A catalyst-free bis(triflyl)ethylation/benzannulation reaction: Rapid access to carbazole-based superacidic carbon acids from alkynols

Article abstract of DOI:10.1039/c9cc08930f

Carbazoles possessing Tf₂CHCH₂ groups were obtained by the reaction of 1-(indol-2-yl)but-3-yn-1-ols with in situ-generated Tf₂CCH₂ through vicinal difunctionalisation of the alkyne moiety, where the vinyl-type c

Full text of DOI:10.1039/c9cc08930f

View Article Online
View Journal
ChemComm
Chemical Communications
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: P. Almendros, I.
Martín-Mejías, C. Aragoncillo, H. Yanai, S. Hoshikawa, Y. Fujimoto and T. Matsumoto, Chem. Commun.,
2020, DOI: 10.1039/C9CC08930F.
Volume 54
This is an Accepted Manuscript, which has been through the
Number
January 2018
Pages 1-112
1
4
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Chemical Communications
rsc.li/chemcomm
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
towards bifunctional catalysts of tunable basicity
rsc.li/chemcomm

Please do not adjust margins
Page 1 of 4
ChemComm
View Article Online
DOI: 10.1039/C9CC08930F
Journal Name
COMMUNICATION
Catalyst-free bis(triflyl)ethylation/benzannulation reaction:
Rapid access to carbazole-based superacidic carbon acids from
alkynols†,‡
Received 00th January 20xx,
Accepted 00th January 20xx
Irene Martín-Mejías,a Cristina Aragoncillo,b Hikaru Yanai,*c Shoki Hoshikawa,c Yuuki Fujimoto,c
Takashi Matsumoto,c and Pedro Almendros*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
[Tf2CR]– was applied to push-pull  systems. Push-pull ethylene 4
with a large distortion around the central C–C bond axis showed
clearly charge-separated characters on the C–C bond in
complementary bonding analysis.9 These examples reveal that - or
-bonding with the [Tf2CR]– moiety is not favourable.
Carbazoles possessing Tf2CHCH2 group were obtained by the
reaction of 1-(indol-2-yl)but-3-yn-1-ols with in-situ-generated
Tf2C=CH2 through vicinal di-functionalisation of the alkyne moiety,
where the vinyl-type carbocation intermediate was selectively
attacked by the indole moiety not by the carbanion moiety.
(A)
(B)
H
N
Carbanions are inherently reactive chemical species and isolations of
carbanion-containing salts are significantly limited. Delocalisation of
the negative charge by electron-withdrawing mesomeric (resonance)
effect is a general approach for the improvement of thermodynamic
stability. Some ‘free’ triphenylmethanides1 and cyclopentadienides2
have been isolated and characterised. Stabilisation of the carbanion
is also achieved by substitution of the anionic carbon atom by
fluorine atom(s) or fluorine-containing substituents. For example,
LPC
Tf2C
N
Tf
R
C
R
R
O
O
F
Tf2C
F
S2
1
2a
C
F
F
i-Pr
*S–C(F )
3
Ph
P
N
H
Tf
Tf
C
C
H
Tf2C
Ph
Ph
N
i-Pr
Farnham and co-workers isolated and characterised
a ‘free’
perfluorocarbanion.3 Carbanions bearing two triflyl groups (Tf =
CF3SO2), which are known to be one of the strongest electron-
withdrawing groups, attract much attention.4 Yanai reported several
intramolecular salts 1–4 (Figure 1A).5–9 For the stability of such
[Tf2CR]– species, orbital interaction between an occupied p orbital of
the anionic carbon atom and adjacent *S–C(F3) orbitals, called
negative hyperconjugation,10 is pointed out to be a key factor (Figure
1B).8 It is easily predictable that nucleophilicity of [Tf2CR]– is strongly
suppressed due to the steric crowding around the anionic carbon
atom and the charge-delocalised nature. Actually, compound 3 was
the first example of well-defined phosphorous carba-betaines, which
did not undergo ring-closure into the corresponding
phosphacyclopropane.8 Recently, chemically inert property of
3
4
Figure
1
(A) Zwitterions bearing [Tf2CR]– structure, (B) Negative
hyperconjugation in [Tf2CR]–.
