Cationic rhodium(I)-catalyzed regioselective tandem heterocyclization/[3+2] cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with alkynes

Article abstract of DOI:10.1002/chem.201103924

Rh^I in two minds: A Rh^I-catalyzed tandem heterocyclization/[3+2] cycloaddition reaction was developed that provides rapid, efficient, and stereoselective access to highly substituted cyclopenta[c]furans from readily available 2-(1-alkynyl)-2-alken-1-ones and alkynes (see scheme). The cationic Rh^I acts as both a Lewis acid and a conventional transition-metal catalyst, providing the first example of a Rh^I species acting as a Lewis acid. Copyright

Full text of DOI:10.1002/chem.201103924

COMMUNICATION
DOI: 10.1002/chem.201103924
Cationic Rhodium(I)-Catalyzed Regioselective Tandem
Heterocyclization/ACHTNUTGRNEUNG[3+2] Cycloaddition of 2-(1-Alkynyl)-2-alken-1-ones with
Alkynes
Hongyin Gao[a] and Junliang Zhang*[a, b]
Organic synthesis has played a major role in the advance-
ment of modern science and technology by enabling the
preparation of natural and designed targets of chemical, bio-
logical, medicinal, environmental, and material impor-
tance.[1] The extensive application of transition-metal cata-
lysts to organic synthesis over the last 40 years has dramati-
cally changed the manner in which organic compounds are
prepared. Among the many transition-metal catalysts used
in organic synthesis, rhodium has played an increasingly im-
portant role because of its wide synthetic utility in carbon–
carbon bond-formation reactions.[2] Rhodium-catalyzed cyc-
lization/cycloaddition reactions, in particular, have proven to
be unrivaled in their ability to construct polycyclic carbo-
and heterocycles.[3] Many research groups have achieved im-
pressive progress in this area. For example, the groups of
Lautens,[4] Nakamura,[5] MascareÇas,[6] and Motherwell[7]
have developed efficient routes to five-membered rings by
use of the intramolecular [3+2] cycloaddition of methylene-
cyclopropanes (MCPs) with various unsaturated compo-
nents. The groups of Wender,[8] Trost,[9] and Yu[10] have also
completed significant work on the rhodium-catalyzed cycli-
zation/cycloaddition of vinylcyclopropanes (VCPs). Evans
and co-workers have described a series of rhodium-cata-
lyzed [2+2+2] and [4+2+2] cycloadditions of 1,6-enynes
with unsaturated compounds.[11] Recently, Tanaka and co-
workers demonstrated an elegant system for enantioselec-
tive [2+2+2] cycloadditions.[12]
alken-1-ones with several nucleophilic components,[13,14] such
as a,b-unsaturated imines, 1,3-diphenylisobenzofuran, and
(E)-1-methyl-3-styryl-1H-indole. These reactions are be-
lieved to proceed via furanyl metal intermediates
A
(Scheme 1, top). It is noteworthy that the cycloaddition
Scheme 1. Previous and current work (LA=Lewis acid, TM=transition
metal).
partners in these reactions are limited to electron-rich unsa-
turated compounds as the dipolarophile and electron-defi-
cient alkynes cannot be introduced. As part of an ongoing
program to develop efficient synthetic approaches to poly-
cyclic furan scaffolds,[15] we turned to a search of some
simple 2C components, such as alkynes, that could react
with 2-(1-alkynyl)-2-alken-1-ones by transition-metal-cata-
lyzed [3+2] cycloaddition reactions (Scheme 1).
We have previously reported a series of efficient and con-
venient routes to fused heterobicyclic compounds by the
transition-metal-catalyzed (i.e, AuI) tandem heterocycliza-
tion/cycloaddition of electron-deficient 2-(1-alkynyl)-2-
Herein, we report a novel cationic rhodium-catalyzed,
[a] H. Gao, Prof. Dr. J. Zhang
highly regioselective tandem heterocyclizaion/ACTHNGUTER[NNUG 3+2] cycload-
Shanghai Key Laboratory of Green Chemistry and
Chemical Processes, Department of Chemistry
East China Normal University
3663 N. Zhongshan Road, Shanghai 200062 (P.R. China)
Fax : (+86)21-6223-5039
dition of electron-deficient 2-(1-alkynyl)-2-alken-1-ones with
electron-rich or electron-deficient alkynes, furnishing highly
substituted cyclopenta[c]furans in good yields with excellent
regioselectivity. To the best of our knowledge, this is the
first example of electron-deficient alkynes as the cycloaddi-
tion component in the transition-metal catalyzed domino re-
action[16] of (1-alkynyl)-2-alken-1-ones, which represent a
novel noncyclopropane 3C component in rhodium chemis-
try. The 3C component in rhodium-catalyzed carbocycliza-
tion reactions is heavily dominated by cyclopropanes and cy-
clopropenes.[17] Moreover, in these reactions the cationic RhI
E-mail: jlzhang@chem.ecnu.edu.cn
[b] Prof. Dr. J. Zhang
State key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103924. It contains prepara-
tive procedures and spectroscopic data for all new compounds.
