Functionalized 2,3-dihydrofurans via palladium-catalyzed oxyarylation of α-allyl-β-ketoesters

Article abstract of DOI:10.1039/c1ob06593a

The palladium-catalyzed reaction of (hetero)aryl bromides, chlorides, and nonaflates with α-allyl-β-ketoesters provides ready efficient access to functionalized 2,3-dihydrofurans. The reaction tolerates several useful substituents including chloro, fluoro

Full text of DOI:10.1039/c1ob06593a

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Functionalized 2,3-dihydrofurans via palladium-catalyzed oxyarylation of
a-allyl-b-ketoesters†‡§
Sandro Cacchi,*a Giancarlo Fabrizi,a Antonella Goggiamani,a Antonia Iazzetti,a David Madec,b Giovanni Polic
and Guillaume Prestatc
Received 19th September 2011, Accepted 14th October 2011
DOI: 10.1039/c1ob06593a
The palladium-catalyzed reaction of (hetero)aryl bromides,
chlorides, and nonaflates with a-allyl-b-ketoesters provides
ready efficient access to functionalized 2,3-dihydrofurans.
The reaction tolerates several useful substituents including
chloro, fluoro, ether, ketone, ester, cyano, and nitro groups.
Scheme 1 Palladium-catalyzed oxyarylation of a-allyl-b-ketoesters with
aryl halides.
The 2,3-dihydrofuran motif is displayed in a large number of
bioactive natural products as well as pharmaceutically important
unnatural compounds such as neo-clerodane diterpenoids1 and
cently reported by J. N. Kim et al.11 who described the construction
aflatoxin B1.2 In addition, 2,3-dihydrofuran derivatives are useful
of 2,3-dihydrofuran rings incorporated into dihydroindenofuran
synthetic intermediates.3 Some of the most convenient approaches
scaffolds through the intramolecular oxyarylation of substrates
to the construction of the 2,3-dihydrofuran ring system are based
that contain both the enolate precursor and a bromobenzene
on the reaction of active methylene compounds4 or ylides5 with
fragment in the same molecule.
Using the reaction of 3-bromoanisole with ethyl 2-allyl-3-
oxobutanoate 2a as a probe for evaluating the reaction conditions,
suitable electrophiles. Functionalized 2,3-dihydrofuran rings have
also been formed via reaction of active methylene compounds
with olefins in the presence of manganese(III)acetate6 or cerium(IV)
