Recent progress in the synthesis of 1,2,3,4,-tetrahydroquinolines

The use of diazonium salts for aryl radical generation and C-H arylation processes has been known since 1896 when Pschorr first used the reaction for intramolecular cyclizations. Meerwein developed it further in the early 1900s into a general arylation method. However, this reaction could not compete with the transition-metal-mediated formation of C(sp2) -C(sp2) bonds. The replacement of the copper catalyst with iron and titanium compounds improved the situation, but the use of photocatalysis to induce the one-electron reduction and activation of the diazonium salts is even more advantageous. The first photocatalyzed Pschorr cyclization was published in 1984, and just last year a series of papers described applications of photocatalytic Meerwein arylations leading to aryl-alkene coupling products. In this Minireview we summarize the origins of this reaction and its scope and applications.

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%202_Aminy_Reakcie/Doplnkove%20studijne%20materialy/Diazonium%20salts/The%20Photocatalyzed%20Meerwein%20Arylation.pdf

The Photocatalyzed Meerwein Arylation: Classic Reaction of Aryl Diazonium Salts in a New Light

Nitrogen heterocycles belong to a highly important class of compounds which are found in various natural products, biologically active structures, and medicinally relevant compounds. Therefore, there is continuing interest in the development of novel synthetic methods for the construction of nitrogen containing heterocycles. Recently, radical insertion reactions into isonitriles have emerged as an efficient and powerful strategy for the construction of nitrogen heterocycles, such as phenanthridines, indoles, quinolines, quinoxalines, and isoquinolines. This review highlights recent advances in this fast growing research area and also includes important pioneering studies in this area.

Recent advances in the synthesis of nitrogen heterocycles via radical cascade reactions using isonitriles as radical acceptors

Radical cyclizations and multicomponent radical cascades in solution were studied, which are initiated by intermolecular radical addition to C-C triple bond in alkynes as well as to related C?N triple bonds in nitriles and isonitriles. The reactions were arranged according to the atom that carries the unpaired electron in the attacking radical and cover main group IV-VI elements. The initial radical addition is highly regioselective and in reactions with substrates possessing several π systems, the alkyne moiety is usually attacked with high preference. This selectivity can be rationalized by the reversibility with which the initial radicaladdition occurs in many cases, in particular with Sn- and S centered radicals. Due to their high reactivity, vinyl radicals that are formed through radical attack at the alkyne moiety in enynes. Alkyl radicals resulting from radical addition to an alkene are far less reactive so that the competing dissociation and regeneration of the starting materials is often faster than the forward reaction steps.

Radical Cascades Initiated by Intermolecular Radical Addition to Alkynes and Related Triple Bond Systems

Radical reactions in organic synthesis

Radical annulations and cyclisations with isonitriles: the fate of the intermediate imidoyl and cyclohexadienyl radicals

Aromatic imines react with phenylacetylene or styrene in an acetonitrile solution of iron(III) chloride to give quinolines 3 or their tetrahydro derivatives 11 together with variable amounts of products 4 arising from the reduction of the imines. The initial atep appears to be a one-electron oxidation to generate iron(II) and an imine radical cation. When the reactions are carried out in the presence of stoichiometric amounts of tetrachloro-p-benzoquinone (chloranil), only quinolines 3 are obtained

Annulation reactions with iron(III) chloride: oxidation of imines

An easily effected aromatic annelation is described involving reaction of O-thioalkyl and O-thioaryl substituted Phenyl radicals with alkynes to give 2-substituted benzo[b]thiophenes; the mechanism is discussed.

Reaction pathways for the cyclization of ortho-thioalkyl and ortho-thioaryl substituted phenyl radicals with alkynes. Reaction of o-methylthioarenediazonium tetrafluoroborates with alkynes to give 2-substituted benzo[b]thiophenes

Reaction of azoarenes with tributyltin hydride

An easily effected aromatic annelation is described, involving the reaction of arylimidoyl radicals with alkynes to give quinolines.

Antonio Tundo

Papers

Radical annulations and cyclisations with isonitriles: the fate of the intermediate imidoyl and cyclohexadienyl radicals

Annulation reactions with iron(III) chloride: oxidation of imines

Reaction pathways for the cyclization of ortho-thioalkyl and ortho-thioaryl substituted phenyl radicals with alkynes. Reaction of o-methylthioarenediazonium tetrafluoroborates with alkynes to give 2-substituted benzo[b]thiophenes

Reaction of azoarenes with tributyltin hydride

Aromatic annelation by reaction of arylimidoyl radicals with alkynes: a new synthesis of quinolines