Br?nsted Acid-Catalyzed Enantioselective Cycloisomerization of Arylalkynes

Article abstract of DOI:10.1002/chem.202003783

The first example of an enantioselective carbocyclization of an alkyne-containing substrate catalyzed by chiral Br?nsted acids was achieved. The use of the 2-hydroxynaphthyl substituent on the alkyne as a directing group constituted the key parameter enabling both efficient regioselective protonation of the carbon–carbon triple bond and chiral induction. The key cationic intermediate could be depicted either as a cationic vinylidene ortho-quinone methide or a stabilized vinyl cation. Atropoisomeric phenanthrenes derivatives were produced in high yields and good enantioselectivities under mild, metal-free reaction conditions in the presence of chiral N-triflylphosphoramide catalysts. The carbenic nature of the cationic intermediate was also exploited to describe an example of alkyne/alkane cycloisomerization.

Full text of DOI:10.1002/chem.202003783

Accepted Article
Accepted Article
Title: Brønsted Acid-Catalyzed Enantioselective Cycloisomerization of
Arylalkynes
Authors: Patrick Yves Toullec, Julien Gicquiaud, Baptiste Abadie,
Kalyan Dhara, Murielle Berlande, Philippe Hermange, and
Jean-Marc Sotiropoulos
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To be cited as: Chem. Eur. J. 10.1002/chem.202003783
Link to VoR: https://doi.org/10.1002/chem.202003783
01/2020

10.1002/chem.202003783
Chemistry - A European Journal
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Brønsted Acid-Catalyzed Enantioselective Cycloisomerization of
Arylalkynes
Julien Gicquiaud,[a] Baptiste Abadie,[a] Kalyan Dhara,[a] Murielle Berlande,[a] Philippe Hermange,[a] Jean-
Marc Sotiropoulos[b] and Patrick Y. Toullec*[a]
Dedicated with gratitude and affection to Prof. Jean-Pierre Genêt
[a]
[b]
Dr Julien Gicquiaud, Baptiste Abadie, Dr Kalyan Dhara, Murielle Berlande, Dr Philippe Hermange and Prof. Patrick Y. Toullec
CNRS, Bordeaux INP, ISM, UMR 5255, University of Bordeaux, F-33400 Talence, France
e-mail: patrick.toullec@u-bordeaux.fr
Prof. Jean-Marc Sotiropoulos
CNRS/UNIV PAU & PAYS ADOUR
Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Matꢀriaux (IPREM, UMR 5254)
Hꢀlioparc, 2 avenue du Prꢀsident Angot, 64053 Pau Cedex 09, France
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: This communication reports the first example of an
enantioselective carbocyclization of an alkyne-containing substrate
catalyzed by chiral Brønsted acids. The use of the 2-
hydroxynaphthyl substituent on the alkyne as a directing group
constitutes the key parameter enabling both efficient regioselective
protonation of the carbon-carbon triple bond and chiral induction.
The key cationic intermediate can be depicted either as a cationic
probably due to the long assumed instability of the
corresponding vinyl cation intermediates.[5b],[11]
Since the seminal report of the use of phosphoric acids in
asymmetric catalysis by Akiyama[12] and Terada[13] in 2004, the
advent and development of chiral Brønsted acids has led to the
emergence of a myriad of new enantioselective transformations
that proceed via mechanisms involving either Brønsted acid
catalysis[14] or hydrogen bond catalysis.[15] Notably, whereas
reactions involving the activation of carbonyl, imines and
Michael acceptors are very well described, enantioselective
hydroelementations of non-activated carbon-carbon multiple
bonds are less numerous, and invoke mechanisms based either
on transient covalent bondings,[16] activation of nucleophilic
partners[17] or more rarely direct protonations.[18] This situation is
directly linked to the low stability of the corresponding
carbocations that results in the formation of side-products and
the lack of interaction between the conjugate base of the
Brønsted acid and the carbocation that induces a weak chiral
induction. So far, chiral Brønsted acids demonstrated their ability
to promote the formation of stabilized allylic,[19] propargylic[20] or
benzylic[21] carbocations, but generation of vinylic carbocations
from alkynes and their use in asymmetric catalysis remains
under-tackled.
To achieve such enantioselective transformations with alkynes,
three problems must be overcome: (i) the lack of reactivity of
alkynes towards BA that requires harsh conditions and/or the
use of strong superacids to be protonated;[22] (ii) the lack of
polarization of unactivated alkynes delivering a mixture of
regioisomeric vinyl cations upon protonation;[23] (iii) the lack of
interaction between the carbocation and the chiral anion,
resulting in poor enantioinductions.[16] In particular, 1-naphthol
substituted alkynyls were selected as promising substrates for
this type of reaction. Indeed, whereas these versatile synthons
vinylidene ortho-quinone methide or
a stabilized vinyl cation.
