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Copper(I) Catalyzed [3+2] Cycloaddition Reaction with Mechanistic Disparity: A DFT Study
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1  Computational Chemistry Group, Department of Chemistry, Faculty of Science, University of Mauritius, Réduit, Mauritius

Abstract: The Huisgen [3+2] cycloaddition (32CA) reaction is one of the most useful convergent strategies for the synthesis of five membered azaheterocycles which are building the blocks for agricultural, industrial and pharmacological materials. Nonetheless, the classical 32CA reactions have certain limitations. While some 1,3 dipoles react readily with dipolarophiles without activation of either components; it is undeniable that many substrate combinations yield no cycloadducts in the absence of promoters. Besides, the control of regio- and stereochemistry is another challenge that the organic chemist encounter.   One method to enhance the reactivity and selectivity of 32CA reactions is to use metal and metalloid catalysts. In particular, transition metal catalyzed 32CA reaction, which also meets the aforementioned criteria, has emerged over the recent years as one of the most attractive synthetic techniques. Yet, a changing trend in catalyst from the rare and expensive metals (Pd, Rh, Ru) to more abundant ones (Cu, Fe, Ni) is being observed in new catalytic 32CA processes.   Studies of catalytic 32CA reaction mechanisms are not trivial because each step in the catalytic cycle is dependent on the subtle interplay between the electronic and steric effects, among others. Hence, an extensive examination of catalytic mechanism is of utmost importance, (1) to optimize reaction parameters, (2) to develop novel catalytic processes, (3) to locate and characterize competitive catalytic pathways, and (4) to enhance the understanding of fundamental reactivity. Nevertheless, synthetic approaches seldom provide a detailed picture of the catalytic 32CA reaction mechanisms and thus, quantum chemical computations which act as a complement, come to its rescue.   Indeed, Cu(I) catalyzed 32CA reaction, which comprises of several experimental studies, figure prominently in the field of organic chemistry. Moreover, based on experiments, some mechanistic postulates on the Cu(I) catalyzed 32CA reactions are available to-date. The aim of our study is to compare two particular Cu(I) 32CA catalyzed mechanisms, namely, (1) the Sharpless proposed mechanism and (2) the Katritzky's proposed mechanism at the B3LYP/6-31G(d) level of theory. An azomethine imine as dipole and a terminal alkyne as dipolarophile act as representative cycloaddition substrates to evaluate the two proposed mechanisms. The DFT study confirms that the two mechanisms studied herein is amenable to the 32CA reaction of azomethine imines and terminal alkynes. However, the reactions of azomethine imines to terminal alkynes have a kinetic preference for the mechanism proposed by Katritzky over the Sharpless one.  
Keywords: catalyzed [3+2] cycloaddition reaction, Copper(I), DFT