trans-FeII(H)2(diphosphine)(diamine) complexes as alternative catalysts for the asymmetric hydrogenation of ketones? A DFT study

Abstract

New insights into the structural, electronic and catalytic properties of Fe complexes are provided by a density functional theory study of model as well as real [Fe(II)(H)(2)(diphosphine)(diamine)] systems. Calculations conducted using several different functionals on the trans- and cis-isomers of [Fe(II)(H)(2)(S-xylbinap)(S,S-dpen)] complexes show that, as with the [Ru(II)(H)(2)(diphosphine)(diamine)] complexes, the trans-[Fe(II)(H)(2)(diphosphine)(diamine)] complex is the more stable isomer. Analysis of the spin states of the trans-[Fe(II)(H)(2)(diphosphine)(diamine)] complexes also shows that the singlet state is significantly more stable than the triplet and the quintet, as with the [Ru(II)(H)(2)(diphosphine)(diamine)] complexes. Calculations of the catalytic cycle for the hydrogenation of ketones using two model trans-[M(II)(H)(2)(PH(3))(2)(en)] catalysts, where M = Ru and Fe, show that the mechanism of reaction as well as the activation energies are very similar, in particular: (i) the ketone/alcohol hydrogen transfer reaction occurs through the metal-ligand bifunctional mechanism, with energy barriers of 3.4 and 3.2 kcal mol(-1) for the Ru- and Fe-catalysed reactions, respectively; (ii) the heterolytic splitting of H(2) across the M[partial double bond, bottom dashed]N bond for the regeneration of the Ru and Fe catalysts has an activation barrier of 13.8 and 12.8 kcal mol(-1), respectively, and is expected to be the rate determining step for both catalytic systems. The reduction of acetophenone by trans-[M(II)(H)(2)(S-xylbinap)(S,S-dpen)] complexes along two competitive reaction pathways, shows that the intermediates for the Fe catalytic system are similar to those responsible for the high enantioselectivity of (R)-alcohol in those proposed trans-[Ru(II)(H)(2)(S-xylbinap)(S,S-dpen)] catalysed acetophenone hydrogenation reaction. Thus the high enantiomeric excess in the hydrogenation of acetophenone could, in principle, be achieved using Fe catalysts.

The NH Functional Group in Organometallic Catalysis

“…Complex 80 b likely served as the active catalyst in the hydrogenation. The reaction was assumed to follow an outer‐sphere mechanistic pathway (Scheme ),48b–d which is analogous to the [RuCl 2 (diphosphine)(diamine)]‐catalyzed ketone hydrogenation shown in Scheme . The NH group was thought to be involved in the hydrogenation via a six‐membered ring transition state 84 .…”

Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η4-C4R2C(O)C4H8}Fe(CO)3] (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) and attempts to prepare Fe(II) hydroxycyclopentadienyl–hydride complexes

“…Ruthenium catalysts are used in a number of asymmetric hydrogen transfer systems. For example, the most effective and efficient class of catalysts for the enantioselective hydrogenation of pro-chiral ketones to chiral alcohols are the ruthenium(II) diphosphine-diaminebased complexes developed by Noyori et al [18,19], and recent practical [20,21] and theoretical [22,23] work has suggested that related iron complexes may be viable alternatives. The dimeric ruthenium complex (Scheme 1), widely known as the Shvo catalyst [24][25][26][27][28][29][30][31], acts as a catalyst for hydrogen transfer reactions between alcohols and aldehydes/ ketones, a process resulting from the scission of the Shvo catalyst into saturated 18-electron and unsaturated 16-electron mononuclear fragments (Scheme 1).…”

Well‐Defined Bifunctional Iron Catalysts for the Hydrogenation of Ketones: Iron, the New Ruthenium

“…DFT calculations on related complexes support the assumption that these complexes are acting as bifunctional catalysts. [12] It is also important to note that Morris and co-workers achieved for the first time an enantioselective hydrogenation, although the conversion and enantioselectivity were modest (40 % conversion, 27 % ee).…”