Continuous flow dynamic kinetic resolution of rac-1-phenylethanol using a single packed-bed containing immobilized CAL-B lipase and VOSO₄ as racemization catalysts

Abstract

Herein, we report a method for the continuous flow DKR of rac-1-phenylethanol catalyzed by CAL-B and VOSO₄, using a single packed-bed containing both catalysts. Taking advantage of continuous flow techniques to circumvent compatibility issues, the DKR of rac-1-phenylethanol could be performed continuously to give enantiopure (R)-esters with excellent conversions and high productivities.

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Introduction

One-pot multistep processes are a current trend in organic synthesis and an attractive strategy for the industrial manufacture of APIs. Performing reactions concurrently or in tandem offers several advantages regarding efficiency and sustainability, since it reduces waste generation and energy consumption by decreasing the number of isolation and purification steps and can also improve reactivity and selectivity by driving equilibrium reactions to completion. This concept has taken advantage of the combination of chemo- and biocatalysis, emerging as a very promising approach in asymmetric catalytic synthesis. The development of such chemoenzymatic one-pot processes, however, is usually very challenging due to the incompatibility between operational conditions required by each catalyst or even mutual inactivation.^1,2 The power of chemoenzymatic one-pot processes in the synthesis of enantiopure compounds has been demonstrated in dynamic kinetic resolution (DKR) reactions, which arose in the 90s^3–7 and are nowadays a well-established method for obtaining chiral alcohols and amines.^8–11 In a chemoenzymatic DKR reaction, the kinetic resolution (KR) of a racemate is performed through an enzyme-catalyzed enantioselective acylation reaction while chemocatalytic racemization of the substrate takes place concurrently (Scheme 1), leading to the product as a single enantiomer with up to 100% yield.^8,9,12–14 The reaction is carried out in organic media and lipases are the most used biocatalysts, since they are tolerant to non-aqueous solvents and present high activity and stereoselectivity towards a wide range of substrates. The racemization step is usually accomplished with the use of Ru(II) and Ir(III) complexes or Pd(0) supported on basic matrices,^15–19 though the use of enzymes,^20,21 Al,²² vanadium-based compounds,^23–26 acid supported on solid matrices, zeolites^27–32 and photogenerated radicals^33–36 has also been reported. In spite of several reports on DKR, there are many drawbacks and challenges that still need to be overcome in this area. The repertoire of racemization catalysts that are compatible with enzymes is narrow and most of them are expensive, non-eco-friendly or not readily available and usually operate under conditions that are harmful to biocatalysts, such as high temperatures and the presence of strong bases.

Scheme 1 Dynamic kinetic resolution.

Continuous flow chemistry has emerged as an enabling technique with great potential for application in multicatalytic processes,^37–40 since it allows reactions to be performed continuously through sequential individual reactors. The use of packed-bed reactors, for instance, enables catalyst compartmentalization, thus circumventing compatibility issues, as well as extends the lifetime of the catalysts by avoiding stirring and allows their reuse, which may be crucial when dealing with expensive catalysts. Additionally, continuous flow processes present advantages over batch procedures such as improved mass and heat transfer, better safety profiles, easier work-up and facilitated automation and scale-up as well as precise control of temperature, pressure and reaction time, leading to higher reproducibility.^38,41–43

In spite of the advantages outlined above, only a few examples of continuous DKR have been reported and the potential of continuous flow techniques in this area still remains underexploited. The examples reported so far include DKR of stereochemically labile substrates, such as benzoins⁴⁴ and thioesters,⁴⁵ semi-continuous DKR of amines catalyzed by lipases and Pd(0)^46,47 or Shvö catalysts,⁴⁸ and DKR of rac-1-phenylethanol using continuous flow reactors packed with immobilized lipases coated with ionic liquids and acid matrices such as zeolites and silica modified with acid groups as racemization catalysts.^27,28,49,50

In our continuous effort to develop continuous flow strategies for chemoenzymatic reactions,^46,51–53 herein we describe the continuous flow DKR of rac-1-phenylethanol catalyzed by immobilized Candida antarctica lipase B (CAL-B) and VOSO₄, a non-expensive and readily available vanadyl oxide salt that operates as a heterogeneous acid catalyst to promote the racemization of benzylic alcohols.⁵⁴ Its use in ordinary chemoenzymatic DKR reactions, however, is hampered due to its incompatibility with lipases. In the present work, we take advantage of a continuous flow approach to address catalyst incompatibility issues in order to develop an efficient, cost-effective and simple method for the lipase- and VOSO₄-catalyzed DKR of rac-1-phenylethanol. By using a packed-bed reactor containing heterogeneous chemo- and biocatalysts, the method described herein allows continuous DKR with easy recovery of the product and catalyst reuse.

