Introduction

The chemical industry has utilized a majority of metals within the periodic table for chemical processes, such as in the manufacturing of chemicals, advanced materials, health care products, fuels and so on. As a society, we currently rely heavily on the use of metals and the intensive mining of these elements could cause environment burden, especially for end-of-life consumer products and industrial processes (Hunt et al. 2015). Inevitably, metals are being depleted, which will create a resource deficit in the future. Considering the environmental, societal and economic effects of intense metal mining on our earth, we need a dramatic change in our chemical processing methods to ensure a more sustainable chemical processes.

Due to the increasing environmental consciousness and more stringent regulations faced by the chemical and pharmaceutical industries, innovation of environmentally benign chemical processes has emerged as an important and demanding field in the modern era of organic synthesis research (Centi and Perathoner 2003; Lavilla et al. 2014). As such, increasing efforts have been devoted in the development of green synthetic endeavours that encompass the use of environmentally friendly reagents, solvents, and catalysts in performing organic processes. In recent years, chemists are faced with the challenge of developing environmentally benign catalysts, whose uses are in favor to maintain a sustainable environment. In fact, designing and developing sustainable catalysts are part of the fundamental aspect of green chemistry, resulting in cleaner, cheaper, and less energy used (Hutchings 2007; Khodabakhshi and Baghernejad 2013; Yu et al. 2020). Hence, cleaner and safer organic processes that emphasize the use of environmentally acceptable catalytic systems, that offer environmental and economic advantages over conventional organic processes are needed by the academia and industrial communities (Chen et al. 2020; Díaz-Sánchez et al. 2019; Kumar et al. 2019; Nongrum et al. 2019; Onuegbu et al. 2011; Paprocki et al. 2019; Peixoto et al. 2020).

Food additives are chemical substances that are used to enhance the quality and safety of food. The use of food additives such as alginic acid (Dekamin et al. 2014), boric acid (Rezayati et al. 2014), tartaric acid (Gratzer et al. 2013), citric acid (Reddy et al. 2018), pectin (Pazhavelikkakath Purushothaman et al. 2018), oxalic acid (Dabiri et al. 2007) and saccharin (Banerjee et al. 2019) in achieving green organic reactions has gained much attention in recent years among researchers. These food additives represent a unique class of catalysts which are green, natural origin, low-cost, commercially available, biodegradable and at the same time have a large application in organic reactions. Though there have been many efficient catalysts being reported, industrial engineers are still faced with the challenge of designing environmentally benign catalysts, which possess a higher reaction rate, efficient recovery of catalysts from the medium, biodegradable, and cost-effectiveness of the catalysts used. Hence, the use of environmentally benign and cost-effective catalysts is highly desirable. Herein, the literature of using food additives as unconventional green catalysts in organic synthesis was surveyed using multiple search terms and keywords to identify the relevant catalysts and their advantages in comparison with classical catalysts, in the development of green and eco-efficient chemical processes, are reviewed here.

Alginic acid

Alginic acid which is also known as an algin or alginate is an ionic polysaccharide that is commonly extracted from the cell walls of brown algae (Kariduraganavar et al. 2014). It can create a vicious gum when dissolves in water due to its capability of absorbing water easily till 200–300 times of its own weight and also soluble in water-miscible solvents such as ketones and alcohols. Besides, alginic acid has a very high demand in the food industry as it is commonly used as a thickener, emulsifiers, and also stabilizers (Kariduraganavar et al. 2014; Titlyanov et al. 2017). Dekamin et al. (2014) reported a highly efficient Hantzsch reaction using alginic acid (a) as a green bio-polymeric catalyst in 1,4-dihydropyridines 4 syntheses (Scheme 1). 1,4-dihydropyridines widely occur in nature, and drug molecules play an essential role in bioactivities and pharmaceutical activities (Wu et al. 2021). This environmentally friendly protocol provided the targeted compounds in high to excellent reaction yields (68–98%) via a Knoevenagel intermediate followed by Michael addition reaction. Overall, this reaction provides a mild and metal-free alternative to previous methods, as no solvents and metal-catalysts were used as well as a faster reaction rate was reported.

Scheme 1
scheme 1

Plausible mechanism of 1,4-dihydropyridines synthesis. Notes: Me (methyl), HOAc (acetic acid), H2O (water), NH2 (nitrogen source)

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On top of that, the same authors also developed a synthesis protocol of polyhydroquinolines 9 via the Hantzsch reaction, which was mediated by alginic acid (Dekamin et al. 2018). The synthesis of polyhydroquinolines commenced with a four-component condensation of ethyl acetoacetate 5, aldehyde 6, ammonium acetate 7, and cyclic diones 8, catalyzed by alginic acid (a) under mild reaction conditions as shown in Scheme 2. The synthesis provided the targeted compounds in high to excellent yields (75–97%) through a Knoevenagel intermediate followed by Michael addition. The increase in the rate of reactions in polyhydroquinolines’ syntheses was attributed to the charged-dipole interaction of alginic acid with reactants’ intermediates. This has led to a high turnover number (9.5) and turnover frequency (22.8 h−1) compared to previous studies. The result indicated that alginic acid catalyst might meet the goal of green chemistry, which provides a more environmentally friendly and biodegradable method for organic catalysis (Hajizadeh and Maleki 2018; Maleki 2014, 2016, 2018 Maleki et al. 2016).

