Enantioselective synthesis of 3-substituted dihydrobenzofurans through iridium-catalyzed intramolecular hydroarylation

Article abstract of DOI:10.1039/d0ob02421j

Intramolecular hydroarylationviaC-H activation is one of the most powerful methods to synthesize carbo- and heterocyclic compounds, whereas we still have room for developing a highly enantioselective variant of the reaction. Here we describe Ir-catalyzed enantioselective intramolecular hydroarylation ofm-allyloxyphenyl ketones. The enantioselective cyclization was efficiently catalyzed by a cationic iridium complex coordinated with a conventional chiral bisphosphine ligand to give benzofurans in high yields with high enantioselectivity. A carbonyl group of ketones functioned as an effective directing group for the C-H activation. In terms of synthetic utility, we also achieved one-pot synthesis of chiral 3-substituted dihydrobenzofurans from readily available allylic carbonates andm-hydroxyacetophenonesviasequential Pd-catalyzed allylic substitution and Ir-catalyzed intramolecular hydroarylation.

Full text of DOI:10.1039/d0ob02421j

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PAPER
Enantioselective synthesis of 3-substituted
dihydrobenzofurans through iridium-catalyzed
intramolecular hydroarylation†
Cite this: Org. Biomol. Chem., 2021,
19, 684
Kana Sakamoto and Takahiro Nishimura
*
Intramolecular hydroarylation via C–H activation is one of the most powerful methods to synthesize
carbo- and heterocyclic compounds, whereas we still have room for developing a highly enantioselective
variant of the reaction. Here we describe Ir-catalyzed enantioselective intramolecular hydroarylation of
m-allyloxyphenyl ketones. The enantioselective cyclization was efficiently catalyzed by a cationic iridium
complex coordinated with a conventional chiral bisphosphine ligand to give benzofurans in high yields
with high enantioselectivity. A carbonyl group of ketones functioned as an effective directing group for
the C–H activation. In terms of synthetic utility, we also achieved one-pot synthesis of chiral 3-substituted
dihydrobenzofurans from readily available allylic carbonates and m-hydroxyacetophenones via sequential
Pd-catalyzed allylic substitution and Ir-catalyzed intramolecular hydroarylation.
Received 3rd December 2020,
Accepted 23rd December 2020
DOI: 10.1039/d0ob02421j
rsc.li/obc
γ-lactones with a quaternary all-carbon stereogenic center.6b
Cui and Wang independently reported Ru-catalyzed enantio-
Introduction
Transition-metal-catalyzed intramolecular direct addition of selective intramolecular hydroarylation of benzaldehyde
aromatic C–H bonds to unsaturated bonds, so-called hydroary- derivatives, which reacted with chiral amines to give imines
lation, has provided efficient and facile routes to cyclic com- functioning as chiral transient directing groups in situ.7 Cui
pounds with perfect atom-economy,1,2 and much attention has successfully developed enantioselective cyclization for the syn-
been paid to the asymmetric cyclization. Following a pioneer- thesis of chiral indoline derivatives, while Wang reported 2,3-
ing Rh-catalyzed direct intramolecular cyclization of 1,5- or dihydrobenzofuran products bearing chiral all-carbon quatern-
1,6-dienes by Murai and co-workers,3 Bergman, Ellman, and ary stereocenters. Most recently, Cavallo and Rueping reported
co-workers reported intramolecular hydroarylation giving intramolecular hydroarylation of m-cinnamyloxyphenyl
hetero- and carbocycles by chiral Rh catalysts (Scheme 1a).4 ketones proceeded with high enantioselectivity in the presence
Substrates containing alkenyl tethers favored endo-cyclization of a cationic iridium catalyst.8,9
to give five-membered rings in an enantioselective manner.
