Positional selectivity in the hydrosilylative partial deoxygenation of

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Positional selectivity in the hydrosilylative partial deoxygenation of disaccharides by boron-catalysts Youngran Seo, and Michel R Gagné ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02992 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Catalysis

Positional selectivity in the hydrosilylative partial deoxygenation of disaccharides by boron-catalysts Youngran Seo1 and Michel R. Gagné1* 1

Department of Chemistry, The University of North Carolina at Chapel Hill, North Carolina

27599, United States ABSTRACT The catalytic partial deoxygenation of the per-silylated gluco-disaccharides maltose, cellobiose, trehalose, and isomaltose by Me2EtSiH and the Lewis acid catalyst B(C6F5)3 have been examined to determine which C–O bonds are favorably removed. In the case of maltose, cellobiose and trehalose, the linking anomeric center is the kinetically preferred site of reduction and cleavage of the disaccharide occurs to yield 1-deoxyglucose and glucose as the primary products. Although 1-deoxyglucose is kinetically inert, glucose continues to reduce by ring opening to sorbitol and is eventually 1,6-deoxygenated to the corresponding tetraol. Isomaltose differs from the other disaccharides in that it has a α-(1´→6) linkage, which is more resistant to cleavage and thus directs reactivity to a ring opening of the reducing sugar and the generation of isomaltitol.

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KEYWORDS: Positional selective, partial deoxygenation, hydrosilylative deoxygenation, glucodisaccharides, B(C6F5)3, polyols.

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INTRODUCTION The goal of producing chemicals from renewable resources such as lignocellulose, starch, and vegetable oils instead of petroleum has inspired considerable research and development.1–6 For example, the versatile furfural and hydroxymethyl furfural (HMF) are accessible from lignocellulose sources,7 and the anhydrosugar isosorbide and fatty acid derivatives from plant oils have been used in the production of polycarbonate material or thermoplastic polymers, respectively.8–10 Because saccharides are inherently chiral and readily available, they have also been viewed as attractive substrates for the synthesis of chiral synthons, with potentially high value returns if they can be appropriately integrated into pharmaceutical or fine-chemical needs.11–16 This allure has led to the development of new approaches to the synthesis of chiral poly-, hetero- or carbocyclic compounds from biomass. One common theme for biomass conversion to fine chemicals is the need for selective, partial deoxygenation schemes that do not destroy the extant chirality of a cellulosic precursor. Eliminative routes (e.g. as for HMF) are less useful when the goal is to retain stereocenters.17,18

Scheme 1. Selectivity trends from previous B(C6F5)3-catalyzed hydrosilylative deoxygenation of hexoses; a) SN2 type polyol reduction via silaoxocarbenium (k1o > k2o), b) SN2 type reduction of 2o C−O with neighboring group participation, c) SN1 type reduction via common oxocarbenium (kβ (a) > kα (b)), d) Reduced anomeric ‘OSi’ basicity leads to pyranose ring opening.


