Light-Mediated Cross-Coupling of Anomeric Trifluoroborates

Article abstract of DOI:10.1021/acs.orglett.1c01035

Stereoselective reactions at the anomeric carbon constitute the cornerstone of preparative carbohydrate chemistry. Here, we report stereoselective C-arylation and etherification reactions of anomeric trifluoroborates derived from BMIDA esters. These react

Full text of DOI:10.1021/acs.orglett.1c01035

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Light-Mediated Cross-Coupling of Anomeric Trifluoroborates
Eric M. Miller and Maciej A. Walczak*
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ABSTRACT: Stereoselective reactions at the anomeric carbon
constitute the cornerstone of preparative carbohydrate chemistry.
Here, we report stereoselective C-arylation and etherification
reactions of anomeric trifluoroborates derived from BMIDA esters.
These reactions are characterized by high anomeric selectivities for
2-deoxysugars and broad substrate scope (24 examples), including
disaccharides and trifluoroborates with free hydroxyl groups.
Taken together, this new class of carbohydrate reagents adds the
palette of anomeric nucleophile reagents suitable for efficient
installation of C−C bonds.
axial trifluoroborates could effectively undergo a stereo-
arbohydrates are critical encoders of biological informa-
1
Ction because of their extensive structural complexity. retentive coupling with electrophilic partners. This proposal
was supported by a recent study from the Hirai group19
Saccharides are also involved in a number of important
biological processes.2−4 Among bioactive glycoconjugates, C-
featuring the synthesis of stable silicon-protected potassium
aryl glycosides have recently been targeted due to antitumor5
trifluoroborates of 2-deoxy-β-D-glucose and β-D-galactose.
(e.g., antibiotics 16 and 2) and antidiabetic activities
exemplified by dapagliflozin 37 and C-mannosyl proteins
(4)8 (Scheme 1A). Recent progress in the chemical synthesis
of C-glycosides revealed an array of new transformations to
rapidly assemble diverse structures.9 Current methods
featuring transition metal-catalyzed synthesis of C-aryl glyco-
sides use Co,10 Fe,11 Ni,12 and Zn13 complexes (Scheme 1B),
engaging anomeric halides as viable coupling partners.
Alternatively, redox-active esters emerged as the source of
anomeric radicals14 and, when coupled with a Ni catalyst, can
produce C-aryl glycosides.15 These methods are characterized
by variable selectivities, poor functional group tolerance, and a
narrow scope. In an effort to address these obstacles, we
recently showed high control of selectivity through a
stereoretentive oxidative addition/reductive elimination Pd-
catalyzed processes allowing for a complete control of
stereochemistry using C1 stannanes.9b,16 To further expand
the scope of anomeric nucleophiles, we hypothesized that C1
trifluoroborates could give rise to air-stable glycosyl donors
and could be engaged in C(sp3)−C(sp2) cross-coupling
reactions. Trifluoroborates have found widespread use as a
nucleophilic component in various cross-couplings due to their
stability, low toxicity, and compatibility with numerous
functional groups.17 These reagents have been shown to
partake in light-mediated reactions with aryl halides using
nickel and photoredox catalysts engaged in a dual catalytic
system.17c,18 On the basis of this literature precedent, we
further hypothesized that with a proper control of the
electronic effects either through the modulation of the
protective groups or ligands on the high-valent nickel center,
Here, we report the synthesis and stereoretentive C−C
cross-coupling reactions of α-anomeric trifluoroborates 11 of
common mono- and disaccharides under the photoredox
conditions (Scheme 1C).20 These reagents are a reliable source
of anomeric radicals and can be merged with Ni(II) cocatalyst
to form C-glycosides in excellent anomeric selectivities.
