Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR3)2(CO)(H)2]: predicted scope and regioselectivities.
Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR3)2(CO)(H)2]: predicted scope and regioselectivities.
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DOI：
10.1039/c3dt32962c
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发表时间：
2013-05
影响因子：
4
通讯作者：
S. Macgregor;D. McKay;J. Panetier;M. Whittlesey
中科院分区：
文献类型：
--
作者：
S. Macgregor;D. McKay;J. Panetier;M. Whittlesey
密度泛函理论计算已用于研究氟代芳烃 C6F(6-n)H(n) (n = 0-5) 在 [Ru(NHC)(PR3)2(CO)(H)2] 类型催化剂上加氢脱氟 (HDF) 的范围和选择性。根据我们之前的研究（Angew. Chem., Int. Ed., 2011, 50, 2783），考虑了氢化物配体对氟代芳烃底物进行亲核攻击的两种机制：（i）一步发生 Ru-H/C-F 交换的协同过程； (ii) 逐步途径，其中决定速率的过渡态涉及 HF 和 Ru-σ-氟芳基复合物的形成。金属配位环境的性质，特别是 NHC 配体被发现在促进 HDF 反应和决定该过程的区域选择性方面发挥着重要作用。因此，对于 C6F5H 的反应，完整的实验系统（NHC = IMes，R = Ph）通过（i）更容易的初始 PR3/氟芳烃取代和（ii）NHC N-芳基取代基通过 F·HC 相互作用稳定关键 C-F 键断裂过渡态的能力来促进 HDF。当存在邻位-H取代基时，后一种效应沿较低能量逐步路径最大化，这解释了在C6F5H生成1,2,3,4-C6F4H2的反应中看到的邻位选择性。计算得出的 C6F(6-n)H(n) 底物的 C-F 键解离能 (BDE) 显示随着 n 的增大而普遍增加，并且对存在的邻位 F 取代基的数量最敏感。然而，当包含 Me3SiH 等硅烷作为末端还原剂时，HDF 始终被计算为保持显着放热。计算出的 HDF 势垒通常也随着 n 的增大而增加，并且对于协同路径，C-F BDE 和势垒高度之间存在良好的相关性。发现这两种机制具有互补的区域选择性。对于协同途径，反应针对具有两个邻位 F 取代基的位点，因为这些取代基具有最弱的 C-F 键。相反，由于在 C-F 键断裂过渡态中难以容纳邻位 F 取代基，因此沿着逐步途径的反应指向仅具有一个邻位 F 取代基的位点。计算预测 1,2,3,5-C6F4H2 和 1,2,3,4-C6F4H2 是 [Ru(IMes)(PPh3)2(CO)(H)2] 处 HDF 的可行候选者，并且这将选择性地分别产生 1,2,4-C6F3H3 和 1,2,3-C6F3H3。
Density functional theory calculations have been employed to investigate the scope and selectivity of the hydrodefluorination (HDF) of fluoroarenes, C6F(6-n)H(n) (n = 0-5), at catalysts of the type [Ru(NHC)(PR3)2(CO)(H)2]. Based on our previous study (Angew. Chem., Int. Ed., 2011, 50, 2783) two mechanisms featuring the nucleophilic attack of a hydride ligand at a fluoroarene substrate were considered: (i) a concerted process with Ru-H/C-F exchange occurring in one step; and (ii) a stepwise pathway in which the rate-determining transition state involves formation of HF and a Ru-σ-fluoroaryl complex. The nature of the metal coordination environment and, in particular, the NHC ligand was found to play an important role in both promoting the HDF reaction and determining the regioselectivity of this process. Thus for the reaction of C6F5H, the full experimental system (NHC = IMes, R = Ph) promotes HDF through (i) more facile initial PR3/fluoroarene substitution and (ii) the ability of the NHC N-aryl substituents to stabilise the key C-F bond breaking transition state through F···HC interactions. This latter effect is maximised along the lower energy stepwise pathway when an ortho-H substituent is present and this accounts for the ortho-selectivity seen in the reaction of C6F5H to give 1,2,3,4-C6F4H2. Computed C-F bond dissociation energies (BDEs) for C6F(6-n)H(n) substrates show a general increase with larger n and are most sensitive to the number of ortho-F substituents present. However, HDF is always computed to remain significantly exothermic when a silane such as Me3SiH is included as terminal reductant. Computed barriers to HDF also generally increase with greater n, and for the concerted pathway a good correlation between C-F BDE and barrier height is seen. The two mechanisms were found to have complementary regioselectivities. For the concerted pathway the reaction is directed to sites with two ortho-F substituents, as these have the weakest C-F bonds. In contrast, reaction along the stepwise pathway is directed to sites with only one ortho-F substituent, due to difficulties in accommodating ortho-F substituents in the C-F bond cleavage transition state. Calculations predict that 1,2,3,5-C6F4H2 and 1,2,3,4-C6F4H2 are viable candidates for HDF at [Ru(IMes)(PPh3)2(CO)(H)2] and that this would proceed selectively to give 1,2,4-C6F3H3 and 1,2,3-C6F3H3, respectively.