RSC Advances PAPER Aldehyde–alkyne–amine (A3) coupling catalyzed by a highly efficient dicopper complex† Cite this: RSC Adv.,2015,5, 37737 Hong-Bin Chen,*a Yan Zhaoa and Yi Liao*b I ð Þ ¼ 3 A dicopper(I) complex, Cu 2 pip 2 (pip (2-picolyliminomethyl)pyrrole anion), was utilized to catalyze A Received 17th March 2015 coupling reactions, which led to the formation of propargylamines. Aldehydes, alkynes and amines with a Accepted 13th April 2015 variety of structures have been tested. A low catalyst loading of 0.4 mol% was sufficient to give good to DOI: 10.1039/c5ra04729c excellent yields. The low catalyst loading, broad scope of substrate and easy preparation make this www.rsc.org/advances dicopper complex a useful catalyst for A3 coupling. Introduction stage of the catalytic cycle.19 The intermediate 1a then binds to the imine or iminium ion, which leads to the addition of the Propargylamines are versatile building blocks for the synthesis alkynylide to the imine or iminium ion, and yields the inter- of various nitrogen-containing heterocyclic compounds,1 and mediate 1b (Scheme 1). It is worth mentioning that this important intermediates for the preparation of complex natural mechanism is similar to those of copper-catalyzed alkyne–azide products and biologically active molecules.2 Further, some cycloaddition (CuAAC),20 Kinugasa reaction,21 and Glaser propargylamines have been clinically used or are currently coupling.22 In fact, several dicopper catalysts have been shown tested for the treatment of Parkinson's disease3 and Alzheimer's to be highly efficient for CuAAC.23 To our best knowledge, only disease.4 Classically, propargylamines are synthesized by the one type of dicopper catalyst based on pybox ligands has been nucleophilic addition of a metal alkynylide to C]N electro- reported for A3 coupling.24 The pybox catalysts were used for philes, which oen requires stoichiometric amount of highly asymmetric synthesis and a loading of 5–10% was commonly active organometallic reagents such as organolithium,5 used. Based on the mechanism, the primary function of a Grignard reagents,6 and organozinc reagents,7 and hence is less dinuclear structure should be improving its catalytic efficiency. attractive due to low tolerance of functional groups, harsh Therefore, exploring new dicopper catalysts is important for reaction conditions, and operational complexity. In the past enhance catalytic efficiency as well as understanding the decade, transition-metal catalyzed three-component coupling mechanism of A3 coupling. of an aldehyde, an alkyne, and an amine (generally referred as Previously we reported that a dinuclear copper(I) com- 3 I ð Þ ¼ A -coupling) has received more and more attention due to its plex, Cu 2 pip 2 (pip (2-picolyliminomethyl)pyrrole anion) atom economy, step efficiency, and high chemical selectivity.8 (Scheme 2) efficiently catalyzed the alkyne–azide cycloaddition This reaction was proposed to proceed through the addition of at a low loading of 0.2 mol%.23a The catalyst has a stable an in situ generated metal-alkynylide to an imine (or iminium ion), which is also formed in situ from a reaction between an aldehyde and an amine, and water was formed as the only side product. Transition metal salts and complexes, especially those from coinage metals (Cu, Ag, and Au),8 as well as Zn,9 Ni,10 Fe,11 In,12 Ir,13 Co,14 Mn,15 Bi,16 Hg17 and Cd18 have been developed as the Scheme 1 Proposed catalytic intermediates in A3-coupling.19 catalysts for A3-coupling, among which copper compounds have been studied most. Recently, Heaney and co-workers proposed 3 that in the mechanism of copper(I)-catalyzed A -coupling, a dimeric copper(I) acetylide 1a (Scheme 1) forms in the early aDepartment of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, 361005, PR China. E-mail: [email protected]; Tel: +86-592-2181573 bDepartment of Chemistry, Florida Institute of Technology, Melbourne, FL 32901, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details and I ð Þ characterization data. See DOI: 10.1039/c5ra04729c Scheme 2 Structure of Cu 2 pip 2. This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5, 37737–37741 | 37737 RSC Advances Paper dimeric structure in solutions and is easy to prepare. Given the Even a loading as low as 0.2 mol% gave 90% yield (entry 11). It is similarity between the proposed mechanisms of copper cata- worth mentioning that homogenous A3 reaction oen requires lyzed A3 and alkyne–azide cycloaddition reactions, we hypoth- 5–10 mol% of catalyst loading.8 Further studies showed that the esized that this catalyst could also possess a high activity