Synthesis of 2,2,2,-Trichloroethyl Aryl- and Vinyldiazoacetates by Palladium-Catalyzed Cross-Coupling

Article abstract of DOI:10.1002/chem.201700101

An efficient and convenient synthesis of 2,2,2-trichloroethyl (TCE) aryl- and vinyldiazoacetates was achieved by palladium-catalyzed cross-coupling reactions between TCE diazoacetates and aryl or vinyl iodides. The broad substrate scope allows for rapid and facile formation of TCE aryl- and vinyldiazoacetates, which recently have emerged as versatile reagents for rhodium-carbene chemistry.

Full text of DOI:10.1002/chem.201700101

A Journal of
Accepted Article
Title: Synthesis of 2,2,2,-Trichloroethyl Aryl- and Vinyldiazoacetates by
Palladium-Catalyzed Cross-Coupling
Authors: Huw Davies, Liangbing Fu, Jeffrey Mighion, and Eric Voight
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To be cited as: Chem. Eur. J. 10.1002/chem.201700101
Link to VoR: http://dx.doi.org/10.1002/chem.201700101
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COMMUNICATION
Synthesis of 2,2,2,-Trichloroethyl Aryl- and Vinyldiazoacetates by
Palladium-Catalyzed Cross-Coupling
†
,§
†,§
‡
†*
Liangbing Fu , Jeffrey D. Mighion , Eric A. Voight , and Huw M. L. Davies
Abstract: An efficient and convenient synthesis of 2,2,2-
trichloroethyl (TCE) aryl- and vinyldiazoacetates was achieved by
palladium-catalyzed cross-coupling reactions between TCE
diazoacetates and aryl or vinyl iodides. The broad substrate scope
allows for rapid and facile formation of TCE aryl- and vinyl-
diazoacetates, which recently have emerged as versatile reagents
for rhodium-carbene chemistry.
for the synthesis of ethyl aryldiazoacetates from aryliodides and
[
10a-c]
ethyl diazoacetate (Scheme 1, B).
We envisioned that the
strategy could also be applied to the synthesis of TCE
aryldiazoacetates (Scheme 1, C) and thus, circumvent the
problems of the diazo transfer reactions for generating these
compounds.
Diazo compounds are among the most frequently used
[
1]
precursors for transition-metal-catalyzed carbene reactions.
When decomposed by transition-metals, diazo compounds are
transformed to highly electrophilic metal-carbenes, which can
undergo a variety of useful transformations, including C−H
insertion, cyclopropanation, ylide formation, and various
[
1]
cycloadditions.
Particularly noteworthy are donor/acceptor
generated from the dirhodium
rhodium-carbenes,
tetracarboxylate-catalyzed decomposition of aryldiazoacetates
because they exhibit excellent reactivity and selectivity in
[
2]
various transformations. Recently, the trichloroethyl (TCE)
aryldiazoacetates have emerged as versatile reagents because
the resulting carbenes react more cleanly than other ester
derivatives and enable the functionalization of substrates that
were generally considered unreactive or would have otherwise
Scheme 1. New synthetic route to TCE aryldiazoacetates.
For the palladium-catalyzed coupling to be
a useful
method for the synthesis of TCE aryldiazoacetates, a practical
method would be needed to access the staring material, TCE
diazoacetate 4. The known procedure for its preparation is not
[
3,4]
reacted unselectively.
With the greater popularity of trichloroethyl (TCE)
diazoacetates and more evidence that they are robust reagents
in rhodium-carbene chemistry, a general and efficient route for
their synthesis is required (Scheme 1). Classically,
aryldiazoacetates are synthesized from the corresponding
arylacetates by the Regitz diazo transfer reaction (Scheme 1,
[
11]
amenable to large-scale synthesis.
adapting a reported procedure for the synthesis of 2,2,2-
This was achieved by
[
12]
trifluoroethyl diazoacetate.
Condensation of 2,2,6-trimethyl-
4H-1,3-dioxin-4-one (1) with trichloroethanol to form the
acetoacetate 2, followed by diazo transfer reactions with p-
ABSA to form 3 and hydrolysis/decarboxylation generated 4 in
75% overall yield over three steps (eq. 1). This reaction was
carried out at up to 120 mmol scale.
[
5]
A). In the case of the formation of TCE diazoacetates, the safe
diazo transfer reagent, p-acetamidobenzenesulfonyl azide (p-
[
6]
ABSA), does not give high yields, and instead, the more
reactive and less stable o-nitrobenzenelsulfonyl azide must be
[
3b]
Cl
3
CH
2
OH
p-ABSA
NEt , CH CN
O
O
used.
substituents such as methoxy, the diazo transfer reaction is
Furthermore, if the aryl group has electron-donating
H
3
C
CH
O
3
O
O
toulene
3
3
O
H
3
C
O
CCl
3
H
3
C
O
CCl
3
[
7]
0 °C to rt
N2
ineffective even with o-nitrobenzenesulfonyl azide.
