Decarboxylative C(sp3)?O Cross-Coupling

Article abstract of DOI:10.1002/anie.201808024

Alkyl aryl ethers are an important class of compounds in medicinal and agricultural chemistry. Catalytic C(sp³)?O cross-coupling of alkyl electrophiles with phenols is an unexplored disconnection strategy to the synthesis of alkyl aryl ethers, with the potential to overcome some of the major limitations of existing methods such as C(sp²)?O cross-coupling and S_N2 reactions. Reported here is a tandem photoredox and copper catalysis to achieve decarboxylative C(sp³)?O coupling of alkyl N-hydroxyphthalimide (NHPI) esters with phenols under mild reaction conditions. This method was used to synthesize a diverse set of alkyl aryl ethers using readily available alkyl carboxylic acids, including many natural products and drug molecules. Complementarity in scope and functional-group tolerance to existing methods was demonstrated.

Full text of DOI:10.1002/anie.201808024

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Accepted Article
Title: Decarboxylative C(sp3)-O Cross Coupling
Authors: Runze Mao, Jonathan Balon, and Xile Hu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808024
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10.1002/anie.201808024
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Decarboxylative C(sp3)-O Cross Coupling
Runze Mao, Jonathan Balon, and Xile Hu*
Abstract: Alkyl aryl ethers are an important class of compounds in
medicinal and agro chemistry. Catalytic C(sp3)-O cross coupling of
alkyl electrophiles with phenols is an unexplored disconnection
strategy to the synthesis of alkyl aryl ethers, with the potential to
overcome some of the major limitations of existing methods such as
C(sp2)-O cross coupling and SN2 reactions. Here we employ tandem
photoredox and copper catalysis to achieve decarboxylative C(sp3)-O
coupling of alkyl N-hydroxyphthalimide (NHPI) esters with phenols
under mild conditions. This method could be used to synthesize a
diverse set of alkyl aryl ethers using readily available alkyl carboxylic
acids, including many natural products and drug molecules.
Complementarity in scope and functional group tolerance to existing
methods was demonstrated.
Alkyl aryl ether linkages are ubiquitous in natural products,[1]
pharmaceuticals[2] and agrochemicals.[3] Transition-metal-
catalyzed C(sp2)-O cross coupling has been developed into a
valuable method for the synthesis of alkyl aryl ethers, overcoming
the limitations in scope and functional group tolerance of
traditional approaches such as nucleophilic substitution (SN2)[4-5]
and nucleophilic aromatic substitution (SNAr).[6] However, these
coupling reactions are typically conducted under elevated
temperatures due to certain difficult elemental steps in the
catalytic cycle. For Pd[7-10] and Ni[11] catalysis, oxidative addition
of aryl halides is facile but C(sp2)-O reductive elimination is
problematic (Fig. 1A). Thus, sterically demanding and
complicated phosphine ligands are employed to promote C(sp2)-
O
reductive elimination. For Cu catalysis, C-O reductive
elimination is straightforward but oxidative addition of aryl
electrophiles is challenging (Fig. 1A). As a result, the scope of
electrophiles is largely restricted to aryl iodides and activated aryl
bromides. In an elegant study, MacMillan and co-workers[12]
employed photoredox catalysis to transiently oxidize an
Ni(II)(aryl)(alkoxide) intermediate to Ni(III), thereby facilitating
C(sp2)-O reductive elimination. The C(sp2)-O coupling occurred at
room temperature. Nevertheless, the electrophiles were limited to
(hetero)aryl bromides. This photoredox/Ni dual-catalyzed
approach has also been applied to other C-heteroatom (S, P, N)
coupling reactions.[13-16]
Figure 1. (A) Comparison of C-O cross coupling methodologies for the
synthesis of alkyl aryl ethers; (B) The initially hypothesized catalytic cycle for
decarboxylative C(sp3)-O coupling. FG = functional group, SET = single electron
transfer.
Catalytic C(sp3)-O coupling of alkyl electrophiles with
phenols is an intriguing alternative for the synthesis of alkyl aryl
ethers. The catalytic version of Williamson ether synthesis has the
potential to overcome some of its inherent shortcomings such as
forcing conditions, narrow scope, and poor tolerance. Moreover,
if alkyl electrophiles other than halides can be used as the
coupling partners, base-catalyzed elimination from alkyl
electrophiles will no longer be a major side reaction. In this context
we considered alkyl redox-active esters derived from alkyl
carboxylic acids as desirable alkyl electrophiles (Fig. 1A).
