catalysts Article Biocatalytic Silylation: The Condensation of Phenols and Alcohols with Triethylsilanol Emily I. Sparkes 1,2, Chisom S. Egedeuzu 1,2 , Billie Lias 1,2, Rehana Sung 1, Stephanie A. Caslin 1,2, S. Yasin Tabatabaei Dakhili 1,2, Peter G. Taylor 3, Peter Quayle 2 and Lu Shin Wong 1,2,* 1 Manchester Institute of Biotechnology, University of Manchester, 131 Princess Street, Manchester M1 7DN, UK; [email protected] (E.I.S.); [email protected] (C.S.E.); [email protected] (B.L.); [email protected] (R.S.); [email protected] (S.A.C.); [email protected] (S.Y.T.D.) 2 Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK; [email protected] 3 Faculty of Science, Technology, Engineering and Mathematics, Open University, Walton Hall, Milton Keynes MK7 6AA, UK; [email protected] * Correspondence: [email protected] Abstract: Silicatein-α (Silα), a hydrolytic enzyme derived from siliceous marine sponges, is one of the few enzymes in nature capable of catalysing the metathesis of silicon–oxygen bonds. It is therefore of interest as a possible biocatalyst for the synthesis of organosiloxanes. To further investigate the Citation: Sparkes, E.I.; substrate scope of this enzyme, a series of condensation reactions with a variety of phenols and Egedeuzu, C.S.; Lias, B.; Sung, R.; aliphatic alcohols were carried out. In general, it was observed that Silα demonstrated a preference Caslin, S.A.; Tabatabaei Dakhili, S.Y.; for phenols, though the conversions were relatively modest in most cases. In the two pairs of chiral Taylor, P.G.; Quayle, P.; Wong, L.S. alcohols that were investigated, it was found that the enzyme displayed a preference for the silylation Biocatalytic Silylation: The of the S-enantiomers. Additionally, the enzyme’s tolerance to a range of solvents was tested. Silα Condensation of Phenols and had the highest level of substrate conversion in the nonpolar solvents n-octane and toluene, although Alcohols with Triethylsilanol. the inclusion of up to 20% of 1,4-dioxane was tolerated. These results suggest that Silα is a potential Catalysts 2021, 11, 879. candidate for directed evolution toward future application as a robust and selective biocatalyst for https://doi.org/10.3390/catal11080879 organosiloxane chemistry. Academic Editors: Keywords: silicatein; condensation; silyl ether; organosiloxanes; biocatalysis Evangelos Topakas, Roland Wohlgemuth and David D. Boehr Received: 26 June 2021 1. Introduction Accepted: 20 July 2021 During the multi-step chemical synthesis of complex molecules, silyl ethers are often Published: 22 July 2021 employed for the protection of hydroxyl groups [1–7], where their utility arises from orthogonality to other commonly used acid- and base-labile protecting groups. Typically, Publisher’s Note: MDPI stays neutral the introduction of these silyl groups involves the use of an electrophilic silylating reagent with regard to jurisdictional claims in such as a silyl chloride (i.e., chlorosilane) or triflate, the latter of which is itself produced published maps and institutional affil- from the corresponding silyl chloride [8]. The trimethylsilylation of alcohols has also iations. been demonstrated with N,O-bis-silyl trifluoroacetamide [9], N,N0-bis-silyl urea [10] and hexamethydisilazane [11]. It has also been known for many years that the silylation of alcohols can be affected by hydrosilanes (silyl hydrides), with the dehydrogenative condensation catalysed by transition metals or strong Lewis acids [12–14]. Alternatively, Copyright: © 2021 by the authors. silylation can also be affected by activation of the silanol to attack by an alcohol (or phenol) Licensee MDPI, Basel, Switzerland. under Mitsunobu conditions [15]. This article is an open access article However, in all cases, the necessary reagents are energy intensive to produce, and their distributed under the terms and use results in the generation of stoichiometric amounts of by-products that are hazardous or conditions of the Creative Commons environmentally undesirable (e.g., triflic acid, hydrogen chloride, hydrogen). In this regard, Attribution (CC BY) license (https:// the capability of silylate hydroxy groups through the condensation of the corresponding creativecommons.org/licenses/by/ silanol and alcohol would circumvent the need for harsh reagents and only release water 4.0/). Catalysts 2021, 11, 879. https://doi.org/10.3390/catal11080879 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 10 corresponding silanol and alcohol would circumvent the need for harsh reagents and only Catalysts 2021, 11, 879 2 of 10 release water as the by-product. However, only one example of this dehydrative conden- sation has been reported, catalysed by rare-earth Lewis acids such as Yb(OTf)3 and Sc(OTf)3 [16]. It would therefore be preferable to identify a reaction pathway that better asconforms the by-product. to the principles However, of green only chemistry—in one example ofparticular, this dehydrative the harnessing condensation of biological has beencatalysts reported, that can catalysed be sustainably by rare-earth sourced Lewis and acidsavoid such any asrequirement Yb(OTf)3 and for rare Sc(OTf) metals3 [16 [17].]