Abstract
Several hydride Mn(I) and Re(I) PNP pincer complexes were applied as catalysts for the homogeneous chemoselective hydrogenation of aldehydes. Among these, [Mn(PNP-iPr)(CO)2(H)] was found to be one of the most efficient base metal catalysts for this process and represents a rare example which permits the selective hydrogenation of aldehydes in the presence of ketones and other reducible functionalities, such as C=C double bonds, esters, or nitriles. The reaction proceeds at room temperature under base-free conditions with catalyst loadings between 0.1 and 0.05 mol% and a hydrogen pressure of 50 bar (reaching TONs of up to 2000). A mechanism which involves an outer-sphere hydride transfer and reversible PNP ligand deprotonation/protonation is proposed. Analogous isoelectronic and isostructural Re(I) complexes were only poorly active.
Highlights
One environmentally friendly and sustainable method to prepare alcohols, which are valuable commodities for a large number of fine and bulk chemicals, is the catalytic hydrogenation of carbonyl compounds with dihydrogen.[1]
Iron- and manganese-based catalysts turned out to be highly active for the hydrogenation of carbonyl compounds, imines, and nitriles (Scheme 2).[8−11] In the case of manganese, most hydrogenations proceed at relatively high catalyst loadings and elevated temperatures and, in addition, require large amounts of strong bases as additives
Only iron-based systems proved to be reasonably chemoselective for the reduction of aldehydes, as shown in Scheme 1.12−14 We recently described the application of [Fe(PNPMe-iPr)(CO)(H)(Br)] and [Fe(PNPMe-iPr)(H)2(CO)] as highly active catalysts for the homogeneous hydrogenation of aldehydes (Scheme 1).[15,16]
Summary
One environmentally friendly and sustainable method to prepare alcohols, which are valuable commodities for a large number of fine and bulk chemicals, is the catalytic hydrogenation of carbonyl compounds with dihydrogen.[1] Over the years, many highly efficient and active homogeneous catalysts based on precious and non-precious metals have been described for this purpose (Scheme 1).[2] Especially catalysts which reveal full selectivity for aldehydes over ketones and/or alkenes[3,4] are of Scheme 1. We describe an experimental and theoretical investigation of the chemoselective hydrogenation of aldehydes with dihydrogen using several hydride Mn(I) and Re(I) PNP pincer complexes as catalysts (Scheme 3). Has to be noted that Re pincer complexes have rarely been used in (de)hydrogenation catalysis.[17,18]
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