[*]
R. Mao, J. Balon, Prof. Dr. X.L. Hu*
Laboratory of Inorganic Synthesis and Catalysis, Institute of
Chemical Sciences and Engineering, École Polytechnique Fédérale
de Lausanne (EPFL), ISIC-LSCI
BCH 3305, Lausanne 1015, Switzerland
E-mail: xile.hu@epfl.ch
Website: http://lsci.epfl.ch
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Summary of the effects of reaction parameters and conditions on the
reaction efficiency[a]
Table 2. Scope of the C-O coupling of alkyl redox-active esters
Entry
1[b]
PC
A
Cu catalyst
CuBr/L1
CuBr
Base
Et3N
Et3N
Et3N
Et3N
Et3N
Cs2CO3
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Solvent
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
Yield
16%
19%
22%
5%
2[c]
A
3[c]
A
CuCl
4[c,d]
5[c]
A
Cu
A
CuCl2
9%
6[e]
A
CuCl
trace
trace
32%
44%
21%
62%
82%
94%
0%
7[c]
A
CuCl
8[c]
A
CuCl
DCM
9
A
CuCl
DCM
10
B
CuCl
DCM
11
C
CuCl
DCM
12[f]
13[f,g]
14[f,h]
15[f]
16
C
(CuOTf)2•C6H6
(CuOTf)2•C6H6
(CuOTf)2•C6H6
(CuOTf)2•C6H6
none
DCM
C
DCM
C
DCM
none
C
DCM
0%
DCM
0%
[a] Calibrated GC yield using n-dodecane as an internal standard. [b] Same
condition as the optimized reaction condition in reference 21. Reaction was
carried out with 1a (0.2 mmol), 2a (2 equiv.), A (1 mol%), CuBr (20 mol%), L1
(7.5 mol%), Et3N (5 equiv.) and MeCN (2 mL). [c] Et3N (5 equiv.). [d] Cu (100
mol%). [e] Yield of the O-acylation product is 85%. [f] (CuOTf)2•C6H6 (10 mol%).
[g] DCM (0.05 M). [h] No light. RT = room temperature, PC = photocatalyst, DMF
= N,N-dimethylformamide, DCM = dichloromethane, (CuOTf)2•C6H6 = copper(I)
trifluoromethanesulfonate benzene complex.
These redox-active esters have been employed for remarkable
decarboxylative C-C[17-21] and C-heteroatom (B, Si, N)[22-29] cross-
coupling reactions. However, analogous C-O cross coupling is
hitherto unknown. Our group recently developed synergetic
photoredox and copper catalysis for decarboxylative C(sp3)-N
cross coupling[28-29] We reasoned that a similar strategy might
provide access to the elusive C(sp3)-O coupling due to the more
facile C-O reductive elimination on Cu.[30]
Following the previous C-N coupling, we hypothesized
(Figure 1B) that binding of the phenol substrate to a Cu(I) species
(I) followed by deprotonation would generate a CuI-alkoxide
intermediate (II). This intermediate can trap the alkyl radical
generated by oxidative quenching of the excited photocatalyst
(*PC) with an alkyl NHPI ester to yield a Cu(II) intermediate (III).
The oxidized photocatalyst (PC+) then oxidizes III to give a CuIII
[a] NHPI ester (1 equiv.), phenol (2 equiv.), [Ir(dtbbpy)(ppy)2]PF6 (1 mol%),
(CuOTf)2•C6H6 (10 mol%) and Et3N (2 equiv.) in CH2Cl2 (0.05 M), irradiated at
room temperature for 20 h, isolated yield. [b] NHPI ester (2 equiv.), phenol (1
equiv.), [Ir(dtbbpy)(ppy)2]PF6 (1 mol%), Cu(MeCN)4OTf (40 mol%) and N-
isopropyl-N-methyl-tert-butylamine (1 equiv.) in CH2Cl2 (0.05 M), irradiated at 0-
RT for 20 h, isolated yield.
