Unimolecular net heterolysis of symmetric and homopolar σ-bonds

Anna F. Tiefel, Daniel J. Grenda, Carina Allacher, Elias Harrer, Carolin H. Nagel, Roger J. Kutta, David Hernández-Castillo, Poorva R. Narasimhamurthy, Kirsten Zeitler, Leticia González, Julia Rehbein (), Patrick Nuernberger () and Alexander Breder ()
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Anna F. Tiefel: Universität Regensburg
Daniel J. Grenda: Universität Regensburg
Carina Allacher: Universität Regensburg
Elias Harrer: Universität Regensburg
Carolin H. Nagel: Universität Regensburg
Roger J. Kutta: Universität Regensburg
David Hernández-Castillo: University of Vienna
Poorva R. Narasimhamurthy: Universität Regensburg
Kirsten Zeitler: Universität Leipzig
Leticia González: University of Vienna
Julia Rehbein: Universität Regensburg
Patrick Nuernberger: Universität Regensburg
Alexander Breder: Universität Regensburg

Nature, 2024, vol. 632, issue 8025, 550-556

Abstract: 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 partners1–3. 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 activation1–7. 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.

Date: 2024
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DOI: 10.1038/s41586-024-07622-7

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