Hydrogenation of Imines Catalyzed by Trisphosphine‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co‐Catalytic Acid |
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| Authors: | Dr. Subrata Chakraborty Dr. Olivier Blacque Dr. Thomas Fox Prof. Heinz Berke |
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| Affiliation: | Institute of Chemistry, University of Zürich, Winterthurerstrasse 190, CH‐8057 Zürich (Switzerland) |
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| Abstract: | Hydride complexes Mo,W(CO)(NO)H(mer‐etpip) (iPr2PCH2CH2)2PPh=etpip) ( 2 a,b(syn) , syn and anti of NO and Ph(etpip) orientions) were prepared and probed in imine hydrogenations together with co‐catalytic [H(Et2O)2][B(C6F5)4] (140 °C, 60 bar H2). 2 a,b(syn) were obtained via reduction of syn/anti‐Mo,W(NO)Cl3(mer‐etpip) and syn,anti‐Mo,W(NO)(CO)Cl(mer‐etpip). [H(Et2O)2][B(C6F5)4] in THF converted the hydrides into THF complexes syn‐[Mo,W(NO)(CO)(etpip)(THF)][B(C6F5)4]. Combinations of the p‐substituents of aryl imines p‐R1C6H4CH=N‐p‐C6H4R2 (R1,R2=H,F,Cl,OMe,α‐Np) were hydrogenated to amines (maximum initial TOFs of 1960 h?1 ( 2 a(syn) ) and 740 h?1 ( 2 b(syn) ) for N‐(4‐methoxybenzylidene)aniline). An ‘ionic hydrogenation’ mechanism based on linear Hammett plots (ρ=?10.5, p‐substitution on the C‐side and ρ=0.86, p‐substitution on the N‐side), iminium intermediates, linear P(H2) dependence, and DKIE=1.38 is proposed. Heterolytic splitting of H2 followed by ‘proton before hydride’ transfers are the steps in the ionic mechanism where H2 ligand addition is rate limiting. |
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| Keywords: | hydrides imines ionic hydrogenation molybdenum tungsten |
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