Chem. Eur. J. 2012, 18, 2777 – 2782
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2777

complex plays a dual role, that is, as a Lewis acid and a tran-
sition metal, which is the key to realizing this transforma-
tion. Recently, Doyle and co-workers have published a semi-
nal contribution to the application of RhII complexes as
Lewis acids in organic synthesis.[18] To the best of our knowl-
edge, RhI complexes acting in a dual role have not been re-
ported so far.
(acac=acetylacetone), and [{RhCl(CO)2}2] could not facili-
tate this transformation (Table 1, entries 2–6). We realized
that increasing the Lewis acidity of the rhodium complex
may be important to achieve the heterocyclization as we
found in the palladium catalyzed case.[14a]
Indeed, the complex [{RhSbF
6ACHTNUGTREN(NNUG cod)}2] (generated in situ
from a 1:2 mixture of [{RhCl(cod)}2] and AgSbF6) was
AHCTUNGTRENNUNG
We began our research by examining the cyclization of
ketone 1a and ethyl 3-phenylpropiolate 3a in the presence
of Ph3PAuOTf (5 mol%; generated in situ from a 1:1 mix-
ture of Ph3PAuCl and AgOTf; OTf=triflate) at room tem-
perature. Unfortunately, we obtained a complex mixture
from this reaction (Table 1, entry 1). From this, we reasoned
found to catalyze this reaction at 608C in 1,2-dichloroethane
(DCE), under a carbon monoxide atmosphere, to give the
desired product in 40% yield (Table 1, entry 7). Decreasing
the pressure of carbon monoxide dramatically reduced the
yield of the desired product (Table 1, entries 8 and 9). Other
solvents, such as CH2Cl2, toluene, 1,4-dioxane, and 1,2-dime-
thoxyethane (DME), did not improve the yield
(Table 1, entries 10–13). We next screened different
silver salts in combination with [{RhClACTHNURTGNENG(U cod)}2], and
Table 1. Optimization of the tandem [3+2] cyclization of ketone 1a and alkyne 3a.[a]
found that there was no improvement in the yield if
AgOTf or AgBF4 were employed as the silver salt
(Table 1, entries 14 and 15). The combination of
[{RhClACTHNUGTRNEUNG(cod)}2] with AgPF6 or AgOMs (OMs=me-
sylate) was ineffective for this transformation
(Table 1, entries 16 and 17). The combination of
[{RhCl(CO)2}2] with AgSbF6 (1:2) gave a better
result of 60% yield with a minor amount of an
isomer also being formed (Table 1, entry 18).[22]
After further investigation, a higher yield of the
product could be obtained if the temperature was
increased to 808C (Table 1, entry 19). Eventually,
the combination of [{RhCl(CO)2}2] and AgSbF6 in
the presence of molecular sieves under an atmos-
phere of carbon monoxide at 808C in DCE was
found to give an 89% yield of the product without
any of the isomer being formed (Table 1, entry 20).
The presence of molecular sieves may restrict the
isomerization of the final product.
Conditions[b]
T
[8C]
t
Yield of 4aa
[h]
[%][c]
1[d]
2
3
4
5
Ph3PAuCl/AgOTf
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH2Cl2
toluene
dioxane
DME
DCE
DCE
DCE
DCE
DCE
DCE
DCE
RT
60
60
60
60
60
60
60
60
40
60
60
60
60
60
60
60
60
80
80
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
complex mixture
n.r.
n.r.
n.r.
n.r.
n.r.
