Harnessing the reactivity of poly(methylhydrosiloxane) for the reduction and cyclization of biomass to high-value products

Article abstract of DOI:10.1039/c9gc00705a

Poly(methylhydrosiloxane) (PMHS) has been examined for its ability to reduce and subsequently cyclize carbohydrate substrates using catalytic tris(pentafluorophenyl)borane (BCF). The work herein is the first reported example of the direct conversion of monosaccharides to 1,4-anhydro and 2,5-anhydro products utilizing a hydrosiloxane reducing agent. PMHS is produced from waste products of the silicone industry, making it a green alternative to traditional hydrosilane reducing agents. This work thus contributes to the goal of utilizing renewable feedstocks in the production of fine-chemicals.

Full text of DOI:10.1039/c9gc00705a

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COMMUNICATION
Harnessing the Reactivity of Poly(methylhydrosiloxane) for the
Reduction and Cyclization of Biomass to High-Value Products
Nicholas M. Hein, Youngran Seo, Stephen J. Lee, and Michel R. Gagné*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Poly(methylhydrosiloxane) (PMHS) has been examined for its neighboring group participation. For example, treatment of per-
ability to reduce and subsequently cyclize carbohydrate Et3Si protected glucose with EtMe2SiH under BCF catalysis
substrates using catalytic tris(pentafluorophenyl)borane (BCF). results in rapid ring opening via reduction at the anomeric carbon
The work herein is the first reported example of the direct (C1), followed by the slow but selective deoxygenation at the
conversion of monosaccharides to 1,4-anhydro and 2,5-anhydro sterically most accessible primary carbon centers, affording 1,6-
products utilizing a hydrosiloxane reducing agent. PMHS is deoxysorbitol as the major product (Scheme 1a).5 Further studies
produced from waste products of the silicone industry, making it demonstrated that a variety of silyl-protected sugar alcohols
a green alternative to traditional hydrosilane reducing agents. afford 1,6-deoxy products when treated with excess EtMe2SiH
This work thus contributes to the goal of utilizing renewable and catalytic BCF, with further reduced species being minor by-
products.6 Interestingly, galactitol yielded a 1,2,6-deoxy triol as
the major product. Mechanistic studies elucidated the formation
of a transiently formed tetraol that quickly cyclizes from O2 to
C5 (with inversion at C5) before experiencing selective
reduction at C2 (Scheme 1b). Later work demonstrated the
ability of catalytic quantities of silylium to cyclize a variety of
sugar alcohols to high-value 1,4-anhydro products in good to
excellent yields (Scheme 1c).7 Effective catalysis was achieved
by generating silylium in situ through hydride abstraction from
EtMe2SiH with trityl cation. The use of BCF as a Lewis acid led
to poor conversions and to the occasional formation of 1,4-
anhydro products, such as in the cyclization of sorbitol to 1,4-
anhydrosorbitol. In situ monitoring by 19F{1H} NMR
spectroscopy traced the poor performance of BCF to its
preferential coordination to the cyclic ether of the 1,4-anhydro
product, which inhibits its ability to activate silane.8 Besides the
reaction shown in Scheme 1a, few hydrosilylative reductions
starting from cyclic sugars have been documented. One reported
case uses in situ generated (C6F5)2BH as a catalyst for the
reductive ring-opening of per-silylated sugars to linear sugar
alcohols with tetramethylhydrosiloxane (TMDS) as the reductant
(Scheme 1d).9 However, reaction rates were sluggish for fully
oxygenated sugars (e.g. TMS-protected D-glucose converted to
TMS-sorbitol in only 14% yield after 14 days).
feedstocks in the production of fine-chemicals.
Introduction
Alternative methods to produce liquid fuels and chemical
commodities are needed to decrease dependence on fossil
resources. The highly functionalized and stereochemically rich
nature of cellulosic biomass makes it especially attractive in the
production of fine chemicals, as a high degree of functionality
can be translated to products that are desirable as synthons to a
variety of industries.1-4 However, the over oxygenated nature of
biomass requires selective deoxygenation protocols to arrive at
useful synthons. The metal-free deoxygenation of carbohydrates
has been studied extensively by our group through the
combination of catalytic tris(pentafluorophenyl)borane (BCF)
and hydrosilane reducing agents. Site selectivity in C–O bond
reduction for specific carbohydrates can be achieved by
exploiting subtle steric effects and, in some instances,
[*] Dr. N. M. Hein, Dr. Y. Seo, Prof. Dr. M. R. Gagné
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290 (USA)
E-mail: mgagne@unc.edu
Dr. S. J. Lee
U.S. Army Research Office
A drawback to the reduction chemistry described above is its
reliance on hydrosilanes, which suffer from poor atom-economy.