Atropoisomeric phenanthrenes derivatives were produced in high
yields and good enantioselectivities under mild, metal-free reaction
conditions in the presence of chiral N-triflylphosphoramide catalysts.
The carbenic nature of the cationic intermediate was also exploited
to describe an example of alkyne/alkane cycloisomerization.
Activation of carbon-carbon unsaturations by carbophilic Lewis
acid appeared during the last two decades as a new synthetic
tool for hydrofunctionalization transformations.[1] The metallic
catalysts employed exhibit a high affinity for carbon-carbon
unsaturations, activating them towards outer sphere anti
nucleophilic attack upon complexation. Although this carbophilic
reactivity was observed for a set of late transition metals, the
metals exhibiting the greatest reactivity feature gold, platinum
and mercury,[2] characterized by their high cost and/or their
relative low availability and/or their toxicity. Considering that the
development of asymmetric transformations involving this type
of activation would require additional chiral and often highly
expensive ligands,[3] the search for new, alternative, catalytic
and metal-free methodologies that could provide the same
reactivity is of upmost interest.
In this context, the analogy of reactivity between carbophilic
transition metals and Brønsted acids (BA) has been well
established,[4] and use of the latter for protonation of alkynes [5]
[6]
and activation towards nucleophilic attack is known for a long
are known to react in base-catalyzed inverse electron-demand
time and would represent an organocatalytic alternative for the
development of asymmetric variants of reactions involving the
electrophilic activation of alkynes. To date, most of the Brønsted
acid-catalyzed cyclizations of substrates bearing an alkyne
function have been reported to proceed through the intermediate
[24]
[4+2] cycloadditions
and intramolecular hydroarylation of
alkynes[25] via a neutral, axially chiral vinylidene ortho-quinone
methide intermediate (VQM) (Scheme 1, eq. 1, intermediate
A),[26] the group of Tan recently reported that chiral phosphoric
acids could catalyze the intermolecular addition of naphthols to
1-alkynylnaphthols.[27] According to DFT calculations, the
asymmetric induction results in this case from the formation of a
generation
of
secondary/tertiary
carbenium
ions,[7]
oxocarbenium ions[8] or ketiminium/ketenium ions,[9] but fewer
examples deal with the direct protonation of an alkyne,[10]
1
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cationic chiral VQM intermediate via an enantioselective 1,5-
proton shift, and involves the bifunctional activation of both
phenanthrene 2a was observed in excellent yields (Table 1,
entries 2-3). As previously noted,[28] the chemo- and
electrophile and nucleophile reaction partners (Scheme 1, eq. 2). regioselectivity of the transformation was excellent and the
We thus considered that the reaction of 1-alkynylnaphthols with
chiral Brønsted superacids would deliver a cationic intermediate
that can be either depicted as a protonated VQM C or as a vinyl
cation C’ stabilized by conjugation (Scheme 1, eq. 3) exhibiting
an enhanced electrophilicity that would allow the intramolecular
attack of weak nucleophiles such as non-activated aromatic
systems, delivering chiral atropisomeric products even in the
absence of concomitant nucleophilic interaction with the catalyst
by hydrogen bonding.
Following our recent investigations showing that the
cycloisomerization of 2-alkynylbiaryls leading to phenanthrenes
can be accomplished in the presence of catalytic amounts of
BA,[28] we decided to study the enantioselective carbocyclization
of such substrates bearing 2-naphthol as potential directing
group in presence of chiral Brønsted superacids. Indeed, the
reaction would lead under very mild and metal-free conditions to
the corresponding atropisomeric[29] phenanthrenyl-2-naphthols
after intramolecular attack by weak aryl nucleophiles.
formation of benzofuran resulting from the O-attack of the
naphthol hydroxyl on the alkyne was not observed. The
reactivity observed with catalyst 7a was rather exciting, as its
acidity was in the pKa range of known chiral Brønsted
superacids[31] such as N-triflylphosphoramide (pKa(MeCN) ~ 6).[32]
To test this hypothesis, the cycloisomerization of naphthol 1a
was engaged in the presence of various chiral Brønsted acids.
Using 5 mol% of catalyst 7b in CH2Cl2 at room temperature, the
formation of 2a was observed in 97% yield with an encouraging
62.5:37.5 e.r (Table 1, entry 4). Modifying the 3,3-disubstitution
pattern of the 2,2-binaphtol moiety of the catalyst resulted in
diminished activities and enantioselectivities for 7c and 7d,
whereas a significant improvement of selectivity was observed
with 7e. Indeed, despite delivering 2a in a low yield of 18%, a
promising e.r of 82.5:17.5 was measured (Table 1, entries 5-7).
