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Brønsted and Lewis Solid Acid Catalysts in the Valorization of Citronellal

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Brønsted and Lewis Solid Acid Catalysts in the Valorization of Citronellal catalysts Article Brønsted and Lewis Solid Acid Catalysts in the Valorization of Citronellal Federica Zaccheria 1, Federica Santoro 1, Elvina Dhiaul Iftitah 2 and Nicoletta Ravasio 1,* 1 CNR Institute of Molecular Science and Technology, Via Golgi 19, 20133 Milano, Italy; [email protected] (F.Z.); [email protected] (F.S.) 2 Chemistry Department Faculty of Science, Brawijaya University Jl. Veteran, Malang 65145, Indonesia; [email protected] * Correspondence: [email protected]; Tel.: +39-02-5031-4382 Received: 30 July 2018; Accepted: 12 September 2018; Published: 22 September 2018 Abstract: Terpenes are valuable starting materials for the synthesis of molecules that are of interest to the flavor, fragrance, and pharmaceutical industries. However, most processes involve the use of mineral acids or homogeneous Lewis acid catalysts. Here, we report results obtained in the liquid-phase reaction of citronellal with anilines under heterogeneous catalysis conditions to give tricyclic compounds with interesting pharmacological activity. The terpenic aldehyde could be converted into octahydroacridines with a 92% yield through an intramolecular imino Diels–Alder reaction of the imine initially formed in the presence of an acidic clay such as Montmorillonite KSF. Selectivity to the desired product strongly depended on the acid sites distribution, with Brønsted acids favoring selectivity to octahydroacridine and formation of the cis isomer. Pure Lewis acids such as silica–alumina with a very low amount of alumina gave excellent results with electron-rich anilines like toluidine and p-anisidine. This protocol can be applied starting directly from essential oils such as kaffir lime oil, which has a high citronellal content. Keywords: solid acids; acidic clays; terpenes; citronellal; octahydroacridines; heterogeneous catalysis 1. Introduction Terpenes represent a class of natural compounds suited for the synthesis of several types of molecules useful for the industrial production of intermediates for fragrances, flavors, and pharmaceuticals [1]. α-pinene, e.g., one of the main components of turpentine, can be used as a starting material for the synthesis of β-santalol and sandalwood fragrances. These are valuable alternatives to toxic nitro-musks and low-biodegradability polycyclic musks that are among the commonly used fragrances in European laundry detergents, fabric softeners, cleaning agents, and cosmetic products, therefore ubiquitously present in the aquatic environment [2]. In previous years, the use of terpenes has been widely investigated in the polymer industry because of the strong need for renewable and biodegradable materials in this sector [3,4]. Among terpenic molecules, citronellal, citral, limonene, carene, and pinene are common because of their large occurrence in essential oils and in oils derived from agro-industrial residues. Citronellal and citral offer noteworthy opportunities in synthetic strategies because of their condensation and addition reactions by virtue of their unsaturated aldehydic structure. The abundance of these two molecules in essential oils such as kaffir lime, citronella, lemongrass, and krangean oil, promotes their use as raw materials for the sustainable synthesis of different chemicals, specifically N-containing ones. A Schiff base with antibacterial activity was recently synthesized through acid catalysis from citronellal, [5] while benzimidazole derivatives of both aldehydes were obtained by using microwave irradiation [6]. In this case, the aldehyde reacts with o-phenylenediamine to give the benzimidazol Catalysts 2018, 8, 410; doi:10.3390/catal8100410 www.mdpi.com/journal/catalysts Catalysts 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 2 of 10 Catalysts 2018, 8, 410 2 of 10 obtained by using microwave irradiation [6]. In this case,, the aldehyde reacts with o-phenylenediamine to give the benzimidazol moiety, but when an aromatic monoamine reacts with moiety,an unsaturated but when aldehyde an aromatic it gives monoamine an N-aryl imine reacts that with can an further unsaturated react with aldehyde the electron it gives-rich an N-arylalkene iminemoiety that through can further the Povarov react with reaction, the electron-rich which isis alkene anan intramolecularintramolecular moiety through iminoimino the Povarov Diels–Ald reaction,er reaction which [7 is] an(Scheme intramolecular 1). imino Diels–Alder reaction [7] (Scheme1). HN N HN N SchemeScheme 1.1. General intramolecular PovarovPovarov reaction.reaction. ThisThis classclass ofof organicorganic reactionsreactions hashas attractedattracted interestinterest asas aa usefuluseful routeroute ofof accessaccess toto