We recently reported Ir-catalyzed intermolecular hydroaryla-
Meanwhile, asymmetric exo-selective hydroarylation has been tion of 2H-chromene with aromatic ketones.10,11 The reaction
still underdeveloped (Scheme 1b). Cramer and co-workers involves olefin isomerization of 2H-chromene into 4H-chro-
developed enantioselective intramolecular hydroarylation to mene, and then, chemo-, regio-, and enantioselective addition
give exo products using a chiral Rh/Cp complex.5 The reaction of aromatic C–H bond to 4H-chromene proceeds to give 2-aryl-
gives dihydrobenzofurans containing methyl-substituted qua- chromanes with high enantioselectivity. We next focused on an
ternary stereocenters. Shibata and co-workers also reported intramolecular reaction via C–H activation catalyzed by the Ir
asymmetric exo-cyclization of N-alkenylindoles to give chiral complex. Herein we report enantioselective intramolecular cycli-
1-substituted-2,3-dehydro-1H-pyrrolo[1,2-a]indoles.6a
zation of m-allyloxyphenyl ketones catalyzed by a cationic
Moreover, Shibata found that enantioselective intramolecular iridium/chiral bisphosphine catalyst (Scheme 1c). Chiral dihy-
formal C–H conjugate addition of 4-methyl-1-aryl-2-methyl- drobenzofurans, which are important scaffolds in natural pro-
fumarates proceeded by a chiral Ir complex and gave chiral ducts and bioactive compounds,12 are provided by the reaction
in a highly enantioselective manner. Moreover, we successfully
synthesized dihydrobenzofurans from readily available allylic
Department of Chemistry, Graduate School of Science, Osaka City University,
carbonates and m-hydroxyacetophenones via formal inter-
Sumiyoshi, Osaka 558-8585, Japan. E-mail: tnishi@sci.osaka-cu.ac.jp
molecular annulation by combining Ir-catalyzed intramolecular
†Electronic supplementary information (ESI) available: Experimental procedures
hydroarylation with Pd-catalyzed allylic substitution.
and compound characterization data. See DOI: 10.1039/d0ob02421j
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(CF3)2C6H3], which are typical reagents for the catalytic hydro-
arylation,10 in toluene at 80 °C for 13 h gave dihydrobenzo-
furan 2a in 55% yield accompanied by the formation of vinyl
ether 3a. The selective 5-exo-cyclization occurred to give 2a and
the 6-endo-cyclized product was not observed at all.
Unfortunately, however, neither increase of amount of the
catalyst or longer reaction time had no effect on the yields or
selectivity of the products at that stage. In contrast, when cin-
namyloxy ether 1b, which has an internal alkene moiety, was
treated under the same reaction conditions, the corresponding
cyclized product 2b was obtained in 71% yield without for-
mation of the isomerized product 3b (Scheme 2b). The results
prompted us to develop asymmetric variant of the reaction of
1b as a model substrate.
The enantioselectivity obtained with conventional chiral
bisphosphine ligands in the Ir-catalyzed 5-exo-cyclization of 1b
are shown in Table 1. The use of (R)-binap13 gave 2b in 67%
yield with 64% ee (entry 1). Segphos14 and difluorphos15 both
improved the reactivity and enantioselectivity, giving 2b in 88
and 92% yields with 73 and 84% ee, respectively (entries 2 and
3). The reaction using (R)-MeO-biphep16 resulted in the for-
Table 1 Ligand screeninga
Scheme 1 Transition-metal-catalyzed
asymmetric
intramolecular
hydroarylation.
Results and discussion
To begin our study of intramolecular hydroarylation, we con-
ducted the reaction of m-allyloxybenzophenone (1a) by using a
cationic iridium complex (Scheme 2a). Treatment of 1a in the
presence of [IrCl(cod)]2 (5 mol% of Ir, cod = 1,5-cycloocta-
diene), (rac)-binap (6 mol%), and NaBArF4 [10 mol%, ArF = 3,5-
Entry
Ligand
Yieldb (%)
eec (%)
1
2
3
4
(R)-Binap
(S)-Segphos
(R)-Difluorphos
(R)-MeO-biphep
(S,S)-Chiraphos
(S,S)-QuinoxP*
(S,S)-QuinoxP*
67
88
92
3
61
49
85
64 (+)
73 (–)
84 (+)
—
67 (+)
97 (–)
97 (–)
5
6
7d
a Reaction conditions: 1b (0.10 mmol), [IrCl(cod)]2 (5 mol% of Ir), (S,
S)-QuinoxP* (6 mol%), and NaBArF (10 mol%) in toluene (0.2 mL) at
4
80 °C for 18 h. b Determined by 1H NMR analysis using benzyl phenyl
ether as an internal standard. c Determined by HPLC analysis with a
chiral stationary phase column: chiralcel OJ-H. d At 100 °C.
Scheme 2 Intramolecular hydroarylation of allyl ethers 1.
