Controlling Sugar Deoxygenation Products from Biomass by Choice of

Jun 13, 2019 - The feedstocks from biomass are defined and limited by nature, but through the choice of catalyst, one may change the deoxygenation out...
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Research Article Cite This: ACS Catal. 2019, 9, 6648−6652

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Controlling Sugar Deoxygenation Products from Biomass by Choice of Fluoroarylborane Catalyst Youngran Seo, Jared M. Lowe, and Michel R. Gagne*́ Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States

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ABSTRACT: The feedstocks from biomass are defined and limited by nature, but through the choice of catalyst, one may change the deoxygenation outcome. We report divergent but selective deoxygenation of sugars with triethylsilane (TESH) and two fluoroarylborane catalysts, B(C6F5)3 and B(3,5-CF3)2C6H3)3 (BAr3,5‑CF3). To illustrate, persilylated 2-deoxyglucose shows exocyclic C−O bond cleavage/reduction with the less sterically congested BAr3,5‑CF3, whereas endocyclic C−O bond cleavage/reduction predominates with the more Lewis acidic B(C6F5)3. Chiral furans and linear polyols can be selectively synthesized depending on the catalysts. Mechanistic studies demonstrate that the resting states of these catalysts are different. KEYWORDS: fluoroarylborane, selective deoxygenation, hydrosilyative reduction, biomass, sugar he strong Lewis acidity of fluoroarylboranes makes them broadly effective catalysts.1,2 Depending on the number of fluorines and their positioning, the Lewis acidity of these planar structures can be modified and estimated from computations or measured experimentally (Gutmann−Beckett3,4 or Childs5 methods).6,7 The most well-developed perfluoroarylborane, B(C6F5)3, shows a Lewis acid strength comparable to that of BF3 but is employable in substoichiometric quantities to numerous classic Lewis acid accelerated reactions (e.g., aldol,8,9 conjugate addition,10 Diels−Alder,11 etc.).12 Moreover, its ability to activate hydrosilanes in the presence of oxygenated substrates by the Piers mechanism13 enables unique C−O-based catalysis.14−17 Another perfluoroarylborane, HB(C6F5)2 (Piers’ borane),18,19 has been directly utilized for selective deoxygenation reactions20 or as a precursor to other fluoroarylborane Lewis acids, taking advantage of its alkene and alkyne hydroboration abilities to assemble diverse catalysts.21,22 Less used are the homoleptic tris(trifluorophenyl)boranes, BAr2,4,6‑F and BAr3,4,5‑F, which are weaker Lewis acids.23,24 Compared to B(C6F5)3, B(3,5CF 3 ) 2 C 6 H 3 ) 3 (BAr 3,5‑CF3 ) is a little stronger by the Gutmann−Beckett method but presents as the weaker Lewis acid using the Childs method.25,26 Although computations show BAr3,5‑CF3 to be intrinsically weaker than B(C6F5)3, the absence of ortho-F groups creates a more sterically accessible boron site and, of interest to us, the possibility of purturbing the selectivity of challenging C−O activation reactions through catalyst choice. We recently showed that fluoroarylborane catalysts can be combined with PPh3 to enable chemoselective late-stage functionalization reactions on a number of natural products, including the glycosylated macrolactone natamycin (e.g., Figure 1a).21 In this context, B(C6F5)3 selectively catalyzed

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© XXXX American Chemical Society

Figure 1. Known reactivity differences between B(C6F5)3 and BAr3,5‑CF3.

enoate conjugate reduction (circled in Figure 1a), while BAr3,5‑CF3 catalyzed a lactol elimination (rectangles in Figure 1a). Although different silanes (R3SiH vs R2SiH2) contributed to the diverging chemoselectivities in natamycin, other case studies directly compare the reactivity of B(C6F5)3 and Received: April 17, 2019 Revised: June 10, 2019 Published: June 13, 2019 6648

