Introduction

β-Citronellal is a renewable raw material that plays a key role in various synthesis organic processes [1]. Through different condensation reactions, such as acetalization, ketalization, etherification, and esterification, aldehydes like β-citronellal are platform molecules easily converted to fine chemicals such as fragrance ingredients, pharmacies, building blocks in the synthesis of drugs, and agrochemicals [2,3,4].

Acetalization is one of the most used condensation reactions to protect carbonyl groups in aldehydes or ketones, which is an important step in several routes of organic synthesis. Liquid mineral acids like sulfuric, nitric, and hydrochloric, as well as organic acids such as trifluoracetic are effective catalysts in acetalization reactions, however, despite being cheap, those acids are corrosive and difficult to reuse, requiring still steps of neutralization steps that lead to a generation of waste and residues [5, 6]. Solid-supported catalysts can answer this demand, nonetheless, besides the laborious synthesis, they can be leached and deactivated during the processes [7,8,9]. Lewis’s acid catalysts have also been demonstrated to be an option, such as metal triflates, however, despite their efficiency, they are sensitive to water, which is a co-product of the reaction [10,11,12]. Noble metal complexes and titanium tetrachloride were also effective catalysts in acetalization or ketalization reactions [13,14,15]. Nonetheless, none of these catalysts was used in acetalization reactions of terpene aldehydes.

In previous works, we have evaluated the performance of simple and commercially available transition metal salts (i.e., chloride, sulfate, nitrate) in different monoterpene transformations, likewise esterification, oxidation, acetalization, and ketalization, which were very efficient homogeneous catalysts [16,17,18,19,20,21,22,23]. In almost all of these works, iron (III) salts were always the most active and selective catalysts. Similarly, tin(II) salts have also deserved a highlight in acid-catalyzed reactions [24,25,26,27]. Glycerol, monoterpenes, fatty acids, and benzoic acid were the main substrates and these reactions. Particularly, metal chloride salts are commercially available, inexpensive, and water-tolerant Lewis’s acids, which makes more attractive their use as catalysts in processes of valorization of renewable raw material such as β-citronellal.

In this work, the goal was to investigate the activity of different transition metal cations in alkyl alcohol acetalization reactions with β-citronellal. The effects of main reaction parameters such as time, temperature, catalyst load, and type of alcohol were assessed. Other iron salts with different anions were also evaluated and compared to iron(III) chloride.

Experimental section

Chemicals

All chemicals were acquired from commercial sources. β-citronellal and alkyl alcohols were all Sigma-Aldrich (99 wt%). The salts FeCl3·6H2O, Fe2(SO4)3·5H2O were Dinamica. FeSO4·7H2O and FeCl2·4H2O were Analítica. Fe(Acac)3 and alkyl alcohols were acquired from Sigma Aldrich. NiCl2·6 H2O and MnCl2·2H2O (Neon), CoCl2·2H2O (Vetec), CuCl2 (Dinamica), and ZnCl2 (Exode). All the salts with 97–99 wt% purity. Alkyl alcohols with 99 wt% purity.

Identification of main reaction products

The reaction products were identified in a Shimadzu GC-2010 gas chromatographer coupled with a MS-QP 2010 mass spectrometer, operating at impact electronic mode (70 eV), within the m/z range of 50 to 450.

Catalytic tests

Catalytic runs were carried out in a three-necked glass reactor (ca. 25 mL), fitted with a reflux condenser and sampling septum. Typically, β-citronellal (1.00 mmol) was magnetically stirred, solved in CH3OH (10 mL), and heated to 298 K. The addition of the metal salt catalyst (5.0 mol%) started the reaction.

The progress reaction was followed for 2 h, periodically collecting aliquots and analyzing them in GC equipment (Shimadzu 2010, FID), fitted with a capillary column (Carbowax 20 M, 30 m length, 0.25 mm i.d., 0.25 mm film thickness). The temperature profile used in the GC analyses was as follows: 80 °C (3 min), heating rate (10 °C min−1) until 240 °C. The temperatures of the injector and detector were 250 °C and 280 °C, respectively. Equations 1 and 2 were used to calculate conversion and selectivity.

$${{{\text{\% conversion}}\;{ = }\;\left( {{\mathbf{A}}_{{\mathbf{i}}} {-} \, {\mathbf{A}}_{{\mathbf{r}}} } \right)} \mathord{\left/ {\vphantom {{{\text{\% conversion}}\;{ = }\;\left( {{\mathbf{A}}_{{\mathbf{i}}} {-} \, {\mathbf{A}}_{{\mathbf{r}}} } \right)} {{\mathbf{A}}_{{\mathbf{i}}} \times 100}}} \right. \kern-0pt} {{\mathbf{A}}_{{\mathbf{i}}} \times 100}}$$
(1)