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The B(C6F5)3-catalyzed hydrosilylative (e.g. Me2EtSiH, Et3SiH, etc.) deoxygenation has emerged as a powerful non-eliminative method for the selective removal of C−O bonds in carbohydrates.19,20 The B(C6F5)3 catalyst generates a silylium equivalent (R3Si+) ion paired to a reductant (H−B(C6F5)3¯), and together they activate and reduce C−O bonds.21–24 Persilylated linear hexoses are converted to 1,6-deoxytetraols by the selective cleavage of the primary C−O bonds, a process that proceeds by first forming a disilyloxonium ion (C−OSi2+) intermediate, followed by cleaving the C−O bond with the ion paired H–B(C6F5)3¯ nucleophile (Piers mechanism).21 1,6-Deoxytetraols can be further reduced to triols by selective reduction of a single secondary C−O bond, though this requires a change in mechanism and the involvement of Si−THF+ intermediates (Scheme 1(b)). Manipulating the steric bulk of the silyl protecting group and the silane reductant can also afford different chiral polyols from the same substrate.19 Diverse 1,2-diols can also be selectively monodeoxygenated by B(C6F5)3 catalysis to afford 2alkanols using a disilane (R2SiH2); a cyclic siloxane controls the site selectivity in this case (Scheme 1(a)).25 Readily rationalized 1o vs 2o selectivity is also possible in unsaturated carbohydrates,26 such as glucal and galactal, as well as in tosylate reduction.27 Although not extensively discussed herein, the (POCOP)IrH+ catalyst also functions in a manner that is analogous to B(C6F5)3.28–33 When the anomeric center is intact, B(C6F5)3-catalyzed hydrosilylative deoxygenations target this functional group, but in a manner that depends on the exo-oxygen substituent. For example, 1-MeO−glucose is readily demethoxylated to yield 1-deoxyglucose via an oxocarbenium ion (Scheme 1c).34,35 In these cases, the β-glucoside is more reactive than the α-glucoside. The high selectivity for demethoxylation was rationalized by the favorable accessibility and Lewis basicity of the MeO− group for the R3Si+ electrophile. When the basicity of the exo-oxygen is reduced, as in persilylated glucose, the more basic ring oxygen sequesters R3Si+ and pyranose ring opening via a sila-oxocarbenium intermediate dominates to yield sorbitol (Scheme 1(d)).35,36 The reactivity trends delineated above can be summarized as follows: a) in linear hexoses, 1o reduction of silyl ethers is facile to yield 1,6-deoxy products; b) reduction of 2o-silyl ethers is sluggish unless neighboring group participation can access Si−THF+ intermediates; c) when the exo-anomeric substituent is Lewis basic (e.g. 1-MeO−glucose), silylium abstraction of this group yields 1-deoxyglucose via a stabilized oxocarbenium intermediate; and d) when the exo-O


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substituent is a silyl ether, the lower Lewis basicity shifts the reactivity to the pyranose ring oxygen to initiate ring opening through sila-oxocarbenium ions. While this paper was in review Chang and Park reported that the Piers borane, (C6F5)2BH, can silylatively ring open simple sugars and a disaccharide.37

RESULTS AND DISCUSSIONS This investigation extends these reactivity guidelines to disaccharides as a prelude to examining oligo- or poly-saccharide feedstocks. If the silyloxonium → oxocarbenium ion → reduction sequence governs the chemoselectivity of the C–O cleavage, several primary reduction products are possible for a generic gluco-disaccharide. Assuming that acetal reduction precedes carbinol reduction and is dominated by SN1 type mechanisms, four possible primary C−O bond cleavages are possible, two that are ring opening (A and B37), one that reduces the terminal anomeric center (C), and one that cleaves the disaccharide (D), (Scheme 2). Monosaccharide models revealed oxygen Lewis basicity to be a key determinant of comparative reactivity, with ROMeanomeric > ROSianomeric >> ROSi.34 The potential for four diastereomers resulting from various configurations at C1 and C1´ adds an additional level of complexity. Our studies aim to establish the C−O cleavage preferences, first of the primary reduction products and eventually to the downstream products as well.

Scheme 2. Possible primary cleavage sites of a persilylated gluco-disaccharide; A: ring opening at the reducing end, B: ring opening at the non-reducing end, C: cleavage at the terminal anomeric center, D: cleavage at the glycosidic linkage.


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The four disaccharides in Figure 1 vary the anomeric linkages (maltose (α) vs cellobiose (β)), the position of the linkage (maltose (1´→4) vs isomaltose (1´→6)), and for trehalose the 1´→1 linkage eliminates the terminal reducing unit. Each disaccharide was pre-protected as dimethylethylsilyl ethers (Me2EtSi−, represented as Si−) before use to reduce hydrosilane usage and improve solubility in organic solvents. The Me2EtSi− group is large enough to lead to products and substrates that are stable enough for productive transformation.

Figure 1. Selected persilylated disaccharides (Si− = Me2EtSi−). First, persilylated β-maltose was reacted at RT with excess Me2EtSiH and 5 mol% B(C6F5)3. The reaction was clean, and after 24 h 1-deoxyglucose (1) and 1,6-deoxysorbitol (2) were isolated in 96% and 99% yields, respectively (Scheme 3). Since multiple reductions were needed to provide these products, the reaction was monitored in situ by 13C NMR spectroscopy to gain insight into the course of events. After 30 min, the starting material was consumed, and sorbitol (3) was clearly visible by its six sharp peaks in the carbinol region (Figure 2(a)). These peaks were accompanied by a set of broadened resonances that roughly coincided with 1-deoxyglucose (1), which stayed broad as 3 was doubly reduced to 1,6-deoxysorbitol (2), the final products of the reaction (Figure 2(b)). Quenching at any time with Et3N, sharpened the broad peaks to those expected for 1.