Results and discussion

We began our study by carrying out a brief solvent screening for the VOSO₄-catalyzed racemization of (S)-1-phenylethanol in batch (Table 1). Almost complete racemization occurred in all solvents after 1.5 h, where by-product formation was also observed leading to poor selectivity in some cases. The by-products were identified by GC-MS as being stereoisomeric di-benzyl-ethers, already reported by Wuyts⁵⁴ as evidence for a racemization mechanism involving acid catalysis (Scheme 2). Nevertheless, satisfactory selectivity could be achieved by performing the reaction in toluene at 70 °C with a lower catalyst loading (entry 5).

Table 1 VOSO₄-catalyzed racemization of (S)-1-phenylethanol in different solvents

Entry	Catalyst load (mg mL^−1)	Solvent	T (°C)	ee^a (%)	Sel^b (%)
Conditions: (S)-1-phenylethanol (20 μL, 0.16 mmol), solvent (3.3 mL), VOSO4·XH2O (34 or 17 mg), tetracosane (internal standard, 14 mg, 0.04 mmol), 1.5 h. a Determined by GC-FID. b Determined by GC-MS.
1	10.3	Isooctane	80	3	1
2	10.3	Heptane	80	<1	10
3	10.3	Cyclohexane	80	<1	13
4	10.3	Toluene	80	1	26
5	5.1	Toluene	70	2	91

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Scheme 2 VOSO₄-catalyzed racemization of 1-phenylethanol.

After identifying the best solvent system for the racemization step, the efficiency of racemization of (S)-1-phenylethanol under optimal conditions in the presence of potential acyl donors for DKR was assessed, since VOSO₄-catalyzed racemization was previously reported to be inhibited by esters usually used in these reactions, such as vinyl and isopropenyl acetates.⁵⁴ The results are summarized in Table 2. Racemization was shown to be less effective at a lower concentration of vinyl acetate (entry 1) in comparison to higher concentrations of this ester (entries 2 and 3), whereas the opposite effect was observed when the reaction took place in the presence of vinyl decanoate (entries 7–9). Nonetheless, racemization was almost complete at a concentration of 0.1 M for both acyl donors, leading to similar ee values in comparison to the reaction carried out in the absence of esters (Table 1, entry 5). On the other hand, isopropenyl acetate and ethyl acetate were found to inhibit racemization even at concentrations of 0.05 M, where in the presence of the latter, the effect was observed to a lower extent. Methyl 2-methoxyacetate, an ester widely used in enzymatic KR of amines, was also investigated for its effect on VOSO₄-catalyzed racemization of (S)-1-phenylethanol, leading to the desired low ee values at only 0.05 M.

Table 2 Effect of acyl donors on the VOSO₄-catalyzed racemization of (S)-1-phenylethanol

Entry	Acyl donor	Acyl donor conc.	ee^a (%)	Sel^a (%)
Conditions: (S)-1-phenylethanol (20 μL, 0.16 mmol), toluene (3.3 mL), acyl donor, VOSO4·XH2O (17 mg), tetradecane (internal standard, 11 μL, 0.04 mmol), 70 °C, 1.5 h. a Determined by GC-FID.
1	Vinyl acetate	0.05	28	92
2		0.1	2	87
3		0.2	8	96
4	Isopropenyl acetate	0.05	47	82
5		0.1	60	45
6		0.2	74	3
7	Vinyl decanoate	0.05	0	72
8		0.1	6	89
9		0.2	28	100
10	Ethyl acetate	0.05	44	100
11		0.1	20	100
12		0.2	44	100
13	Methyl 2-methoxyacetate	0.05	>1	27
14		0.1	21	82
15		0.2	24	75

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From the results described above, our first attempts to perform the continuous flow DKR of rac-1-phenylethanol were based on the use of vinyl acetate and vinyl decanoate as acyl donors. Vinyl acetate has been used in enzymatic KR of a wide range of alcohols while vinyl decanoate would presumably be a suitable alternative to the former in chemoenzymatic lipase and VOSO₄ co-catalyzed DKR of rac-1-phenylethanol, since its analogue, vinyl octanoate, has been reported as a successful acyl donor in such reactions.⁵⁴ Additionally, ethyl acetate was also screened, since it is readily available and less expensive than vinyl esters. Although it has been shown to inhibit racemization in batch, it was envisioned that performing the reaction under continuous flow conditions could mitigate this effect due to the continuous removal of ethyl acetate or its by-products during the reaction.