While hydrosilanes feature favorable thermodynamic and kinetic
properties for C–O bond reduction, the stable Si–O bonds in the
resulting silanol and siloxane by-products are energy intensive to
P.O. Box 1221, Research Triangle Park, NC 27709 (USA)
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
All 1H and 13C NMR spectra (and data files) reported herein are also available at the
spectroscopic
database
that
we
have
established.
http://gagnegroup.web.unc.edu/sugars-spectroscopy/sugars.
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Scheme 1. a) TES-protected glucose experiences ring-opening and selective reduction at the primary carbon centers when treated with
EtMe2SiH. b) Galactitol selectively forms a triol via cyclization/reduction of a transiently formed tetraol. c) Sugar alcohols cyclize to 1,4-
anhydro products in good yields via catalytic silylium (R3Si+). d) Cyclic sugars reduce to sugar alcohols in the presence of TMDS and in
situ generated (C6F5)2BH.
recycle.10 An alternative, greener strategy is to instead exploit the carbohydrates. Careful control of the PMHS stoichiometry
reactivity of hydrosiloxanes, such as poly(methylhydrosiloxane) allows for the selective isolation of linear sugar alcohols or
(PMHS), which are derived from waste products. PMHS is cyclized products derived therefrom. Cyclization processes
synthesized by the controlled hydrolysis of MeSiHCl2,11-12 which differ across the investigated sugars, indicating that the subtle
is a by-product in the production of Me2SiCl2, a feedstock for the interactions between the substrate and the PMHS chain are
silicone industry (Scheme 2).13 In addition to being synthesized affected by the sugar stereochemistry.
from a waste product, PMHS is cheap, stable to air and moisture,
and is considered nontoxic.12 Numerous examples of reduction Results and Discussion
processes that utilize PMHS exists in the literature,12, 14-20 but an
investigation into its use in the reduction of biomass is relatively Initial studies probed the differences in reactivity between
unexplored.21-26
PMHS chains of variable length. Commercially available TMS-
The work herein explores the reactivity of PMHS with capped PMHS with Mn of 390, corresponding to an average of 4
catalytic BCF in the reduction and subsequent cyclization of Si–H units per chain, was compared to PMHS with Mn 1,700-
3,200 (26-51 monomers) in the reduction and cyclization of
TMS-protected
D-glucose. These two different PMHS chain
lengths are denoted as oligomeric and polymeric, respectively.
Reaction progress was monitored in situ by 13C{1H} NMR
spectroscopy and a comparison of the extent of reactivity based
on the equivalents of PMHS monomer per glucose is shown in
Figure 1. Under catalytic conditions with 10 mol% BCF and
using DCM as the solvent, both forms of PMHS reduce TMS-
glucose to TMS-sorbitol (in under 1 hour), followed by the
slower cyclization to 1,4-anhydrosorbitol (Scheme 3).
However, oligomeric PMHS requires higher equivalents of
monomer per glucose to reach product conversions comparable
D-
1
2
Scheme 2. The Müller-Rochow process for the synthesis of
(CH3)2SiCl2, the main feedstock for the silicone industry.
(CH3)SiHCl2 is a by-product from this process and can be used to
synthesize PMHS.
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Figure 1. A comparison of in situ 13C{1H} NMR spectra monitoring the extent of reaction between oligomeric and polymeric PMHS
towards TMS-D-glucose. Catalysis was performed using 10 mol% BCF with DCM as the solvent and all reactions were assayed after 24
h. a) 1.1 equivalents of oligomeric PMHS results in unreacted TMS-glucose with minor conversion to 1, while polymeric PMHS affords
roughly 50% conversion to 1. b) 2.2 equivalents of oligomeric PMHS results in nearly full conversion to 1 with trace amounts of glucose
remaining. 2.2 equivalents of polymeric PMHS results in a mixture of 1 and the cyclized species 2. c) Full conversion to 1 is achieved with
3.3 equivalents of oligomeric PMHS but further reduction of 2 occurs with 3.3 equivalents of polymeric PMHS. d) 4.4 equivalents of
oligomeric PMHS results in a complex product mixture from the reduction of 2.
