Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
J Am Chem Soc. 2012 Dec 12;134(49):20081-96. doi: 10.1021/ja307279r. Epub 2012 Nov 28.
The anilide-methyl complex (PNP)Sc(NH[DIPP])(CH(3)) (1) [PNP(-) = bis(2-diisopropylphosphino-4-tolyl)amide, DIPP = 2,6-diisopropylphenyl] eliminates methane (k(avg) = 5.13 × 10(-4) M(-1) s(-1) at 50 °C) in the presence of pyridine to generate the transient scandium imido (PNP)Sc═NDIPP (A-py), which rapidly activates the C-H bond of pyridine in 1,2-addition fashion to form the stable pyridyl complex (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(4)) (2). Mechanistic studies suggest the C-H activation process to be second order overall: first order in scandium and first order in substrate (pyridine). Pyridine binding precedes elimination of methane, and α-hydrogen abstraction is overall-rate-determining [the kinetic isotope effect (KIE) for 1-d(1) conversion to 2 was 5.37(6) at 35 °C and 4.9(14) at 50 °C] with activation parameters ΔH(‡) = 17.9(9) kcal/mol and ΔS(‡) = -18(3) cal/(mol K), consistent with an associative-type mechanism. No KIE or exchange with the anilide proton was observed when 1-d(3) was treated with pyridine or thermolyzed at 35 or 50 °C. The post-rate-determining step, C-H bond activation of pyridine, revealed a primary KIE of 1.1(2) at 35 °C for the intermolecular C-H activation reaction in pyridine versus pyridine-d(5). Complex 2 equilibrated back to the imide A-py slowly, as the isotopomer (PNP)Sc(ND[DIPP])(η(2)-NC(5)H(4)) (2-d(1)) converted to (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(3)D) over 9 days at 60 °C. Molecular orbital analysis of A-py suggested that this species possesses a fairly linear scandium imido motif (169.7°) with a very short Sc-N distance of 1.84 Å. Substituted pyridines can also be activated, with the rates of C-H activation depending on both the steric and electronic properties of the substrate.
在吡啶存在下，酰胺-甲基配合物（PNP）Sc（NH[DIPP]）（CH（3））（1）[PNP（-）=双（2-二异丙基膦基-4-甲苯基）酰胺，DIPP=2,6-二异丙基苯基]消除甲烷（k（avg）= 5.13×10（-4）M（-1）s（-1），在 50°C），生成瞬态钪亚氨基（PNP）Sc═N[DIPP]（NC（5）H（5））（A-py），其快速以 1,2-加成方式活化吡啶的 C-H 键，形成稳定的吡啶配合物（PNP）Sc（NH[DIPP]）（η（2）-NC（5）H（4））（2）。 机理研究表明，C-H 活化过程总体上为二级反应：一级反应对钪，一级反应对底物（吡啶）。 吡啶结合先于甲烷的消除，α-氢原子的提取是整体速率决定的[动力学同位素效应（KIE）对于 1-d（1）转化为 2 在 35°C 为 5.37（6），在 50°C 为 4.9（14）]，其活化参数ΔH（‡）= 17.9（9）kcal/mol 和ΔS（‡）=-18（3）cal/（mol K），与缔合型机制一致。 当 1-d（3）与吡啶或在 35 或 50°C 下热解时，未观察到 KIE 或与酰胺质子的交换。 吡啶的 C-H 键活化的后速控步骤，显示出在吡啶中与吡啶-d（5）相比，吡啶的分子间 C-H 活化反应的一级 KIE 为 1.1（2）。 配合物 2 缓慢平衡回到亚胺 A-py，因为同位素（PNP）Sc（ND[DIPP]）（η（2）-NC（5）H（4））（2-d（1））在 60°C 下在 9 天内转化为（PNP）Sc（NH[DIPP]）（η（2）-NC（5）H（3）D）。 A-py 的分子轨道分析表明，该物种具有相当线性的钪亚氨基结构（169.7°），Sc-N 距离非常短，为 1.84Å。 取代的吡啶也可以被活化，C-H 活化的速率取决于底物的空间和电子性质。
