Synthesis of 1,1′- and 2,2′-Bicarbazole Alkaloids by Iron(III)-Catalyzed Oxidative Coupling of 2- and 1-Hydroxycarbazoles

Recent advances in iron-catalysed coupling reactions for the construction of the C(sp2)-C(sp2) bond

The advancement of transition-metal-catalyzed coupling reactions has been demonstrated as a highly effective strategy for the formation of carbon-carbon bonds, which serve as the fundamental basis for organic synthetic chemistry. Given that iron represents one of the most economical and ecologically sustainable metallic elements available, the exploration and enhancement of iron-catalysed coupling reactions have garnered increasing interest within the scientific community. In recent years, numerous iron-catalysed reactions have been reported, showcasing their efficacy in establishing C-C bonds. In this minireview, we present a systematic analysis of C(sp²)-C(sp²) bond formation via iron-catalysed coupling reactions as documented in the extant literature.

Atroposelective Synthesis of 2,2'-Bis(arylamino)-1,1'-biaryls by Oxidative Iron(III)- and Phosphoric Acid-Catalyzed C-C Coupling of Diarylamines

We describe an iron-catalyzed asymmetric oxidative C-C coupling of diarylamines which proceeds at room temperature with air as final oxidant. Using hexadecafluorophthalocyanine-iron(II) as catalyst in the presence of catalytic amounts of an axially chiral biaryl phosphoric acid, the resulting chiral 2,2'-diamino-1,1'-biaryls are obtained in up to 90 % ee as confirmed by chiral HPLC. A detailed mechanism has been proposed with a radical cation-chiral phosphate ion pair as key intermediate leading to the observed asymmetric induction.

μ-Oxo-bis[(octacosafluoro-meso-tetraphenylporphyrinato)iron(iii)] - synthesis, crystal structure, and catalytic activity in oxidation reactions

We describe the synthesis and X-ray crystal structure of μ-oxo-bis[(octacosafluoro-meso-tetraphenylporphyrinato)iron(iii)] [(FeTPPF₂₈)₂O]. This novel iron complex is an efficient catalyst for oxidative biaryl coupling reactions of diarylamines and carbazoles. The asymmetric oxidative coupling in the presence of an axially chiral biaryl phosphoric acid as co-catalyst provides the 2,2'-bis(arylamino)-1,1'-biaryl in 96% ee. The Wacker-type oxidation of alkenes to the corresponding ketones with (FeTPPF₂₈)₂O as catalyst in the presence of phenylsilane proceeds at room temperature with air as the terminal oxidant. For internal and aliphatic alkenes increased ketone/alcohol product ratios were obtained.

Iron-Catalyzed Oxidative C-O and C-N Coupling Reactions Using Air as Sole Oxidant

We describe the oxygenation of tertiary arylamines, and the amination of tertiary arylamines and phenols. The key step of these coupling reactions is an iron‐catalyzed oxidative C−O or C−N bond formation which generally provides the corresponding products in high yields and with excellent regioselectivity. The transformations are accomplished using hexadecafluorophthalocyanine−iron(II) (FePcF₁₆) as catalyst in the presence of an acid or a base additive and require only ambient air as sole oxidant.

Mechanistic Studies on the Hexadecafluorophthalocyanine-Iron-Catalyzed Wacker-Type Oxidation of Olefins to Ketones*

The hexadecafluorophthalocyanine-iron complex FePcF₁₆ was recently shown to convert olefins into ketones in the presence of stoichiometric amounts of triethylsilane in ethanol at room temperature under an oxygen atmosphere. Herein, we describe an extensive mechanistic investigation for the conversion of 2-vinylnaphthalene into 2-acetylnaphthalene as model reaction. A variety of studies including deuterium- and ¹⁸ O₂ -labeling experiments, ESI-MS, and ⁵⁷ Fe Mössbauer spectroscopy were performed to identify the intermediates involved in the catalytic cycle of the oxidation process. Finally, a detailed and well-supported reaction mechanism for the FePcF₁₆ -catalyzed Wacker-type oxidation is proposed.