Here Ai = initial area of GC peak of β-citronellal, and Ar = remaining area of GC peak of β-citronellal.

$${\text{\% }}\,{\text{selectivity}}\,{\text{of}}\,{\text{product}}\, = \,{{{\mathbf{A}}_{{\mathbf{p}}} } \mathord{\left/ {\vphantom {{{\mathbf{A}}_{{\mathbf{p}}} } {\left( {{\mathbf{A}}_{{\mathbf{i}}} - {\mathbf{A}}_{{\mathbf{r}}} } \right)}}} \right. \kern-0pt} {\left( {{\mathbf{A}}_{{\mathbf{i}}} - {\mathbf{A}}_{{\mathbf{r}}} } \right)}} \times 100$$
(2)

Here Ap = product GC peak corrected area, Ai = initial area of GC peak of β-citronellal, and Ar = remaining area of GC peak of β-citronellal.

The difference between the consumed area of the GC peak of β-citronellal (Ai-Ar) and the sum of the GC peak corrected areas of the product (Ap) gave the oligomers selectivity (Eq. 3).

$$\% \,{\text{oligomers}}\, = \,\% \,{\text{conversion}}\,\beta {\text{ - citronellal }} - \sum \% {\text{ detected products}}$$
(3)

Results and discussion

Catalytic tests

The catalytic activity of transition metal salts was evaluated in the condensation of methyl alcohol with β-citronellal, following conditions previously described by the literature [28] (Fig. 1). Notwithstanding the methyl alcohol excess, no acetal traces were detected (omitted by simplification).

Fig. 1
figure 1

Effect of transition metal salt catalyst on the kinetic curves of the acetalization reactions of methyl alcohol with β-citronellal. Reaction conditions: β-citronellal (1.0 mmol), toluene (internal standard), catalyst 5.0 mol %), temperature (298 K), CH3OH (5.0 mL)

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Among all the metal chloride salts evaluated, only FeCl3 was an active catalyst to condense β-citronellal and methyl alcohol into β-citronellyl acetal (Fig. 1). In that reaction, only β-citronellyl acetal was selectively formed (Scheme 1).

Scheme 1
scheme 1

FeCl3-catalyzed condensation of β-citronellal with methyl alcohol

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The activity of metal salts in acid-catalyzed reactions has been associated with the ability of these metal cations to react with the water released during the reaction and or the alcohol solvent to release hydronium cations in solution, which may thyself catalyze the acetalization [29]. However, literature has also accepted that the metal cation itself may coordinate with the carbonyl group of the aldehyde in solution, polarizing its double bond and making the alcohol attack easier [30].

Herein, aiming to check this effect, pH measurements were carried out before and after the addition of salt to the alcohol solution (Table 1). Although it is not exactly a pH measurement since the solvent is not water but methyl alcohol, this measurement shows the amount of H+ ions released in the solution, whatever your origin (i.e., water or alcohol) (Eqs. 4 and 5).s

$${\text{M}}^{{\text{X + }}} + {\text{n}}\,{\text{ROH}} \rightleftharpoons \left[ {{\text{M}}\left( {{\text{ROH}}} \right)_{{\text{n}}} } \right]^{{\text{X+ }}}$$
(4)
$$\left[ {{\text{M}}\left( {{\text{ROH}}} \right)_{{\text{n}}} } \right]^{{{\text{X}} + }} + {\text{ROH}} \rightleftharpoons \left[ {{\text{M}}\left( {{\text{ROH}}} \right)_{{{\text{n}} - 1}} \left( {{\text{RO}}} \right)^{{ + {\text{X}} - 1}} } \right] + {\text{ROH}}_{{2}}^{ + }$$
(5)
Table 1 presents the pH measurements of methyl alcohol solutions obtained after the addition of metal chloride salt. Reaction conditions: methyl alcohol (5 mL), catalyst (5 mol%), initial pH (7.5), 298 K
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Table 1 shows the decisive role played by the H+ (or hydronium) cation, which consists in protonating the carbonyl group of β-citronellal favoring the nucleophilic attack of methyl alcohol, significantly increasing its conversion to acetal.

Effect of FeCl 3 concentration on the reactions of methyl alcohol with β-citronellal

The impact of the FeCl3∙6H2O catalyst concentration on the kinetic curves of β-citronellal condensation with methyl alcohol is displayed in Fig. 2.

