Conformational transmission in trigonal-bipyramidal phosphorus(V) compounds. Enhanced gauche(-) population around the C4'-C5' linkage in 5'-PV phosphorylated tetrahydrofurfuryl model systems

A 300-MHz ‘H NMR study on a number of 5’-phosphorylated (P") tetrahydrofurfuryl model compounds (la,b,3a,b, 4a,b) and their Pv trigonal-bipyramidal (TBP) analogues (lc,d, 3c,d, 4c,d) has been performed. The results show a significantly greater population of the gauche(-) conformation for axially situated tetrahydrofurfuryl around the C4X5, bond in the 5’-Pv TBP tetrahydrofurfuryls with respect to their related 5’-P" compounds which show dominant gauche(+) and gauche(trans) conformations. The conformation analysis of the Pv compounds was hampered by pseudorotation. With model compound 6, in which both equatorial and axial sites that undergo pseudorotation bear a tetrahydrofurfuryl group, a precise analysis was possible resulting in an excess of gauche(-) for tetrahydrofurfuryl in an axial position (318 K, 61 %; 217 K, 80%). The corresponding equatorial location shows a relatively small amount of gauche(-) compared with gauche(+) and gauche(trans) (318 K, 24%; 217 K, 13%). The latter values are similar to the PIv counterpart 5. This conformational transmission in the C4-C5, bond of tetrahydrofurfuryls agrees with quantum chemical calculations. It is suggested that the enhanced charge repulsion between 05a,n d 01i,n the Pv TBP drives the rotation around the C47C5b, ond. The impact for conformational isomerizations in phosphorylated biomolecules (e.g., DNA) is briefly mentioned.