INHIBITION OF POSITIONAL EXCHANGE IN DERIVATIVES OF 2,2-DIHYDRO-1,4,2-DIOXAPHOSPHOLANES

Trimethyl phosphite reacted with pentafluorobenzaldehyde at 0° in pentane and gave a crystalline mixture of cis- and trans-3,5-di(pentafluorophenyl)-2,2,2-trimethoxy-2,2-dihydro-1,4,2-dioxaphospholanes in a 55:45 proportion and in 85% of theory. The variable-temperature 1H nmr spectrum of the cis isomer showed that the three methoxy groups on the trigonal bipyramidal pentavalent phosphorus underwent rapid intramolecular positional exchange in solutions above ca. -100°. This exchange was inhibited at about -110°. The spectrum at -130° showed the presence of two methoxy groups in nonequivalent equatorial positions and one methoxy group in an apical position. The phospholane ring occupied the remaining apical-equatorial positions. The interconversions among the possible stereoisomeric trigonal bipyramids by pseudorotation are discussed. 31P nmr data are given. © 1969, American Chemical Society. All rights reserved.