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Hydrogenation of Imines Catalyzed by Trisphosphine‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co‐Catalytic Acid
Author(s) -
Chakraborty Subrata,
Blacque Olivier,
Fox Thomas,
Berke Heinz
Publication year - 2014
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201402280
Subject(s) - chemistry , heterolysis , medicinal chemistry , iminium , hydride , catalysis , ligand (biochemistry) , molybdenum , imine , stereochemistry , ionic bonding , metal , ion , organic chemistry , biochemistry , receptor
Hydride complexes Mo,W(CO)(NO)H(mer‐etp i p) ( i Pr 2 PCH 2 CH 2 ) 2 PPh=etp i p) ( 2 a,b( syn ) , syn and anti of NO and Ph(etp i p) orientions) were prepared and probed in imine hydrogenations together with co‐catalytic [H(Et 2 O) 2 ][B(C 6 F 5 ) 4 ] (140 °C, 60 bar H 2 ). 2 a,b( syn ) were obtained via reduction of syn / anti ‐Mo,W(NO)Cl 3 (mer‐etp i p) and syn , anti ‐Mo,W(NO)(CO)Cl(mer‐etp i p). [H(Et 2 O) 2 ][B(C 6 F 5 ) 4 ] in THF converted the hydrides into THF complexes syn ‐[Mo,W(NO)(CO)(etp i p)(THF)][B(C 6 F 5 ) 4 ]. Combinations of the p ‐substituents of aryl imines p ‐R 1 C 6 H 4 CH=N‐ p ‐C 6 H 4 R 2 (R 1 ,R 2 =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‐ m ethoxybenzylidene)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 (H 2 ) dependence, and DKIE=1.38 is proposed. Heterolytic splitting of H 2 followed by ‘proton before hydride’ transfers are the steps in the ionic mechanism where H 2 ligand addition is rate limiting.