Amide bond formation is a fundamentally important reaction in organic synthesis, and is typically mediated by one of a myriad of so-called coupling reagents. This critical review is focussed on the most recently developed coupling reagents with particular attention paid to the pros and cons of the plethora of “acronym” based reagents. It aims to demystify the process allowing the chemist to make a sensible and educated choice when carrying out an amide coupling reaction (179 references).

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%208_Aminokyseliny%20a%20peptidy/Doplnkove%20studijne%20materialy/Amide%20bond%20formation/Amide%20bond%20formation%20-%20beyond%20the%20myth%20of%20coupling%20reagents.pdf

Amide bond formation: beyond the myth of coupling reagents

Heterocycles constitute the largest and the most diverse family of organic compounds Among them, aromatic heterocycles represent structural motifs found in a great number of biologically active natural and synthetic compounds, drugs, and agrochemicals Moreover, aromatic heterocycles are widely used for synthesis of dyes and polymeric materials of high value 1 There are numerous reports on employment of aromatic heterocycles as intermediates in organic synthesis 2

Although, a variety of highly efficient methodologies for synthesis of aromatic heterocycles and their derivatives have been reported in the past, the development of novel methodologies is in cuntinious demand Particlularly, development of new synthetic approaches toward heterocycles, aiming at achieving greater levels of molecular complexity and better functional group compatibilities in a convergent and atom economical fashions from readily accessible starting materials and under mild reaction conditions, is one of a major research endeavor in modern synthetic organic chemistry Transition metal-catalyzed transformations, which often help to meet the above criteria, are among the most attractive synthetic tools

Several excellent reviews dealing with transition metal-catalyzed synthesis of heterocyclic compounds have been published in literature during recent years Many of them highlighted the use of a particular transition metal, such as gold,3 silver,4 palladium,5 copper,6 cobalt,7 ruthenium,8 iron,9 mercury,10 rare-earth metals,11 and others Another array of reviews described the use of a specific kind of transformation, for instance, intramolecular nucleophilic attack of heteroatom at multiple C–C bonds,12 Sonogashira reaction,13 cycloaddition reactions,14 cycloisomerization reactions,15 C–H bond activation processes,16 metathesis reactions,17 etc Reviews devoted to an application of a particular type of starting materials have also been published Thus, for example, applications of isocyanides,18 diazocompounds,19 or azides20 have been discussed In addition, a significant attention was given to transition metal-catalyzed multicomponent syntheses of heterocycles21 Finally, syntheses of heterocycles featuring formation of intermediates, such as nitrenes,22 vinylidenes,23 carbenes, and carbenoids24 have also been reviewed

The main focus of the present review is a transition metal-catalyzed synthesis of aromatic monocyclic heterocycles The organization of the review is rather classical and is based on a heterocycle, categorized in the following order: (a) ring size of heterocycle, (b) number of heteroatoms, (c) type of heterocycle, and (d) a class of transformation involved A brief mechanistic discussion is given to provide information about a possible reaction pathway when necessary The review mostly discusses recent literature, starting from 200425 until the end of 2011, however, some earlier parent transformations are discussed when needed

Transition Metal-Mediated Synthesis of Monocyclic Aromatic Heterocycles

"Comprehensive Heterocyclic Chemistry" was published in 1984 and is now available on CD-ROM.

Comprehensive heterocyclic chemistry on CD-ROM

L-Proline: an efficient catalyst for the one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles

Cerium(IV) Ammonium Nitrate as a Catalyst in Organic Synthesis

Ceric ammonium nitrate (CAN) is used as an efficient catalyst for the synthesis of 2,4,5-triaryl-1H-imidazoles via condensation of benzoin/benzil, ammonium acetate, and aromatic aldehydes. The easy work-up, higher yields and shorter reaction time are the advantages of the method presented here.

/pdf/ceric-ammonium-nitrate-catalysed-three-component-one-pot-jltm5tqqhf.pdf

Ceric ammonium nitrate catalysed three component one-pot efficient synthesis of 2,4,5-triaryl-1H-imidazoles

2,4,5-Triaryl-1H-imidazoles were efficiently synthesized from benzoin or benzil, ammonium acetate, and aromatic or heteroaromatic aldehydes in the presence of sodium bisulfite as catalyst. The present protocol offers significant improvements for the synthesis of 2,4,5-triaryl-1H-imidazoles with regard to yield of products, simplicity in operation, and cheap catalyst.

https://rd.springer.com/content/pdf/10.1007%2Fs00706-007-0766-3.pdf

Sodium Bisulfite as an Efficient and Inexpensive Catalyst for the One-pot Synthesis of 2,4,5-Triaryl-1H-imidazoles from Benzil or Benzoin and Aromatic Aldehydes

Manganous chloride (MnCl2·4H2O) has been used as an efficient catalyst for an improved and rapid one-pot synthesis of bis-(4-hydroxycoumarin)methanes in excellent yields using water as a reaction medium. This aqueous mediated reaction of various aromatic and heteroaromatic aldehydes with 4-hydroxycoumarin using catalytic amounts of manganous chloride avoids the use of expensive, corrosive reagents, toxic solvents, and provides operational simplicity.

Water mediated efficient one-pot synthesis of bis-(4-hydroxycoumarin)methanes

Oxalic acid as a catalyst for efficient synthesis of bis-(indolyl)methanes, and 14-aryl-14H-dibenzo[a,j]xanthenes in water

Oxalic acid was found to be a versatile catalyst for the synthesis of 2-aryl-1-arylmethyl-1 H-benzimidazoles and 2-aryl-4,5-diphenyl-1 H-imidazoles in moderate to excellent isolated yields. 2-Aryl-1-arylmethyl-1 H-benzimidazoles were efficiently synthesized from O-phenylenediamine and various substituted aldehydes using 10 mol% oxalic acid. 2-Aryl-4,5-diphenyl-1 H-imidazoles were synthesized from benzil or benzoin, ammonium acetate, and aromatic aldehydes. The advantages of this method are the use of an inexpensive and readily available catalyst, easy workup, and improved yields, and the use of an ethanol-water solvent that is considered to be relatively environmentally benign.

Nagnnath D. Kokare

Papers

Ceric ammonium nitrate catalysed three component one-pot efficient synthesis of 2,4,5-triaryl-1H-imidazoles

Sodium Bisulfite as an Efficient and Inexpensive Catalyst for the One-pot Synthesis of 2,4,5-Triaryl-1H-imidazoles from Benzil or Benzoin and Aromatic Aldehydes

Water mediated efficient one-pot synthesis of bis-(4-hydroxycoumarin)methanes

Oxalic acid as a catalyst for efficient synthesis of bis-(indolyl)methanes, and 14-aryl-14H-dibenzo[a,j]xanthenes in water

One-Pot Efficient Synthesis of 2-Aryl-1-arylmethyl-1H-benzimidazoles and 2,4,5-Triaryl-1H-imidazoles Using Oxalic Acid Catalyst