Microwave-Assisted Catalytic Method for a Green Synthesis of Amides Directly from Amines and Carboxylic Acids

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Article Microwave-Assisted Catalytic Method for a Green Synthesis of Amides Directly from Amines and Carboxylic Acids

Adam P. Zarecki, Jacek L. Kolanowski * and Wojciech T. Markiewicz *

Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Pozna´n,Poland; [email protected] * Correspondence: [email protected] (J.L.K.); [email protected] (W.T.M.); Tel.: +48-61-852-85-03 (ext. 165) (J.L.K.); +48-61-852-85-03 (ext. 180) (W.T.M.) Received: 3 March 2020; Accepted: 9 April 2020; Published: 11 April 2020

Abstract: Amide bonds are among the most interesting and abundant molecules of life and products of the chemical pharmaceutical industry. In this work, we describe a method of the direct synthesis of amides from carboxylic acids and amines under solvent-free conditions using minute quantities of ceric ammonium nitrate (CAN) as a catalyst. The reactions are carried out in an open microwave reactor and allow the corresponding amides to be obtained in a fast and effective manner when compared to other procedures of the direct synthesis of amides from acids and amines reported so far in the literature. The amide product isolation procedure is simple, environmentally friendly, and is performed with no need for chromatographic purification of secondary amides due to high yields. In this report, primary amines were used in most examples. However, the developed procedure seems to be applicable for secondary amines as well. The methodology produces a limited amount of wastes, and a catalyst can be easily separated. This highly efficient, robust, rapid, solvent-free, and additional reagent-free method provides a major advancement in the development of an ideal green protocol for amide bond formation.

Keywords: amide synthesis; direct amidation; microwave; green chemistry; solvent free

1. Introduction Amide bonds are among the most widely abundant and fascinating types of linkages in organic synthesis and nature [1–4]. They constitute the backbone of peptides and proteins and are important elementary linkages in many natural products and polymers. In addition, thanks to their stability in biological environments, they are often used in the construction of various drugs, insecticides, nutraceutics, and chemical tools to study and modify biology [5–8]. Nature uses enzymes to form amides from carboxylic acids and amines in a catalytic reaction. For example, the translation of mRNA to proteins in ribosomes, which is one of the key processes in biology, is catalyzed by peptidic transferase, which extends the peptide chain by linking a new amino-acid in the form of aminoacyl-tRNA with an N-terminus of the peptide through an amide bond [9,10]. To date, many diﬀerent methods have been developed to enable the formation of various amide linkages in synthetic laboratories. In particular, arguably the biggest drive to investigate these methods came from the success of solid-phase peptide synthesis (SPPS) [11]. Nevertheless, most of them require an active form of carboxylic acid (formed either in a separate synthetic step or in situ, for example, in a reaction with a so-called “coupling reagent”), which is subsequently attacked by a nucleophile in the next step [12,13]. Moreover, common coupling reagents (e.g., carbodiimides, phosphonium, or uronium salts [14–16]) are often rather expensive, toxic, and are used in stoichiometric quantities

Molecules 2020, 25, 1761; doi:10.3390/molecules25081761 www.mdpi.com/journal/molecules Molecules 2020, 25, 1761 2 of 9 or in excess, leading to a poor atom economy. This, in combination with the need for purification from various by-products, makes these methods challenging and costly, pushing many scientific groups to investigate faster, more straightforward, and therefore “greener” routes for the formation of amide bonds. Green chemistry focuses on designing products and processes to minimize the environmental and economic impact. This can be achieved by a) avoiding difficult to obtain and hazardous starting materials and additional reactants, b) increasing the efficiency of reactions and minimizing by-product formation (e.g., the use of selective catalysts), c) reducing energy consumption, and d) minimizing hazardous waste (e.g., using safer or even no solvent and ensuring facile isolation). In this context, an ideal amide bond formation reaction would be:

Directly between carboxylic acids and amines and would not require the isolation of intermediates; • Catalytic and atom efficient (e.g., no coupling reagents); • Waste free (e.g., no solvents and minimal purification); and • Fast (no extended reaction times) [17,18]. • Due to the ecological and economic advantage of obtaining amides directly from carboxylic acids and amines, several groups have explored this possibility in recent years. The resulting methods include, for example, reflux in toluene in the presence of a Zr(IV) catalyst [19]. A similar transformation was achieved at a lower temperature (reflux of diethyl ether/toluene) in the presence of molecular sieves, but it also required an expensive catalyst (Hf(IV) or Zr(IV)) and long reaction times measured in days [20]. Microwave-assisted rapid organic reactions are an attractive green technology with great potential to meet many of the above-mentioned criteria [21]. It has been noticed that microwave synthesis increases the yield of the reaction, reduces the formation of wasteful by-products, increases the reproducibility of the reaction, and reduces reaction times. It allows also for the use of solid substrates (provided an adequate melting point of at least one reagent) without the requirement of their previous dissolution [22–24]. Consequently, microwave radiation was recently used for the synthesis of primary amides from carboxylic acids and urea in the presence of catalytic amounts of cerium ammonium nitrate [25,26], as well as for direct amidation, allowing for shortening of the reaction times and more energy-efficient heating [27,28]. However, these methods require a solvent and the presence of near-stoichiometric amounts of TiO2 (33.3 wt%) or excess silica gel, which facilitate the transformation, contributing to significant waste generation. In addition, in order to isolate products, column chromatography was needed.

2. Results and Discussion In this paper, we report a microwave-assisted synthetic protocol for the preparation of amides directly from carboxylic acids and amines in very good to excellent yields at reaction times signiﬁcantly shorter (one to few hours) than those of many previously reported direct amidations. Our protocol also eliminates the need for a solvent or any additional reagents, a signiﬁcant advantage over previously reported methods. In addition, the reaction occurs in the presence of trace amounts (0.1–2 mol%) of ceric ammonium nitrate (CAN), a common catalyst for microwave reactions [29,30], or even without a catalyst at all (for some substrates) (Scheme1). Pure products were isolated through extraction with ethyl acetate/water mixtures (acidic, basic, or saline) without the need for tedious column chromatography. Additionally, this process separated any traces of the catalyst, enabling the recovery of cerium, and minimizing any potential environmental threat and increasing the economics of the process. As such, this methodology is a major leap forward in the endeavor of developing an ideal green synthesis of amide bonds. Molecules 2020, 25, x FOR PEER REVIEW 2 of 9 stoichiometric quantities or in excess, leading to a poor atom economy. This, in combination with the need for purification from various by-products, makes these methods challenging and costly, pushing many scientific groups to investigate faster, more straightforward, and therefore “greener” routes for the formation of amide bonds. Green chemistry focuses on designing products and processes to minimize the environmental and economic impact. This can be achieved by a) avoiding difficult to obtain and hazardous starting materials and additional reactants, b) increasing the efficiency of reactions and minimizing by- product formation (e.g., the use of selective catalysts), c) reducing energy consumption, and d) minimizing hazardous waste (e.g., using safer or even no solvent and ensuring facile isolation). In this context, an ideal amide bond formation reaction would be: • Directly between carboxylic acids and amines and would not require the isolation of intermediates; • Catalytic and atom efficient (e.g., no coupling reagents); • Waste free (e.g., no solvents and minimal purification); and • Fast (no extended reaction times) [17,18]. Due to the ecological and economic advantage of obtaining amides directly from carboxylic acids and amines, several groups have explored this possibility in recent years. The resulting methods include, for example, reflux in toluene in the presence of a Zr(IV) catalyst [19]. A similar transformation was achieved at a lower temperature (reflux of diethyl ether/toluene) in the presence of molecular sieves, but it also required an expensive catalyst (Hf(IV) or Zr(IV)) and long reaction times measured in days [20]. Microwave-assisted rapid organic reactions are an attractive green technology with great potential to meet many of the above-mentioned criteria [21]. It has been noticed that microwave synthesis increases the yield of the reaction, reduces the formation of wasteful by-products, increases the reproducibility of the reaction, and reduces reaction times. It allows also for the use of solid substrates (provided an adequate melting point of at least one reagent) without the requirement of their previous dissolution [22–24]. Consequently, microwave radiation was recently used for the synthesis of primary amides from carboxylic acids and urea in the presence of catalytic amounts of cerium ammonium nitrate [25,26], as well as for direct amidation, allowing for shortening of the reaction times and more energy-efficient heating [27,28]. However, these methods require a solvent and the presence of near-stoichiometric amounts of TiO2 (33.3 wt%) or excess silica gel, which facilitate the transformation, contributing to significant waste generation. In addition, in order to isolate products, column chromatography was needed.

