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Nucleophile Induced Ligand Rearrangement Reactions of Alkoxy- and Arylsilanes

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Nucleophile Induced Ligand Rearrangement Reactions of Alkoxy- and Arylsilanes Edinburgh Research Explorer Nucleophile induced ligand rearrangement reactions of alkoxy- and arylsilanes Citation for published version: Docherty, JH, Dominey, AP & Thomas, SP 2019, 'Nucleophile induced ligand rearrangement reactions of alkoxy- and arylsilanes', Tetrahedron. https://doi.org/10.1016/j.tet.2019.04.062 Digital Object Identifier (DOI): 10.1016/j.tet.2019.04.062 Link: Link to publication record in Edinburgh Research Explorer Document Version: Peer reviewed version Published In: Tetrahedron General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 25. Sep. 2021 Graphical Abstract To create your abstract, type over the instructions in the template box below. Fonts or abstract dimensions should not be changed or altered. Nucleophile induced ligand rearrangement Leave this area blank for abstract info. reactions of alkoxy- and arylsilanes Jamie H. Dochertya*, Andrew P. Domineyb and Stephen P. Thomasa* aEaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, UK. bGSK Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK. 1 Tetrahedron journal homepage: www.elsevier.com Nucleophile induced ligand rearrangement reactions of alkoxy- and arylsilanes Jamie H. Dochertya*, Andrew P. Domineyb and Stephen P. Thomasa aEaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, UK. bGSK Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK. ARTICLE INFO ABSTRACT Article history: The ligand-redistribution reactions of aryl- and alkoxy-hydrosilanes can potentially Received cause the formation of gaseous hydrosilanes, which are flammable and pyrophoric. The Received in revised form ability of generic nucleophiles to initiate the ligand-redistribution reaction of commonly Accepted used hydrosilane reagents was investigated, alongside methods to hinder and halt the Available online formation of hazardous hydrosilanes. Our results show that the ligand-redistribution Keywords: reaction can be completely inhibited by common electrophiles and first-row transition Catalysis metal pre-catalysts. Silane Reduction Redistribution Phenylsilane 1. Introduction Predominantly these reactions have been achieved using platinum- based catalysts, however Earth-abundant alternatives have Hydrosilanes are a highly important class of reagent used in emerged [4]. The utility of hydrosilane reagents has been further widespread applications from organic synthesis to polymer exemplified by their use in other reductive transformations; such production [1, 2]. The formation of carbon-silicon bonds by olefin as the hydrogen-atom transfer mediated reductive hydrosilylation constitutes one of the largest applications of functionalisation of alkenes (Scheme 1, b) [5], carbonyl reduction homogeneous catalysis with the silicones industry forecast to be [6] and nitro-group reduction [7]. worth $18 billion per annum by 2021 (Scheme 1, a) [3]. Scheme 1. Utility of silane reagents in modern synthetic chemistry. a Metal-catalysed hydrosilylation of alkene or alkynes. b Reductive functionalisation of alkenes by hydrogen atom transfer. c Ligand-redistribution reaction of phenylsilane to diphenylsilane, silane and triphenylsilane. d Use of ligand-redistribution as masked hazardous silanes. ——— Corresponding author. e-mail: [email protected]; [email protected] 2 Tetrahedron products that would have normally required the use of gaseous hydrosilane reactants. Given that silane (SiH4, b.p. ˗112 ºC) [19], methylsilane (MeSiH3, b.p ˗57 ºC) [20] and dimethylsilane (Me2SiH2, ˗20 ºC) [21] are hazardous, flammable and pyrophoric gases, the use and 100.0 generation of these reagents presents a safety concern [22]. 90.0 Importantly Hu has shown that while ligand-redistribution occurs 80.0 when using alkoxy-hydrosilanes in the presence of NaOtBu, there 70.0 was no build-up of hazardous gaseous alkyl-hydrosilanes in the presence of the nickel-catalyst [18]. Similarly, Thomas, and 3 60.0 others, have made use of alkoxide salts to enable pre-catalyst 50.0 activation for a range of reductive functionalisation reactions using %PhSiH 40.0 Earth-abundant metals with no build-up or formation of hazardous 30.0 gases under the catalytic reaction conditions [23]. 