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During the development of the oral anticoagulant Apixaban4 (6, Scheme 1), we were compelled to gain further insights into the key amidation step in order to optimize the reaction outcome and build a predictive kinetic model.5 Of central interest to the process team was the evaluation of critical kinetic and thermodynamic parameters to enable optimal control of reaction rates and yields. Herein we report mechanistic studies that shed light on the underlying complexities of the ester amidation promoted by sodium formamide. We began investigating the MeONa-mediated deprotonation of formamide in DMF (eq 1).6 The use of a ReactIR system allowed us to monitor the disappearance of formamide (1710 cm−1) and concomitant formation of its sodium salt (1580 cm−1).7 The deprotonation occurred instantaneously upon addition of MeONa at 0 °C8 to afford Keq = 25 ± 5, indicating that under the reaction conditions (24 equiv of formamide relative to MeONa) sodium formamide is the major sodium- bearing species in solution (>99.8 mol %).9 Next, we explored the equilibria generally represented in eq 2 by completing a sequence of control experiments. Reaction of carboxamide 6 in the absence of formamide with excess sodium diformylamide10 in DMF/MeOH afforded mixtures of carboxamide 6 and ester 4 with [6]/[4] ratios that decreased at higher concentrations of added sodium diformylamide (Figure 1a). Submission of the protonated form of N-acylformamide 5 to the reaction conditions resulted in instantaneous conversion to give mixtures of carboxamide 6 and ester 4 with increasing [6]/ [4] ratios at higher formamide/MeOH proportions (Figure 1b). These observations support a reversible amidation that requires excess formamide to promote the formation of carboxamide 6. Kinetic studies on the amidation of ester 4 were performed using the method of initial rates11 in the presence of excess sodium formamide and formamide relative to MeOH to effectively trap N-acylformamide 5. HPLC analyses of the reaction mixtures revealed a clean decay of ester 4 and simultaneous formation of carboxamide 6 along with trace amounts of the steady-state intermediate 5 in isolated experiments.12 A kinetic isotope effect kH/kD = 1.4 ± 0.1 determined by comparing amidations in formamide/MeOH and formamide-d3/MeOD mixtures is consistent with solvent participation in the reaction coordinate. Monitoring the decay of 4 over a range of substrate, sodium formamide, formamide, and MeOH concentrations using DMF as the cosolvent affords first-order dependencies in substrate and sodium formamide along with saturation kinetics in formamide and MeOH 2). Within the saturation regime in solvent, the reaction orders are consistent with the rate law in eq 3 and the general rate- determining step in eq 4, where m denotes the aggregation state of sodium formamide, S represents the coordinating solvents, and n defines the solvation number.13 The rate dependencies at low MeOH and formamide concentrations will be discussed in the context of computational studies below. rate =k ′[4]1[HCONHNa]1[MeOH]0[HCONH2]0 (3) [(HCONHNa)m Sn] + 4 → [(HCONHNa)m Sn(4)] ≠ (4) A Hammett plot for the analogous amidation of p-substituted methyl benzoates gives ρ = 2.2 ± 0.3, indicating the transfer of negative charge from sodium formamide to ester 4 in the rate- Figure 1. (a) Plot of [6]/[4] molar ratios versus [(CHO)2NNa] for the conversion of carboxamide 6 (0.011 M) to ester 4 in 6.5 M DMF/ MeOH after 24 h at 20 °C. (b) Plot of [6]/[4] ratios versus formamide mole fraction (χ) for the reaction of N-acylformamide 5 (0.010 M) with MeONa (0.67 M) in formamide/MeOH mixtures after 24 h at 20 °C. experiments.12 A kinetic isotope effect kH/kD = 1.4 ± 0.1 determined by comparing amidations in formamide/MeOH and formamide-d3/MeOD mixtures is consistent with solvent participation in the reaction coordinate. Monitoring the decay of 4 over a range of substrate, sodium formamide, formamide, and MeOH concentrations using DMF as the cosolvent affords first-order dependencies in substrate and sodium formamide along with saturation kinetics in formamide and MeOH (Figure Figure 2. Plots of initial rates for the amidation of ester 4 by HCONHNa at 0 °C versus (a) [4], (b) [HCONHNa], (c) [MeOH], (d) [HCONH2]. Rate dependencies on [4] and [HCONHNa] were measured in [MeOH] = 0.46 M and [HCONH2] = 4.8 M. Rate dependencies on [MeOH] and [HCONH2] were measured using [4] = 0.004 M and [HCONHNa] = 0.027 M in [HCONH2] = 4.8 M and [MeOH] = 0.11 M, respectively. Additional data are included in Supporting Information. 2). Within the saturation regime in solvent, the reaction orders are consistent with the rate law in eq 3 and the general rate- determining step in eq 4, where m denotes the aggregation state of sodium formamide, S represents the coordinating solvents, and n defines the solvation number.13 The rate dependencies at determining transition state.14 We investigated the amidation of model substrate 7 using DFT calculations at the B3LYP/6-31+G(d) level of theory. A series of geometries were tested for reactants, intermediates, and transition structures, and the optimized structures were submitted to single-point MP2/6-31+G(d) calculations incor- porating thermal corrections to Gibbs free energy as obtained from the frequency analysis at the B3LYP/6-31+G(d) level and PCM corrections for formamide as the solvent (ΔG, 298.15 K, 1.0 atm).15 Admittedly, the choice of implicit solvation and monomeric sodium formamide to simplify the unattainable combination of aggregates and solvates provides results that must be interpreted with caution. The discussion will focus on three aspects: (i) the deprotonation of formamide, (ii) the nature of the rate-determining transition structure, and (iii) the evaluation of the