Scheme 2
scheme 2

Alginic acid-mediated polyhydroquinolines syntheses via multi-component Hantzsch reaction. Notes: NH4OAc (ammonium acetate), NH3 (ammonia), HOAc (acetic acid), H2O (water)

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On the other hand, alginic acid (a) was also employed as a green catalyst in a study demonstrated by Maleki et al. (2019b), who reported the efficient and green synthesis of 2-amino-3-cyano-4H-pyran derivatives 13 as shown in Scheme 3, in which aldehydes 11, 5,5-dimethyl-1,3-cyclohexanedione 10, and malononitrile 12 were reacted in one-pot reaction in the presence of alginic acid nanocomposite (a). It was reported that the current synthesis has led to higher reaction yields (83–95%). Previously, the identical reaction suffered from long reaction times, difficult workup, and mediocre yields. Ironically, this protocol offers a mild reaction condition, green and cost-effective nano-catalyst, shorter reaction time, easy workup with a simple procedure, and the catalyst was recoverable after five consecutive runs. In addition, the author also reported that the efficiency of the alginic acid nanocomposite, outperformed the single catalysts (alginic acid and zinc ferrite), though both can catalyzed the reaction individually. The higher catalytic effect of alginic acid nanocomposite compared to individual catalyst was due to the synergy effect demonstrated by the nanocomposite employed in the protocol.

Scheme 3
scheme 3

Synthesis of derivatives of 2-amino-3-cyano-4H-pyran. Reaction condition: dimedone (1.0 mmol), aldehyde (1.0 mmol), malononitrile (1.0 mmol), and catalyst (alginic acid nanocomposite, 5 mg) in ethanol (2 mL) at room temperature

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The same approach was employed by Srivastava et al. (2015) using alginic acid (a) as a green catalyst in catalysing reaction between indoles 15 and isoquinoline-1,3,4-triones 14 to form the hydroxyindolyl derivatives 17 as depicted in Scheme 4. Satisfactory reaction yields were recorded (57–82%) when the reactions were performed at 65 °C. In the past, the synthesis of hydroxyindolyl derivatives 17 employed metal-salt catalysts, harmful and non-bio-degradable reagents. These methods are unfavorable due to the use of non-bio-degradable catalysts that can cause harm to the environment.

Scheme 4
scheme 4

Alginic acid-mediated isoquinoline-1,3,4-trione syntheses in water via Friedel–Crafts reaction of indoles. Notes: H2O (water)

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Boric acid

Boric acid (Fig. 1) is a food preservative employed in preserving the shelf life of meat products (Arslan et al. 2008). It is also used in starch gelatinization purposes and flavour enhancement (Yiu et al. 2008). Recently, boric acid is highly sought after in organic synthesis due to the eco-friendly, stability, excellent solubility in water, and inexpensive nature of this chemical substance. Gogoi et al. (2014) described a good example of boric acid (b) catalyzed ipso-hydroxylation process that employed hydrogen peroxide as an oxidant (Scheme 5), with turnover number (8.8) and turnover frequency (15.1 h−1) reported for the optimized reaction condition. In this work, aryl boronic acids were all converted into the desired product 19 (80–95% yield) via oxidation reaction. The use of a greener approach such as the one described here is desired as it is metal-free, base-free, and the reaction can be carried out under room temperature conditions to prepare the corresponding phenols.

Fig. 1
figure 1

Boric acid appears in white crystalline solid at room temperature

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Scheme 5
scheme 5

The Synthesis of phenol. Reaction condition: aryl boronic acid (0.5 mmol), boric acid (0.1 mmol), hydrogen peroxide (2 mL) in ethanol (2 mL) at room temperature

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Apart from that, Nath and Chaudhuri (2008) have investigated a bromination protocol using boric acid as a green catalyst. The reaction was carried out involving potassium bromide as the bromide source and hydrogen peroxide as an oxidizing agent in the presence of boric acid (b) and ethanol as solvent (Scheme 6). This site-selective bromination of organic substrates has led to the high yields of desired products 21 and 23. Based on the previous protocols for bromination of aryl compounds, most of these methods are unsustainable such as the use of corrosive and toxic reagents, which at times result in a large amount of waste production, leading to a 50% reduction of the atom efficiency (Vyas et al. 2003). However, the present work offers attractive advantages, such as cost-effective, eco-friendly, non-toxic, recyclable, safe to handle methods, and the reaction can be carried out effectively under mild conditions.

