Decarboxylative Benzylation of Aryl and Alkenyl Boronic Esters

Article abstract of DOI:10.1002/anie.201800829

The copper-catalyzed decarboxylative benzylation of aryl and alkenyl boronic esters with electron-deficient aryl acetates is reported. The oxidative coupling proceeds under mild, aerobic conditions and tolerates a host of potentially reactive electrophilic functional groups that would be problematic with traditional benzylation methods (aryl iodides and bromides, protic heteroatoms, aldehydes, Michael acceptors). A reaction pathway in which a benzylic nucleophile is generated by aryl acetate decarboxylation and in turn is intercepted by the catalyst to form diarylmethane products is supported by mechanistic studies.

Full text of DOI:10.1002/anie.201800829

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Accepted Article
Title: Decarboxylative Benzylation of sp2-Organoboron Reagents
Authors: Patrick J Moon, Anis Fahandej-Sadi, Wenyu Qian, and Rylan
J. Lundgren
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800829
Angew. Chem. 10.1002/ange.201800829
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10.1002/anie.201800829
Angewandte Chemie International Edition
COMMUNICATION
Decarboxylative Benzylation of sp2-Organoboron Reagents
Patrick J. Moon, Anis Fahandej-Sadi, Wenyu Qian, Rylan J. Lundgren* [a]
Abstract: The Cu-catalyzed decarboxylative benzylation of aryl and
alkenyl boronic esters with electron-deficient aryl acetates is reported.
The oxidative coupling proceeds under mild, aerobic conditions and
tolerates a host of potentially reactive electrophilic functional groups
that would be problematic with traditional benzylation protocols (aryl
iodides and bromides, protic heteroatoms, aldehydes, Michael
acceptors). A reaction pathway in which a benzylic nucleophile is
generated via aryl acetate decarboxylation and in turn is intercepted
by the catalyst to form diarylmethane products is supported by
mechanistic studies.
undergo efficient Cu-promoted arylation using organoboronic
esters.8 Malonic half-esters are oxidatively coupled with a
decarboxylation event occurring from the arylated intermediate to
yield aryl acetic acid derivatives.9 We were inspired by the work
of Tunge,10 Lui,11 and Zhu12 who demonstrated that nitrophenyl
acetates could be decarboxylatively trapped with allyl, aryl, and
alkenyl electrophiles under Pd-catalysis at high temperatures
(100–140 °C) (Figure 1a). A mild oxidative method to prepare
diarylmethane structures13 from aryl acetic acids could provide a
useful
alternative
strategy
to
Friedel-Crafts,14
and radical
electrophile/nucleophile
cross-coupling,15
benzylations.16 Herein, we report an oxidative coupling reaction
between aryl or alkenyl boronic esters and nitrophenyl acetates
(Figure 1b). The decarboxylation event precedes oxidative C–C
bond formation, with the Cu catalyst assuming a dual role in
mediating the oxidative coupling process and stabilizing substrate
Carboxylic acids are ubiquitous functional groups found in
organic molecules and can act as versatile building blocks in
synthesis. A number of strategies have been developed to use
carboxylates as reactive groups in carbon–carbon bond forming
processes via the extrusion of CO2. Approaches to enable
decarboxylative functionalizations include nucleophile generation
by enzymatic or organocatalytic activation;1 thermally-promoted
reactions at transition metals;2 C–C bond homolysis induced by
single-electron oxidation to generate radicals;3 or by use of
chemical pre-activation of the carboxylate in combination with
transition metal or photoredox catalysts.4 The astute application
of these concepts has allowed for a myriad of valuable coupling
reactions to be invented.5 The reactivity patterns and functional
group compatibility of decarboxylative transformations often
compliment C–C bond forming methodologies that use more
traditional electrophile/nucleophile pairs, expediting small
molecule synthesis. Thus, expanding the scope of decarboxy-
lative transformations remains an important challenge. This
endeavor requires both improved approaches towards the
decarboxylative generation of reactive intermediates and an
expansion of the coupling manifolds that can productively engage
such species.
