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Nitrile Synthesis with Aldoxime Dehydratases: a Biocatalytic Platform with Applications in Asymmetric Synthesis, Bulk Chemicals, and Biorefineries

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Nitrile Synthesis with Aldoxime Dehydratases: a Biocatalytic Platform with Applications in Asymmetric Synthesis, Bulk Chemicals, and Biorefineries molecules Review Nitrile Synthesis with Aldoxime Dehydratases: A Biocatalytic Platform with Applications in Asymmetric Synthesis, Bulk Chemicals, and Biorefineries Pablo Domínguez de María Sustainable Momentum, SL, Av. Ansite 3, 4–6, 35011 Las Palmas de Gran Canaria, Canary Islands, Spain; [email protected]; Tel.: +34-6-0956-5237 Abstract: Nitriles comprise a broad group of chemicals that are currently being industrially produced and used in fine chemicals and pharmaceuticals, as well as in bulk applications, polymer chemistry, solvents, etc. Aldoxime dehydratases catalyze the cyanide-free synthesis of nitriles starting from aldoximes under mild conditions, holding potential to become sustainable alternatives for industrial processes. Different aldoxime dehydratases accept a broad range of aldoximes with impressive high substrate loadings of up to >1 Kg L−1 and can efficiently catalyze the reaction in aqueous media as well as in non-aqueous systems, such as organic solvents and solvent-free (neat substrates). This paper provides an overview of the recent developments in this field with emphasis on strategies that may be of relevance for industry and sustainability. When possible, potential links to biorefineries and to the use of biogenic raw materials are discussed. Keywords: biocatalysis; green chemistry; nitriles; aldoxime dehydratases; sustainability Citation: Domínguez de María, P. Nitrile Synthesis with Aldoxime Dehydratases: A Biocatalytic Platform with Applications in Asymmetric Synthesis, Bulk 1. Aldoxime Dehydratases as Biocatalysts for the Cyanide-Free Synthesis of Nitriles Chemicals, and Biorefineries. Nitriles comprise an important group of chemicals that are widely spread in industry Molecules 2021, 26, 4466. in a broad range of sectors, being used as products, solvents, polymers, commodities, or https://doi.org/10.3390/ as starting materials for the production of other chemicals such as amines, amides, etc. molecules26154466 (Figure1). A nitrile group (–C ≡N) displays a linear geometry with sp hybridization of the triple bond. Depending on the application, their annual production may range from kilo Academic Editor: Gianpiero Cera scale low-volume, high-value applied to pharmaceuticals and fine chemicals, to millions of tons worldwide for solvents and commodities [1–3]. Several synthetic routes have been Received: 1 July 2021 proposed for the industrial production of nitriles, typically starting from petroleum-based Accepted: 22 July 2021 alkenes (e.g., ethene, butadiene), and proceeding via hydrocyanation (using hazardous Published: 24 July 2021 (hydrogen) cyanide), or via ammonoxidation (using ammonia and alkanes), or via amide dehydration [1–3]. These syntheses have been successfully established at a commercial Publisher’s Note: MDPI stays neutral level and largely optimized over decades. with regard to jurisdictional claims in The quest of sustainable industrial chemical strategies that may serve as alternatives to published maps and institutional affil- current processes is nowadays a matter of intense research. Some approaches focus on using iations. renewable resources as starting materials, such as lignocellulose, CO2, wastes, etc., as well as (bio)catalytic alternatives that may lead to more environmentally friendly production in terms of higher atom economy, less waste production, and energy savings [4,5]. With respect to nitriles, developing versatile cyanide-free routes that could be (preferably) Copyright: © 2021 by the author. performed using biogenic resources and under mild reaction conditions are of particular Licensee MDPI, Basel, Switzerland. interest. Hence, biocatalysis may become an important alternative, provided that optimized This article is an open access article enzymatic routes are set with high substrate loadings and productivities aligned with distributed under the terms and industrial interests [5–10]. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Molecules 2021, 26, 4466. https://doi.org/10.3390/molecules26154466 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 8 N N N N F HOOC Levocabastine Citronellyl nitrile Acetonitrile (Pharmaceutical) (Fragrances) (Solvent) OH Molecules 2021, 26, 4466 Molecules 2021, 26, x FOR PEER REVIEW 22 of 8 8 N O N N N N N N N FuronitrileF Chiral alcohol-nitriles AdiponitrileHOOC (Polymers) (Food additive) (Fine Chemicals) Levocabastine Citronellyl nitrile Acetonitrile Figure 1. Selected examples of occurrence of nitriles with different uses as solvents, pharmaceuticals, food additives, or as (Pharmaceutical) (Fragrances) (Solvent) polymer precursors [1–3]. The quest of sustainable industrial chemical strategies that may serveOH as alternatives to currentN processes is nowadays a matter ofO intense research. Some approaches focus on N using renewable resources as starting materials, such as lignocellulose, CO2, wastes,N etc., N as well as (bio)catalytic alternatives that may lead to more environmentally friendly pro- duction in terms of higher atom economy, less waste production, and energy savings [4,5]. Adiponitrile Furonitrile Chiral alcohol-nitriles With respect to nitriles, developing versatile cyanide-free routes that could be (preferably) (Polymers) (Food additive) (Fine Chemicals) performed using biogenic resources and under mild reaction conditions are of particular Figureinterest.Figure 1. Selected 1. Hence,Selected examples examplesbiocatalysis of occurrence of occurrenceof may nitriles become with of different nitriles an usesimportant with as solvents, different alternative,ph usesarmaceuticals, as solvents, provided food additives, pharmaceuticals, that or as opti- polymer precursors [1–3]. mizedfood additives, enzymatic or as routes polymer are precursors set with [high1–3]. substrate loadings and productivities aligned with industrial interestsThe [5–10].quest of sustainable industrial chemical strategies that may serve as alternatives OverOver the the last to years, current a processes biocatalytic is nowadays platform a matter for ofthe intense synthesis research. of Some nitriles approaches has emerged focus on asas a a promising promising alternative alternativeusing renewable in in terms terms resources of of sustainability sustainability as starting materials, (use (use ofsuch of renewable renewable as lignocellulose, resources, resources, CO2, wastes, cyanide- cyanide- etc., free,free, mild mild conditions) conditions)as well and andas (bio)catalytic industrial industrial alternatives interest interest (highthat (high may productivity productivity lead to more environmentally and and substrate substrate friendly loading). loading). pro- duction in terms of higher atom economy, less waste production, and energy savings [4,5]. AldoximeAldoxime dehydratases dehydratasesWith respect catalyze catalyze to nitriles, the the developing dehydratio dehydration versatilen of of oximescyanide-free oximes in in several routes several that reaction reaction could be media,(preferably) media, af- af- fordingfording nitriles nitriles at at performedmild mild temperatures temperatures using biogenic and and resources am ambientbient and pressure.under mild Thereaction used used conditions substrate substrate are ofaldoximes aldoximes particular cancan be be in situ (andinterest. straightforwardly)straight Hence,forwardly) biocatalysis formed may become via condensationan important alternative, of hydroxylamine provided that with opti- aldehydes,aldehydes, whichwhichmized cancan be enzymaticbe derived derived routes either either are from set from with classic classichigh petroleumsubstrate petroleum loadings refineries refineries and productivities (e.g., (e.g., hydroformy- hydro-aligned lation) [1–3] or fromwith renewable industrial interests resources [5–10]. such as alcohols or carboxylic acids, among other formylation) [1–3] orOver from the renewablelast years, a biocatalytic resource platforms such foras thealcohols synthesis or of carboxylicnitriles has emerged acids, amongoptions other [4] (Scheme optionsas a1 promising).[4] (Scheme alternative 1). in terms of sustainability (use of renewable resources, cyanide- free, mild conditions) and industrial interest (high productivity and substrate loading). Aldoxime dehydratases catalyze the dehydration of oximes in several reaction media, af- Aldehydes fording nitriles at mild temperatures and ambient pressure. The used substrate aldoximes H R N can be in situ (and straightAldoximeforwardly) dehydratase formed via condensation of hydroxylamine with Alkenes OH aldehydes,R N which can be derived either from classic petroleum refineries (e.g., hydro- Acids formylation) [1–3] or fromWaterorSolvent-free renewable resources such as alcoholsNitriles or carboxylic acids, amongAldoxime other options [4] (Scheme 1). (Cyanide-free) Petroleum-refinery or Aldehydes H Biorefinery Aldoxime dehydratase R N Alkenes OH R N SchemeScheme 1. 1. ConceptualConceptual approach approachAcids using using aldoxime aldoxime dehydratases dehydratases to afford nitriles WaterorSolvent-free under mild reaction conditions.Nitriles Aldoxime (Cyanide-free) Petroleum-refinery or TheThe first first aldoxime dehydratases were were characterized two two decades ago by the Asano Biorefinery groupgroup when when working working with with soil soil samples and is isolatingolating microorganisms able to grow with aldoximesaldoximesScheme 1. Conceptual as as the approachcarbon using source aldoxime [[11–13].11 –dehydratases13].
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