Air Oxidation in the Dark

Alkylation of 68 with triethyloxonium tetrafluorborate leads to three stereo-isomeric 4.4 -pero3Q -phosphinic acid ethyl esters 69a-c. One 69a) could be isolated as a pure substance, m.p. 178—180 C the existence of the two others, m.p. 145—152 °C, is proved by H-NMR, although they could not be fully separated. 

The three isomers are cis/trans isomers with respect to the substituents -phenyl and -0-0 at C-4, and -OC2H5, = 0 at the P atom. For convenience, we use the E/Z nomenclature recommended for ethylene diastereoisomer assignment. If we replace the plane in ethylenes through the sp and sp atoms by a plane vertical to the phosphacyclohexadiene (2,4) ring (thought of in first approximation as planar) through C—4 and the phosphorus atom, we have to denote the isomer with -0—0 at C—4 and = O at P on the same side of this plane as Z , and that with -0-0 at C—4 and -OC2H5 at P on different sides as E diasteroisomer. 

69 therefore should give three stereoisomers E—E, E—Z and Z—Z, all of which we have found. In 68, which loses its stereoisomerism by epimerization of the P substituents through hydrogen bridges of the phosphinic acid, the three steric diastereoisomers cannot de detected. 

For assignment, we apply the same arguments as are used on for the diastereo-isomeric 4-hydroxy-phosphinic-esters 72 Z and 72 E, i e. H-NMR shifts of the protons at C-3 and C—5 and the protons of the CH3 groups of the P-O-CH2—CH3 group (Table 8)  

Reduction of a mixture of 69a-c or 69a with zinc dust in acetic acid yields two diastereoisomeric 4-hydroxy-phosphinic acid ethyl esters 70 Z and E, or only 70 E. 

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