Gold(I)-catalyzed enantioselective synthesis of functionalized indenes

Article abstract of DOI:10.1002/anie.201001089

Chemical Equation Presented Can you dig it? An asymmetric synthesis of functionalized 1H-indenes from easily available ortho-(alkynyl)styrene derivatives under mild reaction conditions has been achieved. The reactions proceed through an unprecedented and selective 5-endo-dig gold(l)-catalyzed cycloisomerization or alkoxycyclization, if water or an alcohol is present (see scheme).

Full text of DOI:10.1002/anie.201001089

Angewandte
Chemie
DOI: 10.1002/anie.201001089
Chiral indenes
Gold(I)-Catalyzed Enantioselective Synthesis of Functionalized
Indenes**
Alberto Martꢀnez, Patricia Garcꢀa-Garcꢀa, Manuel A. Fernꢁndez-Rodrꢀguez, Fꢂlix Rodrꢀguez,
and Roberto Sanz*
Molecules containing the 1H-indene scaffold show a wide
range of biological activities,[1] and possess great interest as
functional materials[2] as well as precursors of metallocene
complexes for catalytic polymerization processes.[3] As a
result, several methods have been developed for their syn-
thesis.[4] Despite the unquestionable interest of optically
active indenes bearing a stereogenic center at C1, as far as
we know, only two closely related strategies for the enantio-
selective synthesis of these compounds from achiral sub-
strates have been published.[5–8] Both strategies are based on
the use of boronic acid derivatives as starting materials and
dicationic PdII complexes as the catalyst. Thus, enantioen-
riched 1-arylindenes have been obtained in a cascade 1,4-
addition-aldol condensation process,[5] whereas 1H-indenes
bearing a CH2COR group at the C1 position are formed from
ortho-boronate substituted cinnamic ketones and internal
alkynes.[6,7] The scarcity of general methods for the synthesis
of optically active indenes (in particular from achiral sub-
strates) motivated us to initiate a project in this field. Our
premise was the use of easily available starting materials and,
therefore, we fixed our attention on the catalytic cyclization
of ortho-(alkynyl)styrene derivatives. In this context, it should
be taken into consideration that the skeletal rearrangement of
ortho-(alkynyl)styrenes catalyzed by several metallic com-
plexes has been described to afford naphthalene derivatives
through a 6-endo cyclization process [Scheme 1, Eq. (1)].[9–13]
However, a careful examination of all these publications
showed that reactions with o-(alkynyl)styrenes where the
terminal carbon atom of the alkene was disubstituted were
Scheme 1. Skeletal rearrangement of ortho-(alkynyl)styrenes. Previous
work and proposed pathway.
not reported. So, we envisaged that o-(alkynyl)styrenes
possessing a highly substituted alkene moiety and an internal
acetylene could favor the 5-endo reaction pathway—owing to
better stabilization of the exocyclic carbocationic intermedi-
ate—to form the desired indene skeleton with a stereogenic
center at C1 [Scheme 1, Eq. (2)].[14] Herein we report our
results on this unprecedented metal-catalyzed 5-endo-dig
cyclization of o-(alkynyl)styrenes and the application of this
reaction in the synthesis of enantiomerically enriched
indenes.
For the initial experiments we selected 2’,2’-dimethyl o-
(phenylethynyl)styrene 1a as a model substrate (Scheme 2).
As a result of their excellent ability to activate alkynes,[15]
several complexes derived from coinage metals and platinum
were tested as catalysts for the desired transformation.
