Chiral iminium ions—generated upon condensation of α,β-unsaturated aldehydes and amine catalysts—are used extensively by chemists to make chiral molecules in enantioenriched form. In contrast, their potential to absorb light and promote stereocontrolled photochemical processes remains unexplored. This is despite the fact that visible-light absorption by iminium ions is a naturally occurring event that triggers the mechanism of vision in higher organisms. Herein we demonstrate that the direct excitation of chiral iminium ions can unlock unconventional reaction pathways, enabling enantioselective catalytic photochemical β-alkylations of enals that cannot be realized via thermal activation. The chemistry uses readily available alkyl silanes, which are recalcitrant to classical conjugate additions, and occurs under illumination by visible-light-emitting diodes. Crucial to success was the design of a chiral amine catalyst with well-tailored electronic properties that can generate a photo-active iminium ion while providing the source of stereochemical induction. This strategy is expected to offer new opportunities for reaction design in the field of enantioselective catalytic photochemistry.

中文翻译：

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亚胺离子的可见光激发可实现对映体的对映选择性催化β-烷基化

化学家广泛使用由α，β-不饱和醛与胺催化剂缩合生成的手性亚胺离子，以使手性分子富化。相比之下，它们吸收光并促进立体控制的光化学过程的潜力仍未开发。尽管事实是亚硝酸根离子吸收可见光是自然发生的事件，触发高等生物的视觉机制。本文中，我们证明了手性亚胺离子的直接激发可以解锁非常规的反应途径，从而实现无法通过热活化实现的对映体的对映体选择性催化光化学β-烷基化。化学过程中使用了容易获得的烷基硅烷，这些烷基硅烷对传统的共轭添加物具有顽固性，并在可见光二极管的照明下发生。成功的关键是设计一种具有良好定制电子特性的手性胺催化剂，该催化剂可以产生光敏亚胺离子，同时提供立体化学诱导的来源。该策略有望为对映选择性催化光化学领域的反应设计提供新的机会。