Solvent effects on reactions of hydroxide ion with phosphorus(V) esters. A quantitative treatment

Abstract

Second‐order rate constants of reactions of HO− with phosphate, phosphinate and thiophosphinate esters, (PhO)2PO.OC6H4NO2‐p, Ph2PO.OC6H4NO2‐p, Ph2PO.SPh, Ph2PO.SC6H4NO2‐p and Ph2PO.SEt, go through minima with decreasing water content of H2O‐MeCN or H2O‐t‐BuOH. The rate decrease is due to stabilization of the non‐ionic ester on addition of organic solvent to H2O. This inhibition is partially offset by stabilization of the anionic transition states and in the drier solvents partial desolvation of HO− increases rates.

Mapping transition state structures for thiophosphinoyl group transfer between oxyanionic nucleophiles in water and aqueous ethanol solvents

“…Decreasing the polarity of the medium moves the concerted TS in the opposite direction, towards dissociative structures, which arises because less polar solvents stabilize the IS of the reaction of these phosphorus esters with anionic oxygen nucleophiles, thereby decreasing their reaction rates. 24–27 Unlike typical alkyl substrates whose S N 2 reactions with charged nucleophiles undergo acceleration in less polar solvents due to the desolvation and consequent increased basic strength and nucleophilicity of the nucleophile, 57–59 polar S N 2(P) reactions of charged nucleophiles with neutral phosphorus esters, including (thio)phosphinates, are slower in less polar solvents due to the preferential solvation and stabilization of the phosphorus esters resulting from their decreased activity coefficients in such solvents. 1,24–27…”

“…24–27 Unlike typical alkyl substrates whose S N 2 reactions with charged nucleophiles undergo acceleration in less polar solvents due to the desolvation and consequent increased basic strength and nucleophilicity of the nucleophile, 57–59 polar S N 2(P) reactions of charged nucleophiles with neutral phosphorus esters, including (thio)phosphinates, are slower in less polar solvents due to the preferential solvation and stabilization of the phosphorus esters resulting from their decreased activity coefficients in such solvents. 1,24–27…”

“…1 by decreasing the activity coefficient of the neutral aryl phosphinothioate ester substrates. 24–27 The Hammond response to the stabilization of the IS is to destabilize the products by raising the energy of the product end of Fig. 1 according to the bold arrow S, which is tantamount to an x ↔ y movement of the TS along the concerted reaction coordinate.…”

“…The change in the structure of the concerted TS in water for the reaction of 3a-g to a looser one in 70% water–30% ethanol raises the possibility of the existence of a spectrum of transition states for the reactions of these aryl phosphinothionate esters with oxyanionic nucleophiles in the continuum of polarities between pure water and pure ethanol, obtained by the incremental addition of ethanol to water. Although solvent effects on the nucleophilic reactions of a variety of phosphorus esters have been reported, 24,25,30–34 the effects of solvent polarity on TS structures specifically have not received commensurate attention so far. 1 Such information about uncatalyzed phosphoryl transfer systems and related processes could be useful in understanding the choices which reacting species make in biological reactions when confronted with alternative reaction routes.…”

Reactions of aryl dimethylphosphinothioate esters with anionic oxygen nucleophiles: transition state structure in 70% water–30% ethanol

“… 51 In some cases, the stabilization of the IS by the less polar solvent is partially offset by the stabilization of the TS. 52 The hydrolysis of 4-nitrophenyl diphenylphosphate 52 and a number of phosphinate esters 51,53,54 have been shown to be faster in aqueous solvents than in binary aqueous organic solvents. The reactions of 3a–g with the different oxyanionic nucleophiles in 70% water–30% ethanol reported in this paper are all slower than the corresponding reactions in water, 2 the ratio of the second-order rate constants of these reactions in water and aqueous ethanol solvent, , ranging from 13.5 to 323.9 at 25 °C (see Tables 1 and 2 ), consistent with the effects of solvents on the reactions of phosphinate esters pointed out above.…”