to those obtained with polymeric PMHS. For example, 3.3 the least involved and costly to obtain polyol products free of Si-
equivalents of oligomeric PMHS are required to fully convert byproducts, and in reasonable yields. Table 1 displays the
TMS-glucose to sorbitol, compared to nearly complete number of equivalents of polymeric PMHS required to yield
conversion when 2.2 equivalents of polymeric PMHS are used. linear sugar alcohols or to convert to cyclized product. The initial
Since the number of Si–H equivalents needed in the oligomeric
PMHS roughly coincides with the size of an average chain, it
suggests that once the first Si–H has reacted, a second one is not
accessible in these short chains. The lower equivalents needed to
reach similar conversions with polymeric PMHS implies that a
more efficient use of Si–H groups is possible. When 3.3
equivalents of polymeric PMHS or 4.4 equivalents of oligomeric
PMHS are used, further deoxygenation of
2 is apparent by the
increase in the number of upfield carbon signals. Following
deprotection and purification by column chromatography, 6-
deoxy-1,4-anhydrosorbitol
3 was isolated from this mixture,
along with recovered . Due to the enhanced reactivity of
2
polymeric PMHS, along with its lower cost, the optimization of
reaction conditions was pursued with this material.
A select number of TMS-protected sugars were investigated
for their reactivity with polymeric PMHS under BCF catalysis.
Reactions were performed in DCM with 10 mol% loading of
BCF and reaction progress was monitored in situ by 13C{1H}
NMR spectroscopy. Samples were then deprotected using
Dowex® resin in methanol, followed by hydrolysis of the PMHS
byproduct by heating in water to 50 °C to separate the sugar
products from the siloxane. These conditions were found to be
Scheme 3. When reduction of glucose is performed with 3.3
equivalents of oligomeric PMHS, full conversion to 1 is
achieved. Treatment of glucose with 3.3 equivalents of
polymeric PMHS results in a complex product mixture, from
which 2 and 3 can be isolated following deprotection.
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free method of siloxane removal described above, along with
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percent purity, are reported in Table 1 for DthOeI:o1p0t.1im03i9z/eCd9GpCro0t0o7c0o5lA.
The ESI contains the 1H and 13C{1H} NMR spectra for the crude
products after siloxane hydrolysis. Each product has been
previously reported and can be purified in accordance with
Table 1. Products resulting from the reduction and
cyclization of monosaccharides under BCF catalysis with
PMHS. Workup to remove siloxane byproducts is column
free
established protocols.7, 9, 27 While TMS-
xylose both afford 1,4-anhydro products, TMS-
instead yields the 2,5-anhydro isomer . TMS-
D
-glucose and TMS-
-mannose
-galactose
D-
D
7
D
cannot be reduced to the sugar alcohol galactitol under the
conditions studied, and instead always yields a mixture of 1,6-
anhydrogalactofuranose
8 and 1,4-anhydrogalactitol 9, even
when 1.1 equivalents of PMHS are used in the reaction (see ESI).
The order of addition for reagents was varied in the case of TMS-
galactose reactivity; premixing PMHS and BCF minimizes the
over reduction of
9 observed when TMS-D-galactose was added
before the PMHS.
An important mechanistic consideration when comparing the
differences in reactivity shown in Table 1 is whether the sugar is
tethered to the PMHS chain following the initial reduction.
Evidence from the reduction of TMS-D-mannose to mannitol
indicates that the mannitol product is not protected by the PMHS
chain, as would be expected in a simple mechanism (Scheme 4a).
Instead, a symmetric 13C{1H} NMR spectrum consisting of three
oxygenated carbon signals and three TMS carbon signals is
observed in situ. A PMHS protecting group would break
symmetry and six unique oxygenated carbon signals would result
(Scheme 4a). Formation of per-TMS-mannitol in the reaction
was confirmed by spiking the reaction mixture with authentic
compound. This provided a sample with complete overlap of
resonances in the 13C{1H} NMR spectrum. Since BCF is known
to catalyze silyl group exchange between hydrosilanes and silyl
ethers/siloxanes28-29, we hypothesized that silyl exchange might
be facile, and that the resulting PMHS-protected sugar might not
be visible due to its now polymeric nature. Depending on the
extent of the exchange, this process could liberate half an
equivalent (or more) of per-TMS-mannitol (Scheme 4b).