Commercial immobilized CAL-B (Novozym 435) was chosen as the biocatalyst for the kinetic resolution step due to its availability, robustness and well-known stereoselectivity and activity towards the proposed substrate. In order to circumvent the previously reported incompatibility between CAL-B and VOSO₄,⁵⁴ a continuous flow packed-bed reactor was designed so that four layers of the immobilized enzyme and three layers of VOSO₄ were alternately disposed along a glass column and physically separated by a thin cotton layer (Fig. 1). For such a set-up, which allows a sequence of 4 KR and 3 racemization alternate steps, up to 93.8% of the substrate could be converted to a product with 100 ee.⁵⁷ The reactor was heated to 70 °C and a toluene solution containing rac-1-phenylethanol (0.1 M) and an acyl donor (0.1 M) was pumped through the packed-bed at different rates. Samples were collected from the output of the column at three different times and were analyzed for conversion (C) and enantiomeric excess of the product (ee) after 2 hours in order to secure steady state conditions (Tables 3 and 4).

Fig. 1 Packed-bed reactor for CAL-B and VOSO₄-catalyzed DKR of rac-1-phenylethanol under continuous flow conditions.

Table 3 CAL-B and VOSO₄-catalyzed DKR of rac-1-phenylethanol using ethyl acetate and vinyl acetate as acyl donors under continuous flow conditions

Entry	Acyl donor	Res. time (min)	C (%)	Sel^b (%)	ee^c (%)
Conditions: toluene, rac-1-phenylethanol (0.1 M), acyl donor (0.1 M), packed-bed reactor containing immobilized CAL-B (4 layers of 500 mg) and VOSO4·XH2O (3 layers of 500 mg), flow rates of 0.5, 1.0 and 1.5 mL min^−1, 70 °C. a Determined by GC-FID through determination of the substrate concentration using a calibration curve. b Determined by GC-FID and calculated from the relative peak areas of the product and by-products. c Determined by GC-FID.
1	Ethyl acetate	5.2	29	95	99
2		7.8	39	89	99
3		15.6	47	91	96
4	Vinyl acetate	5.2	92	68	99
5		7.8	75	88	89
6		15.6	75	87	66

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Table 4 CAL-B and VOSO₄-catalyzed DKR of rac-1-phenylethanol using vinyl decanoate as the acyl donor under continuous flow conditions

Entry	Res. time (min)	C (%)	Sel^b (%)	ee^c (%)	Y (%)
Conditions: toluene, rac-1-phenylethanol (0.1 M), acyl donor (0.1 M), packed-bed reactor containing immobilized CAL-B (4 layers of 500 mg) and VOSO4·XH2O (3 layers of 500 mg), flow rates of 0.5, 1.0 and 1.5 mL min^−1, 70 °C. a Determined by GC-FID through determination of the substrate concentration using a calibration curve. b Determined by GC-FID and calculated from the relative peak areas of the product and by-products. c Determined by GC-FID of 1-phenylethanol obtained from hydrolysis of the isolated product. d Calculated for the isolated product obtained from the reaction solution collected from the output of the reactor for 2 h of operation.
1	5.2	64	92	99	67
2	7.8	61	92	99	65
3	15.6	96	89	99	55

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The use of ethyl acetate as an acyl donor in the continuous flow DKR of rac-1-phenylethanol led to the product with high enantiopurity and conversions below 50% in all cases (entries 1–3), thus resembling KR reactions and being consistent with racemization inhibition as suggested by previous experiments (Table 2).

