德国雷根斯堡大学Alexander报道了对称和均极性σ键的单分子网异解。相关研究成果发表在2024年8月14日出版的《自然》。

共价σ键的单分子异解是许多化学转化不可或缺的一部分，包括S_N1-、E1-和1,2-迁移反应。第一个近似是，键异裂过程中电子密度的不均匀再分配是由两个离开的键合伴侣的极性差异决定的。这意味着，如果σ键由两个相同的基团组成（即对称σ键），则其从S₀、S₁或T₁态的单分子裂变仅在热或光化学激活后发生同质化。

为了迫使对称σ键进入异裂流形，必须通过双分子非共价相互作用进行共活化。这些策略仅适用于易受这种极化效应影响的σ键成分，并且在多功能分子中往往存在低效的化学选择性。

该文中，研究人员报告了通过受激双重电子转移（SDET）对对称和同极σ键（即具有相似电负性和相等离去基团能力的σ键）的净异裂。如Se-Se和C-Seσ键所示，对称和同极键最初经历热均裂，然后是光化学SDET，最终导致净异裂。

两个关键因素使这一过程可行且具有综合价值：（1）光激发可能只发生在初始自由基对成员中的一个中，从而导致巧合的对称性破坏，甚至对称σ键的净异裂。（2） 如果形成不相同的自由基，每个自由基可以在不同的波长下被激发，从而使净异裂解具有高度的化学特异性，并与常规异裂解正交。

这一特征在一系列非典型S_N1反应中得到了证实，在这些反应中，硒化物表现出SDET诱导的核逸度，与电负性更强的卤化物或重氮化合物的核逸性相媲美。

附：英文原文

Title: Unimolecular net heterolysis of symmetric and homopolar σ-bonds

Author: Tiefel, Anna F., Grenda, Daniel J., Allacher, Carina, Harrer, Elias, Nagel, Carolin H., Kutta, Roger J., Hernndez-Castillo, David, Narasimhamurthy, Poorva R., Zeitler, Kirsten, Gonzlez, Leticia, Rehbein, Julia, Nuernberger, Patrick, Breder, Alexander

Issue&Volume: 2024-08-14

Abstract: The unimolecular heterolysis of covalent σ-bonds is integral to many chemical transformations, including SN1-, 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 partners1,2,3. This means that if a σ-bond consists of two identical groups (that is, symmetric σ-bonds), its unimolecular fission from the S0, S1, or T1 states only occurs homolytically after thermal or photochemical activation1,2,3,4,5,6,7. To force symmetric σ-bonds into heterolytic manifolds, co-activation by bimolecular noncovalent interactions is necessary4. These tactics are only applicable to σ-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 σ-bonds (that is, those with similar electronegativity and equal leaving group ability3) by means of stimulated doublet–doublet electron transfer (SDET). As exemplified by Se–Se and C–Se σ-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 breaking8 and consequently net heterolysis even of symmetric σ-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 SN1 reactions, in which selenides show SDET-induced nucleofugalities3 rivalling those of more electronegative halides or diazoniums.

DOI: 10.1038/s41586-024-07622-7

Source: https://www.nature.com/articles/s41586-024-07622-7