RESUMO
We found that bis(neopentylglycolato)diboron (B2nep2) served as a bidentate ligand and a one-electron reducing agent for early transition metal chlorides to afford MCl4(B2nep2). Treatment of B2nep2 with MCl5 (M = Nb and Mo) produced MCl4(B2nep2) via two successive reactions, coordination of B2nep2 to the metal center and one-electron reduction from M(V) to M(IV), while coordination of B2nep2 to MCl4 (M = Zr, Ti) was observed without reduction of the central metals. DFT studies for the reduction of NbCl5 by B2nep2 clarified the initial formation of seven-coordinated and B2nep2-ligated Nb(V) species, NbCl5(B2nep2), and one chloride on niobium(V) moves to the Lewis acidic boron center to generate NbCl4[(B2nep2)Cl]. The chloride on the boron atom of NbCl4[(B2nep2)Cl] is trapped by the second B2nep2 to give [NbCl4(B2nep2)][ClB2nep2]. After the formation of [ClB2nep2]- as an anionic sp2-sp3 diboron adduct, one-electron reduction of the niobium(V) center produces NbCl4(B2nep2) along with [ClB2nep2]Ë as a plausible diboron species, whose decomposition affords ClBnep and B2nep2. The reduction of metal halides in the presence of B2nep2 was exemplified by green LED irradiation of TiCl4(B2nep2), producing chloride-bridged titanium(III) species, (B2nep2)TiCl2(µ-Cl)2TiCl2(B2nep2).
RESUMO
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.
RESUMO
α-Diimine niobium complexes serve as catalysts for deoxygenation of benzyl ethers by silicon tetrachloride (SiCl4) to cleanly give two equivalents of the corresponding benzyl chlorides, where SiCl4 has the dual function of oxygen scavenger and chloride source with the formation of a silyl ether or silica as the only byproduct. The reaction mechanism has two successive trans-etherification steps that are mediated by the niobium catalyst, first forming one equivalent of benzyl chloride along with the corresponding silyl ether intermediate that undergoes the same reaction pathway to give the second equivalent of benzyl chloride and silyl ether.
RESUMO
A high-valent d0 niobium(v) complex, (α-diimine)NbCl3 (1), bearing a dianionic redox-active α-diimine ligand served as a catalyst for a hydrodehalogenation reaction of alkyl halides in the presence of PhSiH3. During the catalytic reaction, the redox-active α-diimine ligand allowed the complex to reversibly release and accept one-electron through switching its coordination mode between a dianionic folded form and a monoanionic planar one.
RESUMO
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.