However, no reaction was observed with metal complexes
such us AgSbF6, PtCl2, [PtCl2(cod)], CuI, AuCl3, AuCl, or
[AuCl(Ph3P)] (cod = cycloocta-l,5-diene). Encouragingly, the
reaction proceeded to completion within 30 minutes to yield
the indenyl derivative 2a in a high yield (88% of isolated
product), when it was performed in CH2Cl2 at room temper-
ature in the presence of the cationic gold(I) complex
generated in situ from 5 mol% of [AuCl(Ph3P)] and
[*] A. Martꢀnez, Dr. P. Garcꢀa-Garcꢀa, Dr. M. A. Fernꢁndez-Rodrꢀguez,
Dr. R. Sanz
ꢂrea de Quꢀmica Orgꢁnica, Departamento de Quꢀmica
Facultad de Ciencias, Universidad de Burgos
Pza. Misael Baꢃuelos s/n, 09001-Burgos (Spain)
Fax: (+34)947-258-831
E-mail: rsd@ubu.es
Homepage: http://www.ubu.es/paginas/grupos_investigacion/
cien_biotec/sintorg/uk/index.htm
Dr. F. Rodrꢀguez
Instituto Universitario de Quꢀmica Organometꢁlica
“Enrique Moles”, Universidad de Oviedo
C/Juliꢁn Claverꢀa, 8, 33006, Oviedo (Spain)
[**] We are grateful to the MEC/FEDER (CTQ2007-61436/BQU, FPU
grant to A.M., Ramꢄn y Cajal and Juan de la Cierva contracts to
M.A.F.-R. and P.G.-G.) and Junta de Castilla y Leꢄn (BU021A09) for
financial support. We also thank Dr. A. Mendoza (Universidad de
Oviedo) for his assistance with the X-ray crystallographic analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001089.
Scheme 2. Initial experiments and proof of concept.
Angew. Chem. Int. Ed. 2010, 49, 4633 –4637
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Communications
5 mol% of AgSbF6.[16] Moreover, when this reaction was
Once we had demonstrated the feasibility of our method
for the preparation of indene derivatives through a gold-
catalyzed 5-endo-dig cyclization,[18] we turned to our original
goal, the control of the stereogenic center at C1. In this
context it should be noted that despite the tremendous
activity in the use of gold in homogeneous catalysis, asym-
metric gold-catalyzed reactions are still scarce.[19] Most of
these stereoselective processes are related to the enantiose-
lective p-activation of allenes[20] and very few examples have
been reported about asymmetric gold-catalyzed processes
involving alkyne activation.[21] To the best of our knowledge,
no examples have been reported on the enantioselective
cycloisomerization or alkoxycyclization of o-(alkynyl)styr-
enes.[22]
conducted in the presence of five equivalents of methanol we
observed the formation of compound 3aa in excellent yield
(90%; Scheme 2). The high selectivity of these reactions
should be noted as we did not observe the formation of
naphthalene derivatives coming from a 6-endo-dig type
cyclization or any other product coming from 5-exo additions
of the alkene to the triple bond.
A catalytic cycle that explains the formation of indenes 2a
and 3aa is shown in Scheme 3.[17] The reaction is initiated by
coordination of the cationic gold complex to the triple bond of
Taking into account the relative success with the use of
chiral biphosphines with biphenyl skeletons as ligands in gold-
catalyzed enantioselective reactions, we prepared several
dinuclear chiral gold(I) catalysts with (R)-binap (L1), (S)-H8-
binap (L2), (S)-segphos (L3), (S)-3,5-xylyl-segphos (L4), (S)-
DTBM-segphos (L5), MeO-biphep (L6), (S)-3,5-xylyl-MeO-
biphep (L7), and (S)-DTBM-MeO-biphep (L8) as ligands
(Scheme 5), according to known procedures.[21a]
Scheme 3. Proposed mechanisms for the synthesis of indenes.
the starting o-(alkynyl)styrene 1a to give intermediate 4.
Intramolecular addition of the alkene moiety selectively leads
to the cationic intermediate 5, which can be represented as
the two resonance structures 5a and 5b, through a 5-endo-dig
cyclization as we anticipated. Elimination of a proton in 5
(path a) furnishes the vinyl gold intermediate 6, which after a
protodemetalation reaction gives the indene 2a. Alterna-
tively, in the presence of methanol, trapping of the carbocat-
ion 5a or a nucleophilic attack on 5b (path b) accounts for the
formation of vinyl gold intermediate 7. Further protodeme-
talation furnishes compound 3aa and thus regenerating the
catalytic species.
