Reaktivität nicht-koordinierter Borole
Zusammenfassung der Projektergebnisse
The placement of a three-coordinate boron atom into a conjugated cyclic system allows for unique properties, such as high electron affinity, mobility, and bathochromically shifted optical spectra, as compared to the corresponding all-carbon analogs, and the low-lying lowest-unoccupied molecular orbital (LUMO) helps to account for these properties. For these and reasons related to their antiaromaticity, boroles, or boracyclopentadienes, have captured our attention in recent years. While the boron substituents have been shown to significantly impact the electronics and properties of the boroles, the influence of the remaining ring substituents is largely unknown. This is in part due to the lack of a suitable precursor, which allows the direct functionalization of the borole ring substituents. We have sought to overcome this limitation by the preparation of a borole derivative containing boryl side groups, which can be easily converted to many other functionalities. In the course of this project, a new stannole with ring-appended pinacolboryl groups was synthesized, which we considered ideal for subsequent metallacycle transfer reactions. Unfortunately, the attempted tin-boron exchange using a broad range of di- and trihaloboranes did not result in the desired boroles. In one particular case, with dichloro(phenyl)borane, the reaction selectively produced a 1-boracyclopent-3-ene derivative by phenyl migration and without the expected elimination of dimethyltin dichloride. The molecular structure of the product indicated that the unusual reactivity can be attributed to intramolecular Sn−O interactions between the tin and the oxygen atoms of a pinacolboryl group. The elimination of dimethyl dichloride from the partially unsaturated boracycle can be induced by treatment with pyridine bases, providing base-stabilized cyclic boraalkenes, which can alternatively be viewed as structural isomers of base-stabilized boroles. In addition, we have discovered an alternative reaction pathway for the ring expansion reaction of boroles with organic azides that typically leads to 1,2-azaborinines, BN analogs of benzene. By employing ortho-substituted phenyl azides in the reaction, the formation of 1,2-azaborinine-substituted azo dyes was observed, in which the three-nitrogen-atom rearrangement of the former azide remained intact. With the help of mechanistic DFT calculations, this divergent pathway has been shown to proceed from initial addition of the less nucleophilic terminal N atom of the azide to the Lewis acidic borole. For the synthesis of borole-based di- and triradicals, we have made use of the comproportionation approach, which involves the reaction between a neutral and a fully reduced borole derivative. Following this procedure, model diradicals with antiferromagnetic and ferromagnetic coupling units could be efficiently synthesized. However, the high instability of the resulting species, especially of the systems with a triplet ground state, severely hampered their characterization and suggests that future efforts to prepare high-spin molecules based on the borole framework will prove challenging.
Projektbezogene Publikationen (Auswahl)
- “Antiaromaticity to Aromaticity: From Boroles to 1,2-Azaborinines by Ring Expansion with Azides”. Chem. Eur. J. 2014, 20, 9858-9861
H. Braunschweig, C. Hörl, L. Mailänder, K. Radacki, J. Wahler
(Siehe online unter https://doi.org/10.1002/chem.201403101) - “Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides”. Angew. Chem. Int. Ed. 2015, 54, 6347-6351; Angew. Chem. 2015, 127, 6445-6449
H. Braunschweig, M. A. Celik, F. Hupp, I. Krummenacher, L. Mailänder
(Siehe online unter https://doi.org/10.1002/anie.201500970) - “Ring Expansions of Boroles with Diazo Compounds: Steric Control of C or N Insertion and Aromatic/Nonaromatic Products”. Chem. Eur. J. 2015, 21, 17844-17849
H. Braunschweig, F. Hupp, I. Krummenacher, L. Mailänder, F. Rauch
(Siehe online unter https://doi.org/10.1002/chem.201503048) - “Abnormal Tin-Boron Exchange in the Attempted Synthesis of a Borylated Borole”. Chem. Eur. J. 2017, 23, 16167-16170
H. Braunschweig, M. Dömling, S. Kachel, H. Kelch, T. Kramer, I. Krummenacher, C. Lenczyk, S. Lin, Z. Lin, C. Possiel, K. Radacki
(Siehe online unter https://doi.org/10.1002/chem.201703810) - “Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides”. Chem. Eur. J. 2017, 23, 8006-8013
H. Braunschweig, M. A. Celik, T. Dellermann, G. Frenking, K. Hammond, F. Hupp, H. Kelch, I. Krummenacher, F. Lindl, L. Mailänder, J. H. Müssig, A. Ruppert
(Siehe online unter https://doi.org/10.1002/chem.201700749)
