CONFORMATIONAL PREFERENCE AND DONOR ATOM INTERACTION LEADING TO HEXACOORDINATION VS PENTACOORDINATION IN BICYCLIC TETRAOXYPHOSPHORANES

New bicyclic tetraoxyphosphoranes all containing a six-membered oxapho sphorinane ring, C6H8(CH2O)(2)P(OC12H8)(OXyl) (1), (C6H4O)(2)P(OC12H8) (OXyl) (2), CH2[(t-Bu)(2)C6H2O](2)P(OC12H8)(OXyl) (3), O2S[(t-Bu)MeC6H 2O](2)P(OC12H8)(OXyl) (4), and S[(t-Bu)MeC6H2O](2)P(OC12H8)(OXyl) (5), were synthesized by the oxidative addition reaction of the cyclic pho sphine P(OC12H8)(OXyl) (6) with an appropriate diol in the presence of N-chlorodiisopropylamine. X-ray analysis revealed trigonal bipyramida l (TBP) geometries for 1-4 where the dioxa ring varied in size from si x-to eight-membered, With a sulfur donor atom as part of an eight-memb ered 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 hexacoordinat ed structure via a P-S interaction. Ring constraints are evaluated to give an order of conformational flexibility associated with the (TBP) tetraoxyphosphoranes 4 > 3 similar to 1 > 2 which parallels the degree of shielding from P-31 NMR chemical shifts: 4 > 3 > 1 > 2. The six- a nd 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-(TH)-H-1 NMR data give barriers to xylyl group rotation about the C-OXyl bond. The geometry o f 5 is located along a coordinate from square pyramidal toward octahed ral to the extent of 60.7%. Achieving hexacoordination in bicyclic tet raoxyphosphoranes of reduced electrophilicity relative to bicyclic pen taoxyphosphoranes appears to be dependent on the presence of a suffici ently strong donor atom.