Ni-Catalyzed Reductive C-O Bond Arylation of Oxalates Derived from α-Hydroxy Esters with Aryl Halides

Article abstract of DOI:10.1021/acs.orglett.9b00174

A Ni-catalyzed reductive cross-coupling of α-hydroxycarbonyl compounds modified with oxalyl groups and aryl halides has been developed that furnishes α-aryl esters under mild conditions and tolerates a variety of functionalized aryl halides bearing electron-withdrawing and -donating groups. This work highlights C-O bond fragmentation on secondary alkyl carbon centers that generates α-carbonyl radicals.

Full text of DOI:10.1021/acs.orglett.9b00174

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ni-Catalyzed Reductive C−O Bond Arylation of Oxalates Derived
from α‑Hydroxy Esters with Aryl Halides
Mengyu Gao, Deli Sun, and Hegui Gong*
Center for Supramolecular Chemistry and Catalysis and Department of Chemistry, Shanghai University, 99 Shang-Da Road,
Shanghai 200444, China
S
* Supporting Information
ABSTRACT: A Ni-catalyzed reductive cross-coupling of α-hydrox-
ycarbonyl compounds modified with oxalyl groups and aryl halides
has been developed that furnishes α-aryl esters under mild conditions
and tolerates a variety of functionalized aryl halides bearing electron-
withdrawing and -donating groups. This work highlights C−O bond
fragmentation on secondary alkyl carbon centers that generates α-carbonyl radicals.
unctionalization of strong C(sp3)−O bonds remains a
F
challenge.1 Thus far, Barton-deoxygenative alkyl C−O
bond fragmentation represents the most widely used protocol
that effectively generates alkyl radicals by homolytic scission of
C(sp3)−O bonds under photo- or radical-initiator-induced
conditions.2 As such, relatively forcing conditions adopting UV
irradiation or toxic tin hydride are employed.2−7 Recently, Ir-
and Ru-catalyzed visible-light-induced conditions have been
engaged in forging C−O bond cleavage of alkyl (primary,
secondary, and tertiary) oxalate salts and tertiary alkyl N-
phthalimidoyl (NP) oxalyl esters, respectively.3−6 A similar
photoreductive deoxygenation protocol was also extended to
C−O bond scission/radical cyclization of α-hydroxyl groups
within hemiethyl oxalyl tartrate derivatives.7 We, on the other
hand, disclosed a Zn/MgCl2-mediated and Ni-promoted C−O
bond fragmentation strategy that allows dialkyl oxalates to be
used as tertiary alkyl radical precursors. The mild reaction
conditions are compatible with Ni-catalyzed reductive
couplings.8 This permits the formation of all-carbon quaternary
centers by the coupling of tertiary alkyl oxalates with aryl
chlorides. However, under equivalent reaction conditions,
secondary and primary alkyl oxalates are not suitable; only
alcohols used for the preparation of oxalates were detected. To
address this problem, we visualize that oxalates derived from α-
hydroxyl esters (Ox-α-HE) may be viable for creation of
secondary and primary alkyl radicals due to their stabilization
by a neighboring carbonyl group.
Figure 1. Arylation of α-carbonyl compounds.
compounds with aryl-metallic nucleophiles, e.g., silanes,13
Grignard reagents,14 or boron compounds.15,16 Finally,
reductive coupling of α-haloesters with aryl halides has also
been developed.17 However, none of these studies exploits C−
O bond couplings. Thus, we believe the present work
represents one of the extraordinarily rare examples of catalytic
processes for cross-electrophile arylation of secondary C−O
electrophiles and aromatic halides (Figure 1), which offers a
complementary means to α-aryl and vinyl esters by C−O bond
cleavage strategy. It differs from the reported C−O activation
protocols in the cross-electrophile couplings that involve in situ
conversion of secondary C−O to C−halide bonds or
manipulation of benzylic C−O bonds (mostly primary ones).18
Our investigation commenced with the coupling of methyl
4-bromobenzoate (1) with 1-methoxy-1-oxo-2-propanyl meth-
yl oxalate (2) to form methyl 4-(1-methoxy-1-oxopropan-2-yl)
benzoate (3). As outlined in Table 1, the optimal reaction
conditions comprised the use of NiI2 (10 mol %) as the
precatalyst, 2-(4,5-dihydro-1H-imidazol-2-yl)pyridine (L1) as
the ligand, and Zn, MgCl2, and pyridine (Py) in DMA. The
yield of 3 was obtained in 86% (Table 1, entry 1).20 A trace
amount of biaryl product arising from 1 was observed.
Herein, we report that under Ni-catalyzed, Zn/MgCl2-
mediated C−O bond scission conditions, efficient α-arylation
and vinylation of Ox-α-HE with aryl and vinyl halides are
readily achieved and offer α-aryl and vinyl esters. It is known
that α-aryl carboxylic acid derivatives are the core structure for
numerous biologically active compounds, such as naproxen,
ibuprofen, and flurbiprofen, which are widely used as
nonsteroidal anti-inflammatory drugs.9 Therefore, a number
of important catalytic protocols have been developed to access
this important structural core (Figure 1).10 These include
arylation of enolates or Reformasky reagents with aryl halides
or aryl nucleophiles11,12 and coupling of α-halocarbonyl
Received: January 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00174
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization for the Coupling of 1 with 2
Table 2. Scope of Aryl Halides for the Coupling with 2
a b
,
Using the Standard Method
b
entry
variation
yield (%)
c
1
none
86
2
3
4
5
6
7
8
9
10
11
12
13
14
15
NiCl2 instead of NiI2
NiBr2 instead of NiI2
L2 instead of L1
L3 instead of L1
L4 instead of L1
Mn instead of Zn
MgBr2 instead of MgCl2
NiI2 (5%)/L1 (10%)
1.2 equiv of 2
without NiI2
without MgCl2
without Py
without L1
80
70
71
d
70
trace
45
9