To support the proposed mechanism we performed the
labeling experiment shown in Scheme 4. Thus, when 1a was
treated in the presence of five equivalents of D2O under the
catalytic conditions previously described we observed the
exclusive formation of the deuterated compound [D]-3ab in
89% yield (92% deuterium incorporation at C3). Interest-
ingly, this experiment also served to demonstrate that water
could be used as a nucleophilic partner in this new reaction.
Scheme 5. Chiral ligands (L1–L8) screened in the gold(I)-catalyzed
enantioselective cycloisomerization of o-(alkynyl)styrenes 1.
Initial efforts were focused on selecting an efficient chiral
catalyst for the transformation of o-(alkynyl)styrene 1a into
1-alkenyl-1H-indene 2a (Table 1). Pleasingly, all chiral gold
complexes tested associated with the silver salt AgSbF6
allowed complete conversions to 2a in one hour at room
temperature (Table 1, entries 1–8). The best result with
respect to the ee value was obtained using the gold complex
bearing the ligand L7 (Table 1, entry 7). So, further optimi-
zation was performed with this complex. The influence of the
silver salt was then investigated (Table 1, entries 7, 9, and 10),
and silver tosylate gave the best result. Finally, by lowering
the temperature to À308C we obtained the indene 2a with
82% ee in a reasonable reaction time (Table 1, entries 10–13).
At lower temperature only a slight improvement in the
enantioselectivity was observed, while the reaction became
sluggish (Table 1, entry 14).
Under the optimized catalytic conditions, [L7(AuCl)2]
associated with the silver salt AgOTs in CH2Cl2, we examined
the scope of this enantioselective reaction (Table 2). As
shown, the reaction is tolerant towards a variety of o-
(alkynyl)styrenes 1 bearing different substituents at the
aromatic ring (R1, R2), at the alkene terminal carbon atom
(R3, R4), and at the alkyne moiety (R5). High yields and
enantioselectivities were observed for starting materials 1a–f
Scheme 4. Labeling experiment.
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Table 1: Optimization of the reaction conditions for the asymmetric
synthesis of indene 2a.[a]
Table 3: Gold(I)-catalyzed enantioselective synthesis of oxygen-function-
alized 1H-indenes 3.[a]
Entry
L*/AgX
x [mol%]
T [8C]
t [h]
ee [%][b]
Entry
1
R1 R2 R3 R4 R5
R6
3
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
L1/AgSbF6
L2/AgSbF6
L3/AgSbF6
L4/AgSbF6
L5/AgSbF6
L6/AgSbF6
L7/AgSbF6
L8/AgSbF6
L7/AgOTf
L7/AgOTs
L7/AgOTs
L7/AgOTs
L7/AgOTs
L7/AgOTs
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5
25
25
25
25
25
25
25
25
25
25
0
À20
À30
À40
1
1
1
1
1
1
1
1
3
À10
35
24
40
24
36
41
29
50
60
70
76
82
85[c]
1
2
1a
1a
1a
1a
1a
1b
1b
H
H
H
H
H
H
H
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
Me Ph
Me Ph
Me Ph
Me Ph
Me Ph
Me Ph
Me Ph
Me Ph
Me Ph
Me 3aa
H
Et
allyl 3ad
iPr 3ae
Me 3ba
99
93
88
94
72[e]
93
88
98
80
87
77
90
91
88
90
95
94
88(>98)
86
81
3ab
3ac
3[d]
4[d]
5[d]
6
80
92(98)
82(>98)
86
84(>98)
88(>98)
80
7
8
9
F
H
3bb
Me 3ca
3cb
Me 3da
3 db
9
1c -OCH2O-
1c -OCH2O-
1d
1d
1 f
10
11
12
13
14
6
H
24
48
80
120
10
11
12
13
14
15
16
17
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Br
Br
-(CH2)3- Ph
-(CH2)3- Ph
5
5
5
H
84
H
H
H
H
H
H
Me 3-Th Me 3 fa
Me 3-Th 3 fb
Me nBu Me 3ga
Me nBu
Me Ph
Me Ph
75(>98)
78(>98)
30
28
80(>98)
80(>98)
1 f
H
1g
1g
1h
1h
[a] Reactions conditions: 2’,2’-dimethyl o-(phenylethynyl)styrene 1a
(0.05 mmol) in CH2Cl2 (0.2 mL) until complete conversion. [b] Deter-
mined by HPLC on a chiral stationary phase using a Chiracel OJ column
(eluent: n-hexane/iPrOH (90:10), flow: 1 mL minÀ1. [c] 77% conversion
H
3gb
Me 3ha
3hb
H
1
as estimated by H NMR spectroscopy.
