Singlet sulfur

Cyclic disulfides.1 Singlet sulfur is generated when 2,2 -dithiobenzoylbiphenyl is heated at 80-130° with formation of 9,10-diphenylphenanthrene (equation I). The S2 can be trapped by a Diels-AIder reaction with 1,3-dienes to form cyclic disulfides in 60-85% yield. 

There is also evidence that COS itself may bring about the electronic relaxation of singlet sulfur, as well,... 

That the mercaptan product arises exclusively from singlet sulfur atom reactions was proven in experiments with added CO2. The data in Table VI clearly demonstrate the suppressing effect of COj on the mercaptan yield, with a concomitant and equivalent rise in episulfide production. Further substantiation comes from mercury-photosensitization studies with COS in which it was found that the sulfur atoms produced by this means yield only episulfide, no mercaptans being formed with ethylene or with any of the other olefins studied. 

With butadiene-1,3, the major reaction product is vinyl thiacyclopro-pane. Some thiophene may also be formed in the primary process, with the upper limit of importance of this primary step being 9%. Thus, for the reaction of both triplet and singlet sulfur atoms with butadiene-1,3, addition occurs to at least 91% at the 1,2 position. 

Photodissociation of thietane 1-oxide in the gas phase yields cyclopropane and ethylene. It is suggested that singlet sulfur monoxide is produced except in the mercury-atom-sensitized decomposition that is supposed to yield triplet sulfur... 

Studies on vinyl thiol (or ethene thiol) are scarce. It has been detected experimentally, and its formation from ethylene and singlet sulfur atom has been predicted theoretically [15]. Its conformations have been studied both experimentally [16] and theoretically [17). Using it, in a comparative way, in our study focused on vinyl alcohol seemed relevant. 

CYCLOADDITION Aluminum chloride. Ceric ammonium nitrate. N,N-Diethyl-l-propy-nylamine. Dimethyl aoetylenedicarboxylate. N,N-Dimethyl-0-ethylphenylpropioi-amidium tetrafluoroborate. Ethylaluminum dichloride. Nitrobenzene. Oxygen, singlet. Sulfur dioxide. Trichloroacetyl isocyanate. Trimethylsilylbromoketene. 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Compound A undergoes hydrolysis of its acetal function in dilute sulfuric acid to yield 1,2-ethanediol and compound B (CgHg02), mp 54°C. Compound B exhibits a carbonyl stretching band in the infrared at 1690 cm and has two singlets in its H NMR spectrum, at 8 2.9 and 6.7, in the ratio 2 1. On standing in water or ethanol, compound B is converted cleanly to an isomeric substance, compound C, mp 172—173°C. Compound C has no peaks attributable to carbonyl groups in its infrared spectrum. Identify compounds B and C. 

Both CSs and CSs were also successfully generated by the fragmentation of ionized 4,5-dioxo-2-thioxo-l,3-dithione (65) and 2-thioxo-l,3-dithiole (66) (90JA3750). Tire three sulfur atoms in the anion and cation radicals were chemically equivalent, suggesting that they take the D h (or C2u) form (67 or 68). On the other hand, under similar conditions, 3-thioxo-1,2-dithiole (69) yielded two isomeric cation radicals the (or 2 ) form and the carbon disulfide 5-sulfide form (70). Ab initio calculations on three electronic states of CS3 at the 6-31G -l-ZPVE level indicated that the C21, form (68) was more stable than the carbon disulfide 5-sulfide form (70) in the neutral (both singlet and triplet states) and the anion radical states, but 68 was less stable than 70 in the radical cation state. 

The same equatorial preference is also manifested in the 3,3-disubstituted thietane oxides66,194. Thus, the NMR spectra of 5e,f contain two Me singlets at 1.23 and 1.30 ppm and two methylene multiplets at 3.03 and 3.53 ppm (in CDC13). The large difference in the chemical shifts of the axial and equatorial a-methylene hydrogens is characteristic of an axial nonbonded electron pair on sulfur (conformation 5e equation 73). This conformational preference is corroborated by the small differences in the chemical shifts of the two methyl groups, and fits the contention that 1,3-diaxial interactions are responsible for this ultimate result. Certainly, these interactions are greater in the conformer 5f. 

All of these reactions are exothermic but some of them violate the law of spin conservation since S2 is a triplet molecule while all species on the right side of the above equations are singlet molecules in the ground state. Therefore, a radical-chain reaction has been assumed. S3 may also be formed from S2 by picking a sulfur atom up from a di- or polysulfide in the a-layer. 

Pyrolysis of the parent thiirane oxide 16a monitored by microwave spectroscopy led to the conclusion that the sulfur monoxide is generated in its triplet ground state, although the singlet state ( A) cannot be excluded completely (equation 8). A later study presented evidence, based on the stereoselective addition to dienes of sulfur monoxide generated from thiirane oxide as well as thermochemical data, that the ground state S is formed exclusively ° . ... 

The latter do not yield l,3-dichalcogen-2,4-phosphasilacyclobutanes if they are treated with excess elemental sulfur and tellurium. Compound 23a has been characterized by its 31P-NMR spectrum, which shows a singlet signal at 8 = -126.0, and by l25Te satellites with /(P,Te) = 222 Hz. The composition of 23b,c was proved by mass spectrometry and their... 

Sulfur tetrafluoride appears as two broad singlets at room temperature, as one broad singlet at 85 °C, and (when dry) as two sharp triplets at -30 °C. SF6, with its symmetrical octahedral geometry, appears as a sharp singlet at all temperatures. The activation energy for pseudorotation of SF4, which interconverts its axial and equatorial fluorines, is 12kcalmor1.3... 

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