Silver(I) versus gold(I) catalysis in benzannulation reaction: A versatile access to acridines

Article abstract of DOI:10.1055/s-2008-1078178

A silver/gold-catalysed benzannulation reaction is described. In the presence of catalytic amounts of gold and/or silver salts, the reaction of silyl enol ethers onto alkynes occurs under mild conditions to produce the corresponding polycyclic aromatic systems (acridine, quinoline or naphthalene cores) in good to high yields. Among the catalysts investigated, AgOTf has been chosen as a general catalyst for this reaction which likely proceeds through silver(I) activation of the alkynyl moiety leading to a subsequent cycloisomerisation reaction. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1078178

LETTER
2513
Silver(I) versus Gold(I) Catalysis in Benzannulation Reaction: A Versatile
Access to Acridines
S
ilver-Catalyse
d
h
Benzannul
o
ationReactio
m
n
as Godet, Philippe Belmont*
Laboratoire de Synthèse et Méthodologie Organiques, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires (ICBMS), Uni-
versité Lyon 1, CNRS, UMR 5246, Bâtiment CPE, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France
Fax +33(4)72432963; E-mail: philippe.belmont@univ-lyon1.fr
Received 4 June 2008
Table 1 Transition-Metal-Catalysed Benzannulation Reactiona
Abstract: A silver/gold-catalysed benzannulation reaction is de-
OTBS
OTBS
scribed. In the presence of catalytic amounts of gold and/or silver
salts, the reaction of silyl enol ethers onto alkynes occurs under mild
conditions to produce the corresponding polycyclic aromatic sys-
tems (acridine, quinoline or naphthalene cores) in good to high
yields. Among the catalysts investigated, AgOTf has been chosen as
a general catalyst for this reaction which likely proceeds through
silver(I) activation of the alkynyl moiety leading to a subsequent cy-
cloisomerisation reaction.
cat. (5 mol%)
solvent
OMe
N
N
OMe
1
2
Entry Catalyst
Solvent
Temp Conversion
(°C)
(%)c
Key words: silver, gold, benzannulation, acridine, quinoline
b
1
2
3
4
5
6
7
[Rh(CO)2Cl]2
Toluene reflux 56
AuCl3
CH2Cl2
r.t.
0d
Polycyclic aromatic systems (PAS) made of fused aro-
matic rings such as naphthalene, anthracene, acridine and
quinoline are of particular interest due to their physical
and biological properties.1–8
Physical properties are typically illustrated among others1
by superconductivity2 and fluorescence.3 PAS have a
broad range of biological applications from dyes (ethidi-
um bromide)4 to therapeutics as anticancer agents (acri-
dine derivatives,5 carbazoles6), antibiotics (tetracycline or
quinolone derivatives)7 or antimalarial agents (quinoline
or acridine derivatives).8
AuCl3
C2H4Cl2 50
12d
AuCl(PPh3)
C2H4Cl2 reflux 0d
AuCl(PPh3)–AgSbF6
AgSbF6
C2H4Cl2 50
C2H4Cl2 50
C2H4Cl2 50
>95e
>95e
<5d
TFAf
a Concentration of the reaction mixture was 0.01 M.
b The amount of the catalyst used was 10 mol%.
c Conversion was determined by 1H NMR.
d Overnight reaction.
e Reaction time: 1 h.
Benzannulation has gained considerable interest since a
single step is required to create an aromatic moiety.
Benzannulation reactions9 are carried out through Brønst-
ed acid10 or Lewis acid11 catalysis, under anionic12 or rad-
ical conditions,13 with vinyl ketenes,14 transition metals
catalysis (Rh,15,16 Ru,15–17 Pt,15 Pd,15,18 Au,15,19 Co,20 In,21
Ga,22 Cu,23 Ni,16 Ag24), thermal conditions,25 under
pyrrolysis26 or using stoichiometric metallic systems
(Ag,15 Cr,27 W,28 Te,29 Yb,30 Ni31). Among transition met-
als, gold is highly valuable and efficient due to the wide
range of reaction outcomes.32 We thus first turned our at-
tention to gold-catalysed benzannulation reactions in or-
der to access acridine derivatives under smooth
conditions.
f TFA: trifluoroacetic acid.
uene but the introduction of up to 10 mol% of catalyst un-
der inert atmosphere was mandatory in most cases and
poor to fair yields ranging from 40% to 70% were ob-
tained. These first results led us to envision new catalytic
systems.
