Synthesis of Chiral-Substituted 2-Aryl-ferrocenes by the Catellani Reaction.

A palladium-catalyzed and norbornene-mediated methodology has been developed for the synthesis of chiral 2-aryl-ferroceneamides from chiral 2-iodo-N,N-diisopropylferrocencarboxamide, iodoarenes, and alkenes using a JohnPhos ligand and potassium carbonate as a base in dimethylformamide at 105 °C. The developed three-component coupling protocol allows the compatibility of electron-withdrawing fluoro, chloro, ester, and nitro and electron-donating methyl, methoxy, dimethoxy, benzyl ether-substituted iodo-benzenes, other iodoarenes, such as iodo-naphthalene, heteroarenes, such as iodothiophene, and terminating substrates, such as methyl, ethyl, tert-butyl acrylates, and substituted styrenes with 2-iodo-N,N-diisopropylferrocencarboxamide. Furthermore, the developed three-component Catellani method proceeded with the retention of the configuration of the planar chiral ferrocene, which depends on the role of the participating carbon-iodine bond in ferrocene. Consequently, the developed protocol enabled the formation of densely substituted chiral 2-aryl ferroceneamides, exhibiting good to excellent enantioselectivity. The conversion of an ester of the synthesized chiral 2-aryl ferroceneamides has also been carried out to further accommodate the easily expendable acid and alcohol functionalities.

Copper-Mediated Selective Mono- and Sequential Organochalcogenation of C-H Bonds: Synthesis of Hybrid Unsymmetrical Aryl Ferrocene Chalcogenides.

An 8-aminoquinoline-directed, copper/1,10-phenanthroline-mediated selective mono-organothiolation of the C-H bond in ferroceneamide has been developed using aryl/alkyl disulfide substrates. The sequential ferrocene C-H organochalcogenation (chalcogen = S, Se, and Te) has also been established for the synthesis of novel hybrid unsymmetrical aryl chalcogenides with the aid of a catalytic amount of Cu(OAc)2 under ambient reaction conditions. The developed protocol results in a broad functional group tolerance to allow alkyl-, aryl-, heteroaryl-, bromo-, chloro-, and nitro-containing diorgano dichalcogenides as coupling partners. Furthermore, the 8-aminoquinoline directing group is easily removed to afford the aldehyde functionality after C-H organochalcogenation. A mechanistic understanding of the copper-mediated selective mono-organothiolation reaction suggests that the rigid bicoordinated 1,10-phenanthroline ligand and freshly generated copper(II) from Cu(I) in the less polar solvent acetonitrile are crucial to the selective mono-C-H functionalization of ferroceneamide.

Copper-Catalyzed 8-Aminoquinoline Assisted Aryl Chalcogenation of Ferroceneamide with Aryl Disulfides, Diselenides, and Ditellurides.

A synthetic method has been developed for the synthesis of unsymmetrical ferrocene aryl chalcogenides by C-H activation of ferroceneamide using 8-aminoquinoline as a directing group, aryl dichalcogenides, and copper(II) catalyst in the presence of silver acetate oxidant at 80 °C in DMSO. The developed methodology is quite general and mild to access three unsymmetrical diaryl chalcogenides (sulfide, selenide, and telluride) and also amenable to aryl dichalcogenides with sensitive bromo, chloro, and nitro functionalities. Further, ferrocene aryl tellurides have been obtained in moderate yields for the first time by the developed C-H activation approach. In the mechanistic part, it seems that the presence of a rigid chelating directing ligand is crucial for the aryl chalcogenation reaction of C-H bond under copper catalysis as the nonrigid bidentate directing ligands were unsuccessful for the transformation.

Synthesis of Unsymmetrical Diaryl Acetamides, Benzofurans, Benzophenones, and Xanthenes by Transition-Metal-Free Oxidative Cross-Coupling of sp3 and sp2 C-H Bonds.

A chemo- and regioselective intermolecular sp3 C-H and sp2 C-H coupling reaction for C-C bond formation is described to access unsymmetrical diaryl acetamides under TM-free conditions from sec- and tert-arylacetamides and nitroarenes using tert-butoxide base in DMSO at room temperature. The coupling partners with sensitive functionalities such as chloro, bromo, hydroxy, and cyano were also amenable to the developed reaction. Synthesized α-(2/4-nitroaryl) phenylacetamides have been transformed into biologically important benzofurans, xanthenes, diaryl indoles, and unsymmetrical benzophenones by novel routes without applying a transition metal. Overall, an economical, yet efficient, strategy has been devised to access unsymmetrical diarylacetamides with the possibility of their further elaboration into a variety of biologically important heterocycles. Mechanistic understanding suggests that the reaction proceeds by a nucleophilic addition of a phenylacetamide carbanion, which is generated in the presence of tert-butoxide base, to the para or ortho (if para is substituted) position of nitrobenzene. The formed α-(4-nitrocyclohexa-2,4-dien-1-yl) phenylacetamide anion intermediate oxidized by a basic solution of DMSO or atmospheric oxygen led to the desired sp3 C-H and sp2 C-H coupled α-(2/4-nitroaryl) phenylacetamides.

Metal free sulfenylation and bis-sulfenylation of indoles: persulfate mediated synthesis.

