Selective Photocatalytic C-F Borylation of Polyfluoroarenes by Rh/Ni Dual Catalysis Providing Valuable Fluorinated Arylboronate Esters
Selective Photocatalytic C-F Borylation of Polyfluoroarenes by Rh/Ni Dual Catalysis Providing Valuable Fluorinated Arylboronate Esters
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DOI：
10.1021/jacs.8b09790
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发表时间：
2018-12-19
影响因子：
15
通讯作者：
Marder, Todd B.
中科院分区：
文献类型：
--
作者：
Tian, Ya-Ming;Guo, Xiao-Ning;Marder, Todd B.
报道了一种高选择性和通用的光催化 C-F 硼化方案，该方案采用铑联苯配合物作为三重态敏化剂和镍催化剂 [Ni(IMes)(2)] (IMes = 1,3-二甲咪唑啉-2-亚基)用于 C-F 键活化和脱氟硼化过程。该串联催化剂系统在可见光（蓝色，400 nm）下工作，并在室温下用 B(2)pin(2) 实现多种氟代芳烃的硼基化，具有优异的产率和高选择性。当 B(2)pin(2) 存在时，直接照射中间 C-F 键氧化加成产物反式-[NiF(Ar-F)(IMes)(2)] 会导致非常快速的分解。这种破坏性途径可以通过光激发铑联苯络合物间接激发镍（II）络合物的三重态来绕过。机理研究表明，用作光敏剂的 Rh 联苯配合物的超长寿命三重激发态可实现有效的三重态能量转移至反式-[NiF(Ar-F)(IMes)(2)]，从而导致 NHC 配体之一解离。这与当前大多数光催化转化形成对比，后者采用过渡金属作为激发态单电子转移剂。我们之前报道过，[Ni(IMes)(2)] 的 C(芳烃)-F 键活化在室温下很容易，但 B(2)pin(2) 的金属转移步骤与高能垒有关。因此，这种三线态能量转移最终导致金属交换步骤以及整个硼化过程的速率常数大大提高。虽然添加氟化物源（例如 CsF）可以提高产率，但这并不是绝对必要的。我们将这种产量增强效应归因于（i）形成 B(2)pin(2) 阴离子加合物，即 FB(2)pin(2)(-)，作为硼化/金属转移过程中有效的、亲核性更强的 {Bpin(-)} 转移试剂，和/或 (ii) 通过形成 [F(2)Bpin](-) 捕获路易斯酸性副产物 FBpin，以避免形成显着的NHC-FBpin 的量以及反应混合物中 {Ni(NHC)(2)} 物质的分解。
A highly selective and general photocatalytic C-F borylation protocol that employs a rhodium biphenyl complex as a triplet sensitizer and the nickel catalyst [Ni(IMes)(2)] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the C-F bond activation and defluoroborylation process is reported. This tandem catalyst system operates with visible (blue, 400 nm) light and achieves borylation of a wide range of fluoroarenes with B(2)pin(2) at room temperature in excellent yields and with high selectivity. Direct irradiation of the intermediary C-F bond oxidative addition product trans-[NiF(Ar-F)(IMes)(2)] leads to very fast decomposition when B(2)pin(2) is present. This destructive pathway can be bypassed by indirect excitation of the triplet states of the nickel(II) complex via the photoexcited rhodium biphenyl complex. Mechanistic studies suggest that the exceptionally long-lived triplet excited state of the Rh biphenyl complex used as the photosensitizer allows for efficient triplet energy transfer to trans-[NiF(Ar-F)(IMes)(2)], which leads to dissociation of one of the NHC ligands. This contrasts with the majority of current photocatalytic transformations, which employ transition metals as excited state single electron transfer agents. We have previously reported that C(arene)-F bond activation with [Ni(IMes)(2)] is facile at room temperature, but that the transmetalation step with B(2)pin(2) is associated with a high energy barrier. Thus, this triplet energy transfer ultimately leads to a greatly enhanced rate constant for the transmetalation step and thus for the whole borylation process. While addition of a fluoride source such as CsF enhances the yield, it is not absolutely required. We attribute this yield-enhancing effect to (i) formation of an anionic adduct of B(2)pin(2), i.e., FB(2)pin(2)(-), as an efficient, much more nucleophilic {Bpin(-)} transfer reagent for the borylation/transmetalation process, and/or (ii) trapping of the Lewis acidic side product FBpin by formation of [F(2)Bpin](-) to avoid the formation of a significant amount of NHC-FBpin and consequently decomposition of {Ni(NHC)(2)} species in the reaction mixture.