the combination of nitroxide-mediated radical polymerization (NMRP) and CuAAC click reaction. The synthesis procedure was relatively complex compared with other strategies. 1 - [4-(Chloromethyl)phenyl] ethyl alkoxyamine was used to mediate the styrene polymerization, followed by successive azidation and oxidative cleavage with ammonium cerium(IY) nitrite in the presence of propargyl alcohol. The azide and alkyne groups were then introduced to each end of the polymer. Finally, the cyclization reaction was carried out in toluene with CuBr and PMDETA as catalyst at 100°C (Scheme 35). The cyclization results showed about 64% click product, as derived from Gaussian curve fitting. 

To date, several protocols for Knoevenagel additions to protected carbohydrates or carbohydrate-derived molecules have been reported in the literature [36]. For a manganese(III)- or cerium(IV)-mediated radical addition of malonates to protected carbohydrates, see Reference 37. For early reactions of protected carbohydrates with malonic acid in basic medium (pyridine), see Reference 6j. 

The cerium(IV)-mediated generation of heteroatom radicals by oxidation of anions such as azides was discovered many years ago [10], However, Nair et al. applied this strategy for a C-S bond-forming reaction by oxidation of ammonium thiocyanate le only recently [11], Addition of thiocyanate radical to indole 18 provides an intermediate radical, which is further oxidized to the cation by CAN. Loss of proton from the cationic intermediate provides the substituted arene 19 in excellent yield (Scheme 6). 

Compared to the intramolecular aromatic alkylation with nucleophilic radicals, the analogous process with electrophilic radicals is far less common. Citterio carefully studied the Mn(OAc)3-mediated intramolecular homolytic aromatic substitution of various dialkyl malonates [71, 73]. He showed that the reaction is well suited for the formation of 5- (see 45), 6- (see 46) and 7-membered benzanellated rings (see 47). For cyclizations forming a 6-membered ring, high yields were obtained in the alkylation of electron-rich as well as electron-poor arenes. However, the formation of the 7-membered ring occurred only with electron-rich arenes. Cerium(IV) ammo-... 

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