Iron-Catalyzed Homogeneous Hydrogenation of Alkenes under Mild Conditions by a Stepwise, Bifunctional Mechanism

Abstract

Hydrogenation of alkenes containing polarized CC double bonds has been achieved with iron-based homogeneous catalysts bearing a bis­(phosphino)­amine pincer ligand. Under standard catalytic conditions (5 mol % of (PNHPiPr)­Fe­(H)2(CO) (PNHPiPr = NH­(CH2CH2PiPr2)2), 23 °C, 1 atm of H2), styrene derivatives containing electron-withdrawing para substituents reacted much more quickly than both the parent styrene and substituted styrenes with an electron-donating group. Selective hydrogenation of CC double bonds occurs in the presence of other reducible functionalities such as −CO2Me, −CN, and N-heterocycles. For the α,β-unsaturated ketone benzalacetone, both CC and CO bonds have been reduced in the final product, but NMR analysis at the initial stage of catalysis demonstrates that the CO bond is reduced much more rapidly than the CC bond. Although Hanson and co-workers have proposed a nonbifunctional alkene hydrogenation mechanism for related nickel and cobalt catalysts, the iron system described here operates via a stepwise metal–ligand cooperative pathway of Fe–H hydride transfer, resulting in an ionic intermediate, followed by N–H proton transfer from the pincer ligand to form the hydrogenated product. Experimental and computational studies indicate that the polarization of the CC bond is imperative for hydrogenation with this iron catalyst.

Recent Advances in Hydrometallation of Alkenes and Alkynes via the First Row Transition Metal Catalysis

“…[318] More recently, Jones and co-workers showed a five-coordinate iron complex [143-FeH(CO)] bearing a bis(phosphino)amine ligand, which could hydrogenate a variety of alkenes into alkanes in the absence of base. [318] More recently, Jones and co-workers showed a five-coordinate iron complex [143-FeH(CO)] bearing a bis(phosphino)amine ligand, which could hydrogenate a variety of alkenes into alkanes in the absence of base.…”

Manganese‐Catalyzed Hydrogen‐Autotransfer C−C Bond Formation: α‐Alkylation of Ketones with Primary Alcohols

“…However,w ed on ot think that this option is very probable and instead propose that the basic medium promotes the formation of an amidate,which undergoes abstraction of the b-hydrogen atom, as shown in transition state III.S uch an intramolecular ligand-assisted mechanism leads to aldehyde 11 and the intermediate IV,w hich is protonated to form the manganese hydride V.Analdol condensation between 11 and the enolate resulting from the starting ketone 4 affords the a,b-unsaturated compound 12 with the release of amolecule of H 2 O. Finally,hydrogenation by species V yields the desired ketone 6/7 and regenerates the catalytic active species I. Previously,organometallic hydride complexes (e.g.Ru, Ir,as well as Fe)w ith pincer ligands have been shown to reduce polarized multiple bonds. [17] Thus,w ea ssume that our reaction proceeds through Mn À Hh ydride transfer from complex V to the b-position in aM ichael-type process followed by NÀHp roton transfer to the corresponding enolate. [4f] To confirm the participation of the NÀHm oiety of the pincer ligand in this transformation, we performed the alkylation of acetophenone (4a)w ith benzyl alcohol (5a)i n the presence of the corresponding N-methylated manganese complex 3' ' (Scheme 5a).…”

From Ruthenium to Iron and Manganese—A Mechanistic View on Challenges and Design Principles of Base‐Metal Hydrogenation Catalysts

“…Usually, this type of bifunctional iron complexes exhibit only a very limited activity in the hydrogenation of C−C bonds. Very recently, Jones and coworkers have suggested a bifunctional mechanism for the hydrogenation of polar C=C bonds, catalyzed by the iron pre‐catalyst 12 and analogue complexes . The catalytic activity of 12 is significantly lower than found for state‐of‐the‐art iron catalysts for olefin hydrogenation.…”