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O

SiO OSi O

b

O

SiO OSi

OSi

OSi 5% B(C6 F5)3 Me2 EtSiH (7.0 eq.)

OSi a

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OSi

O

SiO

OSi

SiO CH 2Cl2, 24 h, rt.

OSi Si = Me2EtSi Si-maltose

SiO

OSi 1 (96%)

SiO

OSi 2 (99%)

OSi Si O

SiO

SiO

OSi OSi

SiO SiO

SiO OSi Si −1 +

OSi 3

Scheme 3. Hydrosilylative deoxygenation with maltose, α-Glc(1´→4)Glc. These data indicate that the first cleavage site is “a” to give 1-deoxyglucose and per-Si-glucose, which is further reduced to 3, and eventually to 2 (Scheme 3); per-Si-glucose is rapidly converted to sorbitol with B(C6F5)3/Me2EtSiH (Figure S1), thereby establishing a pathway to 2 via 3. When only 2.05 equivalents of Me2EtSiH is used, the reaction halts at 1 and 3 (Figure S2). Me2PhSiH was typically equally effective, while other bulkier silanes were not as reactive (Figure S3). Under no set of experimental conditions were we able to detect the intermediacy of “b” cleaving first. This product, maltitol, reacts as expected to cleave the anomeric C−O bond to yield sorbitol and 1-deoxyglucose.38

Figure 2. 150 MHz

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C NMR spectra of a) after 0.5 h; b) after 24 h from the hydrosilylative

reduction of Si-maltose with 5 mol% B(C6F5)3 and 7.0 equiv. of Me2EtSiH. The resistance of 1 to further reduction was somewhat puzzling when compared to the ringopening reduction of per-Si-glucose and the reduction of the primary silyl ethers of sorbitol. The stability of 1-deoxyglucose was previously noted in studies examining the global deoxygenation


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of glucose by an Ir-catalyst.34 Experiments to reduce 1-deoxyglucose with B(C6F5)3 and Me2EtSiH confirmed its resistance to additional reduction (over 24 h).35

Figure 3. 150 MHz 13C NMR spectra at -70 oC of per-Me2EtSi-1-deoxyglucose in the presence of 4 equiv. of Me2EtSiH and a) 10 mol% B(C6F5)3; or b) 50 mol% B(C6F5)3. Interestingly, studies to source the lack of reactivity with 1-deoxyglucose provided information explaining the broad resonances for 1 under in situ reduction conditions. A combination of experiments established that broadening of the

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C resonances only occurs in the presence of

B(C6F5)3 and excess Me2EtSiH (Figure S4), suggesting that the combination of B(C6F5)3 and 1 heterolyze the silane to yield Si− −1+ and H−B(C6F5)3¯. Confirming this assessment was a low temperature

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C NMR spectrum of a mixture of 1, 50 mol% of B(C6F5)3, and 4 equiv. of

Me2EtSiH (Figure 3).35 At room temperature, a severely broadened spectrum is observed, but at –70 °C, the silicon exchange dynamics are slowed enough to resolve free 1 and a species we assign as Si− −1+ (Figure S5). The combination of 1 and B(C6F5)3 therefore constitute a frustrated pair that is capable of heterolyzing the silane to Si− −1+ and H–B(C6F5)3¯ (confirmed by 19F NMR spectroscopy, Figure S6). Si− −1+ is sufficiently electrophilic to transfer Me2EtSi+ to a number of reactive compounds (including 3) but is apparently itself not susceptible to reduction by the H– B(C6F5)3¯ counterion.39 With an outline for the sequence of deoxygenations operating in maltose, we then subjected βpersilylated cellobiose (β-Glc(1´→4)Glc) to the deoxygenation conditions. If cellobiose follows the same pathway as maltose, the products should be the same even if the rates are different due to the change in anomer. Since our reported rate differences for α- and β-MeO−glucose were


