Nitrogen-centered heterocyclic radicals

Sulfur/Nitrogen-Centered Heterocyclic Radicals—Thiazyls... 

The addition/elimination mechanism (equation 11) for formation of phenoxyl radicals by OH reaction with phenols is now documented for a vast number of substituted phenols , catechols , resorcinol , hydroquinones and hydroxylated heterocyclics . The addition/elimination mechanism is also operative in OH reactions with anilines to yield the nitrogen-centered anilinyl radicals (equation i2) 408-in... 

The 5-dig-mode of cyclization has been applied in the synthesis of N-heterocycles. For example, treatment of the /i-allenyl dithiosemicarbazide 37 with Bu3SnH and AIBN in hot benzene furnishes the substituted 3H-pyrrole 38 in 41% yield and the isomeric heterocycle 39 in 30% yield (Scheme 11.13) [68], Iminyl radical 40 is formed via Bu3Sn addition to the thiocarbonyl group of the radical precursor 37 and fragmentation of the adduct (not shown). Nitrogen-centered radical 40 adds 5-dig-selectively to provide substituted allyl radical 41. The latter intermediate is trapped by Bu3SnH to furnish preferentially product 38 with an endocydic double bond. 

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. 

A radical cascade reaction has been accompHshed by StaHnski and coworkers which converts dipeptide derivatives to nitrogen-containing heterocycles (Scheme 6) [9]. In this work, N-bromobenzyl-hf-propargyl-substituted dipeptides such as 10 were subjected to Stork s catalytic procedure with tributyltin hydride [10]. An aryl radical is formed followed by a 1,5-hydrogen shift, generating the a-centered carbon radical 11. 5-Fxo-dig radical cyclization... 

Isonitriles as useful trap in radical reactions leading to heterocycles 01MI2. Nitrogen-centered radicals 01MI11. 

The same Romanian group has measured ESR spectra for the cation- and anion-radicals 82 and 83, respectively, of 2-nitrodibenzo[l,4]dioxin. As expected, the cation-radical is heterocycle-centered, showing a very small substituent-nitrogen hyperfine splitting, whereas the opposite is true 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... 

The presence in the molecule of imidazoline nitroxides of an additional nitrogen atom or of an N-oxide group in combination with functional groups in position 4 of the heterocycle allows com-plexation, chelation, and cyclometalation without participation of the radical center. This made it possible to investigate for the first time the magnetic resonance spectra of multispin systems. 

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