[a] Reactions conditions: o-(alkynyl)styrene derivative 1 (0.3 mmol),
nucleophile (30 equiv), X=OTs with ROH and X=SbF6 with H2O, in
CH2Cl2 (1.2 mL) at À308C for 2–4 days. [b] Yield of isolated product
based on starting material 1. [c] Determined by HPLC on a chiral
stationary phase, see the Supporting Information; ee value after
recrystallization in brackets. [d] Reaction conducted at À208C. [e] 12%
of 2a was also formed.
Table 2: Gold(I)-catalyzed enantioselective synthesis of 1-alkenyl-1H-
indenes 2.[a]
alkynes 1a–f,h in the presence of several alcohols (Table 3,
entries 1, 3–6, 8, 10, 12, and 16) or water (Table 3, entries 2, 7,
9, 11, 13, and 17). Primary and secondary alcohols, as well as
water, were successfully employed as nucleophiles in this
transformation. By using isopropanol as the nucleophile, the
isopropoxy derivative 3ae was obtained with the highest
ee value, though the competitive formation of 2a occurred to
a small extent. As expected, alkyl-substituted alkyne 1g led to
lower enantioselectivities (Table 3, entries 14 and 15). Grat-
ifyingly, oxygen-functionalized 1H-indenes 3 can be obtained
as a single enantiomer by recrystallization (Table 3, entries 1,
5, 6, 8, 9, 12, 13, 16, and 17). Moreover, the absolute
configuration of product 3ha was determined to be R by using
single-crystal X-ray analysis,[23] and the configuration of the
remaining products were assigned by analogy.
Entry
1
R1
R2
R3 R4
R5
2
Yield [%][b] ee [%][c]
1
1a
1b
H
H
H
F
H
H
H
Me Ph
Me Ph
Me Ph
2a
2b
2c
2d
2e
81
84
84
93
96
81
80
82
77
86
81
80(92)
68
20
2[d]
3[d]
4
1c -OCH2O-
1d
1e
1 f
1g
H
H
H
H
H
H
H
H
-(CH2)3-
-(CH2)4-
Ph
Ph
5
6
H
H
Me 3-Th 2 f
Me nBu 2g
7[d]
[a] Reactions conditions: o-(alkynyl)styrene derivative 1 (0.3 mmol) in
CH2Cl2 (0.6 mL) at À308C for 3–4 days. [b] Yield of isolated product
based on the starting material 1. [c] Determined by HPLC on a chiral
stationary phase, see the Supporting Information; ee value after
recrystallization in brackets. [d] Reaction conducted at À208C. 3-Th=
3-thienyl, Ts=4-toluenesulfonyl.
In conclusion, we have developed an asymmetric gold-
catalyzed cycloisomerization or alkoxycyclization of o-(alky-
nyl)styrenes that provides enantiomerically enriched func-
tionalized 1H-indene derivatives in high yields and with
ee values of up to 92%, which can be improved to > 98%
after a simple recrystallization. The combined catalytic
system, consisting of a gold complex with the atropisomeric
electron-rich ligand 3,5-xylyl-MeOBIPHEP (L7) and silver
salts, efficiently promotes this enantioselective cyclization
under mild reaction conditions. Notably, the reactions
reported here represent the first examples of metal-catalyzed
where R5 is an aromatic or heteroaromatic group (Table 2,
entries 1–6). However, for alkyl-substituted alkyne 1g a lower
enantioselectivity was observed (Table 2, entry 7). In addi-
tion, the possibility of increasing the enantiomeric excess
value of the final products by a simple recrystallization has
been demonstrated (Table 2, entry 5).
