Catalysis Sciences & Engineering, Short talk
CE-014

Sodium Zincate Mediated Dihydrogen Activation and Catalytic Imine Hydrogenation

T. J. Mathers1, E. Hevia1*
1Department of Chemistry, Biochemistry, and Pharmacy; University of Bern

The catalytic activation of dihydrogen (H2) and subsequent hydrogenation of unsaturated substrates is a cornerstone transformation in synthetic chemistry. Once considered the exclusive domain of transition metals, this reactivity has more recently been extended to main group systems, including Frustrated Lewis Pairs2 and Group 2 metal amide complexes,3 which have demonstrated the enormous potential of these systems in catalytic applications. By contrast, despite early reports that nBuLi can activate H2, the application of Group 1 metal compounds in this field remains very limited, largely due to the formation of insoluble hydride salts [MH]∞.4

Breaking new ground in this area, we report the activation of H2 by combining the highly basic amide NaTMP (TMP = 2,2,6,6 tetramethylpiperidide) with the sterically encumbered Zn(TMP)2. By switching on bimetallic cooperativity, this system can heterolytically cleave H2 under mild reaction conditions (1 bar, 25 °C, 5 min) to furnish the hydrocarbon-soluble [NaZnH2(TMP)TMEDA]2 catalyst (1). (TMEDA= N,N,N’,N’ tetramethylethylenediamine) (Figure 1). Furthermore, conducting the reaction in a one-pot fashion enables the reduction of a series of imines to the corresponding amines in high yields. Remarkably, this H2 activation and hydrogenation reactivity has been upgraded to catalytic regimes, by working under relatively mild reaction conditions (15 bar, 100 °C, 2 h) compared to other zinc based systems.5 By combining spectroscopic studies with the isolation of key reaction intermediates and DFT calculations, mechanistic insights on how these transformations take place will be presented, advancing the understanding of this unique Na/Zn cooperation.

[1] G. C. Welch, R. R. S. Juan, J. D. Masuda, D. W. Stephan, Science 2006, 314 (5802), 1124-1126.

[2] H. Bauer, M. Alonso, C. Färber, H. Elsen, J. Pahl, A. Causero, G. Ballmann, F. De Proft, S. Harder. Nat. Catal. 2018, 1 (1), 40-47.

[3] P. A. A. Klusener, L. Brandsma, H. D. Verkruijsse, P. v. R. Schleyer, T. Friedl, R. Pi, Angew. Chem. Int. Ed. 1986, 25, 465-466.

[4] D.W. Stephan, Angew. Chem. Int. Ed. 2013, 52, 9831-9835.