Synthesis and Photophysical Properties of 3-Substituted-1H-Indazoles: A Pd-Catalyzed Double C-N Bond Formation Strategy via 1,6-Conjugate Addition

A Pd-catalyzed cascade process for the direct synthesis of 3-substituted-1H-indazole employing p-quinone methide (p-QM) and arylhydrazine through Pd-catalyzed double C-N bond formation via 1,6-conjugate addition is reported. This reaction strategy affords efficient and practical access to synthetically important diverse 3-substituted-1H-indazoles in good yields. The photophysical properties of the synthesized 3-substituted-1H-indazoles are investigated, and some of them showed very good fluorescence properties with quantum yields up to 85%. Also, the synthesized 3-substituted-1H-indazole exhibits an acid-sensitive fluorescence turn-off activity.

Electrochemical Regioselective Sulfenylation of 2H-Indazoles with Thiols in Batch and Continuous Flow

A novel electrochemical cross-dehydrogenative C-S bond coupling of aryl thiols with 2H-indazole is reported. Thiol-functionalized 2H-indazoles were synthesized under catalyst-, oxidant-, and metal-free conditions with innocuous hydrogen as the sole byproduct at ambient temperature. Furthermore, continuous electrochemical flow conditions using a graphite/Ni flow cell were used to obtained 3-(arylthio)-2H-indazole compounds on a gram scale within the residence time of 39 min. Detailed mechanistic studies including control experiments and cyclic voltammetry are provided to support the radical-radical cross-coupling pathway.

Recent advances in C-H functionalization of 2H-indazoles

2H-Indazoles are one class of the most important nitrogen-containing heterocyclic compounds. The 2H-indazole motif is widely present in bioactive natural products and drug molecules that exhibit distinctive bioactivities. Therefore, much attention has been paid to access diverse 2H-indazole derivatives. Among them, the late-stage functionalization of 2H-indazoles via C-H activation is recognized as an efficient approach for increasing the complexity and diversity of 2H-indazole derivatives. In this review, we summarized recent achievements in the late-stage functionalization of 2H-indazoles, including the C3-functionalization of 2H-indazoles through transition metal-catalyzed C-H activation or a radical pathway, transition metal-catalyzed ortho C2'-H functionalization of 2H-indazoles and remote C-H functionalization at the benzene ring in 2H-indazoles.

"On Water" Palladium Catalyzed Direct Arylation of 1H-Indazole and 1H-7-Azaindazole

The C3 direct arylation of 1H-indazole and 1H-7-azaindazole has been a significant challenge due to the lack of the reactivity at this position. In this paper, we describe a mild and an efficient synthesis of new series of C3-aryled 1H-indazoles and C3-aryled 1H-7-azaindazoles via a C3 direct arylation using water as solvent. On water, PPh₃ was effective as a ligand along with a lower charge of the catalyst Pd(OAc)₂ (5 mol%) at 100 °C, leading to C3-aryled 1H-indazoles or C3-aryled 1H-7-azaindazoles in moderate to good yields.

Visible-light-mediated direct C3-arylation of 2H-indazoles enabled by an electron-donor-acceptor complex

A mild visible-light-mediated, photocatalyst-free arylation of 2H-indazoles was developed. The formation of an electron donor-acceptor complex by 2H-indazoles and aryl diazonium salts in the presence of pyridine allows the direct arylation of 2H-indazoles under visible-light irradiation. This process provides an efficient route for the synthesis of C3-arylated-2H-indazoles, which are important scaffolds of various bioactive compounds.

Continuous-Flow Visible Light Organophotocatalysis for Direct Arylation of 2H-Indazoles: Fast Access to Drug Molecules

A continuous-flow homogeneous photocatalytic method has been devised for the direct arylation of 2H-indazoles. This visible-light-promoted approach directly accesses a wide range of structurally diverse C3-arylated scaffolds of biological interest in a fast (1 min), single-step reaction by using eosin Y as an organophotocatalyst. Furthermore, a microreactor technology is also employed for the fast synthesis of liver X receptor inhibitor drugs with very good yields under metal-free conditions, whereas the reported methods required multiple steps and much longer reaction times (18-24 h).

Photocatalysis in a multi-capillary assembly microreactor: toward up-scaling the synthesis of 2H-indazoles as drug scaffolds

Visible-light-promoted direct arylation of 2H-indazoles using phenyldiazonium salt enabled a single-step and fast synthesis (<1 min) of C3 arylated products in high yields (>65%) in an eosin Y immobilized capillary microreactor.

An optimized procedure for direct access to 1H-indazole-3-carboxaldehyde derivatives by nitrosation of indoles

The nitrosation of indoles under slightly acidic conditions and reverse addition conditions leads to the preparation of the corresponding indazole-3-carboxaldehydes in high yields and greatly minimizes side reactions.

