Five-membered heterocycles radicals derived from

As expected from simple MO considerations, the radical cations of five-membered heterocycles, e.g. the blue species formed from furan, pyrrole, thiophene, and their alkyl derivatives, are n ions. The semi-occupied orbital is the n orbital with the heteroatom in the nodal plane (ai), see Scheme 2, structure 1. In radical cations of a,ca-bis-(l-pyrrolyl) alkanes the charge remains localized on a single ring, rather than being delocalized over both units [5, 10, 11]. 

The five-membered hypervalent iodine heterocycles, benziodoxoles, are commonly used as convenient radical precursors [3,33]. The main advantage of benziodoxoles over the non-cyclic hypervalent iodine reagents is the higher thermal stability allowing the preparation of otherwise unstable derivatives with I-Br, I-OOR, I-N3, and I-CN bonds. The stable cyanobenziodoxoles 36-38 are prepared in one step by the reaction of cyanotrimethylsilane with the respective hydroxybenziodoxoles 35 (Scheme 16) [34, 35], or from acetoxybenziodoxole... 

HeterocycUzation. Captodative radicals are generated from a-heterosubstituted a-chloroacetic esters by treatment with CuCl bipy. Such a radical can be trapped in-tramolecularly by an alkene (e.g., a 3-butenyl group) attached to the a-heteroatom. Formation of five-membered ring heterocycles is a favorable process. Thus methyl 3-chloromethyltetrahydrofuran-2-carboxylate has been obtained in 75% yield as a mixture of cis and trans isomers (ratio 64 36). The glycine analog gives both pyrrolidine and piperidine derivatives. ... 

The mass spectra of a number of heterocyclic compounds have been reported. The fragmentation patterns of thiophosphoryl derivatives of phosphorinanes are sensitive to stereochemistry, thus for the series (95) facile loss of the HS radical is indicative of an axial PS bond. The seven-membered heterocycles (96 Ch = S, Se) undergo a remarkable migration of sulphur or selenium from phosphorus to carbon with ring cleavage. While exocyclic P-C bonds of five-and six-membered heterocycles in the phosphonic class may be readily cleaved with retention of the phosphorus ring system, the seven-membered ring (97) exhibits facile expulsion of a phosphorus radical. ... 

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... 

---