Sterically Congested Ligands: Synthesis and Solution Conformation of Bisphosphite Ligand Precursors

The Synthesis and Conformation of Sterically Congested Seven-membered Rings Containing Tetracoordinate Germanium(IV): Determination of the ΔG* For Ring Inversion1

“…The protons of the adjacent gem-dimethyl groups were also observed to be anisochronous. These observations are consistent with slow rotation about the stereoaxis (C−C bond connecting the two aryl groups) on the NMR time scale …”

“…A reasonable explanation for these observations is that below the T C , ring inversion of the seven-membered rings is slow on the NMR time scale, which leads to observable diastereoisomerism because of the presence of two independent stereoaxes (sp 2 −sp 2 C−C single bond connecting the two aryl rings). Below the T C , two atropisomers would be expected to be observable with relative absolute configurations of ( R*, R* ) and ( R*, S *), which refers throughout this paper to the relative configurations the two stereoaxes. , The observation of two pairs of equivalent tert -butyl substituents at 110 °C is consistent with rapid ring inversion of the dibenzo[ d,f ][1,3,2]dioxagermepin rings. Conformational averaging, because of rapid ring inversion and rotation of exocyclic bonds at 120 °C, leads to an overall C 2 v symmetry for 3 , which renders the corresponding pairs of tert -butyl protons in each ring homotopic…”