Change in rate-determining step in an E1cB mechanism during aminolysis of sulfamate esters in acetonitrile

2-Difluoromethoxy-Substituted Estratriene Sulfamates: Synthesis, Antiproliferative SAR, Antitubulin Activity, and Steroid Sulfatase Inhibition

2-Difluoromethoxyestratriene derivatives were designed to improve potency and in vivo stability of the drug candidate 2-methoxyestradiol (2ME2). Compound evaluation in vitro against the proliferation of MCF-7 and MDA MB-231 breast cancer cells, as inhibitors of tubulin polymerisation and also steroid sulfatase (STS) both in cell lysates and in whole cells, showed promising activities. In antiproliferative assays 2-difluoromethoxyestradiol was less potent than 2ME2, but its sulfamates were often more potent than their corresponding non-fluorinated analogues. The fluorinated bis-sulfamate is a promising antiproliferative agent in MCF-7 cells (GI50 0.28 μM) vs the known 2-methoxyestradiol-3,17-O,O-bissulfamate (STX140, GI50 0.52 μM), confirming the utility of our approach. Compounds were also evaluated in the NCI 60-cell line panel and the fluorinated bis-sulfamate derivative displayed very good overall activities with a sub-micromolar average GI50 . It was a very potent STS inhibitor in whole JEG-3 cells (IC50 3.7 nM) similar to STX140 (4.2 nM) and additionally interferes with tubulin assembly in vitro and colchicine binding to tubulin. An X-ray study of 2-difluoromethoxy-3-benzyloxyestra-1,3,5(10)-trien-17-one examined conformational aspects of the fluorinated substituent. The known related derivative 2-difluoromethyl-3-sulfamoyloxyestrone was evaluated for STS inhibition in whole JEG-3 cells and showed an excellent IC50 of 55 pM.

Evidence of Rate Limiting Proton Transfer in an SNAr Aminolysis in Acetonitrile under Synthetically Relevant Conditions

An early synthetic step in the synthesis of adavosertib, AZD1775, is the S_NAr reaction between 4-fluoronitrobenzene and 1-methylpiperazine in acetonitrile. A simple kinetics-based design of four reaction profiling experiments was used to investigate the kinetics of the reaction for the purpose of building a kinetic model. Fitting of the reaction profile data from two experiments conducted at 70 °C with a different excess of 1-methylpiperazine showed the reaction to follow a third-order rate law with a second-order dependence upon 1-methylpiperazine. This was rationalized in terms of the reaction following a rate-limiting proton transfer mechanism (base catalyzed) in which the progress to product is driven by a proton transfer involving a second molecule of 1-methylpiperazine. The experimentally determined entropy of activation of -180 J K^-1 is consistent with this mechanism. The formation of a low level impurity was found to be due to the presence of traces of piperazine in the 1-methylpiperazine, which was shown to react approximately 15 times faster than 1-methylpiperazine at 70 °C. The rate constants for the 1-methylpiperazine catalyzed reaction of piperazine, 1-methylpiperazine, and the piperazine derived impurity were found to correlate in a Brønsted type analysis with the pK_a's (acetonitrile) of the amine nucleophile.

Nucleofugality hierarchy, in the aminolysis reaction of 4-cyanophenyl 4-nitrophenyl carbonate and thionocarbonate. Experimental and theoretical study

Brønsted plot of the reaction of 9 with secondary alicyclic amines. The concave upward non-linear plot is in accordance with two parallel mechanistic pathways.

