Abstract
We report on an operationally simple catalytic method for the tungsten-catalyzed hydrogenation of quinolines through the use of the easily handled and self-contained precursor [WCl(η5-Cp)(CO)3]. This half sandwich complex is indefinitely storable on the bench in simple screw-capped bottles or stoppered flasks and can, if required, be prepared on a multi-gram scale while the actual catalytic transformations were performed in the presence ofa Lewis acid in order to achieve both decent substrate conversions and product yields. The described method represents a facile and atom-efficient access to a variety of 1,2,3,4-tetrahydroquinolines that circumvents the use of cost-intensive and oxygen-sensitive phosphine ligands as well as auxiliary hydride reagents.
Highlights
The versatile coordination chemistry of tungsten as well as the catalytic activity of certain complexes thereof are well documented [1] and a vast array of compounds incorporating this 5d transition metal has already found widespread application in numerous highly relevant organic transformations such as alkene and alkyne metathesis reactions [2,3,4,5,6,7,8,9,10,11,12]
(CO)3] that is formed upon oxidative ligation [112,113] of a proton stemming from acetic acid. This tungsten hydride species is back-formed through reaction of precat with molecular hydrogen during the course of the catalytic transformation in order to bring about the actual quinoline reduction
Inspired by a recent report on manganese(I)-catalyzed hydrogenation of N-heteroarenes published by the Beller group [101], we initially tested the catalytic activity of the tungstenbased piano-stool complex [WCl(g5-Cp)(CO)3] for this particular reaction
Summary
The versatile coordination chemistry of tungsten as well as the catalytic activity of certain (organometallic) complexes thereof are well documented [1] and a vast array of compounds incorporating this 5d transition metal has already found widespread application in numerous (industrially) highly relevant organic transformations such as alkene and alkyne metathesis reactions [2,3,4,5,6,7,8,9,10,11,12]. Berke and coworkers reported on a nitrosyltagged tungsten PNP pincer complex that was shown to effect the homogeneous hydrogenation of nitriles [33] and carbon dioxide [34] These findings foreshadowed and paralleled tremendous progress in the field of non-noble-metal-catalyzed redox-transformations based on Fe, Co, or Mn [35,36,37,38,39,40] with an emphasis on active metal–ligand assemblies that incorporate the aforementioned PNP donor atom set [41,42,43,44,45,46,47,48,49,50,51,52,53]. We report on the hitherto unknown homogeneous tungsten-based reduction of quinolines with molecular hydrogen functioning as the reducing agent
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