C–H functionalization represents an area of considerable
150 °C
H
3
C
O
1
2
3
1
20 mmol
N
2
[
8,9]
research interest,
and it has recently been applied to the
6% aq. KOH
O
CCl
3
H
7
5 % (three steps)
(1)
[
10]
synthesis of diazo compounds. Wang and co-workers recently
developed a palladium-catalyzed C–H functionalization strategy
Et
2
O, rt
O
4
The optimal conditions for the cross-coupling were first
explored using 1-bromo-4-iodobenzene as the standard
substrate. Under the reaction conditions reported by Wang and
†
[
]
]
L. Fu, Dr. J. D. Mighion,
Prof. Dr. H. M. L. Davies
Department of Chemistry, Emory University
3 4 2 3
co-workers (Pd(PPh ) (5 mol%)/Ag CO (10 mol%)), variable
yields (42-83%) of the coupling product 6a were obtained.
Attempts at using other palladium and ligand combinations were
unsuccessful. The standard Wang conditions use both palladium
and silver salts and both of these metal salts could potentially
1
515 Dickey Drive, Atlanta, Georgia 30322, United States
E-mail: hmdavie@emory.edu
Dr. E. A. Voight
Research & Development, AbbVie.
‡
[
[
1
North Waukegan Road, North Chicago, Illinois 60064, United
[
13]
catalyze the decomposition of diazo compounds.
The
States
§]
L.F. and J.D.M contributed equally.
possibility that the variable yields were caused by product
decomposition was investigated by means of ReactIR
experiments: When the diazo product 6a was treated with
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
palladium tetrakis(triphenylphosphine) (5 mol%) for 4 h, it
underwent decomposition. After an induction period of about 2 h,
thiophene-,
benzothiazole-,
pyridine-,
and
pyrimidine-
Pd(PPh3)4 (5 mol%),
PPh3 (10 mmol%), Ag2CO3 (50 mol%) R
O
O
6
a decomposed rapidly and was virtually all consumed after 4 h.
I
H
+
In contrast, when an additional 10 mol% of triphenylphosphine
was added to the mixture, the decomposition of 6a was greatly
retarded. Consequently, the cross coupling reactions were
repeated using an additional 10 mol% of triphenylphosphine and
under these conditions, 6a was generated in 89% yield with
consisted yields between runs.
R
O
CCl3
O
CCl3
NEt3, toluene, r.t.
N2
N2
5a-y
4
6a-y
Br
Cl
F
O
O
O
O
CCl3
O
O
CCl3
O
CCl3
N2
N2
N2
6a
6b
6c
89% yield
81% yield
85% yield[a]
[
a]
O2N
MeO
Me
MeO2C
F3C
Table 1. Optimization of the reaction conditions.
O
O
O
CCl3
O
CCl3
O
CCl3
Br
N
2
Br
O
N2
N2
N2
[
Pd] (5 mol%), Ag
2
CO
3
(50 mol%)
6d
6e
6f
O
CCl
3
+
H
75% yield[a]
90% yield
92% yield
O
CCl
3
NEt
3
, toluene, r.t.
I
O
N
2
5a
4
6a
AcO
O
O
[
b]
O
O
entry
[Pd]
Pd(PPh
additional ligand
yield (%)
CCl3
O
CCl3
O
CCl3
N2
1
2
3
4
5
3
)
4
-
42-83
N2
N2
6g
6h
6i
[
[
[
c]
c]
c]
Pd
Pd
Pd
2
2
2
(dba)
(dba)
(dba)
3
dppe
Sphos
-
-
-
77% yield
76% yield
91% yield
3
3
O
O
N
PCy
PPh
3
3
O
O
Pd(PPh
3
)
4
89
O
CCl3
O
CCl3
O
CCl3
N2
[
a] Standard reaction conditions: aryl halide 5a (1.0 mmol), TCE EDA 4 (1.3
N2
N2
6j
6k
6l
mmol), NEt (1.3 equiv), Ag CO (0.5 equiv), Pd(PPh (5 mol %), in 4 mL of
3
2
3
3
)
4
82% yield
88% yield
70% yield
toluene at room temperature for 4 h. [b] Isolated yield. [c] Trace amount of
product.