1
Integration of the H NMR signals for TMS-mannitol from in
situ reduction of per-TMS-mannose relative to a cyclooctane
internal standard gave a yield of 55%, providing support for this
hypothesis. To demonstrate silyl exchange between two different
silyl protected sugars in the presence of BCF and PMHS, an
NMR scale reaction containing in situ produced TMS-mannitol
(from TMS-mannose reduction with 10% BCF and 1.5 equiv.
PMHS) was spiked with per-EtMe2Si-mannitol. The resulting
13C{1H} NMR spectrum contained broadened signals in the C–
O region due to the complex mixture of silyl protected products
obtained from exchange between TMS and EtMe2Si groups. A
similarly broadened 13C{1H} NMR spectrum resulted from
treating equimolar amounts of premixed TMS-mannose and
EtMe2Si-mannose with 1.5 equivalents of PMHS under BCF
catalysis (see ESI). The control experiment where equimolar
amounts of TMS-mannitol and EtMe2Si-mannitol standard are
treated with 10% BCF in the absence of PMHS predominantly
yields sharp, discernable peaks for the two protected forms in the
13C{1H} NMR spectrum. Together these results indicate that
silyl exchange under catalytic conditions is facile and, moreover,
that the hydrides of PMHS are needed to facilitate silyl exchange.
Reaction conditions: Premix substrate with 10 mol% BCF in
CD2Cl2 (0.6 mL), followed by addition of PMHS. aReaction
complete after 1 hour. bReaction complete after 24 hours.
d
cReaction complete after 36 hours. Reaction complete after 24
hours, combined yield of 8 and 9 (~1:1), for cleanest reactivity
PMHS and BCF were premixed before addition to the TMS-
galactose.
reduction of the protected sugar to the linear sugar alcohol is
rapid (complete in under 30 minutes), but the subsequent
cyclization requires at least 24 hours to reach completion.
Reactivity can be halted at the sugar alcohol and further
conversion to cyclized product does not occur unless the
specified equivalents of PMHS are added. For example, when
monitoring the reduction of TMS-D-glucose to sorbitol with 1.8
equivalents of PMHS by in situ 13C{1H} NMR spectroscopy,
reactivity stops at the initial reduction to TMS-sorbitol and
cyclization does not occur even after 3 days. To ascertain how
PMHS reductions perform across a swath of biomass-derived
starting materials, yields of product obtained using the column
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Scheme 4. a) The activation and subsequent reduction of TMS-D-mannose by PMHS and BCF would yield TMS-mannitol tethered to the
PMHS chain (not observed). b) To support the observation of per-TMS-mannitol in situ, silyl group metathesis is invoked between PMHS
and TMS, with transiently produced trimethylsilane driving the process.
The above analysis suggests that the cyclization of sugar
alcohols occurs in an intermolecular fashion with PMHS derived
silylium rather than intramolecularly from a tethered siloxane
such as that shown in Scheme 4a. The O4 to C1 cyclization
process yielding
the silylium catalyzed cyclization of silyl-protected sorbitol.7
However, the rationale for the O5 to C2 cyclization of to is
2 and 5 has been previously demonstrated by
6
7
less clear. Previous work established that under silylium
catalyzed conditions, protected mannitol cyclizes O4 to C1,
yielding 1,4-anhydromannitol (Scheme 5a).7 The conversion of
mannitol to
7 via O5 to C2 cyclization has been previously
reported under debenzylative cyclization conditions30 or via
boronium chelated precyclization intermediates.27 The latter
example of 1,2-boronium chelation driving the activation of the
secondary position in sugar alcohols (boxed intermediate in
Scheme 5b) suggests that PMHS derived silylium may be prone
to accessing hypervalent silicon21 that similarly chelates the
substrate and alters the stereoselectivity of the cyclization
(Scheme 5b). Since hypervalency of PMHS silicon has been used
to explain the means by which catalytic poly(ethylene glycol)
crosslinks PMHS chains by homodehydrocondensation, this
hypothesis is not entirely without merit.31
Scheme 5. a) Previous work has demonstrated that TMS-mannitol
cyclizes O4 to C1 with catalytic silylium (EtMe2Si+). b) The
formation of a transient 1,2-chelate between mannitol and
hypervalent silicon may explain the preference for O5 to C2
cyclization.
