Donor functionalized iron(II) N-Heterocyclic carbene complexes in transfer hydrogenation reactions

Two piano-stool iron(II) complexes bearing N-heterocyclic carbene ligands outfitted with acetamide- and amine-pendant arms [Cp*Fe(NHC(R))(CO)I] {Cp* = eta(5)-tetramethylcyclopentadienyl; R = CH2CONEt2(3), (CH2)(2)NEt2(4)}, have been prepared and fully characterized. Their catalytic activity in transfer hydrogenation (TH) of ketones usingiPrOH as a hydrogen source and catalytic amounts of base (LiOtBu) has been explored, along with that of previously reported [CpFe(NHC(R))(CO)I] {R =nBu (5), (CH2)(2)OH (6), Et (7), and (CH2)(3)OH (8)} complexes containing hydroxyl and nonfunctionalized alkyl arms. Complex3displayed the highest catalytic activity of the whole series3-8, reaching a TOF(50)value of 533 h(-1). NMR monitoring of the stoichiometric reaction of3with LiOtBu, allowed the identification of a new species3'containing a deprotonated amidate moiety, which has been fully characterized by(1)H,C-13, and(15)N NMR. Finally, a green protocol for the reduction of ketones through TH using glycerol as a hydrogen source, under microwave irradiation in the presence of catalytic amounts of3and base has been developed.