Scheme 6
scheme 6

Production of bromoorganic compounds. Reaction condition: substrate (1.0 mmol), boric acid (0.05 mmol), hydrogen peroxide (3.0 mmol), potassium bromide (1.2 mmol) in water (2.5 mL) or ethanol (2.5 mL). Notes: H+ (hydrogen ion), OMe (methoxy)

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Furthermore, an efficient and regioselective ring-opening of epoxide reaction was also observed using boric acid as a catalyst (Halimehjani et al. 2012). Under this reaction, the 2,3-epoxypropyl ether 24 and aniline 25 were selected as model substrates and reacted in the presence of boric acid (b) (Scheme 7). In most cases, a satisfactory yield (93–100%) of derivatives 26 was obtained and various aromatic amines were tolerated under this protocol. Numerous efficient catalysts have been reported for a similar reaction. Yet, some drawbacks have been identified from those procedures such as the use of costly catalysts, hazardous organic solvents, moisture-sensitive reactions, and so on. Alternatively, boric acid could act as a substitute due to its desirable characteristics such as inexpensive, water-soluble, easy to handle, commercially available, and environmentally friendly.

Scheme 7
scheme 7

Reaction of ring-opening of epoxides. Reaction condition: ring-opening of epoxides (2.5 mmol), aromatic amines (3.0 mmol), boric acid (0.3 mmol), glycerol (1–2 drops) in water (5 mL) within 10-18 h at 35 °C. Notes: h (hours), °C (degree Celsius)

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Another example was demonstrated by Hosseinzadeh et al. (2019), whom prepared the 2-amino-4,6-diarylnicotinonitriles 32 via a multicomponent reaction using boric acid (b) as a catalyst (Scheme 8). This environmentally benign protocol provided the targeted compounds in high yields (86–96%) via Knoevenagel condensation followed by air oxidation cyclization. Although various works of literature have been made on the synthetic protocol for 2-amino-4,6-diarylnicotinonitriles, these approaches suffered from the use of harsh reaction conditions, toxic solvents, with low reaction yields recorded. Consequently, the use of boric acid as a catalyst in the afore-mentioned reaction is superior to other protocols due to its excellent solubility in water, inexpensive catalyst, chemically stable and nontoxic nature, recyclable, and easy to handle. Besides, the current procedure takes place under a solvent-free condition which helps in pollution reduction, prevention of solvent waste, energy efficient, enhances the products yield and high atom economy. This is of scientific importance as no waste reactants were generated at the end of chemical processes (Maleki 2012, 2013; Maleki and Aghaei 2017; Maleki et al. 2018, 2019a).

Scheme 8
scheme 8

Tentative mechanism for the production of 2-amino-4,6-diarylnicotinonitrile. Reaction condition: aromatic aldehyde (1.0 mmol), malononitrile (1.5 mol), acetophenone (1.0 mmol), ammonium acetate (NH4OAc, 1.0 mmol), boric acid (1.0 mmol). Notes: Ar (Aromatic), H (hydrogen)

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Tartaric acid

Tartaric acid (Fig. 2) is one of the food additives that is added to enhance taste, acidity regulator, and also provide antioxidant properties in foods (Marchitan et al. 2010). Over the years, chemists have developed an eco-friendly method of organic reactions using tartaric acid as a catalyst. According to the literature, tartaric acid was found to be an efficient catalyst in mediating the synthesis of piperidines 36 via a multi-component reaction (Aboonajmi et al. 2015). This reaction catalyzed by tartaric acid (c) involved aromatic aldehydes 33, β-ketoesters 35, and anilines 34 was carried out at room temperature (Scheme 9). Anilines as electron-donating groups reacted effectively with aldehydes to afford the piperidines in satisfactory yields (39–91%) via a domino process. Prior to that, the identical organic process suffered from low yields, the uses of toxic and expensive catalysts, and complicated workup procedures. Alternatively, this method offers more advantages which are simplicity in the workup procedure, cheap and the use of a biodegradable catalyst.

Fig. 2
figure 2

Chemical structure of tartaric acid that is naturally found in grapes

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Scheme 9
scheme 9

Synthesis of functionalized piperidines. Reaction condition: aromatic amine (2.0 mmol), aromatic aldehyde (2.0 mmol), and β-ketoester (1.0 mmol) with the presence of tartaric acid (0.075 g) in methanol (5 mL) at room temperature

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Likewise, a green synthesis using tartaric acid for the formation of poly-substituted quinolines 39 was reported lately (Mohamadpour et al. 2018). The reaction comprised of ketone 38 (1.0 mmol) and 2-aminobenzophenone 37 (1.0 mmol) in the presence of tartaric acid (c) as shown in Scheme 10 under neat condition. This one-pot four-component reaction gives excellent isolated yields (91–94%) via Friedländer condensation. Moreover, it has overcome some of the limitations encountered in previous protocols such as the uses of expensive and toxic catalysts, tedious workup procedure, the use of highly acidic conditions, prolonged reaction time, and low yields of isolated products. The current innovative method is scientifically significant, as it employed a non-toxic catalyst and column-free procedure. Furthermore, the poly-substituted quinoline synthesis mediated by tartaric acid was found to be in excellent reaction yield, which indicated that the current catalyst can act as a green alternative to other catalysts for the identical organic reaction (Ahmad Ruslan et al. 2020). Previous methods in the synthesis of poly-substituted quinoline and their yields are summarized in Table 1.

