An Organocatalytic Azide–Aldehyde [3+2] Cycloaddition: High‐Yielding Regioselective Synthesis of 1,4‐Disubstituted 1,2,3‐Triazoles

An organocatalytic azide–aldehyde [3+2] cycloaddition (organo‐click) reaction of a variety of enolizable aldehydes is reported. The organo‐click reaction is characterized by a high rate and regioselectivity, mild reaction conditions, easily available substrates with simple operation, and excellent yields with a broad spectrum of substrates. It constitutes an alternative to the previously known CuAAC, RuAAC, and IrAAC click reactions.     

Rhodium(II)‐Catalyzed Intramolecular Annulation of 1‐Sulfonyl‐1,2,3‐Triazoles with Pyrrole and Indole Rings: Facile Synthesis of N‐Bridgehead Azepine Skeletons

A convenient and efficient synthetic method has been developed to construct highly functionalized N‐bridgehead azepine skeletons, which are of great importance in biological and pharmaceutical industry. The reaction proceeds through a rhodium(II) azavinyl carbene intermediate, which initiated the intramolecular C? H functionalization with pyrrolyl and indolyl rings. A variety of azepine derivatives were obtained in moderate to good yields under mild reaction conditions with high chemoselectivity. Several interesting derivatizations of the resulting products demonstrate that this method is synthetically valuable and useful.     

RhII‐Catalyzed [3+2] Cycloaddition of 2 H‐Azirines with N‐Sulfonyl‐1,2,3‐Triazoles

Rh^II‐catalyzed intermolecular [3+2] cycloaddition of 2 H‐azirines with N‐sulfonyl‐1,2,3‐triazoles is disclosed, in which a series of fully functionalized pyrroles is produced via rhodium azavinyl carbene intermediates. A distinct feature of this reaction is that the azavinyl carbene serves as a [2 C] equivalent, instead of as [1 C] or aza‐[3 C] synthons, which have been reported previously in cyclopropanations and [3+n] cycloadditions. Moreover, this methodology has also been successfully applied in the total synthesis of URB447 as well as the formal synthesis of Atorvastatin (Lipitor).     

Microwave‐Assisted Palladium‐Catalyzed Direct Arylation of 1,4‐Disubstituted 1,2,3‐Triazoles with Aryl Chlorides

Treatment of 1,4‐disubstituted 1,2,3‐triazoles with aryl chlorides in the presence of potassium carbonate under palladium catalysis and microwave irradiation at 250 °C for 15 min leads to arylation of the triazole at the 5‐position. A variety of functional groups, including ester and hydroxy groups, are compatible. The procedure is suitable for the regioselective preparation of trisubstituted triazoles. Microwave irradiation accelerates the reaction, thus allowing the rapid synthesis of trisubstituted triazoles, which are difficult to synthesize selectively.     

Recent Developments in Azide‐Free Synthesis of 1,2,3‐Triazoles

1,2,3‐Triazoles, as one of the most significant nitrogen‐containing heterocycles due to their extensive use in biology, material science and organic synthesis, have aroused great interest. 1,2,3‐Triazoles are commonly synthesized by metal‐catalyzed azide–alkyne cycloaddition and organocatalytic azide–carbonyl cycloaddition, which indispensably employ the toxic and potentially explosive azides. The azide‐free synthetic approaches provide a powerful and straightforward alternative to the assembly of diverse 1,2,3‐triazoles without the use of azides. In this review, we summarize the recent development of the construction of 1,2,3‐triazoles under azide‐free conditions.     

Amine‐Catalyzed [3+2] Huisgen Cycloaddition Strategy for the Efficient Assembly of Highly Substituted 1,2,3‐Triazoles

An enamine‐catalyzed strategy has been utilized to fully promote the Huisgen [3+2] cycloaddition with a broad spectrum of carbonyl compounds and azides, thereby permitting the efficient assembly of a vast pool of highly substituted 1,2,3‐triazoles. In particular, the employment of commonly used and commercially available carbonyl compounds has resulted in the introduction of a diverse set of functional groups, such as alkyl, aryl, nitrile, ester, and ketone groups, at the 1‐, 4‐, or 5‐positions of the 1,2,3‐triazole scaffold. This approach might be manipulated to access more useful and sophisticated heterocyclic compounds. Most significantly, the reaction process exhibits complete regioselectivity, with the formation of only one regioisomer.     