Scheme 1. Direct amidation from carboxylic acid and amine as a substrate.

This robust reaction was investigated in more detail to evaluate the role of the environmental conditions and chemical components of the process. The aim of these eﬀorts was to maximize the reaction eﬃciency, while minimizing the time and energy needed for the reaction to occur, making this transformation as “green” as possible. We selected p-toluidine and an appropriate acid as model substrates and varied the concentration of the catalyst and reaction conditions. A summary of the results of these experiments is provided in Tables1–3. The optimal conditions of the reactions for each acid substrate are marked in red.

Table 1. Optimization of the reaction conditions for phloretic acid and p-toluidine.

Entry Catalyst t [h] T [◦C] Yield [%] 1 2 mol% 2 60–65 trace 2 2 mol% 2 80–85 30 3 2 mol% 2 100–105 88 4 2 mol% 2 120–125 95 5 2 mol% 2 160–165 94 6 2 mol% 0.5 120–125 52 7 none 2 120–125 71 8 none 5 120–125 85 9 0.1 mol% 2 120–125 79 10 0.1 mol% 3 120–125 87 11 0.1 mol% 5 120–125 93

Table 2. Optimization of the reaction conditions for phenylacetic acid and p-toluidine.

Entry Catalyst t [h] T [◦C] Yield [%] 1 none 2 120–125 73 2 none 2 160–165 87 3 2 mol% 2 120–125 71 4 2 mol% 2 160–165 98

Table 3. Optimization of the reaction conditions for benzoic acid and p-toluidine.

Entry Catalyst t [h] T [◦C] Yield [%] 1 2 mol% 2 80–85 trace 2 2 mol% 2 100–105 trace 3 2 mol% 2 120–125 18 4 2 mol% 2 140–145 45 5 2 mol% 2 160–165 75 6 2 mol% 5 160–165 95 7 none 2 160–165 trace 8 0.1 mol% 2 160–165 trace

