Radicals from Nitrogen Heterocycles

The electrochemistry of S-N and Se-N heterocycles has been reviewed comprehensively. The emphasis is on the information that electrochemical studies provide about the redox properties of potential neutral conductors. To be useful as a molecular conductor the 4-1, 0, and -1 redox states should be accessible and the neutral radical should lie close to the centre of the redox spectrum. The chalcogen-nitrogen heterocycles that have been studied in most detail from this viewpoint... 

The majority of sequential radical reactions deal with cyclizations as the key steps. The constructions of carbocycles, oxygen, and nitrogen heterocycles using (TMSlsSiH as a mediator are many and represents the expansion and importance of these synthetic approaches. For example, Nicolaou and coworkers found that (TMSlsSiH serves as a superior reagent in the radical-based approach toward the synthesis of azadirachtin, an antifeedant agent currently used as an insecticide, and in other related systems. ° ° Here below we collected a number of reactions mostly from the recent work in the area of intramolecular reactions. 

Protonated nitrogen heterocycles can be carbalkoxylated" by treatment with esters of a-keto acids and Fenton s reagent. Pyridine is carbalkoxylated at C-2 and C-4, for example. The attack is by "COOR radicals generated from the esters ... 

The homolytic acylation of protonated heteroaromatic bases is, as with alkylation, characterized by high selectivity. Only the positions a and y to the heterocyclic nitrogen are attacked. Attack in the position or in the benzene ring of polynuclear heteroaromatics has never been observed, even after careful GLC analysis of the reaction products. Quinoline is attacked only in positions 2 and 4 the ratio 4-acyl- to 2-acylquinoline was 1.3 with the acetyl radical from acetaldehyde, 1.7 with the acetyl radical from pyruvic acid, and 2.8 with the benzoyl radical from benzaldehyde. 

Imines have been employed as radical acceptors in the preparation of nitrogen heterocycles [95AJC291]. A highly selective 6-endo-trig radical cyclization onto a chiral aldehyde imine has been developed by Werstiuk et al. Reductive cyclization of 218 provides a 3.8 1 diastereomeric mixture of the tetrahydroisoquinoline 219. A minor amount of 220, a product arising from a 5-exo cyclization is also produced. 

Electron Density. Electron density calculations for 1,6-naphthyridine have been made for comparison with those of other nitrogenous heterocycles and for the rationalization of ionization phenomena and electronic spectra.676 840 1126 1173 Electron Spin Resonance. Radicals derived from 1,6-naphthyridine have been studied,1329 especially with respect to their ESR spectra.1083 Infrared/Raman Spectra. Assignments for the major bands in IR/Raman spectra of 1,6-naphthyridine and related compounds have been reported.44 1124 1251 Ionization. Theoretical calculations for the protonation of 1,6-naphthyridine have been carried out for comparison with those of the other naphthyridines and related heterocycles.806 813... 

Recently, radical species have attracted much attention as useful intermediates for carbon-carbon bond formations. By contrast, in organic synthesis, radicals centered on heteroatoms have not been widely utilized for construction of molecular skeletons with carbon-heteroatom bond formation. In this section we will discuss the generation of alkylideneaminyl radicals, conventionally called iminyl radicals from oxime derivatives by electron transfer reactions. AU -lideneaminyl radicals thus created are utilized for making nitrogen-containing heterocycles. 

Tetrahydropyrrolizines (210) and (211) can be formed via radical scission of either bond a or b in the intermediate vinyl-aziridine (208) but yields are moderate at best and the major product is the monocyclic pyrroline (209), formed by a 1,5-homo-dienyl rearrangement. The route therefore could provide a useful access to the pyrrolizidine framework if conditions could be identified which would allow more controlled and efficient breakdown of (208). The synthesis of substitute pyrrolines by ring expansion of vinyl aziridine derivatives has also been accomplished in a palladium-catalysed reaction. N -Tosyl-2-vinyl five- and six-membered nitrogen heterocycles (213) ace obtained in generally high yields under mild conditions from precursor dienyl nitrogen heterocycles (212) in the presence of a catalytic amount of [Pd(PPh ) ]. The complete diene unit is required for the... 

The Antonchick group demonstrated the oxidative coupling of nitrogen heterocycles with alkanes. °° Combination of phenyliodine(lll) bis(trifluoroacetate) with TMSNj is a known method of generating azide radicals. The azide radical abstracts a hydrogen atom from an alkane to form a carbon-centered radical. The carbon-centered... 

---