Observation of Pseudorotamers of Two Unconstrained Wittig Intermediates, (3<i>RS</i>,4<i>SR</i>)- and (3<i>RS</i>,4<i>RS</i>)-4-Cyclohexyl-2-ethyl-3,4- dimethyl-2,2-diphenyl-1,2λ⁵-oxaphosphetane, by Dynamic ³¹P NMR Spectroscopy: Line-Shape Analyses, Conformations, and Decomposition Kinetics

Abstract

The two typical, that is unstabilized and unconstrained, oxaphosphetane diastereomers (3RS,4SR)- (3) and (3RS,4RS)-4-cyclohexyl-2-ethyl-3,4-dimethyl-2,2-diphenyl-1,2λ5-oxaphosphetane (5) have been prepared selectively by deprotonation of (1RS,2SR)- (6) and (1RS,2RS)-(2-cyclohexyl-2-hydroxy-1-methylpropyl)ethyldiphenylphosphonium iodide (7). The X-ray structure analysis of 7 established the relative configurations of 6 and 7 and consequently those of 3 and 5. Pseudorotation of the 1,2-oxaphosphetanes 3 and 5 and their alkene formations have been studied by 31P NMR spectroscopy. The conformations of the resolved pseudorotamers at the pentacoordinate trigonal bipyramidal phosphorus atom are identified by 1H, 13C, and 31P NMR data. For both diastereomers, the two rotamers with equatorial position of the ethyl substituent dominate in pseudorotation. Line-shape analyses provided the rate constants and activation parameters of pseudorotations. The results represent the first experimental data for pseudorotation of unstabilized and unconstrained 1,2-oxaphosphetanes. Oxaphosphetanes 3 and 5 stereoselectively decompose to (Z)- and (E)-2-cyclohexylbut-2-ene, respectively. At −30 °C, pseudorotations are faster than alkene formations by a factor of ca. 1011, and at −20 °C, the half-life of the trans isomer 5 in alkene formation is approximately 8 times longer than that of the cis isomer 3.