Synthesis of Al(I)-based Frustrated Lewis Pairs and Reactivity in Activation of Small Nitrogen-containing Molecules
Final Report Abstract
Four different catalytic processes with easily accessible or commercially available main groupbased initiators and catalysts were developed. Simple bipyridinium- and phenanthroliniumderived carbon-based Lewis acid initiators were accessed in a facile two-step synthesis and successfully utilized in silylium ([SiR3]+)-catalyzed hydrodefluorination (HDF) of a range of aryland alkyl- substituted mono-, di- and trifluoromethyl groups. The novel carbonbased initiators abstract hydride from silane to afford catalytically active [SiR3]+. The activity of the dicationic initiators correlated well with the calculated hydrogen ion affinites (HIAs). In a further catalytic procedure the in situ generation of [SiR3]+ with a trityl ([CPh3]+)-based initiator was exploited to reduce haloboranes (R2BCl) to hydridoboranes (R2BH) in the presence of Et3SiH. The protocol proceeds via a borinium cation. A small R2BH-library was successfully synthesized from the R2BCl congeners. More reactive haloboranes like X-9-BBN (X = F, Cl; 9-BBN = 9-borabicyclononane) were found to directly react with Et3SiH without initiator. From this finding, a system consisting of 9-BBN-catalyst and sacrificial silane was developed and employed in Friedel Crafts (FC) benzylation with benzyl fluorides as well as in the reduction of chlorophosphines (R2PCl) to phosphines (R2PH) and diphosphines (R4P2). In both cases the X-9-BBN (X = F, Cl), formed in the C-F- or P-Cl-activation step, is recycled to 9-BBN by reduction with silane. The catalytic FC benzylation of arenes with 9-BBN provided the corresponding 1,1-diarylmethanes in good to excellent yields. R2PCl-reduction by 9-BBN/silane is improved by addition of [nEt4N]Cl. The chloride catalyst accelerates hydrochlorination of PhSiH3 and thus the formation of R4P2. In general, aryl-substituted chlorophosphines afford primarily diphosphines, while secondary phosphines predominate when alkyl-substituted precursors are used. Use of the combined catalysts reduced reaction time and temperature, thus providing a scalable and facile protocol for the preparation of R4P2 and R2PH.
Publications
- Bipyridinium and Phenanthrolinium Dications for Metal-Free Hydrodefluorination: Distinctive Carbon-Based Reactivity. Chem. Eur. J. 2021, 27, 11730-11737
K. I. Burton, I. Elser, A. E. Waked, T. Wagener, R. J. Andrews, F. Glorius, D. W. Stephan
(See online at https://doi.org/10.1002/chem.202101534) - 9-BBN and Chloride Catalyzed Reduction of Chlorophosphines to Phosphines and Diphosphines. Chem. Commun. 2022, 58, 1740-1743
I. Elser, R. J. Andrews, D. W. Stephan
(See online at https://doi.org/10.1039/d1cc06439h) - Probing B–X to B–H conversions and applications in C–F bond activation catalysis. Dalton Trans. 2022, 51, 17962-17966
A. Yeganeh-Salman, I. Elser, K. L. Bamford, D. Ebanks, D. W. Stephan
(See online at https://doi.org/10.1039/d2dt03588j)
