Nuclear magnetic resonance studies of 4,4-bis(trifluoromethyl)-1,2- oxaphosphetans; through-space coupling as a guide to ground-state structures of cyclic phosphoranes

Abstract

Hydrogen-1, ³¹P, ¹³F, and ¹³C n.m.r. studies have been completed on three 4,4-bis(trifluoromethyl)-1,2-oxaphosphetans, (1)–(3). A number of interesting features have been observed. Compounds (1)–(3) have chiral centres at phosphorus and consequently induced non-equivalences in trifluoromethyl groups on (F₃C)₂CHO substituents are observed in the ¹⁹F n.m.r. spectra. Trifluoromethyl groups in the phosphetan ring system also have the expected non-equivalence. Compound (2) shows observable intramolecular exchange at room temperature in the ¹⁹F and ¹H n.m.r. spectra. The chiral centre at phosphorus is eliminated during the intramolecular exchange which probably occurs via a bis[bis(trifluoromethyi)methoxy] equatorially substituted intermediate. Long-range couplings [⁸J(FF)], interpreted as through-space interactions, have been found in all the compounds; the long-range couplings may be used diagnostically in the assignment of ground-state structures to these trigonal-bipyram-idal phosphoranes. Couplings between groups have been assigned on the basis of selective homo- and hetero-nuclear-decoupling experiments.