Spiro-λ4-sulfanes with intramolecular sulfur–oxygen interactions: Syntheses and molecular structures

Abstract Three novel N -acetylated spiro- λ 4 -sulfanes with five-membered spirorings (fused with aromatic rings) and with two N heteroatoms (in ( 1 )) or with N and O heteroatoms (in ( 2 ) and ( 3 )) in axial positions have been prepared and their molecular structures determined by X-ray diffraction. The molecular structures of compounds ( 1–3 ) show trigonal bipyramidal geometry about the central sulfur atom. The rather long axial S–N bonds (1.93 A) in the symmetric spiro- λ 4 -sulfane ( 1 ) exhibit the usual hypervalent character, whereas the S–N (1.73 and 1.80 A) and S–O (2.23 and 2.07 A) bond lengths in the unsymmetric spiro- λ 4 -sulfanes ( 2 ) and ( 3 ) correspond to elongated covalent S–N bonds and highly polarized S–O hypervalent bonds, respectively. Each structure exhibits usual S–Car bond lengths (1.79–1.81 A). The axial N-S-N/O and the equatorial Car-S-Car angles lie in the intervals 173–179° and 98–103°, respectively. The conformations of ( 1–3 ) including the single- cis –single- trans isomerism of the endocyclic and exocyclic amide parts, the shape of the spirorings, and the relative positions of the equatorial aromatic rings are discussed in detail. In ( 1–3 ) the exocyclic carbonyl-oxygen approaches the central sulfur atom by 2.90, 2.73 and 2.71 A, respectively, leading to an effective intramolecular sulfur–oxygen interaction of 1,4-type. The S⋯O close contacts, however, do not alter appreciably the trigonal bipyramidal geometry about the central sulfur atom.