DOI: 10.1021/acscatal.9b01578 ACS Catal. 2019, 9, 6648−6652

Research Article

ACS Catalysis

(1.5 equiv), and BAr3,5‑CF3 yielded the product of exocyclic C− O bond cleavage/reduction, 1,2-deoxyglucose 1 (79%). B(C6F5)3, in contrast, afforded pentatol 2 (86%), the product of a ring-opening cleavage/reduction of the endocyclic anomeric C−O bond. With B(C6F5)3 and TESH (2.5 equiv), the ringopened 2 was further reduced to 3 in high yield (91%, Scheme 2a). This divergent behavior was mirrored with TES-2deoxygalactose, affording either 4 or 5 using 10 mol % BAr3,5‑CF3 or B(C6F5)3, respectively. When using B(C6F5)3, the corresponding pentatol (2-deoxygalactitol) was not isolated as it was prone to deoxygenate at C1; the optimum procedure thus used 2.5 equiv of TESH and pushed the conversion to 5.32 The galacto- series has been previously noted to be especially susceptible to sequential cyclization/reduction protocols via 1,4-anhydrogalactitol intermediates.14,30 For the TES-2-deoxysugars, the choice of catalyst thus controls which anomeric C−O bond is activated (exo- vs endo-). The comparison between the intrinsically stronger B(C6F5)3 and the less congested BAr3,5‑CF3 continued with other persilylated pyranoses (Table 1).33 In general, most of Table 1’s entries worked best with TES-protected starting materials, because small silyl protecting groups were too reactive and tended to give product mixtures. TES-1-OMe-glucose converts to 1-deoxyglucose (6) regardless of the catalyst (entry 1). Other TES-1-OMe-sugars, however, showed deoxygenation patterns that depended on the catalyst. In the case of the fully oxygenated sugars in entries 2−4, the preference is still for a single exocyclic OMe/OSi acetal reduction by the BAr3,5‑CF3 catalyst. The behavior of B(C6F5)3, however, appeared more variable with net endocyclic C−O reduction being favored in entries 2 and 4. In the case of entry 3, C1 reduction is accompanied by an unusual C6 deoxygenation to form 10 (97%). It is mechanistically revealing, however, that in each of these latter cases, the 1-deoxypyranoses are detected as transient intermediates by 13C{1H} NMR spectroscopy, implying that for fully oxygenated cyclic sugars, both catalysts first target the exocyclic OMe/OSi group. It is the higher reactivity of the B(C6F5)3 catalyst that then differentiates the two fluoroarylboranes. In the case of 6-deoxyhexo- and xylo-pyranoses, pyranose ring opening reductions are the norm with B(C6F5)3 while furanoses are dominant under BAr3,5‑CF3-catalyzed conditions (entries 5−7). For example, TES-6-deoxyglucose ring opens to 6-deoxyglucitol 12 with B(C6F5)3, while the ring is maintained through exocyclic C−O bond cleavage with BAr3,5‑CF3 (entry 4). TES-6-deoxy-L-galactose gives complex mixtures with both catalysts; however, the less congested Me2EtSi-protected form was found to selectively convert to the previously unreported furan 13 with BAr3,5‑CF3 (48 h)32 and 1-deoxy-D-galactitol (8) with B(C6F5)3 (1 h, entry 5).34 Several entries in Table 1 deserve additional comment. For example, while BAr3,5‑CF3 transforms both TES-xylose and TES-1-OMe-xylose to furan 14, B(C6F5)3 instead converts each to a different ring-opened product (entries 6 and 7). The diminished Lewis basicity of the exocyclic OSi group in TESxylose apparently biases C−O activation to the more basic ring oxygen, and ring opening ensues to provide TES-xylitol 15. The small size and high basicity of the OMe group in TES-1OMe-xylose, on the other hand, unexpectedly changes the reaction outcome, and enantioenriched 1-deoxyxylitol (16) results from B(C6F5)3 catalysis (Scheme 3a). The optical activity of 16 indicates that direct 1-demethoxylation to the

BAr3,5‑CF3, such as imine reduction or hydroboration with pinacolborane (HBPin, Figure 1b),27,28 and aldehyde reduction with Hantzsch ester (Figure 1c).29 In these cases BAr3,5‑CF3 displayed the superior reactivity despite being the weaker Lewis acid. Herein, we report significantly different selectivities for the B(C6F5)3- and BAr3,5‑CF3-catalyzed silylative deoxygenation of biomass derived sugars. Our comparative studies began with the simple ethers isochroman, 2-methoxyethylbenzene, and 2-methoxytetrahydropyran (Scheme 1). Each was reacted with 5 mol % Scheme 1. Fluoroarylborane (BArF)-Catalyzed Reduction of Simple Ethers