The product distribution resulting from TMS-
reduction is more puzzling. When the reaction mixture resulting
from treatment of TMS- -galactose with 2 equivalents of PMHS
D-galactose
D
monomer is monitored in situ by 13C{1H} NMR spectroscopy,
one observes unreacted starting material and 1,4-
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Scheme 6. Proposed mechanism for the production of 8 and 9
from TMS-D-galactose.
Scheme 7. Treatment of TMS-cellobiose or TMS-maltose with
5.0 equivalents of PMHS with 10% BCF in DCM yields 10 as
the major product.
anhydrogalactitol
however, the starting material begins to convert to the non-
reduced 1,6-anhydrogalactofuranose , eventually generating a
near 1:1 ratio of reduced and non-reduced . It appears that the
change from a reductive to a non-reductive path is linked to
PMHS consumption, but we have been unable to precisely
identify the source of this change in reactivity as PMHS is in
excess throughout. Considering that the formation of 8 appears
to be linked to PMHS consumption, reactivity was probed using
more than two equivalents of PMHS. However, at higher
equivalencies further reduction of both 8 and 9 occurs, resulting
in a complex product mixture with poor control in product
selectivity.
9 after 1 hour (see ESI). Beyond this time,
13C{1H} NMR spectroscopy. While full consumption of
disaccharide is achieved with higher equivalents of PMHS, the
sorbitol that is produced is also likely deoxygenated to a volatile
8
9
8
species, as
1 or products resulting from the further reduction of
1
are absent after deprotection and work-up. The reactivity
observed in the case of disaccharide reduction, including the
resistance of 10 to further reduction, has been observed by our
group previously and is not significantly different from
previously reported results.33
Conclusions
PMHS has been successfully utilized as a greener alternative to
traditional hydrosilanes for the reduction and cyclization of
carbohydrates to high-value products using metal-free catalysis.
The reported reactions are the first examples by which cyclic
pyranose sugars can be converted directly to 1,4-anhydro or 2,5-
anhydro-products using hydrosiloxane reductants. The
cyclization of in situ generated mannitol to 2,5-anhydroglucitol
has not been previously observed using a silylium source and is
unique to the reactivity of PMHS. These transformations are
important contributions to the goal of utilizing renewable
resources for the production of fine chemicals.
To enhance the greenness of the investigated
transformations, toluene was tested as an alternative solvent to
dichloromethane. Acceptable solvent choices are limited, as
oxygenated solvents that could potentially be derived from
biomass would interfere with the desired chemistry.
Nevertheless, the CHEM21 solvent guide gives toluene a better
environmental score compared to dichloromethane and is listed
as a “problematic” solvent compared to “hazardous” for DCM.32
TMS-
monomer (equivalents shown to give good yields of
with 10% BCF loading in d8-toluene. Under these conditions,
reduction to TMS-sorbitol occurred smoothly, with only traces
of
being observed by in situ 13C{1H} NMR spectroscopy (24
hours). When the equivalents of PMHS monomer were increased
to 3.5, still remained the major product, with a slight increase
D-glucose was treated with 2.3 equivalents of PMHS
2
in DCM)
1
2
Conflicts of interest
There are no conflicts to declare.
1
in over-reduced byproducts being observed (see ESI). The
change to toluene does lower the kinetic reduction potential, but
it also enables a selective protocol to sorbitol.
Notes and references
The disaccharides β-cellobiose and β-maltose were also
investigated for their reactivity with polymeric PMHS. Full
consumption of TMS-protected disaccharide was achieved using
5.0 equivalents of PMHS per mole of substrate. However, the
only isolated product was 1-deoxyglucose 10 (Scheme 7). When
the reduction of TMS-cellobiose was investigated using 2.5
equivalents of PMHS per mole of disaccharide, a mixture of
This research was partially supported by the U.S. Department of
Energy (Basic Energy Science, DE-FG02-05ER15630) and the
U.S. Army Research Office (ARO) (W911NF-15-2-0119). Dr. N.
M. Hein was supported by an NRC Research Associateship from
the ARO. The mass spectrometer used in these studies was
purchased with the support of the National Science Foundation
under Grant No. CHE-1726291. We thank the University of North
Carolina’s Department of Chemistry Mass Spectrometry Core
Laboratory, especially Dr. Brandie Ehrmann, for her assistance
with mass spectrometry analysis.
starting material, 10, and TMS-sorbitol 1 was observed in situ by
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