E Inversion, Non-equivalence, and Configuration

The non-equivalence of the ester protons in the monomethyl- and monophenyl-phosphinic ester function, as in (44, Ch = chalkogen), has been studied. Compounds of type (45) have some interesting stereochemistry. They are prepared from racemic secondary butyl alcohol, and the presence of three signals in the P n.m.r. spectrum confirms that the phosphorus atom is the centre of pseudo-asymmetry. A 1 2 1 triplet is observed which is attributed to the presence of equal amounts of two mesa forms, (45) and (46), which have different values of Sp (outer peaks), and two racemic forms, (47) and (48), which have identical values of 8p (central peak). 

spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P derivatives the isomer possessing the bulkiest P-substituent in an axial 

Mikolajczyk, M. Para, A. Ejchart, and J. Jurczak, Chem. Comm., 1970, 654. 

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