Computational Study Illustrating NCN-Palladium(IV) Involvement in Generating Pd0 Species to Facilitate Pd0/PdII Heck-Type Catalysis with Diphenyliodine(III) Species

Density functional theory has been applied in a mechanistic study of the role of pincer complex PdII(NCN–N,C,N)­(O2CPh-O) ([NCN]- = [2,6-(Me2NCH2)2C6H3]−) (3) in Heck-type catalysis in the presence of diphenyliodine­(III) triflate as the oxidative arylating agent for CH2CHAr and bicarbonate as the base to afford PhCHCHAr (Ar = p-BrC6H4). The initially formed palladium­(IV) complex PhPd­(NCN–N,C,N)­(OBz···HOCO2–O,O) (9) (ΔG‡ 31.6 kcal/mol) undergoes Ph···Cipso reductive elimination to form PdII{NC­(Ph)­N–N,C,N}­(OBz···HOCO2–O,O) (11) (ΔG‡ 25.6 kcal/mol), which is reduced by bicarbonate to form palladium(0) species. Reduction to Pd0 occurs via deprotonation of one NMe2 group by bicarbonate to provide a “–CH2–N­(Me)–CH2–PdII″ moiety (ΔG‡ 23.6 kcal/mol) followed by nucleophilic attack on this moiety by bicarbonate to give a Pd0 product with a “–CH2–NMe­(CH2OCO2H)″ group (ΔG‡ 14.5 kcal/mol). The Pd0 complex undergoes exceptionally facile oxidative addition by Ph2I­(HCO3) (ΔG‡ = 5.1 kcal/mol). Modeling the Pd0 complex as [Pd­(benzene)­(O2CPh)]− provides a similar result (ΔG‡ = 5.6 kcal/mol), allowing entry to PhPdII species to be able to undergo migratory insertion for CH2CAr (ΔG‡ = 14.4 kcal/mol) and β-hydride elimination (ΔG‡ = 16.2 kcal/mol) processes of Pd0/PdII Heck-type catalysis. Activation barriers for reduction of PdIV to Pd0, and in the Heck-type process, are lower than the initial oxidation to form PdIV species, ensuring that only a small quantity of PdII(NCN)­(OBz) (3) is consumed, in accord with its presence on completion of catalysis. Computational studies of PdIV-mediated Heck-type catalysis revealed energetically unfavorable processes and a preference for the formation of CH2C­(Ar)­Ph rather than the experimentally reported PhCHCHAr. This study reveals the role of a pincer complex as a precatalyst, the oxidation of PdII to PdIV followed by reductive elimination, the role of bicarbonate in reducing PdII to Pd0, the extremely facile oxidative addition of a diaryliodine­(III) reagent to Pd0, and the selectivity differences in migratory insertion for PdII and PdIV centers.

本研究采用密度泛函理论（Density Functional Theory），对钳形配合物（pincer complex）PdII(NCN–N,C,N)(O2CPh-O)（其中[NCN]- = [2,6-(Me2NCH2)2C6H3]-，记为物种3）在Heck型催化（Heck-type catalysis）中的作用机制开展系统性研究。该催化体系以三氟甲磺酸二苯基碘(III)（diphenyliodine(III) triflate）作为CH2=CHAr的氧化性芳基化试剂，以碳酸氢盐（bicarbonate）作为碱，最终生成产物PhCH=CHAr（Ar = p-BrC6H4）。

首先生成的钯(IV)配合物PhPd(NCN–N,C,N)(OBz···HOCO2–O,O)（物种9，吉布斯自由能活化能垒ΔG‡=31.6 kcal/mol）会发生Ph···Cipso还原消除（reductive elimination），生成PdII{NC(Ph)N–N,C,N}(OBz···HOCO2–O,O)（物种11，ΔG‡=25.6 kcal/mol）；随后该物种被碳酸氢盐还原为钯(0)物种。还原过程具体为：碳酸氢盐先对一个NMe2基团发生去质子化，形成“–CH2–N(Me)–CH2–PdII”片段（ΔG‡=23.6 kcal/mol），随后碳酸氢盐对该片段发起亲核进攻，得到带有“–CH2–NMe(CH2OCO2H)”基团的钯(0)产物（ΔG‡=14.5 kcal/mol）。

该钯(0)配合物可与Ph2I(HCO3)发生极易进行的氧化加成（oxidative addition）反应（ΔG‡=5.1 kcal/mol）；若将钯(0)配合物建模为[Pd(benzene)(O2CPh)]-，则得到相近的计算结果（ΔG‡=5.6 kcal/mol），由此可得到PhPdII物种，其可参与CH2=CAr的迁移插入（migratory insertion）与β-氢消除（β-hydride elimination）过程，完成Pd0/PdII型Heck催化循环。

PdIV还原为Pd0的活化能垒，以及Heck型催化过程中的活化能垒，均低于初始氧化生成PdIV物种的能垒，这意味着仅有少量的PdII(NCN)(OBz)（物种3）被消耗，与催化完成后该物种仍可被检测到的实验结果相符。

针对PdIV介导的Heck型催化的计算研究表明，该过程在热力学上并不有利，且更倾向于生成CH2=C(Ar)Ph，而非实验中报道的PhCH=CHAr。本研究阐明了钳形配合物作为预催化剂的作用机制、PdII氧化为PdIV后发生还原消除的过程、碳酸氢盐在PdII还原为Pd0中的关键作用、二芳基碘(III)试剂极易与Pd0发生氧化加成的特性，以及PdII与PdIV中心在迁移插入反应中的选择性差异。