Oxidative cross-coupling reactions can enable selective bond-
formation between two distinct nucleophilic partners.6 The
inherent compatibility towards electrophilic functional groups
displayed by oxidative coupling processes provides a significant
benefit in comparison to traditional cross-coupling reactions. The
Cu-promoted coupling of aryl boronic acids and N- or O-
heteroatom nucleophiles illustrates the practical utility of oxidative
coupling reactions, finding widespread use in synthetic and
medicinal chemistry.7 We discovered that activated methylenes
a Pd-catalyzed decarboxylative benzylation of electrophiles
[Pd], E–X
CO2R
E
thermal
decarboxylation
Ar
Ar
100–140 ºC
E–X: allyl, aryl and alkenyl electrophiles
b Cu-catalyzed benzylation of aryl/alkenyl boronic esters
Cu(OAc)2
Nuc–[B]
CO2M
Nuc
Ar
Ar
rt–40 ºC, air
Nuc:
aryl/alkenyl
M
Ar
– CO2
[Cu], Nuc–[B]
[O]
[decarboxylation under mild, aerobic conditions]
reactive intermediate generation controlled by [M] / OAc–
[M = K, Li, Na, Cu, Zn]
c Reaction Development: Role of Select Parametersa
Ar–B(neop)
75 mol% Cu(OAc)2
NO2
NO2
NO2
Me
CO2K
Ar
DMA 35 ºC, air
1a 1.25 equiv. [standard conditions]
2a
1a'
entry deviation from standard conditions
conv. 1a (%)b 2a / 1a' (%)
1
2
3
4
5
6
7
8
9
none
75
<2
29
32
25
<2
27
31
<2
31
26
113
30
acid instead of K-salt, 3 equiv. NEt3
acid instead of K-salt, 1.5 equiv. K2CO3
rt instead of 35 ºC
96
>123
65
9
under N2 instead of air
36
93
Cu(OTf)2 instead of Cu(OAc)2
30% Cu(OAc)2 instead of 75%
Ar–B(pin) instead of Ar–B(neop)
Pd, Co, Ni, Fe, or Zn instead of Cu
<2
<2
51
>123
>123
31-119
[a]
P. J. Moon, A. Fahandej-Sadi, W. Qian, Prof. R. J. Lundgren
Department of Chemistry
38
23-76
University of Alberta
Edmonton, Alberta, T6G 2G2, Canada
E-mail: rylan.lundgren@ualberta.ca
Figure 1. (a) Pd-catalyzed coupling of electrophiles with aryl acetates. (b)
Oxidative arylation of aryl acetates (this work). (c) Effect of select reaction
parameters on the coupling. a0.20 mmol scale, 0.2 M, 21 h, yields and conv.
determined by calibrated 1H NMR bConv. 1a out of 125%, Ar = 3-Br-C6H4.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201800829
Angewandte Chemie International Edition
COMMUNICATION
in DMA solvent. These insights can be applied to other metal-
catalyzed reactions of carboxylates to enable reactivity under mild
conditions.
cross-coupling manifolds or thermally-driven decarboxylations
(Figure 2a). Diarylmethanes featuring aryl bromides (2a, 2w) and
iodides (2j), nitriles (2b), NH-amides (2e, 2u), alcohols (2aa),
esters (2h, 2i), ketones (2m), aldehydes (2l), and Michael
acceptors (2i) can be smoothly generated under the standard
conditions. Substrates with Lewis-basic nitrogen-heterocycles (2r,
2s, 2u, 2x) or those featuring multiple functional groups (2q, 2z,
2aa) can be used without significant complication.
Decarboxylative alkenylation of the aryl acetate can also be
achieve using vinyl boronic ester partners (2ab, 2ac).17b
A range of 2- and 4-nitrophenyl acetates engage in productive
cross-coupling reactions (3a-3j, Figure 2b). For these substrates,
it was more convenient and economical to employ the
corresponding Cu bis(carboxylate) salts with KOAc additives (see
below for a discussion). Under mild conditions, a series of
polysubstituted diarylmethanes can be generated featuring
electron-donating and withdrawing groups, including bromides,
chlorides, and fluorides, as well as nitrogen and sulfur-containing
heterocycles.
Reaction development studies focused on enabling the
oxidative cross-coupling of 2-nitrophenyl acetate (1a) and an
electrophile-functionalized aryl boronic ester derivative (Figure
1c). Optimal conditions were achieved using the K-aryl acetate
salt and neopentyl boronic ester, with 75 mol % Cu(OAc)2 in DMA
under air to yield the desired diarylmethane 2a in 75% yield (entry
1). Under other conditions, considerable hydrodecarboxylation of
the acid was observed to form nitrotoluene 1a’. This was
surprising in light of the aggressive reaction conditions typically
used to promote decarboxylation from electron-poor aryl
acetates.11-12 Use of the K-carboxylate salt was essential; when
the free acid was used in combination with NEt3 or K2CO3, or
when reduced amounts of Cu(OAc)2 were employed, formation of
nitrotoluene outpaced product formation (entries 2, 3, and 7).