Total Synthesis of the Natural Carbazoles O-Demethylmurrayanine and Murrastanine A, and of a C4,C4′ Symmetric Murrastanine A Dimer from N-Phenyl-4,5-dimethylene-1,3-oxazolidin-2-one

The synthesis of natural carbazoles O-demethylmurrayanine and murrastanine A starting from the title exo-heterocyclic diene­ is described. In the synthesis of murrastanine A, its symmetric C4,C4′ dimer can be obtained as the sole product under rather mild conditions. In all cases, the key intermediate is the same diarylamine. The carbazole nucleus is obtained through a Pd-promoted cyclization of the appropriate diarylamine. For the synthesis of O-demethylmurrayanine, the cyclization takes place on a silylated derivative. The crystal structures of murrayanine, two diarylamines, and two non-natural carbazole intermediates are also presented.

Iron-Catalyzed Wacker-type Oxidation of Olefins at Room Temperature with 1,3-Diketones or Neocuproine as Ligands*

Herein, we describe a convenient and general method for the oxidation of olefins to ketones using either tris(dibenzoylmethanato)iron(III) [Fe(dbm)₃ ] or a combination of iron(II) chloride and neocuproine (2,9-dimethyl-1,10-phenanthroline) as catalysts and phenylsilane (PhSiH₃ ) as additive. All reactions proceed efficiently at room temperature using air as sole oxidant. This transformation has been applied to a variety of substrates, is operationally simple, proceeds under mild reaction conditions, and shows a high functional-group tolerance. The ketones are formed smoothly in up to 97 % yield and with 100 % regioselectivity, while the corresponding alcohols were observed as by-products. Labeling experiments showed that an incorporated hydrogen atom originates from the phenylsilane. The oxygen atom of the ketone as well as of the alcohol derives from the ambient atmosphere.

Chemo- and regioselective cross-dehydrogenative coupling reaction of 3-hydroxycarbazoles with arenols catalyzed by a mesoporous silica-supported oxovanadium

Cross-dehydrogenative coupling between 3-hydroxycarbazoles and 2-naphthols has been achieved by using a mesoporous silica-supported oxovanadium catalyst.

Cross-dehydrogenative coupling between 3-hydroxycarbazoles and 2-naphthols has been achieved by using a mesoporous silica-supported oxovanadium catalyst.

Chemo- and enantioselective hetero-coupling of hydroxycarbazoles catalyzed by a chiral vanadium(v) complex

The catalytic enantioselective oxidative hetero-coupling of arenols using a chiral vanadium(v) complex has been developed.

Catalytic and Aerobic Oxidative Biaryl Coupling of Anilines Using a Recyclable Heterogeneous Catalyst for Synthesis of Benzidines and Bicarbazoles

In this study, a heterogeneous rhodium-catalyzed oxidative homocoupling reaction of anilines utilizing molecular oxygen as the sole oxidant is reported. Employing a commercially available and recyclable Rh/C catalyst enabled the oxidative dimerization of various anilines, including N,N-disubstituted and N-monosubstituted anilines, as well as diarylamines, triarylamines, and carbazoles. Additionally, the catalytic protocol was extended to the ortho-ortho coupling of anilines, affording 2,2'-diaminobiphenyls with high regioselectivity. Notably, the developed approach provides rapid access to diversely functionalized benzidines and diaminobiphenyls in an operationally simple, practical, and environmentally friendly manner.

Total Synthesis of Pyrolaside B: Phenol Trimerization through Sequenced Oxidative C-C and C-O Coupling

A facile method to oxidatively trimerize phenols using a catalytic aerobic copper system is described. The mechanism of this transformation was probed, yielding insight that enabled cross-coupling trimerizations. With this method, the natural product pyrolaside B was synthesized for the first time. The key strategy used for this novel synthesis is the facile one-step construction of a spiroketal trimer intermediate, which can be selectively reduced to give the natural product framework without recourse to stepwise Ullmann- and Suzuki-type couplings. As a result, pyrolaside B can be obtained expeditiously in five steps and 16 % overall yield. Three other analogues were synthesized, thus highlighting the utility of the method, which provides new accessibility to this area of chemical space. A novel xanthene was also synthesized through controlled Lewis acid promoted rearrangement of a spiroketal trimer.