. It wouldSilicatein- thereforeα be(Sil preferableα), an enzyme to identify responsible a reaction for pathwaythe condensation that better of conforms inorganic to Si–O the principlesbonds in marine of green demosponges chemistry—in [18,19], particular, has thealso harnessing previously of been biological shown catalysts to catalyse that canthe becondensation sustainably of sourced a variety and of avoid organosilanols any requirement with aliphatic for rare metalsalcohols [17 to]. produce the corre- spondingSilicatein- silylα ether(Silα [20].), an enzymeThis enzyme responsible is monomeric for the condensation and does not of require inorganic any Si–O co-factors bonds infor marine activity, demosponges so may offer [18 a ,relatively19], has also benign previously alternative been shownbiocatalytic to catalyse approach the condensa-for silyla- tiontion. of However, a variety the of general organosilanols substrate with scope aliphatic of this enzyme alcohols is to currently produce not the well corresponding established silyland etherrequires [20]. further This enzyme investigation is monomeric before and synthetically does not require useful any methods co-factors can for be activity, subse- soquently may offer developed. a relatively benign alternative biocatalytic approach for silylation. However, the generalIn this substratestudy, the scope substrate of this scope enzyme from isth currentlye perspective not wellof the established alcohol component and requires was furtherinvestigated investigation by surveying before the synthetically enzyme’s ability useful to methods form triethylsilyl can be subsequently ethers with developed. a range of phenolsIn this and study, aliphatic the alcohols. substrate In scopeaddition, from Sil theα’s perspectivetolerance for ofa variety the alcohol of polar component and non- waspolar investigated solvents was by tested surveying to examine the enzyme’s the optimum ability reaction to form conditions triethylsilyl for Sil ethersα in organic with a α rangemedia. of In phenols the original and aliphatic report of alcohols. Silα-catalysed In addition, condensations, Sil ’s tolerance a preference for a variety for phenolic of polar and nonpolar solvents was tested to examine the optimum reaction conditions for Silα hydroxy groups was found [20]. However, more recent studies found that the hexahisti- in organic media. In the original report of Silα-catalysed condensations, a preference dine affinity tag used for isolation of the enzyme was also contributing to nonspecific ca- for phenolic hydroxy groups was found [20]. However, more recent studies found that talysis [21]. Thus, in order to more accurately determine the substrate scope and catalysis the hexahistidine affinity tag used for isolation of the enzyme was also contributing to mediated by the enzyme active site itself, in this study we used an enzyme construct con- nonspecific catalysis [21]. Thus, in order to more accurately determine the substrate scope sisting of the ribosomal chaperone protein trigger factor fused to the N-terminus and a and catalysis mediated by the enzyme active site itself, in this study we used an enzyme Strep II tag fused to the C-terminus (henceforth referred to as TF-Silα-Strep). construct consisting of the ribosomal chaperone protein trigger factor fused to the N- terminus and a Strep II tag fused to the C-terminus (henceforth referred to as TF-Silα-Strep). 2. Results and Discussion 2.2.1. Results Triethylsilylation and Discussion of Phenols 2.1. TriethylsilylationThe triethylsilylation of Phenols of various substituted phenols was first examined to investigate the effectThe triethylsilylation of the substituents of variouson the enzyme-catalysed substituted phenols reaction. was first Chloro, examined methyl to investigate and meth- theoxy effect substituents of the substituents were chosen on theto represent enzyme-catalysed electron-donating reaction. Chloro, and -withdrawing methyl and methoxy groups, substituentsand all three werepositions chosen (ortho to represent, meta and electron-donating para) were investigated. and -withdrawing The condensation groups, reactions and all threewere positionscarried out (ortho using, meta theand procedurepara) were