With such background, we were interested in the ring-closing
reaction of vinyl-type carbocation INT-1 bearing a [Tf2CR]– moiety as
the counter anion and a nucleophilic indole moiety (Eqn. 1). As a
pioneering work, Alcaide and Almendros reported a gem-
bis(triflyl)cyclobutene synthesis through the stepwise (2+2)
cycloaddition of several alkynes with Tf2C=CH2,11,12 which was in-
situ-generated from 2-fluoropyridinium salt 2a7 developed by Yanai
(Eqn. 2).13 In the case of INT-1 derived from 1-(indol-2-yl)but-3-yn-
1-ols 5 with Tf2C=CH2, two reaction paths would be possible as
follows; path a) cyclobutene formation through C–C bond forming
reaction between the anionic carbon atom and the cationic C4
atom; path b) carbazole formation induced by nucleophilic attack of
the indole moiety on the C4 atom. Such reaction system would
allow a further understanding of chemical behaviour of the [Tf2CR]–
species. In this paper, we report that the latter path selectively
proceeds to give Tf2CHCH2-decorated carbazoles as final products.
Carbazoles are an important class of natural products, biologically
active molecules, and advanced materials.14 Therefore, synthetic
interests in them are renewed. In particular, transition metal-
catalysed cyclisations of indolyl-3-alkyn-1-ols 5 have been
a. Instituto de Química Orgánica General, IQOG-CSIC, Juan de la Cierva 3, 28006-
Madrid, Spain; E-mail: palmendros@iqog.csic.es
b. Grupo de Lactamas y Heterociclos Bioactivos, Departamento de Química Orgánica,
Unidad Asociada al CSIC, Facultad de Química, Universidad Complutense de
Madrid, 28040-Madrid, Spain
c. School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1
Horinouchi, Hachioji, Tokyo 192-0392, Japan; E-mail: yanai@toyaku.ac.jp
†Dedicated to Prof. Benito Alcaide on the occasion of his retirement.
‡Electronic Supplementary Information (ESI) available: Computational details,
experimental procedures, characterization data of new compounds,
crystallographic details, and copies of NMR spectra for all new compounds. CCDC
1911534. For ESI and crystallographic data in CIF (CCDC 1911534) or other electronic
format see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
COMMUNICATION
Journal Name
d e v e l o p e d t o c o m p l e m e n t c l a s s i c a l Scheme 1 Reactions of indolyl-3-alkyn-1-ols 5a–c with 2a.
View Article Online
DOI: 10.1039/C9CC08930F
R3
This work
expected by-products including 3-bis(triflyl)ethylated indole and
a
R3
4
Tf2C CH2
2a
4
cyclobutene products were not obtained. For the present molecular
transformation, reaction solvents are critical: the use of DMF led to
complexation of the reaction, while the reaction in ethanol or
dichloromethane was not productive due to low solubility of 2a.
During a screening with several pyridinium salts 2b–e, we re-found
that non-fluorinated salt 2e17 did not react with 5a under similar
conditions and other 2-substituted pyridinium salts caused very slow
conversion of 5a.7b Unfortunately, in a case of terminal alkyne 5b,
2,2-bis(triflyl)ethylation of the indole 3-position (C3’ atom)
selectively occurred to give indole acid 7b-H, which was isolated as
the corresponding triethylammonium salt 7b after column
chromatography on acidic silica gel pre-treated by Et3N for
neutralisation.16 Similar result was observed in the reaction of
phenylalkyne 5c. These results imply that alkyne-selectivity observed
in the reaction of 5a is enhanced by the electron-donating aryl group
on the alkyne terminus. Indeed, by applying the common Lewis acids
such as ZnI2 and InCl3, we observed no evidences to form the
carbazoles from 5c. Adduct 7c gave only a complex mixture by
heating at 80 °C. On this basis, we conclude that 3-alkylated products
7 are formed in an irreversible manner and the possible carbazoles 6
do not generate from them.