By using vinyl acetate, conversions up to 92% and 99% ee could be achieved at a residence time of 5.2 min, whereas at longer residence times, lower enantiomeric excesses and conversions were observed (entries 4–6). The incompatibility of the bio- or chemocatalyst with the acyl donor (or its side products released during the reaction, namely acetic acid and acetaldehyde) could possibly account for these results as well as inhibition of VOSO₄ racemization by vinyl acetate, which was reported in previous work.⁵⁴ As observed, such effects would be greater at longer residence times due to longer contact time of the reactant solution with the catalysts. One could also suggest possible degradation of the product promoted by the catalysts, but this hypothesis is not consistent with the finding that higher conversions were obtained for the reaction with ethyl acetate at longer residence times.

Continuous flow DKR of rac-1-phenylethanol using vinyl decanoate as the acyl donor led to the product with 99% ee and moderate conversions (Table 4). Higher conversions were obtained from the analysis of samples collected at early operation times in all cases, but conversions determined after 60 and 120 minutes were similar, indicating that the steady state was already reached after 60 minutes of operation. After work-up and purification by flash chromatography, enantiopure (R)-phenylethyl decanoate could be obtained in 67% yield by performing the reaction at a flow rate of 1 mL min⁻¹ and a residence time of 7.8 min (entry 2). Interestingly, the highest conversion but the lowest isolated yield were obtained for the reaction carried out at the longest residence time (entry 3). It is important to point out that the obtained conversion is higher than the maximum theoretical conversion expected for the set-up used, which corresponds to 93.8% considering a product with 100% ee.⁵⁷ Because the conversions reported herein were calculated on the basis of substrate consumption, formation of by-products probably accounts for the observed high value. Moreover, conversion was determined from a sample collected after 2 h of operation, whereas the yield, which was found to be 55%, was calculated from the isolated product obtained by collecting the reaction solution from the output of the reactor for 2 h of operation. Therefore, the discrepancy between conversion and yield may also come from variations in the reaction efficiency throughout the operation time.

In an attempt to increase the reaction yield, the chemoenzymatic DKR of rac-1-phenylethanol was performed continuously at 1 mL min⁻¹ using a higher concentration of vinyl decanoate (0.15 M, 1.5 eq.). The solution collected from the reactor for 2 h of operation was evaporated and the residue was purified by flash chromatography to obtain 2.71 g (82% yield) of the product (R)-phenylethyl decanoate with 90% ee and a space-time-yield of 1.35 g h⁻¹. The yield and enantiomeric excess obtained are slightly lower in comparison to those for the batch reaction described by Wuyts et al.,⁵⁴ which afforded (R)-phenylethyl octanoate with 99% ee and 93% yield in 3.5 h. However, as reported by these authors, physical separation of lipase and VOSO₄ was found to be necessary, thus the reaction was performed in a batch reactor where the biocatalyst was confined in a rotating Inox basket inside a glass reactor containing the chemocatalyst. Moreover, the reported batch reaction was performed with a 10× lower substrate concentration (0.0127 M) and required a two-step addition of the acyl donor. Methods for continuous flow DKR of rac-1-phenylethanol have already been reported by Lozano and co-workers^27,28 and were found to give (R)-1-phenylethanol esters with good yields (76–98%) and excellent enantiomeric excesses (91–98%) by employing CAL-B and modified acid resins. A vanadium-based continuous flow DKR of rac-1-phenylethanol was also previously reported by Poppe and co-workers with good selectivities of ∼94% and moderate yields (63%).⁵⁶

As an alternative approach, a continuous flow DKR of rac-1-phenylethanol using vinyl decanoate as the acyl donor was performed by recirculating the reactant solution through a semi-continuous loop-like reactor in which the bio- and chemocatalysts were displaced in two distinct packed-bed reactors. This set-up is useful to overcome the incompatibility between the catalysts and may be especially suitable for thermosensitive biocatalysts, since the kinetic resolution and racemization steps are allowed to occur in different compartments and under different temperatures. By performing the reaction with 20 mL of a toluene solution of rac-1-phenylethanol (0.1 M) and vinyl decanoate (0.15) for 3 h at 100 μL min⁻¹ and residence times of 15 min in the packed-bed containing CAL-B and 5 min in that containing VOSO₄, a conversion of 93% and 90% ee were achieved. GC-MS analysis showed peaks for the substrate, product and by-products with relative areas of 8%, 82% and 10%, respectively.

