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首页> 外文期刊>Chemistry: A European journal >Mechanism of the Pd-catalyzed decarboxylative allylation of α-imino esters: Decarboxylation via free carboxylate ion
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Mechanism of the Pd-catalyzed decarboxylative allylation of α-imino esters: Decarboxylation via free carboxylate ion

机译：Pd催化α-亚氨基酯的脱羧烯丙基化的机理：通过游离羧酸根离子的脱羧

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摘要

The Pd-catalyzed decarboxylative allylation of α-(diphenylmethylene) imino esters (1) or allyl diphenylglycinate imines (2) is an efficient method to construct new C(sp ~3)-C(sp ~3) bonds. The detailed mechanism of this reaction was studied by theoretical calculations [ONIOM(B3LYP/ LANL2DZ+p:PM6)] combined with experimental observations. The overall catalytic cycle was found to consist of three steps: oxidative addition, decarboxylation, and reductive allylation. The oxidative addition of 1 to [(dba)Pd(PPh _3) _2] (dba=dibenzylideneacetone) produces an allylpalladium cation and a carboxylate anion with a low activation barrier of +9.1 kcal mol ~(-1). The following rate-determining decarboxylation proceeds via a solvent-exposed α-imino carboxylate anion rather than an O-ligated allylpalladium carboxylate with an activation barrier of +22.7 kcal mol ~(-1). The 2-azaallyl anion generated by this decarboxylation attacks the face of the allyl ligand opposite to the Pd center in an outer-sphere process to produce major product 3, with a lower activation barrier than that of the minor product 4. A positive linear Hammett correlation [ρ=1.10 for the PPh _3 ligand] with the observed regioselectivity (3 versus 4) supports an outer-sphere pathway for the allylation step. When Pd combined with the bis(diphenylphosphino)butane (dppb) ligand is employed as a catalyst, the decarboxylation still proceeds via the free carboxylate anion without direct assistance of the cationic Pd center. Consistent with experimental observations, electron-withdrawing substituents on 2 were calculated to have lower activation barriers for decarboxylation and, thus, accelerate the overall reaction rates.
机译：Pd催化的α-（二苯基亚甲基）亚氨基酯（1）或烯丙基二苯基甘氨酸亚胺（2）的脱羧烯丙基化是构建新的C（sp〜3）-C（sp〜3）键的有效方法。通过理论计算[ONIOM（B3LYP / LANL2DZ + p：PM6）]结合实验观察，研究了该反应的详细机理。发现整个催化循环包括三个步骤：氧化加成，脱羧和还原烯丙基化。 1 [[dba）Pd（PPh _3）_2]（dba =二亚苄基丙酮）的氧化加成生成烯丙基钯阳离子和羧酸根阴离子，其活化势垒低至+9.1 kcal mol〜（-1）。以下速率确定的脱羧反应是通过溶剂暴露的α-亚氨基羧酸羧酸根阴离子而不是具有+22.7 kcal mol〜（-1）的活化势垒的O-连接的烯丙基钯羧酸盐进行的。由这种脱羧作用生成的2-氮杂烯丙基阴离子在外球过程中攻击与Pd中心相对的烯丙基配体的表面，从而产生主要产物3，其活化势垒低于次要产物4的活化势垒。与PPh _3配体的ρ= 1.10的相关性与观察到的区域选择性（3对4）支持了烯丙基化步骤的外球途径。当Pd与双（二苯基膦基）丁烷（dppb）配体结合用作催化剂时，脱羧仍然通过游离的羧酸根阴离子进行，而没有阳离子Pd中心的直接帮助。与实验观察结果一致，计算得出2上的吸电子取代基具有较低的脱羧活化势垒，从而加快了总反应速率。

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