Synthesis, structural characterization, and conformational bias in solution of a sterically congested pyrophosphite: experimental and computational evidence for restricted rotation about an sp3-sp3 P-O single bond

The synthesis and structural characterization of the sterically congested pyrophosphite 6-[(2,4,8,10-tetrakis(1,1-dimethylethyl)-dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)oxy]-2,4,8,10-tetrakis(1,1-dimethylethyl)-dibenzo[d,f][1,3,2]dioxaphosphepin, 3, is described. In solution at room temperature, a single species was observed that was consistent with a pyrophosphite structure without any evidence for the tautomeric diphosphine monoxide. Below the coalescence temperature (T(C)), 0 degrees C, three atropisomers were observed with relative absolute configurations of (R,R,R), (R,S,R), and (R,R,S). Ring inversion of the seven-membered rings below the T(C) is slow on the NMR time scale, which leads to observable diastereoisomerism because of the presence of two independent stereoaxes (sp2-sp2 C-C single bond connecting the two aryl rings). Additionally, a rotation about an exocyclic P-O single bond connecting the two seven-membered rings, which constitutes a third stereoaxis, is slowed on the NMR time scale. In the X-ray crystal structure of 3, the solid-state conformation was found to be the same as the major conformation in solution below the T(C), namely, the (R,R,S) atropisomer. The results of a conformational search, performed with a specifically parametrized AMBER force field, were in agreement with the 31P NMR assignment of the major (R,R,S) atropisomer, which was found to be an energy minimum. Additionally, we could independently assign the relative configuration of the minor isomers based on the calculated results.