2-Hydroxycarbazole and 4-hydroxycarbazole are important chemicals with extensive applications in optoelectronic materials and pharmaceutical field. State of the art yield of 2-hydroxycarbazole is ∼30% and the reaction time is typically in hours or days. Herein, we developed a green route for the continuous and high-throughput synthesis of 2-hydroxycarbazole and 4-hydroxycarbazole photochemical intramolecular cyclization of 3‑hydroxy-2′‑chloro-diphenylamine using a self-designed millimeter scale photoreactor, which was designed based on sizing-up and numbering-up strategies for a decent liquid holdup (6.8 mL) and fabricated femtosecond laser engraving technique. The photochemical synthesis was carried out continuously under the illumination of 365 nm UV-LED with dimethyl sulfoxide as solvent and potassium -butoxide as catalyst. It was found that under optimized conditions a 2-hydroxycarbazole yield of 31.6% and a 4-hydroxycarbazole yield of 11.1% were obtained with a residence time of 1 min. Compared to semi-batch operations, the reaction time was shortened by 1–2 orders of magnitude. As a result, a throughput of 11.3 g/day 2-hydroxycarbazole and 4.0 g/day 4-hydroxycarbazole can be achieved from the photoreactor. It was proposed that the short reaction time and high product yield are resulted from higher photon transfer rates and more uniform photon distribution provided by the millimeter scale photoreactor, which enhances the reaction rates and mitigates overreaction.

中文翻译：

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毫米级光反应器高效连续合成2-羟基咔唑和4-羟基咔唑

2-羟基咔唑和4-羟基咔唑是重要的化学品，在光电材料和医药领域有着广泛的应用。 2-羟基咔唑的现有技术收率约为 30%，反应时间通常为数小时或数天。在此，我们开发了一种使用自主设计的毫米级光反应器连续高通量合成2-羟基咔唑和4-羟基咔唑光化学分子内环化3-羟基-2'-氯-二苯胺的绿色路线。关于适当持液量 (6.8 mL) 的尺寸和编号策略以及制造的飞秒激光雕刻技术。以二甲亚砜为溶剂，丁醇钾为催化剂，在365 nm UV-LED照射下连续进行光化学合成。结果表明，在优化条件下，停留时间为1 min时，2-羟基咔唑收率为31.6%，4-羟基咔唑收率为11.1%。与半间歇操作相比，反应时间缩短了1-2个数量级。结果，光反应器可实现11.3克/天的2-羟基咔唑和4.0克/天的4-羟基咔唑的吞吐量。有人提出，短的反应时间和高的产物收率是由于毫米级光反应器提供的更高的光子转移率和更均匀的光子分布，从而提高了反应速率并减轻了过度反应。