20.0 2. Results and discussion 10.0 0.0 These precedents prompted us to consider the factors 0 200 400 600 800 1000 important to hydrosilane ligand-redistribution reactions. One Time (minutes) important question was whether any generic nucleophile could be t t t • LiO Bu • NaO Bu • KO Bu • NaHMDS • KHMDS • LiAlH4 used to initiate ligand-redistribution. In answering this possibility we selected phenylsilane as a test substrate in combination with various nucleophiles (Figure 1). Using 1H NMR, the reaction progress of the ligand-redistribution reaction could be directly monitored. Lithium tert-butoxide proved to be a slow initiator for the reaction, only achieving 10% conversion after 13 hours. Lithium aluminium hydride displayed significantly better 100.0 reactivity, and steadily converted phenylsilane to a mixture of 90.0 diphenylsilane, silane and triphenylsilane over the course of ca. 17 80.0 hours to 80% total conversion. Hexamethyldisilazide (HMDS) anions of both sodium and potassium reacted faster than both 70.0 previous reagents, but achieved less overall conversion, plateauing 60.0 at ca. 70%. Sodium- and potassium tert-butoxide demonstrated the 50.0 best reactivity towards the ligand-redistribution reaction, both 40.0 showing the fastest rate of reaction and highest conversion. %Conversion 30.0 Reaction profiling of all components of the reaction mixture 20.0 containing phenylsilane and sodium tert-butoxide (5 mol%) 10.0 showed concomitant consumption of phenylsilane and formation 0.0 of silane and diphenylsilane. The latter is subsequently consumed 0 100 200 300 400 500 600 as triphenylsilane is formed (Figure 1, bottom). Time (minutes) • PhSiH3 • SiH4 • Ph2SiH2 • Ph3SiH Figure 1. Ligand-redistribution reaction of phenylsilane with various nucleophiles, monitoring consumption of PhSiH3 (top). Reaction conditions: 8 PhSiH3 (0.4 mmol), additive (2.5 mol%) in d -THF (0.8 M). Monitoring formation of distribution products over time (bottom). Reaction conditions: t 8 PhSiH3 (0.4 mmol), NaO Bu (5 mol%) in d -THF (0.8 M). Mass balance <100% due to gas loss. Conversions are relative to PhSiH3. While hydrosilane reagents show a wide breadth of reactivity in reductive transformations, they have often been observed to undergo ligand-redistribution reactions (Scheme 1, c) [8]. Most commonly ligand-redistribution reactions of arylsilanes have been observed using metal complexes as catalysts; including: Ir [9], Pd [10], Sm [11], Mo [12], Co [13], Ti [14], Yb [15] and Ru [16]. Similarly, a number of hydride sources, including LiAlH4, KH and NaH, have proven effective towards ligand-redistribution [17]. The common feature in all ligand-redistribution reactions of phenylsilane is the formation of a mixture of products containing: diphenylsilane, silane (SiH4) and triphenylsilane. Scheme 2. Redistribution of various alkoxysilanes with NaOtBu. General Hu has shown that stable alkoxy-hydrosilane reagents reaction conditions: alkoxysilane (0.4 mmol), NaOtBu (1-5 mol%) in d8-THF can be used as surrogates to access volatile and challenging to 1 29 handle alkyl-hydrosilanes (Scheme 1, d) [18]. Using this method, (0.8M). Products identified by H and Si NMR. Observed ratios of products in solution, not accounting for gas dissolution. olefin hydrosilylation could be used to access primary hydrosilane 3 100.0 a 100.0 90.0 80.0 80.0 70.0 3 60.0 3 60.0 40.0 50.0 %PhSiH %PhSiH 40.0 20.0 30.0 0.0 20.0 0 50 100 150 200 10.0 Time (minutes) 0.0 • 2.5 mol% • 3.75 mol% • 5 mol% b 0 100 200 300 400 500 30.0 Time (minutes) Et Et 25.0 • BIPCoCl2 • BIPFeCl2 • Ethyl benzoate • MeCN • Benzaldehyde • none 2 20.0 SiH Figure 2. Ligand-redistribution reaction of phenylsilane with NaOtBu retarded 2 15.0 t by added hydride acceptors. Reaction conditions: PhSiH3 (0.4 mmol), NaO Bu %Ph 10.0 (2.5 mol%) in d8-THF (0.8 M) and hydride acceptor added at 10 minutes, denoted by dashed black line. 5.0 Additionally we tested a range of alkoxy-hydrosilane reagents in 0.0 combination with sodium tert-butoxide to both assess the rate of 0.0 100.0 200.0 300.0 400.0 ligand-redistribution and identify the products of these reactions Time (minutes) (Scheme 2). Methyldiethoxysilane underwent ligand- • 0.6 M • 0.8 M • 1.0 M redistribution to quickly generate methylsilane and c 100.0 methyltriethoxysilane. The analogous dimethylethoxysilane reacted to form dimethylsilane and dimethyldiethoxysilane. 80.0 Triethoxysilane rapidly underwent redistribution to form silane and tetraethoxysilane. Tetramethyldisiloxane (TMDS) reacted 60.0 significantly slower to give a complex mixture of products, as %PhSiH3 1
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