On the basis of the perceived stability of methyliminodi-
acetic acid boronate (BMIDA) esters,21 we attempted the
synthesis of C1 boronates derived from 2-deoxysugars and fully
oxygenated monosaccharides (for details, see the Supporting
Information). Using MIDA proved vital, as it allowed for the
synthesis of stable anomers that could be successfully purified
on silica. With a diverse group of monosaccharides in hand, we
attempted to increase structural diversity of anomeric MIDA
boronates by chemical glycosylation with commonly used
glycosyl donors (Scheme 2). To this end, two BMIDA glycosyl
acceptors 12h and 12f were tested. We found that
trichloroacetimidates, such as 14 and 16, could be activated
with TMSOTf (5 mol %) and produced disaccharides 18a and
18b excellent yields. To test more electrophilic conditions, we
employed thioglycoside 15 and prepared BMIDA disaccharide
18b in 81% using NIS as the activator. We were also interested
to test milder, transition metal-catalyzed reactions to expand
Received: April 12, 2021
Published: May 24, 2021
https://doi.org/10.1021/acs.orglett.1c01035
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4289−4293
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the scope of glycosyl donors. To this end, acceptor 12h was
treated with 1,2-anhydro sugar 17 in the presence of a gold
catalyst previously reported by Yu.22 Although this reaction
was slow (∼30 h at 23 °C), we were able to produce
disaccharide 18c in 52% yield with exclusive β-selectivity. On
the basis of these preliminary studies, we conclude that
BMIDA esters are compatible with Lewis acid activators and
can be considered as a mildly deactivating group when located
at the anomeric carbon in a glycosyl acceptor. These results
also highlight the utility of BMIDA esters as convenient
surrogates of more active boron donors such as boronic acids
and trifluoroborates that may not survive electrophilic
activation.
Scheme 1. C-Glycosides and Selected Synthetic Methods
Next, we engaged anomeric boronates in C−C cross-
coupling reactions. Initially, we tested reactions with MIDA
boronates but these reagents proved to be too stable under
slow-release Pd-catalyzed cross-coupling conditions.21a To
overcome this problem, potassium trifluoroborates were
studied and 2-deoxy-α-D-glucose 13a and iodobenzene were
selected as the model system (Table 1). Evaluation of several
established photocatalyst revealed that PC3 significantly
outperformed other Ir- and Ru-based complexes (entries 1−
7). Inspired by the prior work for the Molander group, we
focused on PC3 as our initial photocatalyst, Cs2CO3 (1.5
equiv) as a base in 1,4-dioxane and combination of 5 mol %
NiCl2·DME (DME = dimethoxyethane) with 5 mol % dtbpy
(dtbpy = 4,4′-di-tert-butyl-2,2′-bipyridine).18d After stirring for
24 h, we isolated 67% of 19 was formed exclusively as the α-
anomer and 12% of β-hydride elimination glucal 20 (entry 3).
Interestingly, PC6 (entry 6) showed encouraging yield of 19
(69%) but significantly suppressed elimination.
Furthermore, we looked at the role of base focusing on PC3
as the catalyst of choice. We found that the base proved to be
essential in increasing the overall yield and reducing the extent
of elimination product with CsF as the best additive furnishing
19 in 83% (entry 7). Sodium carbonate (entry 8), potassium
tert-butoxide (entry 9), and potassium phosphate (entry 10)
a
Scheme 2. Transformations of Anomeric MIDA Boronates
a
Reagents and conditions: (a) KHF2 (5.0 equiv), MeOH, 70 °C, 18 h; (b) 14 (1.2 equiv), 12h (1.0 equiv), TMSOTf (0.05 equiv), 4 Å MS,
CH2Cl2, −20 °C, 2 h; (c) 15 (1.2 equiv), 12h (1.0 equiv), NIS (1.2 equiv), AgOTf (0.2 equiv), 4 Å MS, CH2Cl2, −45 °C, 2 h; (d) AgOTf (0.10
equiv), Au(PPh3)Cl (0.10 equiv), 12h (1.0 equiv), 17 (1.5 equiv), 4 A° MS, CH2Cl2, −78 to 23 °C, 30 h; (e) 14 (1.2 equiv), 12f (1.0 equiv),
TMSOTf (0.05 equiv), 4 Å MS, CH2Cl2, −20 °C, 2 h.
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a
b
Table 1. Optimization of C−C Cross-Coupling
Scheme 3. Scope of C-Arylation
a
b
87% isolated yield with 0.15 mmol of 13a. For all compounds
tested, only one anomer (α: β > 99: 1) was observed by 1H NMR in
crude reaction mixtures. Reagents and conditions taken from Table 1,
entry 6. Aryl iodides used for all entries unless stated otherwise.
substituted phenol gave the corresponding glycoside 22e in
an acceptable 69% yield similar to electron-poor (22f, 22h)
and electron-rich (22g) meta-substituted functional groups,
which provided clean conversion into products in 73−91%.
