Base catalyzed sustainable synthesis of phenyl esters from carboxylic acids using diphenyl carbonate

Abstract

Phenyl esters were obtained in moderate to high yields by reaction of aliphatic and aromatic carboxylic acids with one equivalent of diphenyl carbonate in the presence of catalytic amounts of tertiary amine bases, such as DBU, TBD and DMAP under neat conditions at elevated temperatures (>100 °C).

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Introduction

Diphenyl carbonate (DPC) is a sustainable and environmentally benign reagent mainly, used as phosgene-substitute for the synthesis of polycarbonates.¹ Several eco-friendly procedures for the synthesis of DPC are reported,² including the oxidative carbonylation of phenol in the presence of palladium catalysts³ and the transesterification of dimethyl carbonate (DMC) with phenol using diverse catalysts.⁴ Moreover, transesterification reactions of dimethyl oxalate with phenol to DPC are described.⁵ A modern and promising method is the direct synthesis of DPC from phenol and carbon dioxide.⁶ DPC is often used for the synthesis of bisphenol A derived polycarbonates,⁷ but also found application in the preparation of other aliphatic and aromatic polycarbonates.⁸ Moreover, DPC is used for the protection of sugars and other vicinal diols as cyclic carbonates.⁹ Generally, however, the application of DPC as sustainable and environmentally-friendly reagent in organic chemistry is rarely described. A few articles reported the use of DPC for the synthesis of heterocycles,¹⁰ as well as for N-carbonylations of amines and N-heteroaromatics.¹¹ Moreover, in 1998 Chiriac et al. reported that DPC is a suitable reagent for the synthesis of phenyl esters from carboxylic acids.¹² They demonstrated that aromatic carboxylic acids react with DPC in the presence of one equivalent of 4 (dimethylamino)pyridine (DMAP) as base and pyridine as solvent to the corresponding phenyl esters in moderate to good yields.

Typically, phenyl esters are prepared by reacting phenols with carboxylic acid chlorides or anhydrides.¹³ Besides, coupling reagents such as phosphorus pentoxide,¹⁴ carbodiimides,¹⁵ 2-chloro-1-methylpyridinium iodide (Mukaiyama reagent),¹⁶ chlorosulfonyl isocyanate,¹⁷ as well as numerous conventional and modern reagents were successfully applied for the synthesis of phenyl esters.¹⁸ Direct acid catalyzed esterifications of carboxylic acids with phenol are described as well,¹⁹ but difficult to perform and only low yields are obtained and side reactions occur. Moreover, transesterification reactions of esters with phenols are reported.²⁰ Another useful method for the preparation of phenyl esters is the well-known Mitsunobu reaction.²¹ However, these procedures rely on the use of activated carboxylic acid derivatives or an in situ activation and thus produce a large amounts of waste.