Initially, for phloretic acid, reactions were performed with ceric ammonium nitrate (2 mol%) as the catalyst and without any solvent, with temperatures varying from 60–65 ◦C to 160–165 ◦C in 20-◦C intervals (Table1, entries 1–5). As shown in Table1, an improvement in the yield of the reaction (assuming the same reaction times and other conditions being constant) was observed with an increasing temperature until 120–125 ◦C, above which no further difference could be noticed. Next, a decrease in reaction times was investigated (Table1, entry 4 and 6), showing that 2 h is the optimal Molecules 2020, 25, 1761 4 of 9 time for this particular reaction. We also tested the influence of the amount of catalyst on the yield of the reaction (Table1, entry 7–11) and showed, remarkably, that even in its absence, the reaction occurs in very good yields, which can be further improved by increasing the reaction times. In fact, the same yield can be obtained with 0.1 mol% of CAN and 5 h of reaction as with 2 mol% CAN in 2 h. Therefore, it is now up to the chemist to decide whether, for his or her needs, it is better to continue a reaction for longer but avoid the use of a catalyst, or reduce the reaction time by half or more by adding a catalyst. Similar behavior was observed for another alkyl-carboxylic acid, phenylacetic acid (Table2). The reaction was shown to yield the desired product without the use of a catalyst within 2 h, and an increase of the temperature from 120–125 ◦C to 160–165 ◦C further improved the yield. However, in order to ensure near-quantitative transformation of the substrates to the products, the reaction had to be performed in the presence of 2 mol% of a catalyst for 2 h and in 160–165 ◦C (higher than in the case of phloretic acid). We also used an aryl-carboxylic acid, benzoic acid, and performed a similar screen of the conditions (Table3). We found that in order to obtain near-quantitative yields similar to those with phloretic acid, 2 mol% of catalyst and a temperature of 160–165 ◦C (as in the case of phenylacetic acid) was required, together with a prolonged (5 h) reaction time. Interestingly, for this substrate, the lack of a catalyst (or trace amounts of it) inhibited the reaction (unlike in the case of the alkyl-carboxylic acids described above). This indicates that reactions with benzoic acid derivatives occur significantly slower, but the conditions can be adapted to obtain higher yields. To further investigate the role of CAN in this reaction, we monitored its progress at different time points with NMR (the reaction between benzoic acid and benzylamine was chosen as a model; Figure S1). Initially, no reaction occurs if catalyst is not present and the reaction mixture does not heat up in a microwave (i.e., does not reach the desired temperature). In the presence of CAN, signals from the desired product (amide) start to appear already after 30 min from the beginning of the reaction (according to the NMR; Figure S1), reaching the maximum in 2 h. Further heating does not lead to any change. This confirms the important role of CAN in the reaction of an amine with benzoic acid under our experimental conditions, as also reported previously for other amide bond formation protocols. Nevertheless, to date, the exact mechanism of its involvement remains the subject of debate. One possible hypothesis could point to the catalytic mechanism in which cerium(IV) ion in CAN acts as a Lewis acid or indirectly leads to the formation of a Brønsted acid [31]. It is also possible to postulate an additional role of CAN as an acidic catalyst through the “liberation” of nitric acid, acting analogously to carboxylic acid in a mechanism proposed by Charville et al. [32]. In either case, acidic catalysis, whether involving the activation of carbonyl carbon in carboxylic acid and/or the formation of a complex acting as a better leaving group, seems to be the most plausible explanation. Nevertheless, deeper analysis of this mechanism would require significant experimental and theoretical effort, which is outside the scope of this manuscript, but could constitute a separate publication in the future. To further validate the protocol, we selected a range of aromatic (anilines) and aliphatic (benzylamines and alkylamines) amines, which differ in their nucleophilicity (with phenyl rings bearing H, Me, OMe, or F substituents). Benzoic acid and phloretic acid were used as models of aromatic and aliphatic carboxylic acids, respectively. The latter also bears a phenolic group, demonstrating the compatibility of the protocol with such functionalities. In addition, phloretic acid has been reported, together with its corresponding primary amide, as a new promising bioactive scaffold, but the number of its derivatives remains limited [33]. The results of these experiments are summarized in Table4. In order to enable a direct comparison of the yields of these reactions between different acid substrates, we decided to select identical intermediate reaction conditions for all experiments, which were as follows: 160–165 ◦C, 2 h, 2 mmol (1 equivalent (eq) of carboxylic acid and 2.1 eq of amine, 2 mol% of a catalyst. In the case of the reactions of benzylamine and p-methylbenzylamine with phloretic acid as well as putrescine (diamine) with both acids, it was found that the use of excess acid was preferred to ensure full conversion of the amine (Supplementary Materials). This allowed for an extraction-based purification, which otherwise Molecules Molecules2020, 25, x 2020 FOR, 25PEER, x FOR REVIEW PEER REVIEW 4 of 9 4 of 9 for longerfor butlonger avoid but the avoid use theof a use catalyst, of a catalyst, or reduce or thereduce reaction the reaction time by timehalf byor morehalf or by more adding by addinga a catalyst.catalyst. Similar behaviorSimilar behavior was observed was observed for another for anotheralkyl-carboxylic alkyl-carboxylic acid, phenylacetic acid, phenylacetic acid (Table acid 2). (Table The 2). The reactionreaction was shown was toshown yield to the yield desired the desiredproduct product without without the use theof ause catalyst of a catalystwithin 2within h, and 2 anh, and an increaseincrease of the temperature of the temperature from 120–125 from 120–125 °C to 16 °C0–165 to 16 °C0–165 further °C improvedfurther improved the yield. the However, yield. However, in in order toorder ensure to near-quantitativeensure near-quantitative transformation transformation of the substrates of the substrates to the products, to the products, the reaction the reaction had had to be performedto be performed in the presence in the presence of 2 mol% of 2of mol% a catalyst of a catalystfor 2 h and for 2in h 160–165 and in 160–165 °C (higher °C (higherthan in thethan in the case of phloreticcase of phloretic acid). acid). We alsoWe used also an used aryl-carboxylic an aryl-carboxylic acid, benzoic acid, benzoicacid, and acid, performed and performed a similar a similarscreen ofscreen the of the conditionsconditions (Table 3). (Table We found 3). We that found in orderthat in to order obtain to near-quantitative obtain near-quantitative yields similar yields tosimilar those to with those with phloreticphloretic acid, 2 mol% acid, 2of mol% catalyst of catalystand a temperature and a temperature of 160–165 of 160–165 °C (as in °C the (as case in theof phenylacetic case of phenylacetic acid) acid) was required,was required, together together with a prolonged with a prolonged (5 h) reaction (5 h) reaction time. Interestingly, time. Interestingly, for this substrate,for this substrate, the lack the lack of a catalystof a catalyst(or trace (or amounts trace amounts of it) inhibited of it) inhibited the reaction the reaction (unlike in(unlike the case in the of thecase alkyl-carboxylic of the alkyl-carboxylic acids describedacids described above). above).This indicates This indicates that reactions that reactions with benzoic with benzoicacid derivatives acid derivatives occur significantly occur significantly slower, slower,but the butconditions the conditions can be adaptedcan be adapted to obtain to higher obtain yields.higher Toyields. further To investigatefurther investigate the role theof role of CAN inCAN this reaction,in this reaction, we monitored we monitored its progress its progress at different at different time points time withpoints NMR with (the NMR reaction (the reaction betweenbetween benzoic benzoicacid and acid benzylamine and benzylamine was chosen was chosenas a model; as a model;Figure S1).Figure Initially, S1). Initially, no reaction no reaction occurs ifoccurs catalyst if catalystis not present is not presentand the andreaction the reaction mixture mi doesxture not does heat not up heatin a microwaveup in a microwave (i.e., does (i.e., does not reachnot the reach desired the desiredtemperature). temperature). In the presence In the presence of CAN, of signals CAN, fromsignals the from desired the desiredproduct product (amide) (amide) start to appearstart to alreadyappear alreadyafter 30 aftermin from 30 min the from beginn theing beginn of theing reaction of the reaction (according (according to the NMR; to the Figure NMR; Figure S1), reachingS1), reaching the maximum the maximum in 2 h. Furtherin 2 h. Furtherheating heatingdoes not does lead not to anylead change.to any change.This confirms This confirms the the importantimportant role of roleCAN of in CAN the reactionin the reaction of an amine of an withamine benzoic with benzoicacid under acid ourunder experimental our experimental conditions,conditions, as also reportedas also reported previously previously for other for amide other bond amide formation bond formation protocols. protocols. Nevertheless, Nevertheless, to to date, thedate, exact the mechanism exact mechanism of its involvement of its involvement remains remain the subjects the ofsubject debate. of Onedebate. possible One possible hypothesis hypothesis could pointcould to point the catalyticto the catalytic mechanism mechanism in which in cerium(IV)which cerium(IV) ion in CANion in acts CAN as aacts Lewis as a acid Lewis or acid or indirectlyindirectly leads to leadsthe formation to the formation of a Brønsted of a Brønsted acid [31]. acid It is [31]. also It possible is also possible to postulate to postulate an additional an additional role of roleCAN of as CAN an acidic as an catalystacidic catalystthrough through the “liberation” the “liberation” of nitric of acid, nitric acting acid, analogouslyacting analogously to to carboxyliccarboxylic acid in acida mechanism in a mechanism proposed proposed by Charville by Charville et al. [32]. et al.In [32].either In case, either acidic case, catalysis, acidic catalysis, whetherwhether involving involving the activation the activation of carbonyl of carbonyl carbon incarbon carboxylic in carboxylic acid and/or acid theand/or formation the formation of a of a complexcomplex acting as acting a better as aleaving better leavinggroup, seemsgroup, to seems be the to most be the plausible most plausible explanation. explanation. Nevertheless, Nevertheless, deeper deeperanalysis analysis of this mechanismof this mechanism would requirewould requiresignificant significant experimental experimental and theoretical and theoretical effort, effort, which iswhich outside is outsidethe scope the of scope this manuscript, of this manuscript, but could but constitute could constitute a separate a separate publication publication in the future. in the future. To furtherTo furthervalidate validate the protocol, the protocol, we selected we selected a range a ofrange aromatic of aromatic (anilines) (anilines) and aliphatic and aliphatic (benzylamines(benzylamines and alkylamines) and alkylamines) amines, amines,which differwhich in differ their innucleophilicity their nucleophilicity (with phenyl (with phenylrings rings bearing bearingH, Me, H,OMe, Me, or OMe, F substitu or F substituents). Benzoicents). Benzoic acid and acid phloretic and phloretic acid were acid used were as usedmodels as modelsof of aromaticaromatic and aliphatic and aliphatic carboxylic carboxylic acids, respectiacids, vely.respecti Thevely. latter The also latter bears also a bearsphenolic a phenolic group, group, demonstratingdemonstrating the compatibility the compatibility of the protocol of the protocol with such with functionalities. such functionalities. In addition, In addition, phloretic phloretic acid acid has beenhas reported, been reported, together together with its withcorresponding its corresponding primary primary amide, asamide, a new as promisinga new promising bioactive bioactive scaffold,scaffold, but the numberbut the number of its derivatives of its derivatives remains remains limited [33].limited [33]. The resultsThe resultsof these of experiments these experiments are summarized are summarized in Table in 4.Table In order 4. In toorder enable to aenable direct a direct comparisoncomparison of the yields of the of yields these of reactions these reactions between between different different acid substrates acid substrates, we decided, we decided to select to select identicalidentical intermediate intermediate reaction reaction conditions conditions for all experiments, for all experiments, which were which as werefollows: as follows: 160–165 160–165 °C, 2 °C, 2 h, 2 mmolh, 2(1 mmol equivalent (1 equivalent (eq) of carboxylic (eq) of carboxylic acid and acid 2.1 andeq of 2.1 amine, eq of 2amine, mol% 2of mol% a catalyst. of a catalyst. In the case In the case of the reactionsof theMolecules reactions of benzylamine2020, 25of, 1761benzylamine and p-methylbenzylamine and p-methylbenzylamine with phloretic with phloretic acid as wellacid as putrescinewell as putrescine 5 of 9 (diamine)(diamine) with both with acids, both it acids, was foundit was thatfound the that use theof excessuse of acidexcess was acid preferred was preferred to ensure to fullensure full conversionconversion of the ofamine the amine(Supplementary (Supplementary Materi als).Materi Thisals). allowed This allowed for an forextraction-based an extraction-based purification,purification,(in which case ofotherwise which standard otherwise (in protocol) case (inof requiredstandardcase of standard extraprotocol) column protocol) requir chromatographyed requir extraed column extra for columnchromatography the separation chromatography of the mono for the separationfor (benzylamines)the separation of the mono of and the (benzylami diamide mono (benzylami (putrescine)nes) andnes) diamide products. and diamide (putrescine) (putrescine) products. products.