We chose quinoline derivative 1 as a model substrate. Ex-
posure to a catalytic amount (5%) of [Rh(CO)2Cl]2 pro-
moted its conversion to 2 in a 56% yield (Table 1, entry
1). Thereafter, we first turned our attention to gold species
such as AuCl3 and AuCl(PPh3).34 At room temperature
(Table 1 entry 2), gold(III) salt did not give the desired
product, whereas at 50 °C (Table 1, entry 3), a poor but,
nonetheless, encouraging conversion rate of 12% was ob-
tained. With gold(I), no better results were achieved since
no benzannulation took place either at room temperature
or in refluxing 1,2-dichloroethane (Table 1, entry 4). We
then turned our attention to activated gold(I) complexes.35
With AuSbF6(PPh3) [formed in situ from AuCl(PPh3) and
AgSbF6 in equimolar quantities], we obtained the desired
polycyclic system, as a single product and in high conver-
Previous work from our group33 reported the synthesis of
acridines in moderate yields via a rhodium(I)-catalysed
benzannulation reaction, inspired by an earlier report from
Dankwardt.15 Best results were obtained in refluxing tol-
SYNLETT 2008, No. 16, pp 2513–2517
x
x
.x
x
.2
0
0
8
Advanced online publication: 10.09.2008
DOI: 10.1055/s-2008-1078178; Art ID: G20008ST
© Georg Thieme Verlag Stuttgart · New York

2514
T. Godet, P. Belmont
LETTER
Table 2 Screening of Silver Speciesa
Table 3 Silver-Catalysed Synthesis of Substituted Acridines
OTBS
OTBS
Catalystb
AgSbF6
AgPF6
Time (h)
1
Conversion (%)c
pKA
<0
R2
R3
R2
R3
AgOTf (5 mol%)
DCE, 50 °C
>95
>95
>95
>95
>95
NA
NA
NA
NA
NA
N
N
R1
1
<0
R1
AgOTf
AgNO3
AgCO2CF3
Ag2SO4
AgF
0.5
<0
Entry
R1
R2, R3
Yield (%)
quant
quant
47b
1.5
<0
1
2
CH2OMe
Ph
H, H
H, H
H, H
H, H
H, H
H, H
H, H
H, H
H, H
0.5
<0
overnight
overnight
overnight
overnight
overnight
2
3
Fca
3.2
4.8
10.3
15.7
4
t-Bu
95
AgOAc
Ag2CO3
Ag2O
5
TMS
92
6
cyclopropyl
CH2OTHP
CH2N(Me)Bn
4-py
98
7
89
a Reaction conditions: catalyst (5 mol%), C2H4Cl2, 50 °C.
b OTf: trifluoromethanesulfonate. OAc: acetate.
c Determined by 1H NMR. NA: Not Applicable.
8
97
9
quant
95
10
11
Bu
OMe, H
OMe, H
sion rate (>95%, Table 1, entry 5). Interestingly, control
experiments with only the silver species led to the same
results within one hour (Table 1, entry 6). Finally, the ad-
dition of 5 mol% of TFA led to traces of the acridine prod-
uct, ruling out the possibility of a Brönsted acid catalysed
reaction (Table 1, entry 7).
2-MeO2CC6H4
92
a Fc: ferrocenyl.
b The yield was 72% based on the recovered starting material.
Table 4 Application to Related Polycyclic Systems
Few investigations concerning the catalytic applications
of silver have been reported in the literature.36,37 We thus
became interested in the behaviour of silver species and
screened different silver-based catalysts on the same mod-
el reaction (Table 2).
