Observation of Pseudorotamers of Two Unconstrained Wittig Intermediates, (3RS,4SR)- and (3RS,4RS)-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

The two typical, that is unstabilized and unconstrained, oxaphosphetane diastereomers (3 RS,4 SR )- ( 3 ) and (3 RS,4 RS )-4-cyclohexyl-2-ethyl-3,4-dimethyl-2,2-diphenyl-1,2λ 5 -oxaphosphetane ( 5 ) have been prepared selectively by deprotonation of (1 RS,2 SR )- ( 6 ) and (1 RS,2 RS )-(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 31 P NMR spectroscopy. The conformations of the resolved pseudorotamers at the pentacoordinate trigonal bipyramidal phosphorus atom are identified by 1 H, 13 C, and 31 P 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. 10 11, 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 .