Kinetic study on aminolysis of aryl X-substituted-cinnamates in acetonitrile: differential medium effect determines reactivity and reaction mechanism

A kinetic study on nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted-cinnamates (1a–1f) and Y-substituted-phenyl cinnamates (2a–2g) with a series of alicyclic secondary amines in MeCN at 25.0 ± 0.1 °C is reported. The Brønsted-type plots for the reactions of 1a–1f are linear with βnuc = 0.47∼0.50, indicating that the bond formation between the amine nucleophile and the electrophilic center is advanced slightly in the transition state. The Brønsted-type plot for the reactions of 2a–2g with piperidine is also linear with βlg = –0.66, which is a typical βlg value for reactions reported previously to proceed through a concerted mechanism. Furthermore, the Hammett plot correlated with σ– constants results in much better linearity than that correlated with σo constants, implying that expulsion of the leaving group is advanced in the rate-determining step (RDS). Thus, the reactions are concluded to proceed through a concerted mechanism. The Hammett plots for the reactions of 1a–1f consist of two intersecting straight lines, whereas the corresponding Yukawa–Tsuno plots exhibit excellent linear correlations with ρX = 0.62∼0.71 and r = 0.65∼0.68. Apparently, the nonlinear Hammett plots are not due to a change in the reaction mechanism (or the RDS) but are caused by stabilization of the substrate possessing an electron-donating group (EDG) in the cinnamoyl moiety through resonance interactions between the EDG and the C=O bond of the substrate. Medium effects on reactivity and reaction mechanism are also discussed.

Unexpected medium effect on the mechanism for aminolysis of aryl phenyl carbonates in acetonitrile and H2O: transition-state structure in the catalytic pathway

Upward curvature in the kinetic plots of pseudo first-order rate constants (kobsd) vs. [amine] for the aminolysis of aryl phenyl carbonates (5a–5j) in MeCN demonstrates that these reactions proceed via a zwitterionic tetrahedral intermediate (T±) that partitions between catalyzed and uncatalyzed routes to give the products. Yukawa–Tsuno plots for the reactions of 5a–5j with piperidine result in excellent linear correlations with ρY = 4.82 and r = 0.47 for the uncatalyzed reaction versus ρY = 2.21 and r = 0.21 for the catalyzed reaction. Brønsted plots for reactions of 4-(ethoxycarbonyl)-phenyl phenyl carbonate (5f) with a series of cyclic secondary amines exhibit excellent linear correlations with βnuc = 0.87 and 0.58 for the uncatalyzed and catalyzed reactions, respectively. The ΔH‡ and ΔS‡ values are 0.92 kcal/mol and –50.1 cal/mol K, respectively, for the catalyzed reaction of 5f with piperidine. Deuterium kinetic isotope effects found for reactions of 5f with piperidine/deuterated piperidine are 0.84 (uncatalyzed) and 1.42 (catalyzed). Multi-parameter analysis supports a concerted catalytic pathway involving a six-membered cyclic transition state rather than a traditionally accepted stepwise pathway with an anionic intermediate. The current unexpected results, where T± is the essential central intermediate in this aminolysis, contrast with previous calculation studies that deemed T± unstable in gas phase or MeCN.

Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines

A kinetic study on SNAr reactions of 1-aryloxy-2,4-dinitrobenzenes (1a–1h) with a series of cyclic secondary amines in 80 mol% water – 20 mol% DMSO at 25.0 ± 0.1 °C is reported. The plots of kobsd versus amine concentration curve upward except for the reactions of substrates possessing a strong electron-withdrawing group in the leaving aryloxide with strongly basic piperidine. The curved plots indicate that the reactions proceed through both uncatalytic and catalytic routes. Linear Brønsted-type plots have been obtained for the uncatalyzed and catalyzed reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with βnuc = 0.84 and 0.78, respectively. The Yukawa–Tsuno plot for the uncatalyzed reactions of 1a–1h with piperidine results in an excellent linear correlation with ρ = 1.66 and r = 0.31. In contrast, rate constants for catalyzed reactions are independent of the electronic nature of the substituent in the leaving group. The current SNAr reactions have been proposed to proceed via a zwitterionic intermediate (MC±) that partitions to products through uncatalytic and catalytic routes. The catalyzed reaction from MC± has been concluded to proceed through a concerted mechanism with a six-membered cyclic transition state (TScycl) rather than via a stepwise pathway with a discrete anionic intermediate (MC−), the traditionally accepted mechanism. Medium effects on the reactivity and reaction mechanism are discussed. Particularly, hydrogen bonding of the amines to water precludes formation of kinetically significant dimers found in some aprotic solvents; no explicit role for water in the catalytic transition state is required or proposed. The specific stabilization of the leaving aryloxides substituted with strong electron-withdrawing groups accounts for the lack of the catalytic pathway in these systems (1a–1c) with piperidine nucleophile.

Crystal structure of 2,4-di-nitro-phenyl 4-methyl-benzene-sulfonate: a new polymorph

The title compound, C13H10N2O7S, was synthesized via a nucleophilic substitution reaction between 2,4-di-nitro-phenol and p-toluene-sulfonyl chloride. This crystal structure is a polymorph of CSD entry WUVYUH [Vembu et al. (2003). Acta Cryst, E59, o378-380]. The aromatic substituents on the sulfonate group are oriented gauche to one another with a C-O-S-C torsion angle of -62.0 (3)°. The supra-molecular features that contribute to the crystal stability are offset π-π [centroid-centroid distance = 3.729 (2) Å] and multiple C-H⋯O inter-actions.

Comparison of aminolysis of 2-pyridyl and 4-pyridyl x-substituted benzoates in acetonitrile: evidence for a concerted mechanism involving a cyclic transition state

A kinetic study on reactions of 2-pyridyl X-substituted benzoates (6a-i) with a series of cyclic secondary amines in MeCN is reported. The Hammett plot for the reaction of 6a-i with piperidine consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρX = 1.28 and r = 0.63, indicating that the nonlinear Hammett plot is not caused by a change in the rate-determining step but rather by resonance stabilization of substrates possessing an electron-donating group (EDG) in the benzoyl moiety. The Brønsted-type plots are linear with βnuc = 0.59 ± 0.02, which is typical of reactions reported to proceed through a concerted mechanism. A cyclic transition state (TS), which forces the reaction to proceed through a concerted mechanism, is proposed. The deuterium kinetic isotope effect of 1.3 ± 0.1 is consistent with the proposed mechanism. Analysis of activation parameters reveals that ΔH(‡) increases linearly as the substituent X changes from an electron-withdrawing group (EWG) to an EDG, while TΔS(‡) remains nearly constant with a large negative value. The constant TΔS(‡) value further supports the proposal that the reaction proceeds through a concerted mechanism with a cyclic TS.

Concurrent primary and secondary deuterium kinetic isotope effects in anilinolysis of O-aryl methyl phosphonochloridothioates

The nucleophilic substitution reactions of Y-O-aryl methyl phosphonochloridothioates with substituted anilines (XC(6)H(4)NH(2)) and deuterated anilines (XC(6)H(4)ND(2)) are investigated kinetically in acetonitrile at 55.0 degrees C. The Hammett and Brønsted plots for substituent (X) variations in the nucleophiles are biphasic concave downwards with a break region between X = H and 4-Cl. The deuterium kinetic isotope effects (DKIEs) are primary normal (k(H)/k(D) = 1.03-1.30) for stronger nucleophiles (X = 4-MeO, 4-Me and H), and extremely large secondary inverse (k(H)/k(D) = 0.367-0.567) for weaker nucleophiles (X = 4-Cl, 3-Cl and 3-NO(2)). The cross-interaction constants are negative (rho(XY(H)) = -0.95 and rho(XY(D)) = -1.11) for stronger nucleophiles, while positive (rho(XY(H)) = +0.77 and rho(XY(D)) = +0.21) for weaker nucleophiles. These kinetic results indicate that the mechanism changes from a concerted process involving frontside nucleophilic attack for stronger nucleophiles to a stepwise process with a rate-limiting leaving group expulsion from the intermediate involving backside attack for weaker nucleophiles. A hydrogen-bonded, four-center-type transition state (TS) is suggested for a frontside attack, while a trigonal bipyramidal pentacoordinate TS is suggested for a backside attack. The unusually small DKIEs, as small as or equal to 0.4, for weaker nucleophiles seem to be ascribed to severe steric congestion in the TS.