Scheme 10
scheme 10

Proposed mechanism in the formation of poly-substituted quinolines via Friedländer condensation. Reaction condition: ketone (1.0 mmol), 2-aminobenzophenones (1.0 mmol), tartaric acid (0.15 mmol) at 70 °C under solvent-free condition. Notes: H2O (water), Ph (phenyl), mol% (mole percent), °C (degree Celsius), cat- (cation)

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Table 1 Comparison of previous methods and their reactions’ yields in the synthesis of poly-substituted quinolines
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In another work, tartaric acid was employed to synthesize the poly-substituted dihydropyrrol-2-ones (Mohamadpour et al. 2016). Under the optimized reaction condition, formaldehyde 43, dialkyl acetylenedicarboxylate 41, and amines (aliphatic 40 or aromatic 42) were reacted in the presence of tartaric acid (c) to afford the desired products 44 (Scheme 11) in excellent yields (74–93%), with 15 mol% of catalyst loading under ambient temperature. It is noteworthy that the previous methods involved the use of expensive and toxic catalysts, tedious work-up, prolonged reaction times, but resulted in low yields of products. In contrast, the current developed method is known to be environmentally benign, cost-effective, highly efficient, shorter reaction times, and led to high yields of products.

Scheme 11
scheme 11

Synthesis of poly-substituted dihydropyrrol-2-ones. Reaction condition: amine (1.0 mmol), dialkyl acetylenedicarboxylate (1.0 mmol), formaldehyde (1.5 mmol), tartaric acid (0.15 mmol). Notes: Ar (Aromatic), OH (hydroxy), mol% (mole percent)

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Tartaric acid (c) was also being employed as a catalyst in the green synthesis of 2,3-dihydroquinazolin-4(1H)-ones 47, which involved 2-aminobenzamide 46 and various aromatic aldehydes 45 (Scheme 12) (Singh et al. 2018). In the proposed mechanism, tartaric acid aided in activating the carbonyl group of the aldehydes, which further enhances the electrophilic character of aldehyde followed by cyclization, and the desired products were recorded in high to excellent yields (85–95%). The use of costly catalysts, prolong reaction times, and harsh reaction conditions are among the disadvantages associated with the previous protocols. This approach is more superior to the former protocols as the catalyst used is far safer, efficient, and eco-friendly compared to the previous protocols.

Scheme 12
scheme 12

Synthesis of 2,3-dihydroquinazolin-4(1H)-ones. Reaction condition: aromatic aldehyde (1.0 mmol), 2-aminobenzamide (1.0 mmol), tartaric acid (0.2 mmol) in water (5 mL) at room temperature. Notes: H+ (hydrogen ion)

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Citric acid

Citric acid is a natural preservative, anti-caking agent, acidity regulator and antioxidant agent in food processing industry. In the food industry, citric acid is usually employed in the production of fruit juice, jelly, candies, marmalade, as well as aid in suppressing the browning process in fruits (Navarro et al. 2013). According to a recent literature, many organic reactions have been achieved by using citric acid as a catalyst (Bafti and Khabazzadeh 2014). This catalyst is found to be low-cost, stable, non-volatile as well as eco-friendly additive compared to other catalysts in catalyzing identical organic reactions. One of the remarkable examples was reported in the synthesis of 1,8-dioxo-octahydroxanthenes 50 (Navarro et al. 2013). The reaction takes place between aldehyde 49 and dimedone 48 in the presence of citric acid (d). A plausible mechanism for the synthesis is presented in Scheme 13. At the end of the synthesis, high yields (73–85%) of the desired compounds were obtained via the intramolecular cyclization step. This protocol was found to be greener and efficient, as the conventional methods described in previous literatures suffer from several limitations including the need for catalyst preparation, use of expensive catalysts and reagents, tedious reaction workup, harsh reaction condition, and prolonged reaction times. In the present method, citric acid was selected as a green catalyst as it is non-volatile, cost-effective, readily available, and reusable. Besides, this procedure offers a milder reaction condition and minimizing the production of wastes.

