Improved Second Generation Iron Pincer Complexes for Effective Ester Hydrogenation

Abstract

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

Isomerization of Allylic Alcohols to Ketones Catalyzed by Well‐Defined Iron PNP Pincer Catalysts

“…[11] The groups of Schneider, [11g] Guan, [12] and Beller [13] independently reportedt he synthesis of iron compound A (Scheme 2), which is an effective catalyst for the hydrogenation of esters, [12][13][14] nitriles, [15] and heterocycles. [11] The groups of Schneider, [11g] Guan, [12] and Beller [13] independently reportedt he synthesis of iron compound A (Scheme 2), which is an effective catalyst for the hydrogenation of esters, [12][13][14] nitriles, [15] and heterocycles.…”

Hydrogenation of Esters to Alcohols Catalyzed by Defined Manganese Pincer Complexes

“…[8] Whereas in recent years,m ajor developments have been achieved with iron and cobalt complexes both for hydrogenation and dehydrogenation reactions,m anganese is much less explored. [11] Since then, an improved secondgeneration iron pincer complex [12] and different cobalt com-plexes [13] have been published for effective ester hydrogenation. [10] Furthermore,o ur group and Guan and co-workers independently described hydrogenation reactions of non-activated esters with PNP iron pincer complexes.…”

Cobalt Pincer Complexes for Catalytic Reduction of Carboxylic Acid Esters

“…During our studies on the catalytic behavior of aliphatic iron, or manganese pincer complexes we observed the beneficial effect of carbonyl ligands that are coordinated to the metal centre. Therefore, we tested various, carbon monoxide‐containing cobalt pincer species 5 , 6 and 8 in this model reaction.…”

“…So far, all these cobalt catalysts have been tested for a relatively small number of esters with a limited tolerance of functional groups leaving space for improvements. Here, based on our experiences in non‐noble‐metal‐catalyzed reductions, we report the development of a more general applicable cobalt catalyst for the hydrogenation of various carboxylic acid esters.…”