N-containingN-containing polycyclicpolycyclic structures,structures, such asas substituted tetrahydroquinoline and octahydroacridineoctahydroacridine (OHAs) derivatives.derivatives. OctahydroacridineOctahydroacridine systemssystems are are an an interesting interesting class class of of biologically biologically active active molecules molecules in the in fieldthe field of drugs of drugs and pharmaceuticals, and pharmaceuticals, acting as acting gastric as acid gastric secretion acid inhibitors secretion [ 8inhibitors] in the prevention [8] inin thethe of senileprevention dementia of senile [9]. dementia [9]. DespiteDespite severalseveral varyingvarying methodsmethods available offering access to thethe octahydroacridineoctahydroacridine skeleton,skeleton,, suchsuch as as the the Beckmann Beckmann rearrangement rearrangement of oxime of oxime sulfonat sulfonate,e,, [10] the [10 catalytic] the catalytic hydrogenation hydrogenation of acridine of,, acridine,[11] and Friedel [11] and–Crafts Friedel–Crafts acylation,, acylation,[12] the simplest [12] the one simplest remains one the remains acid catalyzed the acid imino catalyzed Diels– iminoAlder Diels–Alderreaction of 2-azadienes. reaction of Because 2-azadienes. of their Becausepoor reactivity of their in poorthe Povarov reactivity reaction, in the 2- Povarovazadienes reaction, need to 2-azadienesbe activated need through to be coordination activated through with acidic coordination catalysts with that acidic enhance catalysts their that electron enhance-defici theirent electron-deficientcharacter. Both Lewis character. and Brønsted Both Lewis acids have and Brønstedbeen used acids,, but,, havedespite been their used, effectiveness but, despite,, many their of effectiveness,these catalysts many show of disadvantages these catalysts,, showsuch as disadvantages, multistep procedures, such as multistep long reaction procedures, times long,, useuse reaction ofof inertinert times,atmosphere, use of inertexpensiveness, atmosphere, and expensiveness, tedious work and- tediousup. There work-up.fore, developing Therefore, developinggreen and green efficient and efficientcatalysts catalysts for this reaction for this reactionis an important is an important challenge. challenge. AnAn efficient efficient synthesis synthesis of of OHAs OHAs starts starts from from citronellal citronellal and and N-arylamines N-arylamines [13,14 [13] (Scheme,14] (Scheme 2). This2). ThisLewis Lewis-acid-acid catalyzed catalyzed imino imino Diels– Diels–AlderAlder reaction reaction is the is most the mostatom- atom-economiceconomic way to way OHAs, to OHAs, with withhigh highyields yields and, and,in some in some cases, cases, high highstereoselectivity. stereoselectivity. R + O NH2 O NH2 N H Scheme 2. Reaction of citronellal and anilines into the corresponding octahydroacridine.octahydroacridine. During our recent studies on reductive amination of ketones, [15] the attempts to synthesize During our recent studies on reductive amination of ketones,, [15] the attempts to synthesize amine derivatives of C=O-containing terpenes led us to obtain OHAs structures. Herein, we report amine derivatives of C=O-containing terpenes led us to obtain OHAs structures. Herein, we report our results about the synthesis of OHAs from citronellal promoted by solid acid catalysts. our results about the synthesis of OHAs from citronellal promoted by solid acid catalysts. 2. Results and Discussion 2.. Results and Discussion Previously, we have reported on the use of pre-reduced Cu/SiO2 (Cu/Si) catalysts in the Direct Previously, we have reported on the use of pre-reduced Cu/SiO22 (Cu/Si) catalysts in the Direct Reductive Amination (DRA) of aromatic ketones [15]. This is a significant reaction allowing one to Reductive Amination (DRA) of aromatic ketones [15]. This is a significant reaction allowing one to obtain secondary amines in one step starting from a ketone, which is still carried out in the presence obtain secondary amines in one step starting from a ketone,, which isis stillstill carriedcarried outout inin thethe presence of unfavorable reagents such as NaBH3CN or NaBH(OAc)3. The use of a heterogeneous catalyst of unfavorable reagents such as NaBH33CN or NaBH(OAc)33.. TheThe useuse ofof aa heterogeneousheterogeneous catalystcatalyst Catalysts 2018, 8, 410 3 of 10 Catalysts 2018, 8, x FOR PEER REVIEW 3 of 10 basedbased on a non non-noble-noble metal can yield up to 98% of the amine at 100 100 °C◦C in in a a few few hours. hours. Unfortunately, Unfortunately, under the same conditions conditions,, aliphatic terpenic ketones,ketones, in particular menthone, a major component of dementholized mintmint oil,oil, reactreact slowly,slowly, requiring requiring a a longer longer time time span, span although, although selectivity selectivity to to the the amine amine is excellentis excellent (Scheme (Scheme3). 3). O NH2 Cu/SiO2
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