we initiated our study by examining the influence of ligands, bases,
ammonium nitrate7 and by treating diazo compounds with olefins
and solvents in the presence of Pd2(dba)3 at 100 ◦C in 0.08 M
in the presence of copper-8 or rhodium9 catalysts. Palladium catal-
solutions. Low to moderate yields of 3a were obtained using
ysis has also found its place in this field.10 Nevertheless, despite
Cs2CO3 and XantPhos12,13 in 1,4-dioxane by increasing the catalyst
the number of methods developed, the search for more general
loading from 0.01 to 0.025 equiv (Table 1, entries 1–7). With dppf13
and versatile synthetic approaches to this class of compounds
or dppb13 ligands or in the presence of Pd(PPh3)4 no evidence of 3a
was obtained and the Mizoroki–Heck derivative 4a was isolated
continues to be an active area of research thus justifying efforts
to develop more general and versatile procedures, particularly
when these procedures accommodate considerable functionalities
and use readily available starting materials. Here we report
as the main product (Table 1, entries 8–10). A 52% yield of 3a was
isolated with SPhos,13,14 but 4a still formed in significant amounts
(Table 1, entry 11). Substituting K3PO4 for Cs2CO3 led to a lower
such a method allowing for ready access to functionalized 2,3-
yield and lower oxypalladation/Mizoroki–Heck reaction selectivity
dihydrofurans 3 through the palladium-catalyzed oxyarylation of
(Table 1, entry 12). Pleasingly, an increase of both the yield and the
a-allyl-b-ketoesters 2 (Scheme 1). Related chemistry has been re-
oxypalladation/Mizoroki–Heck reaction selectivity were observed
by switching to RuPhos13,15 (Table 1, entry 13). Higher yield and
aDipartimento di Chimica e Tecnologie del Farmaco, Sapienza, Universita`
selectivity were observed using MeCN as solvent (Table 1, entry
14) whereas toluene and DMF gave unsatisfactory results (Table
di Roma, P.le A. Moro 5, 00185 Roma, Italy. E-mail: sandro.cacchi@
uniroma1.it; Fax: +39 (06) 4991 2780; Tel: +39 (06) 4991 2795
bUniversite´ de Toulouse, UPS, LHFA, baˆt. 2R1, 118 route de Narbonne,
31062 Toulouse. CNRS, LHFA UMR 5069, 31062 Toulouse, France.
E-mail: madec@chimie.ups-tlse.fr; Tel: +33 5 61 55 62 87
1, entries 15 and 16). Further optimization studies revealed that
the best results could be obtained by using 0.025 equiv Pd2(dba)3,
0.05 equiv RuPhos, and 1.2 equiv Cs2CO3 in a more concentrated
0.25 M MeCN solution at 100 ◦C. Under these conditions, 3a
was isolated in 79% yield in 2.5 h and no Mizoroki–Heck product
was observed (Table 1, entry 17). Cs2CO3 proved superior to other
bases such as K3PO4, K2CO3, and NaHCO3 (Table 1, entries 18–
20).
cUPMC Univ Paris 06, Institut Parisien de Chimie Mole´culaire (UMR
CNRS 7201), FR 2769, C. 183, 4 Place Jussieu, 75005 Paris, France.
E-mail: giovanni.poli@upmc.fr; Tel: +33 1 44 27 55 72
† This article is dedicated to Professor Alfredo Ricci on the occasion of his
retirement.
‡ This work is the fruit of a collaborative project from COST, Action D40
“Innovative Catalysis: New Processes and Selectivities”.
§ Electronic supplementary information (ESI) available: A complete de-
scription of experimental details and product characterization. See DOI:
10.1039/c1ob06593a
However, when 4-bromoanisole, a model of electron-rich aryl
bromides, was treated with 2a under the best conditions found
for 3-bromoanisole, a longer reaction time was required (very
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Table 1 The influence of ligands, solvents, and bases on the reaction of
3-methoxybromobenzene 1a with ethyl 2-allyl-3-oxobutanoate 2aa
Table 2 Synthesis of functionalized 2,3-dihydrofurans 3 from ethyl 2-
allyl-3-oxobutanoate 2aa
Yield%b
Aryl and heteroaryl
Entry halide 1
Time (h)
3
4
1
2
3
4
5
6
7
8
3-MeOC6H4Br
4-MeOC6H4Br
2.5
7
4.5
3.5
5
7
5
24
4.5
2
79
3a
3b
3b
3b
3c
3d
3d
3e
3e
3f
traces
8
4a
4b
4c
44
64c
57c
33c
65
15
—
28
—
—
7
—
—
—
4-MeOC6H4Cl
