Synthesis of Glycosyl Fluorides by Photochemical Fluorination with Sulfur(VI) Hexafluoride

Article abstract of DOI:10.1021/acs.orglett.0c03915

This study describes a new convenient method for the photocatalytic generation of glycosyl fluorides using sulfur(VI) hexafluoride as an inexpensive and safe fluorinating agent and 4,4′-dimethoxybenzophenone as a readily available organic photocatalyst. This mild method was employed to generate 16 different glycosyl fluorides, including the substrates with acid and base labile functionalities, in yields of 43%-97%, and it was applied in continuous flow to accomplish fluorination on an 7.7 g scale and 93% yield.

Full text of DOI:10.1021/acs.orglett.0c03915

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Letter
Synthesis of Glycosyl Fluorides by Photochemical Fluorination with
Sulfur(VI) Hexafluoride
Sungjin Kim, Yaroslav Khomutnyk, Anton Bannykh, and Pavel Nagorny*
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ABSTRACT: This study describes a new convenient method for
the photocatalytic generation of glycosyl fluorides using sulfur(VI)
hexafluoride as an inexpensive and safe fluorinating agent and 4,4′-
dimethoxybenzophenone as a readily available organic photo-
catalyst. This mild method was employed to generate 16 different
glycosyl fluorides, including the substrates with acid and base labile
functionalities, in yields of 43%−97%, and it was applied in
continuous flow to accomplish fluorination on an 7.7 g scale and
93% yield.
Scheme 1. Summary of Prior and Current Studies
lycosyl fluorides have been of great importance to the
synthesis of oligosaccharides and glycoconjugates, as well
G
as studies of the enzymatic reactions.1−3 Its small molecular
weight (MW), low toxicity, and simple methods for scavenging
make the fluoride anion an ideal leaving group for the
glycosylation reactions.2 The use of glycosyl fluorides is often
advantageous, because of their high thermal and chemical
stability, in particular, to water and chromatography. For
example, Miller group has taken advantage of glycosyl fluoride
stability in aqueous media to achieve selective and mild
glycosylations in water.4 At the same time, the activation of
glycosyl fluorides with Lewis acids that have high affinity to the
F− anion may result in powerful glycosylative agents, which
was recently highlighted by Montgomery and co-workers, who
observed rapid glycosylations of various sterically hindered
acceptors with tris(pentafluorophenyl)borane (BCF) cata-
lysts.5
Many studies have focused on improving the synthesis of
glycosyl fluorides; however, only few practical methods are
available. The primary way to generate these species is based
on deoxyfluorination of the anomeric position of sugars using
(diethylamino)sulfur(IV) trifluoride (DAST).6 DAST exhibits
a great reactivity profile; however, its high toxicity,
corrosiveness and potential explosiveness pose limitations for
its use, in particular, on a large scale.7 Other methods such as
fluorination with corrosive and toxic HF·pyridine as the
solvent or cosolvent (>50%)1,8 require a plastic or a metal
vessel, special work-up conditions, and substrates that can
survive an acidic environment.
While SF6 has been sporadically used to achieve
fluorination,9 SF6 activation only recently was achieved under
mild and catalytic reaction conditions. Thus, the recent study
by Jamison and co-workers disclosed a photoredox activation
of SF6 with Ir(III)-based catalysts resulting in deoxyfluorina-
tion of allylic alcohols (Scheme 1A).10 This study suggested
that the reduction of SF6 leads to unidentified sulfur fluoride
species (SFn) that fluorinates the substrate. Subsequently,
Rueping and co-workers (Scheme 1B)11 and Braun and
Kemnitz12 described reductive activations of SF6 that resulted
in the stoichiometric reagents that could be used for
deoxyfluorination, and Wagenknecht described photoactiva-
tions of SF6 that resulted in SF5 group transfer.13
Received: November 26, 2020
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© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Unlike many other fluorinating agents, SF6 is an inexpensive
and safe-to-handle gas produced on a large scale. The
utilization of SF6 represents an important challenge,14 because
of its chemical inertness, and has great significance, because of
its potency as a greenhouse gas.15
but it did not show any catalytic activity (Table 1, entry 3).