Table 4. TableSummaryTable 4. Summary 4.of Summaryamides of synthesized amides of amides synthesized within synthesized the within scope within the of scope thethis scopework. of this of thiswork. work.

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21 85 22 98 21 85 22 98 21 85 22 98 21 85 22 98 21 85 22 98 232121 968585 242222 889898 21 2323 21 85 9696 85 22 2424 22 98 8888 98 21 232121 96858585 242222 22 889898 98 23 96 24 88 23 96 24 88 2323 9696 2424 8888 Analysis of the reaction yields with23 23 respect 23 to the96 chemical96 96 character24 24 of 24the substrates88 88 points88 to AnalysisAnalysis ofof thethe reactionreaction yieldsyields withwith23 23 respectrespect toto thethe chemicalchemical9696 chcharacteraracter24 of of the24the substratessubstrates88 pointspoints88 toto someAnalysis interesting of thetrends. reaction The yieldshighest with yields respect (above to the 90%)90%) chemical werewere observedobservedcharacter forforof the thethe substrates reactionsreactions pointsbetweenbetween to somesomeAnalysis interestinginteresting of the trends.trends. reaction TheThe yields highesthighest with yieldsyields respect (above(above to the 90%)90%) chemical werewere observedchobservedaracter foroffor the thethe substrates reactionsreactions pointsbetweenbetween to phloreticsomesomeAnalysisAnalysis interestinginteresting acid ofandof the the trends.trends. anilines, reactionreaction TheThe with yields yields highesthighest little withwith effect yieldsyields respectrespect of (above(above the toto electron-withdrawing/electron-donating thethe 90%)90%) chemicalchemical werewere observedchobservedcharacteraracter offorforof thethe thethe substratessubstrates reactionsreactions character pointspointsbetweenbetween toto phloreticsomephloreticAnalysisAnalysis interesting Analysisacidacid of the andofand thereactiontrends. anilines,ofanilines, reactionthe reaction Theyields withwith yieldshighest withlittlelittle yields with effectrespecteffectyields with respect ofof (abovetorespect thethe the to electron-withdrawing/electron-donatingelectron-withdrawing/electron-donating thechemical 90%) to chemicalthe were chemical ch aracterobserved character ch ofaracter the forof thesubstratesthe of substrates thereactions substrates points character characterpointsbetween to points to to ofsomephloreticsome theAnalysis interestinginterestinganalogous acid ofand the aminetrends.trends. anilines, reaction (the TheThe with somewhat yields highesthighest little with effectyieldsyields higher respect of (above(above the yields to electron-withdrawing/electron-donating the 90%)90%) for chemical pwerewere-methoxyaniline-methoxyaniline observedobservedcharacter foroffor overover the thethe substrates preactionsreactions-fluoroaniline-fluoroaniline character pointsbetweenbetween areare to someofphloreticsomeof theinterestingthesome interestinganalogousanalogous acid interestingAnalysis andtrends. aminetrends.amine anilines, The oftrends. (the the(theThe highest with reaction somewhat somewhatThehighest little highestyields yields effectyields higherhigher (above yields of with (above the yieldsyields respect90%) (aboveelectron-withdrawing/electron-donating 90%) for forwere to 90%) ppwere-methoxyaniline the-methoxyaniline observed chemicalwere observed observed for character forthe overover thereactionsfor p preactions ofthe-fluoroaniline-fluoroaniline the reactions between substrates character between between areare points to notphloreticsomeofphloretic the significant interestinganalogous acidacid andand enough trends.amine anilines,anilines, for (theThe with with reliable somewhathighest littlelittle conclusions). effecteffectyields higher ofof (above thethe yields electron-withdrawing/electron-donatingelectron-withdrawing/electron-donating The 90%) for yields pwere-methoxyaniline-methoxyaniline wereobserved similar for over over forthe pthereactions-fluoroaniline-fluoroaniline phloretic character characterbetween acid areare phloreticnotofphloreticnot the significantphloreticsignificant acidanalogoussome acid and interestingacidand anilines,enoughenough amine andanilines, anilines, with(theforfor trends. with reliable reliablesomewhatlittle with little Theeffect conclusions).little conclusions).effect highest higherof effect the of yieldstheelectron-withdrawing/electron-donating yields of electron-withdrawing/electron-donating theTheThe (above forelectron-withdrawing/electron-donating yieldsyields p-methoxyaniline 90%) werewere were similarsimilar observed over forfor pthethe for-fluoroaniline phloretic thephloretic character reactions character characteracid acid are between reactionsofphloreticnotof thethe significant analogousanalogous acidwith and alkylaminesenough amineamine anilines, (thefor(the with and reliablesomewhatsomewhat benzyliclittle conclusions).effect higherhigher amines of the yieldsyields bearing electron-withdrawing/electron-donating The forfor yields electron-withdrawingpp-methoxyaniline-methoxyaniline were similar overover for groups, ppthe-fluoroaniline-fluoroaniline phloretic while character other acid areare of thereactionsnotofreactions the analogoussignificantof analogousthephloretic withwith analogous amine alkylaminesalkylaminesenough acidamine (the andamine for(the somewhat anilines, andand reliable (thesomewhat benzylicbenzylic somewhat with higherconclusions). littlehigher aminesamines yieldshigher eff ectyields bearingbearing offor Theyields the forp -methoxyanilineyields electron-withdrawing electron-withdrawingelectron-withdrawing pfor-methoxyaniline pwere-methoxyaniline similar over over for/pelectron-donating-fluoroaniline groups, groups,over thep-fluoroaniline phloreticp-fluoroaniline whilewhile are other otheracid character are are of benzylicnotofreactionsnotreactions the significantsignificant analogous amineswithwith alkylaminesalkylaminesenoughenough gaveamine lower (theforfor andand reliablereliable somewhatyields. benzylicbenzylic conclusions).conclusions).The higher amines aminestendency yields bearingbearing TheThe forfor for yieldsyields higher higherelectron-withdrawingelectron-withdrawingp-methoxyaniline were were yieldsyields similarsimilar withwith over benzylaminesforbenzylaminesfor groups, groups, pthethe-fluoroaniline phloreticphloretic whilewhile bearingbearing other otheracidacid are notbenzylicreactions notbenzylicsignificant notsignificant the significant amineswithamines analogousenough alkylamines enough gavegave enoughfor aminelower lower forreliable and reliable for (theyields.yields. benzylic reliableconclusions). somewhat conclusions).TheThe amines conclusions). tendencytendency higher The bearing The yields yieldsforfor The yields higherhigherelectron-withdrawing for wereyieldsp -methoxyaniline were yields yieldssimilar were similar withwith similarfor benzylaminesforbenzylaminesthe groups, over theforphloretic pphloreticthe-fluoroaniline while phloretic bearingacidbearing other acid acid are not electron-poorreactionsnotbenzylicreactions significant amineswithwith phenyl alkylaminesalkylaminesenough gave rings lowerfor overandandreliable yields. benzylicbenzylicelectron-rich conclusions).The aminesamines tendency analog bearingbearing The ues,forfor yields higherobservedhigherelectron-withdrawingelectron-withdrawing were yieldsyields for similar phloretic withwith forbenzylaminesbenzylamines groups,acid,groups, the translatesphloretic whilewhile bearingbearing other other acidalso reactionselectron-poorbenzylicreactionselectron-poorreactions withsignificant amineswith alkylamines phenylwithphenyl alkylamines gave enoughalkylamines ringsrings lower and foroveroverand benzylic yields. reliable and electron-richbenzylicelectron-rich benzylic Theamines conclusions). amines tendency analogbearingaminesanalog bearing ues, Theues,for bearingelectron-withdrawing higherobserved yieldsobservedelectron-withdrawing electron-withdrawing wereyields forfor similar phloretic phloreticwith groups, forbenzylamines acid,groups, theacid, phloreticgroups, while translatestranslates while other bearingwhile acid other alsoalso reactionsother tobenzylicreactionstoelectron-poorbenzylic reactionsreactions aminesamineswith withwith phenyl alkylamines gave gavebenzoicbenzoic rings lowerlower acid.acid.overand yields.yields. benzylicelectron-richInIn general,general, TheThe amines tendencytendency reactions reactionsanalog bearing ues,forfor withwith higherhigherobservedelectron-withdrawing benzoicbenzoic yieldsyields for acidacid phloretic withwith gavegave benzylaminesbenzylamines groups,acid,significantlysignificantly translates while bearingbearing lowerlowerother also benzylictoelectron-poorbenzylicto reactionsreactionsbenzylic amineswith amines with alkylamineswith aminesphenylgave gave benzoicbenzoic lower ringsgave lower and yields.acid.overacid.lower yields. benzylic electron-richInIn Theyields. general,general, The tendency amines Thetendency reactions reactionsanalog tendency bearing for ues, forhigher withwith electron-withdrawing observedhigherfor benzoicyieldsbenzoichigher yields for with yieldsacid acidphloretic with benzylamines gave gavewith benzylamines