Mechanistic Insights into the B(C6F5)3-Initiated Aldehyde-Aniline-Alkyne Reaction to Form Substituted Quinolines

Article abstract of DOI:10.1021/acs.organomet.7b00174

A substoichiometric quantity of the Lewis acid B(C₆F₅)₃ is sufficient to initiate the aldehyde-amine-alkyne reaction, in a one-pot methodology that enables the synthesis of a range of functionalized quinolines. Optimization studies revealed that key requirements for the high-yielding tricomponent reaction initiated by B(C₆F₅)₃ at raised temperatures include an excess of the in situ generated imine (which acts as a hydrogen acceptor) and an alkyne substituent able to stabilize positive charge buildup during the cyclization. Mechanistic experiments revealed that under these conditions B(C₆F₅)₃ is acting as a Lewis acid-assisted Br?nsted acid, with H₂O-B(C₆F₅)₃ being the key species enabling catalytic quinoline formation. This was indicated by deuterium labeling studies and the observation that the cyclization of N-(3-phenylpropargyl)aniline using B(C₆F₅)₃ under anhydrous conditions afforded the zwitterion [(N-H-3-B(C₆F₅)₃-4-Ph-quinolinium], which does not undergo protodeboronation to release B(C₆F₅)₃ and the quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an operationally simple and effective methodology for the production of functionalized quinolines.

