The NMR spectra of four-membered carbocyclic ring systems

Abstract

A summary of proton-proton coupling constant data for cyclobutanes, cyclobutanones, cyclobutenes and cyclobutenones is presented. In cyclobutanes, vicinal cis and trans coupling constants vary widely (4·6–11·5 c/s and 2·0–10·7 c/s, respectively), whereas the range of geminal coupling constants is smaller (−10·9 to −14·4 c/s. in the absence of a π-contribution). If the cyclobutane ring is incorporated into the strained bicyclo[2.1.1]-hexane system, then the geminal coupling constant is much smaller (−5·4 to −8·4 c/s) and a characteristic, large (6·7–8·1 c/s) 4σ-bond coupling between endo protons of the cyclobutane ring is observed. The vinylic coupling constant in cyclobutenes varies from 2·5–4·0 c/s (perhaps negative in sign) and the vicinal coupling between protons attached to adjacent sp2 and sp3 carbon atoms is very small (∼0–1 c/s). Values of J13CH are tabulated. Chemical shift data indicate that protons in cyclobutanes and cyclobutenes resonate at slightly lower field than those in the larger cycloalkanes and cycloalkenes.