[10545-99-0] · Cl2S · Sulfur Dichloride · (MW 102.97)
Physical Data: mp -121 °C; bp 59 °C; d (20 °C) 1.621 g cm-3; n
Solubility: sol dichloromethane, n-hexane, CCl4, benzene; reacts violently with H2O, acetone, DMSO, DMF.
Form Supplied in: dark-red liquid. Techgrade (80%), major impurity is S2Cl2. Also 1.0 M in CH2Cl2. Very pungent, chlorine-like odor. Readily decomposes to S2Cl2 and Cl2.
Preparative Methods: can be synthesized from S(5) or S2Cl2.4
Purification: pure SCl2 can be obtained from Disulfur Dichloride and Chlorine by distillation from Phosphorus(III) Chloride.
Handling, Storage, and Precautions: distilled SCl2 is stable for a few days in glass with a few drops of PCl3. Air and H2O sensitive. Incompatible with acids, bases, alcohols, plastics, amines. Avoid contact with metals. Corrosive; lachrymator. Harmful if swallowed, inhaled, or absorbed through the skin. Inhalation can be fatal. Possible carcinogen.
Sulfur dichloride is an extremely reactive electrophile towards alkenes. Under dilute conditions, sulfur dichloride reacts with unconjugated dienes (eq 1) to give dichloro sulfur-bridged bicyclic compounds.1a-c,5
The proposed 1,2-episulfonium ion intermediate (eq 2) explains the selectivity of chlorine addition and their high lability. Yields for the addition of SCl2 to dienes1 and for subsequent halide displacements are closely tied to how readily the sterically demanding 1,2-episulfonium ion is formed.6,7
Common transformations of the SCl2-diene addition products are chloride replacement,6a chloride displacement to form episulfides,6b,7a and oxidation of the sulfide bridge to a sulfone bridge.1 In a few cases, the addition of SCl2 has been extended to conjugated dienes.8
The reactivity of sulfur dichloride towards alkenes is exploited in the synthesis of thiaalkanams and thiaisoalkanams in which SCl2 is used as a sulfur transfer agent to nonconjugated diene systems (eq 3).
Select examples of five and seven-membered sulfur-nitrogen heterocycles can be synthesized in modest yield.9,10
Lewis acid catalysis of sulfur dichloride additions to nitriles leads to the formation of 1,2,4-thiadiazoles.2 As seen in eq 4, a major side reaction of this process is the chlorination of the aromatic substituents.
Lewis acid catalysts for this reaction are Tin(IV) Chloride, Aluminum Chloride, Iron(III) Chloride and Antimony(V) Chloride. Nitriles with a-hydrogens do not yield any thiadiazole product, but rather give disulfur species from deprotonation (eq 5).2
Conjugated imines such as 1-t-butyl-4-phenyl-1-aza-1,3-butadiene react with 0.5 equiv of sulfur dichloride to give isothiazoles11 in high yield, 86-91%, based on sulfur (eq 6). Spectroscopic evidence verifies that two equivalents of the imine are required to form the 2:1 salt intermediate before cyclization.
Unlike 1-azadienes, 2-azadienes react with high yields in a 1:1 molar ratio and show no spectroscopic evidence of a 2:1 salt intermediate (eq 7).
Other applications include the synthesis of dialkynyl sulfides, (RC&tbond;C)S,12 the heterocyclic adducts of alkynes,13 the synthesis of novel macrocyclic polysulfur compounds,13b and one-pot syntheses of benzothiophenes.14 Considerable literature has also been devoted to the formation of episulfides from hindered alkenes,15 with limited success (see also Disulfur Dichloride).
Brian C. Austad
University of Wisconsin-Madison, WI, USA