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Enantioselective Henry Reaction Catalyzed by Copper(II) Complex of Bis(Trans-Cyclohexane-1,2-Diamine)-Based Ligand

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Enantioselective Henry Reaction Catalyzed by Copper(II) Complex of Bis(Trans-Cyclohexane-1,2-Diamine)-Based Ligand catalysts Article Enantioselective Henry Reaction Catalyzed by Copper(II) Complex of Bis(trans-cyclohexane-1,2-diamine)-Based Ligand David Tetour, Marika Novotná and Jana Hodaˇcová * Department of Organic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic; [email protected] (D.T.); [email protected] (M.N.) * Correspondence: [email protected]; Tel.: +420-220-444-173 Abstract: Copper(II) complex of the ligand possessing two enantiomerically pure trans-cyclohexane- 1,2-diamine units proved to be an efficient catalyst for the enantioselective Henry reaction of aromatic aldehydes with nitromethane. The effect of various reaction conditions on yield and enantioselectivity of the Henry reaction was studied. The results suggest that only one cyclohexane-1,2-diamine unit is involved in catalysis of the Henry reaction. Keywords: Henry reaction; enantioselective catalysis; trans-cyclohexane-1,2-diamine; copper(II) complexes 1. Introduction Base-catalyzed nucleophilic addition of nitroalkane to carbonyl compound is an im- portant carbon–carbon bond-forming reaction [1–3]. The reaction is named Henry reaction according to its discoverer [4] and because of its similarity with the aldol reaction it is also referred as the nitroaldol reaction. A significant feature of the Henry reaction is a possibility to conduct the reaction enantio- or diastereoselectively. Chiral products of Citation: Tetour, D.; Novotná, M.; the Henry reaction, 2-nitroalcohols, are important intermediates in syntheses of many Hodaˇcová,J. Enantioselective Henry biologically active compounds, such as antiasthmatic drug (R)-salmeterol [5], antibiotics Reaction Catalyzed by Copper(II) L-acosamine [6] and bestatin [7], or fungicide (S)-spirobrassinin [8]. In the past, develop- Complex of Bis(trans-cyclohexane-1, ment of the field of stereoselective Henry reaction had been rather slow [9]. One major 2-diamine)-Based Ligand. Catalysts 2021, 11, 41. https://doi.org/ obstacle is that the reaction components, nitroalkane and a carbonyl compound, lack of 10.3390/catal11010041 suitable points for attachment of chiral auxiliaries. In addition, the reversible nature of the reaction and relatively easy racemization at the stereogenic center (located at the α-position Received: 2 December 2020 to the nitro group) have hampered the progress in stereocontrol of the Henry reaction. Accepted: 28 December 2020 In general, enantioselectivity of the reaction can be controlled by chirality of a substrate, Published: 31 December 2020 chiral auxiliary, or catalyst. Among these approaches, employment of the chiral catalyst is highly advantageous since the usually expensive and heavily accessible enantiomerically Publisher’s Note: MDPI stays neu- pure compound is used in less than stoichiometric amount. tral with regard to jurisdictional clai- The first catalyst-controlled enantioselective Henry reaction was reported by Shibasaki [10] ms in published maps and institutio- in 1992. A large number of catalysts capable of catalyzing the nitroaldol reaction enan- nal affiliations. tioselectively has been developed since then, including both metal catalysts [11] and organocatalysts [12]. For example, copper(II) complexes of bis(oxazolidine) ligands [13], lanthanide complexes of 1,10-binaphthalene-2,20-diol derivatives [14], dinuclear zinc(II) complexes of C -symmetric alcohols [15], and copper(II) complexes of chiral ethane-1,2- Copyright: © 2020 by the authors. Li- 2 censee MDPI, Basel, Switzerland. diamine derivatives [16,17] were successfully used in the metal-based catalysis. Chiral This article is an open access article guanidines [18], bis(thioureas) [19,20] or cinchona alkaloids [21] can serve as examples of distributed under the terms and con- efficient organocatalysts. ditions of the Creative Commons At- Recently, mononuclear copper complexes of secondary amines derived from enan- tribution (CC BY) license (https:// tiomerically pure trans-cyclohexane-1,2-diamine have been reported to achieve modest creativecommons.org/licenses/by/ to excellent enantioselectivities in the Henry reaction [22–30]. Zhang et al. [31] have de- 4.0/). veloped efficient binuclear copper(II) complex of the C2-symmetric ligand, in which two Catalysts 2021, 11, 41. https://doi.org/10.3390/catal11010041 https://www.mdpi.com/journal/catalysts Catalysts 2021,, 11,, 41x FOR PEER REVIEW 2 of 10 Catalysts 2021, 11, x FOR PEER REVIEW 2 of 10 oped efficient binuclear copper(II) complex of the C2-symmetric ligand, in which two chiraloped efficientβ-amino binuclearalcohols arecopper(II) connected complex at the of 1,4-positions the C2-symmetric