A syn‐Selective Aza‐Aldol Reaction of Boron Aza‐Enolates Generated from N‐Sulfonyl‐1,2,3‐Triazoles and 9‐BBN‐H

A syn‐selective aza‐aldol reaction of boron aza‐enolates, generated from N‐sulfonyl‐1,2,3‐triazoles and 9‐BBN‐H, is reported. It provides a sequential one‐pot procedure for the stereoselective construction of 1,3‐amino alcohols, having contiguous stereocenters, starting from terminal alkynes.     

Catalyst‐Dependent Chemoselective Formal Insertion of Diazo Compounds into C−C or C−H Bonds of 1,3‐Dicarbonyl Compounds

A catalyst‐dependent chemoselective one‐carbon insertion of diazo compounds into the C?C or C?H bonds of 1,3‐dicarbonyl species is reported. In the presence of silver(I) triflate, diazo insertion into the C(=O)?C bond of the 1,3‐dicarbonyl substrate leads to a 1,4‐dicarbonyl product containing an all‐carbon α‐quaternary center. This reaction constitutes the first example of an insertion of diazo‐derived carbenoids into acyclic C?C bonds. When instead scandium(III) triflate was applied as the catalyst, the reaction pathway switched to formal C?H insertion, affording 2‐alkylated 1,3‐dicarbonyl products. Different reaction pathways are proposed to account for this powerful catalyst‐dependent chemoselectivity.     

Rhodium(II)‐Catalyzed Intramolecular Cycloisomerizations of Methylenecyclopropanes with N‐Sulfonyl 1,2,3‐Triazoles

A novel rhodium(II)‐catalyzed tandem cycloisomerization of methylenecyclopropanes (MCPs) with N‐sulfonyl 1,2,3‐triazoles is disclosed. The reaction produces a series of highly functionalized polycyclic N heterocycles via a rhodium imino carbene intermediate. A distinct feature of this divergent synthesis is that different types of substrates control the reaction pathways. Moreover, several interesting transformations of these products to construct diazabicyclo[3.2.1]octane derivatives are also reported.     

An Enolate‐Mediated Organocatalytic Azide–Ketone [3+2]‐Cycloaddition Reaction: Regioselective High‐Yielding Synthesis of Fully Decorated 1,2,3‐Triazoles

An enolate‐mediated organocatalytic azide–ketone [3+2]‐cycloaddition (OrgAKC) reaction of a variety of enolizable arylacetones and deoxybenzoins with aryl azides was developed for the synthesis of fully decorated 1,4‐diaryl‐5‐methyl(alkyl)‐1,2,3‐triazoles in excellent yields with high regioselectivity at 25 °C for 0.5–6 h. This reaction has an excellent outcome with reference to reaction rate, yield, regioselectivity, operation simplicity, and availability of substrates and catalyst. This reaction has advantages over the previously known metal‐mediated reactions.     

Copper(II) Triflate Catalyzed Amination of 1,3‐Dicarbonyl Compounds

A method to prepare α,α‐acyl amino acid derivatives efficiently by Cu(OTf)₂+1,10‐phenanthroline (1,10‐phen)‐catalyzed amination of 1,3‐dicarbonyl compounds with PhI?NSO₂Ar is described. The mechanism is thought to initially involve aziridination of the enolic form of the substrate, formed in situ through coordination to the Lewis acidic metal catalyst, by the putative copper–nitrene/imido species generated from the reaction of the metal catalyst with the iminoiodane source. Subsequent ring opening of the resultant aziridinol adduct under the Lewis acidic conditions then provided the α‐aminated product. The utility of this method was exemplified by the enantioselective synthesis of a precursor of 3‐styryl‐2‐benzoyl‐L ‐alanine.     

FeCl3‐Catalyzed Addition of 1,3‐Dicarbonyl Compounds to Aromatic Olefins

A direct, intermolecular addition of 1,3‐dicarbonyl compounds to styrenes in the presence of FeCl₃ as an inexpensive and disposable catalyst has been developed for the straightforward and practical synthesis of arylated diketones and ketoesters. The reactions proceed under mild conditions for most substrates (50–80 °C), and no strong acid or base is required. The synthetic value of the method is demonstrated by 15 examples, including the synthesis of the current pharmaceutical drug warfarin in one step and 42 % yield from commercially available substrates.