Conclusions

Continuous flow techniques were successfully applied to the chemoenzymatic DKR of rac-1-phenylethanol in order to circumvent bio- and chemocatalyst incompatibility issues, thus allowing for the use of a combination of catalysts that is not possible in ordinary batch reactions. Thereby, a method for the continuous DKR of rac-1-phenylethanol using CAL-B and the non-expensive, readily available heterogeneous catalyst VOSO₄ for the racemization step was developed for the first time, leading to enantiopure esters with high enantiopurity and moderate conversions. By using this method, 2.718 g (82% yield) of (R)-phenylethyl decanoate with 90% ee could be obtained continuously from rac-1-phenylethanol in 2 h. Alternatively, vinyl acetate could be used as an acyl donor leading to the desired chiral product with 92% conversion and 99% ee with a space-time-yield of 0.96 g h⁻¹. Alternatively, the product could be obtained with 99% ee and 65% yield by using an equimolar amount of vinyl decanoate.

Experimental section

General

All solvents were purchased from Vetec. Isopropenyl acetate, vinyl acetate, methyl 2-methoxyacetate, vinyl decanoate, tetradecane, tetracosane, rac-1-phenylethanol, (S)-1-phenylethanol and VOSO₄·XH₂O were purchased from Sigma-Aldrich and immobilized Candida antarctica lipase B (Novozym 435) was purchased from Novozymes. All reagents and solvents were used as-received.

Reactions in batch were carried out in 4 mL vials heated in silicon carbide plates and under gentle stirring using a magnetic bar.

Reactions under continuous flow conditions were performed using an Asia Flow System purchased from Syrris, which consists of a syringe pump, a heater and a glass column with adjustable ends.

GC-MS analysis was performed on a Shimadzu GC-MS-QP2010 Plus chromatograph-mass spectrometer equipped with an AOC-20i autosampler and a 5-(phenyl)methylpolysiloxane column (29.6 × 0.25 mm ID) using helium as carrier gas. The injector temperature was set at 250 °C. GC-MS temperature program used in the analysis of racemization experiments: 100 °C | 2 min → 280 °C, 20 °C min⁻¹ | 15 min. GC-MS temperature program used in the analysis of DKR reactions: 60 °C | 2 min → 65 °C, 1 °C min⁻¹ | 1 min → 280 °C, 20 °C min⁻¹ | 15 min.

Chiral GC analysis was performed on a Shimadzu GC-2010 chromatograph equipped with a FID detector, an AOC-20i autosampler and a chiral β-Dex-325 column using hydrogen as carrier gas. The injector temperature was set at 230 °C. GC-FID temperature program: 117 °C | 5 min → 126 °C, 3 °C min⁻¹ | 0 min → 200 °C, 20 °C min⁻¹ | 15 min.

¹H- and ¹³C{¹H}-NMR spectra were recorded on a 300 MHz Bruker instrument at 300 and 200 MHz, respectively. Chemical shifts are expressed in ppm downfield from tetramethylsilane (TMS). Signal multiplicities are indicated by the letters s (singlet), d (doublet), t (triplet), q (quartet) and m (multiplet).

Preparation of rac-1-phenylethyl decanoate⁵⁵

To a solution of decanoic acid (231 μL, 1.2 mmol) in dichloromethane were added N,N-dicyclohexylcarbodiimide (227 mg, 1.1 mmol), N,N-dimethylaminopyridine (122 mg, 1.0 mmol) and rac-1-phenylethanol (121 μL, 1.0 mmol). The solution was stirred overnight and a white precipitate was then observed. After consumption of rac-1-phenylethanol (TLC), the suspension was filtered and the filtrate was washed with water (3 × 10 mL), 5% acetic acid solution (3 × 10 mL) and water again (3 × 10 mL). The organic phase was dried over Na₂SO₄ and evaporated under reduced pressure. The crude product was purified by flash chromatography to give rac-1-phenylethyl decanoate as a colourless oil (180 mg, 65% yield).

Preparation of rac-1-phenylethyl acetate

To a solution of rac-1-phenylethanol (0.66 mmol) in dichloromethane (10 mL) were added triethylamine (1.32 mmol), 4-dimethylaminepyridine (26 mg, 0.20 mmol) and acetic anhydride (128 μL, 1.32 mmol). The reaction mixture was stirred at room temperature for 2 h and then quenched by addition of water (30 mL). The mixture was extracted with dichloromethane (3 × 10 mL) and the organic phase was dried over Na₂SO₄ and evaporated to give rac-1-phenylethyl acetate, which was used for analytical method development without further purification.