These results led us to the investigate more difficult pyridine-
based couplings, such as 2-iodopyridine (22i) and 2-fluoro-4-
iodopyridine (22j), which gave the expected products
establishing a simple one-step protocol for the synthesis of
pyridine glycals. Following these results, we focused on more
challenging substrates such as the ones with a formyl group
(22k, 82%) and 4-iodophenyalanine (22l, 73%). For all
compounds shown in Scheme 3, exclusive α selectivity was
a
General conditions: 13a (0.15 mmol), (Ph-I) (0.10 mmol), PC3
(0.025 equiv), NiCl2·DME (0.05 equiv), dtbpy (0.05 equiv), CsF (1.5
equiv), 1,4-dioxane (2.0 mL), blue LED (40W), 23 °C, 24 h, N2.
1
showed reduced yields with an increase in elimination.
Although additional studies on the role of the base in
photoredox reactions is needed, we also established that the
counterion combination is essential, and fluoride ion itself is
not sufficient to suppress elimination, as demonstrated by the
results with KF and LiF (entries 11 and 12, respectively).
Finally, we explored solvents other than 1,4-dioxane−acetone
(entry 13) and tetrahydrofuran (entry 14) produced
comparable although slightly diminished yields. For all
conditions shown in Table 1, a single anomer (α) was
observed.
With suitable conditions in hand (Table 1, entry 6), our
focus was to determine the extent of functional group tolerance
(Scheme 3). Electron-rich 4-iodoanisole afforded 22a in 88%
with no loss of stereoselectivity. Electron-deficient cyano
(22b) and ester (22c) groups gave almost quantitative yields
(96% and 94%, respectively). These results suggest that
electron withdrawing groups furnish better yields likely
because of increasing the rate of reductive elimination and
the ability to stabilize generated aryl radical.23 Unprotected 4-
iodobenzyl alcohol produced 22d cleanly in 72% yield
signifying that fully deprotected saccharides could be tolerated
with these reaction conditions (vide infra). The meta-
observed (>99:1 α:β, H NMR).
After uncovering the broad functional-group tolerance of an
assortment of iodoarenes, we proceeded to apply our
conditions to incorporate other sugars (Scheme 4). D-Glucose
with a free hydroxyl group (24a) or protected as an acetate
(24b) is a viable substrate for photoredox C(sp3)−C(sp2)
cross-coupling. Similarly, 2-deoxy-D-galactose (24c) and
dideoxysugars derived from D-arabinose (24e) and L-fucose
(24f) furnished high selectivities in modest to high yields. To
our delight, disaccharides are also competent reagents in
photocatalytic C-arylation and afforded 24d in 63%. Both, D-
galactose (24g) and D-glucose (24h, 24i) regardless of the
nature of the protective group located at C2 resulted in a
mixture of α:β products, with the smaller hydroxyl groups
resulting in slightly improved axial preference (2.3−3.7:1).
Because the proposed activation method proceeds through a
radical species, we also wondered if C1 trifluoroborates can be
enaged in C−S24 and C−Se25 bond-forming reactions
(Scheme 5). To our delight, the formation of both ethers 26
and 28 proceeded smoothly from 13a and eletrophilic sulfur
(25) or selenium (27) sources without a nickel cocatalyst.
In summary, we introduced here a new class of C1-
trifluoroborates that are competent reagents for the formation
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Scheme 4. Scope of Photoredox C-Arylation with Pyranosyl
Trifluoroborates
AUTHOR INFORMATION
Corresponding Author
■
Maciej A. Walczak − Department of Chemistry, University of
Colorado, Boulder, Colorado 80309, United States;
orcid.org/0000-0002-8049-0817;
Email: maciej.walczak@colorado.edu
Author
Eric M. Miller − Department of Chemistry, University of
Colorado, Boulder, Colorado 80309, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01035
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(U01GM125284) and the National Science Foundation
(CHE-1753225). We thank Kathryn Baugh (University of
Colorado) for assistance with the preparation of MIDA
boronates.
DEDICATION
Scheme 5. Scope of Photoredox Etherification of 2-Deoxy-D-
glucose Trifluoroborate
■
a
This paper is dedicated to Professor Samuel J. Danishefsky on
the occasion of his 85th birthday.
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01035.
Copies of NMR spectra, detailed experimental proce-
dures, and computation data (PDF)
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