A kinetic study of the alkaline hydrolysis of four-coordinated phosphorus (PIV) compounds. Impact of conformational transmission in the transiently formed five-coordinated (PV-trigonal-bipyramidal) intermediates on reaction rate and product distribution

It can be shown that alkaline hydrolysis of four-coordinated phosphorus (PIV) compounds is substantially accelerated if the trigonal-bipyramidal (TBP) five-coordinated phosphorus (PV) intermediate in the reaction can exhibit conformational transmission. With the compounds 5a-c, it was shown that this acceleration is an entropy effect. Pseudorotation in the PV-TBP intermediate is facilitated if conformational transmission occurs, and therefore the entropy loss during build-up of the PV-TBP is reduced. This evidently results in a lower free energy of activation for the reaction. For the phosphate triester systems 6a and 6b, it was also found that the occurrence of conformational transmission speeds up the hydrolysis reaction, although the effect is less pronounced than for compounds 5a-c. The cyclopentylmethoxy group (6a) and the tetrahydrofurfuryloxy group (6b) show slight preferences for equatorial and axial positioning in the transient PV-TBP, respectively. This study shows for the first time that reaction rate and product selectivity of hydrolyses of phosphate esters can be determined in part by the conformational transmission effect.