Transition-Metal-Catalysed Borane Dehydrocoupling and its Mechanistic Implications
Final Report Abstract
This project was proposed in order to fully explore the synthetic possibilities of borane dehydrocoupling, which was little known at the outset and developed by us into practical processes by our group before the project funding began. The reaction has inherent synthetic value as it creates an electron-precise boron-boron bond - which are very difficult to construct selectively - with only hydrogen as the byproduct. Unfortunately, many of our attempts to extend these reactions to other systems were thwarted by unexpected side-reactions and the unforeseeable reluctance of the reagents to undergo B-B coupling. Despite these difficulties, we were able to make significant advances in the field, including: (a) the synthesis of a number of interesting new transition metal dihydroborate complexes in which the boron atom bears organyl groups, (b) the synthesis and study of new low-valent transition metal complexes bearing cyclic (alkyl)(amino)carbenes (CAACs) as possible precursor complexes for dehydrocoupling, and most importantly, (c) the isolation of transition metal complexes bearing highly novel low-valent boron ligands from dehydrogenation, including diborene, borylene and "slipped" Z-type borane ligands. The latter discoveries confirm the dehydrocoupling of borane units, and, fascinatingly, also its reverse reaction, the hydrogenolysis of B-B bonds. These findings were published in three full papers (J. Am. Chem. Soc., Chem. Eur. J. and Organometallics). Further unpublished work has uncovered other systems that undergo dehydrocoupling reactions, which we are currently pursuing.
Publications
- Aryldihydroborane Coordination to Iridium and Osmium Hydrido Complexes. Organometallics 2015, 34, 5709–5715
N. Arnold, S. Mozo, U. Paul, U. Radius, H. Braunschweig
(See online at https://doi.org/10.1021/acs.organomet.5b00788) - Building Electron-Precise Boron-Boron Single Bonds: Imposing Monogamy on a Promiscuous Element. Chem. Cat. Chem. 2015, 7, 1630–1638
H. Braunschweig, R. D. Dewhurst, S. Mozo
(See online at https://doi.org/10.1002/cctc.201500219) - Correlations and Contrasts in Homo- and Heteroleptic Cyclic (Alkyl)(amino)carbene-Containing Pt0 Complexes. Chem. Eur. J. 2015, 21, 12357–12362
N. Arnold, H. Braunschweig, P. B. Brenner, M. A. Celik, R. D. Dewhurst, M. Haehnel, T. Kramer, I. Krummenacher, T. B. Marder
(See online at https://doi.org/10.1002/chem.201502048) - Unprecedented Borane, Diborane(3), Diborene, and Borylene Ligands via Pt-Mediated Borane Dehydrogenation. J. Am. Chem. Soc. 2016, 138, 76–79
N. Arnold, H. Braunschweig, R. D. Dewhurst, W. C. Ewing
(See online at https://doi.org/10.1021/jacs.5b11315) - Formation and Reactivity of Electron-Precise B—B Single and Multiple Bonds. Angew. Chem. Int. Ed. 2017, 56, 96–115
M. Arrowsmith, H. Braunschweig, T. E. Stennett
(See online at https://doi.org/10.1002/anie.201610072)
