Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 636371, Singapore.
J Phys Chem B. 2021 Aug 5;125(30):8550-8557. doi: 10.1021/acs.jpcb.1c04536. Epub 2021 Jul 21.
In the heterobiaryl cross-coupling reaction between aryl halides (Ar-X) and -methylpyrrole (-MP) catalyzed by rhodamine 6G (Rh6G) under irradiation with visible light, a highly active and long-lived (millisecond time range) rhodamine 6G radical (Rh6G) is formed upon electron transfer from ,-diisopropylethylamine (DIPEA) to Rh6G. In this study, we utilized steady-state and time-resolved spectroscopy techniques to demonstrate the existence of another electron-transfer process occurring from the relatively electron-rich -MP to photoexcited Rh6G that was neglected in the previous reports. In this case, the radical Rh6G formed is short-lived and undergoes rapid recombination (nanosecond time-range), rendering it ineffective in reducing Ar-X to aryl radicals Ar that can subsequently be trapped by -MP. This is further demonstrated two model reactions involving 4'-bromoacetophenone and 1,3,5-tribromobenzene with insignificant product yields after visible-light irradiation in the absence of DIPEA. The unproductive quenching of photoexcited Rh6G by -MP leads to a lower concentration of photocatalyst available for competitive charge transfer with DIPEA and hence decreases the efficiency of the cross-coupling reaction.
在可见光照射下，由罗丹明 6G（Rh6G）催化的芳基卤化物（Ar-X）和 - 甲基吡咯（-MP）的杂芳基交叉偶联反应中，- ，- 二异丙基乙胺（DIPEA）向 Rh6G 转移电子会形成高活性和长寿命（毫秒时间范围）的 Rh6G 自由基（Rh6G）。在这项研究中，我们利用稳态和时间分辨光谱技术证明了先前报道中忽略的另一个电子转移过程的存在，该过程从相对富电子的 -MP 转移到光激发的 Rh6G。在这种情况下，形成的自由基 Rh6G 寿命短，会迅速重组（纳秒时间范围），从而无法将 Ar-X 还原为芳基自由基 Ar，随后 Ar 可以被 -MP 捕获。这进一步通过涉及 4'-溴苯乙酮和 1,3,5-三溴苯的两个模型反应得到证明，在没有 DIPEA 的情况下，可见光照射后产物产率没有显著提高。-MP 对光激发的 Rh6G 的非生产性猝灭导致可用于与 DIPEA 进行竞争电荷转移的光催化剂浓度降低，从而降低了交叉偶联反应的效率。
