Highly enantioselective tandem enzyme–organocatalyst crossed aldol reactions with acetaldehyde in deep-eutectic-solvents

Abstract

Deep-eutectic-solvents (DES), e.g. choline chloride and glycerol, have emerged as promising bio-based and cost-effective reaction media. Herein, the first concept of tandem catalysis of enzymes and organocatalysts in such environmentally-friendly DES is reported, focusing on enzymatic in situ acetaldehyde production combined with highly enantioselective crossed aldehyde–aldehyde C–C bond formation organocatalysis. This leads to an integrative concept with straightforward product recovery and promising catalyst recycling, enabling the synthesis of highly valuable optically active building blocks under mild reaction conditions.

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A current trend in sustainable chemistry is the development of highly integrated domino processes, e.g. multi-enzymatic pathways^1,2 or organocatalyst–enzyme combinations.³ By using these concepts simplified synthetic operational units, together with reduced waste production – as less work-up procedures are applied – can be reached. Another asset of these strategies is that the use of volatile and toxic reagents may be optimized by establishing in situ production methods, which enable the controlled formation of these chemicals exclusively for the intended synthesis.^2b,d,e In this context, and dealing with organocatalytic processes, acetaldehyde remains as a potentially useful but still highly challenging substrate, due to its intrinsic volatility and reactivity, being prone to undergo a vast array of competing reactions like self-oligomerizations, condensations, etc.⁴ For these reasons several creative approaches for the in situ formation and subsequent use of acetaldehyde have been reported in the literature so far.⁵ Among them, the use of vinyl esters and lipases is useful because this approach enables the set-up of synthetic concepts both at aqueous^2d and at non-aqueous conditions,⁶ depending on the particular needs of a given synthetic protocol. Furthermore, other important aspects for organocatalysis are the role that solvents play in the reactivity and selectivity of these reactions, as well as the option of recycling the organocatalyst, derived by the fact that in many cases high catalyst loadings need to be added. Aligned with these challenges, the combination of organocatalysis and ionic liquids has opened novel areas focusing on different strategies such as catalyst-tag-immobilization, tailored solvents for controlled reactivities, set-up of mono-, di-, or even multi-phasic systems, etc.⁷

Apart from these ionic liquids, very recently deep-eutectic-solvents (DES) were reported as biodegradable and cost-effective neoteric solvents for a wide number of applications, including solvents for biocatalysis and separation technologies among other promising uses.^8,9 DES are formed by mixing and complexation of primarily quaternary ammonium salts (e.g. choline chloride) with hydrogen-bond-donor (HBD) compounds (alcohols, carboxylic acids, etc.). The mixing of them creates a disruption on the crystalline structure of the quaternary ammonium salt, triggering a depression in the melting point and thus generating liquids at room temperature. Typical examples reported in the literature comprise choline chloride and urea, glycerol or other bio-based polyols, sugars and carboxylic acids as HBDs.^8–10 Albeit DES are typically regarded as green solvent with largely reduced E-factor for their syntheses,¹¹ their potential toxicology has been discussed recently, and further research in that line is still needed.¹²

Similar to ionic liquids, DESs are excellent solvents for many substrates, providing a promising bio-based and effective media for reactions. In this respect, DESs have been efficiently used for biocatalytic reactions, e.g. for peptide synthesis using substrates with different polarities.^9b,d Surprisingly, no examples have been reported dealing with highly enantioselective organocatalytic reactions in DES media yet. Considering that organocatalysts bear hydrogen-bond-donor groups (e.g. amine, alcohols, carboxylic acids, etc), and therefore would dissolve and remain within the DES upon extractive product downstream – conducted with an organic solvent, typically ethyl acetate forming a clear second phase with most of the DESs –, this communication explores for the first time the tandem catalysis of enzymes and organocatalysts in bio-based, environmentally-friendly DESs as a fully integrated and highly enantioselective concept with straightforward product recovery and promising catalysts recycling.

To probe the concept, a choline chloride–glycerol DES (1 : 2 mol/mol) was formed and used as reaction media.^10b Immobilized lipase B from Candida antarctica (CAL-B) was used as the biocatalyst for in situ acetaldehyde formation, and different proline-based organocatalysts were assessed for the crossed aldehyde–acetaldehyde aldol reaction, using p-nitrobenzaldehyde as benchmark.¹³ Immobilized CAL-B catalyzes the transesterification of vinyl acetate with iso-propanol, affording iso-propyl acetate – or eventually another kinetic resolution approach if a racemic alcohol, instead of iso-propanol, were used –, and producing the enol (4), which rapidly tautomerizes to acetaldehyde. In the same DES media, an organocatalyst performs the enantioselective aldehyde–aldehyde C–C bond formation. The subsequent addition of ethyl acetate enables the efficient extraction of the product by the formation of a second phase. After phase decantation – permitting the reuse of both catalysts and DES –, sodium borohydride is added to the ethyl acetate phase in order to reduce the aldol addition product to the corresponding 1,3-diol. The overall concept is depicted in Scheme 1.

Scheme 1 Tandem enzyme-organocatalyst assessed in this communication. The direct addition of acetaldehyde in large surplus led to low yields and large by-product formation.