69
57
ND
ND
55
17
75
e
e
c
1 (3 mmol)
a
Reaction conditions: 1 (0.15 mmol), oxalate (1.5 equiv), Ni(10 mol
%), ligand (15 mol %), MgCl2 (2 equiv), Zn (3 equiv), Py (2 equiv),
b
and DMA (1 mL). NMR yield using 2,5-dimethylfuran as the
c
d
e
internal reference. Isolated yields. 0% ee was observed. Not
1
detected by H NMR.
Alternation of Ni catalysts and ligands did not result in
improvement (entries 2−6). Both Zn and MgCl2 were proven
necessary (entries 7 and 8). A control experiment indicated
that lowering the loading of NiI2 to 5 mol % and L1 to 10 mol
% provided 3 in 69% yield (entry 9). Use of 1.2 equiv of 2
resulted in a decrease in the yield (entry 10). No reaction was
observed in the absence of the Ni catalyst, MgCl2, or Zn
(entries 11 and 12). Without Py or ligand L1, the reaction
yield diminished drastically (entries 13 and 14). The reaction
was also performed at a 3 mmol scale, which delivered 3 in
75% yield (entry 15).
With the optimized conditions in hand (Table 1, entry 1),
we sought to probe the scope of aryl bromides for the coupling
with 2 (Table 2). The arenes bearing electron-withdrawing
groups generally furnished good to excellent yields, as is
evident in products 5−18. Whereas the ortho-ester led to 5 in
50% yield, the meta-substituted analogue generated 6 in 80%
yield, suggesting substitution patterns play an important role.
Aryl bromide decorated with an acrylate moiety furnished 11
in 60% yield. The reactions tolerated amide proton as in 12.
Arylation of naphthyl bromides effectively delivered 13 and 14.
The esters 17−19 were precursors of marketed α-aryl
propanoic acids for inflammatory disease treatment which
were obtained in moderate to excellent yields.9 Coupling of β-
bromostyrene with oxalate 2 also gave 20 in 46% yield. A low
yield was detected for conjugated dienyl bromide as evident for
the preparation of 21.
a
b
c
Reaction conditions as in Table 1, entry 1. Isolated yield. NMR
d
yield using 2,5-dimethylfuran as the internal reference. 2 equiv of
oxalate were used.
arylation event, as evident in 29−34. When bromo- and
iodobenzene were subjected to standard reaction conditions,
the latter delivered 4 in 57% yield, whereas the former
generated 4 in a low yield.
Finally, we used methyl 4-bromobenzoate (1) and 4-
iodoanisole to couple with a wide range of Ox-α-HEs (Table
3). The ester moieties constituting n-butyl, benzyl, cyclohexyl,
phenyl, and 4-chlorophenyl groups were compatible with
reaction conditions, as exemplified by 35−40. Increase of the
size of primary alkyl groups at the α-position of the Ox-α-HEs
(with respect to the methyl group as in 2) did not result in
appreciable changes in the coupling efficiency (e.g., 41−47).
However, the introduction of secondary alkyl groups in the α-
position of the Ox-α-HEs significantly reduced the yields as
shown by the examples of 48−50. The more flexible isopropyl
group was less effective than cyclopentyl. Gratifyingly, the
coupling protocol was suitable for lactone and ketone scaffolds
as evidenced by the examples of 51−54. By contrast, amide
was not competent for this coupling event.19
For arenes substituted with electron-donating groups, aryl
iodides were better performers (Table 2). The yields for 22−
34 were good to excellent. The coupling of heteroaryl iodides
was examined. Indoles and imidazoles were suited for this
To gain insight into this Ni-catalyzed reaction, we first
measured the reduction potential of 2, which was determined
to be −1.247 V (vs SCE in CH3CN).19 Similar to our previous
studies on tertiary alkyl oxalates, we reason that Zn (∼0.76 V
B
DOI: 10.1021/acs.orglett.9b00174
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Table 3. Scope of Oxalates Using the Standard Method
Figure 2. Dependence of the ratio of 58 to 59 on the catalyst loading
using a radical clock. Each ratio is averaged on the basis of five
independent runs.
using Zn as the reductant. The present method tolerates a wide
range of functional groups and displays excellent compatibility
for a wide set of α-alkyl-substituted esters, which provides an
alternative to the current method to α-aryl carbonyl
compounds. Preliminary mechanistic studies support that
alkyl radicals were involved in the catalytic process.
a
b
c
Reaction conditions as in Table 1, entry 1. Isolated yield. NMR
d
yield using 2,5-dimethylfuran as the internal reference. 2 equiv of
LiCl were added.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00174.
■
in water) is thermodynamically disfavored to reduce the
oxalates. It is MgCl2 that activates the oxalate and stabilizes the
radical anion intermediate by a chelation effect, which allows
Zn to reduce Ox-α-HE.8 To test whether the present C−O
bond cleavage event involves formation of alkyl radicals,
cyclopropyl-containing oxalate 55 was subjected to the
reaction conditions (eq 1). The ring-opening product 56 was
S
Detailed experimental procedures; characterization of
new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hegui_gong@shu.edu.cn.
ORCID
obtained in 52% yield, consistent with a proposal of the
participation of a radical intermediate. None of the
corresponding cyclopropane-containing product was observed.
To further verify the reaction mechanism, compound 57 was
used to serve as a radical clock to test whether a radical-chain
process is viable. The coupling of 57 with methyl 4-
bromobenzoate (1) was carried out at varying concentrations
of the catalyst loading (eq 2). A linear dependence of the ratio
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 and 21572127).
■
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