for A3 reaction time can be decreased without affect the yields. As reaction. Herein, we reported our study on the catalytic behav- shown in entry 12 and 13 in Table 1, 98% yield was obtained 3 iors of this dicopper(I) compound for a series of A reactions aer 2 h and there was no signicant difference between 2 h with different substrates. and 4 h reactions. The low catalyst loading and short reaction I ð Þ time clearly proves the high activity of Cu 2 pip 2. I ð Þ The catalytic activity of Cu 2 pip 2 was compared to another Results and discussion dicopper(I) compound (CuOAc)2, which has also been reported to be a high-efficiency catalyst for CuAAC reaction.23c However, A model reaction with benzaldehyde, phenylacetylene and even when 2 mol% of (CuOAc) was used, which was 5 times piperidine was studied rst to evaluate the catalytic activity of 2 more than that of CuI ðpipÞ , the corresponding A3-coupling CuI ðpipÞ and to optimize the reaction condition. The reaction 2 2 2 2 only gave 67% yield (entry 14). was performed in reuxing toluene for 12 hours in the presence The structural effects of the aldehyde reactants were studied of 1 mol% of CuI ðpipÞ . The coupling reaction proceeded well 2 2 using the optimized reaction condition. Aromatic and aliphatic and the expected propargylamine was isolated with nearly aldehydes with a variety of structures were examined using quantitative yield (Table 1, entry 1). Other solvents were also piperidine and phenylacetylene as the other reactants, and the tested for the reaction (entries 2–7). The bath temperature was results are summarized in Table 2. All the aromatic aldehydes set to 110 C, and thus the reaction was either at this temper- tested showed high reactivity. Electron-withdrawing or electron- ature or the boiling points of the solvents. Non-polar solvents donating substituents at ortho-, meta-, or para-position showed toluene and dioxane gave much better yields than the polar almost no effects, and excellent yields were obtained from the solvents including DMF, DMSO, EtOH and MeCN. The corresponding aldehydes (entries 1–9). Sterically hindered moderate yield obtained by using THF (entry 3) may also be due aromatic aldehydes including 2,6-dichlorobenzaldehyde and to the its low boiling point. 2,6-dimethoxybenzaldehyde also reacted well, and gave the The loading of the catalyst was optimized by conducting the corresponding products with yields of 88% and 98% reaction with 1–0.2 mol% of loading using toluene as the – I ð Þ solvent (entry 1, 8 11). The results revealed that Cu 2 pip 2 is highly active for A3-coupling. When the catalyst loading was Table 2 Reaction yields of different aldehydes under optimized reduced to 0.4 mol%, the reaction still gave 98% yield (entry 10). reaction conditionsa Table 1 Optimization of reaction conditions of A3 couplinga Entry R Yieldb (%) 1C6H5 98 b Entry Solvent Cat. (%) t (h) Yield (%) 2 4-FC6H4 97 3 3-FC6H4 98 1 Toluene 1 12 99 4 2-FC6H4 98 2 MeCN 1 12 61 5 4-MeOC6H4 96 3 THF 1 12 71 6 3-MeOC6H4 97 4 Dioxane 1 12 80 7 2-MeOC6H4 97 5 EtOH 1 12 37 8 1-Naphthyl 95 6 DMF 1 12 32 9 2-Naphthyl 97 7 DMSO 1 12 25 10 2,6-Cl2C6H3 88 8 Toluene 0.8 12 99 11 2,6-(MeO)2C6H3 98 9 Toluene 0.6 12 99 12 C4H9 99 10 Toluene 0.4 12 98 13 Me2CHCH2 99 11 Toluene 0.2 12 90 14 C7H15 97 12 Toluene 0.4 4 97 15 c-Pentyl 91 13 Toluene 0.4 2 98 16 c-Hexyl 94 14c Toluene 2 2 67 17 2-Furyl 89 18 2-Thiophenyl 81 a Reaction conditions: benzaldehyde (1.0 mmol), piperidine (1.0 mmol), phenylacetylene (1.2 mmol), catalyst, solvent (3 mL). b Isolated yield. a Reaction conditions: aldehyde (1.0 mmol), piperidine (1.0 mmol) and c b (CuOAc)2 was used as the catalyst. phenylacetylene (1.2 mmol) in toluene (3 mL). Isolated yield. 37738 | RSC Adv.,2015,5, 37737–37741 This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances respectively (entry 10 and 11). Aliphatic aldehydes including Table 4 Effect of different amines on A3 coupling under optimization a pentanal, isopentanal, octanal, cyclopentanecarboxaldehyde reaction conditions and cyclohexanecarboxaldehyde, were also excellent substrates for this catalytic reaction. Yields above 90% were obtained for all the tested aliphatic aldehydes aer the reactions proceeded 2 hours (entries 12–16). Heteroaromatic aldehydes including 2- furaldehyde and 2-thenaldehyde were also examined and afforded the corresponding products with yields of 89% and 81% respectively (entries 17 and 18). Entry R R NH Yieldb (%) The structural effects of alkynes were studied using benzal- 1 2 dehyde and piperidine as the other reactants, and the results 1 Morpholine 96 are summarized in Table 3.