Stereoselective Preparation of α- C-Vinyl/Aryl Glycosides via Nickel-Catalyzed Reductive Coupling of Glycosyl Halides with Vinyl and Aryl Halides

Article abstract of DOI:10.1021/acs.orglett.8b03567

Facile preparation of the α-C-vinyl and -aryl glycosides has been developed via mild Ni-catalyzed reductive vinylation and arylation of C1-glycosyl halides with vinyl/aryl halides. Good to high α-selectivities were achieved for C-glucosides, galactosides, maltoside, and mannosides, which were dictated by the employment of pyridine type ligands. As such, the present work represents unprecedented control for a high level of α-selectivity for C-vinyl-glucosides using cross-coupling approaches and offers hitherto optimal α-selective preparation of C-aryl glucosides via catalyst-controlled coupling strategies.

Full text of DOI:10.1021/acs.orglett.8b03567

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Stereoselective Preparation of α‑C‑Vinyl/Aryl Glycosides via Nickel-
Catalyzed Reductive Coupling of Glycosyl Halides with Vinyl and
Aryl Halides
Jiandong Liu and Hegui Gong*
School of Materials Science and Engineering, Center for Supramolecular Chemistry and Catalysis and Department of Chemistry,
Shanghai University, 99 Shang-Da Road, Shanghai 200444, China
S
* Supporting Information
ABSTRACT: Facile preparation of the α-C-vinyl and -aryl
glycosides has been developed via mild Ni-catalyzed reductive
vinylation and arylation of C1-glycosyl halides with vinyl/aryl
halides. Good to high α-selectivities were achieved for C-
glucosides, galactosides, maltoside, and mannosides, which
were dictated by the employment of pyridine type ligands. As
such, the present work represents unprecedented control for a
high level of α-selectivity for C-vinyl-glucosides using cross-coupling approaches and offers hitherto optimal α-selective
preparation of C-aryl glucosides via catalyst-controlled coupling strategies.
catalyzed Negishi method to realize catalytic alkylation and
arylation of C1-glycosyl halides.2 In this context, the Ni-
catalysts provide fully saturated C-aryl glucosides with high β-
selectivities. Knochel described an additional notable example
using Ar2Zn as the nucleophiles leading to excellent β-
selectivity for C-aryl glucosides in the absence of a metal
catalyst.3 By contrast, the achievements of moderate α-
selectivities via arylation of glucosyl bromides have recently
been disclosed using Fe- and Co-catalyzed Negishi and
Kumada conditions (Scheme 1).4,5 In a different strategy,
Walczak demonstrated a Pd-catalyzed Stille coupling protocol
to access α- and β-C-aryl glucosides based on the stereo-
retentive reactions of glycosyl-Sn with aryl electrophiles.6
Albeit with considerable progress, the concurrent methods do
not show effective control of the stereochemistry for C-vinyl
glucosides. In addition, arylation of glucosyl halides only offers
no to moderate α-selectivities under catalyst-controlled
coupling conditions.
tereoselective construction of C-glycosides remains a
Schallenge, in particular for those fully oxygen saturated
C-glucosides.1 This topic has evoked growing interest in the
development of transition-metal-catalyzed cross-coupling
methods based upon the manipulation of glycosyl C1(sp3)-
nucleophiles or -electrophiles (Scheme 1).2−6 The coupling of
organometallic reagents with readily accessible C1-glycosyl
halides has proven to be beneficial for such purposes.3−6 The
́
seminal work revealed by Gagne highlights the first Ni-
Scheme 1. Cross-coupling Methods to C-Aryl- and Vinyl-
glucosides2−6
Herein we present Ni-catalyzed reductive coupling of
glycosyl halides with vinyl and aryl halides using pyridine
and N,N-dimethyl aminopyridine (DMAP) as ligands, enabling
stereoselective preparation of α-C-vinyl and -aryl glycosides
under mild reaction conditions.7 To the best of our knowledge,
this work provides thus far optimum diastereoselective results
for the preparation of α-C-aryl and -vinyl glucosides using a
metal-catalyzed coupling strategy.
We selected acetyl-protected glucosyl bromide 1 with E-(2-
bromovinyl)benzene 2 as the model substrates for optimiza-
tion. Extensive examinations revealed that a combination of
Ni/Py/MgCl2/Zn in DMF/CH3CN at 0 °C to be optimal,
with which the C-vinyl glucoside 3a was obtained in 81% yield
Received: November 7, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03567
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
with an α/β ratio of 8.3:1 (method A1, Table 1, entry 1).8
Alternation of the reaction temperatures and solvents did not
a b
,
Table 1. Optimization for the Formation of 3a
entry
variation from the standard conditions
yield % (α/β)
c
1
none
81% (8.3:1)
2
3
4
25 °C instead of 0 °C
DMF
CH3CN
76% (6:1)
20% (7.2:1)
trace
5
w/o MgCl2
trace
6
7
8
w/o Ni(ClO4)•6H2O
w/o pyridine
DMAP
N.D.
