Platinum-catalysed bisindolylation of allenes: A complementary alternative to gold catalysis

Article abstract of DOI:10.1002/chem.201103337

Pt versus Au: Platinum-catalysed addition of nucleophiles to allenes follows a distinctly different pathway to the process catalysed by gold(I) complexes; the platinum catalyst leads to different products with indoles involving a bisindolylation reaction, whereas gold(I) gives allyl indoles from a single addition (see scheme).

Full text of DOI:10.1002/chem.201103337

COMMUNICATION
DOI: 10.1002/chem.201103337
Platinum-Catalysed Bisindolylation of Allenes: A Complementary
Alternative to Gold Catalysis
Marꢀa Paz MuÇoz,*[a, b] Marꢀa C. de la Torre,[b] and Miguel Angel Sierra[c]
Indoles and bisACHTUNGTRENNUNG
(indolyl)alkanes[1] are present in a large va-
riety of natural products isolated from terrestrial and
marine natural sources and have shown to have very impor-
tant biological activities,[2] such as antibacterial, antimicrobi-
al, anti-inflammatory, antibiotic, anti-carcinogenic, genotox-
icity or DNA-damaging properties (Figure 1).[3] The synthe-
ses of bisACHTUNGTRENNUNG(indolyl)alkanes have been classically carried out
by condensation of indole derivatives with different alde-
hydes and ketones in the presence of stoichiometric
amounts of Lewis or protic acids.[4]
Scheme 1. Pt-catalysed dihydroalkoxylation[6] versus Au-catalysed hydro-
alkoxylation[7] of allenes.
lysed one (b, Scheme 1) in the outcome and in the mecha-
nism, in which allyl ethers are obtained via a Au-vinyl com-
plex intermediate.[7]
We found that other nucleophiles were also able to react
with this new distinct reactivity pattern in the presence of
platinum catalysts, and we described the first intermolecular
addition of indole nucleophiles to allenes to give the [2+1]
Figure 1. Examples of bisACTHNUGRTNEUNG(indolyl)alkanes with biological activity.
Allenes are rather unique structures that contain two p
orbitals perpendicular to each other. The synthetic utility of
allenes is extensive and new chemo-, regio-, stereo- and po-
sitional-selective transformations have been developed over
the past 15 years.[5] We have recently reported the platinum-
catalysed dihydroalkoxylation of monosubstituted allenes, in
which two molecules of an alcohol are added to the terminal
carbon of the allene moiety with complete saturation of the
second double bond via Pt-zwitterionic carbene intermedi-
ates (a, Scheme 1).[6] This reaction differs from the Au-cata-
bisACTHNUGRTENUNG(indolyl)-adducts, in which two molecules of indole are
bonded through the C3 to the terminal carbon of the allene
with complete saturation of the other double bond (a,
Scheme 2).[8,9] Preliminary studies[6] indicated that the Pt-
catalysed reaction again shows complementarity with the
AuI-catalysed process reported recently by Widenhoefer
et al. (b, Scheme 2).[10,11] Closer examination of the reaction
reinforced this assumption and optimised conditions led to
smooth bisindolylation (see the Supporting Information for
details).[12] The two-fold reactivity observed in our reaction
has also been reported for the gold(I)-catalysed reaction of
[a] Dr. M. P. MuÇoz
School of Chemistry
University of East Anglia
Earlham Road, Norwich, NR4 7TJ (UK)
Fax : (+44)1603592003
E-mail: M.Munoz-Herranz@uea.ac.uk
[b] Dr. M. P. MuÇoz, Dr. M. C. de la Torre
IQOG-CSIC, Juan de la Cierva, 3. 28006
Madrid (Spain)
[c] Prof. M. A. Sierra
Facultad de Ciencias Quꢀmicas
Universidad Complutense de Madrid
Avda. Complutense s/n. 28040, Madrid (Spain)
Supporting information (including experimental procedures, charac-
terization of all the products, 1H and 13C NMR spectra, and additional
tables, deuteration experiments and other information) for this article
is available on the WWW under http://dx.doi.org/10.1002/
chem.201103337.
Scheme 2. Pt-catalysed bisindolylation[6] versus Au-catalysed hydroaryla-
tion of allenes.[10]
Chem. Eur. J. 2012, 18, 4499 – 4504
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4499

indoles and furans with al-
kynes.[13] In the case of the
gold-catalysed reaction with al-
kynes and heteroarenes, the
Markovnikov product is observ-
ed,[13b] whereas in the case of
acceptor-substituted
allenes
a two-fold hydroarylation with
Markovnikov regioselectivity is
observed and in this case the
first addition is faster than the
second.[13c] However, to the best
of our knowledge, this is the
Scheme 3. Deuteration experiments.
first example of platinum-catalysed bisindolylation of al-
lenes.
was used as an additive (Scheme 3). The deuteration in the
platinum-catalysed dihydroalkoxylation of allenes was
shown to be greater than 98%. If acetals are intermediates
in the formation of the bisindoles, the deuterium from the
first dihydroalkoxylation should be retained because these
positions are not involved in the reaction to form the bisin-
doles, so the fact that the deuterium incorporation is low in
the final products rules out the hypothesis of acetals being
intermediates of the bisindolylation reaction.
The substitution on the indole and the allene were studied
by using the optimum conditions obtained under platinum
catalysis. Table 1 shows the bisindolylation process is general
under these conditions and it works well for unsubstituted
indoles, as well as N-alkyl and differently substituted indole
derivatives (see entries 1–6 and 8–10, Table 1).[15] The 2-
methyl-substituted indole 3 f gave the 3-allyl-indole 5af in
It was found that methanol is a key additive in this pro-
cess (an increased of yield from 23 to 62% was found in the
reaction of 1a with indole 3a when three equivalents of
MeOH were added).[6] At first sight, a straightforward ex-
planation for this observation would be a stepwise process
in which a previously reported Pt-catalysed dihydroalkoxyla-
tion reaction occurs,[6] followed by indolylation through
known simple pathways. However, it is important to note
that the reaction works without the addition of methanol,
albeit in lower yields; most likely methanol stabilises the
zwitterionic intermediates involved in the course of the re-
action or it could also take the role of a proton shuttle, as
reported for small water and water–alcohol clusters.[14] How-
ever, this possibility was excluded because of the results ob-
tained in the deuterium labelling experiments carried out
and detailed in the paragraph below. Besides, we have ob-
served that reaction of isolated dimethyl acetal 2a in the
presence of three equivalents of indole 3a under our Pt-cat-
51% yield, as well as the expected bisACTHNUTRGNEUNG(indolyl) derivative
4af (entry 5, Table 1). This derivative was also observed in
the reaction with 5-OH indole derivative 3i (entry 8,
Table 1), indicating a pronounced influence of the substitu-
ent on the reactivity. When the C3 position of the indole
alysed conditions, gives bisACTHNUTRGENUGN(indolyl) 4aa in 58% yield. The
reaction also works without a Pt catalyst, in THF at 708C,
but only 77% conversion was achieved after three days.
However, we have never observed the formation of acetal
2a during the Pt-catalysed reaction of 1a and 3a in the pres-
ence of MeOH, even at short reaction times (45 min, 2 or
4 h), which rules out the hypothesis of this being the main
reaction pathway under our reaction conditions.
The reaction using indole 3a and MeOD gave the corre-
sponding bisindole 4a with no deuterium incorporation in
any position. Moreover, deuteration studies were also per-
formed with C3-[D]indoles with and without MeOH/D. Al-
though the same deuteration pattern was observed in the
formation of products 4[D2] as for the Pt-catalysed dihy-
droalkoxylation of allenes (one deuterium added to the in-
ternal (more substituted) carbon from the 3 position of the
indole transferred in the first step to the internal carbon to
form the platinum-carbene intermediate key in the reaction,
and one deuterium incorporated in the central carbon of the
allene from the last step of protonolysis of the platinum
complex (see later, Scheme 7),[6] deuterium incorporation
was low in all the conditions tested and the excess of indole
3[D] left over in the reaction was recovered with partial or
total loss of deuterium in all the cases, even when MeOD
was blocked, the bisACTHNGUTERN(UNG indolyl) product from attack at C2 po-
sition was obtained (4ah, entry 7, Table 1). Pyrrole 3l also
gave the [2+1] adduct 4al under these platinum-catalysed
conditions, although in a lower yield (entry 11, Table 1).[16]
The reaction also works with allenes with different func-
tionalities (entries 1–5, Table 2).[17] In contrast to the dihy-
droalkoxylation of allenes described previously,[6] the bisin-
dolylation reaction of 1,3- and 1,1-disubstituted allenes also
worked under the platinum-catalysed reaction conditions to
give the bisACTHNUGRTNEUNG(indolyl) products 4, albeit in lower yields (en-
tries 6 and 7, Table 2).
Moreover, the products from the platinum-catalysed bisin-
dolylation of allenes are not limited to the addition of the
same indoles and cross-products could be obtained by reac-
tion of the allenes with different indoles, which increases the
utility of this reaction and gives the possibility of new bis-
AHCTUNGTREGUN(NN heteroarene) compounds. One example is shown in
Scheme 4.
The reaction of N-(2,3-butadienyl)indole derivatives 6
(Scheme 5) was also studied in the presence of platinum cat-
alysis and again, we found a divergent reactivity compared
with the reactivity found with gold.[18] The platinum-cata-
lysed reaction of 6a and the 3-methyl derivative 6b gave the
4500
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4499 – 4504

Platinum-Catalysed Bisindolylation of Allenes
COMMUNICATION
Table 1. Pt-Catalysed bisindolylation of cyclohexylallene 1a.
Entry
Indole 3[b]
Products
Yield
[%][a]
Entry
Indole 3[b]
Products
Yield
[%][a]
1
3b
4ab
90
7
3h
4ah 10[e]
2
3
3c
3d
4ac
4ad
75
60
8
9
3i
3j
4ai 33[f]
4aj 62[g]
4
5
6
3e
3 f
3g
4ae[c] 50
10
11
3k
4ak 46
4af
44[d]
3l
4al 12
4ag
67
[a] Conversion >98% unless otherwise stated; isolated product yield. [b] X=H, unless otherwise stated. [c] 12% of dimethyl acetal (2a) obtained.
1
[d] 51% of 5af obtained. [e] Conversion <98%. [f] 15% of 5ai obtained. [g] Traces of 5aj observed by H NMR spectroscopy.
Scheme 5. Platinum-catalysed reaction of allene-indole derivatives 6.
Scheme 4. Platinum-catalysed cross reaction of allene 1a with two differ-
ent indoles.
6-endo cyclisation products but, in contrast to the clean
gold-catalysed reaction in which only derivative 7 is formed,
we obtained a mixture of two isomers differing in the posi-
tion of the double bond in different ratios, depending on the
Chem. Eur. J. 2012, 18, 4499 – 4504
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4501

M. Paz MuÇoz et al.
Table 2. Pt-Catalysed bisindolylation of allenes 1.
tion reaction was then studied using the N-(2,3-butadi-
AHCTUNGTREGeNNNU nyl)indole derivatives 6 in the presence of an external
indole (Scheme 6). When the reaction of 6a was carried out
in the presence of free indole under the conditions described
above, instead of the expected intra-intermolecular reaction,
again a mixture of the allylic 6-endo cyclic product 7a (from
the attack at the less activated C2 position of the indole)
and an inseparable regioisomer 7a’ (with the conjugated
double bond), was obtained in 17% yield (Scheme 6).
Entry Allene 1
Product 4
Yield
[%][a]
A mixture of two inseparable isomers 7b and 7b’, from
the 6-endo cyclisation, was also obtained with the 3-methyl
indole derivative 6b under the standard conditions in
a higher yield. However, in this case, along with the mixture
of 7b and 7b’, the expected intra/intermolecular product 8
was obtained as a sole bisindole isomer, as well as traces of
1
1b
4ba
95
the trisACHTUNGERTN(NUNG indolyl) derivative 9ba (Scheme 6). The analogue
2
3
4
5
1c
1d
1e
1 f
4ca
4da
4ea
74
42
32
tris(indolyl) derivative 9ca was the only product of the reac-
AHCTUNGTRENNUNG
tion of 2-methyl-substituted derivative 6c, in which the C2
carbon on the indole was blocked for the 6-endo cyclisation
(Scheme 6).
In summary, we have described the first platinum-cata-
lysed intermolecular reaction of indoles with allenes to give
bisACTHNUGRTENUNG(indolyl)alkanes. This reaction differs both in outcome
and mechanism from the gold-catalysed version in which
allyl-indoles are obtained. The preliminary studies suggest
that the reaction does not proceed through a stepwise pro-
cess with formation of the acetal as the first step when car-
ried out in the presence of methanol. Besides the mecha-
nism involving zwitterionic intermediates stabilised by
a polar environment, as proposed for the dihydroalkoxyla-
tion of allenes (Scheme 7),[6] insertion of platinum into the
C3-H of the indole,[8] a reversible reaction involving vinyl
indole intermediates,[20] or a gold-type-catalysed reaction[10]
(Figure 2) could be taking place during the process as com-
4 fa 17[b]
6
7
1g
1h
4ga 13[c]
peting pathways to the formation of the bisACHTNUTRGNEUNG(indolyl) deriva-
tives, which would explain the low deuterium incorporation
observed, the loss of deuterium in the starting deuterated
indole and the formation of allyl indoles 5.
Therefore, further studies on the mechanism of the bisin-
dolylation reaction and the analogous dihydroalkoxylation
of allenes, are needed to fully understand this new mode of
reactivity, as well as the differences with the gold(I)-cata-
lysed version. Studies to understand these processes could
open the possibility of new divergent reactions in organic
chemistry, the extension of the methodologies to other
4ha
7
[a] Conversion >98% unless otherwise stated; isolated product yield.
[b] Conversion <100%. [c] Low yield possible due to the volatility of 1g.
solvent used (7 and 7’
Scheme 5. See the Supporting
Information for details). The
unreacted 2-methyl derivative
6c was recovered in all the con-
ditions tested (6c in the pres-
ence of AuI gives trimeric-mac-
rocyclic structures).[18]
Given the different reactivity
observed under platinum catal-
ysis, the possibility of an intra-
Scheme 6. Platinum-catalysed intra-intermolecular reaction of allene-indole derivatives 6 in the presence of an
intermolecular[19]
bisindolyla- external indole 3.
4502
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4499 – 4504

Platinum-Catalysed Bisindolylation of Allenes
COMMUNICATION
[1] T. S. Kam in Alkaloids, Chemical and Biological Perspectives (Ed.:
S. W. Pelletier), Pergamon, Amsterdam, 1999, Vol. 4.
[2] a) R. J. Sundberg in The Chemistry of Indoles, Academic Press, New
York, 1970; b) J. E. Sexton in Indoles (The Chemistry of Heterocy-
clic Compound V 25), Wiley, New York, 1983; c) R. J. Sundberg, in
Indoles, Academic Press, New York, 1996.
[3] R. Veluri, I. Oka, I. Wagner-Dçbler, H. Laatsch, J. Nat. Prod. 2003,
66, 1520–1523.
[4] For selected examples, see: a) J. S. Yadav, B. V. S. Reddy, G. Sa-
theesh, Tetrahedron Lett. 2004, 45, 3673–3676; b) R. Nagarajan, P. T.
Perumal, Chem. Lett. 2004, 33, 288–289; c) D. StC. Black, D. C.
Craig, N. Kumar, Tetrahedron Lett. 1991, 32, 1587–1590.
[5] Reviews: a) D. R. Taylor, Chem. Rev. 1967, 67, 317; b) A. S. Hashmi,
Angew. Chem. 2000, 112, 3737–3740; Angew. Chem. Int. Ed. 2000,
39, 2285; c) S. Ma, Chem. Rev. 2005, 105, 2829 and references cited
therein.
[6] M. P. MuÇoz, M. C. de La Torre, M. A. Sierra, Adv. Synth. Catal.
2010, 352, 2189–2194.
[7] a) N. Nishina, Y. Yamamoto, Tetrahedron Lett. 2008, 49, 4908–4911;
b) Z. Zhang, R. A. Widenhoefer, Org. Lett. 2008, 10, 2079–2081;
c) Z. Zhang, S. Du, A. S. Fisher, R. A. Widenhoefer, Tetrahedron
2009, 65, 1794–1798; d) N. Nishina, Y. Yamamoto, Tetrahedron
2009, 65, 1799–1808; e) D.-M. Cui, K. R. Yu, C. Zhang, Synlett 2009,
1103–1106; f) D.-M. Cui, Z.-L. Zheng, C. Zhang, J. Org. Chem.
2009, 74, 1426–1427; g) R. S. Paton, F. Maseras, Org. Lett. 2009, 11,
2237–2240; h) M. S. Hadfield, A.-L. Lee, Org. Lett. 2010, 12, 484–
487. For another example describing a different outcome when com-
paring Au and Pt, see: A. S. K. Hashmi, E. Kurpejovic, W. Frey,
J. W. Bats, Tetrahedron 2007, 63, 5879–5885.
Scheme 7. Preliminary mechanistic hypothesis for the platinum-catalysed
bis(indolylation) of allenes.
[8] For an intermolecular Pt-catalysed reaction of allenols in the pres-
ence of indoles, see: W. Kong, Y. Yu, G. Chen, C. Fu, S. Ma, Org.
Lett. 2009, 11, 1213–1216.
Figure 2. Other possible intermediates in the platinum-catalysed bisindo-
lylation of allenes.
[9] For a Sc-catalysed bis(indolylation)of allenic ketones, see: S. Ma, S.
Yu, Org. Lett. 2005, 7, 5063–5065.
carbon-based nucleophiles and the synthesis of novel and
more complex structures not obtained so far containing im-
portant motifs found in natural products with potential bio-
logical activity.
[10] K. L. Toups, G. T. Liu, R. A. Widenhoefer, J. Organomet. Chem.
2009, 694, 571–575. For the Au-catalysed intermolecular hydroaryla-
tion of allenes, see: M. A. Tarselli, A. Liu, M. R. Gagnꢃ, Tetrahe-
dron 2009, 65, 1785–1789. For the Pd-catalysed version see: Z.
Fang, C. Fu, S. Ma, Chem. Eur. J. 2010, 16, 3910–3913.
[11] For the Au-catalysed intramolecular hydroarylation of allenes with
indoles, see: a) H. C. Shen, Tetrahedron 2008, 64, 3885–3903; b) Z.
Zhang, C. Liu, R. E. Kinder, X. Han, H. Qian, R. A. Widenhoefer,
J. Am. Chem. Soc. 2006, 128, 9066–9073; c) C. Liu, R. A. Widenhoe-
fer, Org. Lett. 2007, 9, 1935–1938; d) R. A. Widenhoefer, Chem.
Eur. J. 2008, 14, 5382–5391. For a Pt-catalysed reaction of indolyl al-
lenols, see: e) W. Kong, C. Fu, S. Ma, Chem. Commun. 2009, 4572–
4574. For a Pd-catalysed example, see: f) E. M. Beccalli, A. Bernas-
coni, E. Borsini, G. Broggini, M. Rigamonti, G. Zecchi, J. Org.
Chem. 2010, 75, 6923–6932. For the Au-catalysed intramolecular hy-
droarylation of allenes, see: g) M. A. Tarselli, M. R. Gagnꢃ, J. Org.
Chem. 2008, 73, 2439–2441; h) D. Weber, M. A. Tarselli, M. R.
Gagnꢃ, Angew. Chem. 2009, 121, 5843–5846; Angew. Chem. Int. Ed.
2009, 48, 5733–5736; i) W. Kong, C. Fu, S. Ma, Eur. J. Org. Chem.
2010, 6545–6555. For the Pt version, see: j) J. Mo, P. H. Lee, Org.
Lett. 2010, 12, 2570–2573. For an example of inter- and intramolecu-
lar iodoarylation of allenes, see: k) J. Barluenga, E. Campos-Gꢂmez,
A. Minatti, D. Rodriguez, J. M. Gonzꢄlez, Chem. Eur. J. 2009, 15,
8946–8950.
Experimental Section
General procedure for the bisindolylation of allenes: A solution of the
corresponding allene (1 equiv) in dry THF (0.2 mm) and MeOH
(3 equiv) was added to PtCl2 (5 mol%) and the corresponding indole (3
equiv) under Ar. The reaction mixture was heated at 708C for 20 h. The
reaction was then cooled to room temperature, filtered through Celite
and washed with Et2O and CH2Cl2. The solvent was evaporated and the
crude purified by flash column chromatography on silica gel using
hexane/Et2O as eluent.
Acknowledgements
The present work was started at the IQOG-CSIC with the support of
a Ramꢂn y Cajal Award and a grant from the Spanish Government
(CTQ2010-20714-02), to whom the author would like to thank. Funding
by the University of East Anglia (UEA) is gratefully acknowledged. The
author wants to thank Dr. Goss (UEA) for the supply of some indole de-
rivatives and the EPSRC National Mass Spectrometry Service Centre,
Swansea, for accurate HRMS analysis.
[12] Traces of the 3-allyl indole 5aa
were observed when the reaction
was carried out in less polar sol-
vents. When only one equivalent
of indole was used, compound
4aa was the major product in all
the cases tested (see the Support-
ing Information for details).
[13] The two-fold reactivity observed in our reaction has also been re-
ported for the gold(I)-catalysed reaction of indoles and furans with
Keywords: allenes · bisindolylation · catalysis · gold ·
indoles · platinum
Chem. Eur. J. 2012, 18, 4499 – 4504
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4503

M. Paz MuÇoz et al.
alkynes, but to the best of our knowledge, this is the first example of
platinum-catalysed bisindolylation of allenes; a) Z. Li, Z. Shi, C. He,
J. Organomet. Chem. 2005, 690, 5049–5054; b) A. S. K. Hashmi,
M. C. Blanco, Eur. J. Org. Chem. 2006, 4340–4342; c) A. S. Hashmi,
L. Schwarz, J.-H. Choi, T. M. Frost, Angew. Chem. 2000, 112, 2382–
2385; Angew. Chem. Int. Ed. 2000, 39, 2285–2288; d) C. Ferrer,
C. H. M. Amijs, A. M. Echavarren, Chem. Eur. J. 2007, 13, 1358–
1373; e) J. Barluenga, A. Fernꢄndez, F. Rodrꢀguez, F. J. FaÇanꢄs, J.
Organomet. Chem. 2009, 694, 546–550. For a Pt-catalysed version,
see: S. Bhuvaneswari, M. Jeganmohan, C.-H. Cheng, Chem. Eur. J.
2007, 13, 8285–8293. For a Pd-catalysed version, see: W. Lu, C. Jia,
T. Kitamura, Y. Fujiwara, Org. Lett. 2000, 2, 2927–2930. For a Ru-
catalysed version, see: V. Cardieno, J. Francos, J. Gimeno, Chem.
Commun. 2010, 46, 4175–4177. For a GaIII-catalysed example, see:
J. S. Yadav, B. V. S. Reddy, B. Padmavani, M. K. Gupta, Tetrahedron
Lett. 2004, 45, 7577–7579.
[16] For an example of intramolecular reaction of pyrrole and allenes,
see: Z. Liu, A. S. Wasmuth, S. G. Nelson, J. Am. Chem. Soc. 2006,
128, 10352–10353.
[17] Allenes 1a and 1g are commercially available (Aldrich) and were
used without further purification. Allenes 1b, 1c, 1d, 1e and 1 f,
were prepared from the corresponding alkynes by Crabbꢃ reaction,
using the microwave version: a) P. Crabbꢃ, B. Nassim, M.-T. Robert-
Lopes, Org. Synth., Coll. 1990, Vol. 7, p. 276; 1985, Vol 63, p. 203;
b) H. Nakamura, T. Sugiishi, Y. Tanaka, Tetrahedron Lett. 2008, 49,
7230–7233. Allene 1h was prepared using a modified Crabbꢃ reac-
tion: J. Kuang, S. Ma, J. Am. Chem. Soc. 2010, 132, 1786–1787.
[18] J. Barluenga, M. Piedrafita, A. Ballesteros, A. L. Suꢄrez-Sobrino,
J. M. Gonzꢄlez, Chem. Eur. J. 2010, 16, 11827–11831.
[19] For a Pd-catalysed intra/intermolecular alkene difunctionalisation
with an alcohol and an indole, see: T. P. Pathak, K. M. Gligorich,
B. E. Welm, M. S. Sigman, J. Am. Chem. Soc. 2010, 132, 7870–7871.
[20] G. Fridkin, N. Boutard, W. D. Lubell, J. Org. Chem. 2009, 74, 5603–
5606.
[14] C. M. Krauter, A. S. Hashmi, M. Pernpointner, ChemCatChem 2010,
2, 1226–1230.
[15] When more electron-withdrawing groups were attached or close to
the nitrogen of the indole (such us, P=Ts, Ac, or 7-azaindole), com-
peting hydroalkoxylation from the reaction with the methanol pres-
ent, or no reaction took place.
Received: October 24, 2011
Published online: March 13, 2012
4504
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4499 – 4504