40
34
complex mixture
18
trace
trace
trace
38
A
U
A
U
A
U
A
U
6
N
7
A
U
8[e]
A
U
9[d]
A
U
10
A
U
11
12
13
14
15
16
17
18
19
20
A
U
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
U
21
A
U
n.r.
n.r.
60
78
89
With the optimized reaction conditions in hand,
our attention turned to the scope of the rhodium-
catalyzed tandem [3+2] cycloaddition of various
A
ACHTUNGTRENNUNG
N
1
2
R
=
3
2
electron-deficient eneynes
1
with alkyne 3a
A
=
3
(Table 2). Both electron-rich and electron-deficient
aromatic groups could be introduced onto the
alkyne (substituent R3, Table 2, entries 1–5). It
seems that alkyne substituent R3 needs to be an ar-
omatic group to obtain a reasonable product yield.
The olefin substituent (R2) can also be both elec-
[a] All reactions were carried out with 1a (0.3 mmol) and 3a (0.9 mmol); MS=molec-
ular sieves. [b] Under an atmosphere of CO (balloon). [c] Yield of the isolated prod-
uct. [d] Under an atmosphere of N2 (balloon). [e] Under an atmosphere of CO/N2
(balloon, CO/N2 =1:4).
that frequently used Lewis acids were not applicable for this
system, and a novel catalytic pattern was highly desired.
We envisaged that ketone 1 might react with rhodium to
generate furanyl-fused cyclorhodium intermediate B, fol-
lowed by regioselective insertion of the electron-deficient
alkyne 3 into the alkyl–rhodium or vinyl–rhodium bond.
This could then be followed by reductive elimination to
afford 4H-cyclopenta[c]furan 4 (Scheme 1, bottom). There-
fore, we turned our attention to a series of rhodium cata-
lysts. Surprisingly, several commonly used rhodium catalysts,
tron-rich and electron-deficient aromatic groups (Table 2,
entries 6–9). It is noteworthy that a halogen atom on the ar-
omatic ring stays untouched under the reaction conditions
(Table 2, entry 6). The ketone substituent (R1), which can be
either an aliphatic or aromatic group, has little impact on
the yield of this transformation (Table 2, compare entries 1
and 10).
After the investigation of the scope with respect to
eneynes 1, we examined the scope toward the alkyne com-
ponent of this rhodium-catalyzed transformation (Table 3).
The ester substituent on the alkyne component has little
impact on the yield of the transformation (compare Table 2,
including
[{RhCl
G
(cod=1,5-cyclooctadiene),
(PPh3)3], [Rh(cod)(acac)]
[RhCl(CO)
G
E
N
ACHTUNGTRENNUNG
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Chem. Eur. J. 2012, 18, 2777 – 2782

Regioselective Tandem Heterocyclization/ACTHNUTRGNEU[GN 3+2] Cycloaddition Reaction
COMMUNICATION
Table 2. Tandem [3+2] cyclization of various ketones
3-phenylpropiolate 3a.
1 and ethyl
single-crystal X-ray diffraction of compound 4dc, which is
the cycloadduct of ketone 1d and electron-deficient alkyne
3c [Eq. (2)].[19] To our delight, aromatic alkynyl aldehyde
and ketone substrates can also be efficiently introduced into
the catalytic system [Eq. (3)]. These results dramatically
broaden the scope of this rhodium-catalyzed cascade reac-
tion.
R1
R2
R3
t
Product Yield
[%][a]
[h]
2
1
2
3
4
Me Ph
Me Ph
Me Ph
Me Ph
Ph
4-MeC6H4
4-OMeC6H4 1c
4-NO2C6H4
1-naphthyl
Ph
1a
1b
=
4aa
4ba
4ca
4da
4ea
4 fa
4ga
4ha
4ia
89
66
72
75
80
71
89
97
77
85
3
2
=
3
2
=
3
2
1d
1e
1 f
1g
1h
=
3
2
5[b] Me Ph
=
3
2
6
7
8
9
10
Me 4-ClC6H4
Me 4-OMeC6H4 Ph
Me 4-OMeC6H4 1-naphthyl
Me 4-OMeC6H4 4-OMeC6H4 1i
Ph Ph Ph 1j
=
3
2
=
3
2
2
2
=
4ja
3
[a] Isolated yield. [b] 10 mol% of the catalyst was employed.
Table 3. Tandem [3+2] cyclization of ketone 1a and various electron-
deficient alkynes 3.
