A highly efficient protocol for the synthesis of benzimidazole‐substituted arylboronic acids was developed via aerobic oxidative cyclization of 1,2‐aryldiamines and formyl‐substituted aryl MIDA (N‐methyliminodiacetic acid) boronates using potassium iodide as a nucleophilic catalyst. Furthermore, a one‐pot protocol for the synthesis of benzimidazole‐substituted arylboronic acids from 1,2‐phenylenediamines and formyl‐substituted arylboronic acids was developed without the isolation of any intermediates. The resulting boronic acids were further subjected to Suzuki–Miyaura coupling reactions without isolation, leading to diaryl‐substituted benzimidazoles with only one separation step.
Visible light irradiation of N‐bromosuccinimide serves as an effective means to convert methyl 2‐(azidomethyl)‐3‐arylpropenoates and 2‐(azidomethyl)‐3‐arylacrylonitriles to the corresponding iminyl radicals via α‐hydrogen abstraction and subsequent extrusion of dinitrogen. Thus formed iminyl radicals then undergo intramolecular ortho attack on the aryl ring, affording methyl quinoline‐3‐carboxylates and quinoline‐3‐carbonitriles respectively.
The combination of benzyl bromide, sodium hydroxide and 15‐crown‐5 in tetrahydrofuran is shown to be an efficient method for installing benzyl groups at both the 4‐ and 6‐positions regioselectively directly from peracetylated N‐trichloroacetyl‐protected glucosamine and galactosamine. Application of this benzylation strategy proved to significantly shorten the synthetic route to hyaluronic acid tetra‐ and hexamers.
The activation of 1,2‐diols through formation of boronate esters was found to enhance the selective oxidation of 1,2‐diols to their corresponding α‐hydroxy ketones in aqueous medium. The oxidation step was accomplished using dibromoisocyanuric acid (DBI) as a terminal chemical oxidant or an electrochemical process. The electrochemical process was based on the use of platinum electrodes, methylboronic acid [MeB(OH)2] as a catalyst and bromide ion as a mediator. Electro‐generated OH− ions (EGB) at the cathode acted as a base and “Br+” ion generated at the anode acted as an oxidant. Various cyclic and acyclic 1,2‐diols as substrates were selectively oxidized to the corresponding α‐hydroxy ketones via their boronate esters by the two oxidative methods in good to excellent yields.
The paper describes the oxidative Heck arylation of various allylic amines using arylboronic acids for the preparation of tetrasubstituted alkenes. As oxidant the commercially available 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) is used and coupling reactions occur under very mild conditions at room temperature. The densely substituted alkenes are formed in good to excellent yields with complete control of the diastereoselectivity. Substrate scope with respect to the allylic amine and the arylboronic acid is investigated.
A concise synthesis of 1‐naphthols via cyclization of o‐iodoacetophenones and methyl ketones has been realized under very mild conditions. The cyclization process is initiated by a rare copper‐catalyzed arylation of simple methyl ketones with ortho‐iodoacetophenones.
Free NH 2‐(aminomethyl)indoles have been prepared via copper‐catalyzed cyclization of 3‐(ortho‐trifluoroacetamidophenyl)‐1‐propargylic alcohols in the presence of primary or secondary amines. The synthesis has been developed into a simple and very efficient domino three‐component Sonogashira cross‐coupling/cyclization/substitution process that, omitting the isolation of 3‐(ortho‐trifluoroacetamidophenyl)‐1‐propargylic alcohols, provides access to this class of compounds by treating 2‐iodotrifluoroacetanilides, propargylic alcohols, and primary or secondary amines with a copper/palladium catalyst system.
An efficient and practical copper‐catalyzed domino synthesis of benzo[4,5]imidazo[1,2‐a]pyrimidin‐4(10H)‐ones has been developed. The protocol uses N‐(2‐halophenyl)‐3‐alkylpropiolamides and cyanamide as the starting materials, inexpensive copper(I) iodide and pipecolinic acid as the catalyst and ligand, and the corresponding products were obtained in moderate to good yields.
An N‐phthalimide‐protected (ortho‐aminophenyl)propiolate was prepared by the carboxylation of the corresponding silylalkyne precursor with carbon dioxide. The obtained propiolate was further transformed to various 3,4‐fused 2‐quinolones via cycloadditions and subsequent deprotection/lactam formation.
The oxidizing system of tert‐butyl hydroperoxide (TBHP) and tetrabutylammonium iodide (TBAI) is capable of generating α‐(arylaminocarbonyl)iminyl radicals from ethyl 2‐(N‐arylcarbamoyl)‐2‐iminoacetates. These iminyl radicals preferably undergo intramolecular ipso attack on the benzene ring to give azaspirocyclohexadienyl radicals, which are readily captured by molecular oxygen under an oxygen atmosphere to yield azaspirocyclohexadienones. In the absence of oxygen, the reaction affords quinoxalin‐2‐one products. This oxidizing system is also effective to convert α‐aryl‐α‐azido‐N‐arylamides to the corresponding iminyl radicals under basic conditions (sodium tert‐butoxide, t‐BuONa), and the subsequent cyclization of these iminyl radicals results in the formation of azaspirocyclohexadienone products in high yields under an oxygen atmosphere. Plausible mechanisms are proposed to rationalize the experimental results, and factors influencing the reactions are discussed.