Next, we proceeded with testing the derivatives of thiazine
because some of them have been used for the photoactivation
of SF6 with UV-A LED by Wagenknecht and co-workers13
(Table 1, entries 4 and 5). Both N-phenylphenothiazine and
Methylene Blue demonstrated fair catalytic activity that was
comparable with Ir(ppy)2(dtbpy)PF6. Subsequently, we
evaluated benzophenone (Table 1, entry 6), since this
compound is often an indispensable catalyst for various
photochemical transformations.16 Benzophenone was also
found to promote the fluorination with both Blue and UV-A
(λmax = 365 nm) LEDs, although the yield was found to be
higher with the UV-A LED. This is not surprising, because the
n → π* band of benzophenone has a λmax value of ∼340 nm.16
It is known that additional substitution on benzophenones may
increase the λmax (cf. Figures SI 9−SI 11 in the Supporting
Information), affect the lifetime of the triplet state, and
increase the reduction potential of the benzophenone-derived
ketyl radicals.18 Therefore, six other benzophenone derivatives
(Table 1, entries 7−13) were tested. Among these six
photocatalysts, Michler’s ketone, 4,4′-dimethoxybenzophe-
none, and 4-fluoro-4′-methoxybenzophenone showed en-
hanced catalytic activity with 4,4′-dimethoxybenzophenone
providing the highest yield (60%; see Table 1, entry 12).
Considering its low cost and high catalytic activity, we
subsequently employed 4,4′-dimethoxybenzophenone
(DMBP) as our default photocatalyst and proceeded to
further optimize the reaction parameters, such as reaction
stoichiometry, base, solvent, light intensity, irradiation surface,
and reaction vessel (cf. Tables SI 2−SI 4). These optimizations
permitted us to reduce the catalyst loading to 30 mol % and
resulted in the enhanced formation of 2a (72% isolated yield,
95% BRSM, α:β = 13:1; see Table 1, entry 13).
Building on the aforementioned studies, this manuscript
describes a mild, safe, and efficient fluorination of 16 protected
carbohydrates with SF6 using commercially available UV-A
LED source (λmax = 365 nm) and inexpensive 4,4′-
dimethoxybenzophenone as the photocatalyst. Importantly,
all of the substrates and products were found to be stable
under the reaction conditions, which permitted to carry gram-
scale fluorination reactions both in batch and continuous flow.
Based on preliminary mechanistic studies, we propose that this
reaction proceeds through the formation of SF4 that is formed
in trace quantities and either fluorinates the substrate or gets
further reduced to SnFm or elementary sulfur under the
photochemical conditions.
Our studies commenced by subjecting the disarmed 2,3,4,6-
tetra-O-acetyl-α-D-mannose 1a to the fluorination reaction
condition previously developed by Jamison and co-workers
(Table 1, entry 1).10 Excitingly, 1a showed no signs of
a
Table 1. Photocatalyst Screening
b
entry
photocatalyst
light source
blue LED
yield (%)
38
c
1
Ir(ppy)2(dtbbpy)PF6 (5
mol %)
2
3
4
5
6
7
8
9
Eosin Y
Rose Bengal
blue LED
7
−
UV-A LED
UV-A LED
UV-A LED
UV-A LED
UV-A LED
UV-A LED
UV-A LED
UV-A LED
Methylene Blue
N-phenylphenothiazine
benzophenone
Michler’s ketone
xanthone
33
43
33
47
25
7
With the optimized conditions in hand, the evaluation of the
substrate scope was performed next (cf. Scheme 2, as well as
Table SI 5 in the Supporting Information). First, we
investigated the formation of other disarmed peracetylated
a
9-fluorenone
Scheme 2. Substrate Scope Studies
10
4-fluoro-4′-
56
methoxybenzophenone
11
4-chloro-3′-
UV-A LED
29
methoxybenzophenone
12
13
4,4′-dimethoxybenzophenone UV-A LED
60
d
4,4′-
UV-A Flood
Lamp
72 (95%
e
dimethoxybenzophenone
BRSM)
a
Reactions in entries 1−13 were performed on 0.1 mmol scale, with
40 mol % catalyst, 20 equiv of DIPEA in 0.033 M DCE for 20 h with
b
UV-A LED (λmax = 365 nm) or blue LED (λmax = 452 nm). 19F
NMR yield of the major α-anomer with α,α,α-trifluorotoluene as an
c
internal standard. 5 mol % of Ir(ppy)2(dtbbpy)PF6 and 3 eq. of
d
DIPEA were used. Performed with 30 mol % of the catalyst, 10 equiv
e
of DIPEA in a plastic syringe as the reaction vessel. Isolated yield,
α:β = 13:1.
