Robert G. Webster

Studies of heterocyclic compounds. Part 23. Diazo-coupling–deformylation of 3,4-dialkyl-1-aryl-6-oxa-6aλ4-thia-1,2-diazapentalenes: synthesis of 3,4-dialkyl-1,6-diaryl-6aλ4-thia-1,2,5,6-tetra-azapentalenes

Partial desulphurisation of 3,4-dialkyl-1-aryl-6,6aλ4-dithia-1,2-diazapentalenes with mercury(II) acetate affords 3,4-dialkyl-1-aryl-6-oxa-6aλ4-thia-1,2-diazapentalenes, which couple with arenediazonium tetrafluoroborates with accompanying deformylation to give 3,4-dialkyl-1,6-diaryl-6aλ4-thia-1,2,5,6-tetra-azapentalenes, a new class of four-electron three-centre bonded compound. Reversible exchange of diazo-groups takes place between 3,4-dialkyl-1,6-diaryl-6aλ4-thia-1,2,5,6-tetra-azapentalenes and arenediazonium tetrafluoroborates in solution. It is proposed that the formation and the exchange reactions of 3,4-dialkyl-1,6-diaryl-6aλ4-thia-1,2,5,6-tetra-azapentalenes proceed by way of 1,2,3-thiadiazolium intermediates.

Studies of heterocyclic compounds. Part 22. Reactions of 1,6-dioxa-6aλ4-thiapentalenes

Electrophilic substitution of 1,6-dioxa-6aλ4-thiapentalene involves attack at position(s) 3 (and 4) and gives either normal substitution products or products of substitution with rearrangement. Bromination of 1,6-dioxa-6aλ4-thiapentalene invariably gave 3,4-dibromo-1,6-dioxa-6aλ4-thiapentalene. Iodination with iodine and silver acetate could be controlled to give either 3,4-di-iodo- or 3-iodo-1,6-dioxa-5aλ4-thiapentalene. Tritylation with trityl perchlorate in the presence of calcium carbonate gave 3-trityl-1,6-dioxa-6aλ4-thiapentalene. Acetoxymercuration in acetic acid afforded an insoluble bisacetoxymercurio-derivative quantitatively. The position of substitution was established by comparative 1H n.m.r. spectral studies. Attempts to formylate, acetylate, and nitrate 1,6-dioxa-6aλ4-thiapentalene were unsuccessful. 1,6-Dioxa-6aλ4-thiapentalene couples with p-nitrobenzenediazonium tetrafluoroborate with rearrangement to give 1-p-nitrophenyl-6-oxa-6aλ4-thia-1,2-diazapentalene-3-carbaldehyde, the first derivative of the 6-oxa-6aλ4-thia-1,2-diazapentalene system to be reported. 1,6-Dioxa-6aλ4-thia-pentalenes show aldehyde reactivity. 3,4-Dibromo- and 3,4-di-iodo-1,6-dioxa-6aλ4-thiapentalene reacted with methylamine in aqueous acetonitrile to give 3,4-dibromo-6-methyl- and 3,4-di-iodo-6-methyl-1-oxa-6aλ4-thia-6-azapentalene, respectively, the first derivatives of the 1-oxa-6aλ4-thia-6-azapentalene system to be reported. 3-Iodo-1,6-dioxa-6aλ4-thiapentalene reacted with methylamine to give a mixture of 3-iodo-6-methyl- and 4-iodo-6-methyl-1-oxa-6aλ4-thia-6-azapentalene, which were not separated. 3-lodo-1,6-dioxa-6aλ4-thiapentalene underwent halogen–metal exchange with n-butyl-lithium at –70 °C. The resulting 3-lithio-derivative reacted in situ with dimethylformamide or with the Vilsmeier reagent (Me2[graphic omitted]CHCl·PO2Cl2–) to give 1,6-dioxa-6aλ4-thiapentalene-3-carbaldehyde in low yield.

Studies of heterocyclic compounds. Part 27. Routes to 1,6-dioxa-6aλ4-thia-2-azapentalenes, 1,6-dioxa-6aλ4-selena-2-azapentalenes, and 1-oxa-6aλ4-thia-2,5,6-triazapentalenes

Partial desulphurisation of 3,4-dialkyl-1-oxa-6,6aλ4-dithia-2-azapentalenes with mercury (II) acetate gives 3,4-dialkyl-1,6-dioxa-6aλ4-thia-2-azapentalenes. 3,4-Dimethyl-1-oxa-6,6aλ4-diselena-2-azapentalene likewise gave 3,4-dimethyl-1,6-dioxa-6aλ4-selena-2-azapentalene. Peracid oxidation of 1-oxa-6,6aλ4-dithia-2-azapentalenes afforded 3-nitromethylene-3H-1,2-dithioles in low yield. 3,4-Dialkyl-6-oxa-6aλ4-thia-1,2-diazapentalenes in which the reactive position 3 is blocked, react with nitrous acid to give 3,4-dialkyl-1-oxa-6aλ4-thia-2,5,6-triazapentalenes by nitroso-deformylation, together with smaller amounts of the corresponding 5-nitromethylene-5H-1,2,3-thiadiazoles by nitro-deformylation. Peracid oxidation of 3,4-dialkyl-1-oxa-6aλ4-thia-2,5,6-triazapentalenes gave the corresponding 5-nitromethylene-5H-1,2,3-thiadiazoles in modest yield. 1,6-Dioxa-6aλ4-selena-2-azapentalenes and 1-oxa-6aλ4-thia-2,5,6-triazapentalenes are two new classes of four-electron, three-centre bonded compound.

