Building the phosphoindigo backbone by oxidative coupling of phosphindolin-3-ones with selenium dioxide

Oxidative coupling of racemic 1-ethoxy-1-oxophosphindolin-3-one (1) and its 5-CF3-derivative 6 with SeO2 furnishes 1,1'-diphosphaindigo derivatives 5 and 7 as bis-phosphinic esters, i.e. as P-V-compounds. Like indigo and thioindigo, 5 and 7 exist in the E-configuration; the crude products of 5 and 7 are mixtures of isomers that are trans- and cis-configurated with respect to the relative orientation of the ester groups oat phosphorus. The structure of the centrosymmetric E-P(R)P'(S) isomer [(E)-trans-isomer] of 5 was determined by X-ray crystallography. Ester cleavage of 5, followed by addition of triethylamine to bis-phosphinic acid 9 (the 1,1,1',1'-tetroxide of phosphoindigo), furnishes the related bis-triethylammonium salt 10 as a crystalline hydrate that exhibits an extended hydrogen bonding network.