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Table 3. Late-Stage decarboxylative etherification of natural products and drugs containing carboxylic acids
[a] NHPI ester (1 equiv.), phenol (2 equiv.), [Ir(dtbbpy)(ppy)2]PF6 (1 mol%), (CuOTf)2•C6H6 (10 mol%) and Et3N (2 equiv.) in CH2Cl2 (0.05 M), irradiated at room
temperature for 20 h, isolated yield. [b] NHPI ester (2 equiv.), phenol (1 equiv.), [Ir(dtbbpy)(ppy)2]PF6 (1 mol%), Cu(MeCN)4OTf (40 mol%) and N-Isopropyl-N-
Methyl-tert-butylamine (1 equiv.) in CH2Cl2 (0.05 M), irradiated at 0-RT for 20 h, isolated yield.
alkyl alkoxide intermediate (IV) and regenerates the PC. IV should
readily undergo C(sp3)-O reductive elimination to give the alkyl
aryl product while regenerating the initial CuI catalyst (I). Here we
report the successful development of such a C(sp3)-O cross
coupling methodology, which exhibits attractive features such as
mild conditions, no ancillary ligand, broad scope, and functional
group tolerant. Furthermore, the method could be applied for late-
stage functionalization of various natural products and drug
molecules.
(0.05 M) as the solvent, blue LED irradiation, 20 h. A yield of 94%
was obtained under these conditions (Table 1, entry 13). Light,
photocatalyst, and copper catalyst were all essential for the
coupling (Table 1, entries 14-16).
The scope of the decarboxylative C(sp3)-O cross coupling
method was explored employing the optimized conditions
described above. As shown in Table 2, a myriad of phenols
containing electron-donating (3a-3d), electron-neutral (3c), and
electron-withdrawing (3e-3j) substituents were coupled to give the
corresponding ethers in moderate to excellent yields. Notably,
aryl halide (3e-3i) or aryl boronic ester (3k) groups were tolerated
in the coupling, demonstrating its excellent chemoselectivity and
orthogonal functional group compatibility relative to C(sp2)-O
coupling methods. Other important functional groups such as
ketone (3j), cyano (3l), trifluoromethoxyl (3m), and polycyclic
aromatic ring (3n) were also tolerated. Although GC (Gas
Chromatography) analysis indicated a high yield of 3m, its volatile
nature led to a decreased isolated yield (54%). The modest yield
of 3l (49%) was probably due to coordination of copper by the
cyano group of the substrate. The coupling was successful for
various secondary alkyl NHPI esters, including those containing
cyclic alkyl groups (4a-4c), heterocyclic alkyl group (4d), and
acyclic alkyl groups (4e-4g). To our delight, some tertiary NHPI
esters, which generated non-planarizable radicals, could be
coupled in good to excellent yields (5a-5f). These bulky ether
products would be difficult to access using using classic
Williamson ether synthesis. Planarizable radical sources, like
NHPI esters derived from pivalic acid and 1-methylcyclohexanoic
acid, only afforded the desired products in less than 30% yields.
For the coupling of primary alkyl NHPI esters, a modification
of reaction conditions was necessary (Table S2). The optimized
conditions were NHPI ester (2 equiv.), phenol (1 equiv.),
[Ir(dtbbpy)(ppy)2]PF6 (1 mol%) as the photocatalyst,
Cu(MeCN)4OTf (40 mol%) as the copper catalyst, and N-
isopropyl-N-methyl-tert-butylamine (1 equiv.) as the base in
We began our investigation by subjecting cyclohexyl NHPI
ester 1a and guaiacol 2a to the conditions previously employed
for decarboxylative C(sp3)-N cross coupling of NHPI esters with
anilines21 (Table 1, entry 1; and Table S1, entry 1). The desired
ether 3a was formed, albeit in a low yield (16%). The coupling was
then optimized by varying reaction parameters (Table S1, SI) and
a summary of key observations is shown in Table 1. Interestingly,
omission of the ligand was slightly beneficial (Table 1, entry 2).
Although various Cu(I) salts could serve as catalysts, the nature
of their counter anions had an influence in the yields (Table 1,
entries 2-3; Table S1, entries 7-10). CuCl was the best copper
catalyst among preliminary optimization (Table 1, entry 3). Cu(II)
and Cu(0) catalysts gave lower yields (Table 1, entries 4-5). When
Et3N was replaced by an inorganic base, Cs2CO3, O-acylation of
phenol instead of O-alkylation was observed (Table 1, entry 6).
CH2Cl2 was the best solvent (Table 1, entries 6-8; Table S1,
entries 19-21), and decreasing the amount of base increased the
yield (Table 1, entry 9). Changing the photocatalyst from
[Ru(bpy)3](PF6)2 to an Ir-based photocatalyst could be beneficial
(Table 1, entries 9-11; Table S1, entries 23-26), and the best
photocatalyst was [Ir(dtbbpy)(ppy)2]PF6 (Table 1, entry 11).