We have also examined the enantioselective alkoxycycli-
zation of o-(alkynyl)styrenes 1 (Table 3). Again, high yields
and enantioselectivities were observed for aryl-substituted
Angew. Chem. Int. Ed. 2010, 49, 4633 –4637
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Communications
T. M. Judge, R. B. Gammill, J. Org. Chem. 1999, 64, 1733 – 1737;
b) P. Dubꢂ, F. D. Toste, J. Am. Chem. Soc. 2006, 128, 12062 –
12063.
cyclizations of o-(alkynyl)styrenes through a 5-endo-dig
mechanism. o-(Alkynyl)styrene derivatives had been widely
used as precursors of naphthalene derivatives and this work
further expands the utility of these starting materials, thus
demonstrating their ability to act as simple precursors of
(enantiopure) indenes.
[9] Tungsten-catalyzed 6-endo cyclizations: a) K. Maeyama, N.
Iwasawa, J. Org. Chem. 1999, 64, 1344 – 1346; b) T. Miura, N.
Iwasawa, J. Am. Chem. Soc. 2002, 124, 518 – 519.
[10] Rhodium- and palladium-catalyzed 6-endo cyclizations: J. W.
Dankwardt, Tetrahedron Lett. 2001, 42, 5809 – 5812.
[11] Platinum-catalyzed 6-endo cyclizations: a) B. Martꢃn-Matute, C.
Nevado, D. J. Cꢀrdenas, A. M. Echavarren, J. Am. Chem. Soc.
2003, 125, 5757 – 5766; b) V. Mamane, P. Hannen, A. Fꢄrstner,
Chem. Eur. J. 2004, 10, 4556 – 4575. See also reference [10].
[12] Gold-catalyzed 6-endo cyclizations: T. Shibata, Y. Ueno, K.
Kanda, Synlett 2006, 411 – 414. See also reference [10]. A related
cycloisomerization of enyne arene–chromium complexes has
also been reported: C. Michou, S. Liu, S. Hiragushi, J. Uenishi,
M. Uemura, Synlett 2008, 1321 – 1324.
Experimental Section
General procedure for the gold(I)-catalyzed enantioselective syn-
thesis of 1H-indenes 2 and 3: AgSbF6 (10 mol%, 5.1 mg) or AgOTs
(10 mol%, 8.4 mg) was added to a solution of [L7(AuCl)2] (5 mol%,
17.4 mg) in dry CH2Cl2 and the mixture was stirred for 5–10 min and
cooled to À308C or À208C (see Table 2 and Table 3 for the suitable
Ag salt and temperature for each substrate). The nucleophile
(30 equiv, 9 mmol), when appropriate, was added, followed by a
solution of the corresponding o-(alkynyl)styrene derivative
1
[13] Ruthenium-catalyzed 6-endo cyclizations: H.-C. Shen, S. Pal, J.-
J. Lian, R.-S. Liu, J. Am. Chem. Soc. 2003, 125, 15762 – 15763.
[14] Liu and co-workers have reported the ruthenium-catalyzed
cycloisomerization of 2’,2’-disubstituted o-(ethynyl)styrenes to
afford, 2-alkenyl-1H-indenes as major adducts. The reaction is
proposed to occur through a cascade process initiated by 5-endo
cyclization of the initially formed ruthenium–vinylidene species
and subsequent “methylenecyclopropane–trimethyleneme-
thane” rearrangement: R. J. Madhushaw, C.-Y. Lo, C.-W.
Hwang, M.-D. Su, H.-C. Shen, S. Pal, I. R. Shaikh, R.-S. Liu, J.
Am. Chem. Soc. 2004, 126, 15560 – 15565. These results are in
sharp contrast with the total selectivity to give naphthalene
derivatives through a 6-endo cyclization reaction observed with
the same ruthenium catalyst when non disubstituted o-(alky-
nyl)styrenes are used as starting materials (see reference [13]).
[15] For excellent reviews on metal-catalyzed cycloisomerization
reactions of enyne derivatives, see: a) E. Jimꢂnez-Nfflæez, A. M.
