Analysis of Linear Free‐Energy Relationships Combined with Activation Parameters Assigns a Concerted Mechanism to Alkaline Hydrolysis of X‐Substituted Phenyl Diphenylphosphinates

Abstract

A kinetic study is reported for alkaline hydrolysis of X-substituted phenyl diphenylphosphinates (1 a-i). The Brønsted-type plot for the reactions of 1 a-i is linear over 4.5 pK(a) units with beta(lg)=-0.49, a typical beta(lg) value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with sigma(o) and sigma(-) constants are linear but exhibit many scattered points, while the corresponding Yukawa-Tsuno plot results in excellent linear correlation with rho=1.42 and r=0.35. The r value of 0.35 implies that leaving-group departure is partially advanced at the rate-determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO(-) ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (D(N) + A(N)) mechanism is also ruled out on the basis of the small beta(lg) value. As the substituent X in the leaving group changes from H to 4-NO(2) and 3,4-(NO(2))(2), DeltaH++ decreases from 11.3 kcal mol(-1) to 9.7 and 8.7 kcal mol(-1), respectively, while DeltaS++ varies from -22.6 cal mol(-1) K(-1) to -21.4 and -20.2 cal mol(-1) K(-1), respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1 a-i.

Multiparameter kinetic analysis of alkaline hydrolysis of a series of aryl diphenylphosphinothioates: models for P=S neurotoxins

“…The data in parentheses for the reactions of the P=O centered substrates 1a , 1b , and 1i were taken from Um et al…”

“…The Brønsted‐type plots for the reactions of 1a to 1i and 2a to 2i are linear with β lg = −0.49 ± 0.04 ( R 2 = 0.980) and −0.43 ± 0.04 ( R 2 = 0.975), respectively. A linear Brønsted plot with β lg = −0.5 ± 0.1 is typical for reactions reported to proceed via a concerted mechanism . We have reported that the reactions of 1a to 1i proceed through a concerted mechanism, partly on the basis of such linear Brønsted‐type plots with β lg = −0.49 .…”

“…The uncertainty in the k OH − values is estimated to be less than 3 % based on replicate runs. The k OH − values for the reactions of 2a to 2i with OH − are summarized (Tables and ) together with those reported previously for the corresponding reactions of 1a to 1i (P=O analogues of 2a‐i ) for comparison. Activation parameters (Δ G ‡ , Δ H ‡ , and T Δ S ‡ ) were calculated from the kinetic data for the reactions performed at 5 temperatures using the Eyring equation and are summarized (Table ).…”

“…A linear Brønsted plot with β lg = −0.5 ± 0.1 is typical for reactions reported to proceed via a concerted mechanism . We have reported that the reactions of 1a to 1i proceed through a concerted mechanism, partly on the basis of such linear Brønsted‐type plots with β lg = −0.49 . Thus, one can propose that the current reactions of 2a to 2i proceed also through a concerted mechanism, although expulsion of the leaving group may be slightly less advanced for the reactions of 2a to 2i than for the corresponding reactions of 1a to 1i based on the magnitude of the β lg values, ie, −0.43 (P=S) versus −0.49 (P=O), a difference of 0.06 that, admittedly, is only slightly beyond the statistical errors in the respective slopes (±0.04).…”

A Kinetic Study on Nucleophilic Substitution Reactions of Phenyl Y-Substituted-Phenyl Carbonates with Z-Substituted-Phenoxides: Effect of Modification of Nonleaving Group from Benzoyl to Phenyloxycarbonyl on Reactivity and Reaction Mechanism

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] As shown in Scheme 1, aminolysis of esters is generally known to proceed through a stepwise pathway with a zwitterionic tetrahedral intermediate T ± , in which the rate-determining step (RDS) is dependent on the basicity of the incoming amine and the leaving group, or through a concerted mechanism depending on the reaction conditions (e.g., the nature of electrophilic center and reaction medium).…”