Sulphur Heterocyclic Radicals

Sulphur Heterocyclic Radicals.—Most studies in this field have used theoretical methods to derive the spin-density distribution and hyperfine splitting constants. In a few cases the unrestricted Hartree-Fock (UHF) formalism has been applied (PPP approximation). (The inclusion of d-orbitals slightly improved the spin-density results in the p-model. )... 

A condition necessary for a radical to be observable is the favourable position of the dismutation equilibrium Ox+Red 2Sem. This equilibrium between the oxidized form (Ox) and reduced form (Red) and the radical (Sem) depends on the molecular electronic repulsion integral Ja n of the singly occupied molecular orbital of the radical. Values have been computed and listed for a great variety of sulphur heterocyclic radicals. Radicals with extensively conjugated skeletons would be expected to have dismutation constants with lower values, and this has been found. On the other hand, small radicals are likely to dimerize or attack other species. The highest spin density for the unpaired electron is probably at the site of the radical dimerization, Le. the 2-position of the 1,3-dithiole radical. The spin-density distribution may also be responsible for other radicals. The pathway for the polarographic reduction of 1,2-dithiolium cation (87) or trithiones (88) via cation radicals has recently been discussed by CNDO/2... 

The numerous amino-adds which are met in the products of advanced tryptic hydrol)rsis are of quite different constitution. Some possess only an add function and an amine function others are di-adds and mono-amines, or mono-adds and di-amines. Some have an alcohol function others are sulphur products. Finally, these compounds are sometimes of straight chain, sometimes of dosed chain constitution. Yet, we may say that all answer to the general formula, R — CHCNHj) — COjH, which shows that there is at least a grouping NHj fixed on the C adjacent to carboxyl, R bdng an acyl, an aromatic or heterocyclic radical. The following is a list of these compounds ... 

Photolysis of allyl pent-4-enyl sulphides such as (9) gave mixtures of five- and six-membered sulphur heterocycles via unsaturated thiyl radicals. Oxidative decarboxylation of the disulphide (10), and intramolecular trapping of the intermediate carbon radical to give the thiolan (11), indicated a biochemical pathway for the formation of the C(2)—S bond of penicillins. ... 

Photoelectron spectra have been reported for a number of sulphur heterocycles, including thiolan, thian, 1,3- and 1,4-dithian, " 1,4-oxa-thian, " and 7-thiabicyclo[2,2,l]heptane. The effects have been compared with those of the corresponding oxygen heterocycles. Lone-pair interactions in six-membered-ring heterocycles having the heteroatoms para seem to occur primarily via through-bond mechanisms. " When the heteroatoms are meta the interaction is mainly through-space. " The e.s.r. spectrum of free radicals a to sulphur in thians and thiolans has been measured and interpreted. Electron-diffraction data have been obtained and used to C. B. Quinn, J. R. Wiseman, and J. C. Calabrese, J. Amer. Chem. Soc., 1973 95, 6121. 

Few experimental and theoretical electronic spectra are available for sulphur heterocycles. Zahradnik et al. have demonstrated the applicability of the open-shell SCF-LCI method of Longuet-Higgins and Pople in calculations of, for example, the cation radicals of (120), (121), and heteroanalogues. ... 

In intramolecular arylations, a new bond is created between two aromatic moieties of the same molecule or between an aromatic nucleus and an atom of a side-chain. Many intramolecular arylation reactions of homocyclic and heterocyclic aromatic halides have been studied mainly in view of their synthetic applications, and it is not always clear which mechanistic pathway is followed. The reaction may start with homolytic or heterolytic dissociation of the carbon-halogen bond and proceed by attack of the aryl radical or aryl cation on another part of the molecule. Electrocyclization followed by elimination of hydrogen halide is another possibility. Especially when heteroatoms such as nitrogen, sulphur or phosphorus are involved, the initial step may be a nucleophilic attack on the carbon atom bearing the halide atom. 

A number of heterocyclic S—N ambident anions have also been shown to react via the sulphur atom with 2-halo-2-nitropropanes by the S l mechanism5,116. The authors suggest116 that the addition of ambident anions to alkylnitro radicals (CR2N02) takes place under kinetic control via the more nucleophilic centre. 

Shine and Mach (1965) have reported the spectrum of the pheno-thiazine radical-cation (45), generated by treatment with sulphuric acid of either the parent compound or its neutral radical (i.e. the conjugate base) on treatment with water. Radical-cations of related heterocycles have been studied using similar methods (Shine and Small, 1965 Shine and Davies, 1965). 

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. ... 

The organization of this Chapter is a radical change from the arrangement of the corresponding parts of Volume 5. This Reporter has tried to review, from the standpoint of sulphur chemistry rather than of heterocyclic chemistry, the highlights of current work which in the previous Volume comprised Chapters 6, 10, 11, 12, and 13, and parts of Chapters 7 and 8. It is not, of course, comprehensive the main criteria used in selection were novelty, utility, and theoretical interest. 

Non-classical Thiophens.—Thiophens which formally require quadricovalent bonding about one or more sulphur atoms are usually called non-classical . Numerous examples have been described during the past decade (see Vol. 5, pp. 282-3). Tetraphenylthieno[3,4-c]thiophen (55) and its radical-cation and -anion have now been subjected to a detailed spectroscopic examination, and the results have been rationalized by M.O. models. Non-classical thiophens usually need several aromatic substituents for reasonable stability examples in which the stability is conferred, at least in part, by fusion to polycyclic nitrogen heterocycles, e.g. (56) and (57), have been described, but the degree of stability is not great (57), for example, has only a transient existence. ... 

This value is appropriate for nitration in sulphuric acid, i.e. for circumstances in which the nitrogen atom is protonated. Comparison with values derived for the nitrogen atom from nucleophilic and radical substitution ae show that the heterocyclic component of the ring should make the greatest effect upon electrophilic substitution and the least upon radical substitution, as is the case. 

Synthesis of compound 68 by oxidative decarboxylation of adamantane carboxylic acid is described in [25], Initially, for the synthesis of 2-(l-adamantyl)benzimidazole (68), adamantan radical was obtained in sulphuric acid diluted by silver nitrate in the presence of ammonia persulfate. Then, the synthesized radical was inserted directly into the heterocycle. After N-methylation, 2-(l-adamantyl)benzimidazole (68) was transferred in l-methyl-2-(l-adamantyl)-benzimidazole (79). The experiments carried out on chicken embryo revealed high antiviral activity of both obtained compounds [25] ... 

With the exception of the above routes, other reported preparations of these heterocycles followed the more conventional routes, such as intramolecular displacement by a thiolate ion or other nucleophilic sulphur function, intramolecular addition of a thiol radical to a carbon-carbon double bond, and transannular addition of SClg to dienes. 

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