R7
R6
t
4
Yield
[%][a]
[h]
2
1
2
3
CO2Me
CO2Me
CO2Me
CO2Me
Ph
Ph
3b
3c
3d
3e
3 f
3g
=
4ab
4ac
4ad
4ae
4af
89
78
59
81
78
79
3
4-MeC6H4
4-MeOC6H4
1-cyclohexenyl
Ph
2
2
2
4[b]
5
=
A proposed mechanism that accounts for this rhodium-
catalyzed cascade [3+2] cycloaddition is depicted in
Scheme 2. There is a competition between formation of het-
erocoordination compound IA (rhodium center coordinated
with ketone 1 and alkyne 3) and formation of the homo-
coordination compound (rhodium center coordinated with
3
2
=
3
2
6
4-MeC6H4
4-MeC6H4
=
4ag
3
[a] Isolated yield. [b] 10 mol% of the catalyst was employed.
entry 1 with Table 3, entry 1). Both aliphatic and electron-
rich aromatic groups could be introduced as the alkyne sub-
stituent R6 (Table 3, entries 2–4), but the electron-rich aryl-
substituted one gives a relatively lower yield. To our delight,
electron-rich alkynes 3 f and 3g can also react with ketone
1a under the catalysis of the rhodium catalyst to give [3+2]
cycloadducts 4af and 4ag in 78 and 79% yield, respectively
(Table 3, entries 5 and 6).
To our surprise, none of the desired product was detected
when more electron-rich alkyne 3h was employed as the
alkyne component under the standard conditions [Eq. (1)].
We envisaged that 3h may hinder the coordination of the
rhodium catalyst to the electron-deficient alkyne moiety in
1a. To prove this hypothesis, substrate 1i, with a slightly
more electron-rich alkyne was prepared and subjected to
the reaction conditions with alkyne 3h. This reaction did
indeed occur very well to give the corresponding product
4ih [Eq (1)]. The structure of the product of this rhodium-
catalyzed [3+2] cycloaddition was confirmed by use of
Scheme 2. Proposed mechanism (EWG=electron-withdrawing group).
Chem. Eur. J. 2012, 18, 2777 – 2782
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2779

J. Zhang and H. Gao
two molecules of alkyne 3, not shown). Under appropriate
conditions, the metal center may simultaneously coordinate
with the alkyne moiety of substrate 1 and alkyne 3. This
would be followed by intramolecular nucleophilic attack by
the oxygen atom of ketone 1 on the cationic RhI-activated
alkyne to give furanyl–rhodium carbocation IB. In this step,
the cationic RhI acts as a Lewis acid, whereas it will later
play the conventional role of a transition metal. The oxida-
tive addition of intermediate IB would afford a furanyl-
fused cyclorhodium intermediate IC. This may be followed
by regioselective insertion of the alkyne to afford interme-
diate ID or IE. Subsequent reductive elimination would pro-
duce the final product 4 and regenerate the rhodium cata-
lyst. Otherwise, strongly electron-donating substituents on
the alkyne, such as in alkyne 3h, prefer homocoordination
under the catalysis of rhodium, which will shut down the re-
action. An alternative reaction pathway involving the direct
oxidative cyclometalation of IA leading to IC cannot be
ruled out by the present data. In this reaction, the cationic
RhI complex not only acts as a traditional transition metal
(for oxidative addition and insertion), but also as a Lewis
acid (for heterocyclization). The facts that phosphine ligands
stop the reaction and the replacement of the counteranion
of the RhI complex with a weaker one facilitates the reac-
tion both support this hypothesis.
Intermolecular competition experiments with differently
substituted alkynes 3c and 3g gave the corresponding prod-
ucts 4ac and 4ag in 48 and 37% yield, respectively, indicat-
ing that the electronic nature of the alkyne has a little
impact on the reaction rate [Eq. (4)]. Moreover, no [3+2]
cycloadducts were detected in the presence of methanol
under the standard reaction conditions [Eqs. (5), (6), and
(7)]. These experiments indicate that furanyl–rhodium car-
bocation intermediate IB can be easily trapped in the pres-
ence of nucleophiles, such as methanol. However, the furan-
yl-fused cyclorhodium intermediate IC can be formed by
rapid oxidative addition of intermediate IB in the absence
of nucleophiles.
sorption bands of these products appear in the region of 270
to 370 nm, depending on the electron-donating or electron-
withdrawing ability of the substituents. In CH2Cl2, these
compounds exhibit fluorescence in the range 430 to 490 nm.
