Ring opening and reclosure in a cyclic phosphorane.

Cyclic phosphates and certain related compounds undergo hydrolysis millions of times faster than their acyclic analogs, and specifically may undergo rapid hydrolysis of ester groups that are external to the ring.'-4 These unusual rate effects have recently been explained5 by postulating that the trigonal bipyramidal intermediates, formed during hydrolysis, undergo an internal inversion, or "pseudorotation" process. Such an inversion or pseudorotation has been well established for PF5 and for alkyl fluorophosphoranes6' 7 and was recently demonstrated for the cyclic alkyl oxyphosphorane, I, by nuclear magnetic resonance (NMR) measurements.8 At low temperature, I is frozen into the conformation shown; by contrast, at room temperature the methoxyl groups undergo a rapid interchange of position. Although this exchange almost certainly takes place by pseudorotation, prior publications9' 10 had suggested that a process of this sort might occur by an opening and reclosure of the ring, to form a zwitterionic intermediate with equivalent methoxyl groups. In this communication, we report the interconversion of the stereoisomers II and III by pseudorotation at room temperature, and demonstrate that at elevated temperatures (120-160°) a second process occurs that makes the methyl (as contrasted to the methoxyl) groups of II and III equivalent. TheNMR measurements thus allow a clear distinction to be made between the low-temperature and high-temperature processes; they simultaneously confirm the reality of the mechanism previously proposed by Ramirez and Desai9 and by Kukhtin et al.,'0 and strongly reinforce our interpretation of the low-temperature process as pseudorotation.8