Despite a lower postulated pKa, catalyst 7f[33] exhibited no
activity under our standard reaction conditions, whereas
phosphoric acid 7g and disulfonimide[34] 7h showed no reactivity
(Table 1, entries 8-10). This demonstrates that aryl-substituted
alkynylnaphthols 1 are weaker Lewis bases compared to the
tertiobutyl analogues used by the group of Tan (Scheme 2, eq.
2).[27] The use of catalysts with other diol backbones such as 7i
and 7j was not beneficial in terms of both activity and
enantioselectivity (Table 1, entries 11,12). Selecting catalyst 7e,
a screening of solvent, temperature, catalyst loading and
concentration was performed (see supporting information for
details). Increasing the concentration to 0.25 M improved both
the yield and the enantioselectivity of the cycloisomerization
product (Table 1, entry 14). Lowering the temperature to -10°C
resulted in a complete loss of catalytic activity (Table 1, entry 16).
As expected, the use of the 2-naphthol moiety as a directing
group turned to be mandatory to reach high activity and
selectivity. Indeed, employing substrate 3a (where the hydroxyl
was substituted by a methoxy group) produced racemic cyclized
phenanthrene 4a in a very low yield (Table 1, entry 17). In the
same way, no cyclization to 6a was observed in the presence of
7a when using substrate 5a, which possessed a benzylic alcohol
function instead of the hydroxyl group (Table 1, entry 18). Thus,
stabilization of the vinyl cation by conjugation with the aryl
substituent or creation of hydrogen bonding between the catalyst
and the substituent are not sufficient factors to observe an
enantioselective cycloisomerization, the combination of the two
factors is needed to enable the chiral induction.
Scheme 1. Reactivity of neutral vs cationic VQM intermediates towards
nucleophilic attack of aromatic systems
In the initial model experiments, substrate 1a was cyclized in
presence of 5 mol% of a selection of catalysts. In the presence
of quinine as control experiment, no conversion of the substrate
was monitored (Table 1, entry 1). As anticipated, this result
showed that neutral VQM intermediates are not reactive enough
towards weak nucleophiles such as a phenyl group.[30] On the
contrary, employing achiral CF3SO3H or 7a, the formation of
2
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Table 1. Optimization of reaction conditions.[a]
products in good to excellent yields (52-99%) and high
enantioselectivities (86.5:13.5 to 91:9 e.r.). The reaction of a
substrate bearing an isopropyl substituent at the meta position of
the aryl nucleophile delivered a mixture of regioisomers 2f and
2f’ in a 4:1 ratio with good enantioselectivities (86:14 and 85:15
e.r., respectively). The 3,5-disubstitution was also well tolerated
(2g). In contrast, with an aryl nucleophile possessing an ortho
substituent, a significant drop of enantioselectivity was observed
(2h). Interestingly, the presence of an electron-withdrawing
substituent on the nucleophilic ring resulted in a diminished
conversion but an excellent enantioselectivity (2i, 96:4 e.r.). In
contrast, the presence of electron-donating substituents, such as
a 4-methoxy group, resulted in diminished enantioselectivities
(2j-k). Halogen substituents were well tolerated and resulted in
cyclized products in high yields and moderate (2l-m) to good
(2n) enantioselectivities (up to 90:10 e.r.).
Table 2. Substrate scope.[a]
Entry[a]
1
Catalyst
quinine
CF3SO3H
7a
Conc.
Yield[b]
0
e.r.[c]
0.05 M
0.05 M
0.05 M
0.05 M
0.05 M
0.25 M
0.05 M
0.05 M
0.05 M
0.05 M
0.05 M
0.05 M
0.1 M
-
2
99
99
99
71
4
-
3
-
4
(R)-7b
(R)-7c
(R)-7d
(R)-7e
(R)-7f
(R)-7g
(R)-7h
(S)-7i
62.5:37.5
54:46
52.5:47.5
82.5:17.5
n. d.
5
6
7
18
traces
0
8
9
-
10
11
12
13
14
15
16[d]
17[e,f]
18[g]
traces
8
n. d.