An electrochemical method for the decarboxylative coupling of α‐keto acids with ortho‐phenylenediamines was developed. The reaction proceeded smoothly in aqueous solution under air and metal catalyst‐free conditions to afford 2‐substituted benzimidazoles in good yields. Benzothiazoles could also be synthesized by this protocol.
A straightforward, tandem, copper‐catalyzed N‐arylation–condensation has been developed using chiral cyclic 1,2‐diamines and ortho‐haloaryl aldehydes or ketones. The corresponding chiral tricyclic 1,4‐benzodiazepines were synthesized in high yields. Subsequently, the 1,4‐diazepines have been converted to a new class of tetracyclic N‐fused imidazobenzodiazepines (ImBDs) using the van Leusen reaction. A one‐pot sequential strategy has also been demonstrated for the synthesis of ImBDs. The synthetic utility of 1,4‐benzodiazepines and ImBDs is described.
A novel and efficient oxidative annulation of 2‐aminobenzamides with nitromethane has been developed for the chemoselective synthesis of N‐substituted 1,2,3‐benzotriazine‐4(3H)‐ones in moderate to excellent yields under transition metal‐free conditions. Two N N bonds were constructed in one pot via C N cleavage of nitromethane, which was selectively employed as the nitrogen synthon. The preliminary mechanistic studies revealed that this protocol proceeded under hypoiodite catalysis generated in situ.
A rhodium(I)‐catalyzed direct C H bond olefination of pyridyl‐substituted arenes with readily available vinyl carboxylic acids has been realized. This reaction occurred efficiently without the need for any external oxidant, affording the ortho‐olefinated products in high yields and excellent regioselectivities. Diversely substituted vinyl carboxylic acids behaved as efficient olefination reagents under the reaction conditions, and a range of functional groups in both coupling partners was well tolerated. Mechanistic studies indicated that a decarbonylation step is involved in this catalytic process, and pivalic anhydride [(t‐BuCO)2O] acts as the activator of the carboxylic acids for the in situ generation of highly active anhydrides.
An asymmetric tandem Michael addition–lactonization between ortho‐nitrovinylphenols and azalactones was investigated for constructing 3,4‐dihydrocoumarin backbones with a quaternary amino acid moiety. Under the catalysis of the chiral squaramide derived from L ‐tert‐leucine, a wide range of substituted (E)‐2‐(2‐nitrovinyl)phenols and azalactones were well tolerated in this tandem reaction to provide the corresponding biologically significant 3,4‐dihydrocoumarin derivatives in excellent yields with high levels of diastereo‐ and enantioselectivity.
A synthetic strategy has been developed for the synthesis of 2‐dialkylaminoquinolines from easily available quinoline N‐oxides, tertiary amines, diisopropyl H‐phosphonate and carbon tetrachloride (CCl4) in one pot under metal‐free conditions at room temperature.
The combination of bis(cyclooctadiene)nickel [Ni(COD)2] and 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene hydrochloride (IPr⋅HCl) effectively catalyzes coupling of fluoroarenes with amines in the presence of sodium tert‐butoxide (t‐BuONa). Activated, unactivated and deactivated fluoroarenes as well as fluoropyridines can react with cyclic or acyclic aliphatic amines. The reactions tolerate various functional groups in the fluorides including PhC(O), C(O)NEt2, CF3, OMe and vinyl groups.
Earth‐abundant and inexpensive titanium can catalyze alkyne iminoamination, which generates tautomers of 1,3‐diimines. Upon treatment with base (DBU) and malononitrile, the multicomponent coupling product is converted to 2‐amino‐3‐cyanopyridines in a one‐pot procedure in good to modest yields. There is substantial control of regioselectivity for the substituents on the pyridine ring and on the 2‐amino group. Several studies were done that provide significant evidence for a Dimroth rearrangement mechanism for 2‐aminopyridine formation, including isolation of a 2‐imino‐1,2‐dihydropyridine intermediate that undergoes rearrangement under the reaction conditions.
A simple and efficient domino protocol has been developed for the preparation of biologically important benzamides, 2,2‐diazidobenzofuran‐3(2H)‐ones and benzoxazolones from various structurally and electronically divergent acetophenones and ortho‐hydroxyacetophenones in the presence of molecular iodine, sodium azide and sodium bicarbonate at 100 °C in good to excellent yields.
An efficient copper‐catalyzed α‐aminoxylation of ketones with 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) is presented for the synthesis of 2‐aryloxy‐1‐aryl‐2‐(2,2,6,6‐tetramethylpiperidin‐1‐yloxy)ethanones in moderate to excellent yields. It is noteworthy that the copper/iron (Cu/Fe) catalyst can be recovered and reused several times with high catalytic reactivity.