decomposition under these conditions, and the reaction
proceeded to 38% conversion of 2a after 20 h. However, the
significant deceleration of the reaction progression after 12 h,
and the high price and low availability of Ir(ppy)2(dtbpy)PF6,
prompted us to investigate more cost-effective organic
photocatalysts,16 using commercially available LED light
sources (Table 1, entries 2−13). Fluorescein derivative Eosin
Y17 was able to activate SF6 to form 2a in low conversions
(Table 1, entry 2). A related dye, Rose Bengal, was also tested,
a
Reactions were performed in plastic syringes on 0.1 mmol scale, with
30 mol % of the DMBP catalyst for substrates 1a−1d and 20 mol % of
the DMBP catalyst for substrates 1e−1p, DIPEA (10 equiv), DCE
(0.03 M), rt, for 20 h. The yields are the average of duplicate
experiments, and in all cases the actual isolated yields were within
2% from the average yield.
B
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fluorides such as D-glucose derivative 2b, D-galactose derivative
2c, and L-rhamnose derivative 2d. Similar to the D-mannose
derivative 2a, the transformations leading to 2b and 2c were
relatively slow, because of the disarmed nature of the
substrates, and provided the products in yields of only 43%−
53% after 20 h, albeit with good BRSM yields. At the same
time, more reactive 6-deoxy sugar 1d provided significantly
higher yield for product 2d (70% yield, 77% BRSM).
would be more feasible in continuous flow, we have performed
7.7 g scale fluorination of 1e in continuous flow using the setup
depicted in Scheme 3B. Thus, the solution of 1e, photocatalyst
(DMBP), and DIPEA in DCE was cycled with pressurized
SF6(100 psi) through the loop containing Teflon tubing that
had been irradiated by the UV-A lamp and a back-pressure
regulator (BPR) using a standard prep HPLC pump. The
irradiated solution was returned back to the container with 1e,
and the resulting mixture was reintroduced back to the
photochemical reactor for the total duration of 120 h. These
conditions led to the formation of 7.2 g of 2e (93% yield), and
0.5 g of 1e was recovered (99.8% BRSM).
Subsequently, we performed the evaluation of armed donors
1e−1p. These armed donors are significantly more reactive,
and their interconversion to products 2e−2p was efficiently
performed with only 20 mol % of the DMBP catalyst (cf.
Scheme 2). The fluorination of the benzylated hexoses 1e, 1f,
and 1g proceeded in good yields (85%, 69%, and 81%,
respectively) to provide the resultant glycosyl fluorides as
mixtures of the α:β anomers. Interestingly, the reactions of 1e
and 1g favored the formation of the more valuable β-anomer
with ∼3:1 to 4:1 selectivities. The preference for the β-anomer
was the general trend that was observed for other substrates
lacking axial C2 substitution such as 2i, 2j, 2n, and 2p. Similar
to 1d, the benzylated derivatives of deoxysugars such as 1h, 1i,
and 1j were also viable substrates for the fluorination reaction
providing the glycosyl fluorides in yields of 68% (83% BRSM),
80% (87% BRSM), and 80% (99% BRSM), respectively.
In addition, the benzylated ribose derivative 1k was
fluorinated to provide primarily β-anomer of glycosyl fluoride
in 13:1 dr and 78% yield (90% BRSM). Finally, the application
of this chemistry to substrate with acid-labile linkages (1i−1p)
was investigated. This included the successful formation of D-
mannose benzylidene acetal-containing derivative 2l (66%
yield, α:β = 7.8:1), disaccharide 2o (70%, α:β = 1:3.4) as well
as sugar derivatives 2m (73%, α:β = 36:1), 2n (72%, α:β =
1:2.4), and 2p (70%, α:β = 1:3.7) carrying the multiple p-
methoxybenzyl (PMB) group protections. These results
demonstrate that fluorination with SF6 is mild and does not
affect the majority of the protecting groups used in
carbohydrate chemistry.
Several control experiments were performed using substrate
1g to elucidate the mechanism of this transformation, and the
tentative mechanism is depicted in Figure 1A.
The scaleup of the photochemical experiments in the batch
is notoriously challenging and requires further optimization of
various experimental parameters. While the setup depicted in
Scheme 3A was not optimized, it was successfully used for the
1.0 g scale fluorination of 1g to produce product 2g in 93%
yield (0.93 g), using a standard glass tube and an extended
irradiation time (166 h). Arguing that larger-scale reactions
Figure 1. (A) Tentative reaction mechanism. (B) Catalyst
decomposition products and side-products resulting from SF6. (C)
Direct fluorination with sodium ketylate. (D) Light on/off control
experiment.