Under N2, less than one turnover of Cu catalyst was observed
(entry 5, 25% yield 2a).17 Cu(OAc)2 outperformed other Cu-
catalysts tested (see the SI for details). Other metal (II) acetates
(Pd, Co, Ni, Fe and Zn) were ineffective in promoting the coupling
process, with varying amounts of non-productive substrate
decarboxylation observed (entry 9).
The aerobic coupling can be scaled to deliver grams of
product (Figure 3a). It was beneficial and more cost-effective to
react the carboxylic acid with CuSO4•5H2O and to subject the
resultant Cu(II)bis(carboxylate) salt to the standard reaction
conditions with the addition of KOAc. The Cu reagent can be
isolated by simple filtration and is less hydrolytically sensitive than
the corresponding K-species.17c
The oxidative benzylation process tolerates electronically
diverse aryl boronic ester partners and a host of reactive
functional groups that would be poorly compatible with traditional
a
O
NO2
NO2
Br
CN
CF3
OMe
Cu(OAc)2
(neop)B
N
H
CO2K
Ar'
Ar'
DMA
35 ºC, air
2b: 66%
2e: 52%
2a: 71%
2c: 69%
2d: 64%
CO2Et
I
Ac
OMe
2n: 71%
Me
2f: 65%a
Cl
CO2Et
OCF3
2k: 71%
CHO
2l: 52%
Cl
2g: 58%
2j: 55%
2h: 63%
2i: 61%
2m: 50%
O
Cl
CO2Et
H
N
N
Cy
S
SMe
2o: 49%
SiMe3
OMe
2q: 70%
N
Cl
N
O
2p: 61%a
2r: 51%a
2s: 47%
2t: 56%
2u:
55%
2v: 55%
O
NBoc
Br
O
N
N
OMe
H
N
N
OH
F3C
S
Me
2z: 48%
O
O
2y: 66%
Br
O
2w: 57%a
2aa: 41%a
2ac: 55%a
Br
CO2Et
NO2
2x: 59%
2ab: 54%
b
1.0 equiv
Ar–B(neop)
KOAc
NO2
NO2
NO2
Br
Br
Br
N
CO2 Cu
Ar
O2N
Ar
Ar'
O2N
DMA
room temp, air
Cl
O
O
2
OMe
Br
Cl
Cl
3a: 72%b
3d: 66%
3b: 66%
3c: 67%
F
F
NO2
OMe
NO2
Cl
O2N
S
S
F
O2N
OMe
O2N
O2N
OMe
Br
OMe
F
3f: 53%c
3h: 54%c
3i: 54%a,c
3j: 52%c
3e: 70%
3g: 53%
Figure 2. Cu-promoted decarboxylative benzylation: (a) Aryl/alkenyl boronic ester scope (b) Nitrophenyl acetate scope, see Fig 1c for condition. acalibrated 1H
NMR yield. bat 40 °C. c0.5 equiv. carboxylate, 1.5-2.0 equiv. Ar–B(neop), 0.5 equiv. KOAc. See SI for full details.
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10.1002/anie.201800829
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COMMUNICATION
a Potential pathways for decarboxylation and C−C bond formation
a simplified gram-scale reaction [1.6 g product]
NO2
NO2
[Cu], [O]
Ar'−B(OR)2
CO2[M]
Ar'
1. CuSO4•5 H2O
aq. NaOH
Path I:
[arylation then
decarboxylation]
– CO2
Ar
Ar
2. Ar–B(neop) 1.0 equiv
KOAc, DMA, rt in air
CO2H
Cl
Br
CO2[M]
– CO2
Ar'
Cl
Ar
Ar
1.5 equiv.