N.D.
trace
9
Bpy (10%)
20% (2.2:1)
24% (2.8:1)
30% (1.2:1)
20% (1:1)
N.D.
63% (7:1)
74% (8.1:1)
75% (8.8:1)
10
12
13
14
15
16
17
dtbBpy (10%)
tBu-Terpy (10 mol %)
iPr-Pybox (10 mol %)
NiCl2
Ni(acac)2
1 (0.15 mmol), 2 (0.15 mmol)
Figure 1. Selective preparation of α-C-vinyl-glycosides using method
A1 (as in Table 1, entry 1); yield refers to isolated yield, and ratio in
parentheses refers to α/β ratio. (a) 1 equiv of vinyl bromide and 2
equiv of 1 were used. (b) 1 equiv of Bu4NI and 10% of Ni were used,
25 °C.
c
1 (3 mmol scale)
a
b
Method A1: see entry 1. Yield determined by 1H NMR
spectroscopy using 2,5-dimethylfuran as an internal reference. The
c
ratios in the parentheses refer to α/β ratios. Isolated yields.
yield better results (entries 2−4). Without Ni catalyst,
pyridine, or MgCl2, only trace amounts of 3a were generated
(entries 5−7).8 Other ligands and nickel sources did not
enhance the coupling efficiency (entries 8−15). No reaction
took place when we used Ni(OTf)2; only hydrolysis and
elimination of glucosyl halide 1 to glucal were detected. An
equimolar mixture of 1 and 2 delivered 3a in 74% yield
without eroding the selectivity (entry 16). Finally, the reaction
can be scaled up on a gram scale (entry 17).
Next, a survey of the scope of vinyl bromides was performed
(Figure 1). Couplings of 1 with a range of vinyl bromides were
subjected to the optimized method A1. As shown in Figure 1,
the aryl-conjugated vinyl bromides produced 3b−3g in good
to excellent yields with good to high α selectivities. In contrast,
(Z)-1-(2-bromovinyl)-4-methoxybenzene provided a mixture
of E/Z isomers of 3b (E/Z = 1:3) in a comparable yield and α/
β selectivity (Table S1).8,9 The dienyl bromide was also
effective which furnished 3h in 60% yield (α/β = 5:1). While
coupling of α-vinyl bromides was generally unsuccessful, 3i
derived from 2-bromo-1H-indene was obtained in a moderate
yield with good α-selectivity. The alkyl-decorated vinyl
bromide resulted in 3j and 3k in good yields with 4:1 α/β
ratios. High α-selectivities were also observed for galactoside
(e.g., 4a−d), mannoside 5, and maltoside 6. Of note was that
facile preparation of α-4d was also achieved from the coupling
of Ac-protected galactosyl bromide, which can be considered
as a key intermediate to the C-analogue of bacterial glycolipid
BbGL2.10
We applied the vinylation conditions to the arylation of 1
with methyl 4-iodobenzoate. Using method A1, 7a was
obtained in a low yield with poor reproducibility. This is not
surprising, as vinyl halides often display distinctive reactivities
from aryl counterparts under reductive coupling conditions.11
Modification of method A1 using 1 in excess, 20% of Ni and
80% of DMAP, and DMA/THF as the solvent (method A2,
Figure 2) showed the yield can be promoted to 85% with a 6:1
α/β ratio. A small amount of HBr was necessary to initiate the
reaction, possibly for activation of Zn. The coupling of 4-
methyl benzoate with per-O-acetyl glucopyranosyl iodides (an
iodo analog of 1) only resulted in a trace amount of 7a; glucal
(∼37%) and hydrolysis (∼42%) of the iodide were detected as
the major products. Unfortunately, when Ac was replaced with
Bn, no reaction occurred, due to rapid hydrolysis of the
glycosyl bromide. The use of a chloro analog did not provide
an appreciable result; most of the chloride remained intact
(85%). The generality of this method was also manifested by
the examples of 7b−d containing electron-withdrawing groups
in the arenes. For iodobenzene and other electron-rich aryl
iodides, the yields were poor although the α-selectivities
remained unaffected (e.g., for 7e). With method A2, galactoside
8a was obtained in good yield and high α-selectivity. Whereas
B
DOI: 10.1021/acs.orglett.8b03567
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Letter
arylation method A3 displayed excellent substrate compatibility,
allowing effective incorporation of 6-indole and 3-thiophenyl
groups into 7h−i. Good to high α-selectivities were also
observed for galactosides and mannosides 8b−c and 11 using
method A3.