Scheme 13
scheme 13

Formation of 1,8-dioxo-octahydroxanthenes Reaction condition: aldehyde (0.5 mmol) and dimedone (1.0 mmol) with the presence of citric acid (2 mL). Notes: H2O (water)

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The role of citric acid as a green catalyst was also investigated in the synthesis of 3-pyrrolin-2-ones 54 (Ahankar et al. 2016). These reactions involved aromatic aldehydes 51, aniline 52, and diethyl acetylene-dicarboxylate 53 were carried out successfully under microwave irradiation, with citric acid (d) as catalyst. Scheme 14 shows the mechanism of formation for the 3-pyrroline-2-ones. The elimination of ethanol and citric acid molecules at the final stage of the reaction resulted in high yields (80–92%) of the targeted products. In many cases, the use of harsh reaction conditions, non-eco-friendly catalysts, toxic solvents, and low yields of end-products were commonly encountered in the previous method for the identical organic reaction. With the aid of citric acid, this green catalyst helps to increase reactions yields within a short reaction time. Moreover, this green catalyst is inexpensive and eco-friendly, commercially available additives, and resulted in high purity of the targeted products.

Scheme 14
scheme 14

Mechanism of formation of 3-pyrrolin-2-ones. Reaction condition: aromatic aldehyde (1.0 mmol), aniline (1.0 mmol), diethyl acetylene-dicarboxylate (1.0 mmol), citric acid (2.0 mmol) in ethanol (4 mL). Notes: Ph (phenyl), H2O (water)

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In addition, Shaikh et al. (2014) also demonstrated the preparation of amidoalkyl naphthols 58 using citric acid (d). Amidoalkyl naphthols have been attracting interest among researchers due to their useful pharmacological and biological properties. Previously, the use of unsustainable methods have been employed in the amidoalkyl naphthols syntheses, such as the use of molecular iodine (Das et al. 2007), ferric hydrogen sulfate (Shaterian et al. 2008), strontium bis(trifluoromethanesulfonate) (Shaikh et al. 2014), disulfonamide catalyst (Ghorbani-Vaghei and Malaekehpour 2010) and magnetic nanoparticle-supported sulfuric acid (Safari and Zarnegar 2013). However, some of these methods required prolonged reaction times, high catalyst loading, the use of expensive and corrosive reagents, and strongly acidic conditions. Due to these problems, citric acid has been attracting the attention of chemists as an alternative green catalyst. Under the improved protocol, citric acid (d) catalyzed the three-component reaction involving aromatic aldehyde 56, acetamide 57, and 2-naphthol 55 under the neat condition to synthesize the desired compound in high yields (87–94%) as shown in Scheme 15. Moreover, citric acid is more favourable than other catalysts, as it is a biodegradable, cheap, non-corrosive, and non-toxic. In this experiment, the calculated turnover number (8.7) and turnover frequency (34.8 h−1) demonstrated that the optimized reaction condition was achieved with the use of 10 mol% of citric acid as catalyst and heated under 120 °C.

Scheme 15
scheme 15

Synthesis of amidoalkyl-2-naphthols. Reaction condition: 2-naphthol (1.0 mmol), aromatic aldehyde (1.0 mmol), amide (1.2 mmol), citric acid (10 mol%) at 120 °C under solvent-free conditions. Notes: °C (degree Celsius), mol% (mole percent)

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Next, a citric acid-mediated synthesis of hexabenzyl hexaazaisowurtzitane 61 was developed (Shokrollahi et al. 2016) via the condensation of benzylamine 59 with glyoxal 60 (Scheme 16). The synthesis of hexabenzyl hexaazaisowurtzitane was reported to have the best results when only 5 mol% of citric acid (d) catalyst was used and with acetonitrile–water as the solvent. Besides, the reaction takes 5 min with ultrasonic power of 150 W producing high yields (89%) of the desired hexabenzyl hexaazaisowurtzitane. Previous methodologies utilized formic acid as a catalyst for the synthesis of hexabenzyl hexaazaisowurtzitane. However, the formic acid has been proven to be corrosive to most alloys including stainless steel (Shokrollahi et al. 2016; Singh et al. 2018). Therefore, citric acid is a more favourable catalyst as it is non-toxic and biodegradable, as well as well known to catalyze a variety of organic reactions. When coupled with ultrasound irradiation, the reaction was found to have good reaction yields and completed within a shorter reaction time as demonstrated in this work.

Scheme 16
scheme 16

Mechanism of formation of hexabenzyl hexaazaisowurtzitane. Reaction condition: benzylamine (8.5 mmol), glyoxal (3.75 mmol), citric acid (0.05 mmol) in acetonitrile (7.75 mL) and water (0.775 mL) under conventional stirring and ultrasonic radiation condition. Notes: H+ (hydrogen ion), H2O (water), Ph (phenyl)