32:68
67:33
81:19
86:14
76.5:23.5
-
(R)-7j
(R)-7e
(R)-7e
(R)-7e
(R)-7e
(R)-7e
7a
20
73
90
84
0
0.25 M
0.5 M
0.05 M
0.25 M
0.25 M
4
50:50
-
0
[a] 0.1 mmol of 1a in CH2Cl2 at rt. [b] yields determined by 1H NMR. [c] (R):(S)
enantiomeric ratios determined by chiral HPLC. Configuration of the R-
enantiomer determined by circular dichroism (see SI). [d] reaction performed
at -10°C. [e] substrate 3a used. [f] 10 mol% of catalyst used [g] substrate 5a
used. conc: concentration.
[a] The reaction was performed with 0.1 mmol of substrate 1, 5 mol% of
catalyst 7e at r.t. in CH2Cl2 (0.25 M) for 70h. [b] Reaction run at 6°C using 10
mol% of catalyst 7e. [c] Reaction run for 100h, 40% conversion. [d] Reaction
run for 24h. [e] Reaction run at 40°C.
With these optimized conditions in hand, we next examined the
scope of the transformation (Table 2). The reaction proceeded
well with a variety of alkyl and aryl substituents on the para
position of the aryl nucleophile (2b-2e), furnishing atropisomeric
3
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The introduction of a substituent on the naphthol ring was also
investigated. Remarkably, in the case of substrate 1o
possessing two alkyne functions on the naphthol moiety, the
electrophilic activation occurred selectively on the carbon-carbon
triple bond placed on position 1. The substitution on the internal
aryl ring was also studied and moderate enantioselectivities
were obtained both for the 4-trifluoromethyl and the 3,5-dimethyl
substitution patterns. Finally, the use of a substituted phenol as
a directing group was also evaluated: the cycloisomerization
delivered product 2r in low yield (14%) and enantioselectivity
(77.5:22.5 e. r.). This result obtained with unoptimized
conditions/catalytic system already demonstrates that this new
mode of activation of alkynes using Brønsted acid catalysts is
not restricted to 2-naphthol directing groups.
delivering a stabilized single pseudo diastereoisomer C that
cyclizes upon attack of the nucleophilic aryl substituent.[27]
Alternatively, a non-selective protonation step may result in the
formation of two pseudo diastereoisomeric ion pairs C’ in
equilibrium upon rotation along the naphthol/vinyl cation axis. In
this case, the enantioinduction would result from a nucleophilic
attack of the aryl group on the more stable of these two
intermediates. The latter hypothesis is backed up by the
relationship linking reaction rate and enantioselectivity with
nucleophilic strength, as demonstrated by the trend observed
with substrates 1j, 1a and 1i bearing –OMe, -H and –CO2Me
groups at the para position of the aryl nucleophile. Indeed, full
conversion was achieved for the first substrate in less than 24h,
whereas 40h was needed for the second one and only 40%
conversion of 1j was obtained after 100h. Concomitantly, the
enantiomeric excesses increased from 5%, to 80% and 92%,
respectively. Interestingly, the same trend was also observed
with substrates 2t and 2u, which are devoid of undesired
electronic effects onto the alkyne originating from the
substitution of the nucleophilic aryl moiety (87% e.e. for -C6H5 vs
9% e.e for -C6H4-OCH3, respectively).
We also investigated the cycloisomerization of alkyl-substituted
alkyne substrates (Table 3). The cyclization of substrate 1s
proceeded
smoothly
to
deliver
the
atropisomeric
dihydronaphtalene 2s in good yield and a disappointing 26% ee.
Considering the results obtained in the intermolecular
hydroarylation of alkynes with the tert-butyl substituent,[27] we
studied the reactivity of a substrate bearing a cyclopropyl group
at the propargylic position: the tricycle 2t was obtained with an
excellent yield and a very good enantioselectivity. Once more,
the nucleophilicity of the aryl nucleophile has a strong influence
on the selectivity: substrate 1u bearing a 4-methoxyphenyl
substituent reacts in the presence of 7e to give the product 2u
with a very low ee.
Table 3. Cycloisomerization of alkyl-substituted alkynes.[a]
Scheme 2. Proposed mechanism of the catalytic asymmetric
carbocycloisomerization of 1a catalyzed by N-triflyl phosphoramide Brønsted
acids.
[a] The reaction was performed with 0.1 mmol of substrate 1, 5 mol% of
catalyst 7e at r.t. in CH2Cl2 for 40h. [b] [1s] = 0.1 M. [c] [1t-u] = 0.25 M.
To assess the vinyl cation character of intermediate C/C’, we
decided to investigate its ability to promote C-H insertion. Indeed,
as experimentally observed[5] and then rationalized by DFT
studies,[37] vinyl cations possess a carbenic reactivity[38] resulting
in their insertion into inert C-H bonds. We thus prepared
substrate 1v bearing an isopropyl chain and submitted it to
cycloisomerization in the presence of 5 mol% of Brønsted acid
catalyst 7a as model catalyst (Scheme 3). Interestingly, the
reaction delivered indene 8 and polycycle 9 in 40% and 41%,
respectively.