to the ligand,central benzenein which ring. two Theirchiral βbinuclearβ-amino-amino alcoholsalcohols complex are are connectedexhibited connected atsignifican theat the 1,4-positions 1,4-positionstly higher to theenantioselectivities to centralthe central benzene benzene ring. over Theirring. the mononuclearTheirbinuclear binuclear complex one. complex exhibitedBased onexhibited significantly these published significan higher results,tly enantioselectivities higher we decidedenantioselectivities overto prepare the mononuclear overligand the 1 possessingmononuclearone. Based two on one. thesetrans- Basedcyclohexane-1,2-diamine published on these results, published we decided unitsresults, attached to we prepare decided to ligandthe centralto 1preparepossessing benzene ligand unit two 1 trans (Figurepossessing-cyclohexane-1,2-diamine 1) and two study trans- cyclohexane-1,2-diaminethe effect unitsof the attached second tometalunits the attached centralcenter in benzene to the the binuclear central unit (Figure benzene complex1) andunit on study the effect of the second metal center in the binuclear complex on catalytic activity catalytic(Figure 1) activity and study and enantioselectivitythe effect of the secondof the Henry metal reaction.center in the binuclear complex on catalyticand enantioselectivity activity and enantioselectivity of the Henry reaction. of the Henry reaction. NH HN NH HN NH HN NH HN 1 1 Figure 1. Structure of ligand 1. Figure 1. Structure of ligand 1. 2. Results Results 2. Results 2.1. Synthesis Synthesis of of Ligand Ligand 1 2.1. Synthesis of Ligand 1 Preparation ofof ligandligand1 1was was previously previously published published by by Bellemin-Laponnaz Bellemin-Laponnaz et al. et [ 32al.] and[32] andwe used wePreparation used the same the of syntheticsame ligand synthetic 1 strategy was previouslystrategy with some with published experimental some experimentalby Bellemin-Laponnaz modifications. modifications. The et synthesis al. The[32] synthesisandstarted we from used started (1 theS,2 from Ssame)-trans (1 syntheticS-cyclohexane-1,2-diamine,,2S)-trans strategy-cyclohexane-1,2-diamine, with some which experimental was which first monoprotected wasmodifications. first monopro- withThe tectedsynthesisthe tert with-butoxycarbonyl started the tert from-butoxycarbonyl (1 groupS,2S)-trans [33].-cyclohexane-1,2-diamine, Carbamate group [33].2 wasCarbamate condensed 2 whichwas with condensed was terephthaldehyde, first monopro- with ter- tected with the tert-butoxycarbonyl group [33]. Carbamate 2 was condensed with ter- ephthaldehyde,and the resulting and Schiff the resulting base 3 was Schiff reduced base 3 with wasNaBH reduced4. Thewith obtained NaBH4. The amine obtained4 was amineephthaldehyde,deprotected 4 was anddeprotected and tetraamine the resultingand5 wastetraamine condensedSchiff 5base was with 3 condensed was two reduced molar with equivalents with two NaBH molar of4 .equivalents benzaldehydeThe obtained of amine 4 was deprotected and tetraamine 5 was condensed with two molar equivalents of benzaldehydeto give Schiff base, to give whose Schiff two base, imine whose double tw bondso imine were double finally bonds reduced were with finally NaBH reduced4 giving benzaldehyde to give Schiff base, whose two imine double bonds were finally reduced withligand NaBH1 in 36%4 giving overall ligand yield 1 in (Scheme 36% overall1). yield (Scheme 1). with NaBH4 giving ligand 1 in 36% overall yield (Scheme 1). OHC CHO NH2 Boc2O NH2 NaBH4 NH Boc O NH OHC CHO N N NaBH 2 2 2 N N 4 NH NH 2 80 % Boc 80 % NH HN 95 % NH NH 2 80 % Boc 80 % NHBoc BocHN 95 % 2 Boc Boc 2 3 3 CHO CHO 1. 1. TFA NH HN 1. 1. TFA NH HN NH HN ligand 1 NH HN NH HN ligand 1 2. KOH NH H N 2. NaBH4 NH HN 2 2 Boc Boc 2. KOH NH H N 2. NaBH4 Boc Boc 98 %2 2 59 % 4 98 %5 59 % 4 5 Scheme 1. Synthesis of ligand 1. Scheme 1. Synthesis of ligand 1. Contrary to original authors [32], [32], we had to isolate and carefully purify all all interme- diatesContrary in in the the reaction reaction to original sequence. sequence. authors If Ifcrude[32], crude we intermediates intermediateshad to isolate were and were used carefully used in subsequent in purify subsequent all steps, interme- steps, we we were not able to obtain ligand 1 in sufficient purity. werediates not in theable reaction to obtain sequence. ligand 1 Ifin crude sufficient intermediates purity. were used in subsequent steps, we were not able to obtain ligand 1 in sufficient purity. 2.2. Catalytic Studies of the Henry Reaction 2.2. Catalytic Studies of the Henry Reaction 2.2. CatalyticThe reaction Studies of benzaldehydeof the Henry Reaction with 10 molar equivalents of nitromethane was selected for initialThe reaction experiments of benzaldehyde to elucidate with the catalytic10 molar activityequivalents of the of copper(II)nitromethane complex was se- of lectedThe for reactioninitial
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