Multicomponent Reactions (MCRs) are convergent reactions, in which three or more starting materials react to form a product, where basically all or most of the atoms contribute to the newly formed product. In an MCR, a product is assembled according to a cascade of elementary chemical reactions. Thus, there is a network of reaction equilibria, which all finally flow into an irreversible step yielding the product. The challenge is to conduct an MCR in such a way that the network of pre-equilibrated reactions channel into the main product and do not yield side products. The result is clearly dependent on the reaction conditions: solvent, temperature, catalyst, concentration, the kind of starting materials and functional groups. Such considerations are of particular importance in connection with the design and discovery of novel MCRs. (A. Dömling, Org. Chem. Highlights 2004, April 5. Link)
Multicomponent Reactions with Carbonyl Compounds
Some of the first multicomponent reactions to be reported function through derivatization of carbonyl compounds into more reactive intermediates, which can react further with a nucleophile. One example is the Mannich Reaction:
Obviously, this reaction only proceeds if one carbonyl compound reacts faster with the amine to give an imine, and the other carbonyl compound plays the role of a nucleophile. In cases where both carbonyl compounds can react as the nucleophile or lead to imines with the same reaction rate, preforming the intermediates is an alternative, giving rise to a standard multistep synthesis.
Carbonyl compounds played a crucial role in the early discovery of multicomponent reactions, as displayed by a number of name reactions:
Isocyanide-based Multicomponent Reactions
Isocyanides play a dual role as both a nucleophile and electrophile, allowing interesting multicomponent reactions to be carried out. One of the first multicomponent reactions to use isocyanides was the Passerini Reaction. The mechanism shows how the isocyanide displays ambident reactivity. The driving force is the oxidation of CII to CIV, leading to more stable compounds.
This interesting isocyanide chemistry has been rediscovered, leading to an overwhelming number of useful transformations. One of these is the Ugi Reaction:
Both the Passerini and Ugi Reactions lead to interesting peptidomimetic compounds, which are potentially bioactive. The products of these reactions can constitute interesting lead compounds for further development into more active compounds. Both reactions offer an inexpensive and rapid way to generate compound libraries. Since a wide variety of isocyanides are commercially available, an equivalently diverse spectrum of products may be obtained.
Variations in the starting compounds may also lead to totally new scaffolds, such as in the following reaction, in which levulinic acid simultaneously plays the role of a carboxylic acid and a carbonyl compound:
But how can multicomponent reactions be discovered? It's sometimes a simple matter of trial and error. Some very interesting MCRs have even been discovered by preparing libraries from 10 different starting materials. By analyzing the products of each combination (three-, four-, up to ten-component reactions), one is able to select those reactions that show a single main product. HPLC and MS are useful analytical methods, because the purity and mass of the new compounds help to decide rapidly whether a reaction might be interesting to investigate further. (L. Weber, K. Illgen, M. Almstetter, Synlett, 1999, 366-374. DOI)
Links of Interest
Organic Chemistry Highlights: Multicomponent Reactions
Reviews on Multicomponent Reactions
Books on Multicomponent Reactions
Jieping Zhu, Hugues Bienaymé
Hardcover, 468 Pages
First Edition, 2005
ISBN: 3-527-30806-7 - Wiley-VCH
NH4I promotes an efficient indole-to-carbazole strategy under metal-free conditions. The reaction offers high regioselectivity through formal [2 + 2 + 2] annulation of indoles, ketones, and nitroolefins and enables the assembly of a large number of diversified carbazoles with good functional group tolerance.
S. Chen, Y. Li, P. Ni, H. Huang, G.-J. Deng, Org. Lett., 2016, 18, 5384-5387.
An efficient and versatile Pd(II)-catalyzed oxidative three-component cascade reaction of diverse amines, alkyne esters, and alkenes enables the direct synthesis of diverse 2,3,4-trisubstituted pyrroles with broad functional group tolerance and in good to excellent yields.
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A copper-catalyzed three-component tandem reaction enables a convenient and practical synthesis of 1,4-benzothiazines from terminal alkynes, 2-iodophenyl isothiocyanates, and aqueous ammonia.
