Amatore C, Bucaille A, Fuxa A, Jutand A, Meyer G, Ndedi Ntepe A
Ecole Normale Supérieure, Département de Chimie, UMR CNRS 8640, Paris, France.
Chemistry. 2001 May 18;7(10):2134-42. doi: 10.1002/1521-3765(20010518)7:10<2134::aid-chem2134>3.0.co;2-9.
The oxidative addition of phenyl iodide to the palladium(o) generated from [Pd0(dba)2] and n equivalents of AsPh3 (the most efficient catalytic precursor in Stille reactions) proceeds from [(solv)Pd0(AsPh3)2] (solv= solvent). However, the latter is present only in trace concentrations because it is involved in an equilibrium with the major, but nonreactive, complex [Pd0(dba)(AsPh3)2]. As regards the phosphine ligands, dba has a decelerating effect on the rate of the oxidative addition by decreasing the concentration of the reactive species. Relative to PPh3, the effect of AsPh3 is to increase the rate of the oxidative addition of PhI by a factor ten in DMF and seven in THF, independent of the value of n, provided that n > or = 2. In contrast to PPh3, the addition of more than two equivalents of AsPh3 to [Pd0(dba)2] (dba= trans,trans-dibenzylideneacetone) does not affect the kinetics of the oxidative addition because of the very endergonic displacement of dba from [Pd0(dba)(AsPh3)2] to form [Pd0(AsPh3)3]. The complex trans-[PhPdI(AsPh3)2], formed in the oxidative addition, is involved in a slow equilibrium with the T-shaped complex [PhPdI(AsPh3)] after appreciable decomplexation of one AsPh3. Under catalytic conditions, that is, in the presence of a nucleophile, such as CH2=CH-SnBu3 which is able to coordinate to [Pd0(AsPh3)2], a new Pd0 complex is formed: [Pd0(eta2-CH2=CHSnBu3)(AsPh3)2]; however, this complex does not react with PhI. Consequently, CH2=CH-SnBu3 slows down the oxidative addition by decreasing the concentration of the reactive species [(solv)Pd0(AsPh3)2]. This demonstrates that a nucleophile may be not only involved in the transmetallation step, but may also interfere in the kinetics of the oxidative addition step by decreasing the concentration of reactive Pd0.
苯基碘对由[Pd⁰(dba)₂]和n当量的三苯基砷（Stille反应中最有效的催化前体）生成的零价钯的氧化加成反应，是从[(溶剂)Pd⁰(AsPh₃)₂]（溶剂 = 溶剂）开始的。然而，后者仅以痕量浓度存在，因为它与主要的但无反应性的配合物[Pd⁰(dba)(AsPh₃)₂]处于平衡状态。关于膦配体，二亚苄基丙酮通过降低反应性物种的浓度，对氧化加成速率有减速作用。相对于三苯基膦，三苯基砷的作用是在N,N - 二甲基甲酰胺中使苯基碘的氧化加成速率提高10倍，在四氢呋喃中提高7倍，只要n≥2，与n的值无关。与三苯基膦不同，向[Pd⁰(dba)₂]（二亚苄基丙酮 = 反式，反式 - 二亚苄基丙酮）中加入超过两当量的三苯基砷不会影响氧化加成的动力学，因为二亚苄基丙酮从[Pd⁰(dba)(AsPh₃)₂]中被置换形成[Pd⁰(AsPh₃)₃]的反应是非常吸热的。在氧化加成中形成的配合物反式 - [PhPdI(AsPh₃)₂]，在一个三苯基砷明显解络合后，与T形配合物[PhPdI(AsPh₃)]处于缓慢平衡。在催化条件下，即在存在亲核试剂（如能够与[Pd⁰(AsPh₃)₂]配位的CH₂=CH - SnBu₃）的情况下，会形成一种新的零价钯配合物：[Pd⁰(η² - CH₂=CHSnBu₃)(AsPh₃)₂]；然而，这种配合物不与苯基碘反应。因此，CH₂=CH - SnBu₃通过降低反应性物种[(溶剂)Pd⁰(AsPh₃)₂]的浓度来减缓氧化加成。这表明亲核试剂不仅可能参与转金属化步骤，还可能通过降低活性零价钯的浓度来干扰氧化加成步骤的动力学。