4-Me2NC6H4Br
3-MeC6H4Br
4c
79c
54
4-MeC6H4Br
2-MeC6H4Br
9
10
11
73c
70c
—
Yield%b
Ligand or catalyst
Entry system (equiv)
Time
(h)
—
Base
Solvent
3a
4a
1
2
3
4
5
6
7
8
XantPhos (0.02)
XantPhos (0.03)
XantPhos (0.03)
XantPhos (0.03)
XantPhos (0.03)
XantPhos (0.05)
XantPhos (0.05)
dppf (0.05)
dppb (0.05)
Pd(PPh3)4 (0.05) Cs2CO3 1,4-dioxane
SPhos (0.05)
SPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
RuPhos (0.05)
Cs2CO3 1,4-dioxane
5
20c
—
—
—
—
—
—
—
60
65
30
28
28
13
—
25
—
—
8
Cs2CO3 1,4-dioxane 5.5
Cs2CO3 1,4-dioxane 3.5
Cs2CO3 1,4-dioxane 5.5
Cs2CO3 1,4-dioxane
Cs2CO3 1,4-dioxane
Cs2CO3 1,4-dioxane
27c
25d
27d,e
24d,f
31g
12
13
14
15
16
17
18
19
20
21
22
23
24
PhBr
2
95
82
71
70
92
94
66
91
91
86
92
81
37
3g
3h
3h
3i
—
—
—
—
—
—
—
—
—
traces
—
—
—
8
3
3
4-PhC6H4Br
4-PhC6H4ONf
4-FC6H4Br
1.5
0.5
5
2.5
5.5
3
1.25
1
1.25
2.5
4.5
24
23g,h
g
Cs2CO3 1,4-dioxane 2.5
Cs2CO3 1,4-dioxane
—
—
3-F3CC6H4Br
3-F3CC6H4Cl
4-ClC6H4Br
4-EtO2CC6H4Br
4-MeCOC6H4Br
4-NCC6H4Br
2-NCC6H4Br
3-O2NC6H4Br
3j
g
9
6
3
3j
3k
3l
3m
3n
3o
3p
3q
10
11
12
13
14
15
16
17
18
19
20
—
Cs2CO3 1,4-dioxane 22
K3PO4 1,4-dioxane 24
Cs2CO3 1,4-dioxane 2.5
Cs2CO3 MeCN
Cs2CO3 toluene
Cs2CO3 DMF
Cs2CO3 MeCN
52g
32g
65g
4n
2.5
1
1.5
2.5
1
2
9
74g
44g
55g
79g,e
60e,g
55e,g
tracese,g,i
K3PO4
K2CO3
MeCN
MeCN
—
—
25
26
2.5
5.5
57c
51
3q
3r
—
—
NaHCO3 MeCN
a Unless otherwise stated, reactions were carried out on a 0.5 mmol scale
at 100 ◦C in a 0.08 M solution, under a nitrogen atmosphere, using 1 equiv
of 1a, 1.2 equiv of 2a, and 1.2 equiv of base. b Yields are given for isolated
products. c 0.01 equiv Pd2(dba)3. d 0.015 equiv Pd2(dba)3. e 0.25 M. f 0.05
M. g 0.025 equiv Pd2(dba)3. h 1 equiv of 1a, 2 equiv of 2a. i 2a was recovered
in 52% yield.
27
28
0.75
2.25
74c
75
3r
3s
—
7
4s
likely because of the slower, rate determining oxidative addition
step), early formation of palladium black was observed, and the
corresponding 2,3-dihydrofuran derivative 3b was isolated only in
44% yield. The amount of RuPhos was then increased to 0.1 equiv
to favor the solubilization of the palladium catalyst. To our delight,
the desired dihydrofuran derivative was isolated in a satisfactory
64% yield (Table 2, entry 3). Consequently, we decided to employ
0.05 equiv RuPhos with neutral or electron-poor aryl halides and
0.1 equiv RuPhos with electron-rich aryl halides.
Using these conditions, we next explored the scope and gener-
ality of the process (Table 2). In general, clean formation of 2,3-
dihydrofurans 3 was observed with a variety of neutral, electron-
poor, and electron-rich aryl bromides. The reaction tolerates
several useful substituents including chloro, fluoro, ether, ketone,
ester, cyano, and nitro groups. The ability to incorporate the chloro
substituent (Table 2, entry 18) is particularly interesting since it can
be used for further synthetic manipulations via transition metal-
catalyzed coupling reactions. Only 4-bromo-N,N-dimethylaniline,
among the aryl bromides investigated, provided the corresponding
a Unless otherwise stated, reactions were carried out on a 0.5 mmol scale, in
a 0.25 M MeCN solution, under a nitrogen atmosphere at 100 ◦C, using 1
equiv of 1, 1.2 equiv of 2a, 1.2 equiv of Cs2CO3, 0.025 equiv of Pd2(dba)3,
and 0.05 equiv of RuPhos. b Yields are given for isolated products. c 0.1
equiv RuPhos.
2,3-dihydrofuran in low yield along with significant amounts of
the Heck product (Table 2, entry 5). Ortho substituents such
as o-methyl and o-cyano groups (Table 2, entries 10 and 22,
respectively) are also well tolerated. However, a bulkier o-ketal
aryl bromide failed to give the desired 2,3-dihydrofuran derivative
(Table 2, entry 11).
The method can be extended to aryl nonaflates (Table 2,
entry 14) and chlorides, although electron-poor aryl chlorides
afford oxyarylation products in excellent yields (Table 2, entry 17)
whereas electron-rich aryl chlorides are less successful substrates
(Table 2, entry 4).