Results and discussion

Here, we report a catalytic procedure to obtain phenyl esters of different carboxylic acids. Tertiary amine bases were used as catalyst under neat conditions, which offers significant advantages to similar procedures, such as the one described by Chiriac and coworkers.¹² In a first experiment, we performed the reaction of castor-oil derived 10-undecenoic acid 2a²² with DPC 1 (1 eq.) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 1 eq.) as strong organic base by heating the mixture to 100 °C (Scheme 1). Immediately, an acute gas formation occurred and subsequent analysis by thin layer chromatography (TLC) indicated a complete conversion of the carboxylic acid under release of phenol. After purification of the crude reaction mixture by column chromatography, the phenyl ester 3a was obtained in nearly quantitive yields. Inspired by this result, other tertiary amine bases were investigated in stoichiometric and catalytic quantities (Table 1, entry 1–6). Potassium carbonate, as a cheap inorganic base, did not induce the phenyl ester formation. Similar to reactions with pyridine, the formation of 3a was very slow and after 8 hours reaction time only traces of the respective phenyl ester were obtained (entries 2 and 3). Applying 1,4-diazabicyclo[2.2.2]octane (DABCO) as amine base in stoichiometric quantities led to full conversion after seven hours (entry 4). As also observed by Chiriac,¹² DMAP gave very good results in stoichiometric amounts. Indeed, similar to DBU, full conversion was reached within 15 minutes using DMAP (entry 5). Also the strong cyclic guanidine base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)²³ resulted in a complete conversion to 3a in short reaction times (entry 6). Next, we were interested if also catalytic quantities of the successfully tested bases can be applied for the phenyl ester formation. However, reactions with 0.2 equivalents of DBU at 100 °C only showed a very slow conversion of the carboxylic acid (entry 7). In order to accelerate the reaction, higher temperatures were were applied to favor the phenyl ester formation. If 0.4 equivalents of DBU were used at 130 °C, after one hour, a complete conversion to 3a was detected (entry 8), while reactions with 0.2 equivalents of DBU showed full conversion after three hours (entry 9). Interestingly, a further increase of the temperature to 160 °C significantly reduced the reaction time without observing the formation of side products (entries 10–13). Thus, full conversion was achieved after one hour utilizing only 0.2 equivalents of DBU. Reducing the amount of DBU to only 3.0 mol% also yielded complete conversion, although longer, but still acceptable, reaction times were needed (entry 13). These results clearly demonstrate that the temperature has a strong influence on the reaction time and, more importantly, phenyl ester formation using DPC is possible by using only catalytic amounts of amine bases.

Scheme 1 Synthesis of phenyl 10-undecenoate 3a by the reaction of 10 undecenoic acid 2a with DPC 1 in the presence of different bases under neat conditions.

Table 1 Results for the synthesis of phenyl 10-undecenoate 3a using different bases and conditions

Entry	Base [eq.]	T [°C]	Complete conversion reached in^a
a Reactions were monitored by thin layer chromatography (TLC) and the conversions were determined by ^1H NMR.
1	DBU [1]	100	15 min
2	K2CO3 [1]	100	Very slow
3	Pyridine [1]	100	Very slow
4	DABCO [1]	100	7 hours
5	DMAP [1]	100	15 min
6	TBD [1]	100	15 min
7	DBU [0.2]	100	Very slow
8	DBU [0.4]	130	1 hour
9	DBU [0.2]	130	3 hours
10	DBU [0.2]	160	1 hour
11	DBU [0.1]	160	2 hours
12	DBU [0.05]	160	4 hours
13	DBU [0.03]	160	8 hours

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As a next step, several other aliphatic and aromatic carboxylic acids were investigated for their performance in this catalytic phenyl ester formation. Similarly to 2a, oleic acid 2b and stearic acid 2c were fully converted in the presence of 5.0 mol% DBU after eight hours at 160 °C. After purification by column chromatography, phenyl oleate 3b (82%) and phenyl stearate 3c (90%) were obtained in good yields (Scheme 2). The phenolic acid derivative 2d, derived from renewable ferulic acid,²⁴ was also successfully converted to the corresponding phenyl ester 3d (yield 79%). In case of levulinic acid 2e, a valuable renewable platform chemical,²⁵ the preparation of the corresponding phenyl ester 3e was less successful and pure 3e was isolated only in a yield of 44%. Lower temperatures and/or applying higher amounts of DBU did not lead to improved yields in this case.

Scheme 2 Synthesis of phenyl esters 3a–l from carboxylic acids 2a–l with DPC 1 and DBU as catalyst under neat conditions. 10 mol% of DBU and two equivalents of DPC 1 were used.

Moreover, the fatty acid derived dicarboxylic acids sebacic acid 2f,²² azelaic acid 2g,²⁶ and adipic acid 2h (ref. 27) were successfully converted to the corresponding diphenyl esters 3f–h. After removal of phenol via vacuum distillation, the diphenyl esters 3f–3h were obtained as colorless solids in good yields of almost 80% by simple recrystallization. It is noteworthy that the phenol can be recycled by simple vacuum distillation and almost recovered quantitatively (93%), thus increasing the sustainability of this process. Unfortunately, the conversions of amino acid derivatives, such as Boc- or Cbz-protected glycine, were not successful. Under the applied conditions, numerous side-reactions, e.g. polymerizations and fragmentations occurred, and only traces of the desired amino acid phenyl ester derivatives could be isolated. Nevertheless, several other functional groups, such as ketones, aldehydes and double bonds, are well tolerated in this reaction.