In accordance with these results, a postulated mechanism was
suggested in Scheme 2 for the reaction of 1a in the presence of
BA catalysts. At the initial stage of the catalytic cycle, the
presence of the naphthol directing group both increases the
basicity of the alkyne function and polarizes the carbon-carbon
triple bond leading to a regioselective protonation that delivers
an intermediate that can be viewed either as (i) a protonated
VQM[27, 35] C or (ii) a stabilized vinyl cation[36] C’ that can react as
a stronger electrophile compared to its neutral counterpart A
(Scheme 1). The origin of the enantioselectivity observed with a
chiral catalyst may result from
a chiral protonation step
4
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carbon 1 – carbon 2 bond (rotamers C1 and C2) and of the
carbon 3 - carbon 4 bond (C3 and C4) (Scheme 4, a). The
barrier of rotation between C1 and C3 corresponds to a ΔG‡ of
26.4 kcal/mol, showing that cationic VQM C exhibit a low
configurational stability at room temperature on the time scale of
the reaction, and that isomerization can occurs through rotation
around the vinyl cation/naphthol axis of mesomer C’. The
representation of the molecular orbitals (depicted in scheme 4, b
for rotamer C4 as an example) clearly shows a vacant p orbital
(corresponding to the LUMO) strongly localized on the C3
central carbon and orthogonal to a π bond (corresponding to the
HOMO). Such description is fitting perfectly with a carbenoïd
structure, in accordance to the experimental results observed in
C-H activation reactions (Scheme 3)
In conclusion, we have developed a catalytic asymmetric
carbocyclization of 2-alkynylbiphenyls to atropisomeric
phenanthrenes catalyzed by a Brønsted superacid catalyst. This
method, based on the use of the 2-naphtholyl substituent,
provides a metal-free alternative to the carbophilic late transition
metals Lewis acids for cycloisomerization involving the
electrophilic activation of alkynes as the initial step of the
catalytic cycle. Mechanistic studies showed that the carbenic
nature of the stabilized vinyl cation intermediate can be used to
initiate an alkyne/alkane cycloisomerization reaction. Current
studies are underway in our group to optimize the steric and
electronic features for both the directing group and the catalyst
in order to obtain higher levels of selectivity. The scope of
nucleophiles that can be applied in such transformations is also
under evaluation and results will be reported in due course.
Scheme 3. Brønsted acid-catalyzed alkyne-alkane cycloisomerization.
The postulated mechanism involves the protonation of the
carbon-carbon triple bond to give the stabilized vinyl cation D.
Further intramolecular C-H insertion at the benzylic position
leads to the formation of carbocation E, that can react either via
elimination of a proton to give indene 8, or via O-addition of the
naphthol hydroxyl to give 9. Remarkably, when indene 8 was
treated in the presence of Brønsted acid 7a, no formation of 9
was observed.
Acknowledgements
This work was supported by the Centre National de la
Recherche Scientifique (CNRS) and the Ministère de
l’Enseignement Supꢀrieur et de la Recherche. The Agence
Nationale de la Recherche is kindly acknowledged for a
research grant (ANR-19-CE07-0010). The “Direction du
Numꢀrique” of the Universitꢀ de Pau et des Pays de l′Adour,
MCIA (Mésocentre de Calcul Intensif Aquitain) and CINES under
allocation A005080045 made by Grand Equipement National de
Calcul Intensif (GENCI) are acknowledged for computational
facilities. J. G. acknowledges the Ministère de l’Enseignement
Supérieur et de la Recherche for
a PhD grant. K. D.
acknowledges the Idex Bordeaux for a postdoctoral grant.
Yutaka Okazaki and Antoine Scalabre (CBMN, university of
Bordeaux) are acknowledged for recording CD spectra.
Alexandre Karnat is acknowledged for the preparation of
compound 2i.
Scheme 4. a) Four rotamers found on the PES (main distances in Ångström),
b) HOMO and LUMO of rotamer C4.
Keywords: asymmetric catalysis • Brønsted acid •
cycloisomerization • atropoisomer • alkyne
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Chiral N-triflyl phosphoramides catalyze the asymmetric carbocyclization of alkynyl aryl substrates. The introduction of a 2-naphtolyl
substituant on the alkyne enables the chemo- and regioselective protonation of alkynes and delivers atropisomeric phenanthrenes
and dihydronaphtalenes enantioselectively.
7
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