groups, acid,significantly significantlybenzylamines translates while bearing bearing otherlowerlower also bearing benzylic (althoughelectron-poorbenzylic(althoughtoelectron-poorto reactionsreactions amines stillstill withwith phenylphenylsatisfactory)satisfactory) gave benzoicbenzoic ringsrings lower yieldsyieldsoveracid.acid.over yields. electron-richelectron-rich In Inthanthan general,general, The inin thethe tendency casecase reactions reactionsanaloganalog ofof phloreticphloreticues,ues,for withwith higherobservedobserved benzoic benzoic acid.acid. yields TheforThefor acid acid phloretic phloretic higherhigherwith gavegave benzylamines yieldsyields acid,acid,significantlysignificantly ofof translatestranslates thisthis reactionreactionbearing lowerlower alsoalso electron-poor(althoughtoelectron-poor(although reactionselectron-pooramines stillstillphenyl with satisfactory)phenylsatisfactory) gave ringsphenylbenzoic lower rings over rings yieldsacid. yields.yieldsover electron-rich over electron-rich In thanthan The general, electron-rich inin tendency thethe analog casereactionscase analog forues, of ofanalog phloretic higherphloreticues, observed with ues,observed yields benzoic acid.observedacid. forwith ThephloreticforThe acid phloretic higher benzylaminesforhigher gavephloretic acid, yieldsyields acid,significantly translates acid, of oftranslates bearing thisthis translates reaction reactionalso lower electron-poor also also fortoelectron-poorfor(althoughto(although reactions reactionsalkylalkyl stilloverstillover withwith phenyl satisfactory)satisfactory) aromaticaromatic benzoicbenzoic rings carboxylic carboxylic yieldsacid.yieldsoveracid. electron-richIn In thanthan general,general, acidsacids inin thethe were werecasereactions casereactionsanalog ofofconfirmedconfirmed phloreticphloreticues, withwith observed benzoicbenzoic acid.acid.byby additional additional forTheThe acidacid phloretic higherhigher gavegave experiments experimentsyieldsyields acid,significantlysignificantly ofoftranslates thisthis reactionwithwithreaction lowerlower also 4-4- to reactionsfor(althoughtofor reactionsalkyltoalkyl reactionsphenyl withstilloverover with satisfactory) benzoic ringsaromatic aromaticwith benzoic over benzoicacid. carboxylic electron-richcarboxylicyieldsacid. In acid. general, Inthan general, Inacids acidsin analogues, general, thereactions werecasewerereactions ofreactions confirmed confirmed observedwithphloretic with benzoic with benzoic acid.forbyby phloreticbenzoic acid additionaladditionalThe acid highergave acidgave acid, significantly experiments yieldsexperimentsgave translatessignificantly ofsignificantly this lower alsoreactionwithwith lower to 4-4-lower reactions methylpentanoic(althoughtofor(althoughfor reactions alkylalkyl stillstilloverover with satisfactory)satisfactory) aromaticaromatic acid benzoic and phenylacetic carboxyliccarboxylic yieldsacid.yields In thanthan general, acids acidsinin acid, thethe werecasewereeachcasereactions of ofwithconfirmedconfirmed phloreticphloretic onewith aniline benzoic acid.acid.byby additional additional(anisidine)TheThe acid higherhigher gave andexperimentsyieldsexperimentsyields significantly one ofof benzylamine thisthis reactionreactionwithwith lower 4-4- (althoughmethylpentanoicfor(althoughmethylpentanoic alkyl(although stillwith stillover satisfactory) benzoic satisfactory)still aromatic acidacid satisfactory) acid. andand yields phenylaceticcarboxylic phenylaceticyields In than general, yields than in theacidsthan in reactionsacid,acid, thecase in were eachcase eachthe of phloreticwith case ofwithwithconfirmed phloretic of benzoic oneone phloretic acid. anilineaniline acid.by acidThe acid. additional (anisidine)The(anisidine) higher gave Thehigher significantly yields higher andexperimentsandyields of oneyieldsone this of benzylaminelower benzylaminethis reaction of withreactionthis (although reaction 4- still (for(although(methylpentanoicforp-fluorobenzylamine)-type-fluorobenzylamine)-type alkylalkyl stilloverover satisfactory) aromaticaromatic acid and carboxylic phenylaceticcarboxylicyields aminesamines than (Table(Table acids acidsin acid, the 4,4, were bottomwerebottomcaseeach of withconfirmedconfirmed phloreticenen tries).tries).one aniline Indeed, Indeed, acid.byby additional additionalThe(anisidine) bothboth higher acidacid experimentsandexperimentsyields substratessubstrates one of benzylaminethis gavegave reactionwithwith veryvery 4-4- for (methylpentanoicfor(palkylp-fluorobenzylamine)-type-fluorobenzylamine)-type foralkyl over satisfactory)alkyl over aromatic over aromatic acid yieldsaromatic andcarboxylic phenylaceticcarboxylic than aminesamines carboxylic in acids the (Table(Table acids case acid, were 4,acids4, of bottomwereeachbottom phloretic confirmed were withconfirmed enen tries). acid.onetries).confirmed byaniline The Indeed, Indeed, byadditional higher additional(anisidine)by both bothadditional yields acidacidexperiments andexperiments of substratessubstrates this oneexperiments reaction benzylamine with gavegave with for 4- veryverywith alkyl4- 4- over highmethylpentanoicfor(methylpentanoic(p-fluorobenzylamine)-type-fluorobenzylamine)-type alkyl yields over of the aromatic acid acidreactions, andand phenylaceticcarboxylicphenylacetic similaraminesamines to (Table(Table theacids acid,acid, ones 4,4, werebottomeachbottomeach observed withwithconfirmed enen tries).onetries).one for anilineanilinephloretic Indeed,Indeed,by additional (anisidine)(anisidine) acid.bothboth acidacid andexperimentsand substratessubstrates oneone benzylaminebenzylamine gavegave with veryvery 4- methylpentanoichigh(methylpentanoichighp-fluorobenzylamine)-type methylpentanoic yieldsyieldsaromatic ofof theacidthe carboxylic acidreactions,reactions, and acidand phenylacetic andphenylacetic acidssimilaraminessimilar phenylacetic were to(Tableto acid, thethe confirmed acid, oneseach ones4, acid, bottomeach observedwithobserved byeach with oneadditionalen withtries). one anilineforfor phloreticanilineonephloretic Indeed, experiments (anisidine)aniline (anisidine) bothacid.acid. (anisidine) acidand with and substratesone 4-methylpentanoic andone benzylamine benzylamineone gave benzylamine very acid and (methylpentanoichigh(pp-fluorobenzylamine)-type-fluorobenzylamine)-type Whileyields aof variation the acidreactions, andin the phenylacetic aminessimilaramines yields to(Table(Tableof thethe acid, ones reactions4,4, bottombottomeach observed with s eneneems tries).tries).one for to anilinephloretic Indeed,Indeed,stem from(anisidine) bothacid.both the acid acid electronic and substratessubstrates one propertiesbenzylamine gavegave veryvery of (p-fluorobenzylamine)-typehigh(p-fluorobenzylamine)-type (WhileyieldsWhilep-fluorobenzylamine)-type aofa variationvariation the reactions, inin amines thethe similaramines yieldsyields (Tableamines to of(Tableof the the the4, (Table bottomones reactions4,reactions bottom observed4, enbottom tries). s seneemseemstries). for en Indeed, toto tries).phloretic Indeed,stemstem Indeed,both fromfrom acid.both acid thethe both acid substrates electronicelectronic acidsubstrates substrates gavepropertiesproperties gave very gave very ofof very thehigh(thehighp-fluorobenzylamine)-type carboxyliccarboxylic Whileyieldsyields aofof variation theacidacidthe reactions,reactions, substratessubstrates in the similaraminessimilar (e.g.,(e.g.,yields preferencepreference to to(Tableof the thethe onesones 4,reactions bottom forfor observedobserved alaliphaticiphatic seneemstries). forfor vs.vs. to phloreticphloretic aromatic), aromatic),Indeed,stem from acid.bothacid. otherother the acid electronic factorsfactors substrates (e.g.,(e.g., properties solubility,solubility,gave very of highthehighthe yields carboxyliccarboxylichighWhile yields of yields the aof variation acidtheacidreactions, of reactions, the substratessubstrates reactions,in similar the similar (e.g.,yields(e.g., tosimilar preferencethepreference toof theonesthe to onesreactionsthe observed for forones observed al aliphatic iphaticobserved s eemsfor phloreticfor vs.vs. to phloretic aromatic),foraromatic),stem phloretic acid. from acid. other otherthe acid. electronic factors factors (e.g.,(e.g., properties solubility,solubility, of conformationalhighthethe carboxyliccarboxylic WhileWhileyields aaof variationvariation theacidacid flexibility reactions, substratessubstrates inin thethe ofsimilar yields(e.g.,(e.g.,yields thethe preference preference to ofofsubstrates,substrates, the thethe ones reactionsreactions forfor observed polarity, polarity,alaliphaticiphatic sseemseems for vs.vs. CAN-complexationCAN-complexationtoto phloretic aromatic),aromatic),stemstem fromfrom acid. other other thethe electronicelectronic factorsfactors capacitycapacity (e.g.,(e.g., propertiesproperties ofsolubility,ofsolubility, amineamine ofof conformationaltheconformationalWhile carboxylicWhile a Whilevariation a variation acid aflexibility flexibilityvariation substrates in the in theyields inofof (e.g., yields the thethe of yields preference the substrates,ofsubstrates, thereactions of reactionsthe for reactions