Manganese(I)-Catalyzed Chemoselective Transfer Hydrogenation of the C=C Bond in Conjugated Ketones at Room Temperature.

Chemoselective transfer hydrogenation of C=C bond in α,ß-unsaturated ketones is demonstrated at room temperature employing a manganese(I) catalyst and half an equivalent of ammonia-borane (H3 N-BH3 ). A series of mixed-donor pincer-ligated Mn(II) complexes, (tBu2 PN3 NPyz )MnX2 [κP ,κN ,κN -(N-(di-tert-butylphosphaneyl)-6-(1H-pyrazol-1-yl)pyridin-2-amine)MnX2 ] {X=Cl (Mn2), X=Br (Mn3), X=I (Mn4)} were synthesized and characterized. Amongst the Mn(II) complexes, (Mn2, Mn3, Mn4) and Mn(I) complex, (tBu2 PN3 NPyz )Mn(CO)2 Br (Mn1) screened; the Mn1 acts as an efficient catalyst for the chemoselective C=C bond reduction in α,ß-unsaturated ketones. Various synthetically important functionalities like halides, methoxy, trifluoromethyl, benzyloxy, nitro, amine, and unconjugated alkene and alkyne groups, including heteroarenes, were compatible and provided saturated ketones in excellent yields (up to 97 %). A preliminary mechanistic study highlighted the crucial role of metal-ligand (M-L) cooperation through the dearomatization-aromatization process in catalyst Mn1 for the chemoselective C=C bond transfer hydrogenation.

Room temperature chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds by using a well-defined mixed donor Mn(i) pincer catalyst.

Chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds in α,ß-unsaturated ketones, aldehydes and imines is accomplished at room temperature (27 °C) using a well-defined Mn(i) catalyst and 5.0 bar H2. Amongst the three mixed-donor Mn(i) complexes developed, κ3-(R2PN3NPyz)Mn(CO)2Br (R = Ph, iPr, t Bu); the t Bu-substituted complex ( tBu2PN3NPyz)Mn(CO)2Br shows exceptional chemoselective catalytic reduction of unsaturated bonds. This hydrogenation protocol tolerates a range of highly susceptible functionalities, such as halides (-F, -Cl, -Br, and -I), alkoxy and hydroxy, including hydrogen-sensitive moieties like acetyl, nitrile, nitro, epoxide, and unconjugated alkenyl and alkynyl groups. Additionally, the disclosed method applies to indole, pyrrole, furan, thiophene, and pyridine-containing unsaturated ketones leading to the corresponding saturated ketones. The C[double bond, length as m-dash]C bond is chemoselectively hydrogenated in α,ß-unsaturated ketones, while the aldehyde's C[double bond, length as m-dash]O bond and imine's C[double bond, length as m-dash]N bond are preferentially reduced over the C[double bond, length as m-dash]C bond. A detailed mechanistic study highlighted the non-innocent behavior of the ligand in the ( tBu2PN3NPyz)Mn(i) complex and indicated a metal-ligand cooperative catalytic pathway. The molecular hydrogen (H2) acts as a hydride source, whereas MeOH provides a proton for hydrogenation. DFT energy calculations supported the facile progress of most catalytic steps, involving a crucial turnover-limiting H2 activation.

3 d Transition Metal-Catalyzed Hydrogenation of Nitriles and Alkynes.

Selective hydrogenation of nitriles and alkynes is crucial considering the vast applications of reduced products in industries and in the synthesis of bioactive compounds. Particularly, the late 3d transition metal catalysts (manganese, iron, cobalt, nickel and copper) have shown promising activity for the hydrogenation of nitriles to primary amines, secondary amines and imines. Similarly, semihydrogenation of alkynes to E- and Z-alkenes by 3d metals is adequately successful both via the transfer hydrogenation and by using molecular hydrogen. The emergence of 3d transition metals in the selective synthesis of industrially relevant amines, imines and alkenes makes this protocol more attractive. Herein, we provide a concise overview on the late 3d transition metal-catalyzed hydrogenation of nitriles to amines and imines as well as semihydrogenation of alkynes to alkenes.

Nickel-Catalyzed Regioselective C(2)-H Difluoroalkylation of Indoles with Difluoroalkyl Bromides.

Regioselective C(2)-H difluoroalkylation of C-3 unsubstituted indoles with commonly available fluoroalkyl bromides is successfully achieved employing a simple nickel catalyst system, (DME)NiCl2 /Xantphos. This methodology shows excellent regioselectivity and exhibits a broad substrate scope. Various functional groups, such as -OMe, -F, and -Br, are tolerated on the indole backbone to give the difluoroalkylated products in moderate to good yields. Preliminary mechanistic findings demonstrate that the reaction is homogeneous in nature and involves a radical manifold. Synthetic utility of this nickel-catalyzed method is demonstrated by synthesizing melatonin receptor antagonist Luzindole derivative.