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Pectin

Pectin is a naturally occurring polymer that has applications in the fields of medical, pharmaceutical, and biotechnology industries. In the food and beverage industry, pectin is used as thickening, bulking and colloidal stabilizer agent. The source of commercial pectin is from the citrus rind (Thakur et al. 1997; Sriamornsak 2007). Over the past years, the use of pectin in organic synthesis has received considerable attention among chemists due to its biodegradable and green nature as an organic catalyst. One of the examples was found on the synthesis of tetrahydrobenzo[b]pyran 65 and 3,4-dihydropyrano[c]chromene 67 (Kangani et al. 2016) using this environmentally benign catalyst. A one-pot three-component reaction of benzaldehyde 62 with malononitrile 63, and dimedone 64 or 4-hydroxycoumarin 66 exclusively lead to the targeted products, 65 and 67, respectively (Scheme 17). Scheme 18 shows the plausible mechanism for the production of 65. The great interest in this catalyst was due to the green and biodegradable property of pectin (e) as an organic catalyst. Furthermore, the tetrahydrobenzo[b]pyran 65 and 3,4-dihydropyrano[c]chromene 67 syntheses mediated by pectin (e) were found to be in excellent reaction yields, which indicated that the current catalyst can act as green alternative to other catalysts for the identical organic reactions (Table 2).

Scheme 17
scheme 17

Synthesis of tetrahydrobenzo[b]pyran 65 and 3,4-dihydropyrano[c]chromene 67. Reaction condition: aromatic aldehyde (1.0 mmol), malononitrile (1.0 mmol), dimedone (1.0 mmol) or 4-hydroxycoumarine (1.0 mmol), pectin (0.05 g) in water:ethanol or ethanol solvent system at room temperature

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Scheme 18
scheme 18

Mechanism of formation for tetrahydrobenzo[b]pyran 65. Notes: the reaction were catalyzed by pectin in ethanol–water solvents system. Ar (Aromatic)

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Table 2 Comparison of previous methods and their reaction yields in the syntheses of tetrahydrobenzo[b]pyran 65 and 3,4-dihydropyrano[c]chromene 67
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The presence of food additives as catalysts in organic reactions also has the utility for organic reactions to be performed at room temperature. One such reaction was identified in the pectin (e) catalyzed oxopyroles and furan-2(5H)-one's syntheses (Kangani et al. 2018). The result of this finding showed that the reaction can be carried out successfully at room temperature, which was desirable in minimizing the economic and environmental impact. Scheme 19 shows a proposed mechanism for the synthesis of dihydro-2-oxopyroles 72.

Scheme 19
scheme 19

Tentative mechanism of dihydro-2-oxopyroles syntheses. Reaction condition: mixture of amine (1.0 mmol), dialkyl acetylene-dicarboxylate (1.0 mmol), aldehyde (1.5 mmol), pectin (0.05 g) in water:ethanol (1:3) at room temperature. Notes: Ar (Aromatic)

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Oxalic acid

Oxalic acid or commonly known as oxalate is a compound that occurred naturally in plants. Oxalic acid is usually found in vegetables such as spinach, carrots, rhubarb and bamboo shoots. Dabiri et al. (2007) prepared quinolines 75 via the Friedländer annulation in the presence of oxalic acid (f) as an organic catalyst. The reaction pathway involved 2-aminobenzophenone 73 and ethyl acetoacetate 74 which leads to the formation of quinolines 75 under solvent-free condition as shown in Scheme 20. The result showed that the desired compounds can be efficiently furnished and obtained in excellent yields (88–95%) via condensation reaction followed by cyclodehydration steps. The uniqueness of employing oxalic acid in mediating this reaction including eco-friendly nature, inexpensive and solvent-free condition, which makes this catalyst more superior than the use of toxic catalysts and solvents in the previous studies. Table 3 summarizes the previous methods and their yields in the synthesis of quinoline.

Scheme 20
scheme 20

Synthesis of quinolines. Reaction condition: 2-aminobenzophenone (1.0 mmol), ethyl acetoacetate (1.1 mmol), oxalic acid (0.1 mmol), heated at 80 °C under solvent-free conditions for 2 h. Notes: h (hours), °C (degree Celsius), Ph (phenyl), mol% (mole percent)

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Table 3 Comparison of previous methods and their reaction yields in the syntheses of quinoline 75
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The use of oxalic acid for organic transformation is recommended by chemists as the reaction can be carried out in a solvent-free condition. One of these examples was demonstrated by Sarkate et al. (2013), which successfully employed oxalic acid (f) as a green catalyst for the synthesis of 1,5-benzodiazepines 78 under the solvent-free condition at 80 °C. The reaction commenced by reacting o-phenylenediamine 76 and α, β-unsaturated ketones 77 as presented in Scheme 21. Excellent yields (76–96%) of the desired product were achieved within a short reaction time. A plausible mechanism for the synthesis of 1,5-benzodiazepines is illustrated in Scheme 22. In the reported literature, previous methods have suffered from several drawbacks and limitations in using costly and toxic catalysts or reagents, associated with tedious work-up and resulting in low product yields. Alternatively, the present method developed by the authors offers advantages which includes a more environmentally friendly procedure, cost-effective catalysts, easy work-up, shorter reaction time, and reduction of waste production.