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with the Ir catalyst,34,35 we first tested a 1:1 mixture of 1-MeO−α- and β-glucose to the B(C6F5)3catalyzed conditions (Figure S7).38 Although a slight excess of silane was required to achieve a similar conversion, the β-MeO− converted to 1 while α-MeO− stayed mostly unreacted. Reacting cellobiose under the same conditions (5 mol% B(C6F5)3, 7.0 equiv. Me2EtSiH) yielded a 1:1 mixture of 1 and 3 after 24 h. Maltose and cellobiose were directly compared by reacting a 1:1 mixture of the two with 5 mol% of B(C6F5)3 and 2.0 equiv. of Me2EtSiH. By in situ

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C

NMR spectroscopy, Si-maltose was completely converted to 1 and 3 while Si-cellobiose was largely untouched (Scheme 4 and Figure S8). In addition to inverting the anomer selectivity observed with simple methyl glucosides, the preference for cleaving the α-anomer in the disaccharide is significantly larger than observed for the acid-catalyzed hydrolysis, which also favors maltose over cellobiose.40

Scheme 4. B(C6F5)3-catalyzed hydrosilylative reduction of a 1:1 mixture of Si-maltose and Sicellobiose. We also determined the relative C−O cleavage rates of Si-maltose, Si-cellobiose and Si-βglucose by treating 1:1 mixtures of the sugars to the B(C6F5)3-catalyzed reaction with 1.4 or 2.4 equiv. of silane (Figures S9 and S10).41 In this way Si-maltose was determined to be significantly more reactive than Si-glucose (glucose largely untouched although other minor products are present after silane consumption), which in turn is more reactive than Si-cellobiose; Si-glucose is completely converted to sorbitol before Si-cellobiose reacts. In both cases, the O-silyl anomeric centers are largely unreacted until the disaccharides are cleaved to monomers. These pairwise experiments in combination with reactions on the individual sugars lead to the relative hydrosilylative deoxygenation reactivities outlined in Figure 4 (“a” fastest, “e” slowest).42

Figure 4. Relative hydrosilylative reduction in Si-maltose, Si-cellobiose and Si-β-glucose.


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A time course for the deoxygenation of Si-cellobiose was obtained by adding Et3N to the individual reactions at the indicated time points and analyzing the quenched solutions by 1H NMR spectroscopy (Figure 5 and Figures S11 and S12). The glycosidic bond cleaves within a half hour to yield an unchanging amount of 1-deoxyglucose, along with sorbitol and a small amount of pentatol. Over the next 10 h the sorbitol is consumed, the pentatol transiently builds, and eventually converts to 1,6-deoxysorbitol (2). The intermediate pentatol was presumed to be either 1-deoxysorbitol or 6-deoxysorbitol. Since Si-glucose was never observed, it is clear that it is consumed quickly to yield sorbitol. The relative deoxygenative rates for cellobiose were determined to be β-C1´→O4 ≥ β-C1−O5 > C1−O1 or C6−O6 (i ≥ ii > iii (or iv) > iv (or iii); Figure 5(a)).

Figure 5. Conversion of hydrosilylative deoxygenation with a) Si-cellobiose and b) Si-trehalose. This same procedure was used to monitor the deoxygenation of the slower reacting nonreducing disaccharide trehalose, which required a 10 mol% catalyst loading. Figure 5(b) reveals that its reduction also provides the same 1:1 mixture of partially deoxygenated polyols 1 and 2 (Figures S13 and S14), however, the slow glycosidic bond cleavage ensures that few intermediates build at intermediate times.


