Probing the limits of tetraalkylthiuram disulfide synthesis by direct reaction of secondary amines with CS2: The structures of Cy2NC(S)SSC(S)NCy2 and Cy2NC(S)SSSSC(S)NCy2

Reaction of Cy₂NH with CS₂ in NaOH/H₂O, followed by oxidation with I₂, yields a complex mixture that includes the tetracyclohexylthiuram disulfide, Cy₂NC(S)SSC(S)NCy₂, in mixture with S₈ and the tetrasulfide Cy₂NC(S)SSSSC(S)NCy₂. Direct reaction of 2 eq of Cy₂NH with CS₂ in EtOH, followed by I₂ oxidation, does not yield isolable Cy₂NC(S)SSC(S)NCy_2, even though other tetraalkylthiuram disulfides are effectively produced by this protocol. Deprotonation of Cy₂NH with tBuLi, followed by treatment with CS₂ and then oxidative coupling with I₂ yields Cy₂NC(S)SSC(S)NCy₂ in ∼74% yield, suggesting secondary amines that are branched at the α carbon may generally require this lattermost approach to the corresponding tetraalkylthiuram disulfide. Evaporation of an Et₂O solution of the crude product mixture obtained from the CS₂/NaOH/H₂O reaction yielded a triclinic form of Cy₂NC(S)SSC(S)NCy₂, which contained two independent molecules in its asymmetric unit with opposite C₂ handedness. This same product mixture contained Cy₂NC(S)SSSSC(S)NCy₂, which was identified crystallographically in monoclinic C2/c and is the first such tetrasulfide to be characterized by X-ray diffraction. A second polymorph of Cy₂NC(S)SSC(S)NCy₂, obtained by evaporation of an acetone solution, is disordered across a mirror plane in monoclinic P2₁/m. This monoclinic polymorph of Cy₂NC(S)SSC(S)NCy₂ is the only tetraalkylthiuram disulfide found to crystallize on a mirror plane.