Vinylindoles, radical-cations

Cycloadditions between 2-vinylindoles as heterodienes and acceptor substituted enamines can be induced by formation of 2-vinylindole radical cations either via anodic oxidation or photoelectron transfer. In this way pyridoindoles and indolo-naphthyridines are formed in one step under complete regio- and stereochemical control [Eq. (24b)] [248b]. 

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]. 

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]. 

Formation of a cation radical of the diene may catalyze Diels-Alder reactions with certain dienophiles thus oxidation of substituted vinylindoles to the cation radical in the presence of a push-pull dienophile may form different heterocyclic compounds by the Diels-Alder reaction, for instance pyrido[l,2i7]indoles from vinylindoles and yS-enamino-esters [33]. Similarly, anodic oxidation in MeCN of certain oxazolidines in presence of vinyl ether leads to derivatives of oxazepines in a catalytic reaction in which the ring-opened radical cation of the oxazolidine adds to the vinyl ether and the radical cation of the resulting oxazepine oxidizes the oxazolidine [34]. 

In the electron-transfer-sensitized reaction of the 2-vinylindole 27 with dienes it has been found that the indole rather than the diene usually [41c], although not always [41b], becomes the diene partner in the 2-1-4 cycloaddition reaction styrenes also add to 27 in this manner and the initial adducts are further oxidized to the carbazole 28 under the reaction conditions (Scheme 11) [41c]. Formation of the indole radical cation and its attack on the styrene to give intermediate 29 explains the regioselectivity observed. 

Guertler et al. (1996) described a wide range of cycloaddition reaction between 2-vinyl indoles acting as heterodienes and cyclic or acyclic enamines bearing acceptor groups in (3 positions. The reaction was induced by the formation of 2-vinylindole cation-radicals through anodic oxidation. The synthesis of 4a-carbomethoxy-6-cyano-5,7-dimethylindolo[l,2-a]-l,2,3,4,4a,12a-hexahydro-1,8-naphthyridine can serve as an example (Scheme 7.24). 

Indoles, which are especially electron-rich and thus unsuitable for ordinary Diels-Alder reactions, have performed successfully in the cation-radical reaction as dienophiles (Scheme 44)107 and as dienes (Scheme 45)126. Interestingly, the site of annulation (across the C—C or the C—N bond) in vinylindole cation radicals (functioning as dienes for eneamine dienophiles) may be manipulated by varying the substituent on the enamine and thereby altering its push-pull nature (Scheme 45). 

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