Sulfonyl-Containing Eight-Membered Rings Varying in Ring Conformation in Oxyphosphoranes. Hexacoordination vs Pentacoordination¹

Abstract

The new cyclic phosphine O 2 S[(t-Bu)MeC 6 H 2 O] 2 PPh (1) was used to prepare the new cyclic tetraoxyphosphoranes O 2 S[(t-Bu)MeC 6 H 2 O] 2 PPh(O 2 C 6 Cl 4 ) (2) and O 2 S[(t-Bu)MeC 6 H 2 O] 2 PPh(O 2 C 6 H 4 ) (3) by oxidative addition reactions. In similar reactions, the new cyclic pentaoxyphosphoranes O 2 S[(t-Bu)MeC 6 H 2 O] 2 P(OCH 2 CCl 3 ) 3 (4), O 2 S-[Me 2 C 6 H 2 O] 2 P(OPh) 3 (5), and O 2 S[(t-Bu)MeC 6 H 2 O] 2 P(OC 6 F 5 ) 3 ( 6) were formed from acyclic phosphites and the appropriate diol. Yields ranged from 51% to 66%. X-ray studies revealed hexacoordinated representations for 2 and 6 while 3-5 are pentacoordinate in trigonal bipyramidal geometries. For 3 and 4, the eight-membered rings are positioned diequatorially in anti chair conformations, whereas for 5, this ring occupies axial-equatorial sites in a syn twist-boat form. Tetraoxyphosphorane 3 is unique in locating the least electronegative ligand, the phenyl group, in an axial position. Oxygen donor action from the sulfone SO 2 group led to a displacement from a square pyramid to an octahedron to the extent of 82.2% for the highly fluorinated member 6 in comparison with 27.9% for the tetrachlorocatecholate derivative 2. These two pentaoxyphosphoranes provide the longest and shortest P-O distance thus far from a sulfone donor group, 2.646 (5) Å for 2 and 1.936 (7) Å for 6. In comparison with oxyphosphoranes having bridging sulfur atoms in the eight-membered ring, it is apparent that sulfur atom coordination is greater than oxygen atom coordination from the sulfone group as measured by the tendencies toward hexacoordination. 31 P and 1 H NMR data indicate the presence of two isomeric forms for each of the phosphoranes 2-4. Structural comparisons are made with related cyclic phosphoranes containing sulfur atoms and sulfone groups relative to steric and electronic factors that influence the formation of hexacoordination.

Hexacoordinate Phosphorus via Donor Interaction. Implications Regarding Enzymatic Reaction Intermediates

“…Cyclic phosphoranes with methylene 30,39 or sulfur [30][31][32][33][34][35][36] bridges in place of the sulfone group lack evidence for isomeric representations. Since the sulfur atom acts as a stronger donor in comparison to the sulfone group, 40 perhaps this factor acts to increase the…”

The Effect of Very Bulky Groups on the Equilibrium of Penta- and Hexacoordinated Phosphoranes

“…5,6 In the latter penta-hexa coordinate equilibrium, we observed only the pentacoordinated phosphorane in the solid state, but the nature and strength of the hexacoordinated form remained unknown. To investigate this problem more extensively, we synthesized new diphenols consisting of very bulky groups.…”

“…Sulfur dichloride (80%, Aldrich) was purified according to literature method. 7 N-chlorodiisopropylamine 6 was synthesized according to our earlier methods. Solvents were purified according to standard procedures.…”

“…5,6,13 For example, when there were no bulky groups, there was no donor interaction between oxygen and phosphorus as seen in the phosphorane 12 (Scheme 4). 6 In phosphorane 12, the eight-membered ring has a twistsyn conformation with a P O distance of 3.233(5)Å.…”

New Trends in Hexacoordinated Phosphorus Chemistry

“…If the sulfur atom is oxidized to a sulfonyl moiety, then oxygen donation occurs to the phosphorus as observed in compounds 49a-49h (Fig. 10) [61, [64][65][66]. In most examples, the existence of an equilibrium between penta-and hexacoordinated phosphorus was demonstrated in solution.…”