B(C6F5)3 or BAr3,5‑CF3 under hydrosilylative deoxygenation conditions. In situ 13C{1H} NMR analysis revealed that both catalysts gave the same products (A and B) from isochroman and 2-methoxyethylbenzene, respectively, though the rates with BAr3,5‑CF3 were considerably slower (Scheme 1a and Figure S1 in the Supporting Information). In the case of 2methoxytetrahydropyran, both catalysts predominantly give the pyranose ring-opened diol C, though small amounts of THP from cleavage at “b” are observed with BAr3,5‑CF3 and traces of D are detected with B(C6F5)3 (Scheme 1b and Figure S2). Control experiments show that B(C6F5)3 can catalyze a single C−O bond reduction in THP itself, whereas BAr3,5‑CF3 is unreactive (Figure S2). The comparison was then extended to complex biomassderived sugar alcohols. To avoid the nonreductive condensative cyclizations that can accompany Lewis acid catalysis, the catalysts and silane were premixed prior to addition of substrate.30 Significant differences in selectivity were observed when using 5 or 10 mol % B(C6F5)3 and BAr3,5‑CF3 to reduce the gluco- and galacto-based 2-deoxy sugars (Scheme 2).31 After 1 day, the combination of TES-2-deoxyglucose, TESH Scheme 2. Catalyst-Dependent Reduction of TES-2deoxysugars

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DOI: 10.1021/acscatal.9b01578 ACS Catal. 2019, 9, 6648−6652

Research Article

ACS Catalysis Table 1. Selective Reduction with BAr3,5‑CF3 or B(C6F5)3a

Scheme 3. Comparison of the Optical Activity of Tetraol 16 and Plausible Intermediates for 16

Scheme 4. Divergent Reduction Pathways with TES versus TMS-1-OMe-galactose

a

Indicated yields are of chromatographically purified materials and are averages of a minimum of two experiments. b1 g of the starting sugar was used. c5 mol % boron catalyst used. d46% of starting material recovered. eSi = SiMe2Et, Me2EtSiH was applied. f1 h reaction time.

good yield suggested that catalytic C6 deoxygenation occurs by the illustrated transannular C6 activation/addition and requires access to the 1C4 conformation (Scheme 4b). Because silylium activation of O6 (to form II) should raise the energy of an axial C6 even further in the 1C4 conformer, we presume that it is a steric effect that is biasing the reaction to C1 cleavage. Mechanistic studies to assess the resting state of the two catalysts were also conducted. 1H and 19F{1H} NMR spectroscopic monitoring of the TESH reduction of TES-2deoxyglucose revealed differences in the speciation of the boron catalysts. In the case of B(C6F5)3, the conjugate hydride (C6F5)3B−H− was the only observable boron species, while BAr3,5‑CF3 rested as either the free Lewis acid or as a complex with a nonchemical shift perturbing Lewis base such as the 1,2deoxyglucose product (Figure S4). The countercation to (C6F5)3B−H− is likely similar to I (Scheme 4).36 Independently generated BAr3,5‑CF3−H − has diagnostic spectral signatures,37 and it is not observed during in situ catalysis experiments. In hydrogenations with Hantzsch ester, BAr3,5‑CF3 was found to also not build up the borohydride while B(C6F5)3 does.29,38 The above in situ observations demonstrate that the catalysts speciate quite differently. The consequences of these behaviors are that B(C6F5)3-catalyzed reactions have little free

achiral xylo-pyran is not viable, and the chiral furan likely precedes ring opening (Scheme 3b). A number of the entries in Table 1 stimulated additional experiments to probe the mechanism. The comparison between TMS- and TES-protected 1-OMe-galactose was illuminating (Scheme 4 a). As shown in entry 2 of Table 1, TES-1-OMe-galactose reacts with TESH and B(C6F5)3 to give 1-deoxygalactitol (8). By contrast, the TMS-protected form, Me2EtSiH, and 10 mol % B(C6F5)3 generated the unusual 1,6deoxygalactose 17 (Scheme 4). These divergent outcomes suggested that the 4C1 and 1C4 chair conformers of the putative common intermediate Si-7+ might somehow be playing a role. The difference in energy between the two chair conformations was computed using DFT methods using TMS-7 and TES-7 as models, and as expected the 4C1 chair is more stable, with the 4 C1/1C4 gap being 0.46 and 2.6 kcal/mol, respectively (Figure S3). These calculations coupled with experiments showing that silylium catalysis alone (generated by the Lambert method)35 will convert TMS-7 to TMS-1,5-3,6-anhydrogalactose (18) in 6650