Scheme 21
scheme 21

Synthesis of 1,5-benzodiazepines. Reaction condition: o-phenyldiamine (10.0 mmol), α, β-unsaturated ketones (10.0 mmol), oxalic acid (0.1 mmol), heated at 80 °C under solvent-free conditions. Notes: °C (degree Celsius)

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Scheme 22
scheme 22

Mechanism of formation for 1,5-benzodiazepines catalyzed by oxalic acid

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Another example of this approach was found in the preparation of 2-(substituted phenyl) phthalazin-1(2H)-ones 81 using oxalic acid (Sangshetti et al. 2015). A model reaction of substituted phenyl hydrazine 80 and phthalaldehydic acid 79 was established using water as a solvent in the presence of oxalic acid (f) (Scheme 23). This environmentally benign protocol afforded the targeted compound in excellent yields (95–98%) via dehydrative cyclization. The presence of oxalic acid in the synthetic route helps in the protonation at the carbonyl carbon of phthalaldehydic acid (stage I). Several synthetic methods for the synthesis of phthalazinones have been identified in the literatures. However, these synthetic methods involve harsh reaction conditions, prolonged reaction times, and required strong acids and bases. The use of oxalic acid in organic reactions is economical, mild, and easy to handle. In addition, the advantage of this catalyst is also associated with a marked decrease in reaction time and only requires water as the sole solvent.

Scheme 23
scheme 23

Mechanism of 2-(substituted phenyl) phthalazin-1(2H)-one’s syntheses. Reaction condition: phthalaldehydic acid (10.0 mmol), substituted phenyl hydrazine (10.0 mmol), oxalic acid (0.1 mmol) in water (15 mL) as solvent. Notes: H2O (water)

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Saccharin

Over the past, saccharin plays a role as an artificial sweetener to enhance the sweet taste of the food (Tripathi et al. 2006). Besides, it is also generally used as a calorie-free additive in beverages and food products (Larsen 2012). Recently, saccharin has gained attention among researchers as it has high catalytic applicability in organic transformations as it is easily available, requires very mild reaction condition, and efficiently water-soluble. Bhandari and Gaonkar (2015) was the first to report on the synthesis of pyrroles derivatives achieved using saccharin as a green catalyst. The synthesis of N-substituted 2,5-dimethylpyrroles 84 involved the reaction of various hydrazides 83 and hexane-2,5-dione 82 in the presence of saccharin (g) (Scheme 24). This synthetic protocol leads to good reaction yields (80–92%) of the targeted products via Paal-Knorr condensation, with turnover number (3.4) and turnover frequency (6.9 h−1) reported for the optimized reaction condition. A close inspection of previous methods for the Paal-Knorr condensation, they often involved the use of strong acids, expensive and toxic reagents. The present work offers more advantages compared to the previous methods where the use of saccharin displayed a cost-effective, non-toxic, and easy to handle method. Moreover, it is recyclable for another two runs without significant loss in its activity.

Scheme 24
scheme 24

Synthesis of N-substituted 2,5-dimethylpyrroles. Reaction condition: hexane-2,5-dione (6.0 mmol), hydrazide (4.0 mmol), saccharin (1.0 mmol) in methanol (6 mL) while stirred for 30 min at room temperature via Paal-Knorr condensation reaction, Notes: Me (methyl), min (minute)

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Another saccharin-mediated approach was identified in the work of Mohamadpour et al. (2016). An efficient solvent-free 3,4-dihydropyrimidine-2-(1H)-ones 88 method was developed, when urea/thiourea 86, aromatic aldehydes 85, and alkyl acetoacetate 87 were reacted in one-pot in the presence of saccharin (g) as catalyst. Under this protocol, the turnover number (4.6) and turnover frequency (18.2 h−1) were reported for the optimized reaction condition. In addition, various types of benzaldehydes were tolerable and reacted following the Biginelli manner, which afforded the targeted compounds in high yields (80–93%). The solvent-free and the use of the inexpensive catalyst of this protocol provides attractive advantages compared to previous protocols. The mechanism for the formation of 88 catalyzed by saccharin is illustrated in Scheme 25. A multicomponent reaction involving phthalamide 89, aldehydes 91, hydrazine hydrate 90, and malononitrile 92 under solvent-free conditions were reported by the same author to afford pyrazolo[1,2-b]-phthalazine-5,10-dione 93 in the presence of saccharin (Scheme 26). Surprisingly, this green approach afforded an excellent yield (82–93%) of the desired products within a shorter reaction time. Generally, the use of saccharin is much safer than previous protocols for the identical organic reaction.