Since phenyl esters are “active esters”, they are valuable intermediates for the synthesis of versatile alkyl esters, thioesters and amides.²⁸ Under neutral and acidic conditions, they are quite stable and can even be considered as protective groups in peptide chemistry,²⁹ but under basic conditions or enzyme catalysis, phenyl esters hydrolyze quickly in the present of water,³⁰ or can be transesterified with alcohols to alkyl esters.^28,31 Moreover, in the presence of acidic catalysts, phenyl esters can undergo the so-called Fries rearrangement to obtain acyl phenols.³² This reaction can also be induced by light, known as the so-called photo-Fries rearrangement.³³

To verify the reactivity, diverse transesterification reactions were performed by reacting 3a with different alcohols 5a–d in the presence of catalytic amounts of TBD at higher temperatures and under solvent-free conditions (Scheme 3). The reaction of 3a with a slight excess of benzyl alcohol 5a and 5.0 mol% of TBD under neat conditions led to full conversion after 90 minutes at 120 °C and pure benzyl 10 undecenoate 6a was isolated in a yield of 91% (Table 2, entry 1). For the transesterification with alcohols exhibiting low boiling points, reactions were performed in a microwave reactor. By applying a temperature of 140 °C under neat conditions, the reactions of 3a with allyl alcohol 5b, n-butanol 5c and isopropanol 5d yielded full conversion to the corresponding alkyl esters 6b–d after 60 minutes of reaction time (entry 2–4). Contrary, the reaction of 3a with isopropanol 5d to 6d was completed after three hours, probably due to steric hindrance. After purification via column chromatography, alkyl 10-undecenoates 6b–d were obtained in almost quantitative yields. Moreover, the isolation of the phenyl esters is not necessary for the additional conversion with alcohols to the corresponding alkyl esters. In a one-pot procedure, an excess of a primary or secondary alcohol can add directly to the phenyl ester and stirring at higher temperature gives the wished alkyl ester derivative. Unfortunately, tertiary alcohols such as tert-butanol did not react with the phenyl esters to the desired tert-butyl ester derivatives. Other basic catalysts as well as high temperatures also did not yield the corresponding tert-butyl esters.

Scheme 3 Transesterifications of phenyl 10-undecenoate 3a with alcohols 5a–d and TBD as catalyst under neat conditions.

Table 2 Results for the transesterification of phenyl 10-undecenoate 3a with alcohols 5a–d

Entry	Product	Conditions^a	Yield^b [%]
a MW = microwave reactor.b Isolated yields after column chromatography.
1	6a	120 °C/90 min	91
2	6b	MW/140 °C/60 min	95
3	6c	MW/140 °C/60 min	95
4	6d	MW/140 °C/3 hours	93

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Moreover, phenyl esters can be coupled with ammonia as well as with primary or secondary amines to obtain the corresponding amide derivatives. In a one-pot procedure, the synthesized phenyl 10 undecenoate 3a was subsequently reacted with an excess of ammonia (7 N solution in methanol 7a) to obtain 10-undecenamide 8a (Scheme 4). Complete conversion of 3a was observed at room temperature after three hours. After recrystallization in n-hexane/ethyl acetate mixture, 8a was obtained in a yield of 69%.

Scheme 4 Amidation reactions of phenyl esters 3a and 3i with ammonia 7a, primary amine 7b and pyrrolidine 7c.

