α-Functionalization of Imines via Visible Light Photoredox Catalysis

Abstract

The innate electrophilicity of imine building blocks has been exploited in organic synthetic chemistry for decades. Inspired by the resurgence in photocatalysis, imine reactivity has now been redesigned through the generation of unconventional and versatile radical intermediates under mild reaction conditions. While novel photocatalytic approaches have broadened the range and applicability of conventional radical additions to imine acceptors, the possibility to use these imines as latent nucleophiles via single-electron reduction has also been uncovered. Thus, multiple research programs have converged on this issue, delivering creative and practical strategies to achieve racemic and asymmetric α-functionalizations of imines under visible light photoredox catalysis.

Photoredox-catalyzed multicomponent Petasis reaction in batch and continuous flow with alkyl boronic acids

“…However, the inherent requirement of a directing group to form the borate intermediate and the necessity to stabilize the negative charge on the migrating group severely restrict the broad utilization of this reaction. Moreover, being water the byproduct of imine formation, activation strategies which require dry conditions, as in the case of several photocatalyzed processes, are usually limited to ex situ prepared imines ( Garrido-Castro et al., 2020 ).

Figure 1 Background to Petasis Reaction and activation of boronic acids as radical precursors a.

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Solvent-Free Visible Light Photocatalytic Oxidation Processes Mediated by Transparent Films of an Imine-Based Organic Polymer

“…An emerging trend in modern chemistry is the design of photocatalytic processes to achieve the assembly of a variety of compounds of increasing complexity [1][2][3][4][5]. On a first approximation, the light used to trigger several chemical transformations was in the range of ultraviolet.…”

Sodium Iodide-Triphenylphosphine-Mediated Photoredox Alkylation of Aldimines

“…[9] N Ar 2 图式 1 NaI-TPP-NHPI 酯的光氧化还原反应 Scheme 1 Photoredox reactions of NaI-TPP-NHPI ester EDA complex 亚胺的还原型自由基反应在胺类化合物的合成方 法中占有独特的地位 [16] . 经典方法主要为三烷基硼、二 烷基锌等引发的自由基加成, 而光氧化还原方法是近年 来比较热门的话题 [17] , 主要机理途径可分为从非亚胺 的 前 体 试 剂 经 单 电 子 转 移 (single electron transfer, SET) [18] 或氢原子转移(hydrogen atom transfer, HAT) [19] 产 生自由基, 以及从亚胺经质子偶合电子转移(protoncoupled electron transfer, PCET)产生 α-氨基自由基 [20] , [21] 和 Ru(bpy) 3 Cl 2 催化可见光介导的脱羧烷基化 [22] . 在此基 础上, 通过 NaI-TPP-NHPI 酯 EDA 络合物的光氧化还原 过程引发自由基, 首次建立了该 EDA 络合物的净还原 性光反应体系, 以高产率实现了醛亚胺加成, 并发现可 用于通常反应性较差的伯烷基和苄基自由基的反应 (Scheme 1c).…”