polarity,alspolarity,eemsiphatic seems to s eemsvs.stem CAN-complexationCAN-complexationto aromatic), stem tofrom stem from the otherfrom theelectronic electronic thefactors capacitycapacity electronic properties (e.g., properties solubility,ofof properties amine amineof of of substrates,theconformationaltheconformational carboxyliccarboxylicWhile anda variation acidacid others)flexibilityflexibility substratessubstrates in cannot the ofof (e.g.,yields(e.g., bethethe preferenceexcluded.preference ofsubstrates,substrates, the reactions Importan forfor alpolarity,polarity,aliphaticiphatic seemstly,tly, vs. vs. weCAN-complexationwetoCAN-complexation aromatic),aromatic),stem observedobserved from other other thatthethat electronic factorsininfactors capacityourcapacityour (e.g.,conditions,(e.g.,conditions, properties solubility,solubility,ofof amineamine thethe of the substrates,conformationalthecarboxylicsubstrates, carboxylicthe carboxylic acidandand acid substrates others)others)flexibility acidsubstrates substrates cannotcannot (e.g., of (e.g., preferencebethebe (e.g., excluded. preferenceexcluded.substrates, preference for Importan alImportanfor iphaticpolarity, al foriphatic al vs.tly,tly,iphatic aromatic),vs. weCAN-complexationwe aromatic), vs.observedobserved aromatic), other other thatthat factors other infactorsin capacityourour (e.g., factors conditions,(e.g.,conditions, solubility, (e.g., ofsolubility, amine solubility, thethe reactionconformationalthesubstrates,conformationalsubstrates, carboxylic can andand take acid others)others) flexibilityplaceflexibility substrates even cannotcannot betweenofof (e.g., thebebethe preference excluded. excluded. substrates,substrates,two substrates ImportanforImportan polarity,alpolarity,iphatic,, whichwhichtly,tly, vs. CAN-complexationweareCAN-complexationarewe aromatic), solidssolidsobservedobserved atat room roomother thatthat temperature,temperature,factorsinin capacitycapacityourour (e.g.,conditions,conditions, solubility,of ofprovidedprovided amineamine thethe conformationalreactionsubstrates,conformationalreactionconformational cancan and taketakeflexibility others) placeflexibilityplace flexibility even evencannotof betweenofbetweenthe betheofsubstrates, excluded. the substrates, twotwo substrates, substratessubstrates Importanpolarity, polarity,, , whichwhichpolarity, tly,CAN-complexation we areCAN-complexationare solidsobservedsolidsCAN-complexation atat roomroom that capacity temperature,temperature,in capacityour capacityconditions, of ofamine providedprovided amineof theamine thatsubstrates,conformationalthatreactionsubstrates,reaction thethe melting meltingcancan andand taketake others) others)point pointflexibility placeplace of ofeven even cannotcannot atat least leastofbetweenbetween bethebe oneone excluded. excluded.substrates, twooftwoof themthem substratessubstrates ImportanImportanisis belowbelowpolarity,,, whichwhich thetly,thetly, temperaturetemperature we CAN-complexationweareare observedsolidsobservedsolids atat roomofroomof thatthat thethe temperature, temperature,ininreaction.reaction. capacityourour conditions,conditions, However,However, of providedprovided amine thethe inin substrates,thatreactionsubstrates,that thesubstrates,the andmeltingcanmelting and takeothers) and others)point pointplace others)cannot ofevenof cannot atat leastbetweenleastbecannot excluded.be oneone excluded. be two ofof excluded. themthem substratesImportan Importanisis belowbelow Importan,tly, which the tly,thewe temperature temperaturearewetly,observed solidsobservedwe observed atthat roomofof that theinthe ourtemperature, that inreaction.reaction. our conditions,in ourconditions, However,However, conditions, provided the the inin the suchreactionsubstrates,thatreactionthat thethecases, meltingcanmeltingcan and takethetake others) point yieldspointplaceplace ofeven ofeven seemcannot atat leastbetweenleastbetween to be oneonedrop. excluded. two twoofof Inthemthem substratessubstrates partic Importanisis belowbelowular,,, whichwhich thetly,the temperature temperatureweareyieldsare solidsobservedsolids of atatthe roomroomofof that reactions thethe temperature,temperature, in reaction.reaction. our of conditions, benzoicHowever,However, providedprovided andthe inin reactionsuchthatreactionsuch reaction the cancases,cases, melting cantake the takethecan place pointyields takeyieldsplace even place of even seem seem betweenat evenleast between toto between one drop.twodrop. twoof substrates themInIn twosubstrates particpartic issubstrates below, ular,whichular,, which thethe are, which temperature yieldsaresolidsyields solids are atofof solids room atthethe ofroom at reactionstemperature,reactionsthe room temperature,reaction. temperature, ofof benzoicprovidedHowever,benzoic provided provided andand in phenylaceticthatreactionsuchthatsuch the the cases,cases, meltingcanmelting take thetheacids point pointyieldsplaceyields with ofevenof seemseem atatanisidine leastleastbetween toto oneonedrop.drop. (all twoofof themsubstratesthemInIn substrates particpartic isis belowbelowular, are, which thethesolids) temperature temperatureareyields solidsare ofrelatively atthe roomofof reactionsthethe temperature, reaction.lowerreaction. ofthan benzoicHowever,However, forprovided other and inin thatphenylaceticsuchthatphenylacetic the that themeltingcases, meltingthe the meltingpointacidsacids yieldspoint ofwithwith point at of seem least anisidineatanisidine of least oneatto least one drop.of (all (allthem oneof Insubstratesthemsubstrates of ispartic thembelow is belowular, is aretheare below thethetemperaturesolids)solids) temperatureyieldsthe temperatureareare of relatively relativelyof the the of reactions thereaction. of lower reaction.lowerthe reaction. ofHowever, thanthan benzoicHowever, forfor However, otherin otherand in in reactionssuchthatphenylaceticsuch the cases,cases, melting of thesethe theacids pointyieldsyields acids with of butseemseem atanisidine withleast toto amines,onedrop.drop. (all of themInsubstratesIn which particpartic is arebelowular,ular, areliquid thethesolids) at temperatureyieldsyields room are of oftemperaturerelatively thethe of reactionsreactionsthe reaction.lower (approximately of ofthan benzoicHowever,benzoic for other 50%andand in suchreactionsphenylaceticsuchreactions cases,such cases, theofofcases, these thesetheyieldsacids theyields acidsacids with seemyields butbutseem anisidine to withwithseem drop.to amines,amines, drop. to (allIn drop. particsubstratesIn whichwhich particInular, areparticareular, are liquidtheliquidular, thesolids)yields at attheyields roomroom ofareyields the temperatureoftemperaturerelatively thereactionsof thereactions lowerreactions (approximately(approximatelyof benzoicofthan benzoicof for benzoicand other 50%50%and and vs.phenylaceticsuchreactionsphenylaceticreactions 75% cases, andofof thesethese the acidsacids85% yields acidsacids vs.withwith >90%,butbutseem anisidineanisidine withwith torespectively). amines,amines,drop. (all(all substratesInsubstrates whichwhich partic In arearecontrast,ular, are areliquidliquid thesolids)solids) atattheyields roomroom reactionareare of temperaturetemperaturerelativelyrelatively the of reactions solid lowerlower (approximately(approximately anisidine of thanthan benzoic forfor with otherother 50%50%and 4- phenylaceticvs.reactionsphenylaceticvs. 75%75%phenylacetic andofandacids these acids85%85% with acidsacids vs.vs.with anisidine but>90%,>90%,with anisidine with anisidinerespectively).respectively). (allamines, (allsubstrates (allsubstrates which substratesInIn are contrast,contrast, liquidaresolids) solids)are atthe theare solids)room reaction reactionarerelatively temperature relativelyare relativelyofof lowersolidsolid lower (approximately anisidinethananisidinelower than for than for otherwithwith other for50% 4- 4-other reactionsphenylaceticvs.reactions 75% andofof thesethese acids85% acidsacids vs.with butbut>90%, anisidine withwith respectively). amines,amines, (all substrates whichwhich In are arecontrast, areliquidliquid solids) atatthe roomroom reactionare temperaturetemperaturerelatively of solid lower (approximately(approximately anisidine than for with other 50%50% 4- reactions vs. 75%reactions of theseand of85%acids these vs.but acids >90%,with but amines, respectively). with amines,which are Inwhich liquidcontrast, are at liquid room the atreactiontemperature room temperatureof solid (approximately anisidine (approximately with50% 4- 50% reactionsvs.reactions 75% andofof thesethese 85% acidsacids vs. but>90%,but withwith respectively). amines,amines, whichwhich In arearecontrast, liquidliquid atatthe roomroom reaction temperaturetemperature of solid (approximately(approximately anisidine with 50%50% 4- vs. vs. 75% 75%vs. and 75%and 85% and85% vs. 85% vs.>90%, >90%,vs. respectively).>90%, respectively). respectively). In Incontrast, contrast,In contrast, the thereaction reactionthe reaction of solidof solid of anisidine solid anisidine anisidine with with 4- with 4- 4- vs. 75% and 85% vs. >90%, respectively). In contrast, the reaction of solid anisidine with 4-

