The Structure of Dehydromethionine. An Azasulfonium Salt

The crystal and molecular structure of dehydromethionine has been determined by x-ray crystallographic techniques. The compound crystallizes in the monoclinic space group P2₁/n with a = 5.658 (6) Å, b = 8.844 (6) Å, c = 13.605 (9) Å, α = 90°, β = 92.54 (2)°, γ = 90°, and Z = 4. The atoms other than hydrogen were located using direct methods, and the hydrogen atoms were located by Fourier difference maps. Full-matrix least-squares refinement led to a conventional R factor of 0.044. Key features of the molecular structure are an envelope conformation of the five-membered isothiazolidine ring and trivalent sulfur bonded to pyramidal nitrogen. The geometry about the nitrogen atom suggests the absence of p-d π bonding between the sulfur and nitrogen atoms in the azasulfonium linkage. It is suggested that the geometry and bonding in azasulfonium salts is analogous to that in the isoelectronic sulfonium ylides.