PinB
N
O
O
O
O
O
CCl3
Br
O
CCl3
O
CCl3
N2
N2
N2
0% yield
6n
6o
6
m
9
78% yield
91% yield
O
O
O
O
MeO
O
CCl3 Me
O
CCl3
CCl3
N2
N2
Ph N2
6p
6q
6r
93% yield
89% yield
43% yield
Cl
Cl
Cl
X
O
O
O
O
CCl3
Cl
O
CCl3
O
CCl3
Figure 1. ReactIR experiment. (Blue): 6a in the presence of Pd(PPh
3
)
4
(5
N2
N2
N2
6
s
6t
mol%); (Red): 6a in the presence of Pd(PPh
3
)
4
(5 mol%) and PPh
3
(10 mol%).
X = Me, NDP[b]
X = Br, NDP[b]
7
8% yield
83% yield
See supporting information for details.
Cl
N
Cl
O
O
O
The substrate scope of the reaction was then examined
and the results are summarized in Scheme 2. A range of p-
substituted phenyl derivatives with electron-withdrawing and
electron-donating groups were tolerated (6a-m). The boron
pinacolate group is compatible with this chemistry, generating
N
N
O
CCl3
O
CCl3
O
CCl3
N2
N2
N2
6u
6v
6w
7
6% yield
39% yield[a]
58% yield[a]
S
S
N
O
O
O
Me
CCl3
6
n, which contains
a
useful synthetic handle for further
O
N
O
CCl3
O
CCl3
N2
N2
21% yield
N2
manipulation. Particularly noteworthy is the ready access to the
methoxy, acetamido and amino derivatives (6g, 6l and 6m)
because the methoxy derivative 6g was not effectively formed in
a diazo transfer reaction and amino derivatives have not been
extensively utilized in the chemistry of donor/acceptor carbenes
due to their inefficient synthesis. meta-Substituted derivatives
such as halogen, methyl, and methoxy were also well tolerated
6
x
6y
6z
52% yield[a]
81% yield
Scheme 2. Scope of aryl- and vinyl iodides. Standard reaction conditions: aryl
halide 5a-y (4.0 mmol), EDA (1.3 equiv), NEt (1.3 equiv), Ag CO (0.5 equiv),
Pd(PPh (5 mol %), in 16 mL of toluene at room temperature for 4 h. [a] The
3
2
3
3 4
)
reactions were performed for 12 h. [b] Low conversion and no desired product.
(
6o-q), but ortho-substituted derivatives could not be prepared,
diazoacetates (6v-z), with the pyridine and pyrimidine systems
requiring longer reactions times.
suggesting that the reaction is sensitive to steric effects. TCE
(
(
Z)-styryldiazoacetate could also be synthesized by this method
6r). Both 3,5- and 3,4- disubstituted substrates were also
Interestingly, the cross-coupling reaction of 2-iodopyridine
7
gave a cyclic pyridotriazole product 8 (eq 2). The formation of
compatible with this method (6s-t). Moreover, heterocyclic TCE
this product is proposed to go through the initial diazo formation,
diazoacetates could also be synthesized as illustrated by the
[5,14]
followed by
a
ring closing cyclization process.
This
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Chemistry - A European Journal
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COMMUNICATION
closed/open form equilibrium of pyridotriazoles was utilized in
The cross-coupling reaction generated a number of novel
diazo compounds that had not been previously applied to
enantioselective rhodium-catalyzed reactions. Therefore, some
of the most interesting ones were evaluated in the
[
14]
Rh-catalyzed transannulation by Gevorgyan and co-workers,
and our observation provides an alternative method for the
preparation of pyridotriazoles.
cyclopropanation of styrene using the recently developed
CCl3
[15a]
catalyst Rh
2
(S-p-PhTPCP)
4
(Scheme 4).
The reactions of
O
N
O
CCl3
O
O
N2
Pd(PPh3)4, PPh3
I
TCE aryldiazoacetates with p-morpholine and p-Bpin on the
phenyl ring gave the desired cyclopropanation in good yields
with high levels of enantioselectivity (16a-b). The 3,5-dichloro-
substituted TCE diazoacetate yielded the cyclopropane in 85%
yield with 85% ee (16c). Additionally, heterocycles such as
benzothiazole, thiophene and pyrimidine were tolerated as the
aryl group of the TCE diazoacetates (16d-f). Cyclopropanes
were obtained in moderate yields (85-65%) with excellent levels
of enantioselectivity (99-85%). The asymmetric induction with
the heterocyclic diazo esters is much improved over previous
(2)
O
CCl3
H
+
N2
N
N
Ag2CO3, NEt3
toluene, r.t
N
N
O
82%
4
7
8
The cross-coupling process could be applied to other ester
derivatives that are useful in rhodium-carbene chemistry.
Specifically, besides 2,2,2-trichloroethyl, 2,2,2-trifluoro, 2,2,2-
tribromo, and trimethylsilylethyl aryldiazoacetate could also be
[
3a, 15]
synthesized with this cross-coupling strategy (Table 2).
results of cyclopropanation with the corresponding methyl esters
[
a]
[16]
Table 2. Test of different esters.
