1H (1-D and 2-D) NMR spectra and the conformational states of some 3-monosubstituted saturated five-membered heterocycles

Abstract

We have recorded the1H (1-D and 2-D) NMR spectra of sulfolane, a series of 3-monosubstituted sulfolanes, and 3-acetoxy-1,1-diphenylsilacyclopentane. We have determined the chemical shifts and the proton-proton spin-spin coupling constants of seven ring protons. From analysis of the magnitudes of the vicinal spin-spin coupling constants and consideration of the full-cycle pseudorotation model for five-membered heterocycles, we found that seven conformers are most probable for 3-iodo- and 3-tert-butylsulfolanes. Four of these conformers have the half-chair conformation, three have the envelope conformation with pseudoequatorial substituents. The rest of the 3-monosubstituted sulfolanes (chloro-, bromo-, hydroxy-, and methacryloxysulfolanes) and 3-acetoxy-1,1-diphenylsilacyclopentane also have seven preferred conformations, but with pseudoaxial substituents.