Unimolecular net heterolysis of symmetric and homopolar σ-bonds

• 成果类型: Article
• 署名作者: Tiefel, Anna F.; Grenda, Daniel J.; Allacher, Carina; Harrer, Elias; Nagel, Carolin H.; Kutta, Roger J.; Hernandez-Castillo, David; Narasimhamurthy, Poorva R.; Zeitler, Kirsten; Gonzalez, Leticia; Rehbein, Julia; Nuernberger, Patrick; Breder, Alexander
• 署名单位: University of Regensburg; University of Regensburg; University of Vienna; University of Vienna; Leipzig University; University of Vienna; University of Regensburg
• 刊物名称: Nature
• ISSN/ISSBN: 0028-6131
• DOI: 10.1038/s41586-024-07622-7
• 发表日期: 2024-08-15
• 页码: 550-+
• 关键词: s(n)1 ionic dissociation; theoretical-model; absorption; activation; SPECTRA

摘要

The unimolecular heterolysis of covalent sigma-bonds is integral to many chemical transformations, including S(N)1-, E1- and 1,2-migration reactions. To a first approximation, the unequal redistribution of electron density during bond heterolysis is governed by the difference in polarity of the two departing bonding partners(1-3). This means that if a sigma-bond consists of two identical groups (that is, symmetric sigma-bonds), its unimolecular fission from the S-0, S-1, or T-1 states only occurs homolytically after thermal or photochemical activation(1-7). To force symmetric sigma-bonds into heterolytic manifolds, co-activation by bimolecular noncovalent interactions is necessary(4). These tactics are only applicable to sigma-bond constituents susceptible to such polarizing effects, and often suffer from inefficient chemoselectivity in polyfunctional molecules. Here we report the net heterolysis of symmetric and homopolar sigma-bonds (that is, those with similar electronegativity and equal leaving group ability(3)) by means of stimulated doublet-doublet electron transfer (SDET). As exemplified by Se-Se and C-Se sigma-bonds, symmetric and homopolar bonds initially undergo thermal homolysis, followed by photochemically SDET, eventually leading to net heterolysis. Two key factors make this process feasible and synthetically valuable: (1) photoexcitation probably occurs in only one of the incipient radical pair members, thus leading to coincidental symmetry breaking(8) and consequently net heterolysis even of symmetric sigma-bonds. (2) If non-identical radicals are formed, each radical may be excited at different wavelengths, thus rendering the net heterolysis highly chemospecific and orthogonal to conventional heterolyses. This feature is demonstrated in a series of atypical S(N)1 reactions, in which selenides show SDET-induced nucleofugalities(3) rivalling those of more electronegative halides or diazoniums.