Studies of heterocyclic compounds. Part 25. Stable indolizine and pyrrolo [2,1-b]thiazole carboselenaldehydes

The reaction of indolizines and pyrrolo[2,1-b]thiazoles with phosphoryl chloride in dimethylformamide and treatment of the resulting Vilsmeier salts in situ with aqueous sodium hydrogen selenide gives indolizine and pyrrolo[2,1-b]thiazole carboselenaldehydes, many of which are stable crystalline compounds. 1H N.m.r. and u.v. spectral data of the selenaldehydes are reported. 2,7-Dimethylindolizine-3-carboselenaldehyde and 5,6-dimethylpyrrolo[2,1-b]thiazole-7-carboselenaldehyde were reduced by lithium aluminium hydride–aluminium chloride to the corresponding trimethyl derivatives. 2,7-Dimethylindolizine-3-carboselenaldehyde and 5,6-dimethylpyrrolo[2,1-b]thiazole-7-carboselenaldehyde smoothly underwent Wittig reactions with isopropylidenetriphenylphosphorane in dimethyl sulphoxide–benzene solution at room temperature, and with acetylmethylenetriphenylphosphorane in boiling xylene.

Studies of heterocyclic compounds. Part XVII. Synthesis of 1,6-Dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes

1,6-Dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes, two new classes of hypervalent heterocyclic compound, have been synthesised by the reaction of pyran-4-thiones and -4-selones, respectively, with thallium(III) trifluoroacetate, and treatment of the resulting labile thallium-containing pyrylium trifluoroacetates with water 1,6-Dioxa-6a-thiapentalene was also formed in low yield by ring-opening of pyran-4-thione with sodium hydroxide in aqueous dimethyl sulphoxide, and intramolecular oxidative coupling of the resulting anion with potassium ferricyanide. 1H N.m.r. spectra of symmetrically substituted 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes show real or time averaged C2v symmetry. The pattern of the spectrum of 1,6-dioxa-6a-thiapentalene was unaffected by change of solvent (CDCl3, CS2, or CD3CN) or by lowering the temperature of a solution in CDCl3(to –50°) or CS2(to –90°). The i.r. spectra of 1,6-dioxa-6a-thiapentalene, 1,6-dioxa-6a-selenapentalene, and their 2,5-dimethyl derivatives show no absorption in the double bond region above 1515 cm–1. The significance of the 1H n.m.r. and i.r. spectra of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes is discussed in relation to structure. Thionation of 1,6-dioxa-6a-thiapentalene with phosphorus pentasulphide gave 6a-thiathiophthen. 1,6-Dioxa-6a-thiapentalene underwent sulphur–oxygen exchange in boiling thioacetic acid to give a mixture of 6a-thiathiophthen and 1-oxa-6,6a-dithiapentalene. Thionation of 1,6-dioxa-6a-selenapentalene with phosphorus pentasulphide afforded 6a-selenathiophthen.

Studies of heterocyclic compounds. Part XVI. Mechanism of electrophilic substitution of 6a-thiathiophthens and related compounds: nitrosation with rearrangement