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mation of a trace amount of 2b (entry 4). (S,S)-Chiraphos17
also worked as a ligand to give 2b in a moderate yield with
modest enantioselectivity (entry 5). After testing several other
ligands, we found that (S,S)-QuinoxP* 18 displayed excellent
enantioselectivity (97% ee, entry 6), albeit with the modest
yield. An elevated temperature of 100 °C improved the yield of
2b up to 85% without compromising the enantioselectivity
(entry 7).
Scheme 3 summarizes the results obtained for the enantio-
selective hydroarylation of m-cinnamyloxyphenyl ketones using
(S,S)-QuinoxP* or (S)-difluorphos as a ligand. The intra-
molecular hydroarylation of benzophenones substituted with
both electron-donating and -withdrawing groups all proceeded
smoothly to give cyclized products in good to high yields with
high enantioselectivity (2c–h). Besides benzophenone deriva-
tives, isopropyl ketone 1i and butyl ketone 1j were suitable for
the reaction and gave cyclic products with high enantio-
selectivity. The reactions of m-substituted acetophenone 1k
and α-tetralone derivative 1l also took place to give the corres-
ponding products with high enantioselectivity. In contrast, an
ester group in 1m did not work as a directing group. The reac-
tion of allylic amide 1n either gave no cyclized product 2n. The
present catalytic system can also be applied to several aceto-
phenones
bearing
substituted
cinnamyloxy
groups.
Substituents at para-, meta-, and ortho-positions of the cinna-
myl groups hardly affected the reactivity and enantioselectivity,
thus giving the corresponding cyclized products 2o–t in high
yields with high enantioselectivity. Aromatic ketones having a
2-furyl group (1u) and an alkyl group (1v) instead of the aryl
groups also underwent intramolecular hydroarylation to give
2u and 2v with 85% and 56% ee, respectively, while
α,β-unsaturated ester 1w was inert under the present catalytic
conditions.
To determine the absolute configuration of the cyclization
products 2, dihydrobenzofuran 2b obtained by the present
reaction, was converted into a known compound (Scheme 4).
Haloform reaction of 2b with NaOCl solution gave the corres-
ponding carboxylic acid after acidic work-up. The carboxyl
group was transformed into an aldehyde, and then, Ir-cata-
lyzed deformylation of the aldehyde gave dihydrobenzofuran 4.
The absolute configuration of 4 was determined to be S-(+) by
comparison of its specific rotation with the value reported pre-
viously: 4 [α]D25 +37.4 (c = 0.37, CHCl3) for 97% ee; lit.19 [α]D20
+41.3 (c = 0.75, CHCl3) for 86% ee (S)-4.
As shown in Scheme 5, it was found by chance that a pres-
ence of 4-methoxystyrene (5a) significantly improved the yield
of the cyclized product in a shorter reaction time without parti-
cipating in the reaction. Thus, the yield of 2b was 29% after
1.5 h of the reaction of 1b under the standard reaction con-
ditions, whereas the reaction in the presence of 4-methoxystyr-
ene (5a, 50 mol%) gave 2b in 84% yield with 98% ee. Styrene
(5b) also displayed an enhancement of the reactivity, thus
Scheme 3 Ir-Catalyzed intramolecular hydroarylation of m-cinnamy-
loxyphenyl ketones. Reaction conditions: 1 (0.10 mmol), [IrCl(cod)]2
(5 mol% of Ir), (S,S)-QuinoxP* (6 mol%), and NaBArF (10 mol%) in
4
toluene (0.2 mL) at 100 °C for 18 h. Isolated yields are shown. a(S)-
Difluorphos was used instead of (S,S)-QuinoxP*. b10 mol% of Ir, 12 mol%
(S,S)-QuinoxP*, and 20 mol% of NaBArF were used. c(R)-Difluorphos
4
was used.
giving 2b in 46%. In contrast, 4-chlorostyrene (5c) did not incorporation into 2b was not observed (Scheme 5b). The
influence the reaction. The asymmetric cyclization of 1b in the result implies that a hydridoiridium species formed by ortho-
presence of deuterated 4-methoxystyrene (5a-d3)20 smoothly C–H activation may not react with 5a-d3 intermolecularly.21
A
proceeded to give 2b in 97% yield after 3 h, where a deuterium positive effect of 4-methoxystyrene (5a) was also observed in
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Scheme 4 Determination of the absolute configuration.
Scheme 6 Deuterium-labeling experiments.