R2
CTf2
3'
C
(from
)
3
R2
3
1
R4
b
2
without catalyst
2'
N
R1
R4
N
R1
OH
OH
INT-1
5
via path b
via path a
Tf Tf
R3 CHTf2
R2
R3
R2
(1)
N
R1
R4
R4
N
R1
OH
Tf
Literature works
R2
R1
Tf
R2
R3
F
N
Tf2C CH2
(2)
(3)
R1
CTf2
2a
R3
R2
Au or Pt
(cat.)
R2
N
R1
R4
R4
N
R1
OH
The scope of the bis(triflyl)ethylation/benzannulation reaction
was studied by using a series of indolyl-3-alkyn-1-ols 5 (Scheme 2).
The reaction was applicable to 5-methylindoles 5d and 5e bearing
electron-rich aryl groups including 2,4-dimethoxyphenyl and 2-
thienyl groups on the alkyne terminus to give carbazoles 6d and 6e
in moderate to good yields. 5-Unsubstituted indoles 5f–h and 5-
5
approaches (Eqn. 3).15 The present results clearly demonstrate that
non-ionic Tf2C=CH2 effectively activates the internal alkynes in an
electrophilic fashion. In addition, this is a rare example that attack of
the non-ionic carbon-centred nucleophile is faster than that of the
carbanion.
We first examined the reaction of indolyl-3-alkyn-1-ols 5 with 2-
(2-fluoropyridinium-1-yl)-1,1-bis(triflyl)ethan-1-ide 2a under the
optimised conditions for the cyclobutene-forming reaction (at 25
° C , i n C H 3 C N ) 1 1 ( S c h e m e 1 ) . T h e r e a c t i o n o f ( p -
methoxyphenyl)alkyne 5a with 1.0 equiv of 2a selectively produced
the desired carbazole acid 6a-H, which was isolated as the
corresponding sodium salt 6a in 56% yield after column
c h r o m a t o g r a p h y o n s i l i c a g e l . 1 6 I n t h i s c a s e ,
Ar
Ar CTf2
2a
25 °C
1) , CH3CN
R2
R2
Z
R3 2) silica gel
N
N
R1
R3
R1
OH
5d–n
6d–n
OMe
Z
Et3NH
S
CTf2
MeO
CTf2
R
Me
OMe
OMe
N
N
Me
2
(R = Me) 36% (Z = Na, 0 oC)
6f
(R = H) 42% (Z = Et3NH)
Me
1)
Z
N
6e
Na
6d
56%
CTf2
Me
Me
OMe
OMe
OMe
CH3CN, 25 °C
CTf2
Na
Et3NH
CTf2
Et3NH
CTf2
2) silica gel
N
N
R
CTf2
R
R
OH
Me
5a
Me
6a
recovery
5a
MeO
Cl
of
56%
35%
17%
8%
2a
(Z = F)
N
N
Me
N
61%
80%
89%
95%
2b
2c
2d
2e
(Z = Br)
(Z = I)
Me
Me
o
6i
6j
6g
6h
(R = H)
30%
6k (R = H)
45%
(R = H)
63%
(45
C
)
(Z = SMe)
(Z = H)
(R = MeO) 47%
6l (R = MeO) 38%
(R = MeO) 50%
0%
OMe
OMe
R
R
Et3NH
CTf2
Na
Tf2C
Et3NH
2a
0 °C
1) , CH3CN
MeO
Me
CTf2
Me
Me
3'
2) silica gel
N
N
N
Me
N
OH
OH
Me
Me
Me
6m
Boc
5b
5c
7b
7c
(R = H)
43%
74%
6n
42%
49%
(R = Ph)
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 4
ChemComm
Please do not adjust margins
Journal Name
COMMUNICATION
Scheme 2 Catalyst-free reaction of indolyl-3-alkyn-1-ols 5d–n with 2a.
conditions to form the products 12a and 12b (Eqn. 6; also, see ESI).