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Nuclear Magnetic Resonance Study. Can. J. Chem. 1982, 60, 801− 808. (6) Sivaev, I. B.; Bregadze, V. I. Lewis Acidity of Boron Compounds. Coord. Chem. Rev. 2014, 270−271, 75−88. (7) Morgan, M. M.; Marwitz, A. J. V; Piers, W. E.; Parvez, M. Comparative Lewis Acidity in Fluoroarylboranes: B(o-HC6F 4)3, B(pHC6F4)3, and B(C6F5)3. Organometallics 2013, 32, 317−322. (8) Ishihara, K.; Hananki, N.; Yamamoto, H. Tris(Pentafluorophenyl)Boron as a New Efficient, Air Stable, and Water Tolerant Catalyst in the Aldol-Type and Michael Reactions. Synlett 1993, 1993, 577. (9) Christmann, M.; Kalesse, M. Vinylogous Mukaiyama Aldol Reactions with Triarylboranes. Tetrahedron Lett. 2001, 42, 1269− 1271. (10) Ishihara, K.; Funahashi, M.; Hanaki, N.; Miyata, M.; Yamamoto, H. Tris(Pentafluorophenyl)Boran as an Efficient Catalyst in the Aldol-Type Reaction of Ketene Silyl Acetals with Imines. Synlett 1994, 1994, 963. (11) Ishihara, K.; Hanaki, N.; Funahashi, M.; Miyata, M.; Yamamoto, H. Tris(Pentafluorophenyl)Boran as an Efficient, Air Stable, and Water Tolerant Lewis Acid Catalyst. Bull. Chem. Soc. Jpn. 1995, 68, 1721. (12) Ishihara, K.; Yamamoto, H. Arylboron Compounds as Acid Catalysts in Organic Synthetic Transformations. Eur. J. Org. Chem. 1999, 1999, 527−538. (13) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions. J. Org. Chem. 2000, 65, 3090−3098. (14) Adduci, L. L.; Bender, T. A.; Dabrowski, J. A.; Gagné, M. R. Chemoselective Conversion of Biologically Sourced Polyols into Chiral Synthons. Nat. Chem. 2015, 7, 576−581. (15) Drosos, N.; Morandi, B. Boron-Catalyzed Regioselective Deoxygenation of Terminal 1,2-Diols to 2-Alkanols Enabled by the Strategic Formation of a Cyclic Siloxane Intermediate. Angew. Chem., Int. Ed. 2015, 54, 8814−8818. (16) Chatterjee, I.; Porwal, D.; Oestreich, M. B(C6F5)3-Catalyzed Chemoselective Defunctionalization of Ether-Containing Primary Alkyl Tosylates with Hydrosilanes. Angew. Chem., Int. Ed. 2017, 56, 3389−3391. (17) Hazra, C. K.; Gandhamsetty, N.; Park, S.; Chang, S. Borane Catalysed Ring Opening and Closing Cascades of Furans Leading to Silicon Functionalized Synthetic Intermediates. Nat. Commun. 2016, 7, 13431. (18) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Synthesis, Properties, and Hydroboration Activity of the Highly Electrophilic Borane Bis(Pentafluorophenyl)Borane, HB(C6F5)2. Organometallics 1998, 17, 5492−5503. (19) Parks, D. J.; Spence, R. E.; Piers, W. E. Bis(Pentafluorophenyl)Borane: Synthesis, Properties, and Hydroboration Chemistry of a Highly Electrophilic Borane Reagent. Angew. Chemie - Int. Ed. 1995, 34, 809−811. (20) Zhang, J.; Park, S.; Chang, S. Selective C−O Bond Cleavage of Sugars with Hydrosilanes Catalyzed by Piers’ Borane Generated In Situ. Angew. Chem., Int. Ed. 2017, 56, 13757−13761. (21) Bender, T. A.; Payne, P. R.; Gagné, M. R. Late-Stage Chemoselective Functional-Group Manipulation of Bioactive Natural Products with Super-Electrophilic Silylium Ions. Nat. Chem. 2018, 10, 85−90. (22) Peruzzi, M. T.; Mei, Q. Q.; Lee, S. J.; Gagné, M. R. Chemoselective Amide Reductions by Heteroleptic Fluoroaryl Boron Lewis Acids. Chem. Commun. 2018, 54, 5855−5858. (23) Lawson, J. R.; Wilkins, L. C.; Melen, R. L. Tris(2,4,6Trifluorophenyl)Borane: An Efficient Hydroboration Catalyst. Chem. - Eur. J. 2017, 23, 10997−11000. (24) Carden, J. L.; Gierlichs, L. J.; Wass, D. F.; Browne, D. L.; Melen, R. L. Unlocking the Catalytic Potential of Tris(3,4,5Trifluorophenyl)Borane with Microwave Irradiation. Chem. Commun. 2019, 55, 318−321.

boron Lewis acid but concomitantly high concentrations of silylium. In contrast, the BAr3,5‑CF3 catalyst rests in the Lewis acidic form of the borane and the concentration of any generated silylium species will be small and likely transient. The higher nucleophilicity and lower steric profile of a putative BAr3,5‑CF3−H− almost certainly contributes to this behavior. The inversion in steady-state “B” vs “Si+” Lewis acids for the two catalysts may be the ultimate source of the observed differences in selectivity. Mechanistic studies are ongoing. In summary, the two reported fluoroarylboranes are highly complementary in their ability to site-selectively activate the C−O bonds of pentoses and hexoses for hydrosilylative reduction. Catalyst control over these processes in complex environments is therefore one step closer and provides one potential path to generating diverse high-value chemicals from a limited number of renewable feedstocks.39

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01578.

