Kinetics and mechanism of the hydrolysis of a (5,6)-spirophosphorane. Thermodynamics of the hydrolysis of cyclic five-membered and six-membered phosphonium ions

The cyclic five-membered phosphonium ion 2b (2-(2'-hydroxyethoxy)-2-phenyl-1,3,2-dioxaphospholan-2-ylium) derived from ring-opening of the (5,5)-spirophosphorane 1b (5-phenyl-1,4,6,9-tetraoxa-5-phosphaspiro[4,4]nonane) has been observed in neat CF3SO3H and at > 85% H2SO4. The cation undergoes hydrolysis in the latter solutions, and an extrapolation has been carried out to obtain an estimate for reactivity in 100% water. Hydrolysis rate constants for phenyltrialkoxyphosphonium ions in water are 10(7), 10(0), and 5 x 10(-3) S-1 for cyclic five-membered, cyclic six-membered, and acyclic derivatives respectively; these show an excellent correlation with rate constants for a similar series of phosphate esters. An investigation of the hydrolysis of the (5,6)-spirophosphorane 5 (5-phenyl-8,8-dimethyl-1,4,6,10-tetraoxa-5-phosphaspiro[4,5]decane) provides a clue as to the origins of these rate differences. This phosphorane can in principle hydrolyze via two isomeric cyclic phosphonium ions, the six-membered 14 and the five-membered 15. The former is thermodynamically more stable, being the only cation observed under equilibrating conditions of strong acid. However, the hydrolysis of the spirophosphorane, as well as the hydrolysis of fully formed 14, channels through the cyclic five-membered 15. A thermodynamic breakdown reveals that the 9.5 kcal mol-1 difference in activation free energy for the hydrolysis of five- and six-membered cyclic phosphonium ions is due to a combination of a higher free energy (2.5-4.5 kcal mol-1) for the five-membered cation, and a lower free energy (7-5 kcal mol-1) for the pentacoordinate transition state with the five-membered ring. This analysis also shows that a (5,6)-spirophosphorane is 6-8 kcal mol-1 more stable than a (6,6)-spirophosphorane. Thus, a five-membered ring has a significant stabilizing effect on a pentacoordinated phosphorus structure. The accelerated hydrolysis of cyclic phosphonium ions and phosphate esters with five-membered rings is caused by a combination of this stabilizing effect in the transition state and a destabilizing effect in the ground state associated with ring strain.