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A Zn(OTf)2-mediated solvent-free synthesis of propargylamines proceeds effectively via A3 coupling of aldehydes, amines, and phenylacetylene. The protocol tolerates a broad range of substituted benzaldehydes, enolizable aldehydes, and formaldehyde. Recyclability of the catalyst, low catalyst loading, and use of inexpensive catalyst are the key features.
P. B. Sarode, S. P. Bahekar, H. S. Chandak, Synlett, 2016, 27, 2209-2212.
A general Pd-catalyzed, enantioselective three-component synthesis using readily available sulfonamides, glyoxylic acid derivatives, and boronic acids provides a broad range of α-arylglycines in high yields and excellent levels of enantioselectivity. Incorporation of Pbf-amides gives a racemization-free access to N-unprotected α-arylglycines.
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An unprecedented nucleophilic addition/cyclization/aromatization cascade of simple starting materials, i.e., aromatic alkenes/alkynes, trimethylsilyl cyanide and N,N-disubstituted formamide provides multisubstituted pyrroles in good yields with high regioselectivities.
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A three-component cyclization of 2H-azirines, alkynyl bromides, and molecular oxygen under visible-light photoredox catalysis at room temperature provides a wide range of substituted oxazoles in good yields.
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In a copper-catalyzed regio- and stereoselective borylalkylation of dialkylsubstituted internal alkynes with bis(pinacolato)diboron and alkyl halides, a borylcopper species containing a novel π-accepting N-heterocyclic carbene ligand chemoselectively reacted with unactivated internal alkynes over alkyl halides. The intermediate alkenylcopper species subsequently reacted with alkyl halides, affording the desired products.
T. Itoh, Y. Shimizu, M. Kanai, J. Am. Chem. Soc., 2016, 138, 7528-7531.
A silver-catalyzed sequential formation of two C-C bonds enabels the construction of a series of polysubstituted quinolines from anilines, aldehydes, and alcohols under mild conditions. The transformation is effective for a broad range of substrates, including aliphatic alcohols, arylalkanols, cycloalkanols, and ethylene glycol.
X. Zhang, W. Liu, R. Sun, X. Xu, Z. Wang, Y. Yan, Synlett, 2016, 27, 1563-1568.
A copper(II)-catalyzed oxidative cross-coupling of anilines, primary alkyl amines, and sodium azide provides benzimidazoles in the presence of TBHP at moderate temperature via a domino C-H functionalization, transimination, ortho-selective amination, and a cyclization sequence. The reaction offers broad substrate scope and functional group compatibility.
D. Mahesh, P. Sadhu, T. Punniyamurthy, J. Org. Chem., 2016, 81, 3227-3234.
The use of N-Boc-protected hydrazine in the Ugi tetrazole reaction provides a library of highly substituted 5-(hydrazinomethyl)-1-methyl-1H-tetrazoles in good yield.
P. Patil, J. Zhang, K. Kurpiewska, J. Kalinowska-Tłuścik, A. Dömling, Synthesis, 2016, 48, 1122-1130.
A direct difunctionalization protocol of alkenes with nitriles and thiols under metal-free synthesis conditions provides various β-acetamido sulfides with very good yields simply by using inexpensive molecular iodine as a catalyst, DMSO as a mild oxidant, and readily available thiols as thiolating reagents.
H. Cui, X. Liu, W. Wei, D. Yang, C. He, T. Zhang, H. Wang, J. Org. Chem., 2016, 81, 2252-2260.
A multicomponent reaction involving aryl boronic acids, elemental sulfur, and P(O)H compounds provides valuable S-aryl phosphorothioates and S-aryl phosphorodithioates in excellent yields. Moreover, this method can be easily scaled up.
J. Xu, L. Zhang, X. Li, Y. Gao, G. Tang, Y. Zhao, Org. Lett., 2016, 18, 1100-1103.
A palladium-catalyzed three-component synthesis of arylmethylsulfonamide derivatives from sulfonamides, paraformaldehyde, and arylboronic acids is operationally simple and provides a broad range of structurally interesting sulfonamidomethyl compounds.
T. Beisel, G. Manolikakes, Synthesis, 2016, 48, 379-386.
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