Racemization reactions in batch

Solvent screening. Racemization reactions in batch were performed in 4 mL vials heated to 70 °C or 80 °C in silicon carbide plates. To a solution of (S)-1-phenylethanol (20 μL, 0.16 mmol) and tetracosane (internal standard, 14 mg, 0.04 mmol) in toluene (3.3 mL) at 70 °C or 80 °C was added VOSO₄·XH₂O (17 or 34 mg). The resulting suspension was gently stirred at 70 °C or 80 °C using a magnetic bar. Aliquots of 300 μL were taken before addition of VOSO₄·XH₂O and 1.5 h thereafter were diluted to 1 mL in toluene and analyzed using a GC-FID equipped with a chiral column (β-Dex-325) for enantiomeric excess (ee) and selectivity (Sel) determination. Selectivity was determined using the equation Sel (%) = {[(As_t1.5 + Ar_t1.5)/Ais_t1.5]/[As_t0/Ais_t0]} × 100, where As_t1.5, Ar_t1.5 and Ais_t1.5 are the peak areas of (S)-1-phenylethanol, (R)-1-phenylethanol and tetradecane, respectively, in the chromatograms obtained from aliquots taken 1.5 h after catalyst addition, whereas As_t0, Ar_t0 and Ais_t0 are the peak areas of (S)-1-phenylethanol, (R)-1-phenylethanol and tetradecane, respectively, in the chromatograms obtained from aliquots taken before catalyst addition.

Effect of acyl donors. Racemization reactions in batch were performed in 4 mL vials heated to 70 °C in silicon carbide plates. To a solution of (S)-1-phenylethanol (20 μL, 0.16 mmol) and tetradecane (internal standard, 11 μL, 0.04 mmol) in toluene (3.3 mL) at 70 °C were added VOSO₄·XH₂O (17 or 34 mg) and an ester (0.16, 0.32 or 0.64 mmol). The resulting suspension was stirred at 70 °C using a magnetic bar. Aliquots of 300 μL were taken before addition of VOSO₄·XH₂O and 1.5 h thereafter were diluted to 1 mL in toluene and analyzed using a GC-FID equipped with a chiral column (β-Dex-325) for enantiomeric excess (ee) and selectivity (Sel) determination. Selectivity was determined using the equation Sel (%) = {[(As_t1.5 + Ar_t1.5)/Ais_t1.5]/[As_t0/Ais_t0]} × 100, where As_t1.5, Ar_t1.5 and Ais_t1.5 are the peak areas of (S)-1-phenylethanol, (R)-1-phenylethanol and tetradecane, respectively, in the chromatograms obtained from aliquots taken 1.5 h after catalyst addition, whereas As_t0, Ar_t0 and Ais_t0 are the peak areas of (S)-1-phenylethanol, (R)-1-phenylethanol and tetradecane, respectively, in the chromatograms obtained from aliquots taken before catalyst addition.

Continuous flow reactions. Continuous flow experiments were performed using an Asia Flow System (Syrris) which included a syringe pump, a heater and a glass column. The glass column was packed with immobilized CAL-B and VOSO₄·XH₂O alternately so that a 7.854 mL-packed-bed reactor with 4 layers of enzyme (each layer containing 500 mg of the catalyst and a volume of 1.57 mL) and 3 layers of VOSO₄·XH₂O (each layer containing 500 mg of the catalyst and a volume of 0.39 mL) separated by thin cotton layers (total volume of 0.39 mL) was obtained. The packed-bed reactor was heated at 70 °C and perfused with toluene at a flow rate of 1.0 mL min⁻¹ for 20 minutes before each experiment.

Continuous flow reactions were carried out by pumping a toluene solution of rac-1-phenylethanol (0.1 M) and an acyl donor (0.1 M or 0.15 M) through the packed-bed reactor at flow rates of 0.5, 1.0 and 1.5 mL min⁻¹, which correspond to residence times of 15.6, 7.8 and 5.2 minutes, respectively. Aliquots of 300 μL were collected from the output of the reactor after pumping the reaction solution for twice the residence time and after 60 and 120 minutes and were diluted with toluene to 1.0 mL and analyzed by GC-FID. Conversions were calculated by determining the concentration of 1-phenylethanol in the aliquots using a calibration curve. The percentages of the substrate, product and by-products were given by their relative peak areas in the chromatograms obtained from aliquots taken from the output of the reactor. Enantiomeric excess values for 1-phenylethyl acetate were determined directly using a GC-FID equipped with a chiral column (β-Dex-325). Enantiomeric excess values for 1-phenylethyl decanoate were obtained indirectly after purification procedures by determining the enantiomeric excess values for 1-phenylethanol resulting from its hydrolysis. Isolated yields of (R)-1-phenylethyl decanoate were obtained from the solution collected from the reactor for 120 minutes of operation and added to the toluene collected during the reactor washing (20 minutes at a flow rate of 1 mL min⁻¹) thereafter. The solvent was evaporated and the residue was purified by flash chromatography (petroleum ether : diethyl ether, 95 : 5, R_f = 0.40).