OTBS
OTBS
AgOTf (5 mol%)
DCE, 50 °C
X = CH, N
X
X
Ph
As we previously reported,38 we found a good correlation
between the reactivity of silver species and the pKA of
their counteranion since only the silver salts having coun-
teranions with pKA values below zero were efficient cata-
lysts (Table 2). These silver salts are known for forming
p-complexes with alkynes, thus acting as transition metal
catalysts, and they gave here a quantitative conversion of
quinoline 1 to acridine 2.
Ph
Entry Starting material
Product
Conversion (%)
>95
OTBS
OTBS
1
Ph
Ph
Ph
OTBS
Other silver species such as Ag2CO3 and Ag2O are mainly
characterised through their oxidising properties of oxy-
genated functions.39 They act as Lewis acids, interacting
with oxygen atoms. Such activation does not allow the cy-
cloisomerisation to occur since the alkynyl group activa-
tion seems mandatory.
OTBS
2
>95
N
N
Ph
OTBS
D
OTBS
D
Having established a standard reaction protocol, we ex-
amined the current benzannulation reaction with various
combinations of alkynyl substituents and aromatic cycles
(with or without nitrogen). The results are shown in
Table 3 and Table 4.
cat. (5 mol%)
solvent
D
OMe
40(60)
N
1-d2
N
OMe
2-d2
2-d1
D(H)
Good results are obtained with alkyl (Table 3, entries 1, 4,
6–8 and 10), silyl (Table 3, entry 5) and aryl substituents
(Table 3, entries 2, 9 and 11). It is noteworthy that sensi-
tive protecting groups such as acetal remained untouched
under these conditions (Table 3, entry 7). Sterically hin-
Scheme 1 Isotopic labelling experiment
dered substituents did not disfavour the course of the re-
action (Table 3, entries 4 and 5). Remarkably, a ferrocenyl
Synlett 2008, No. 16, 2513–2517 © Thieme Stuttgart · New York

LETTER
Silver-Catalysed Benzannulation Reaction
2515
OTBS
D
OTBS
CD2
OTBS
D
D
[Ag]
x
H
OMe
N
N
N
OMe
D
H
•
H
1-d2
H
OMe
Scheme 2 Hypothetical formation of an allenyl intermediate
moiety led to the expected product but in a moderate yield
(47%), along with the unreacted starting material (entry
3). Such a result might be explained through a stable com-
plexation of silver to the iron atom of the ferrocenyl
moiety40 and not through a reduction of silver(I).41
OTBS
OTBS
H
R
[Ag]
R
complexation
H
proton
Moreover, we found that the benzannulation reaction pro-
ceeded nicely to form naphthalene (Table 4, entry 1) and
quinoline rings (Table 4, entry 2) with high conversion,
attesting that neither the aromatic nitrogen nor the extra
benzene ring interfere in the reaction.
migration
TBS
O
TBS
H
O
A
H
B
In order to get insights into the reaction mechanism, we
performed the reaction using a D2 labelled silyl enol ether
(1-d2, Scheme 1). Performing this reaction in C2H4Cl2 led
to the formation of a mixture of deuterated and nondeuter-
ated products in a ratio of 40:60, probing the feasibility of
an intra-molecular deuterium 1,3-migration.
R
R
[Ag]
[Ag]
cycloisomerisation
Scheme 3 Proposed mechanism
Moreover, as shown in Scheme 2, the absence of any deu-
terium insertion in the CH2 group a to the alkynyl moiety
apparently rules out the possibility of an allenyl interme-
diate.
OTBS
OTBS
OH
H2O (traces)
AgOTf (5 mol%)
anhyd DCE
50 °C
10–12 h
R
R
These results led us to envision the following reaction
mechanism (Scheme 3). First, we propose that the reac-
tion begins upon complexation of the alkynyl moiety by
silver(I) species to give the silver-alkynyl complex A
(Scheme 3), which is supported by our previous work re-
lated to the carbophilic properties of these silver species.38
A 6-endo-dig cycloisomerisation would then occur, from
the electron-rich alkene towards the electron-deficient
complexed alkyne, thanks to the electronic assistance of
the silyl enol ether, leading to the zwitterionic intermedi-
ate B (Scheme 3). A proton migration driven by the arom-
atisation of the newly generated ring system would then
lead to the desired product and to the regeneration of the
silver catalyst.