The results of deuterium-labeling experiments are shown in
Scheme 6. Treatment of 1l-d under the standard reaction con-
ditions gave 2l-d in 45% yield. The obtained product 2l-d con-
tained deuterium at the benzylic position of the phenyl group
as expected, and H/D exchange (8% D) at the C-3 position of
the dihydrobenzofuran ring was also observed, indicating that
the reaction is accompanied by ortho-C–H activation and sub-
sequent reversible hydrometalation and β-hydrogen elimin-
ation. Intermolecular KIE (kinetic isotope effect)22
was observed in the competitive reaction of a 1 : 1 mixture of 1l
and 1l-d (KIE = 1.5). In parallel cyclization reactions of 1l and
1l-d, the difference of the reactivity was also observed (2l/2l-d =
1.5).
A plausible catalytic cycle of the present reaction is postu-
lated as illustrated in Scheme 7. Oxidative addition of ortho-C–
H bond to a cationic iridium A generates an aryl(hydrido)
iridium(III) species B. Species B undergoes irreversible carbo-
metalation to form intermediate C, and sequential formation
of a C–H bond by reductive elimination gives cyclized product
2b and regenerates species A. Species B might also give inter-
mediates E and F through reversible hydrometalation. Based
on computational studies on Ir-catalyzed hydroarylation, the
carbometalation might be a turn-over limiting step of the
Scheme 5 Effect of 4-methoxystyrene.
the cyclization of 1a. Thus, the reaction of 1a in the presence present reaction,8,23 although we observed KIE of C–H
of 5a using (S,S)-chiraphos gave cyclized product 2a in 62% activation.22
yield with 77% ee accompanied by formation of 18% of iso-
m-Cinnamyloxyphenyl ketones 1, which underwent Ir-cata-
merized product 3a, indicating that the presence of 5a slightly lyzed asymmetric hydroarylation, can be prepared by Pd-cata-
suppressed the olefin isomerization. The use of (S,S)-QuinoxP* lyzed allylic substitution of allylic compounds, such as allylic
resulted in a low enantioselectivity (25% ee), although the for- carbonates and esters with phenol derivatives.24 In this respect,
mation of 3a was well inhibited. At present, the role of 4-meth- we next focused on the synthesis of chiral dihydrobenzofurans
oxystyrene to improve the yield is not yet clear, and the possi- by formal intermolecular annulation of phenol derivatives with
bility of the contribution as a secondary ligand of the iridium allylic compounds through Pd-catalyzed allylic substitution and
species cannot be excluded.
Ir-catalyzed intramolecular hydroarylation. In the first set of
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the allylic substitution, giving 1b in 81% and 86% yields,
respectively (entries 3 and 4). Pd2(dba)3·CHCl3 was a better cata-
lyst precursor than Pd(OAc)2 with L1 (entries 4 and 5).26
We next tried to synthesize dihydrobenzofurans via intra-
molecular allylic substitution of 6 and intramolecular hydroary-
lation in a single operation. Thus, allyl carbonate 6 was added
to a solution of premixed Pd2(dba)3·CHCl3 (1 mol% of Pd) and
L1 (1.2 mol%), and premixed [IrCl(cod)]2 (5 mol% of Ir), (S,S)-
QuinoxP* (6 mol%), and NaBArF4 (10 mol%) in toluene, and the
resulting mixture was stirred at room temperature for 30 min
and heated to 100 °C (Scheme 8a). The reaction resulted in
recovery of the starting material, and several control experi-
ments revealed that NaBArF inhibited the allylic substitution.
4
Therefore, after completion of the allylic substitution in the
presence of the Pd and Ir catalysts without NaBArF , which was
4
monitored by TLC, NaBArF was added to the reaction mixture
4
(Scheme 8b). Unfortunately, however, after the mixture was
heating at 100 °C for 14 h, the formation of the cyclized product
was not observed. Following the results, we next focused on the
one-pot protocol without any work-up of the first stage of the
sequential reactions. Treatment of 6 in the presence of
Pd2(dba)3·CHCl3 and L1 in toluene at room temperature for
30 min, followed by the successive addition of [IrCl(cod)]2, (S,S)-
Scheme 7 Proposed catalytic cycle.
QuinoxP* (6 mol%), and NaBArF to the reaction mixture, gave
experiments, intramolecular allylic substitution was examined
by use of a readily available allyl carbonate 6 to find out an
efficient Pd catalyst giving the corresponding allyl ether 1b
(Table 2). Treatment of 6 with Pd2(dba)3·CHCl3 (dba = dibenzyli-
deneacetone) in toluene at 40 °C for 3 h gave ether 1b in 25%
yield (entry 1). The reaction of 6 in the presence of binap gave
1b in 43% yield (entry 2). Dppf [1,1′-bis(diphenylphosphino)
ferrocene] and ligand L1 25 were found to efficiently promote
4
cyclized product 2b in 64% yield with 97% ee after heating at
100 °C for 16 h (Scheme 8c).