Molecules 2020, 25, 1761 6 of 9 phenylacetic acid, each with one aniline (anisidine) and one benzylamine (p-fluorobenzylamine)-type amines (Table4, bottom entries). Indeed, both acid substrates gave very high yields of the reactions, similar to the ones observed for phloretic acid. While a variation in the yields of the reactions seems to stem from the electronic properties of the carboxylic acid substrates (e.g., preference for aliphatic vs. aromatic), other factors (e.g., solubility, conformational flexibility of the substrates, polarity, CAN-complexation capacity of amine substrates, and others) cannot be excluded. Importantly, we observed that in our conditions, the reaction can take place even between two substrates, which are solids at room temperature, provided that the melting point of at least one of them is below the temperature of the reaction. However, in such cases, the yields seem to drop. In particular, the yields of the reactions of benzoic and phenylacetic acids with anisidine (all substrates are solids) are relatively lower than for other reactions of these acids but with amines, which are liquid at room temperature (approximately 50% vs. 75% and 85% vs. >90%, respectively). In contrast, the reaction of solid anisidine with 4-methylpentanoic acid, which is a liquid at (room temperature (rt), led to the isolation of a product in a very high yield (98%). We also confirmed in preliminary experiments that the protocol developed within this work can be applied to secondary amines. However, the optimized conditions of the reaction, together with the purification protocols, require further elaboration, due to the lower polarity of tertiary amides in comparison to secondary ones. This work is in progress and will be reported in the future in a separate manuscript. In order to further extend the utility of our protocol, while maintaining “green” reaction conditions, we investigated the reactivity in the presence of water as an environmentally friendly solvent. In solvent-free conditions, the minimal amount of the substrates required for a reaction to occur was approximately 200 mg (for lower quantities, the reaction mixture could not be efficiently heated up in a microwave reactor). The addition of water allowed us to perform reactions efficiently down to the scale of a few dozens of milligrams, and also enabled amide bond formation between substrates with higher melting points (Figure S3). The yields of these reactions were comparable to the yields obtained with other similar substrates. The biological activity of all prepared amide derivatives is under investigation in 2D cultures of human cells as well as induced pluripotent (iPS) cells as biological models, and the results of these studies, combining a larger library of amide derivatives, will be published elsewhere.

