Conformational Preference and Donor Atom Interaction Leading to Hexacoordination vs Pentacoordination in Bicyclic Tetraoxyphosphoranes1

Publisher Website
Google Scholar
Abstract
New bicyclic tetraoxyphosphoranes all containing a six-membered oxaphosphorinane ring, C6H8(CH2O)2P(OC12H8)(OXyl) (1), (C6H4O)2P(OC12H8)(OXyl) (2), CH2[(t-Bu)2C6H2O]2P(OC12H8)(OXyl) (3), O2S[(t-Bu)MeC6H2O]2P(OC12H8)(OXyl) (4), and S[(t-Bu)MeC6H2O]2P(OC12H8)(OXyl) (5), were synthesized by the oxidative addition reaction of the cyclic phosphine P(OC12H8)(OXyl) (6) with an appropriate diol in the presence of N-chlorodiisopropylamine. X-ray analysis revealed trigonal bipyramidal (TBP) geometries for 1−4 where the dioxa ring varied in size from six- to eight-membered. With a sulfur donor atom as part of an eight-membered ring in place of a potential oxygen donor atom of a sulfone group as in 4, the X-ray study of 5 showed the formation of a hexacoordinated structure via a P−S interaction. Ring constraints are evaluated to give an order of conformational flexibility associated with the (TBP) tetraoxyphosphoranes 4 > 3 ∼ 1 > 2 which parallels the degree of shielding from 31P NMR chemical shifts: 4 > 3 > 1 > 2. The six- and seven-membered dioxa rings in 1 and 2, respectively, are positioned at axial−equatorial sites, whereas the eight-membered dioxa ring in 3 and 4 occupies diequatorial sites of a TBP. V−T1H NMR data give barriers to xylyl group rotation about the C−OXyl bond. The geometry of 5 is located along a coordinate from square pyramidal toward octahedral to the extent of 60.7%. Achieving hexacoordination in bicyclic tetraoxyphosphoranes of reduced electrophilicity relative to bicyclic pentaoxyphosphoranes appears to be dependent on the presence of a sufficiently strong donor atom.