A method which avoids metal and halogen for the synthesis of 3-arylthioindoles from indoles and diaryl disulfides using ammonium persulfate in methanol has been presented. Moreover, double C-H sulfenylation of indoles at 2 and 3-positions has also been achieved using iodine and ammonium persulfate.

8-Aminoquinoline-Assisted Synthesis and Crystal Structure Studies of Ferrocenyl Aryl Sulfones.

A copper-catalyzed 8-aminoquinoline-directed oxidative cross-coupling of the C-H bond of ferrocene with sodium arylsulfinates has been achieved. The robust copper catalyst tolerates a range of methyl, tert-butyl, bromo, chloro, iodo and nitro functional groups in the phenyl ring, and set the stage for the synthesis of substituted ferrocene sulfones. Furthermore, X-ray crystal structure study on several ferrocenyl sulfones reveals the tetrahedral geometry around sulfur; interestingly, the O-S-O angle is larger than the electropositive substituent C-S-C angle which could be explained by Bent's rule. Further, unusual intramolecular O(S)⋅⋅⋅N(amide) short contacts (2.925-3) and O(S)⋅⋅⋅C=O were also noticed in ferrocenyl sulfones.

Palladium-Catalyzed Removable 8-Aminoquinoline Assisted Chemo- and Regioselective Oxidative sp2-C-H/sp3-C-H Cross-Coupling of Ferrocene with Toluene Derivatives.

The coupling of the C(sp2)-H bond of ferrocene with toluene avoiding in situ functionalization of both coupling partners has been achieved using palladium and iron dual catalysis. This cross oxidative coupling features regioselectivity to primary and secondary C(sp3)-H bonds and chemoselectivity toward the -C(sp3)-H over C-I bond of toluene with gram-scale production. It seems Fe(II) catalyst not only stabilizes the generated benzyl radical but also tames its oxidizing behavior, and consequently transfers it to the palladacycle for C-C coupling.

Transition-Metal-Free Selective Oxidative C(sp3)-S/Se Coupling of Oxindoles, Tetralone, and Arylacetamides: Synthesis of Unsymmetrical Organochalcogenides.

Transition-metal-free synthetic methods have been developed for the preparation of unsymmetrical diaryl and aryl alkyl chalcogenides: sulfones, sulfides, and selenides from the sp3-C-H bond of oxindole, tetralone, arylacetamide, and aryl chalcogenide precursors. Sulfones were obtained from sodium sulfinates using potassium iodide, tert-butyl hydroperoxide in DMSO, and acetic acid. Sulfides and selenides were prepared from diaryl disulfides or diselenides employing potassium tert-butoxide in DMSO. α-Tetralone underwent concomitant chalcogenation and aromatization resulting in 2-chalcogenyl-1-naphthols in one pot.

Transition-metal-free Chemoselective Oxidative C-C Coupling of the sp3 C-H Bond of Oxindoles with Arenes and Addition to Alkene: Synthesis of 3-Aryl Oxindoles, and Benzofuro- and Indoloindoles.

A transition-metal (TM)-free and halogen-free NaOtBu-mediated oxidative cross-coupling between the sp3 C-H bond of oxindoles and sp2 C-H bond of nitroarenes has been developed to access 3-aryl substituted and 3,3-aryldisubstituted oxindoles in DMSO at room temperature in a short time. Interestingly, the sp3 C-H bond of oxindoles could also react with styrene under TM-free conditions for the practical synthesis of quaternary 3,3-disubstituted oxindoles. The synthesized 3-oxindoles have also been further transformed into advanced heterocycles, that is, benzofuroindoles, indoloindoles, and substituted indoles. Mechanistic experiments of the reaction suggests the formation of an anion intermediate from the sp3 C-H bond of oxindole by tert-butoxide base in DMSO. The addition of nitrobenzene to the in-situ generated carbanion leads to the 3-(nitrophenyl)oxindolyl carbanion in DMSO which is subsequently oxidized to 3-(nitro-aryl) oxindole by DMSO.

KO(t)Bu-mediated aerobic transition-metal-free regioselective ß-arylation of indoles: synthesis of ß-(2-/4-nitroaryl)-indoles.

A KO(t)Bu-mediated intermolecular oxidative C-C coupling of nitroarenes with indoles is presented in DMSO at room temperature in an open flask. By using this mild and economical methodology, syntheses of ß-(2/4-nitroaryl)-indoles with sensitive functionalities such as bromo, iodo, cyano, and nitro were achieved chemo- and regioselectively. Synthesized ß-(2/4-nitroaryl) indoles were transformed into densely functionalized biindoles, indoloindoles, and (4-aminoaryl)-indoles which demonstrate post-transformation utility of the developed methodology.

Chemoselective arylation of phenols with bromo-nitroarenes: synthesis of nitro-biaryl-ols and their conversion into benzofurans and carbazoles.

A series of electron withdrawing or donating group substituted phenols were chemoselectively arylated with various substituted bromo-nitroarenes using KO(t)Bu at room temperature via an SNAr pathway. The synthesis of natural alkaloids (carbazoles), dibenzofurans, and a biaryl-indole was achieved from the synthesized nitro-biaryl-ols.