1
2
3
R
AgOTf (5 mol%)
technical grade DCE
50 °C, 10–12 h
Scheme 4 Silver-catalysed deprotection of a silyl alcohol protected
group (R = CH2OMe)
Me
Me
N
N
AuCl3 (10 mol%)
100 °C, toluene
97%
n-C6H13
n-C6H13
Scheme 5 Dankwardt´s work; see refs. 15 and 34
Interestingly, the use of anhydrous 1,2-dichloroethane led
to the exclusive formation of the TBS-protected alcohol
acridine (2, Scheme 4) within 30 minutes, whereas the ad-
dition of traces of water (or the use of technical grade 1,2-
dichloroethane) enabled the complete deprotection of the
alcohol group (3, Scheme 4) in an overnight reaction.42
Acknowledgment
T.G. thanks the ‘Ministère de l’Enseignement Supérieur et de la Re-
cherche’ for a fellowship. P. B. thanks the financial support from the
‘Cluster de recherche Chimie de la Région Rhône-Alpes’ and from
the ‘Association pour la Recherche sur le Cancer’ (ARC).
In conclusion, we have demonstrated that silver salts can
promote catalytically benzannulation reactions.43 The re-
action proceeds under smooth conditions and the choice
of the counteranion is crucial to obtain good to excellent
yield of highly substituted acridine, quinoline, or naphtha-
lene nuclei.
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Synlett 2008, No. 16, 2513–2517 © Thieme Stuttgart · New York

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T. Godet, P. Belmont
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Synlett 2008, No. 16, 2513–2517 © Thieme Stuttgart · New York

LETTER
de Kozak, A.; Aubert, C.; Fensterbank, L.; Malacria, M.
Silver-Catalysed Benzannulation Reaction
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Soc. Rev. 2007, 36, 1395. (b) Nieto-Oberhuber, C.; Munoz,
M. P.; Bunuel, E.; Nevado, C.; Cardenas, D. J.; Echavarren,
A. M. Angew. Chem. Int. Ed. 2004, 43, 2402. (c) Robles-
Machwin, R.; Adrio, J.; Carretero, J. C. J. Org. Chem. 2006,
71, 5023. (d) Lee, S. I.; Kim, S. M.; Choi, M. R.; Kim, S. Y.;
Chung, Y. K. J. Org. Chem. 2006, 71, 9366. (e) Mamane,
V.; Gress, T.; Krause, H.; Fürstner, A. J. Am. Chem. Soc.
2004, 126, 8654. (f) See ref. 32.
(36) For selected examples, see: (a) For a review, please see:
Halbes-Letinois, U.; Weibel, J.-M.; Pale, P. Chem. Soc. Rev.
2007, 759. (b) Menz, H.; Kirsch, S. F. Org. Lett. 2006, 8,
4795. (c) Porcel, S.; Echavarren, A. M. Angew. Chem. Int.
Ed. 2007, 46, 2672. (d) Li, Z.; Capretto, D. A.; Rahaman,
R.; He, C. Angew. Chem. Int. Ed. 2007, 46, 5184.
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(41) Zanello, P. In Ferrocenes; Togni, A.; Hayashi, T., Eds.;
Wiley-VCH: Weinheim, Germany, 1995, 317–430.
(42) Silver-catalysed deprotection of silyl alcohols has already
been reported. See for instance: Orsini, A.; Vitérisi, A.;
Bodlenner, A.; Weibel, J.-M.; Pale, P. Tetrahedron Lett.
2005, 46, 2259.