Fig. 2
figure 2

Effect of FeCl3 6H2O catalyst concentration on the kinetic curves of acetalization reactions of methyl alcohol with β-citronellal. Reaction condition: β-Citronellal (1.0 mmol), CH3OH (5 mL) temperature (298 K)

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An increase in catalyst concentration from 0.6 to 2.5 mol% led to higher conversions, which reached 98%, however, reactions carried out with loads greater than 2.5 mol% achieved almost the same conversion. The pH values were all checked in all the reactions. The pH values were progressively diminished when higher catalyst loads were used (Fig. 3). Conversely, the selectivity was minimally impacted, regardless of the catalyst concentration. In all the runs, a minority products mixture (i.e., nucleophilic addition products of water) was detected (Fig. 3).

Fig. 3
figure 3

Effect of FeCl3∙6H2O catalyst concentration and the pH on the conversion, and selectivity of methyl alcohol acetalization reactions with β-citronellal. Reaction conditions: β-Citronellal (1 mmol), CH3OH (5 mL) temperature (298 K)

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Effect of anion present and oxidation number of the iron catalysts

The impact of the anion present in the iron salt was also investigated, as well as the effect of oxidation number. The kinetic curves are presented in Fig. 4.

Fig. 4
figure 4

Effect of oxidation number and anion on the iron catalyst in the kinetic curves of methyl alcohol acetalization reactions with β-citronellal. Reaction conditions: β-citronellal (1 mmol), iron load (5 mol %), CH3OH (5 mL) temperature (298 K)

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All the catalysts were used at the same iron load. The iron oxidation number plays a key role in the catalytic activity of metal cations, once it affects their Lewis acidity strength [16, 22]. This explains why Fe2(SO4)3·5H2O was a catalyst much more active than FeSO4, notwithstanding both FeSO4·7H2O salt being soluble in the reaction medium and Fe2(SO4)3·5H2O being insoluble.

On the other hand, the nature of the anion was also important for the efficiency of the catalyst. Interestingly, the difference in activity between Fe2+ and Fe3+ chlorides was much lower than the sulfates. Although FeCl3 6 H2O has continued to be the most active (98% conversion), the conversion achieved in the FeCl2-catalyzed reaction was 90%. Conversely, while FeSO4 was basically inactive the Fe2(SO4)3-catalyzed reaction achieved 80% of conversion.

The Fe(Acac)3 salt was soluble, however, it was also an inactive catalyst. It is a consequence of acetylacetonate anion being a strong ligand, hampering the coordination of Fe(III) cation to the carbonyl group of β-citronellal. A similar effect was observed when palladium or tin catalysts containing different ligands were used in acetalization furfural reactions [29, 31].

The pH values of the iron salt solutions were also differently impacted and can be useful to explain the reached. Table 2 summarizes the results.

Table 2 Conversion, selectivity toward methyl acetal β-citronellal, and pH measurements. Reaction conditions: see Fig. 4
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The lower conversions were accomplished in the solutions with lower acidity (i.e., higher pH values, and FeSO4 and Fe(acac)3). Conversely, the most active catalysts were those that triggered the higher decrease in pH value (i.e., Fe2(SO4)3, FeCl2, and FeCl3, respectively). The difference between pH values of Fe2+ salts (chloride and sulfate) can be assigned to the probable hydrolysis of sulfate anion, which consumes protons or hydronium cations.

Insights on the reaction mechanism

Commonly, the acetalization reactions have been carried out using Brønsted acids. In these cases, H+ ions are responsible for carbonyl activation of aldehyde through the protonation step, favoring thus its nucleophilic attack by the hydroxyl group of alcohol.

Herein, the catalyst used was the Fe3+ cations, which can act either polarizing the carbonyl of β-citronellal or generating H+ ions that act as catalysts too. In Scheme 2, the mechanism proposal involving these two catalytic species is depicted.

Scheme 2
scheme 2

The reaction pathway proposed for the β-citronellal acetalization with methyl alcohol containing FeCl3 and H+ ions results from its alcoholysis

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In the first step, the carbonyl group of β-citronellal (1) is activated by the Fe3+ or H+ cations (intermediate 1a), which favor the nucleophilic attack by the methyl alcohol. The hydroxyl group of hemiacetal formed in step II (intermediate 1b), interacts with Fe3+ or H+ cations (intermediate 1c; step III), promoting the exchanges of a proton with a methyl group of another molecule of methyl alcohol, releasing water the dimethyl acetal β-citronellal (2) (step IV).

Influence of temperature on the FeCl3-catalyzed acetalization reactions of β-citronellal with methyl alcohol

To investigate the impact of temperature, FeCl3·6H2O was selected and the reactions were carried out using a low catalyst load to make more visible this effect (Fig. 5).