Unprotected Indazoles Are Resilient to Ring-Opening Isomerization: A Case Study on Catalytic C-S Couplings in the Presence of Strong Base

Indazoles represent a privileged scaffold in medicinal chemistry. In the presence of strong base, however, N-protected indazoles are prone to an undesirable ring-opening reaction to liberate o-aminobenzonitriles. By employing unprotected indazoles with a free N-H bond, isomerization is averted because the heterocycle is deprotonated in situ. We herein report functional group-tolerant and robust C-S couplings of bromoindazoles with thiols of varying electronic nature in the presence of lithium bis(trimethylsilyl)amide at elevated temperatures.

Rh(III)-Catalyzed Regio- and Chemoselective [4 + 1]-Annulation of Azoxy Compounds with Diazoesters for the Synthesis of 2H-Indazoles: Roles of the Azoxy Oxygen Atom

A Rh(III)-catalyzed tandem C-H alkylation/intramolecular decarboxylative cyclization of azoxy compounds with diazoesters for the synthesis of 3-acyl-2H-indazoles is disclosed. The azoxy instead of the azo group enables a distinct approach for cyclative capture, leading to a [4 + 1]-annulation rather than a classic [4 + 2] manner. The azoxy oxygen atom is traceless after annulation, and further removal from the product is not required. This reaction features a complete regioselectivity for unsymmetrical azoxybenzenes and a compatibility of monoaryldiazene oxides.

One-pot synthesis of highly functionalized pyrimido[1,2-b]indazoles via 6-endo-dig cyclization

An efficient synthesis of nitrogen ring junction pyrimido-indazoles has been developed.

BF3·OEt2 mediated metal-free one-pot sequential multiple annulation cascade (SMAC) synthesis of complex and diverse tetrahydroisoquinoline fused hybrid molecules

A highly efficient and distinct BF₃·OEt₂ mediated one-pot metal-free sequential multiple annulation cascade (SMAC) protocol for the synthesis of complex and diverse THIQ fused hybrid molecules has been presented.

TMP-magnesium and TMP-zinc bases for the regioselective metalation of the cinnoline scaffold

A regioselective functionalization of cinnolines in positions 3 and 8 using metalations has been developed. This involves either the use of a frustrated Lewis pair consisting of BF3·Et2O and TMP2Mg·2LiCl or the in situ generated base TMP2Zn·2MgCl2·2LiCl. Successive metalations allow the preparation of 3,8-disubstituted cinnolines. Various functionalizations by acylation, allylation, and cross-coupling reactions with aryl halides or alkenyl iodides were carried out successfully.

A convenient alumination of functionalized aromatics by using the frustrated Lewis pair Et3 Al and TMPMgCl⋅LiCl

A straightforward and efficient alumination of functionalized arenes by using the frustrated Lewis pair Et3 Al and TMPMgCl⋅LiCl (TMP=2,2,6,6-tetramethylpiperidyl) has been developed. In particular, halogenated electron-rich aromatics can be smoothly functionalized by using the frustrated Lewis pair Et3 Al and TMPMgCl⋅LiCl. Compared with previously described alumination methods, this procedure avoids extensive cooling and the need for an excess of base. This in situ procedure has proven to be most practical and allows for regio- and chemoselective metalation of a wide range of aromatics with sensitive functional groups (CONEt2 , CO2 Me, CN, OCONMe2 ) or halogens (F, Cl, Br, I). The resulting aromatic aluminates, which were characterized by using NMR spectroscopy, were subjected to allylations, acylations, and palladium-catalyzed cross-coupling reactions after transmetalation to zinc. It was shown that the nature of the Zn salt used for transmetalation is crucial. Thus, compared with ZnCl2 (2 equiv), the use of Zn(OPiv)2 (2 equiv; OPiv=pivalate) allows the subsequent quenching reactions to be performed with only a slight excess of electrophile (1.2 equiv) and provides interesting functionalized aromatics in good yields.

A Robust Protocol for Pd(II)-catalyzed C-3 Arylation of (1H) Indazoles and Pyrazoles: Total Synthesis of Nigellidine Hydrobromide

C3-arylated indazole and pyrazoles are privileged structural motifs in agrochemicals and pharmaceuticals. C-3 C-H arylation of (1H) indazole and pyrazole has been a significant challenge due to the poor reactivity of the C-3 position. Herein, we report a practical Pd(II)/Phen catalyst and conditions for direct C-3 arylation of indazole and pyrazole with ArI or ArBr without using Ag additives as halide scavengers. The use of toluene, chlorobenzene, trifluoromethylbenzene and mesitylene as the solvent was found to be crucial for the selectivity and reactivity. We further demonstrate the robustness of this protocol through the first total synthesis of Nigellidine hydrobromide as well as expedient preparation of heterocycles structurally related to pesticides and drug molecules.