Free Radical Isomerizations in Acetylene Bromoboration Reaction
The experimentally motivated question of the acetylene bromoboration mechanism was addressed in order to suggest possible radical isomerization pathways for the <i>syn</i>-adduct. Addition–elimination mechanisms starting with a bromine radical attack at the “bromine end” or the “boron en...
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| Format: | Article |
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MDPI AG
2021-04-01
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| Online Access: | https://www.mdpi.com/1420-3049/26/9/2501 |
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| author | Hugo Semrád Ctibor Mazal Markéta Munzarová |
| author_facet | Hugo Semrád Ctibor Mazal Markéta Munzarová |
| author_sort | Hugo Semrád |
| collection | DOAJ |
| description | The experimentally motivated question of the acetylene bromoboration mechanism was addressed in order to suggest possible radical isomerization pathways for the <i>syn</i>-adduct. Addition–elimination mechanisms starting with a bromine radical attack at the “bromine end” or the “boron end” of the C=C bond were considered. Dispersion-corrected DFT and MP2 methods with the SMD solvation model were employed using three all-electron bases as well as the ECP28MWB ansatz. The rate-determining, elimination step had a higher activation energy (12 kcal mol<sup>−1</sup>) in case of the “bromine end” attack due to intermediate stabilization at both the MP2 and DFT levels. In case of the “boron end” attack, two modes of C–C bond rotation were followed and striking differences in MP2 vs. DFT potential energy surfaces were observed. Employing MP2, addition was followed by either a 180° rotation through an eclipsed conformation of vicinal bromine atoms or by an opposite rotation avoiding that conformation, with 5 kcal mol<sup>−1</sup> of elimination activation energy. Within B3LYP, the addition and rotation proceeded simultaneously, with a 9 (7) kcal mol<sup>−1</sup> barrier for rotation involving (avoiding) eclipsed conformation of vicinal bromines. For weakly bound complexes, ZPE corrections with MP2 revealed significant artifacts when diffuse bases were included, which must be considered in the Gibbs free energy profile interpretation. |
| first_indexed | 2024-03-10T11:59:19Z |
| format | Article |
| id | doaj.art-d2801bd0a7074fe6aac5caff910ee3ea |
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| issn | 1420-3049 |
| language | English |
| last_indexed | 2024-03-10T11:59:19Z |
| publishDate | 2021-04-01 |
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| spelling | doaj.art-d2801bd0a7074fe6aac5caff910ee3ea2023-11-21T17:03:36ZengMDPI AGMolecules1420-30492021-04-01269250110.3390/molecules26092501Free Radical Isomerizations in Acetylene Bromoboration ReactionHugo Semrád0Ctibor Mazal1Markéta Munzarová2Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech RepublicDepartment of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech RepublicDepartment of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech RepublicThe experimentally motivated question of the acetylene bromoboration mechanism was addressed in order to suggest possible radical isomerization pathways for the <i>syn</i>-adduct. Addition–elimination mechanisms starting with a bromine radical attack at the “bromine end” or the “boron end” of the C=C bond were considered. Dispersion-corrected DFT and MP2 methods with the SMD solvation model were employed using three all-electron bases as well as the ECP28MWB ansatz. The rate-determining, elimination step had a higher activation energy (12 kcal mol<sup>−1</sup>) in case of the “bromine end” attack due to intermediate stabilization at both the MP2 and DFT levels. In case of the “boron end” attack, two modes of C–C bond rotation were followed and striking differences in MP2 vs. DFT potential energy surfaces were observed. Employing MP2, addition was followed by either a 180° rotation through an eclipsed conformation of vicinal bromine atoms or by an opposite rotation avoiding that conformation, with 5 kcal mol<sup>−1</sup> of elimination activation energy. Within B3LYP, the addition and rotation proceeded simultaneously, with a 9 (7) kcal mol<sup>−1</sup> barrier for rotation involving (avoiding) eclipsed conformation of vicinal bromines. For weakly bound complexes, ZPE corrections with MP2 revealed significant artifacts when diffuse bases were included, which must be considered in the Gibbs free energy profile interpretation.https://www.mdpi.com/1420-3049/26/9/2501free radicalsisomerizationacetylenebromoborationmechanismaddition-elimination |
| spellingShingle | Hugo Semrád Ctibor Mazal Markéta Munzarová Free Radical Isomerizations in Acetylene Bromoboration Reaction Molecules free radicals isomerization acetylene bromoboration mechanism addition-elimination |
| title | Free Radical Isomerizations in Acetylene Bromoboration Reaction |
| title_full | Free Radical Isomerizations in Acetylene Bromoboration Reaction |
| title_fullStr | Free Radical Isomerizations in Acetylene Bromoboration Reaction |
| title_full_unstemmed | Free Radical Isomerizations in Acetylene Bromoboration Reaction |
| title_short | Free Radical Isomerizations in Acetylene Bromoboration Reaction |
| title_sort | free radical isomerizations in acetylene bromoboration reaction |
| topic | free radicals isomerization acetylene bromoboration mechanism addition-elimination |
| url | https://www.mdpi.com/1420-3049/26/9/2501 |
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