Synthesis, structural characterization, and conformational bias in solution of a sterically congested pyrophosphite: Experimental and computational evidence for restricted rotation about an sp(3)-sp(3) P-O single bond.

The synthesis and structural characterization of the sterically congested p yrophosphite 6-[2,4,8,10-tetrakis(1,1-dimethylethyl)-dibenzo[d,f][1,3,2] di oxaphosphepin, 3, is described. hi solution at room temperature, a single s pecies was observed that was consistent with a pyrophosphite structure with out any evidence for the tautomeric diphosphine monoxide. Below the coalesc ence temperature (T-C), 0 degreesC, three atropisomers were observed with r elative absolute configurations of (R*,R*,R*), (R*,S*,R*). and (R*,R*,S*). Ring inversion of the seven-membered rings below the we is slow on the NMR time scale, which leads to observable diastereoisomerism because of the pre sence of two independent stereoaxes (sp(2)-sp(2) C-C single, bond connectin g the two aryl rings). Additionally, a rotation about an exocyclic P-O sing le 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 c onformation in solution below the T-C, namely, the (R*,R*,S*) atropisomer. The results of a conformational search, performed with a specifically param etrized AMBER* force field, were in agreement with the P-31 NMR assignment of the major (R*,R*,S*) atropisomer, which was found to be an energy minimu m. Additionally, we could independently assign the relative configuration o f the minor isomers based on the calculated results.