The reaction did not proceed without a light source, which
reinforces that light irradiation is required to initiate the first
SET step between the excited DMBP and DIPEA.19 This also
suggests that DIPEA cannot reduce SF6 by itself. Similarly, the
reduction of SF6 did not happen in the absence of DIPEA,
which indicates that the presence of this reagent is essential.
Surprisingly, the omission of DMBP did not completely shut
down the fluorination of 1g, and we observed some formation
of 2g without a presence of a photocatalyst. Presumably, the
open aldehyde form of 1g may participate in the SET process
to activate SF6, but further mechanistic investigations are
required for a better understanding of this phenomenon. To
eliminate the possibility of the chain processes initiated by
light, we performed the light on/off experiment depicted in
Figure 1C. After the reaction was irradiated with UV-A light
for 4 h, the light was turned off and the reaction vessel was
covered with aluminum foil for 2 h, and no reaction
progression happened in the absence of light. However, the
Scheme 3. Gram-Scale Fluorination in Batch and
Continuous Flow
C
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formation of 2g was resumed when the reaction was exposed
to the light again. These results suggest that the photo-
excitation of DMBP leads to the formation of its triplet state
(DMBP*), and the observed catalyst decomposition products
such as benzylic alcohol and pinacol adduct provide further
evidence for this step (cf. Figure 1B).19 The resultant DMBP*
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AUTHOR INFORMATION
■
Corresponding Author
Pavel Nagorny − Department of Chemistry, University of
Michigan, Ann Arbor, Michigan 48109, United States;
orcid.org/0000-0002-7043-984X; Email: nagorny@
umich.edu
species undergoes a known oxidation of DIPEA (E1/2(SCE)
∼
0.8 V)15,18 to generate a ketyl radical (E1/2(SCE) = −2.2 V) that
reduces SF6 to [SF6]• − (E1/2(SCE) = −1.9 V).10 The subsequent
reduction of the [SF6]• − radical anion results in the in situ
formation of a strong fluorinating agent, SF4,7a,20 that is likely
to exist in dynamic equilibrium with its fluoride-complexed
form [SF5]−.11 Our attempts to directly detect SF4 or [SF5]−
via low-temperature 19F NMR under the optimized reaction
conditions were not successful, which implies that these
species are transient under the photochemical conditions.
Similarly, the attempts to detect the SF6 reduction products
arising from the reaction of the ketyl radical pregenerated from
DMBP and lithium metal and SF6 were not successful,
although ∼1% of the fluorinated product 2e was observed
when the ketyl reduction of SF6 was immediately followed by
the addition of 1e (cf. Figure 1C, as well as Section VI-f in the
Supporting Information).
Authors
Sungjin Kim − Department of Chemistry, University of
Michigan, Ann Arbor, Michigan 48109, United States
Yaroslav Khomutnyk − Department of Chemistry, University
of Michigan, Ann Arbor, Michigan 48109, United States
Anton Bannykh − Department of Chemistry, University of
Michigan, Ann Arbor, Michigan 48109, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03915
Funding
This work was funded by NIH Common Fund Grant No.
U01GM125274.
This might suggest that unreacted SF4 is further reduced,21
and the 19F NMR analysis of the crude reaction mixture indeed
contains a singlet at −124 ppm that could be contributed to
S2F2 or a related SnF2 species.22 This is in agreement with the
observation that the addition of the PPh3 to the crude reaction
mixture leads to the formation of SPPh3 and F2PPh3. In
Notes
The authors declare no competing financial interest.
REFERENCES
addition, the significant quantities of FSO3 (NHi-Pr2Et)+ were
−
■
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dimethoxybenzophenone as a readily available organic photo-
catalyst. This mild method was employed to generate 16
different glycosyl fluorides, including the substrates with acid
and base labile functionalities, in yields of 43%−97%, and was
applied in continuous flow to accomplish fluorination on a 7.7
g scale. The subsequent studies suggest that this reaction might
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03915.
1H, 13C, and 19F NMR spectra of products; experimental
information; description of the continuous flow experi-
ments; and studies of the reaction mechanism (PDF)
Accession Codes
CCDC 2017701 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
D
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