The reaction of 3a with a slight excess of dec-9-en-1-amine 7b, as representative primary amine, led to full conversion after 15 minutes at 100 °C and under neat conditions to obtain amide derivative 8b (yield 92%) after recrystallization. The reaction with secondary amines is also feasible, but longer reaction times are required and the obtained yields are comparably low. As example, the one pot reaction of phenyl benzoate 3i with pyrrolidine 7c (1.2 equivalents) led to full conversion after one day at 80 °C. Isolation and purification of the crude product afforded the amide derivative 8c in a yield of 48%. These examples demonstrate the use of DPC 1 as coupling agent for the synthesis of amides from carboxylic acids under neat conditions by a straightforward one-pot procedure.

To evaluate the sustainability of the catalytic phenyl ester synthesis, the environmental factor (E factor) was determined and compared to conventional phenyl ester synthesis procedures. The E-factor is defined by the ratio of the mass of waste per unit of generated product.³⁴ As an example, for the reaction of 10-undecenoic acid 2a with DPC 1 and 5.0 mol% of DBU to phenyl 10 undecenaote 3a (yield of 89%) an E-factor of 0.93 was calculated. Considering that phenol is not a waste product and can be recycled in an average yield of 93% (see performed recycling above), the E-factor reduces to a value of 0.51. Industrially, it should further be possible to recycle the released carbon dioxide, which would further reduce the E factor to a value of 0.30. Compared to the procedure of Chiriac et al.¹² using stoichiometric amounts of DMAP and pyridine as solvent, a considerably higher E-factor of 4.15 was estimated. The assumption that product 3a will be synthesized according to a standard procedure^13a from 2a with phenol in the presence of 0.4 equivalents of phosphorus(V) oxychloride and the yield will be the same, a higher E factor of 0.61 was calculated. Moreover, harmful organic solvents are often used and the isolated product yields of this reaction are normally lower. Furthermore, the E-factors of conventional coupling methods such as the Mitsunobu reaction²¹ or other reactions utilizing common coupling reagents (Mukaiyama reagent¹⁶ or chlorosulfonyl isocyanate¹⁷) were calculated. Although, the phenyl ester formation was assumed to be quantitative, high E factors were calculated for these procedures (19.1 for the Mitsunobu reaction, 78.4 applying the Mukaiyama reagent, and 26.9 using chlorosulfonyl isocyanate). The extremely high E factors mainly result from the high amounts of solvents (THF and dichloromethane). By considering that the solvent can be recycled completely for these procedures or the coupling reactions are possible under neat conditions, the values of the E-factors decrease dramatically (E = 1.96 for the Mitsunobu reaction, E = 2.96 with Mukaiyama reagent and E = 1.38 with chlorosulfonyl isocyanate). Nevertheless, the calculated E-factors of conventional phenyl ester synthesis procedures are significantly higher than the ones obtained by our new catalytic procedure with DPC as coupling reagent.

Conclusions

In summary, we developed a novel sustainable approach for the synthesis of phenyl esters. The reactions of aliphatic and aromatic carboxylic acids with equimolar ratios of diphenyl carbonate in the presence of catalytic amounts of tertiary amine bases such as DBU, TBD and DMAP under neat conditions at temperatures over 100 °C gave phenyl esters in moderate to excellent yields. Noteworthy, only 3.0 mol% of catalyst is sufficient to achieve full conversion. Furthermore, in a one-pot procedure, the phenyl esters can be directly converted with primary and secondary alcohols to obtain alkyl esters in high yields. One-pot reactions with primary and secondary amines led to the corresponding amide derivatives. We demonstrated that phenol can be recovered with yields up to 93% via vacuum distillation. Since phenol and carbon dioxide are the only by-products, which can be easily recycled, the introduced catalytic procedure is more sustainable and environmental benign compared to other phenyl ester synthesis protocols. Generally, a more sustainable, efficient and straightforward method for the synthesis of phenyl esters in is described.

Acknowledgements

We kindly acknowledge financial support from the German Federal Ministry of Food, Agriculture and Consumer Protection (represented by the Fachagentur Nachwachsende Rohstoffe; FKZ 22023008; collaborative project: SynRg).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and analytic data of synthesized compounds. See DOI: 10.1039/c5ra10206e

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