Direct Arylation of 2-Methylthiophene with Isolated [PdAr(μ-O2CR)(PPh3)]n Complexes: Kinetics and Mechanism

The palladium-catalyzed direct arylation of aromatic compounds with aryl halides has been proposed to involve an arylpalladium carboxylate intermediate. However, isolated arylpalladium complexes, which undergo C–H bond cleavage of aromatic substrates without the aid of additional activators or promoters, have been scarcely documented. This paper reports that [PdAr­(μ-O2CR)­(PPh3)]n complexes (1: Ar = Ph, 2-MeC6H4, 2,6-Me2C6H3; R = Me, tBu) successfully react with 2-methylthiophene (2) in the absence of additives to afford 5-aryl-2-methylthiophenes (3) in high yields. The reactivity increases with increasing bulkiness of the Ar group, whereas the bulky pivalate ligand (R = tBu) reduces the reactivity as compared with the acetate ligand (R = Me). Complex 1 is in equilibrium with the monomeric species [PdAr­(O2CR-κ2O)­(PPh3)] (5) in solution, as confirmed by IR spectroscopy. Kinetic examinations have suggested that the direct arylation proceeds via 5, which undergoes C–H bond cleavage of 2. Complex 1 serves as a good catalyst for direct arylation of 2 with aryl bromides.钯催化芳基卤化物与芳香化合物的直接芳基化反应，此前被认为涉及芳基钯羧酸酯中间体。然而，无需额外活化剂或促进剂辅助即可介导芳香底物发生碳氢键断裂的分离态芳基钯配合物，相关报道却十分匮乏。本文报道，[PdAr(μ-O₂CR)(PPh₃)]ₙ型配合物（其中1的Ar分别为苯基、2-甲基苯基、2,6-二甲基苯基，R分别为甲基、叔丁基）可在无添加剂的条件下与2-甲基噻吩（2）反应，以高收率得到5-芳基-2-甲基噻吩（3）。该反应的活性随芳基（Ar）基团的位阻增大而升高；而相较于乙酸根配体（R=Me），位阻更大的新戊酸根配体（R=tBu）则会降低反应活性。红外光谱表征证实，溶液中配合物1与单体物种[PdAr(O₂CR-κ²O)(PPh₃)]（5）存在动态平衡。动力学研究结果显示，本次直接芳基化反应经由单体5进行，该单体可完成2-甲基噻吩的碳氢键断裂步骤。配合物1可作为高效催化剂，用于芳基溴化物与2-甲基噻吩的直接芳基化反应。