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 (3RS,4SR)- (3) and (3RS,4RS)-4-cyclohexyl-2-ethyl-3,4-dimethyl-2,2-diphenyl-1,2 lambda(5)-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 P-31 NMR spectroscopy. The conformations of the resolved pseudorotamers at the pentacoordinate trigonal bipyramidal phosphorus atom are identified by H-1,C-13,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 I,2-oxaphosphetanes Oxaphosphetanes 3 and 5 stereoselectively decompose to (Z)- and (E)-2-cyclohexylbut-2-ene, respectively. At -30 degrees C, pseudorotations are faster than alkene formations by a factor of ca. 10(11), and at -20 degrees C, the half-life of the trans isomer 5 in alkene formation is approximately 8 times longer than that of the cis isomer 3.