It is noteworthy that the substituents R6 and R7 on the prod-
uct have a large impact on the fluorescence efficiency (4aa
vs. 4af, 4aa vs. 4ca). Furthermore, the structural change
from 4aa to 4ba and 4ca resulted in a redshift in the fluo-
rescence spectra and an increase in the quantum yield.
In conclusion, we have developed a novel rhodium-cata-
lyzed domino heterocyclization and formal [3+2] cycloaddi-
tion reaction of readily available 2-(1-alkynyl)-2-alken-1-
ones and alkynes, leading to highly substituted 4H-cyclopen-
ta[c]furans in good yields and with excellent regioselectivity,
in which not only electron-deficient, but also electron-rich
alkynes can be applied and 2-(1-alkynyl)-2-alken-1-ones act
as nonpropane 3C components. Mild reaction conditions,
ease of operation, high yields, high regioselectivities, and
wide functional-group tolerance are some of the merits of
this highly efficient protocol. It is noteworthy that the dual
role of the cationic RhI complex is key to realizing this
transformation.
Owing to the broad substrate diversity of the rhodium-
catalyzed cascade reaction, we were intensely interested in
the potential of the polyheterocycle products as electroac-
tive materials. We therefore investigated the photophysical
properties of the polyheterocycle products and the results
are summarized in the Supporting Information.[20,21] The ab-
Experimental Section
Typical procedure for 4H-cyclopenta[c]furan synthesis: AgSbF6 (10.3 mg,
0.03 mmol) was added to
a solution of [{RhCl(CO)2}2] (5.8 mg,
0.015 mmol) in 1,2-dichlorethane (DCE; 2 mL) under a CO (1.0 atm) at-
mosphere, and the mixture was stirred for 10 min at room temperature.
This mixture was then added to a suspension of ketone 1a (73.8 mg,
0.3 mmol), alkyne 3a (156.6 mg, 0.9 mmol), and 4 ꢁ molecular sieves
(150 mg) in DCE (4 mL) under a CO (1.0 atm) atmosphere. After being
stirred for 40 min at 808C, 1a had been completely consumed, as deter-
mined by TLC analysis. The mixture was passed through a short silica gel
2780
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Chem. Eur. J. 2012, 18, 2777 – 2782

Regioselective Tandem Heterocyclization/ACTHNUTRGNEU[GN 3+2] Cycloaddition Reaction
COMMUNICATION
column and then concentrated under reduced pressure. The residue was
purified by flash column chromatography on silica gel to give pure prod-
uct 4aa (112.4 mg, 89%) as a colorless oil. 1H NMR (400 MHz, CDCl3):
d=7.40–7.32 (m, 5H), 7.31–7.23 (m, 4H), 7.22–7.15 (m, 1H), 7.10–6.98
(m, 3H), 6.97–6.90 (m, 2H), 4.98 (s, 1H), 4.05–3.90 (m, 1H), 3.89–3.80
(m, 1H), 2.09 (s, 3H), 0.85 ppm (t, J=6.8 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d=164.3, 147.8, 143.8, 142.7, 139.5, 138.9, 135.7, 131.5, 130.2,
130.1, 128.4, 128.3, 128.1, 128.0, 127.8, 127.7, 127.0, 126.7, 125.9, 59.8,
48.2, 13.5, 12.1 ppm; MS (EI): m/z (%): 420 [M+] (22.65), 347 (100);
HRMS: m/z calcd for C29H24O3: 420.1725; found: 420.1727.
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Acknowledgements
Financial support from the NSFC (20972054), the Ministry of Education
of China, the 973 Program (2011CB808600), and the PhD Program
Scholarship Fund of ECNU (2010032) is greatly appreciated.
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Keywords: cyclization · domino reactions · Lewis acids ·
regioselectivity · rhodium · transition metals
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[22] We assigned the minor isomer to be 4aa’. Compound 4aa was treat-
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MS, affording a 5:1 mixture of 4aa and 4aa’, as determined by
1H NMR spectroscopy of the crude mixture.
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Received: December 14, 2011
Published online: February 9, 2012
2782
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Chem. Eur. J. 2012, 18, 2777 – 2782