Kinetics and Mechanism of the Phenolysis of Asymmetric Diaryl Carbonates

Abstract

The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0 degrees C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Brönsted-type plots for the nucleophilic rate constants (k(N)) are linear, with slopes beta = 0.48 (MPNPC), 0.67 (ClPNPC), 0.41 (MPDNPC), and 0.32 (ClPDNPC). The magnitude of these slopes and the absence of a curvature in the Brönsted plot at pK(a) = 7.1 for the CIPNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and ClPDNPC are more reactive than MPNPC and CIPNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log k(N) in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log k(N) to the basicity of the nonleaving moiety (beta(nlg) = -0.27) is very similar to that of the nucleofuge (beta(lg) = -0.25).

Kinetics and mechanism of the reactions of polyallylamine with aryl acetates and aryl methyl carbonates

“…The values for the sensitivity to the nucleophile ( nuc ¼ 0.45 and 0.71 for acetates and carbonates, respectively) were discussed earlier and those for the sensitivity to the leaving group ( lg ¼ À 0.29 and À0.28) are near the expected value for a concerted mechanism. 23 Scheme 2 shows the transition state for the acetate esters with PAA. Logarithmic plots of the experimental nucleophilic rate constant values against those calculated with Eqns (3) and (4) (not shown) are linear withal a slope of unity and zero intercept.…”

Phenolysis of diaryl thiolcarbonates and thionocarbonates

“…There have been many reports on the kinetics and mechanisms of the phenolysis of esters [1][2][3][4][5][6] and thioesters, [7,8] but less regarding the phenolysis of methyl aryl carbonates, [9] diaryl carbonates, [10][11][12] O-alkyl S-aryl thiolcarbonates [8,13] alkyl aryl thionocarbonates [8,11,14] and diaryl thionocarbonates. [8,11,14] The kinetic studies on the phenolysis of methyl aryl carbonates [9] and O-ethyl S-aryl thiolcarbonates [13] show concerted mechanisms, in contrast to those of methyl aryl thionocarbonates, which show stepwise mechanisms.…”

Kinetics and mechanism of the anilinolysis of S‐aryl N‐arylthiocarbamates in acetonitrile

“…We note, however, that the magnitude of both b X (0.04-0.12) and b Z (from À0.07 to À0.18) is rather small, which is in the range of values (b X ¼ Àb Z ¼ 0-0.4) normally found for a stepwise mechanism where the formation of a zwitterionic tetrahedral intermediate is the rate-determining step. [5,[22][23][24][25][26][27][28][29] It has been shown that the b X values are normally greater than ca. 0.8 for a stepwise mechanism with rate-limiting breakdown of the intermediate, [5][6][7][8][9][10][11][12][13] while they are in the range of 0.4-0.6 for a concerted aminolysis process.…”

“…0.8 for a stepwise mechanism with rate-limiting breakdown of the intermediate, [5][6][7][8][9][10][11][12][13] while they are in the range of 0.4-0.6 for a concerted aminolysis process. [1][2][3][22][23][24][25][26][27][28][29][30][31] Based on the magnitude of Brö nsted coefficients therefore, we propose that reaction (1), proceeds through a zwitterionic tetrahedral intermediate, T AE , 2, the formation of which is rate determining. À0.99 À1.06 À1.12 À1.24 À1.45 a The correlation coefficients were better than 0.991, and standard deviations were less than 0.01 (with an average value of 0.006) in all cases.…”