Semi-continuous DKR of rac-1-phenylethanol. 20 mL of a solution containing rac-1-phenylethanol (0.1 M), vinyl decanoate (0.15 M) and tetradecane (0.04 M) in an Erlenmeyer flask was pumped through a loop-like reactor composed of a packed-bed reactor containing immobilized CAL-B (500 mg, 1.5 mL) connected to a packed-bed reactor containing VOSO₄ (500 mg, 0.5 mL) at a flow rate of 100 μL min⁻¹. The solution was allowed to recirculate through the reactor for 3 h and then the reactor was pumped with toluene for 25 min. A sample of the solution contained in the Erlenmeyer flask after the reaction and the “reactor washing” was analyzed by GC-MS. Conversion was calculated from the relative peak areas of 1-phenylethanol and tetradecane before the beginning of the reaction and after 3 h.

Preparation of (R)-1-phenylethyl decanoate under continuous flow conditions. A solution of rac-1-phenylethanol (0.1 M) and vinyl decanoate (0.15 M) in toluene was pumped at a flow of 1 mL min⁻¹ through a packed-bed reactor containing CAL-B and VOSO₄ at 70 °C, which was previously washed by pumping toluene at 1 mL min⁻¹ for 20 minutes. The reaction solution was pumped through the reactor for 120 minutes and the toluene was pumped at 1 mL min⁻¹ for 20 minutes. The reaction solution and the toluene pumped thereafter were collected from the output of the reactor and evaporated under reduced pressure. The residue was purified by flash chromatography (petroleum ether : diethyl ether, 95 : 5, R_f = 0.40) to give the product as a colorless oil (2.718 g, 82% yield). The enantiomeric excess of the product was determined by chiral GC-FID analysis of 1-phenylethanol resulting from its hydrolysis, as described below, and it was found to be 90%.

¹H-NMR (300 MHz, CDCl₃, 25 °C, TMS): δ = 0.90 (t, J_H–H = 6 Hz, 3H; CH₂CH̲₃), 1.28 (m, 12H; CH₂), 1.49 (d, J_H–H = 6 Hz, 3H; CHCH̲₃), 1.64 (m, 2H; CH₂), 2.34 (t, J_H–H = 6 Hz, 2H; CH̲₂CO), 5.92 (q, J_H–H = 6 Hz, 1H; CH̲CH₃), 7.33 (m, 5H; ArH); ¹³C{H}-NMR (300 MHz, CDCl₃, 25 °C, TMS): δ = 14.29 (C̲H₃CH₂), 22.45 (CH₂), 22.85 (CH₂), 25.17 (CH₂), 29.29 (CH₂), 29.44 (CH₂), 29.61 (CH₂), 32.05 (CH₂), 34.83 (CH₂), 72.19 (CH₃CH̲), 126.25 (Ar), 127.96 (Ar), 128.65 (Ar), 142.06 (Ar), 173.30 (C=O).

Hydrolysis of (R)-1-phenylethyl decanoate. To a round-bottom flask were added (R)-1-phenylethyl decanoate (92 mg, 0.33 mmol), methanol (3.2 mL) and 1.0 M LiOH solution (850 μL). The reaction medium was stirred at room temperature until complete consumption of the ester (as indicated by TLC) and then concentrated under reduced pressure, followed by addition of water (5 mL) and extraction with AcOEt (3 × 5 mL). The organic phase was washed with water (2 × 5 mL) and brine (5 mL), dried over Na₂SO₄ and evaporated to give (R)-1-phenylethanol as a colorless oil (31 mg, 75% yield, 90% ee).

Acknowledgements

The authors thank CNPq, CAPES and FAPERJ for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7re00003k

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