4,4′-Bipyridyl-Catalyzed Reduction of Nitroarenes by Bis(neopentylglycolato)diboron

Abstract

4,4′-Bipyridyl worked as an organocatalyst for the reduction of nitroarenes by bis­(neopentylglycolato)­diboron (B2nep2), followed by hydrolysis to give the corresponding anilines. This reduction proceeded under aerobic conditions without any prepurification of substrates and reagents. We found broad functional group tolerance and compatibility for O- and N-protecting groups under the reaction conditions. The key in this catalytic system was the addition of B2nep2 to 4,4′-bipyridyl to form N,N′-bis­[(neopentylglycolato)­boryl]-4,4′-bipyridinylidene as a deoxygenating reagent of nitroarenes.

Pyridine-Mediated B–B Bond Activation of (RO)2B–B(OR)2 for Generating Borylpyridine Anions and Pyridine-Stabilized Boryl Radicals as Useful Boryl Reagents in Organic Synthesis

“…Treatment of nitroarenes with excess bis(neopentylglycolato)diboron (B2nep2) with 4,4'-bipyridyl (2 mol%) in CH3CN at 100 °C and subsequent hydrolysis gave the corresponding anilines in good to excellent yields (Scheme 35). 41 Other diborons such as B2pin2 and B2cat2 were ineffective for this reaction. This deoxygenation tolerated many functional groups, such as halides, carbonyl, cyano, alkene, and heterocycles, as well as typical O-and Nprotecting groups.…”

Reduction of Nitroarenes via Catalytic Transfer Hydrogenation Using Formic Acid as Hydrogen Source: A Comprehensive Review

“…The first is catalyzed by specific transition metals such as iron, nickel, palladium or platinum in presence of molecular hydrogen under special pressure conditions, [8b,10-16] whereas the second strategy employs hydrogen sources for the transfer hydrogenation process such as hydrides (e. g. NaBH 4 or hydrosilanes, [17][18][19][20][21][22][23][24][25] hydrazine hydrate, [17,[26][27][28][29][30] organic reagents (e. g. alcohol, glycerol or formic acid) [17,[31][32][33] and more recently, boron reagents. [34][35] Other methods based on reduced sulfur reagents like elemental sulfur, [36][37] dithionite [38] or sulfides [39] are useful for the reduction of nitro group, being particularly convenient for the selective reduction of polynitroarenes. From Green Chemistry point of view, the catalytic hydrogenation have received enormous attention by their improvements compared to Bechmap's reduction; [17] however, this type of strategies presents some drawbacks such as the use of specialized equipment and safety issues related to H 2 storage, the manipulation of flammable hydrogen gas and the use of hazardous reagents and sophisticated organometallic complexes/nanometallic catalyst, which limit its practicality and economy.…”

“…As an alternative to the typical catalytic hydrogenation using H 2 for the reduction of nitro group highlights the heterogeneous catalytic transfer hydrogenation (CTH), which emerged as a more controlled strategy because the hydrogen is generated in situ using a hydrogen donor and represents a valuable strategy by their minor procedure risk, simplicity, cost-effective and sustainability. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] The design of this strategy is focused on the selection of the metallic catalyst and the hydrogen sources. The nature of catalyst is very variable and a large number of examples can be found including catalyst based on coinage (Au, Ag, Cu), noble (mostly Pd, Pt and Ru) and non-noble (Ni, Co, Fe) metals.…”

“…NaBH 4 or hydrosilanes, [17–25] hydrazine hydrate, [17,26–30] organic reagents ( e. g . alcohol, glycerol or formic acid) [17,31–33] and more recently, boron reagents [34–35] . Other methods based on reduced sulfur reagents like elemental sulfur, [36–37] dithionite [38] or sulfides [39] are useful for the reduction of nitro group, being particularly convenient for the selective reduction of polynitroarenes.…”

“…morpholine, phosphorous acid); [44] however, the control of the selectivity is even persistent in the catalytic hydrogenation. As an alternative to the typical catalytic hydrogenation using H 2 for the reduction of nitro group highlights the heterogeneous catalytic transfer hydrogenation (CTH), which emerged as a more controlled strategy because the hydrogen is generated in situ using a hydrogen donor and represents a valuable strategy by their minor procedure risk, simplicity, cost‐effective and sustainability [17–35] . The design of this strategy is focused on the selection of the metallic catalyst and the hydrogen sources.…”