Iron-Catalyzed Synthesis, Structure, and Photophysical Properties of Tetraarylnaphthidines

We describe the synthesis and photophysical properties of tetraarylnaphthidines. Our synthetic approach is based on an iron-catalyzed oxidative C-C coupling reaction as the key step using a hexadecafluorinated iron-phthalocyanine complex as a catalyst and air as the sole oxidant. The N,N,N',N'-tetraarylnaphthidines proved to be highly fluorescent with quantum yields of up to 68%.

Iron-Catalyzed Oxidative C-C Cross-Coupling Reaction of Tertiary Anilines with Hydroxyarenes by Using Air as Sole Oxidant

A mild procedure for the oxidative C-C cross-coupling of tertiary anilines with phenols is described which provides the products generally in high yields and with excellent selectivity. The reaction is catalyzed by the hexadecafluorinated iron-phthalocyanine complex FePcF16 in the presence of substoichiometric amounts of methanesulfonic acid and ambient air as sole oxidant.

Anti-inflammatory and cytotoxic carbazole alkaloids from Murraya kwangsiensis

Chemical investigation of the traditional Chinese medicine, Murraya kwangsiensis, led to the isolation of 16 undescribed biscarbazole alkaloids, kwangsines A-M, two undescribed natural products, (+/-)-bispyrayafoline C, and 19 known monomeric analogues. (±)-Bispyrayafoline C and (±)-kwangsines A-C are four pairs of biscarbazole atropisomers, and they were separated by chiral HPLC to obtain the optically pure compounds. The structures of the undescribed compounds were elucidated on the basis of HRESIMS and NMR data analysis. Their absolute configurations were assigned via comparison of the specific rotation, ECD exciton coupling method, as well as comparison of experimental and calculated ECD data. A compound showed significant inhibition on NO production in lipopolysaccharide-stimulated BV-2 microglial cells, and four compounds exhibited moderate cytotoxicities against HepG2 cells, with IC₅₀ values less than 20 μM.

Substituent-Guided Palladium-Ene Reaction for the Synthesis of Carbazoles and Cyclopenta[ b]indoles

An efficient palladium-catalyzed intramolecular Trost-Oppolzer type Alder-ene strategy was developed for the synthesis of carbazoles and cyclopenta[ b]indoles from easily accessible(3-allyl-1 H-indol-2-yl)methyl acetates. This strategy was extended for the synthesis of naphthalenes and dibenzobenzofurans as well. In addition, a short synthesis of antibacterial and antifungal natural product glycozoline and its analogues was also achieved.

C3-C3' and C6-C6' Oxidative Couplings of Carbazoles

9-Substituted carbazoles are widely used units in materials science, and their oxidative reactions have been utilized for the synthesis and characterization of polymers. Though the oxidative mechanism of carbazoles has been known for a few decades, structural definition has remained difficult, because their polymers are generally insoluble with incomplete characterization and unknown dependence of the electrochemical potentials. The oxidative reactions of 9-substituted carbazoles should be carefully considered under specific oxidative conditions; otherwise, structure definitions could be wrong, because the IR and NMR spectra used previously cannot quantitatively analyze 3,3'-coupling and 6,6'-coupling of carbazoles. In this review, the best understanding of the C3-C3' and C6-C6' oxidative couplings of 9-substituted carbazoles is presented, and the benefit of these oxidative reactions from the viewpoints of electrochemical synthesis, film engineering, and the synthesis and processing of polymers is highlighted.

Asymmetric oxidative coupling of hydroxycarbazoles: Facile synthesis of (+)-bi-2-hydroxy-3-methylcarbazole

Asymmetric oxidative coupling reactions of hydroxycarbazoles have been established using a chiral dinuclear vanadium complex. To demonstrate the utility of vanadium-catalyzed reactions, we have used them to synthesize (+)-bi-2-hydroxy-3-carbazole in three steps from cyclohexanone and commercially available aniline derivatives.