Scheme 25
scheme 25

Mechanism of formation for the synthesis of 3,4-dihydropyrimidine-2-(1H)-ones. Reaction condition: aromatic aldehyde (1.0 mmol), urea/thiourea (1.5 mmol), ethyl/methyl acetoacetate (1.0 mmol), saccharin (0.15 mmol) under solvent-free conditions. Notes: H2O (water), H+ (hydrogen ion), Ar (Aromatic)

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Scheme 26
scheme 26

Mechanism of formation for the pyrazolo[1,2-b]-phthalazine-5,10-diones. Reaction condition: phthalimide (1.0 mmol), hydrazine monohydrate (1.0 mmol), aromatic aldehyde (1.0 mmol), malononitrile (1.0 mmol), in the presence of saccharin (0.2 mmol). Notes: Ar (Aromatic), NH3 (ammonia)

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Inspired by the previous study, the same authors (Mohamadpour et al. 2016) have synthesized a series of dihydro-2-oxypyrrole 98 via a one-pot four-component reaction, involving aromatic 94, aliphatic amines 96, formaldehyde 97, and dialkyl acetylenecarboxylate 95 in the presence of saccharin (g) as organic catalyst (Scheme 27). Excellent reaction yields (85–94%) were recorded using saccharin as a green catalyst. A close inspection on the previous methods revealed that the majority of the literatures reported on the use of unsustainable methods, such as the use of tetrachlorobis(tetrahydrofuran)zirconium (Sajadikhah et al. 2016), silica gel-supported tungstic acid (Reddy et al. 2014), Brønsted acidic ionic liquid (Zhang et al. 2015), sulfonic acid functionalized nanoporous silica (Ziarani et al. 2014), and molecular iodine (Ziarani et al. 2014) as catalysts. This has prompted the current investigation towards an environmentally friendly method, which uses saccharin as a green catalyst because it is a cost-effective and non-toxic catalyst but without compromising on catalytic activity.

Scheme 27
scheme 27

Mechanism of formation for the synthesis of dihydro-2-oxypyrrole. Reaction condition: amine (1.0 mmol), dialkyl acetylenedicarboxylate (1.0 mmol), aromatic amine (1.0 mmol), formaldehyde (1.5 mmol), in the presence of saccharin (0.15 mmol). Notes: Ar (Aromatic), OH (hydroxy)

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Last but not least, the use of saccharin was also demonstrated by Moradi and Aghamohammad Sadegh (2017) whereby the synthesis of dihydropyrano[2,3-g]chromenes 102 was carried out by using sodium saccharin. The one-pot five-component reaction between aryl aldehyde 100, 2,5-dihydroxy-1,4- benzoquinone 101, and malononitrile 99 in the presence of sodium saccharin (g) under microwave irradiation was developed to obtain 102 (Scheme 28). This efficient and eco-friendly approach enables the synthesis of 102 in excellent yields (78–90%). The benefits of the current method include simplicity in reaction workup, cost-effective catalysts, and producing high reactions yields within short reaction times. Table 4 summarizes the different methods and their yields performed by the author in the synthesis of dihydropyrano[2,3-g]chromenes 102 (Moradi and Aghamohammad Sadegh 2017).

Scheme 28
scheme 28

Mechanism of dihydropyrano[2,3-g]chromenes synthesis. Reaction condition: malononitrile (2.0 mmol), aryl aldehyde (2.0 mmol), 2,5-dihydroxy-1,4-benzoquinone (1.0 mmol), sodium saccharin (0.1 mmol) under microwave irradiation. Notes: Ar (Aromatic)

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Table 4 Comparison of different methods and their reactions’ yields in the syntheses of dihydropyrano[2,3-g]chromenes 102
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Conclusion

At industry level, the growing demand for the finite elemental resources such as metals for industrial processes has led to concerns over the sustainability of the supply of these elements. Furthermore, intensive mining of these elements has increased the burden on the environment and at times lead to the issue of environmental pollution. Therefore, there are calls to search for greener alternatives to reduce the use of elemental resources for chemical processes in the industries. Food additives, on the other hand, have been gaining interest not only in the food industry but also in the chemical industry as elemental substitutes contributing to sustainable organic reactions. The involvement of food additives as green catalysts for preparative organic reactions has raised the interest of today’s scientists for the development of environmentally benign methodologies. Generally, food additives are regarded as “green catalyst” in organic syntheses and we believed that the next decade will mark a major development of using a green and sustainable method in promoting organic transformations, for both academia and industries alike. Thus, the use of food additives can act as the alternative catalysts in achieving the goals of green and sustainable chemistry, as many benefits and green features described previously by scientists in this field. This review article specifically addresses the use of non-metals, biodegradable and green catalysts, namely the natural food additives that is significant to address the issue of sustainable chemical processes at industrial and academia levels. In recent years, with more researchers increasingly investigating the chemical properties of food additives, their future applications have been evident and prominent in organic catalysis and green nanoparticle synthesis preparation. The current finding of this review identify one of the future challenges in the use of food additives as green catalyst is the stability and miscibility of the food additives, especially in large-scale operation, with deemed to be researched and reported in the future.