We present the first base-free Fe-catalyzed ester reduction applying molecular hydrogen. Without any additives, a variety of carboxylic acid esters and lactones were hydrogenated with high efficiency. Computations reveal an outer-sphere mechanism involving simultaneous hydrogen transfer from the iron center and the ligand. This assumption is supported by NMR experiments.

The catalytic hydrogenation of carboxylic acid derivatives represents an atom-efficient and clean reduction methodology in organic chemistry. More specifically, the selective hydrogenation of nitriles offers the possibility for a green synthesis of valuable primary amines. So far, this transformation lacks of useful, broadly applicable non-noble metal-based catalyst systems. In the present study, we describe a molecular-defined iron complex, which allows for the hydrogenation of aryl, alkyl, heterocyclic nitriles and dinitriles. By using an iron PNP pincer complex, we achieve very good functional group tolerance. Ester, ether, acetamido as well as amino substituents are not reduced in the presence of nitriles. Moreover, nitriles including an α,β-unsaturated double bond and halogenated derivatives are well tolerated in this reaction. Notably, our complex constitutes the first example of an homogeneous catalyst, which permits the selective hydrogenation of industrially important adipodinitrile to 1,6-hexamethylenediamine.

The iron-catalyzed reduction of aromatic nitro compounds to the corresponding anilines applying organosilanes is reported. In the presence of FeX(2)-R(3)P catalysts a series of nitroarenes is selectively reduced tolerating a wide range of functional groups.

It takes two: For the reduction of amides to amines iron catalysts were developed. A combination of two different iron catalysts made possible the challenging reduction of primary amides (see picture).

The performance of well-defined ultrasmall iron(0) nanoparticles (NPs) as catalysts for the selective hydrogenation of unsaturated C-C and C=X bonds is reported. Monodisperse iron nanoparticles of about 2 nm size are synthesized by the decomposition of {Fe(N[Si(CH3)3]2)2}2 under dihydrogen. They are found to be active for the hydrogenation of various alkenes and alkynes under mild conditions and weakly active for C=O bond hydrogenation.

A selection of cobalt(I) and cobalt(II) pincer type complexes with different substitution patterns was tested in the catalytic reduction of carboxylic acid esters to alcohols. The cobalt pincer type complex 4 is suitable for the hydrogenation of aromatic as well as aliphatic and cyclic esters. Mechanistic investigation indicated a metal ligand cooperated reaction pathway.

Hydrogenation of esterst oa lcoholsw ith aw ell-definedi ron iPr2 PNP pincerc omplexh as been recently reported by us and other groups.W en ow introduce an ovel and sterically less hindered Et2 PNP congener that provides superior catalytic activity in the hydrogenation of various carboxylic acid esters and lactones comparedt ot he known complex. Successful hydrogenation proceeds under relatively mild conditions (60 8 8C) with lower catalyst loadings.Keywords: alcohols;c atalytic hydrogenation;esters; iron;p incer complexes Thec atalytic hydrogenation of esterst ot he corresponding alcohols represents an interesting methodology of paramount importance for organic synthesis and fine chemical processes.[1] In the past, this transformation relied on the use of stoichiometrica mounts of inorganic metal hydridess uch as LiAlH 4 ,N aBH 4 and related compounds resulting in the formation of stoichiometrica mounts of waste products followed by complexw ork-upp rocedures .[2] In contrast, catalytic hydrogenation of esters and lactonese mploying H 2 constitutes ac ompletely atom economic, waste-free and environmentally benign transformation. Heterogeneousc atalysts are applied in the hydrogenation of fatty estersb ut these catalysts require high temperature andp ressure.[3] Hence,t he development of milder and more selective catalytic protocols for ester hydrogenation facilitated by well-defined homogeneous complexes constitutes an actual and highly desired research goal. [4] In the early 2000s av ariety of seminal reportso n homogeneous catalysts for the hydrogenation of estersa ppeared in the literature. [5][6][7] Major developments in this field were madeb yt he groups of Milstein, [8] Saudan, [9] Kuriyama, [10] andG usev. [11] In addition, very recently ah ighly active bis-NHC amino pincerR uc omplexh as been developed for ester hydrogenation. [12] Despite the recent progress with these Ru-based materials,t he development of inexpensive,e arthabundant metal catalysts is desirable.I nt his context, iron-based catalysts are of special interest due to their low cost, low toxicity and high abundance. [13] In the past year, significant advancesh ave been achieved in our group [14] ando thers [15] using well-defined ironbased pincerc omplexes (Figure 1), which were successfullya pplied in selective hydrogenation and dehydrogenation reactions.[16] Based on our experience in this redox catalysis, [17] we became interested in the development of similar applications using an iron-based catalyst.Inspired by our recent worko nt he hydrogenation of esters to alcohols [14b] conducted with the isopropyltaggedi ron-PNP pincerc omplex 1,w ef ocused on the preparation of two novelc ongeners 2 and 3 bearing phosphinem otifsw ith different stericd emands (Figure 2). Herein, we address the influence of the alkyl substituents at the phosphorus binding site on

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