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
Microwave irradiation enables an expeditious one-pot, ligand-free, Pd(OAc)2-catalyzed, three-component reaction for the synthesis of 2,3-diarylimidazo[1,2-a]pyridines. This methodology offers high availability of commercial reagents and great efficiency in expanding molecule diversity.
Y. Wang, B. Frett, H.-y. Li, Org. Lett., 2014, 16, 3016-3019.
An operational simple palladium-catalyzed three-component reaction of readily available 2-aminobenzamides, aryl halides, and tert-butyl isocyanide efficiently constructs quinazolin-4(3H)-ones in good yields via a palladium-catalyzed isocyanide insertion/cyclization sequence.
X. Jiang, T. Tang, J.-M. Wang, Z. Chen, Y.-M. Zhu, S.-J. Ji, J. Org. Chem., 2014, 79, 5082-5087.
Arylboration of vinylarenes and methyl crotonate with aryl halides and bis(pinacolato)diboron by cooperative Pd/Cu catalysis gives 2-boryl-1,1-diarylethanes and α-aryl-β-boryl ester in a regioselective manner. The reaction is compatible with various functionalities and can be scaled-up to a gram scale.
K. Semba, Y. Nakao, J. Am. Chem. Soc., 2014, 136, 7567-7570.
A copper(II)-catalyzed intermolecular three-component oxyarylation of allenes using arylboronic acids as a carbon source and TEMPO as an oxygen source proceeded under mild conditions with high regio- and stereoselectivity and functional group tolerance.
T. Itoh, Y. Shimizu, M. Kanai, Org. Lett., 2014, 16, 2736-2739.
An efficient synthesis of 2H-indazole derivatives in a one-pot three-component reaction of 2-chloro- and 2-bromobenzaldehydes, primary amines and sodium azide is catalyzed by copper(I) oxide nanoparticles (Cu2O-NP) under ligand-free conditions in polyethylene glycol (PEG 300) as a green solvent.
H. Sharghi, M. Aberi, Synlett, 2014, 25, 1111-1115.
The odorless and stable solid Na2S2O3·5H2O was used as a convenient and environmentally friendly source of sulfur in a Pd-catalyzed cross-coupling of aryl halides and alkyl halides to deliver aromatic sulfides.
Z. Qiao, J. Wei, X. Jiang, Org. Lett., 2014, 16, 1212-1215.
Microwave-assisted conditions enabled a simple, rapid, one-pot synthesis of arylaminomethyl acetylenes in very good yields using arylboronic acids, aqueous ammonia, propargyl halides, copper(I) oxide and water as the solvent within ten minutes.
Y. Jiang, S. Huang, Synlett, 2014, 25, 407-410.
A multicomponent protocol enables the synthesis of highly substituted imidazole derivatives in excellent yield from various α-azido chalcones, aryl aldehydes, and anilines in the presence of erbium triflate as a catalyst.
K. Rajaguru, R. Suresh, A. Mariappan, S. Muthusubramanian, N. Bhuvanesh, Org. Lett., 2014, 16, 744-747
A three-component reaction of alkynes, elemental sulfur, and aliphatic amines allows a general, straightforward, and atom-economical synthesis of thioamides.
T. B. Nguyen, M. Q. Tran, L. Ermolenko, A. Al-Mourabit, Org. Lett., 2014, 16, 310-313.
A highly practical copper-catalyzed intermolecular cyanotrifluoromethylation of alkenes provides a general and straightforward way to synthesize various useful CF3-containing nitriles, which can be used for the preparation of pharmaceutically and agrochemically important compounds.
Y.-T. He, L.-H. Li, Y.-F. Yang, Z.-Z. Zhou, H.-L. Hua, X.-Y. Liu, Y.-M. Liang, Org. Lett., 2014, 16, 270-273.
A facile formation of C-N, C-O, and C-S bonds from ynals, pyridin-2-amines, and alcohols or thiols enables a transition-metal-free three-component reaction for the construction of imidazo[1,2-a]pyridines.
H. Cao, X. Liu, L. Zhao, J. Cen, J. Lin, Q. Zhu, M. Fu, Org. Lett., 2014, 16, 146-149.
Unusual N-Acyl-N,O-acetals are present in a number of bioactive natural products and can act as a synthetic precursor to unstable reactive N-acylimines. Various N-acyl-N,O-acetals can be prepared under mild conditions mediated by titanium ethoxide (Ti(OEt)4). The method also offers a new strategy to make other O-alkyl-N,O-acetals.
M. Li, B. Luo, Q. Liu, Y. Hu, A. Ganesan, P. Huang, S. Wen, Org. Lett., 2014, 16, 10-13.
An operationally simple and rapid copper-catalyzed three-component synthesis of trisubstituted N-aryl guanidines involving cyanamides, arylboronic acids, and amines is performed in the presence of K2CO3, a catalytic amount of CuCl2·2H2O, bipyridine, and oxygen (1 atm).
J. Li, L. Neuville, Org. Lett., 2013, 15, 6124-6127.
Heating a solution of an aldehyde, an aromatic amine, and a nitroalkane in 20% water-methanol at 60°C for five hours enables an environmentally benign three-component, one-pot synthesis of 2-nitroamines in the absence of a catalyst.
C. G. Piscopo, G. Sartori, J. A. Mayoral, D. Lanari, L. Vaccaro, R. Maggi, Synlett, 2013, 24, 2596-2600.
A Lewis acid palladium-catalyzed reaction of amides, aryl aldehydes, and arylboronic acids enables a practical and general synthesis of α-substituted amides from simple, readily available building blocks.
T. Beisel, G. Manolikakes, Org. Lett., 2013, 15, 6046-6049.
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