2 4
using Rh (S-DOSP) as the catalyst as noted by 16e.
N2
Pd(PPh3)4 (5 mol%),
OR PPh3 (10 mmol%), Ag2CO3 (50 mol%)
Br
Br
+
H
CO2R
I
O
NEt3, toluene, r.t.
N2
5
a
9a-c
10a-c
yield (%)
89
[
b]
entry
R
product
6a
1
2
3
4
TCE
TFE
TBE
10a
58
[
c]
10b
53
TMSE
10c
87
[
a] Standard reaction conditions: aryl halide 5a (4.0 mmol), diazoacetates 9a-c
1.3 equiv), NEt (1.3 equiv), Ag CO (0.5 equiv), Pd(PPh (5 mol %), and
PPh (10 mol %) in 16 mL of toluene at room temperature for 4 h. [b] Isolated
yield. [c] 10 mol% of Pd(PPh and 20 mol % of PPh were used.
(
3
2
3
3 4
)
3
3
)
4
3
The synthetic utility of the transformation was further
demonstrated by functionalizing aryl and vinyl iodides derived
from estrone. These reactions afforded the corresponding diazo
products 13 and 14 in 71% and 25% yield respectively (Scheme
3
). The relatively low yield (25%) for the generation of 14 is
presumably another indication that the cross-coupling reaction is
not very tolerant of sterically demanding iodides.
I
O
Me
H
O
Me
H
Me
H
H
H
H
H
H
H
Scheme 4. Test of new diazo compounds in cyclopropanation reactions. The
reactions were carried out with Rh (S-p-PhTPCP) (0.5 mol %) and styrene
2.0 equiv) in dry dichloromethane, followed by addition of a solution of diazo
compounds 6 (1.0 equiv) in dichloromethane under argon over 2 h at room
temperature. [a] The reaction used 1 mol % of Rh (S-p-PhTPCP)
MeO
I
HO
1
1
estrone
12
2
4
(
N
2
N
2
O
CCl
3
O
CCl
3
[Pd]
[
Pd]
H
H
O
O
4
.
4
4
2
O
O
Me
N
2
CCl3
Me
O
To conclude, a general and efficient route for the synthesis
2,2,2-trichloroethyl (TCE) aryl-, heteroaryl-, and
H
O
H
of
H
H
Cl
3
C
O
H
H
vinyldiazoacetates was achieved by palladium-catalyzed cross-
coupling of the corresponding iodides and diazoacetate. The
successful development of this process required addition of
further phosphine ligand to the reaction mixture to avoid
decomposition of the diazo product. The ready access of the
diazo compounds will enhance the range and flexibility of the
donor/acceptor metallocarbene chemistry.
N
2
MeO
1
3
14
7
1%
25%
Scheme 3. Application to estrone derivative
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Chemistry - A European Journal
10.1002/chem.201700101
COMMUNICATION
Experimental Section
[5]
[6]
a) M. Regitz, Angew. Chem. Int. Ed. 1967, 6, 733; Angew. Chem. 1967,
7
9, 786; b) M. Regitz, J. Hocker, A. Liedhegener Org. Synth., Coll. Vol.
General Procedure for the palladium-catalyzed cross-coupling
V 1973, 179; Vol. 48 1968, 36.
reaction: Pd(PPh
.4 mmol, 10 mol%), aryl iodide (4.0 mmol, 1.0 equiv), Ag
550 mg, 2.0 mmol, 0.5 equiv) were suspended in toluene (16
mL) under argon, followed by addition of NEt (0.73 mL, 5.2
mmol, 1.3 equiv) and 2,2,2-trichloroethyl 2-diazoacetate (1.13 g,
.2 mmol, 1.3 equiv). The resulting reaction was stirred at room
3
)
4
(231 mg, 0.2 mmol, 5 mol%), PPh
3
(106 mg,
J. S. Baum, D. A. Shook, H. M. L. Davies, H. D. Smith, Syn. Commun.
1987, 17, 1709.
0
2
CO
3
(
[7]
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temperature for 4 h and then filtered through a short path of
silica gel, eluting with ethyl acetate. The filtrates were
concentrated and the residue was purified by column
chromatography to give the products.
Acknowledgements
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Keywords: 2,2,2,-trichloroethyl aryldiazoacetates • cross-
coupling • palladium • donor/acceptor carbenes • asymmetric
cyclopropanation
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Liangbing Fu†,§, Jeffrey D. Mighion†,§,
Eric A. Voight‡, and Huw M. L. Davies†*
Page No. – Page No.
Synthesis of 2,2,2,-Trichloroethyl Aryl
and Vinyldiazoacetates by Palladium-
Catalyzed Cross-Coupling
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