View Article Online
DOI: 10.1039/C9CC08930F
A plausible reaction pathway for carbazoles 6-H is shown in
Ar
Ar CTf2
Scheme 3. First, the alkyne moiety of indolyl-3-alkyn-1-ols 5 traps
Tf2C=CH2 generated from reagent 2a to produce the zwitterionic
vinyl-type carbocation INT-1. This initial addition is followed by a 5-
endo-dig carbocyclisation through nucleophilic attack of C2’ atom of
the indole nucleus on the cationic C4 atom to give spirocyclic
indolinium species INT-2.21 Next, fused tricyclic intermediate INT-3
arises through 1,2-alkenyl migration in the spirocyclic nucleus of INT-
2. Further aromatisation including deprotonation of INT-3 and
dehydration of INT-4 should generate 6-H.
2a
1) , CH3CN
I
25 °C
(4)
(5)
2) silica gel
Et3NH
N
N
1
OH
Me
8
OH
Me
5g-I
(Ar = 4-MeOC6H4)
R
37%
R
CTf2
2a
1) , CH3CN
Me
Me
2) Et3N
Et3NH
O
O
(R = 4-MeOC6H4)
(R = Me)
9a
9b
10a
25 °C, 1.5 h
80 °C, 3 h
95%
no reaction
Tf
Ar
Ar
3'
CHTf2
F
N
R2
CTf2
4
R2
4
C
R2
R2
2a
Tf
CTf2
3'
2a
3
(6)
3
1
CH3CN
90 °C
N
R1
R3
R3
N
R1
2
2'
2'
N
R1
N
R1
5-endo-dig
carbocyclisation
Ar
OH
OH
(R1 = Et, R2 = Br)
(R1 = Ac, R2 = H)
89%
INT-1
11a
11b
12a
5
12b
37%
Ar
CTf2
R2
1,2-alkenyl
migration
H
CTf2
4
R2
methoxyindoles 5i and 5j were successfully converted to the desired
carbazoles 6f–j. Likewise, 5-chloroindoles 5k and 5l reacted with
Tf2C=CH2 to give carbazoles 6k and 6l, respectively. Taking into
account of low reactivity of halogenated carbazoles in the direct
aromatic electrophilic substitution (SEAr) reaction with in-situ-
generated Tf2C=CH2 (vide infra), the present results show one of
synthetic advantages. The reactions of 2-methylbut-3-yn-1-ol 5m
and N-Boc indole 5n also worked well to give carbazoles 6m and 6n,
respectively. As shown in Eqn. 4, the reaction of iodoindole 5g-I with
2a gave 1-hydroxycarbazole 8. These examples suggest that the
present reaction is potent for regio-controlled synthesis of highly
substituted carbazoles. The cyclisation methodology initiated by
highly electrophilic Tf2C=CH2 is not limited to the carbazole
formation. For example, the reaction of propargyl ether 9a with 2a
produced 2H-chromene 10a in 95% yield (Eqn. 5). Unfortunately,
dialkyl alkyne 9b was significantly less reactive and no consumption
of that was observed even under heating conditions.
4
3'
2'
N
R1
R3
INT-3
N
R1
R3
OH
OH
Ar
INT-2
–H
Ar CHTf2
CTf2
R2
R2
+H
–H2O
R3
N
R3
N
R1
R1
OH
6-H
INT-4
Scheme 3 Mechanistic explanation.