According to the previous mechanistic studies on the Ni-
catalyzed reductive arylation of alkyl halides,15 we speculate
this glycoside forming protocol may comply with a radical-
chain mechanism, wherein an aryl−NiII intermediate intercepts
an alkyl radical generated from halide abstraction of a NiI
species.8,14 The radical nature of glucosyl was confirmed by the
coupling of 12 with methyl 4-iodobenzate, which resulted in a
cyclization/coupling product 13 in 42% yield (eq 1).
Treatment of methyl 4-iodobenzoate with Ni0 in the
presence of DMAP and dtbBipy led to mono- and bidentated
NiII complexes 14 and 15, respectively,8 which are para-
magnetic and cationic in polar solvents.8,16 Stoichiometric
reactions of 14−15 with 1 were examined (eq 2). For complex
Figure 2. Arylation of C-glycosyl bromides with electron-deficient aryl
iodides: yield refers to isolated yield, and ratio in parentheses refers to
α/β ratio. (a) Ni (20 mol %), DMAP (80 mol %) with trace quantity
of HBr/AcOH. (b) Ni (10 mol %), DMAP (40 mol %), w/o HBr/
AcOH. (c) 25 °C.
arabinoside 9 did not show good selectivity, maltoside 10 were
obtained in 55% yield with a 5.5:1 α/β ratio (Figure 2).
To solve the problem for arylation of electron-rich arenes as
in Figure 2, replacement of DMAP with dtbBpiy and use of 1.5
equiv of 1 and DMA as the solvent furnished 7f in 80% yield
with a 2.8:1 α/β ratio (method A3, Figure 3).12,13 It appears
that bipyridine can boost the arylation efficiency including
those electron-rich arenes, but at the expense of the α
selectivities. Likewise, 7g was obtained in a good yield and
moderate α/β ratio, which is the α-anomer of Ac-protected
commercial drug Canagliflozin for type-2 diabetes.14 The
14, MgCl2 is crucial for the α-selective formation of 7a.8 The
reaction of 15 with 1 produced 7a in 60% or 30% yield with a
2:1 α/β ratio in the presence or absence of MgCl2. These
results are in agreement with a radical-chain mechanism
proposed by Weix, in which an Ar−NiII species can initiate
alkyl radical formation and capture it to yield an Ar−NiIII−
alkyl intermediate that releases aryl−alkyl products upon
reductive elimination.8,15
Our recent studies on the roles of MgCl2 suggest that it may
involve Cl− and I− exchange to form a more stable NiII−Cl
bond than a NiII−I bond, as the latter generally dissociates in
polar solvents to give cationic Ni species.17 For instance, a
reaction of 14 with excess MgCl2 yields a neutral
(DMAP)2NiII−Ar(Cl) species 16 by releasing a DMAP ligand
(eq 2), which may account for the authentic intermediate for
intercepting a glycosyl radical.8
Steric interactions may play a key role in dictating the α/β
selectivities for the glucosides. A boat-like B2,5-comfomer was
proposed as the more stable glucosyl radical intermediate than
the chairlike ones with a free energy difference by 0.57 kcal/
mol.18 The Ar−NiII intermediate can approach the radical via
α or β sides, resulting in an α- or a β-C−Ni bond (eq 3), with
the α-intermediate being more stable due to anomeric
Figure 3. Arylation of C-glycosyl bromides with electron-rich aryl
iodides: yield refers to isolated yield, and ratio in parentheses refers to
α/β ratio. (a) DMA/THF = 1:4 (1 mL).
C
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Organic Letters
Letter
2000, 2, 863. (d) Ahmed, M. M.; O’Doherty, G. A. De novo synthesis
of a galacto-papulacandin moiety via an iterative dihydroxylation
strategy. Tetrahedron Lett. 2005, 46, 4151.
(2) For Ni-catalyzed cross-coupling of glycosyl halides, see:
́
(a) Gong, H.; Sinisi, R.; Gagne, M. R. A Room Temperature Negishi
Cross-Coupling Approach to C-Alkyl Glycosides. J. Am. Chem. Soc.
