Direct Arylation of 2-Methylthiophene with Isolated [PdAr(μ-O2CR)(PPh3)]n Complexes : Kinetics and Mechanism
Direct Arylation of 2-Methylthiophene with Isolated [PdAr(μ-O2CR)(PPh3)]n Complexes : Kinetics and Mechanism
复制标题
2-甲基噻吩与分离的 [PdAr(μ-O2CR)(PPh3)]n 配合物的直接芳基化：动力学和机理
作者：
脇岡正幸;中村友紀;王 奇峰;小澤文幸
钯催化的芳香族化合物与芳基卤化物的直接芳基化反应涉及芳基钯羧酸盐中间体。然而，孤立的芳基钯配合物，其中经历C-H键裂解的芳香族底物没有额外的活化剂或促进剂的帮助下，几乎没有记录。本文报道了[PdAr（μ-O_2CR）（PPh_3）] n配合物（1：Ar = Ph，2-MeC_6H_4，2，6-Me_2C_6H_3; R = Me，tBu）与2-甲基噻吩（2）在无添加剂的条件下反应，高产率地合成了5-芳基-2-甲基噻吩（3）。反应性随着Ar基团的体积增加而增加，而体积庞大的新戊酸酯配体（R =tBu）与乙酸酯配体（R = Me）相比降低了反应性。红外光谱证实，配合物1与单体[PdAr（O2 CR-κ 2 O）（PPh 3）]（5）处于平衡状态。动力学研究表明，直接芳基化是通过5进行的，5经历了2的C-H键断裂。配合物1是2与芳基溴直接芳基化的良好催化剂。
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)]ncomplexes (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). Complex1is 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 via5, which undergoes C–H bond cleavage of2. Complex1serves as a good catalyst for direct arylation of2with aryl bromides.