Highly Chemo- and Stereoselective Transfer Semihydrogenation of Alkynes Catalyzed by a Stable, Well-Defined Manganese(II) Complex

Abstract

Herein we report a manganese catalyzed semihydrogenation of internal alkynes to (Z)-alkenes using ammonia borane as a hydrogen donor. The reaction is catalyzed by a pincer complex of the earth abundant manganese(II) salt in the absence of any additives, base or super hydride. The ammonia borane smoothly reduces the manganese precatalyst [Mn(II)-PNP][Cl] 2 to the catalytically active species [Mn(I)-PNP]-hydride in the triplet spin state. This manganese hydride is highly stabilized by complexation with the alkyne substrate. Computational DFT analysis studies of the reaction mechanism rationalize the origin of stereoselectivity towards formation of (Z)-alkenes.

Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes

“…13 This new generation of Mn complex was also satisfactorily applied for the multicomponent synthesis of pyrimidines. 14 Important advances have also been carried out by Rueping and co-workers, describing unprecedented chemoselective hydrogenation of internal alkynes into (Z)-alkenes by a Mn(I)-PNP hydride, 15 or the generation of "green" methanol by hydrogenation of CO 2 -derived ( poli)-carbonates using well-defined Mn(I)-NNP complexes that work at milder conditions of pressure and temperature. 16 Definitively, these are just a few instances amongst the plethora of organic transformations catalysed by manganese that have been reported in the last lustrum.…”

Chemoselective Hydrogenation of Nitroarenes Using an Air-Stable Base-Metal Catalyst

“…Our interest in manganese catalysis 17 and the lack of reports regarding its application in the hydrogenation of nitroarenes motivated us to address this issue. Recently, the application of manganese, as the third most abundant metal in the Earth’s crust, for the catalytic hydrogenation of organic molecules has considerably increased.…”

Catalytic Transfer Hydrogenation of Arenes and Heteroarenes

“…More recently, ammonia borane complex has received a lot of attention due to its stability, high hydrogen storage capacity (19.6 wt %) and ready availability. For these reasons, the reduction of diverse motifs such as ketones, imines, alkenes and alkynes has been studied using ammonia borane as the hydrogen donor [5–20] . Nevertheless, only two studies have been reported on the reduction of aromatic substrates (Scheme 1 A).…”