In a first set of experiments different reaction parameters were optimized, such as temperature, CAL-B loading and the proportion of vinyl acetate and iso-propanol to enable a proper acetaldehyde production in the biocatalytic step (see ESI†). With optimized conditions in hands, several organocatalysts were assessed. Experiments with proline (8) in different solvents led to low conversions, while in DES more promising conversions (19%) were reached, yet at poor enantioselectivities. Therefore, various organocatalysts^4a were further studied for the benchmark reaction in such choline chloride–glycerol media. Under optimized reaction conditions of temperature, substrate ratio and bio- and organocatalyst loadings, high conversions and enantioselectivities were achieved (Fig. 1).

Fig. 1 Assessment of different organocatalysts for the tandem reaction in DES media. Conditions: (1) 1 mmol p-nitrobenzaldehyde, 3 mmol vinyl acetate, 3 mmol iso-propanol, 0.2 mmol organocatalyst, 1 mg CAL-B, 1 mL glycerol–choline chloride (2 : 1) DES, 48 h, rt; (2) 6 mmol NaBH₄, 2 mL methanol, 1 h, 0 °C.

Consistent with previous literature,^4a trifluoromethyl substituted prolinol 6 resulted a suitable catalyst for the cross aldol reaction with acetaldehyde and led to high isolated yields with excellent enantioselectivities (95%), demonstrating the feasibility of the integrated concept enzyme–organocatalyst in DES, where organocatalysts are fully dissolved and active. Other organocatalysts such as the unsubstituted diarylprolinol 5 or Singh's catalyst 7 displayed lower yields and enantioselectivities, yet still showed remarkable activities in the novel media.

Stimulated by these results, a set of different aromatic aldehydes were assessed in DES for the selective aldehyde–acetaldehyde aldol condensation. The tandem catalysis worked successfully for many of them, affording high yields and excellent enantioselectivities in most of the cases (Table 1). Aromatic 1,3-diols have a broad application range in the pharmaceutical industry and act often as precursors for bioactive agents like anti-HIV agents.¹⁴ Thus, the reported integrated technology might open new research areas in the synthesis of these building blocks.

Table 1 Reaction results with different aromatic aldehydes. Conditions: (1) 1 mmol aromatic aldehyde, 3 mmol vinyl acetate, 3 mmol iso-propanol, 0.2 mmol organocatalyst, 1 mg CAL-B, 1 mL glycerol–choline chloride (2 : 1) DES, 48 h, rt; (2) 6 mmol NaBH₄, 2 mL methanol, 1 h, 0 °C; n.d: not determined

Entry	Product	Conversion/yield (%)	ee (%)
1		92/70	95
2		60/23	94
3		78/65	97
4		76/35	93
5		85/53	99
6		41/14	96
7		2/1	n.d.
8		36/2	n.d.

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In a subsequent step, the recyclability of the DES media containing enzymes and organocatalysts was assessed. To this end, NaBH₄ was added separately and slowly in the ethyl acetate phase at low temperature, after decantation – to avoid contact with the DES and catalysts –, and the media containing catalysts was reused for further reaction cycles (Fig. 2).

Fig. 2 Assessment of the recyclability of the media. Conditions: (1) 1 mmol aromatic aldehyde, 3 mmol vinyl acetate, 3 mmol iso-propanol, 0.2 mmol organocatalyst, 1 mg CAL-B, 1 mL glycerol–choline chloride (2 : 1) DES, 48 h, rt; (2) extraction with EtOAc (3 × 2 mL), 6 mmol NaBH₄, 2 mL methanol, 1 h, 0 °C.

DES and CAL-B could be reused up to 6 cycles without observing any loss of enzymatic activity, with excellent enantioselectivities. In the case of the organocatalyst, yields remained stable when fresh organocatalyst was added (Fig. 2, cycles 1–4), and some loss of the activity was observed when no extra organocatalyst was added (cycles 5 and 6). This suggests that the organocatalyst is partially extracted by ethyl acetate. However, a significant amount of it remained in the DES media for the next cycle. Given the fact that novel organocatalysts with more HBD groups can be designed – to reinforce the affinity for DES – it may be expected that other catalysts with better performances can be found by means of an analogous conceptual DES-based approach.

In summary, novel tandem catalysis of enzymes and organocatalysts in environmentally-friendly deep-eutectic-solvents (DES) has been reported, focusing on enzymatically in situ acetaldehyde production combined with highly enantioselective crossed aldehyde–aldehyde C–C bond formation organocatalysis. As a highly enantioselective integrative concept with straightforward product recovery and promising catalysts recycling, the novel approach enables the synthesis of highly valuable optically active building blocks under mild reaction conditions.

Acknowledgements

Financial support from DFG training group 1166 “BioNoCo” (“Biocatalysis in Non-conventional Media”). Mrs Hannelore Eschmann and Ms Julia Wurlitzer are acknowledged for analytical measurements.

References

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09307k

‡ Present address: Sustainable Momentum, SL Ap. Correos 3517. 35004, Las Palmas de Gran Canaria, Canary Islands, Spain.

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