(43) Typical Procedure: To a flask charged with silyl enol ether
quinoline (0.1 mmol)44 dissolved in anhyd 1,2-dichloro-
ethane (10 mL), was added silver catalyst (5 mol%). The
reaction mixture was stirred at 50 °C until the reaction was
judged complete by TLC analysis (0.5–2 h). The crude
mixture was dissolved in CH2Cl2 and washed with a sat. aq
solution of NaHCO3 (3 ×). The organic phase was dried over
Na2SO4, filtered and the solvents were removed in vacuo. If
needed, the residue was loaded on a silica gel column and
elution with the appropriate mixture of cyclohexane and
EtOAc yielded the pure cyclised products.
(e) Lingaiah, N.; Babu, N. S.; Reddy, K. M.; Prasad, P. S. S.;
Suryanarayama, I. Chem. Commun. 2007, 278. (f) Yamada,
W.; Sugawara, Y.; Cheng, H. M.; Ikeno, T.; Yamada, T. Eur.
J. Org. Chem. 2007, 2604. (g) Oh, C. H.; Yi, H. J.; Lee, J. H.
New J. Chem. 2007, 31, 835. (h) Harrison, T. J.; Kozak,
J. A.; Corbella-Pane, M.; Dake, G. R. J. Org. Chem. 2006,
71, 4525. (i) Arcadi, A.; Alfonsi, M.; Marinelli, F. J.
Organomet. Chem. 2007, 692, 5322. (j) Sweis, R. F.;
Schramm, M. P.; Kozmin, S. A. J. Am. Chem. Soc. 2004,
126, 7442. (k) Asao, N.; Yudha S., S.; Nogami, T.;
Yamamoto, Y. Angew. Chem. Int. Ed. 2005, 44, 5526.
(l) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2004,
126, 9993. (m) Li, Z.; Capretto, D. A.; Rahaman, R.; He, C.
Angew. Chem. Int. Ed. 2007, 46, 5184. (n) Cui, Y.; He, C.
J. Am. Chem. Soc. 2003, 125, 16202. (o) Ding, Q.; Wu, J.
Org. Lett. 2007, 9, 4959. (p) See ref. 38
Selected spectroscopic data for entry 8, Table 3: isolated as
a yellow oil (97%). 1H NMR (300 MHz, CDCl3, 25 °C): d =
9.04 (s, 1 H), 8.20 (app d, 3JH,H = 8.8 Hz, 1 H), 8.02 (app d,
3JH,H = 8.3 Hz, 1 H), 7.77 (s, 1 H), 7.77 (ddd, 3JH,H = 8.7, 6.7
Hz, 4JH,H = 1.4 Hz, 1 H), 7.51 (ddd, 3JH,H = 8.0, 6.6 Hz,
4JH,H = 0.8 Hz, 1 H), 7.43–7.45 (m, 2 H), 7.34 (t, 3JH,H = 7.0
Hz, 2 H), 7.26 (m, 1 H), 7.03 (d, 4JH,H = 1.0 Hz, 1 H), 3.69
(s, 2 H), 3.63 (s, 2 H), 2.26 (s, 3 H), 1.15 (s, 9 H), 0.37 (s, 6
H). 13C NMR (75 MHz, CDCl3, 25 °C): d = 151.6, 150.2,
149.2, 143.2, 139.6, 131.8, 130.5, 129.2, 128.9, 128.9,
128.4, 127.1, 126.0, 125.3, 122.3, 121.3, 111.7, 62.2, 62.2,
42.6, 26.1, 18.7, –4.1. MS (ESI+): m/z (%) = 443 (100) [M +
H]+. HRMS (CI): m/z calcd for C28H35N2OSi+: 443.2519;
found: 443.2522.
(37) In ref. 15, Dankwardt reported the use of 1.1 equivalents of
AgCO2CF3 in nitromethane at r.t.
(38) Godet, T.; Vaxelaire, C.; Michel, C.; Milet, A.; Belmont, P.
Chem. Eur. J. 2007, 13, 5632.
(44) Silyl enol ether quinolines (Table 3) were prepared
following a published procedure. Please see ref. 33.
Synlett 2008, No. 16, 2513–2517 © Thieme Stuttgart · New York