́
2007, 129, 1908. (b) Gong, H.; Gagne, M. R. Diastereoselective Ni-
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Oxygenated C-Alkyl and C-Aryl Glycosides. J. Am. Chem. Soc. 2008,
130, 12177.
interaction of the p-lone electron pair of the oxygen atom with
the antibonding orbital of the C−Ni bond.18 Owing to the
flexibility of the boat conformer, moderate α-selectivities (α/β
< 3:1) are generally observed as evidenced in the previous C-
glucoside forming methods. Similar results were observed in
our studies when bipyridine ligands were used. The labile
pyridine/DMAP ligands however deliver good α-selectivities
(α/β up to 8:1), likely involving dissociation of a ligand from
the Ni center upon formation of the α-Ni−C bond.19 As such,
the repulsive steric interaction via α-attack is reduced, leading
to enhanced α-selectivity. The high α-selectivity for C-
mannosides can be explained by a chairlike mannosyl radical
that leads to preferential formation of α-products due to less
steric hindrance of the α-face.4
In summary, a facile Ni-catalyzed cross-electrophile coupling
method for the construction of C-aryl and -vinyl glycosides has
been developed. The stereoselective outcomes were deter-
mined by both the catalysts and the structures of the
substrates. For C-glucosides, a unique ligand-controlled
diastereoselectivity was observed, wherein good to high α-
selectivities were attained using Py/DMAP ligands, respec-
tively. Steric effects induced by the catalysts and the glucosyl
radical may explain this profound stereoselective result.
(3) Lemaire, S.; Houpis, I. N.; Xiao, T.; Li, J.; Digard, E.; Gozlan, C.;
̀
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Takaya, H.; Orita, A.; Li, H. C.; Shing, T. K. M.; Nakamura, M.
Synthesis of Aryl C-Glycosides via Iron-Catalyzed Cross Coupling of
Halosugars: Stereoselective Anomeric Arylation of Glycosyl Radicals.
J. Am. Chem. Soc. 2017, 139, 10693.
(5) Nicolas, L.; Angibaud, P.; Stansfield, I.; Bonnet, P.; Meerpoel, L.;
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̈
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03567.
■
Corpet, M.; Gosmini, C.; von Wangelin, A. J. Reductive Cross-
Coupling Reactions between Two Electrophiles. Chem. - Eur. J. 2014,
20, 6828. (b) Everson, D. A.; Weix, D. J. Cross-Electrophile Coupling:
Principles of Reactivity and Selectivity. J. Org. Chem. 2014, 79, 4793.
(c) Moragas, T.; Correa, A.; Martin, R. Metal-Catalyzed Reductive
Coupling Reactions of Organic Halides with Carbonyl-Type
Compounds. Chem. - Eur. J. 2014, 20, 8242. (d) Wang, X.; Dai, Y.;
Gong, H. Nickel-Catalyzed Reductive Couplings. Top. Curr. Chem.
(Z) 2016, 374, 43. (e) Richmond, E.; Moran, J. Recent Advances in
Nickel Catalysis Enabled by Stoichiometric Metallic Reducing Agents.
Synthesis 2018, 50, 499.
S
Detailed experimental procedures, characterization of
new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hegui_gong@shu.edu.cn.
ORCID
(8) See the Supporting Information for details.
(9) Formation of the E-products from the Z-vinyl halides may
involve a possible zwitterionic C(carbene)−Ni complex; see:
(a) Huggins, J. M.; Bergman, R. G. Mechanism, regiochemistry, and
stereochemistry of the insertion reaction of alkynes with methyl(2,4-
pentanedionato)(triphenylphosphine)nickel. A cis insertion that leads
to trans kinetic products. J. Am. Chem. Soc. 1981, 103, 3002.
(b) Clarke, C.; Incerti-Pradillos, C. A.; Lam, H. W. Enantioselective
Nickel-Catalyzed anti-Carbometallative Cyclizations of Alkynyl
Electrophiles Enabled by Reversible Alkenyl nickel E/Z Isomerization.
J. Am. Chem. Soc. 2016, 138, 8068.
(10) Kulkarni, S. S.; Gervay-Hague, J. Efficient Synthesis of a C-
Analogue of the Immunogenic Bacterial Glycolipid BbGL2. Org. Lett.
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(11) Liu, J.; Ren, Q.; Zhang, X.; Gong, H. Preparation of Vinyl
Arenes by Nickel-Catalyzed Reductive Coupling of Aryl Halides with
Vinyl Bromides. Angew. Chem., Int. Ed. 2016, 55, 15544.
Hegui Gong: 0000-0001-6534-5569
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support was provided by the Chinese NSF (Nos.
21871173, 21572140, and 21372151).
■
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