3c 62%
Path II:
[M]
b nitro-diarylmethane product functionalizations
[decarboxylation
then arylation]
R
[Cu], [O]
Ar'−B(OR)2
NH2
B(pin)
I
NO2
[G] conv. (%)
b Cation effects on arylacetate decarboxylation
Ar
Ar
Ar
Ar
H
Cu(II)
Zn
0
0
Ar
4e 72%e
Me
Me
Cl
Cl
Cl
CO2[G]
Cl
Cl
Ar
Ar
Ar
4a 94%a
4b 52%a,b
4c 73%a,c
4d 64%a,d
0
– CO2
DMA, 35 °C
K
61
[adtv.] conv. (%)
Figure 3. (a) Gram-scale cross-coupling. (b) Product functionalizations. Ar = 3-
Br-C6H4 R = CO2Me. aZn, NH4Cl. bNaNO2, B2pin2. cNaNO2, KI. dtBuONO, methyl
acrylate, Pd(OAc)2, MeSO3H. eallyl bromide, KOtBu, 18-C-6. See SI for details.
[additive]
Cu(OAc)2 65
CO2K
Ar
Cu(OTf)2
Zn(OTf)2
Zn(OAc)2
0
0
– CO2
DMA, 35 °C
15
M(II)-arylacetates do not readily decarboxylate
Cu(OAc)2 does not inhibit decarboxylation from K-carboxylate
A range of useful product classes can be derived from the
requisite aryl nitro group.18 Reduction to an amino group,
borylation, halogenation, olefination, and methylene allylation can
be accomplished to deliver polyfunctionalized diarylmethanes in
a straight-forward fashion (4a-4e, Figure 3b).
c Proposed phenylacetate equilibrium prior to decarboxylation (Path II)
[CuII]OAc
KOAc
CO2[CuII]
CO2K
K
Ar
Ar
Ar
– CO2
[benzyl nucleophile]
[CuII]OAc
KOAc
[stable]
Two general potential pathways for the decarboxylative
arylation exist. One route consists of nitrophenyl acetate oxidative
arylation followed by decarboxylation (Path I, Figure 4a). An
alternative pathway involves a decarboxylation event to generate
a benzyl nucleophile which undergoes subsequent oxidative
trapping (Path II, Figure 4a). We found both nitrophenyl acetate
decarboxylation and arylation to be dependent on the nature of
the cation and solvent polarity. In DMA at 35 °C, the free acid or
the corresponding Cu(II) or Zn(II) salts were stable, while rapid
decarboxylation was observed for the K-salt (Figure 4b).17d No
decarboxylation or cross-coupling was observed in less polar
solvents. Decarboxylation from the K-carboxylate was not
impeded by the addition of Cu(OAc)2, however Cu(OTf)2
completely suppressed reactivity, as did Zn(OTf)2 and to a lesser
extent Zn(OAc)2 (Figure 4b).
d Effect of KOAc additive on decarboxylative oxidative arylation
Bneop
25%
10%
none
100%
50%
60
45
30
15
0
CO2 Cu
Ar
Br
2 equiv.
2
0.5 equiv.
[KOAc]
– CO2
DMA, 35°C
air
Br
Ar
0
5
10
15
20
Time/(h)
e Diarylmethane carboxylates dimerize under standard reaction conditions
60 mol%
Cu(OAc)2
NO2 Ph
CO2K
NO2 Ph
NO2 Ph
DMA, air, 35 ºC
20 minutes
It is likely that stable Cu-carboxylates are in equilibrium with
reactive K-carboxylates under the standard reaction conditions
and that decarboxylation precedes C–C bond formation (Path II).
We propose a benzylic nucleophile is generated and trapped by
Cu in the presence of aryl boronic ester in a series of steps similar
to those in the Chan-Evans-Lam reaction (Figure 4c).19 No
product formation or decarboxylation is observed when a
Cu(carboxylate)2 salt of the standard substrate is added to aryl
boronic ester, providing evidence that such species cannot
access productive intermediates in the reaction. The addition of
KOAc to mixtures of Cu(carboxylate)2 and aryl boronic ester
induces product formation with increasing rate and reaction
productivity as KOAc loading is increased (Figure 4d). When a
diarylmethyl carboxylate is subjected to the standard conditions
oxidative dimerization is dominant over diarylmethane product
formation. This indicates it is unlikely diarylated acids are formed
under standard condition (Figure 4e). D-labelling studies of the
nitrophenyl acetate also support a decarboxylation event prior to
C–C coupling (see the SI for full details).