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Table 3 The reaction of aryl halides 1 with a-allyl-b-ketoesters 2a
Yield%b
Entry
Aryl halide 1
a-Allyl-b-ketoesters 2 R
Time (h)
3
4
1
2
3
4
5
6
7
8
4-NCC6H4Br
4-MeC6H4Br
4-NCC6H4Br
4-NCC6H4Br
4-MeC6H4Br
4-MeC6H4Br
4-MeC6H4Br
4-NCC6H4Br
4-MeC6H4Br
4-NCC6H4Br
4-MeC6H4Br
4-NCC6H4Br
4-MeC6H4Br
4-NCC6H4Br
4-MeC6H4Br
4-NCC6H4Br
4-MeC6H4Br
2-furyl
2-furyl
Ph
Ph
Ph
Ph
Ph
0.66
2.5
7
2.5
21
24
4
1.25
5.25
1
2.5
24
84
3t
—
—
—
—
10
10
15
—
traces
—
—
—
16
75c
46
3u
3v
3v
84c,d
traces
tracesc
66e
69c
70c
51c
52c
23
4v
3w
3w
3w
3x
3y
4w
4w
4w
4-MeC6H4
4-MeC6H4
4-MeOC6H4
4-MeOC6H4
4-O2NC6H4
4-O2NC6H4
i-Pr
9
4y
10
11
12
13
14
15
16
17
3z
3za
3zb
3zc
3zd
3ze
—
24
11c
90
4zc
4zd
0.75
2.5
46
traces
—
—
i-Pr
83c
—
t-Bu
t-Bu
22
10c
3zf
38
4zf
a Unless otherwise stated, reactions were carried out on a 0.33 mmol scale, in a 0.25 M MeCN solution, under a nitrogen atmosphere at 100 ◦C, using 1
equiv of 1, 1.2 equiv of 2, 1.2 equiv of Cs2CO3, 0.025 equiv of Pd2(dba)3, and 0.05 equiv of RuPhos. b Yields are given for isolated products. c 0.1 equiv
RuPhos. d At 110◦ C. e 0.15 equiv Ruphos.
Heterocyclic halides were briefly investigated and were found
to give the corresponding 2,3-dihydrofurans in moderate to high
yields (Table 2, entries 24–28).
The influence of the substituent diversity of the a-allyl-b-
ketoesters 2 was also explored. Our preparative results are
summarized in Table 3. Although we did not make a systematic
study and generalization from these results have to be drawn with
caution, nevertheless some basic trends can be recognized; i.e.,
the oxyarylation reaction appear to be disfavored by substituents
decreasing the nucleophilicity of the enolate of the b-ketoester
both with electron-rich (Table 3, entry 13) and electron-poor (Table
3, entry 12) aryl halides. Even the presence of bulky R substituents
has a detrimental effect on the reaction outcome (Table 3, entries
16 and 17).
Most probably the reaction proceeds according to a mecha-
nism analogous to that described for related palladium-catalyzed
reactions16 (Scheme 2, path a; ligands are omitted for clarity’s
sake). The oxidative addition of the aryl halide to Pd(0) would
generate the s-arylpalladium complex A, which could undergo
an oxygen displacement with the in situ generated enolate to
afford the adduct B. A subsequent intramolecular oxypalladation
would provide the intermediate C from which the 2,3-dihydrofuran
derivative would form via reductive elimination. However, the
alternative catalytic cycle involving the intermediacy of the car-
bopalladation complex D, which would generate the palladacycle
E and then the 2,3-dihydrofuran product via reductive elimination
(Scheme 2, path b) or that proceeding through an intramolecular
nucleophilic attack of the oxygen on the olefinic moiety activated
by a s-arylpalladium complex (Scheme 2, path c) cannot be ruled
out.
Scheme
2
Proposed mechanism for the palladium-catalyzed syn-
thesis of functionalized 2,3-dihydrofurans from aryl halides and
a-allyl-b-ketoesters.
In conclusion, an efficient palladium-catalyzed oxyarylation
approach to 2,3-dihydrofurans from readily available a-allyl-b-
ketoesters has been developed. The procedure is simple and
provides the desired products in good to excellent yields. Aryl
bromides and chlorides as well as heteroaryl halides can be
used in this chemistry. The new method tolerates several useful
substituents including chloro, fluoro, ether, ketone, ester, cyano,
and nitro groups.
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3081.
13 XantPhos
SPhos = 2-(2¢,6¢-dimethoxybiphenyl)dicyclohexylphosphine; dppf =
1,3-bis(diphenyl-
2-(2¢,6¢-diisopropoxybiphenyl)-
=
9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene;
1,1¢-bis(diphenylphosphino)ferrocene; dppb
=
phosphino)butane; RuPhos
dicyclohexylphosphine.
=
14 T. E. Barder, S. D. Walker, J. R. Martinelli and S. L. Buchwald, J. Am.
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8236 | Org. Biomol. Chem., 2011, 9, 8233–8236
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