Triplex Diels-Alder reaction

Diels-Alder reactionsBoth 1,4-dicyanonaphthalene (DCN) and 2,6,9,10-te-tracyanoanthracene (TCA) have been used as sensitizers to effect photochemical [4 + 2]cycloadditions of electron-rich dienes and electron-rich dienophiles, which do not normally undergo thermal cycloadditions. These cycloadditions are known as triplex Diels-Alder reactions because they are postulated to involve as an intermediate a three-membered complex of sensitizer, dienophile, and diene. This reaction is useful for synthesis of bicyclo[2.2.2]octenes from some silyl enol ethers, alkenes, or arylalkynes. 

In 1983, Jones et al. [104] reported on the photosensitized [4 + 2]-cyclodimeriza-tion of 1,3-cyclohexadiene. They found evidence for a tight complex as an intermediate of the Diels-Alder adducts rather than free radical cations. These findings extended an earlier work of Libman [109] but, however, left a decision open pro or against a triplex intermediate. In a series of papers, Schuster and coworkers elegantly extended these preliminary discoveries to its generalization as the Triplex-Diels-Alder Reaction . 

A triplex has been proposed as an intermediate in some photoreactions. Yang and co-workers [54] found that addition of 1,3-dienes to anthracene excimers leads to different products than does its adition to monomeric excited arenes. Lewis and co-workers [55, 56] found that stilbene excimers can be intercepted by dimethyl fumarate to give an oxetane through a presumed triplex. More recently, Schuster and co-workers [57,58] studied the Triplex Diels-Alder reaction of 1,3-dienes with enol, alkene, and acetylenic dienophiles. Take 9,10-dicyanonaphthalene (DCN)/indene (IN)/l,3-cyclohexadiene (CHD) system as an example ... 

In contrast, until very recently only a few studies of asymmetric photocycloadditions by chiral sensitizers have been reported. Kim and Schuster achieved enantiomeric excesses of up to 23% ee in the sensitized [4+2]-cycloaddition of trcms-P-methylstyrene 34 with cyclohexa-1,3-diene (35), which represents the first reported enantioselective photocycloaddition reaction. This reaction is called the triplex Diels-Alder reaction, as it was shown to occur by formation of a ternary complex of the singlet excited sensitizer, (-)-l,l -fcis(2,4-dicyanonaphthalene) 36, with the diene and the dienophile. The Diels-Alder product 38 is formed in up to 11% chemical yield (Scheme 13) other products are the dimers of cyclohexa-1,3-diene (83%) and [2+2]-cycloadducts of 34 and 35 (5%). ... 

Akbulut, N., Hartsough, D., Kim, J.-L, and Schuster, G. B., Xhe triplex Diels-Alder reaction of 1,3-dienes with enol, alkene and acetylenic dienophiles scope and utility, /. Am. Chem. Soc., 54, 2549, 1989. 

This review article deals primarily with addition reactions of nucleophiles, electrophiles, and neutral radicals to photochemically generated radical ions of organic compounds and some organometallic compounds. Photocyclodimerizations of electron-rich alkenes, photo-Diels-Alder reactions between alkenes and alkadienes via dimer or heterodimer radical cations, and photocycloadditions via triplexes are also included. 

Schuster proposed a triplex as a key intermediate for the inter- and intramolecular photo-Diels Alder reaction in nonpolar solvents in the presence of an electron accepting sensitizer [105-107]. This photoreaction can be applied to an enantioselective photo-Diels Alder reaction between 1,3-cyclohexadiene and propenylbenzene by the use of a chiral sensitizer (Scheme 32) [108]. 

---