3. Materials and Methods

3.1. Chemistry—General Information All chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and Eurisotop Saint-Aubin, France) (NMR solvents) and used without further purification. 1H NMR and 13C NMR spectra were recorded on an AVANCE III Bruker spectrometer (Bruker Corporation, Billerica, Massachusetts, USA) in DMSO-d6 at frequencies of 500 and 126 MHz, respectively. Spectra were processed using MestReNova Version 9.0.1-13254. NMR data were reported as follows: Chemical shift (δ), multiplicity (recorded as br, broad; s, singlet; d, doublet; t, triplet; q, quadruplet, and m, multiplet), coupling constants (J in Hertz, Hz), and integration. The chemical shifts are expressed in parts per million (ppm) and reported in relation to a residual solvent peak (2.50 and 39.5 ppm for 1H and 13C NMR in DMSO-d6, respectively). High-resolution mass spectrometry (HRMS) was performed on a Q-Exactive Orbitrap mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) equipped with a TriVersa NanoMate robotic nanoflow ESI ion source (Advion BioSciences ltd., Ithaca, NY, USA). The nanoelectrospray chips with a nozzle diameter of 5.5 µm were used in order to obtain a stable ion spray and Chipsoft software (ver. 8.3.1.1018) was applied to control the ESI ion source. Microwave experiments were performed in a commercially available open microwave reactor (Magnum II) from ERTEC-Poland Dr. Edward Reszke, Wroclaw, Poland. Microwave power was smoothly regulated in the range from 0 to 750 W at a field frequency of 2.45 GHz. The temperature was monitored by a Molecules 2020, 25, 1761 7 of 9 pyrometer and by monitoring the vapor temperature using a K-type thermocouple. Reactions were carried out at atmospheric pressure in regular glass flasks equipped with a reflux condenser glass topped with a free airflow tube with a drying agent.

3.2. Synthesis of Amides—General Procedure Amine (4.2 mmol), carboxylic acid (2 mmol), and catalyst (ceric ammonium nitrate, CAN) (2 mol%) were added to an empty flask equipped with a reflux condenser at atmospheric pressure and placed in a microwave set to maintain a constant temperature in the range of 160–165 ◦C for a given time period (microwave power set up to 480 W but was smoothly and automatically controlled by the software to keep the target temperature constant). After 2 h, the reaction mixture was allowed to cool to room temperature, and subsequently, 25 mL of ethyl acetate were added. The organic phase was washed with 3 15 mL of 2 M aqueous HCl, 3 15 mL of saturated aqueous NaHCO , and × × 3 3 15 mL of saturated aqueous NaCl; filtered; and solvent from the organic fraction was removed × under reduced pressure to obtain the pure product. The compounds were fully characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry (HRMS) (data shown in Supplementary Materials—characterization of products (amides)—compounds 1–24). As examples, the NMR spectra and HRMS results for two novel compounds that were obtained within the scope of this work are shown below. A complete set of data for all new compounds not reported previously in the literature (2, 4, 6, 8, 12, 14, 16, 18, 20, 22, and 24) and other synthesized compounds is provided in the Supplementary Materials. 3-(4-hydroxyphenyl)-N-phenylpropanamide (2): Compound 2 was obtained in the form of a solid, following the general procedure described above (yield = 94%, 92% and 96%—3 repeats). 1H NMR (500 MHz, DMSO-d6) δ = 9.86 (s, 1H), 9.15 (s, 1H), 7.62–7.54 (m, 2H), 7.28 (t, J = 7.9 Hz, 2H), 7.06–6.99 (m, 13 3H), 6.70–6.62 (m, 2H), 2.80 (t, J = 7.7 Hz, 2H), 2.56 (t, J = 7.7 Hz, 2H). C NMR (126 MHz, DMSO-d6) δ = 171.0, 155.9, 139.7, 131.7, 129.5, 129.1, 123.4, 119.5, 115.5, 38.9, 30.6. HRMS [M + H]+ calcd for C15H15NO2: 242.1175, found: 242.1154. 3-(4-hydroxyphenyl)-N-(p-tolyl)propanamide (4): Compound 4 was obtained in the form of a solid, following the general procedure described above (yield = 89 and 95%—2 repeats). 1H NMR (500 MHz; DMSO-d6) δ = 9.77 (s, 1H), 9.15 (s, 1H), 7.46 (d, J = 8.2 Hz, 2H), 7.08 (d, J = 8.2 Hz, 2H), 7.03 (d, J = 8.2 Hz, 2H), 6.67 (d, J = 8.4 Hz, 2H), 2.79 (t, J = 8.7, 6.7 Hz, 2H), 2.53 (t, J = 8.7, 6.8 Hz, 2H), 2.23 (s, 3H). 13C NMR (126 MHz, DMSO-d6) δ = 170.8, 155.9, 137.2, 132.3, 131.7, 129.5, 129.5, 119.6, 115.5, 38.9, 30.6, 20.9. + HRMS [M + H] calcd for C16H17NO2: 256.1321, found: 256.1332.

4. Conclusions In conclusion, we developed a cheap, universal, high-yielding, and green method for the direct amidation of carboxylic acids with different amines. The use of microwave energy remarkably facilitates this otherwise very difficult direct transformation, without the need for any coupling reagents. Its versatility in terms of the substrates extends from electron rich to electron poor, aliphatic and aromatic amines and carboxylic acids. This, together with the lack of solvents (except water in some cases) and even catalyst, fast reaction kinetics (significantly faster than under standard conditions where direct amidations with catalyst last for days [18,19]), simplicity of purification, and scalability (from dozens of milligrams to grams), combined with the high atom efficiency puts the reported protocol at the forefront of synthetic methodologies for the sustainable formation of amide bonds.

Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/8/1761/s1, Figure S–Figure S: 1H and 13C NMR and HRMS data of all the compounds. Table S1: MS and 1H NMR Spectra for the analysis of by-products measured at diﬀerent times, compound 9. Table S2: 1H NMR Spectra from the product obtained from the procedure for a reaction in water (on a smaller scale). Table S3: Physical state of substrates at RT. Any synthesis that is diﬀerent from the general procedure is described in Supplementary Information. Optimization of reaction conditions has also been included there. Molecules 2020, 25, 1761 8 of 9

Author Contributions: Conceptualization, W.T.M., and J.L.K.; methodology, A.P.Z.; investigation, A.P.Z.; writing—original draft preparation, A.P.Z.; writing—review and editing, J.L.K., W.T.M.; supervision, W.T.M., J.L.K.; project administration, W.T.M.; funding acquisition, W.T.M., J.L.K. All authors have read and agreed to the published version of the manuscript. Funding: We would like to acknowledge technical and infrastructural support from a Laboratory of Mass Spectrometry and Laboratory of NMR at IBCH PAS. J.L.K. acknowledges funding from Foundation for Polish Science (Homing grant no HOMING/2017-4/33) for a project entitled “MultiGATE: dual-analyte responsive fluorescent probes for a real-time multi-parametric sensing in cellular models”. A.P.Z. and W.T.M. received funding from the NCBR (National Centre for Research and Development, Poland) – Strategmed programme STRATEGMED1/233624/5/NCBR/2014 “Low molecular weight epigenetic modulators for activation of pluripotency of cells for regenerative medicine purposes”. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds are not available from the authors.

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