Fig. 5
figure 5

Effect of temperature on the kinetic curves of FeCl3-catalyzed acetalization reactions of β-citronellal with methyl alcohol. Reaction conditions: β-citronellal (1 mmol), FeCl3∙6H2O (0.6 mol %), CH3OH (5 mL)

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An increase in temperature accelerated the reactions, probably due to a greater number of effective collisions between the reactant molecules, resulting in a higher conversion. In addition, the acetal selectivity was higher in the reactions at temperatures greater than 298 K (Fig. 6).

Fig. 6
figure 6

Effect of temperature on the kinetic curves of FeCl3-catalyzed acetalization reactions of β-citronellal with methyl alcohol

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Influence of alcohol on the FeCl 3 -catalyzed acetalization reactions of β-citronellal

To verify how the steric hindrance effects may affect the conversion and selectivity of the acetalization reactions of β-citronellal, primary and secondary alcohols were evaluated. Figure 7 displays the kinetic curves obtained after 2 h of reaction.

Fig. 7
figure 7

FeCl3-catalyzed β-citronellal acetalization reactions with different alcohols. Reaction condition: β-citronellal (1 mmol), FeCl3∙6H2O (5.0 mol%), alcohols (5 mL), temperature (298 K)

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Noticeably, methyl alcohol was the most reactive substrate. Conversely, ethyl and propyl alcohols reacted at the same rate, achieving the same conversion after a 2 h reaction. Possibly, this different behavior can be attributed to the donating electron effect of the methyl group exercised on the hydroxyl group of methyl alcohol. This should enhance its nucleophilicity, favoring their attack on the carbonyl group of β-citronellal, after it has been protonated (i.e., in the presence of H+ ions), or polarized (i.e., in the presence of Fe(III) cations). An increase in the carbon chain size reduces this electronic effect, making ethyl and propyl alcohols less reactive. Consequently, the butyl alcohol had the lowest reactivity.

The steric hindrance is also an effect that drastically reduces the alcohol reactivity. It can be verified in the reaction with isopropyl alcohol, whose acetalization reached the lowest conversion. The reaction selectivity and final conversion after 2 h of reaction are depicted in Fig. 8.

Fig. 8
figure 8

Effect of alcohol on the conversion and selectivity of FeCl3-catalyzed acetalization reactions of β-citronellal. Reaction conditions: β-citronellal (1 mmol), FeCl3·6H2O (5.0 mol%), alcohols (5 mL), temperature (298 K)

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Regardless of the type of alcohol, the alkyl β-citronellyl acetal was always the main product. In alcohols with higher polarity, the oligomerization of β-citronellal was favoured, as well as the formation of minority products. The impact of the addition of FeCl3·6H2O to the alcohol solution was checked (Table 3).

Table 3 Effect of the addition of FeCl3 to the alcohol solution
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Results in Table 3 show that in some cases, there was a competitive effect between pH, electronic and steric effects, which are decisive to explain the alcohol's reactivity. First of all, compared to other alcohols, the greatest decrease in pH value was observed in the methyl alcohol solution, making him the most reactive substrate. Moreover, higher conversions were achieved in the more acidic solutions (methyl and isobutyl alcohol). However, even with a low value of pH, the isopropyl alcohol was less reactive, due to steric hindrance.

Although butyl and isobutyl alcohols are primary alcohols with the same number of carbon atoms, they had different reactivity; the acetalization of β-citronellal was more effective with isobutyl than butyl alcohol. It can be assigned to the greater decrease in pH value achieved after the addition of the FeCl3 catalyst (Table 3).

Conclusions

The iron-catalyzed acetalization reactions of β-citronellal with alkyl alcohols were assessed. Initially, a series of transition metal chlorides were evaluated in reactions of methyl alcohol with β-citronellal, among them, only FeCl3 6 H2O was an effective catalyst. It was ascribed to the higher decrease in pH value triggered by the addition of the FeCl3 catalyst. The activity of FeCl3 was compared to the other Fe3+ or Fe2+ salts containing different anions. The higher oxidation number favored the activity of the catalyst, regardless of the anion present in the salt. On the other hand, for the oxidation number, chloride salts were more reactive than sulfate ones. Once more it could be correlated to the pH values of alcohol solutions of catalysts. An increase in temperature or catalyst load enhances the conversion of reactions. Different alcohols were acetalized with β-citronellal. The carbon chain sizes, besides the electronic and hysteric effects, affected the reactivity of alcohols, being methyl alcohol the most reactive. Surprisingly, isobutyl alcohol was more reactive than shorter-chain alcohols like ethyl and propyl alcohols. Measurement of the pH of its FeCl3·6H2O solution was compatible with this result showing that it reacts strongly with FeCl3. The mild reaction conditions, and the use of a non-corrosive, inexpensive, and commercially affordable catalyst are positive aspects of this process.