Metal‐Free Deoxygenation and Reductive Disilylation of Nitroarenes by Organosilicon Reducing Reagents

Abstract

A metal-free deoxygenation and reductive disilylation of nitroarenes was achieved using N,N'-bis(trimethylsilyl)-4,4'-bipyridinylidene (1) under mild and neutral reaction conditions, and a broad functional group tolerance was possible in this reaction. Mono-deoxygenation, giving a synthetically valuable N,O-bis(trimethylsilyl)phenylhydroxylamine (7 a) as a readily available and safe phenylnitrene source from nitrobenzene, and double-deoxygenation, giving N,N-bis(trimethylsilyl)anilines 8, were easily controlled by varying the amounts of 1 and reaction temperature as well as adding dibenzothiophene (DBTP). Reaction of 2-arylnitrobenzenes with 1 resulted in the formation of the corresponding carbazoles 14 via in situ-generated phenylnitrene species derived by thermolysis of N,O-bis(trimethylsilyl)phenylhydroxylamines 7, followed by their subsequent intramolecular C-H insertion. In addition, the intramolecular N-N coupling reaction proceeded in the reduction of 2,2'-dinitrobiphenyl derivatives by 1, giving the corresponding benzo[c]cinnolines.

Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

“…Silanes and disilanes has long been known to promote the reduction of nitroarenes even in the absence of a metal catalyst and the reaction can be employed for the synthesis of heterocyclic compounds, [215][216][217] but high temperatures are needed unless activated silanes are employed. [217] Iron complexes have been found to catalyze the reduction of nitroarenes to anilines by phenylsilane. [218] A single example of the use of a copper catalyst in the reductive cyclization of a nitrostyrene to indole by a silane was reported in 2011 by Taylor and Correia (Scheme 87).…”

Organocatalytic Reduction of Nitroarenes with Phenyl(2‐quinolyl)methanol

“…Moreover, in the last decades, the focus on green and sustainable methodologies, also associated to the progress of organocatalysis, [7] stimulated the study of metal-free processes. In this context, a great variety of non-metal and transition metal free reagents were exploited for the reduction of nitro aromatics to anilines such as 9,10-dihydroanthracene, [8] 1,4-dihydropyridine systems, [9] elemental sulfur, [10] thiols, [11] Baker's Yeast in basic solution, [12] fullerene/H 2 , [13] glucose, [14] vasicine, [15] trichlorosilane/NR 3 , [16] triethylsilane/B(C 6 F 5 ) 3 , [17] diboron compounds, [18] viologen/Na 2 S 2 O 4 , [19] N,N'-bis(trimethylsilyl)-4,4'bipyridinylidene, [20] 2-aminoquinazolin-4(3H)-one/N 2 H 4 , [21] char-coal/NaBH 4 in aqueous medium, [22] as well as different carbonbased catalysts in the presence of N 2 H 4 , [23] NaBH 4 , [24] isopropanol, [25] or subcritical water [26] as hydrogen sources, and even the system isopropanol/KOH. [27] Recent studies from our laboratory showed the possibility to apply pyridylmethanols as hydrogen donors, mimetic of Hantzsch ester (HEH), for metal free reductions of aromatic/ heteroaromatic nitro compounds to the corresponding amines.…”

Reductive Silylation Using a Bis‐silylated Diaza‐2,5‐cyclohexadiene

“…While it was originally envisioned that elimination of (Me 3 Si) 2 O from MO n by 1 might be a useful approach to MO n −1 , a recent application to support‐bound WO 3 gave instead tetravalent tungsten silyl esters, which is still a valuable 2 electron reduction, but without full elimination of W/O bonds and creation of lower metal coordination number . The method has also been applied to nanoparticle formation, silylation of B−F bonds and of α‐halo carbonyls, and deoxygenation of nitro compounds . In summary, the strengths and limitations of this attractive reagent are only just emerging …”

“…[5c] The method has also been applied to nanoparticle formation, [5g] silylation of BÀFb onds [6] and of a-halo carbonyls, [7] and deoxygenation of nitro compounds. [8] In summary,t he strengths and limitations of this attractive reagent are only just emerging. [5c, 9] Finally,t he combination of Me 3 SiCl and KC 8 has become a popularw ay to functionalize reduced N 2 in metal complexes [10] and is speculated to involve Me 3 Si radicals; [1a, 11] the parasitic production of (SiMe 3 ) 2 is ap roblem with this reagentc ombination, and this might be improved using the bis-silyl transfer reagents 1 or 2.…”