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.