Dioxomolybdenum(VI)‐Catalyzed Reductive Cyclization of Nitroaromatics. Synthesis of Carbazoles and Indoles

Abstract

Reductive cyclization of nitrobiphenyls and nitrostyrenes to carbazoles and indoles, respectively, is carried out by triphenylphosphine under mild conditions catalyzed by a dichlorodioxomolybdenum(VI) complex. A one-pot procedure for the synthesis of regioselectively functionalized indoles has been developed from commercially available onitrobenzaldehydes and phosphoranes.

Pinacol as a New Green Reducing Agent: Molybdenum‐ Catalyzed Chemoselective Reduction of Sulfoxides and Nitroaromatics

“…Interestingly, the present protocol, involving pinacol as reductant, allowed the selective reduction of 2-nitrobiphenyl to 2-aminobiphenyl (entries 4−5), whereas the corresponding reaction with the same dioxomolybdenum(VI) catalyst in the presence of triphenylphosphine as oxygen-acceptor exclusively afforded carbazole. [13] This result shows that under the pinacol-Mo VI catalytic system nitroso intermediates are probably not generated in significant amounts. The synthesis of anilines containing halides occurred in high yields regardless of their substitution pattern (Scheme 2 and Table 3, entries 7−16).…”

“…[10] In this field we have reported the first examples of the Mo-catalyzed deoxygenation of sulfoxides [11] and N-oxides, [12] as well as the reductive cyclization of nitroaromatics. [13] In those processes phosphorous(III)-based reagents were used as oxygen-acceptors. Later on, other authors have also reported similar dioxomolybdenum(VI)-catalyzed reactions using alternative reductants such as silanes, [14] boranes, [15] and hydrogen.…”

Molybdenum‐Catalyzed Deoxygenation of Heteroaromatic N‐Oxides and Hydroxides using Pinacol as Reducing Agent

“…[5c] as well as for the reductive cyclization of nitroaromatics, [9] using phosphorous(III) reagents as oxygen-acceptors. [10] Looking for greener reducing agents in this area, we have more recently found that diols, including glycerol, [11] are also useful reagents for the deoxygenation of some of these compounds.…”

Progresses in Synthetic Methods of Carbazole and Its Derivatives

“…在各种环合反应中, 以硝基为环合基团的应用最为 广泛. Arnáiz 小组 [6] 以各种二取代的硝基联苯为原料, 如 Siamenol 等(Scheme 4). 以叠氮基为环合基团同样是合成咔唑的有效手段.…”