This reaction pathway including 5-endo-dig cyclisation
followed by ring-expansion is supported by a DFT simulation
[PCM(CH3CN)-M06-2X/6-31+G(d) level of theory] of a model
molecule 13, where the hydroxyl group of substrate 5g is
replaced by a hydrogen atom (Figure 2A). For all optimised
geometries, frequency calculations were conducted. Calculated
Gibbs energy difference at 298 K (G298) for the initial
electrophilic attack of Tf2C=CH2 on the alkynic C3 atom of 13 is
only 16.3 kcal mol–1, when the initial state is used as a reference
Tf2CH-type superacidic molecules behave as carbon acids with
unique catalytic activity.18,19 In the synthetic context,5 direct SEAr
reaction of simple carbazoles with Tf2C=CH2 would be attractive to
obtain the carbazole-based acids.20 However, the reactions of
electron-deficient carbazoles 11a and 11b required heating
(A)
(B)
OMe
TS1
4
3
+16.3
2'
3'
2'
5
4
4
TS2
–1.6
+0.3
3'
0
3
2
2'
3'
1
INT-1'
13
+ Tf2C=CH2
2'
N
1'
TS3
–1
2
.
H
H
Me
13
4
3
–19.9
4
9
INT-2'
3'
2'
2'
3'
–34.5
4
INT-3'
2'
3'
Reaction coordinate
Figure 2 (A) Chemical structure of model molecule 13. (B) Reaction profile of the bis(triflyl)ethylation/benzannulation reaction.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
COMMUNICATION
Journal Name
5
6
7
For a review, see: H. Yanai and T. Taguchi, J. FluorinVeieCwhAermtic.l,e 2O0n1lin5e,
174, 108.
H. Yanai, T. Yoshino, M. Fujita, H. Fukaya, A. Kotani, F. Kusu, and T.
Taguchi, Angew. Chem. Int. Ed., 2013, 52, 1560.
(a) H. Yanai, Y. Takahashi, H. Fukaya, Y. Dobashi, and T. Matsumoto,
Chem. Commun., 2013, 49, 10091; (b) H. Yanai, R. Takahashi, Y.
Takahashi, A. Kotani, H. Hakamata, and T. Matsumoto, Chem. Eur. J.,
2017, 23, 8203.
H. Yanai, P. Almendros, S. Takahashi, C. Lázaro-Milla, B. Alcaide, and
T. Matsumoto, Chem. Asian. J., 2018, 13, 1956.
(Figure 2B). A vinyl-type carbocation INT-1’ corresponding to
INT-1 shown in Scheme 3 was found as a local minimum species.
Transition state geometry of the 5-endo-dig cyclisation (TS2) is
very close to INT-1’ and the activation barrier to give INT-2’ is
1.9 kcal mol–1 (from INT-1’). In INT-1’, the C4–C2’ distance
(275.1 pm) is obviously shorter than the C4–C3’ distance (320.7
pm). The low activation barrier and the geometric similarity
between INT-1’ and TS2 support that the 5-endo-dig path rather
than the 6-endo-dig path is kinetically favourable. In addition,
the (2+2) cycloaddition path requires much higher activation
energy (6.3 kcal mol–1 from INT-1’; see, ESI). The following ring-
expansion reaction (TS3) to give INT-3’ is highly exothermic and
requires small activation energy (7.0 kcal mol–1 from INT-2’). For
each step on this pathway, the simulation well agrees with a fact
that the bis(triflyl)ethylation/benzannulation reaction proceeds
under really mild conditions.
DOI: 10.1039/C9CC08930F
8
9
H. Yanai, T. Suzuki, F. Kleemiss, H. Fukaya, Y. Dobashi, L. A. Malaspina,
S. Grabowsky, T. Matsumoto, Angew. Chem. Int. Ed., 2019, 58, 8839.
10 (a) J. D. Roberts, R. L. Webb, and E. A. McElhill, J. Am. Chem. Soc.,
1950, 72, 408; (b) O. Exner and S. Böhm, New J. Chem., 2008, 32, 1449.