Ph NO2
74%
16%
f Improved reaction efficiency at reduced Cu-loadings with Zn(OAc)2
75% Cu(OAc)2
25% Cu(OAc)2 + 50% Zn(OAc)2
25% Cu(OAc)2
Bneop
Br
CO2K
Ar
75
50
25
0
cat. [Cu]
cat. [Zn]
– CO2
DMA, 35 °C
Br
25% Cu(OAc)2 + 50% Zn(OTf)2
75% Zn(OAc)2
Ar
0
6
12
Time-(h)
18
g Room temperature Pd-catalyzed benzylation of aryl bromides
1 equiv. Ph–Br
2.5 mol % [Pd(cin)Cl]2
7.5 mol % XPhos
CO2K
O2N
O2N
DMA, 25 ºC
5a 2-NO2 81%
5a 4-NO2 90%
1.25 equiv.
Given that Cu(OAc)2 plays a dual role, both enabling aerobic
oxidative coupling and stabilizing the reactive nitrophenyl acetate
substrate, the necessity for high Cu-loadings (75 mol %) becomes
Figure 4. Mechanistic studies and process improvements.
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10.1002/anie.201800829
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COMMUNICATION
d) J. X. Qiao, P. Y. S. Lam, in Boronic Acids, Wiley-VCH Verlag
GmbH & Co. KGaA, 2011, pp. 315-361.
evident. At 25 mol % Cu(OAc)2 a terminal yield of <25% is
observed under otherwise standard conditions due to
unproductive decarboxylation of the nitrophenyl acetate (Figure
4f). A reduction in Cu catalyst could be achieved by simply adding
Zn salts to modulate the rate of substrate decarboxylation. The
use of 25% Cu(OAc)2 in combination with 50% Zn(OAc)2 closely
[8]
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2016, 138, 13826-13829; b) A. Fahandej-Sadi, R. J. Lundgren,
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a dramatic
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improvement to reactions conducted with similar Cu(OAc)2
loading in the absence of Zn (Figure 4f).20 In line with the proposal
that acetate is also vital to enable the exchange of metal
carboxylates, the use of Zn(OTf)2 with 25% Cu(OAc)2 provided no
diarylmethane product, as with Zn(OAc)2 alone. Finally, we
reconsidered the established protocols for thermally-driven Pd-
catalyzed decarboxylative benzylation of aryl electrophiles that
proceed only with forcing conditions (140 °C in mesitylene).11
Remarkably, conducting reactions in DMA allowed for smooth
formation of diarylmethane product at room temperature (Figure
4g).
14860-14861.
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nitrophenyl acetates and sp2-organoboronic esters has been
developed. Diarylmethanes containing a number of potentially
reactive electrophilic groups can be prepared via this method. The
nitro functional group of the benzylic partner can be readily
modified to gain access to a diverse range of arylated products.
Process optimization and mechanistic studies uncovered an
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sp2–sp3 couplings.
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[17] Notes: a) Two equiv. of Cu(II) are required per oxidative bond
forming event assuming a Chan-Evans-Lam type mechanism.
b) See the SI for less successful substrates. c) CuSO4•5 H2O:
$18 USD/mol vs Cu(OAc)2 $170/mol, prices from Merck Dec.
20, 2017 for largest available quantity, compare to Pd(OAc)2:
$5000/mol. d) Li, Na, and Cs-nitrophenyl acetates undergo
decarboxylation and arylation under similar conditions. e)
Pivalate can be used instead of acetate, see the SI for details.
f) Batey has reported the Chan-Evans-Lam reaction of
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Acknowledgements
We thank NSERC Canada (DG and I2I to R.J.L., CGS-D
fellowship to P.J.M, PGS-D fellowship to A.F.S.) for support.
Keywords: cross-coupling • decarboxylation • aerobic catalysis •
boron
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10.1002/anie.201800829
Angewandte Chemie International Edition
COMMUNICATION
Layout 2:
COMMUNICATION
cat. Cu(OAc)2
Ar'–B(neop)
Patrick J. Moon, Anis Fahandej-Sadi,
Wenyu Qian, Rylan J. Lundgren *
CO2M
Ar
Ar
Ar'
–CO2
rt - 40 ºC, air
Page No. – Page No.
Cu modulates decarboxylation
mediates C–C bond formation
Decarboxylative Benzylation of sp2-
Organoboron Reagents
Text for Table of Contents
This article is protected by copyright. All rights reserved.