The present one-pot reaction system can be applied to inter-
molecular reaction achieving formal intermolecular annulation
between
m-hydroxyacetophenone
and
allyl
carbonate
(Scheme 9). Thus, treatment of m-hydroxyacetophenone (7a)
with 1.1 equiv. of tert-butyl cinnamyl carbonate (8a) in the pres-
ence of the Pd catalyst in toluene at room temperature for 1 h,
and then sequential addition of the iridium catalyst gave, after
heating, dihydrobenzofuran 2b in 80% yield, whose ee was 98%.
Table 2 Pd-catalyzed allylic substitution of allyl carbonate 6 a
Entry
Pd complex
Ligand
Yieldb (%)
1
2
3
4
5
Pd2(dba)3·CHCl3
Pd2(dba)3·CHCl3
Pd2(dba)3·CHCl3
Pd2(dba)3·CHCl3
Pd(OAc)2
None
(rac)-Binap
Dppf
L1
25
43
81
86
32
L1
a Reaction conditions: 6 (0.10 mmol), Pd complex (5 mol% of Pd), and
ligand (6 mol%) in toluene (0.2 mL) at 40 °C for 3 h. b Determined by
1H NMR analysis using benzyl phenyl ether as an internal standard.
Scheme 8 Pd-catalyzed allylic substitution and Ir-catalyzed asym-
metric intramolecular hydroarylation.
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As shown in Scheme 10, benzophenones with electron-
donating and -withdrawing groups were all good substrates for
the formal intermolecular annulation by two catalytic systems
to give dihydrobenzofuran 2c–f in high yields with high
enantioselectivity. In addition to benzophenones, butyl ketone
also participated in the reaction to give 2j with 98% ee.
m-Hydroxyacetophenones substituted with methoxy and
bromo groups at the aromatic rings also reacted with the car-
bonate 8a smoothly to give dihydrobenzofurans (2k, 2x, and
2y). Substituted cinnamyl carbonates and an allylic carbonate
having a 2-furyl group can also be applied to the reaction with
m-hydroxyacetophenone (7a) to give dihydrobenzofurans 2o,
2q, 2t, and 2u in high yields with high enantioselectivity.
Scheme 9 One-pot synthesis of dihydrobenzofuran by dual catalysis.
Conclusions
In summary, we found that iridium/(S,S)-QuinoxP* complex
catalyzed intramolecular hydroarylation of m-cinnamyloxyphe-
nyl ketones to give 3-substituted dihydrobenzofurans by 5-exo-
cyclization in both high yields and enantioselectivity. A broad
range of m-cinnamyloxyphenyl ketones were applicable to the
present reaction. We also found that the presence of 4-methox-
ystyrene significantly improved the yield of the cyclized
product within a shorter reaction time. Furthermore, in terms
of synthetic utility, we developed the one-pot process to dihy-
drobenzofurans through Pd-catalyzed allylic substitution from
readily available starting materials and Ir-catalyzed intra-
molecular hydroarylation in a highly enantioselective manner.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by JSPS KAKENHI Grant No.
JP19H02721 and JP20J23499. K. S. thanks the JSPS for a
research Fellowship for Young Scientists.
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Scheme 10 Synthesis of dihydrobenzofurans via Pd-catalyzed inter-
molecular allylic substitution and Ir-catalyzed asymmetric intra-
molecular hydroarylation. Reaction conditions: 7 (0.10 mmol), 8 (1.1
equiv.), Pd2(dba)3·CHCl3 (1 mol% of Pd), and L1 (1.2 mol%) in toluene
(0.2 mL) at 40 °C for 1 h and sequential addition of [IrCl(cod)]2 (5 mol%
of Ir), (S,S)-QuinoxP* (6 mol%), and NaBArF (10 mol%) at 100 °C for
4
18 h. Isolated yields are shown. a Initial allylic substitution was performed
at 60 °C. b 10 mol% of Ir, 12 mol% (S,S)-QuinoxP*, and 20 mol% of
NaBArF4 were used. c (S)-Difluorphos was used instead of (S,S)-QuinoxP*.
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