Nitrosation of 6a-thiathiophthens and related compounds occurs with rearrangement. 6a-Thiathiophthens, 1-oxa-6,6a-dithiapentalenes, and 1,6a-dithia-6-azapentalenes rearrange into 1-oxa-6,6a-dithia-2-azapentalenes, and 1,6-dioxa-6a-thiapentalenes into 1,6-dioxa-6a-thia-2-azapentalenes. 6a-Thiathiophthens react with nitrous acid with difficulty unless activated by strongly electron-releasing substituents. 2-t-Butyl-6a-thiathiophthen reacted at position 4 to give 3-formyl-5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene in low yield. 2-Methylthio-5-t-butyl-and 2-dimethylamino-5-t-butyl-6a-thiathiophthen reacted readily at position 3 to give the methyl 3-dithiocarboxylate and the 3-NN-dimethylthiocarboxamide of 5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene, respectively, selective desulphurisation of which afforded the corresponding S-methyl thioester and NN-dimethylcarboxamide. 5-Phenyl-, 5-t-butyl-, and 2,5-dimethyl-1-oxa-6,6a-dithiapentalene reacted smoothly at position 3 to give the corresponding 3-formyl(acetyl)-1-oxa-6,6a-dithia-2-azapentalenes. 6-Methyl-2-phenyl- and 6-methyl-2-t-butyl-1,6a-dithia-6-azapentalene also gave 3-formyl-1-oxa-6,6a-dithia-2-azapentalenes by hydrolysis in situ of the intermediate 3-methyliminomethyl-1-oxa-6,6a-dithia-2-azapentalenes. 1-Oxa-6,6a-dithiapentalenes and 1,6a-dithia-6-azapentalenes in which the reactive position 3(4) is blocked, reacted at position 3(4) with elimination of the formyl or methyliminomethyl group to yield 1-oxa-6,6a-dithia-2-azapentalenes. Nitrosation of 1,6-dioxa-6a-thiapentalene with nitrosyl hexafluorophosphate gave 3-formyl-1,6-dioxa-6a-thia-2-azapentalene, the first reported derivative of the 1,6-dioxa-6a-thia-2-azapentalene system. A mechanism is proposed to account for the various features of the electrophilic substitution of 6a-thiathiophthens and related hypervalent heterocyclic systems. It is proposed that reaction proceeds by way of stable 6π-electron monocyclic cations, such as 1,2-dithiolium and 1,2-oxathiolium.

Studies of heterocyclic compounds. Part XVIII. Protonation of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes: formation of 1,2-oxathiolium and 1,2-oxaselenolium cations

A 1H n.m.r. spectroscopic study of the protonation of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes shows that O- and C(3)-protonation occurs. Equilibrium mixtures are formed which contain the unprotonated heterocycle and the O- and the C(3)-protonated species in varying relative amounts, depending on the nature of the heterocycle and the acid medium. 1,6-Dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalene remained largely unprotonated in trifluoroacetic acid, but the occurrence of H–D exchange at C-3 and C-4 in trifluoroacetic [2H]acid revealed the presence of the 3-formylmethyl-1,2-oxathiolium and -1,2-oxaselenolium cations. The major species in a solution of 2,5-dimethyl-1,6-dioxa-6a-thiapentalene in trifluoroacetic acid were the unprotonated heterocycle and two isomeric 3-(2-hydroxypropenyl)-5-methyl-1,2-oxathiolium cations in a 4 : 6 : 1 ratio. 2,5-Dimethyl-1,6-dioxa-6a-selenapentalene was largely unprotonated in trifluoroacetic acid. In trifluoroacetic acid containing 5%(v/v) perchloric acid 2,5-dimethyl-1,6-dioxa-6a-thia- and 2,5-dimethyl-1,6-dioxa-6a-selenapentalene gave the 3-acetonyl-5-methyl-1,2-oxathiolium and -1,2-oxaselenolium cations, respectively, as the only observable species. The O- and C(3)-protonated 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes are the first members of the 1,2-oxathiolium and the 1,2-oxaselenolium systems to be identified. The structure of the C(3)-protonated 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes is discussed in relation to the mechanism of electrophilic substitution of 1,6-dioxa-6a-thiapentalenes and related hypervalent heterocyclic compounds.

Studies of heterocyclic compounds. Part XIII. Pyrrolo[2,1-b]thiazole thioaldehydes: synthesis and spectral studies

Pyrrolo[2,1 -b]thiazole-5- and 7-thiocarbaldehydes, a class of stable heterocyclic thioaldehydes, have been synthesised by a modification of the Vilsmeier reaction. 6-Methyl- and 2,3,6-trimethylpyrrolo[2,1-b]thiazole-5-[2H]-thiocarbaldehyde have been obtained by using [2H7]dimethylformamide in the Vilsmeier reaction. 1H N.m.r. spectral data for the thioaldehydes in CDCl3 are recorded. Restricted rotation about the ring–CHS bond was detected in 6-methyl-, 2,6-dimethyl-, and 6,7-dimethyl-pyrrolo[2,1-b]thiazole-5-thiocarbaldehyde, by using the change in shape of the 3-H and CHS signals with temperature as a probe. The coalescence temperatures for solutions in (CD3)2SO lie in the range 78–86°. 1H N.m.r. spectral data indicate that pyrrolo[2,1 -b]thiazole-5-thiocarbaldehydes exist either in a syn-configuration (6-methyl-, 2,6-dimethyl-, 6,7-dimethyl-, and 3-methyl-6-t-butyl-) or in an anti-configuration (3,6-dimethyl- and 2,3,6-trimethyl-), and that the 7-thiocarbaldehydes exist in a syn-configuration. Normal ranges for the thioformyl proton chemical shifts are: 5-syn CHS, δ 10·2–10·4; 5-anti CHS, 11·0–11·1; 7-syn CHS, 10·5–10·8 p.p.m. The preparation of some new pyrrolo[2,1-b]thiazoles is described.