5-Benzyl-1H-tetrazols from the reaction of 1-aryl-5-methyl-1H-tetrazoles with 1,2-dehydrobenzene

Asymmetric reactions involving aryne intermediates

Although arynes are usually considered fleeting intermediates, they are highly valuable synthons because they enable the introduction of aromatic rings and the simultaneous formation of new bonds at two sites. Although catalytic reactions using transition metals are excellent method for constructing complex polycyclic aromatic molecules in a single step, the use of asymmetric catalysis for the capture of arynes remains a crucial goal for the progress of aryne chemistry. Catalytic asymmetric reactions of arenes are challenging, requiring sufficient interactions between the neutral and highly reactive short-lived aryne intermediates in a stereo-controlled fashion. In addition, spontaneous decomposition, as well as side reactions, has hindered their development and, until recently, highly enantioselective reactions using arynes had remained elusive. This Review highlights asymmetric reactions using arynes, featuring diastereoselective, enantioselective and catalytic enantioselective reactions.

The Aryne Ene Reaction

Intermolecular aryne ene reactions present opportunities to arylate a wide range of unsaturated substrates in a single step, whilst intramolecular reactions provide expedient access to valuable benzofused carbo- and heterocyclic frameworks. This short review will chart the development of the aryne ene reaction from initial reports that rationalise unexpected byproduct formation in competing [4+2] and [2+2] cycloadditions through to its exploitation in contemporary synthetic methodology.

1 Introduction

2 Alkene Ene Reactions

2.1 Intermolecular

2.2 Intramolecular

3 Alkyne Ene Reactions

3.1 Intermolecular

3.2 Intramolecular

4 Allene Ene Reactions

5 Aromatic Ene Reactions

6 Hetero Ene Reactions

6.1 Enols

6.2 Enamines

6.3 Imines

7 Conclusions

Selectivity between an Alder–ene reaction and a [2 + 2] cycloaddition in the intramolecular reactions of allene-tethered arynes

Substituent-dependent reactivity and selectivity in the intramolecular reactions of arynes tethered with an allene are described.

Construction of 5-Aminotetrazoles via in Situ Generation of Carbodiimidium Ions from Ketones Promoted by TMSN3/TfOH

A novel synthetic approach for the synthesis of 5-aminotetrazoles has been developed by employing simple ketones as substrates. This methodology involved the N₂-extrusion/aryl migration of azido complexes as the key step for the in situ generation of carbodiimidium ion, which could further react with hydrazoic acid and cyclize intramolecularly to provide 5-aminotetrazoles in good to excellent yields. In addition, the regioselectivity of the reaction was studied and rationalized by density functional theory calculations.

Regioselective formation of 1,2,4-triazoles by the reaction of amidrazones in the presence of diethyl azodicarboxylate and catalyzed by triethylamine

A general method for the synthesis of 1,3,5-trisubstituted 1,2,4-triazoles has been developed from reaction of amidrazones with ethyl azodicarboxylate and triethylamine (Mitsunobu reagent) in EtOH. This highly regioselective one-pot process provides rapid access to highly diverse triazoles. The reaction was explained, based on Mitsunobu reagent oxidizing ethanol to acetaldehyde, which would then react with amidrazones to give the substituted 3-methyltriazoles. A [2 + 3] cycloaddition reaction between two oxidized forms of amidrazones produced the second type of triazoles. X-ray structure analyses proved the structure of each type of product.

Domino Aryne Annulation via a Nucleophilic-Ene Process

1,2-Benzdiyne equivalents possess the unique property that they can react with two arynophiles through iteratively generated 1,2- and 2,3-aryne intermediates. Upon rational modification on the second leaving group of these aryne precursors, a domino aryne annulation approach was developed through a nucleophilic-ene reaction sequence. Various benzo-fused N-heterocyclic frameworks were achievable under transition metal-free conditions with a broad substrate scope.

Reactivity of arynes toward functionalized alkenes: intermolecular Alder-ene vs. addition reactions

The selectivity between two different manifolds of reactions of arynes reacting with functionalized alkenes is described.

The synthesis of planar chiral pseudo-gem aminophosphine pre-ligands based on [2.2]paracyclophane

We have synthesized three pseudo-gem [2.2]paracyclophane-derived P,N-ligands and report preliminary activity studies for the amination of aryl bromides and chlorides.

Synthesis of biaryls using aryne intermediates

The synthesis of biaryls from benzyne intermediates offers an alternative strategy to conventional metal-catalyzed cross-coupling approaches.

A review of syntheses of 1,5-disubstituted tetrazole derivatives

This report provides a brief overview of the various representative literature procedures for the synthesis of 1,5-disubstituted tetrazoles (1,5-DSTs) and fused 1,5-disubstituted tetrazoles with more than 120 references. Most of the published methods for the synthesis of 1,5-DSTs include the use of nitriles, amides, thioamides, imidoyl chlorides, heterocumulenes, isocyanates, isothiocyanates, carbodiimides, ketenimines, ketones, amines, and alkenes as the starting materials. The transformation of 1- and 5-substituted tetrazoles into 1,5-DSTs is also covered in this report.

Regioselectivity in the nucleophile trapping of arynes: the electronic and steric effects of nucleophiles and substituents

The regioselectivity in nucleophile trapping is investigated with arynes generated directly from bis-1,3-diynes. The regioselectivity is profoundly influenced by not only the nature of nucleophiles but also the substituents on the arynes, which is the consequence of both the unfavorable steric interaction between the incoming nucleophile and the nearby substituent and the inherent electronic bias induced by different substituents on the arynes.

Development of an intramolecular aryne ene reaction and application to the formal synthesis of (±)-crinine

A general and high yielding annulation strategy for the synthesis of various carbo- and heterocycles, based on an intramolecular aryne ene reaction is described. It was found that the geometry of the olefin is crucial to the success of the reaction, with exclusive migration of the trans-allylic-H taking place. Furthermore, the electronic nature of the aryne was found to be important to the success of the reaction. Deuterium labeling studies and DFT calculations provided insight into the reaction mechanism. The data suggests a concerted asynchronous transition state, resembling a nucleophilic attack on the aryne. This strategy was successfully applied to the formal synthesis of the ethanophenanthridine alkaloid (±)-crinine.

Intramolecular aryne-ene reaction: synthetic and mechanistic studies

Although the chemistry of arynes is well developed, some challenges still remain. The ene reaction of arynes has not gained widespread use in synthesis as a result of poor yields and selectivity. A general, high yielding and selective intramolecular aryne-ene reaction is described providing various benzofused carbo- and heterocycles. Mechanistic data is presented, and a rationale for the resulting stereochemistry is discussed.