(c) G. Raabe, H.-J. Gais, and J. Fleischhauer, J. Am. Chem. Soc., 1996,
118, 4622.
11 (a) B. Alcaide, P. Almendros, I. Fernández, and C. Lázaro-Milla, Chem.
Commun., 2015, 51, 3395; (b) B. Alcaide, P. Almendros, and C. Lázaro-
Milla, Chem. Eur. J., 2016, 22, 8998; (c) B. Alcaide, P. Almendros, and
C. Lázaro-Milla, Adv. Synth. Catal., 2017, 359, 2630.
In conclusion, we successfully developed sequential
bis(triflyl)ethylation/benzannulation reaction to produce
bis(triflyl)ethylated carbazoles from indolyl-3-alkyn-1-ols. This
molecular transformation is triggered by regioselective
electrophilic attack of Tf2C=CH2, which is generated from the 2-
fluoropyridinium salt 2a in an in-situ manner, on the alkyne
moiety of the substrates. The fact that the reaction well
proceeds without any catalyst evidences notably high
electrophilicity of non-cationic Tf2C=CH2. In vinyl-type
carbocations thus generated, the ring-closing reaction between
the cationic carbon atom and indole C2’ atom predominantly
takes place. This event clearly shows chemically inert properties
of [Tf2CR]– in the chemical reaction.
12 B. Alcaide, P. Almendros, and C. Lázaro-Milla, Chem. Commun., 2015,
51, 6992.
13 (a) H. Yanai, M. Fujita, and T. Taguchi, Chem. Commun., 2011, 47,
7245; (b) H. Yanai, H. Ogura, H. Fukaya, A. Kotani, F. Kusu, and T.
Taguchi, Chem. Eur. J., 2011, 17, 11747.
14 For selected reviews, see: (a) M. Li, Chem. Eur. J., 2019, 25, 1142; (b)
X. Zhang and S. Ma, Isr. J. Chem., 2018, 58, 608; (c) G. Sathiyan, E. K.
T. Sivakumar, R. Ganesamoorthy, R. Thangamuthu, and P. Sakthivel,
Tetrahedron Lett., 2016, 57, 243; (d) A. W. Schmidt, K. R. Reddy, and
H.-J. Knölker, Chem. Rev., 2012, 112, 3193; (e) J. Roy, A. K. Jana, and
D. Mal, Tetrahedron, 2012, 68, 6099; (f) H. J. Jiang, J. Sun, and J. L.
Zhang, Curr. Org. Chem., 2012, 16, 2014; (g) J. Li and A. G. Grimsdale,
Chem. Soc. Rev., 2010, 39, 2399; (h) T. Janosik, N. Wahlstrom, and J.
Bergman, Tetrahedron, 2008, 64, 9159; (i) H.-J. Knölker and K. R.
Reddy, Chemistry and Biology of Carbazole Alkaloids, in The Alkaloids,
vol. 65, pp 1–430, Ed. G. A. Cordell, Academic Press, Amsterdam,
2008.
15 (a) Y. Qiu, W. Kong, C. Fu, and S. Ma, Org. Lett., 2012, 14, 6198; (b) A.
S. K. Hashmi, W. Yang, and F. Rominger, Chem. Eur. J., 2012, 18, 6576;
(c) Y. Qiu, J. Zhou, C. Fu, and S. Ma, Chem. Eur. J., 2014, 20, 14589; (d)
J. Zhou, Y. Qiu, J. Li, C. Fu, X. Zhang, and S. Ma, Chem. Commun., 2017,
53, 4722.
Financial support for this work by AEI (MICIU) and FEDER
(Project PGC2018-095025-B-I00) and KAKENHI (17K08224) is
gratefully acknowledged. I. M. thanks MINECO for a predoctoral
contract. We are grateful to M. Moyano for preliminary studies,
Dr. M. P. Ruiz and Dr. C. Lázaro-Milla for helpful discussions, and
Prof. B. Alcaide for continued support.
