R. C. Hopkins
Sep 18, 1968
Citations
0
Influential Citations
4
Citations
Journal
Journal of Molecular Spectroscopy
Abstract
Abstract In order to extend the available NMR data on cyclobutane compounds so that substituent effects on conformation can be elucidated, the NMR spectra of four 1,1-difluoro-2,2-dichlorocyclobutanes and a related cyclobutene-2 compound, all substituted at the 3-position, have been analyzed. The ring protons and fluorines give rise to an ABXY spectrum which markedly changes in complexity as the 3-substituents are varied. It is proposed that the qualitative degree of simplicity of each spectrum is directly related to the extent of puckering of the four-membered ring. In 1,1-difluoro-2,2-dichloro-3-carbomethoxy-3-methylcyclobutane the relative signs of all vicinal HF coupling constants have been determined by 1 H{ 19 F} double resonance to be the same as the sign of the FF coupling constant and probably opposite to the sign of the HH coupling constant. An analysis of the spectrum of this compound at 200°C is consistent with a conformation of the cyclobutane ring which is puckered so that the carbomethoxy group assumes a pseudo-equatorial position. The ring methyl group is coupled to the cis transannular fluorine by 1.5 Hz and to the trans ring proton which, though unresolved, results in line broadening.