Echavarren, Chem. Rev. 2008, 108, 3326 – 3350; b) V. Michelet,
P. Y. Toullec, J.-P. GenÞt, Angew. Chem. 2008, 120, 4338 – 4386;
Angew. Chem. Int. Ed. 2008, 47, 4268 – 4315.
[16] A similar result was obtained using 5 mol% of the bis(trifluor-
omethanesulfonyl)imidate derivative [AuNTf2(Ph3P)] (Tf = tri-
fluoromethanesulfonyl).
[17] For related mechanisms, see: a) M. R. Luzung, J. P. Markham,
F. D. Toste, J. Am. Chem. Soc. 2004, 126, 10858 – 10859; b) L.
Zhang, S. A. Kozmin, J. Am. Chem. Soc. 2005, 127, 6962 – 6963;
c) A. K. Buzas, F. M. Istrate, F. Gagosz, Angew. Chem. 2007, 119,
1159 – 1162; Angew. Chem. Int. Ed. 2007, 46, 1141 – 1144; d) Y.
Horino, T. Yamamoto, K. Ueda, S. Kuroda, F. D. Toste, J. Am.
Chem. Soc. 2009, 131, 2809 – 2811.
[18] The scope of this new transformation was briefly investigated
using different o-alkynyl styrenes 1a–h. See the Supporting
Information for the synthesis of racemic 1-alkenyl-1H-indenes
2a–g and 1-oxygen-functionalized-1H-indenes 3aa–3hb. In
addition, we also looked at the behavior of the corresponding
terminal alkyne, 2’,2’-dimethyl o-(ethynyl)styrene, though the
reaction was sluggish and gave decomposition products.
[19] For reviews, see: a) R. A. Widenhoefer, Chem. Eur. J. 2008, 14,
5382 – 5391; b) N. Bongers, N. Krause, Angew. Chem. 2008, 120,
2208 – 2211; Angew. Chem. Int. Ed. 2008, 47, 2178 – 2181.
[20] For recent examples, see: a) A. Z. Gonzꢀlez, F. D. Toste, Org.
Lett. 2010, 12, 200 – 203; b) R. L. LaLonde, Z. J. Wang, M. Mba,
A. D. Lackner, F. D. Toste, Angew. Chem. 2010, 122, 608 – 611;
Angew. Chem. Int. Ed. 2010, 49, 608 – 611, and references
therein.
(0.3 mmol) in dry CH2Cl2. The resulting reaction mixture was stirred
until complete consumption of starting material 1 (as evident by TLC
or GC-MS analysis). The mixture was diluted with hexanes and
filtered through a pad of silica gel, the solvent was removed and the
crude residue was purified by flash chromatography on silica gel using
mixtures of hexanes and EtOAc as eluents. The corresponding yields
and enantioselectivities of 1H-indenes 2 or 3 are reported in Table 2
and Table 3, respectively.
Received: February 22, 2010
Published online: May 17, 2010
À
Keywords: asymmetric catalysis · C C coupling · cyclization ·
gold · indenes
.
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[21] a) M. P. Muæoz, J. Adrio, J. C. Carretero, A. M. Echavarren,
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[8] For the synthesis of enantioenriched indene derivatives from
nonracemic starting materials, see: a) K. R. Romines, K. D.
Lovasz, S. A. Mizsak, J. K. Morris, E. P. Seest, F. Han, J. Tulinsky,
4636 www.angewandte.org
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Angew. Chem. Int. Ed. 2010, 49, 4633 –4637

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conceptually related platinum-catalyzed alcoxycyclization of
1,6-enynes.
1,5-enynes has been reported. In contrast, gold-catalyzed
enantioselective reactions of 1,6-enynes have been described
(see references [21a,c,d]).
[23] CCDC 766525 (3ha) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[22] ortho-(Alkynyl)styrenes could be considered in some way as 1,5-
enynes. As far as we know neither the gold-catalyzed enantio-
selective cycloisomerization of o-(alkynyl)styrenes neither of
Angew. Chem. Int. Ed. 2010, 49, 4633 –4637
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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