16 The most carbon acids were purified by column chromatography on
acidic or Et3N-pretreated silica gel. Therefore, chemical yields refer to
the weight of isolated salts (see, ESI).
17 L. L. Barber Jr and R. J. Koshar, U.S. Pat., 3 962 342, 1976.
18 (a) T. Akiyama and K. Mori, Chem. Rev., 2015, 115, 9277; (b) C. H.
Cheon and H. Yamamoto, Chem. Commun., 2011, 47, 3043.
19 For selected examples, see: (a) K. Ishihara, A. Hasegawa, and H.
Yamamoto, Angew. Chem. Int. Ed. 2001, 40, 4077; (b) A. Hasegawa, Y.
Naganawa, M. Fushimi, K. Ishihara, and H. Yamamoto, Org. Lett.,
2006, 8, 3175; (c) A. Takahashi, H. Yanai, and T. Taguchi, Chem.
Commun., 2008, 2385; (d) H. Yanai, O. Kobayashi, K. Takada, T. Isono,
T. Satoh, and T. Matsumoto, Chem. Commun., 2016, 52, 3280. (e) T.
Gatzenmeier, M. van Gemmeren, Y. Xie, D. Höfler, M. Leutzsch, and
B. List, Science, 2016, 351, 949; (f) D. Höfler, M. van Gemmeren, P.
Wedemann, K. Kaupmees, I. Leito, M. Leutzsch, J. B. Lingnau, and B.
List, Angew. Chem. Int. Ed., 2017, 56, 1411.
Conflicts of interest
There are no conflicts of interest to declare.
Notes and references
1
(a) W. Schlenk and J. Holtz, Ber. Dtsch. Chem. Ges., 1916, 49, 603; (b)
S. Harder, Chem. Eur. J., 2002, 8, 3229; (c) M. M. Olmstead and P. P.
Power, J. Am. Chem. Soc., 1985, 107, 2174.
2
(a) E. Le Goff and R. B. LaCount, J. Am. Chem. Soc., 1963, 85, 1354; (b)
S. Matsumura and S. Seto, Chem. Pharm. Bull., 1963, 11, 126; (c) R.
Kuhn and D. Rewicki, Angew. Chem., 1967, 79, 648. (d) K. Okamoto,
T. Kitagawa, K. Takeuchi, K. Komatsu, and K. Takahashi, J. Chem. Soc.,
Chem. Commun., 1985, 173. (e) C. D. Gheewala, M. A. Radtke, J. Hui,
A. B. Hon, T. H. Lambert, Org. Lett., 2017, 19, 4227.
20 P. Almendros, H. Yanai, S. Hoshikawa, C. Aragoncillo, C. Lázaro-Milla,
M. Toledano-Pinedo, T. Matsumoto, and B. Alcaide, Org. Chem.
Front., 2018, 5, 3163.
21 A related spirocyclic intermediate has been recently proposed on the
gold-catalysed synthesis of carbazoles: B. Alcaide, P. Almendros, C.
Aragoncillo, E. Busto, C. G. López-Calixto, M. Liras, V. A. de la Peña
O´Shea, A. García-Sánchez, and H. V. Stone, Chem. Eur. J., 2018, 24,
7620.
3
4
(a) W. B. Farnham, Chem. Rev., 1996, 96, 1633; (b) W. B. Farnham, D.
A. Dixon, J. C. Calabrese, J. Am. Chem. Soc., 1988, 110, 2607.
(a) A. Hasegawa, T. Ishikawa, K. Ishihara, and H. Yamamoto, Bull.
Chem. Soc. Jpn., 2005, 78, 1401; (b) D. Höfler, R. Goddard, J. B.
Lingnau, N. Nöthling, and B. List, Angew. Chem. Int. Ed., 2018, 57,
8326; (c) L. Turowsky and K. Seppelt, Inorg. Chem., 1988, 27, 2135.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins