Friedel-Crafts Alkylation of Indoles with p-Quinols: The Role of Hydrogen Bonding of Water for the Desymmetrization of the Cyclohexadienone System

Article abstract of DOI:10.1021/acs.orglett.6b00781

Lewis acid catalyzed Friedel-Crafts alkylation of indoles has been achieved in high yields and selectivities using p-quinols as electrophiles. (S)-Binol-3,3′-(9-anthracenyl)-phosphoric acid was able to catalyze the enantioselective formation of 5-(3-indol

Full text of DOI:10.1021/acs.orglett.6b00781

Letter
pubs.acs.org/OrgLett
Friedel−Crafts Alkylation of Indoles with p‑Quinols: The Role of
Hydrogen Bonding of Water for the Desymmetrization of the
Cyclohexadienone System
†
†
‡
,‡
,†
́
́
Carolina García-García, Laura Ortiz-Rojano, Susana Alvarez, Rosana Alvarez,* María Ribagorda,*
and M. Carmen Carreno*,†
̃
†Departamento de Química Organ
́
ica, Facultad de Ciencias, Universidad Auton
Cantoblanco, 28049 Madrid, Spain
‡Departamento de Química Organ
ica, Facultad de Química, Universidade de Vigo, CINBIO, Lagoas-Marcosende s/n, 36310 Vigo,
́ ́
oma de Madrid, C/Francisco Tomas y Valiente n° 7,
́
Spain
S
* Supporting Information
ABSTRACT: Lewis acid catalyzed Friedel−Crafts alkylation
of indoles has been achieved in high yields and selectivities
using p-quinols as electrophiles. (S)-Binol-3,3′-(9-anthracen-
yl)-phosphoric acid was able to catalyze the enantioselective
formation of 5-(3-indole)-2-cyclohexenone derivatives. Exper-
imental results and theoretical calculations explained the
enantioselectivity based on a transition state where two water
molecules act as a tether joining the p-quinol with the
phosphoric acid and the NH of indole, thus facilitating the desymmetrization of the prochiral cyclohexadienone framework.
reaction of p-cresol and indole in the presence of (diacetoxy-
iodo)benzene (PIDA), which in situ generated the 4-methoxy-4-
methyl-2,5-cyclohexadienone moiety. 3-(5-Methoxy-2-methyl
phenyl)-1H-indole was formed as a result of the aromatization
of the initial 1,4-addition product (Scheme 1).18
he chemistry of indoles has been extensively studied due to
Tthe widespread appearance of these units in a number of
structures ranging from natural products1 to pharmaceutical
compounds.2 Different approaches have been reported for the
synthesis of the heterocyclic system and its functionalization.3 3-
Substituted indoles, the most frequently present moieties in such
structures, are available by Friedel−Crafts (FC) reactions. The
development of efficient asymmetric versions of these alkylations
has been the focus of interest of several groups in the past
decades.4 Lewis acids incorporating chiral ligands, chiral
Brønsted acids, and organocatalysts are currently available to
access optically active electrophilic aromatic substitution
products.5 Alkylation of indoles with enones,6 β,γ-unsaturated-
α-keto esters,7 or nitroalkenes8 is a powerful C−C bond forming
process giving the 1,4-addition products that have also been
achieved using asymmetric catalysts. Alkylidene malonates,9 α′-
hydroxy-α,β-unsaturated ketones,10 and α′-phosphoric enones11
are also useful electrophiles for alkylation of indoles.
Scheme 1. FC Reactions of Indole with p-Quinol Systems
4-Hydroxy-4-alkyl-2,5-cyclohexadienones (p-quinol deriva-
tives) are double Michael-type acceptors12 with a challenging
prochiral cyclohexadienone moiety. Catalytic desymmetrization
of such moieties has been mainly achieved in an intramolecular
fashion via Heck,13 Stetter,14 and Michael type reactions.15,16
The intermolecular catalytic desymmetrization of p-quinols has
been accomplished by Feringa et al. in the Cu-catalyzed
conjugate addition of dialkylzinc reagents, using enantiopure
phosphoramidite ligand.17 Despite the interest in FC alkylations
(FCAlk) of indoles in organic chemistry, the reaction using p-
quinols as alkylating agents remained unexplored to date. A
related FC arylation (FCAryl) reaction has been reported by
In continuation of our work devoted to extending the synthetic
applications of p-quinols,19 we present herein the intermolecular
FC reaction of indoles using p-quinols as electrophilic alkylating
agents, to give the desired 1,4-addition products under mild
conditions in a highly diastereoselective manner. We also report
our attempts to achieve an enantioselective version of the process
and a computational study to rationalize the experimental
observations (Scheme 1).
Received: March 24, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00781
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We initiated our study using p-quinol 1a19a and 1H-indole 2a
as model substrates (Scheme 2). After extensive screening with
Scheme 3. Iron Catalyzed FC Reaction of 2a with 1b−e
Scheme 2. Iron Catalyzed FC Reaction with p-Quinol 1a
version of this FC process. Reaction between 1a and 2a was
chosen as a model to study the desymmetrization of the prochiral
2,5-cyclohexadienone moiety. Initially, we tried iron chiral
catalysts24 generated from FeCl3·6H2O and enantiopure ligands
such as Evan’s bis(oxazolines), Feringa’s phosphoramidites,17
(R)-VAPOL, or chiral iron Lewis acids derived from Salen
derivatives,24 but we did not observed enantioselection. Use of
the (S)-Phanephos ligand gave 3a in 8% ee and low conversion
(20%). Based on previous asymmetric Brønsted acid catalyzed
indole alkylations,25 we checked the model reaction in the
presence of (R)-BINOL derived phosphoric acids 12 (Scheme
4).
different catalysts, solvents, and temperatures,20 we could
establish that the best results were obtained using FeCl3·6H2O
(10 mol %) and CH2Cl2 (0.1 M) as solvent at rt. Under these
conditions, a mixture of 3a and 4a was obtained in a 92:8 ratio
(GC/MS), from which 3a was isolated in 91% yield as a single
diastereomer.21 Compound 3a resulted from the 1,4-addition of
indole to 1a from the less hindered face, containing the
hydroxyl.22 Formation of 4a must result from OH elimination
and enolization of 3a, favored by the acidic conditions. We next
evaluated different substituted indoles under the optimized
conditions (Scheme 2). N-Me indole 2b reacted faster than 2a,
giving a 75:25 mixture of 3b (64%) and 4b (24%). 5-Bromo
indole 2c, 6-fluoro indole 2d, and 6-methoxycarbonyl indole 2e
gave only the FCAlk products 3c, 3d, and 3e (87%, 95%, and 82%
yield). Indoles having an electron-donating group, such as 7-
methyl and 6-methoxy 2f and 2g, gave the FCAlk products 3f and
3g as major products, although variable amounts of aromatic
derivatives 4f and 4g were also formed. A lower yield was
observed in the reaction of 1a with 1,2-dimethylindole, probably
due to the steric hindrance at the C-3 position.
a
Scheme 4. Enantioselective FC of 2a with p-Quinol
a
i
HPLC (Chiralpack IC-0.8 mL/min-15% PrOH).
Other electron-poor systems, such as N-Boc indole as well as
5-nitro and 4-carboxaldehyde 1H-indoles, did not react after long
reaction times and upon heating (60 °C). Other electron-rich
heterocycles were also checked as substrates for alkylation with p-
quinol 1a. No evolution was observed with furan, thiophene, and
2-methylthiophene. Pyrrole gave a complex mixture where the 2-
pyrrole substituted monoalkylation product and the 2,5-
dialkylated pyrrole derivatives were detected in a 1:3 ratio
respectively (see compounds 10 and 11 in the Supporting
Information).23 2,4-Dimethyl pyrrole gave the FCAlk product 5
(60% yield), whereas only the FCAryl derivative 6 (50% yield) was
formed in the reaction with 2-methyl furan (Scheme 2).
Other p-quinols, such 3-methyl-p-quinol 1b, reacted with 2a to
give the FCalk product 7 in 70% yield, resulting from the attack of
2a to the unsubstituted β-carbon of the precursor. 4-Phenyl-p-
quinol 1c gave compound 8 in a low 18% yield. 2,6-Dimethyl-p-
quinol 1d proved to be a poor Michael acceptor, and the reaction
gave the FCAryl derivative 9 in 26% yield. Interestingly, no
reaction occurred with p-quinol methyl ether 1e, showing that
under these conditions the free OH is crucial in the FC process
(Scheme 3).
Reaction with (R)-12a−e (10 mol %) gave the FCAlk 3a in a
>98% conversion on the basis of GC-MS analysis, but
unfortunately with null or poor enantiomeric excess. No reaction
was observed with 4-(2-naphthyl)phenyl phosphoric acid 12f,
while a poor conversion (12%) and 0% ee resulted in the
presence of 1-(4-mesytyl)phenyl substituted 12g. The best result
in terms of conversion and enantioselectivity (>98%, 32% ee) was
obtained using 9-anthracenyl phosphoric acid (S)-12h. Final
tuning23 of other parameters such as solvents, substrate
concentration, temperature, additives,26 and catalyst loading
allowed the best conditions to be established [(S)-12h (10 mol
%), CH3CN (0.5 M), −20 °C] to obtain 3a in a 72% yield and
72% ee. Interestingly, the addition of 4 Å molecular sieves to the
reaction significantly decreased both the rate and enantiose-
lectivity (17%, 10 d, 44% ee). This indicated the important role of
water in the catalytic process.27 The (4R,5R) absolute
configuration of the major enantiomer 3a was established by
comparison of the experimental ECD spectra of pure
enantiomers (separated by chiral-HPLC from an enantioen-
riched mixture).23
The great interest in the alkylation products for further
transformations prompted us to explore the enantioselective
We next explored the scope of the (S)-12h catalyzed FC
reaction varying the heterocycle substitution under the
B
DOI: 10.1021/acs.orglett.6b00781
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Scope of the Enantioselective FC Alkylation
optimized reaction conditions (Scheme 5). Electron-with-
drawing substituted indoles, such as 5-bromo or 6-fluoro indole
2c and 2d gave the FCalk products 3c and 3d in moderate to good
yields and 62% ee and 64% ee. No significant differences were
observed in the reactivity of electron-rich indoles 2f and 2g, since
the reactions were completed in 6 days leading to 3f and 3g in
52% ee and 62% ee, respectively. 2,4-Dimethyl pyrrole reacted
faster (24 h) affording compound 5 in 45% ee (75% yield). In all
cases the FCaryl product 4 was not observed. Methyl 1H-indole-6-
carboxylate did not react under the standard conditions. N-Me
indole gave the FCalk compound 3b in a 50% yield, but with only
an 8% ee, thus indicating the essential role of the NH in the
process. The presence of the free OH in the p-quinol was also
crucial for the reactivity, since using 4-methoxy-p-quinol
derivative 1e gave no reaction.
To confirm the interactions responsible for the enantiose-
lectivity, we developed a computational study of the bond
forming step taking into account three important experimental
observations: (1) the lack of reaction of indole 2a with 4-
methoxy-p-quinol 1e, lacking the free OH; (2) the important role
of traces of water for both reactivity and enantioselectivity; and
(3) the differences in enantioselectivity for the NH free indole
(72% ee) and the N-methylindole (8% ee). Our DFT
investigations were carried out using as a model system the
reaction of 1a and 2a catalyzed by the (R) enantiomer of 12h
leading to the desymmetrization product. The two-point
coordination Akiyama model,28 described for coordination of
the BINOL phosphoric acid catalyst to the two reagents, was
adopted. To reproduce the experimentally observed role of water
in the FC reaction, two molecules of H2O were included in our
studies. In the absence of indole, the water H-bonding-
coordinated system was positioned (in a linear-trans mode) in
a network that also involves the hydroxyl group of p-quinol 1a
and the basic site of (R)-binol-3,3′-(9-anthracenyl)-phosphoric
acid 12h (Figure 1A). In order to conveniently carry out the DFT
study,23 we selected the reaction on the less hindered face (OH
containing face of the p-quinol), as this was experimentally
observed. The approach to the rear face of p-quinol in the 1a-2·
H2O-(R)-12h associated system is blocked by steric interference
of the methyl group. In contrast, the coordination of H2O opens
a cavity in the OH-containing face that can be later occupied by
indole 2a. Without disturbing the water-mediated H-bonding
network, indole N−H also binds the phosphoric acid basic site.
The most stable transition state was calculated to be the one
resulting from the Si-face, pro-R β-carbon attack, shown in Figure
1B, leading to the (4S,5S)-3a enantiomer.
Figure 1. Calculated complexes of (A) 1a-(R)-12h-2·H2O complex and
(B) (4S,5S)-H2O-TSI−II. Relevant atom distances (Å) are shown
(wB97XD/PCM(CH3CN)/6-31G*//wB97XD/6-31G*).29
Interestingly, it was found that the presence of water not only
forms a more compact cavity in (4S,5S)-H2O-TSI−II (H−O
distances 1.76, 1.72, and 1.77 Å) than in the diastereomeric one
(approach to the pro-S β-carbon) but also plays a stabilizing role
in this TS, through an OH−π interaction with the aryl ring of the
phosphoric acid.30 Moreover, a comprehensive analysis of both
transition structures revealed that the lowest-energy transition
state represented in Figure 1B exhibited another important
noncovalent interaction, namely an edge-to-face π−π interaction
(2.78 Å) between the indole (H-7) and an aryl ring of the
catalyst. This type of interaction has been recognized to play
important roles in chemical and biological recognition
processes,31 and also in the enantioselectivity, as shown by the
desymmetrization of 1a. In order to estimate the effect of the
water molecules in the reaction, both the nonassisted and the
water-assisted variants of the Re face, pro-S approach have been
computed.23 Comparison of the energies of activation (in gas
phase) and the structures of the transition states of the FC
reaction indicated that the network of water molecules favored
(by 9 kcal/mol) the 1,4-addition of 2a to 1a. This effect might be
due to the modulation of the acidity of phosphoric acid (R)-12h,
which translates into an increase of the nucleophilicity of indole
at its C3 position. In agreement with these calculations, the
desymmetrization of 1a in the (R)-12h catalyzed reaction with 2a
must occur preferentially by the Re-face, pro-S β-carbon double
bond of 1a to give the (4S,5S)-3a. Thus, the (4R,5R)-3a
enantiomer must be formed in the (S)-12h catalyzed process, as
experimentally observed.
In summary, we have developed an efficient method for the
construction of 5-indolyl substituted-4-hydroxy-4-methyl-2-
cyclohexenone systems in a high π-facial diastereoselective
manner. We demonstrated for the first time that intermolecular
FC alkylations of indoles can be achieved using p-quinols as an
electrophilic partner. The asymmetric version of the process has
been accomplished in the presence of chiral BINOL derived
phosphoric acids as organocatalysts with up to 72% ee. The
computational study of the asymmetric FC reaction confirmed
the important role of water, increasing the nucleophilicity of the
C3 position of indole, as well as the origin of stereoselectiviy
based on π-stacking (π−π and HO−π) and steric interactions.
C
DOI: 10.1021/acs.orglett.6b00781
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2221. (c) Carreno, M. C.; Merino, E.; Ribagorda, M.; Somoza, A.;
̃
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00781.
■
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: carmen.carrenno@uam.es.
*E-mail: maria.ribagorda@uam.es.
*E-mail: rar@uvigo.es.
Notes
(16) (a) Vo, N. T.; Pace, R. D. M.; O’Hara, F.; Gaunt, M. J. J. Am. Chem.
Soc. 2008, 130, 404−5. (b) Corbett, M. T.; Johnson, J. S. Chem. Sci.
2013, 4, 2828−2832. (c) Takizawa, S.; Nguyen, T. M.-N.; Grossmann,
A.; Enders, D.; Sasai, H. Angew. Chem., Int. Ed. 2012, 51, 5423−6.
(d) Gu, Q.; You, S.-L. Org. Lett. 2011, 13, 5192−5195. (e) During the
preparation of this manuscript a gold (I) phosphoric acid catalyzed
hydroamination/Michael addition cascade process was reported: Zhao,
F.; Li, N.; Zhu, Y.-F.; Han, Z.-Y. Org. Lett. 2016, 18, 1506−9.
(17) Imbos, R.; Brilman, M. H. G.; Pineschi, M.; Feringa, B. L. Org. Lett.
1999, 1, 623−626.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
́
This work is dedicated to Prof. Angel Rodriguez de Lera on the
occasion of his 60th birthday. We thank MICINN (Grants
CTQ2011-24783, CTQ2012-37734-C02-01, and CTQ2014-
53894-R) for financial support. C.G.G. and L.O.R. thank MEC
for FPU and FPI fellowships. We thank Centro de Super-
computacion de Galicia (CESGA) for the generous allocation of
́
computing resources.
(18) Ye, Y.; Wang, H.; Fan, R. Synlett 2011, 2011, 923−926.
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Urbano, A.; Carreno, M. C. Chem. Commun. 2013, 49, 3561−3563. (c)
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DOI: 10.1021/acs.orglett.6b00781
Org. Lett. XXXX, XXX, XXX−XXX