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Finally, the disaccharide isomaltose, which contains a sterically less congested, conformationally more mobile primary glycosidic linkage, provides a 139% isolated yield of 2 under conditions of excess silane (Scheme 5). The previous disaccharide results make clear that cleaving the glycosidic bond first inevitably leads to unreactive 1-deoxyglycose (1), implying that another C−O bond is preferentially cleaved in this case. When only 1.05 equiv. of Me2EtSiH was used, Si-isomaltose was converted to a single compound possessing one acetal carbon (98.6 ppm

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C NMR spectroscopy). Because Si-isomaltose is an α-/β-mixture of anomers, the

generation of a single compound suggested that the SiO-sugar was ring opened to converge the two anomers. Exchanging the silyl groups for acetyl groups provided a species spectroscopically identical to known 5 (isomaltitol, Scheme 5).43 Since authentic isomaltitol is expensive ($998/ 500 mg), we confirmed the structure of 4 by comparing it to the 1H and 13C NMR spectra (after deprotection) of the isomaltitol that is present in the commercial sweetener isomalt (a mixture of 6-O-α-D-glucopyranosyl-D-sorbitol and 1-O-α-D-glycopyranosyl-D-mannitol). The results from Scheme 5 imply that 4 is the primary product from reduction of Si-isomaltose, and that it reacts further to yield 2 (but without proceeding through 1-deoxyglucose).

Scheme 5. B(C6F5)3-catalyzed hydrosilylative deoxygenation with Si-isomaltose. *yields >100% indicate that both sugars in the starting material are converted to the same product. Similar reactivity differences between α-(1´→4) and α-(1´→6) glycosidic bonds have also been observed in the Lewis acid catalyzed reductive cleavage of polysaccharides.44 For example, the BF3•Et2O/Et3SiH reduction of permethylated pullulan, a polymer composed of maltotriose units (each containing two α-1,4-linkages) enchained by α-1,6-glycosidic bonds, preferentially occurs at the α-1,4-linked sites to preferentially provide the α-(1´→6) disaccharide (isomaltose).


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The enhanced stabilization provided by the exo-anomeric effect, which is a distinct feature of the α-(1´→6) glycosidic bond, was suggested.45 Summary The B(C6F5)3-catalysed partial deoxygenation of disaccharides shows selectivity trends that depend on configuration and steric effects. The reactivity of persilylated maltose, cellobiose and trehalose are dominated by kinetic cleavage of the anomeric center linking the two gluco-units, to generate two partially reduced polyols, 1 and 2. For reasons that are not yet clear, 1deoxyglucose does not react further. In the case of Si-maltitol, the contra-steric product isomaltitol results despite the isomaltose displaying a more accessible Lewis basic site. The selective reduction of these disaccharides is highly related to its first C−O bond cleavage event.

ASSOCIATED CONTENT Supporting Information. Experimental procedure for deoxygenation of disaccharides, and NMR spectra of per-silylated disaccharides, as well as Figures S1~S17 (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail for M. R. G.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was exclusively supported by the Department of Energy (Basic Energy Science, DEFG02-05ER15630). We especially thank Laura Adduci for performing low temperature NMR experiments on 1-deoxyglucose. REFERENCES


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The (POCOP)Ir−H+/silane catalyst differs from B(C6F5)3 in that it yields 1,6anhydroglucose along with smaller quantities of 1-deoxyglucose and sorbitol from Simaltose. See SI for details (Figure S15, S16, and S17). The diverging behavior of Ir and B catalysts will be the subject of a future contribution.

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We still do not fully understand why 1-deoxyglucose is resistant to hydrosilylative deoxygenation. By contrast, 1,2-deoxyglucose is converted to a triol under similar reaction conditions (See reference 19). Establishing the reason for this stability is a current research focus.

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Capon, B. Chem. Rev. 1969, 69, 407–498.

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Attempts to observe glucose at early times have been unsuccessful, even at lowered reaction temperature or substoichiometric silane amounts. For example, 1.05 equiv. of Me2EtSiH afforded partial conversion to 1 and 2, no glucose.

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Since “b” is slower than “a” and “e” is slower than “d,” it is not possible to directly compare “b” and “e.”

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Wolfrom, M. L.; Thompson, A.; O'Neill, A. N.; Galkowski, T. T. J. Am. Chem. Soc. 1952, 74, 1062–1064.

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Rolf, D.; Bennek, J. A.; Gray, G. R. Carbohydr. Res. 1985, 137, 183–196.

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Javaroni, F.; Ferreira, A. B. B.; da Silva, C. O. Carbohydr. Res. 2009, 344, 1235–1247.


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Positional selectivity in the hydrosilylative partial deoxygenation of

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