Unimolecular net heterolysis of symmetric and homopolar σ-bonds
The unimolecular heterolysis of covalent σ-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 depa...
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Veröffentlicht in: | Nature (London) 2024-08, Vol.632 (8025), p.550-556 |
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Zusammenfassung: | The unimolecular heterolysis of covalent σ-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 σ-bond consists of two identical groups (that is, symmetric σ-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 σ-bonds into heterolytic manifolds, co-activation by bimolecular noncovalent interactions is necessary
4
. 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 ability
3
) 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 breaking
8
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 S
N
1 reactions, in which selenides show SDET-induced nucleofugalities
3
rivalling those of more electronegative halides or diazoniums.
Net heterolysis of symmetric and homopolar σ-bonds by stimulated doublet–doublet electron transfer is reported in a series of atypical S
N
1 reactions, in which selenides show SDET-induced nucleofugalities rivalling those of more electronegative halides or diazoniums. |
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ISSN: | 0028-0836 1476-4687 1476-4687 |
DOI: | 10.1038/s41586-024-07622-7 |