kinetically obscure formyl transfer. The computational study of the reaction between MeONa and formamide reveals a barrierless and exothermic deproto- nation (ΔG = −5.5 kcal•mol−1), in agreement with experimental results (Scheme 2). Transition structure TS1 displays an optimal geometry for the proton transfer with a planar six-membered ring and a virtually linear N−H−O angle.16 The lowest energy pathway calculated for the reaction between sodium formamide and ester 7 is summarized in Figure 3. The calculations depict a slightly exothermic conversion (ΔG = −1.0 kcal•mol−1) involving transition structures of comparable activation energies. Within the small range of energies (～1.5 kcal•mol−1), the first acyl transfer between sodium formamide and ester 7 to give N- acylformamide 9 is rate-determining. However, the calculations are not able to distinguish between the addition of sodium formamide to ester 7 (TS2) and the elimination of MeOH from tetrahedral intermediate 8 (TS3a) because both transitions states exhibit identical activation energies (ΔG⧧ = 23.2 kcal•mol−1).17 The assistance of a molecule of formamide in TS3a reduces the activation energy for the C(O)−OMe bond cleavage relative to the uncatalyzed pathways TS3b and TS3c, the MeOH-mediated cleavage TS3d, and the direct elimination of basic MeONa in TS3e18 (Figure 4). Presumably, formamide stabilizes the departure of the −OMe leaving group in a process followed by the barrierless regeneration of sodium formamide and deprotonation of the resulting N-acylforma- mide.19 Allred and Hurwitz proposed the nucleophilic attack of MeONa to the protonated form of N-acylformamide 9 as the next step in the sequence.3 However, two simple observations challenge this proposal. First, the higher acidity of the conjugated acid of N-acylformamide 9 relative to formamide and MeOH determines its overwhelming existence as the sodium salt 9.20 Second, the virtually complete deprotonation of formamide by MeONa results in a negligible concentration of MeONa. Taken together, these observations suggest the prevalence of a formyl transfer from sodium N-acylformamide 9 to sodium formamide. The examination of the nucleophilic attack of sodium formamide to sodium N-acylformamide 9 affords TS4 along with tetrahedral intermediate 10. Sub- sequently, 10 undergoes a 1,3-proton shift to give isomer 11.21 The most stable transition structure (TS5a) for the conversion of 10 to 11 includes a molecule of MeOH as the proton shuttle between the two amide nitrogens. Attempts to locate transition structures corresponding to the participation of formamide as the proton shuttle (TS5b) or a direct 1,3-proton shift (TS5c) resulted in structures with lower stabilities possibly due to the poor ability of formamide to transfer a proton between two heteroatoms22 and the inadequate geometric arrangement in the four-membered ring,16 respectively (Figure 4). Tetrahedral intermediate 11 displays a lengthened N(H)−C(O) bond (1.51 Å  relative to isomer 10 (1.45 Å en route to its cleavage through TS6. Finally, deprotonation of formamide by the resulting sodium carboxamide 12 is moderately favored to afford the desired carboxamide 13 and sodium formamide. The participation of formamide during the departure of the −OMe group in TS3a is seemingly at odds with the experimental zeroth-order dependence observed at high formamide concentrations (Figure 2d).23 The existence of the sodium formamide reactant as a formamide solvate would offer a plausible explanation for the discrepancy. Indeed, a saturation behavior for the formation of sodium formamide−formamide complexes in DMF/MeOH mixtures (eq 5) is in agreement Figure 4. Alternative transition structures TS3 and TS5. Calculated activation energies (ΔG⧧, kcal•mol−1) are given in parentheses. in a process followed by the barrierless regeneration of sodium formamide and deprotonation of the resulting N-acylforma- mide.19 Allred and Hurwitz proposed the nucleophilic attack of MeONa to the protonated form of N-acylformamide 9 as the next step in the sequence.3 However, two simple observations challenge this proposal. First, the higher acidity of the conjugated acid of N-acylformamide 9 relative to formamide and MeOH determines its overwhelming existence as the sodium salt 9.20 Second, the virtually complete deprotonation of formamide by MeONa results in a negligible concentration of MeONa. Taken together, these observations suggest the prevalence of a formyl transfer from sodium N-acylformamide 9 to sodium formamide. The examination of the nucleophilic attack of sodium formamide to sodium N-acylformamide 9 affords TS4 along with tetrahedral intermediate 10. Sub- sequently, 10 undergoes a 1,3-proton shift to give isomer 11.21 The most stable transition structure (TS5a) for the conversion of 10 to 11 includes a molecule of MeOH as the proton shuttle between the two amide nitrogens. Attempts to locate transition structures corresponding to the participation of formamide as the proton shuttle (TS5b) or a direct 1,3-proton shift (TS5c) resulted in structures with lower stabilities possibly due to the poor ability of formamide to transfer a proton between two heteroatoms22 and the inadequate geometric arrangement in with reports that indicate a coordinating ability for sodium salts that follows the order formamide ≥ DMF > MeOH.24 The persistence of such a complex in transition state TS3a would concur with the observed first-order in sodium formamide and zeroth-order in formamide at concentrations higher than 2 M (eq 6). In contrast, the saturation behavior in MeOH could be traced to the requirement of sufficient MeOH to facilitate the 1,3-proton shift in TS5a, which in the absence of MeOH would be rate-determining (TS5b or TS5c). 各位大俠幫忙翻譯一下 |
4樓2014-01-21 15:49:21
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