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Experimental procedure, analysis data, and NMR spectra for products 1−19; details of the computational studies of Si-1-deoxysugars’ conformations; and Figures S1−S4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michel R. Gagné: 0000-0001-8424-5547 Notes

The authors declare no competing financial interest. All 1H and 13C{1H} NMR spectra (and data files) reported herein are also available at the spectroscopic database that we have established (http://gagnegroup.web.unc.edu/sugarsspectroscopy/sugars). This database currently contains highresolution spectra and raw FID files for more than 75 biomassderived, partially deoxygenated hexoses and pentoses.

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ACKNOWLEDGMENTS This work was exclusively supported by the Department of Energy (Basic Energy Science, DE-FG02-05ER15630). We thank UNC’s Department of Chemistry Mass Spectrometry Core Laboratory, especially Dr. Brandie Ehrmann, for her assistance with the mass spectrometry analysis.

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REFERENCES

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ACS Catalysis (25) Herrington, T. J.; Thom, A. J. W.; White, A. J. P.; Ashley, A. E. Novel H2 Activation by a Tris[3,5-Bis(Trifluoromethyl)Phenyl]Borane Frustrated Lewis Pair. Dalt. Trans. 2012, 41, 9019−9022. (26) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. From B(C6F5)3 to B(OC6F5)3: Synthesis of (C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity. Organometallics 2005, 24, 1685−1691. (27) Yin, Q.; Soltani, Y.; Melen, R. L.; Oestreich, M. BArF3Catalyzed Imine Hydroboration with Pinacolborane Not Requiring the Assistance of an Additional Lewis Base. Organometallics 2017, 36, 2381−2384. (28) Yin, Q.; Kemper, S.; Klare, H. F. T.; Oestreich, M. Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do! Chem. - Eur. J. 2016, 22, 13840− 13844. (29) Hamasaka, G.; Tsuji, H.; Uozumi, Y. Organoborane-Catalyzed Hydrogenation of Unactivated Aldehydes with a Hantzsch Ester as a Synthetic NAD(P)H Analogue. Synlett 2015, 26, 2037−2041. (30) Seo, Y.; Gagné, M. R. Silylium (R3Si+) Catalyzed Condensative Cyclization for Anhydrosugar Synthesis. ACS Catal. 2018, 8, 6993− 6999. (31) 2 mol % of the fluoroaryl borane catalysts was not sufficient for clean conversion. (32) Compounds in dotted boxes have not previously appeared in the literature. (33) Unpublished experiments have shown that the free alcohol is a viable substrate; however, the vigorous H2 evolution that accompanies in situ silylation of the alcohols complicates the experiment. For this reason, preprotection was usually employed. (34) Me2EtSi-8 is fully converted to Si-13 from the reaction of Me2EtSi-6-deoxy-L-galactose, TESH (1.5 equiv), and B(C6F5)3 for 24 h, and then 13 was isolated in 76% yield after deprotection. (35) Lambert, J. B.; Zhao, Y. The Trimesitylsilylium Cation. Angew. Chem., Int. Ed. Engl. 1997, 36, 400−401. (36) Seo, Y.; Gagné, M. R. Positional Selectivity in the Hydrosilylative Partial Deoxygenation of Disaccharides by Boron Catalysts. ACS Catal. 2018, 8, 81−85. (37) Lawrence, E. J.; Oganesyan, V. S.; Hughes, D. L.; Ashley, A. E.; Wildgoose, G. G. An Electrochemical Study of Frustrated Lewis Pairs: A Metal-Free Route to Hydrogen Oxidation. J. Am. Chem. Soc. 2014, 136, 6031−6036. (38) Webb, J. D.; Laberge, V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3. Chem. - Eur. J. 2010, 16, 4895− 4902. (39) Bender, T. A.; Dabrowski, J. A.; Gagné, M. R. Homogeneous Catalysis for the Production of Low-Volume, High-Value Chemicals from Biomass. Nat. Rev. Chem. 2018, 2, 35−46.

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DOI: 10.1021/acscatal.9b01578 ACS Catal. 2019, 9, 6648−6652