The Effect of Very Bulky Groups on the Equilibrium of Penta- and Hexacoordinated Phosphoranes¹

The synthesis of dithio-diphenol 4 and sulfonyl-diphenol 5, both with very bulky groups, provided starting materials for reaction sequences that led to the formation of the very stable hexacoordinated phosphorane 2 and sensitive pentacoordinated phosphorane 3. Hexacoordination was established in 2 by an intramolecular donor interaction at the phosphorus center from an oxygen atom of the sulfonyl group present as part of the eight-membered ring. The solid state structures of 2 and 3 were established by X-ray analysis, as was that of phosphite 1 formed in the reaction sequence leading to 2. In solution, 2 has two forms existing in a dynamic equilibrium between a pentacoordinated and the more dominant hexacoordinated form as determined by ³¹P and ¹⁹F NMR spectroscopy. The high stability of 2 with respect to hydrolysis and alcoholysis reactions suggests that an associative process is responsible as the controlling reaction mechanism.

Keywords:

• Cyclic sulfonyl derivatives
• hexacoordination
• phosphoranes
• steric effects
• X-ray crystallography

The research presented here was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and is gratefully acknowledged. We thank the X-Ray Structural Characterization Laboratory at the Chemistry Department supported by University of Massachusetts and the National Science Foundation (Grant CHE-9974648).

Notes

^a R = Σ|F_o‖−|F_c‖/Σ |F_o|.

^bR_w (F_o ²) = {Σ w(F_o ²− F_c ²)²/Σ wF_o ⁴}^½.

^a Equilibrium denotes the existence of an isomeric penta-hexa coordination equilibrium in the solution. The solid state coordination at phosphorus is given in parentheses as found by X-ray analysis.

^b Bulkier α,α -dimethylbenzyl group is used here.

^c Highly electronegative pentafluorophenoxy groups are used for 11 and 13.

^d Phosphite 13 has P─O donor interaction ¹³ that is as strong as that found in phosphoranes.

^e Compounds 11 and 12 deviate considerably from the Berry coordinate.