Further optimization indicated (CuOTf)2•C6H6 (10 mol%) and
decreased concentration further improved the yield (Table 1,
entries 12-13). The best conditions were [Ir(dtbbpy)(ppy)2]PF6 (1
mol%) as the photocatalyst, (CuOTf)2•C6H6 (10 mol%) as the
copper catalyst, triethylamine (2 equiv.) as the base, and CH2Cl2
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CH2Cl2 (0.05 M), irradiated at 0-R.T. for 20 h. Under these
conditions, various primary alkyl NHPI esters were successfully
coupled (6a-6m). Both non-activated alkyl (6a-6f) and benzyl
groups (6g-6j) were compatible. Alkyl bromide (6k) and
perfluoroalkyl (6l) groups were tolerated, demonstrating the
orthogonal functional tolerance of the current method relative to
SN2 reactions. Note that alkyl aryl ethers containing bulky
ortho,ortho-disubstituted aryl groups (6a, 6c-6f, 6h-6m) were
readily obtained using this coupling method. These products are
diffuclt to access using C(sp2)-O coupling methods which are
inefficient for the activation of bulky aryl halides. Coupling of the
NHPI ester of 6-heptenoic acid, a radical-clock probe, gave the 5-
exo-trig cyclized product (6m), suggesting the intermediacy of
alkyl radical.
Figure 2. Proposed catalytic cycle for decarboxylative C(sp3)-O coupling.
The use of alkyl NHPI esters instead of alkyl halides as
electrophiles makes the current decarboxylative C(sp3)-O
coupling method amenable to the rapid, late-stage modification of
many carboxylic acid-containing natural products and drug
molecules (Table 3). Indeed, fatty acids such as stearic acid (7a),
elaidic acid (7b), oleic acid (7c) and linoleic acid (7d) were
etherified with ease. Moreover, drugs such as chlorambucil (7e),
indomethacin (7f), isoxepac (7g), zaltoprofen (7h), ketoprofen (7i),
naproxen (7j) and ibuprofen (7k) underwent smooth esterification
as well. Natural phenolic compounds such as o-phenylphenol
(OPP, 7l), methylparaben (7m) and raspberry ketone (7n) could
also be used as coupling partners. The successful late-stage
functionalization of these natural products and drugs, many of
which contain sensitive alkene, carbonyl, halide, and heterocyclic
groups, underscores the high chemoselectivity and functional
group tolerance of the current method.
In summary, decarboxylative C(sp3)-O coupling of alkyl
NHPI esters has been achieved using tandem photoredox and Cu
catalysis. This coupling method allows for the rapid
transformation of readily available alkyl carboxylic acids into alkyl
aryl ethers, which are important compounds in medicinal
chemistry. The method provides a new disconnection strategy
and exhibits complementary scope and functional group tolerance
compared to traditional C(sp2)-O coupling and SN2 reactions.
Acknowledgements
This work is supported by the NoNoMeCat Marie Skłodowska-
Curie training network funded by the European Union under the
Horizon 2020 Program (675020-MSCA-ITN-2015-ETN). We
thank Dr. Lei Zhang (EPFL) for assistance in conducting
mechanistic study.
While the detailed mechanism warrants a dedicated study,
a few experiments were conducted to give preliminary insights.
Surprisingly, the fluorescence of [Ir(dtbbpy)(ppy)2]PF6 was not
quenched by guaiacol (2a), triethylamine (Et3N), Cu(MeCN)4PF6,
NHPI ester (1a), mixture of Cu(MeCN)4PF6 and NHPI ester (1a),
or mixture of Cu(MeCN)4PF6 and guaiacol (2a) (Fig. S1, SI). On
the other hand, the fluoresence was quenched by the mixture of
Cu(MeCN)4PF6 and Et3N (Fig. S2, SI) with defined Stern-Volmer
kinetics (Fig. S3, SI). These results suggest that due to the
change of photocatalyst (Ru to Ir) and solvent, the initial step of
the photocatalysis in the present C-O coupling is different from
that of analogous C-N coupling[28] The excited state *IrIII complex
(E1/2 [*IrIII/IrII] = +0.66 vs SCE in MeCN)[31] first reacts with a Cu(I)-
Et3N complex A to give a copper(II) species B (Fig. 2). The unique
function of Et3N compared to other bases is evident here. The
Keywords: photoredox · decarboxylation · C-O coupling ·
copper catalysis · alkylation
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COMMUNICATION
A tandem photoredox and Cu
catalysis has been developed
to enable the cross coupling of
alkyl NHPI esters with
phenols.
R. Mao, J. Balon, X. Hu*
Page No. – Page No.
Decarboxylative C(sp3)-O
Cross Coupling
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