Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada.
Inorg Chem. 2011 Aug 15;50(16):7852-62. doi: 10.1021/ic201062h. Epub 2011 Jul 11.
The mechanism of cleavage of a series of seven O,O-dimethyl O-aryl phosphorothioates (6a-g) promoted by a C,N-palladacycle, (2-[N,N-dimethylamino(methyl)phenyl]-C(1),N)(pyridine) palladium(II) triflate (5:OTf) in methanol at 25 °C was investigated with the aim of identifying catalytically important intermediates. Complete (s)(s)pH/rate profiles (in methanol) were conducted for the cleavage of 6a-g in the presence of 0.08 mM 5. The log k(obs) for the catalyzed methanolysis of 6a increases linearly with (s)(s)pH with a plateau above the (s)(s)pK(a)(1) of 11.16 for formation of 5:(-)OCH3. The profiles for 6b-g are bell-shaped, depending on the apparent ionizations of two acidic groups, with the rate constant maximum of the bell and the (s)(s)pK(a)(1) values shifting to higher (s)(s)pH values as the (s)(s)pK(a)(HOAr) of the leaving group phenol increases. A Brønsted plot of the log k(obs)(max) (the maximum rate constants for cleavage of 6a-g) vs (s)(s)pK(a)(HOAr) exhibits a downward break at ~ (s)(s)pK(a)(HOAr) 13, with the two wings having β(lg) values of 0.01 and -0.96. A model describing the kinetically important species involves a complex series of equilibria: 5:(HOCH(3)):pyr <=> 5:((-)OCH3):pyr + H(+) <=>(6) 5:((-)OCH3):6 + pyr <=> phosphorane 7 → product, where the rate limiting steps change from formation of 5:((-)OCH3):6 to formation of thiophosphorane 7 and then to product formation as the aryloxy leaving groups of 6 get progressively worse. Kinetic experiments indicate that the reaction of 5 with 6e, having a 4-chlorophenoxy leaving group, rapidly produces a transient intermediate, postulated to be the palladacycle-bound 5-coordinate thiophosphorane (7e) that exists long enough to obtain its UV/vis spectrum by stopped-flow spectrophotometry. Detailed analysis of the data sheds light on the origins of a previously reported anomalously large β(lg) of -1.93 for the descending wing of a Brønsted plot (J. Am. Chem. Soc. 2010, 132, 16599). Finally, energetics analysis indicates that the binding of palladacycle to the transition state comprising attack of methoxide on 6e, MeO(-) + 6e, stabilizes the latter by 34.9 kcal/mol, converting that transition state into an observable intermediate.
在 25°C 下，研究了一系列 7 个 O,O-二甲基 O-芳基膦酸硫酯（6a-g）在 C,N-钯环，（2-[N，N-二甲基氨基（甲基）苯基]-C（1），N）（吡啶）钯（II）三氟甲磺酸酯（5：OTf）的促进下的断裂机制，目的是鉴定催化上重要的中间体。在存在 0.08 mM 5 的情况下，在甲醇中对 6a-g 的断裂进行了完全（s）（s）pH/速率曲线的测定。对于 6a 的催化甲醇解的 log k（obs）随（s）（s）pH 线性增加，在 5：（-）OCH3 形成的（s）（s）pK（a）（1）为 11.16 时达到平台。6b-g 的曲线呈钟形，取决于两个酸性基团的表观离解，钟形的速率常数最大值和（s）（s）pK（a）（1）值随着离去基团苯酚的（s）（s）pK（a）（HOAr）值升高而向更高（s）（s）pH 值移动。以 log k（obs）（max）（6a-g 断裂的最大速率常数）与（s）（s）pK（a）（HOAr）的 Brønsted 图显示在~（s）（s）pK（a）（HOAr）13 处发生向下断裂，两翼的β（lg）值分别为 0.01 和-0.96。描述动力学上重要物种的模型涉及一系列复杂的平衡：5：（HOCH（3））：吡啶<=>5：（-）OCH3）：吡啶+H+<=>(6)5：（-）OCH3）：6+吡啶<=>膦酸硫酯 7→产物，其中限速步骤从 5：（-）OCH3）：6 的形成变为硫代膦酸酯 7 的形成，然后变为产物形成，因为 6 的芳氧基离去基团变得越来越差。动力学实验表明，5 与具有 4-氯苯氧基离去基团的 6e 的反应迅速产生瞬态中间体，推测为钯环结合的 5 配位硫代膦酸酯（7e），其存在时间足以通过停流分光光度法获得其紫外/可见光谱。对数据的详细分析揭示了先前报道的 Brønsted 图下降翼异常大的β（lg）为-1.93的起源（J. Am. Chem. Soc. 2010，132，16599）。最后，能垒分析表明，钯环与包括甲氧基进攻 6e 的过渡态的结合[MeO（-）+6e]（++），通过 34.9 kcal/mol 稳定了后者，将该过渡态转化为可观测的中间体。