Full text of DOI:10.1021/acs.organomet.7b00174

Article
pubs.acs.org/Organometallics
Mechanistic Insights into the B(C6F5)3‑Initiated Aldehyde−Aniline−
Alkyne Reaction To Form Substituted Quinolines
Valerio Fasano, James E. Radcliffe, and Michael J. Ingleson*
School of Chemistry, University of Manchester, Manchester M13 9PL, United Kingdom
S
* Supporting Information
ABSTRACT: A substoichiometric quantity of the Lewis acid
B(C6F5)3 is sufficient to initiate the aldehyde−amine−alkyne
reaction, in a one-pot methodology that enables the synthesis
of a range of functionalized quinolines. Optimization studies
revealed that key requirements for the high-yielding
tricomponent reaction initiated by B(C6F5)3 at raised temper-
atures include an excess of the in situ generated imine (which
acts as a hydrogen acceptor) and an alkyne substituent able to stabilize positive charge buildup during the cyclization.
Mechanistic experiments revealed that under these conditions B(C6F5)3 is acting as a Lewis acid-assisted Brønsted acid, with
H2O−B(C6F5)3 being the key species enabling catalytic quinoline formation. This was indicated by deuterium labeling studies
and the observation that the cyclization of N-(3-phenylpropargyl)aniline using B(C6F5)3 under anhydrous conditions afforded
the zwitterion [(N-H-3-B(C6F5)3-4-Ph-quinolinium], which does not undergo protodeboronation to release B(C6F5)3 and the
quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an
operationally simple and effective methodology for the production of functionalized quinolines.
a
uinolines are ubiquitous in natural products, pharma-
Scheme 1. Pathways for the A3 Reaction
Q ceutically active compounds, ligands, and functional
materials.1 Over the past two centuries, multiple routes to
quinolines have been developed since the first synthesis by
Skraup,2 including the Doebner−Von Miller,3 Combes,4
Conrad−Limpach,5 Friedlander,6 and Pfitzinger syntheses.7
Although these methods are effective, they often require
prefunctionalization of the starting materials, and regioselectiv-
ity issues can arise when using 1,3-dielectrophiles. Multi-
component reactions (MCRs), in which three or more
compounds are mixed together in a single step, are efficient
methods to rapidly furnish complex products.8 The MCR
approach has been applied to quinoline syntheses via the
aldehyde−aniline−alkyne reaction (also referred to as the A3
reaction). In this an imine, produced in situ, undergoes
nucleophilic attack by an alkyne to yield a propargylamine.9
The N-aryl propargylamine then reacts further, forming a
quinoline motif upon cyclization/oxidation (Scheme 1).
a
LA = Lewis acid.
reported that B(C6F5)3 activates alkynes toward cyclization to
form complex aromatic carbo- or boracyclic compounds (e.g.,
dibenzopentalene and benzoborolate).14 Similarly, intramolec-
ular hydroaminations15 and oxoborations16 of terminal alkynes
have been reported using stoichiometric B(C6F5)3, allowing the
synthesis of N- and O-heterocyclic aromatic compounds. The
possibility of cyclizing alkynes with catalytic B(C6F5)3 has also
been investigated as reported for 1,5-enynes17 or anilines with a
pendent alkyne (Scheme 2, top).16
Furthermore, in previous work, Stephan and co-workers have
reported that the frustrated Lewis pair (FLP) system N-
benzylidene tert-butylimine/B(C6F5)3 was able to activate
phenylacetylene, yielding the iminium alkynylborate (Scheme
2, bottom).18 Interestingly, upon heating the reaction, the
The first application of the A3 reaction for quinoline
synthesis used catalytic CuCl,10 and since then, multiple
examples have been reported using a range of transition metal
or lanthanide catalysts (generally proposed to form metal
acetylide nucleophiles during the catalytic cycle, Scheme 1a).11
A different pathway proceeds with Brønsted and certain Lewis
acids, which instead activate the imine to nucleophilic attack by
the alkyne, in a variant of the Povarov reaction (Scheme 1b).12
In the past decade, the Lewis acid B(C6F5)3 has attracted
attention as a metal-free alternative to transition metal catalysis,
with one major focus being the functionalization of alkynes.13
On the basis of previous work using B(C6F5)3 we were
interested in determining if B(C6F5)3 was a viable catalyst for
the A3 reaction. Specific literature relevant to this study
Received: March 6, 2017
© XXXX American Chemical Society
A
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a
Scheme 2. (Top) Intramolecular Hydroamination of
Terminal Alkynes with Catalytic B(C6F5)3; (Bottom)
Propargyl Amine Formation Using B(C6F5)3
Table 1. A3 Reaction Initiated by B(C6F5)3
PhCHO
(equiv )
PhNH2
(equiv)
B(C6F5)3
(mol %)
temp
yield 1
b
a
entry
(°C)
(%)
1
2
3
4
5
6
7
8
1
2
3
5
3
3
3
3
3
3
3
3
1.2
2.2
3.2
5.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
20
20
20
20
20
20
20
20
20
20
10
0
80
80
33
52
80
63
80
63
90
70
100
110
120
100
100
100
100
73 (70)
69
authors observed NMR spectra consistent with acetylide
transfer to the iminium cation, but isolation of any propargyl-
amines was unsuccessful. On the basis of the above precedence,
it is feasible that the Lewis acid B(C6F5)3 could be employed as
catalyst to accomplish the A3 reaction due to its ability to
activate both alkyne (acetylide formation) and imine (via
nitrogen coordination) and initiate cyclization of substituted
alkynes.13e Furthermore, B(C6F5)3 can be tolerant of water and
anilines/imines (at raised temperatures); thus the imine can be
formed in situ (producing an equivalent of H2O), which is
necessary for the A3 reaction.19 However, because strong
Brønsted acids also can catalyze the A3 reaction, H2O−
B(C6F5)3 is also a potential initiator, as it has a similar pKa to
HCl (in MeCN),20 and this Brønsted acid has been used
recently by Maron et al. to catalyze a number of conversions.21
Herein we report the B(C6F5)3-initiated A3 reaction for the
synthesis of substituted quinolines and our mechanistic
investigations to identify the key initiator.
66
c
9
65
d
10
72
11
12
65
<5
a
Reactions run in sealed tubes under ambient atmosphere (1 equiv of
b
alkyne used in each reaction). Conversion relative to the alkyne and
calculated by 1H NMR spectroscopy versus cyclohexane as an internal
c
standard (in parentheses = isolated yield). Reaction in PhCl.
Reaction in o-dichlorobenzene.
d
(entries 5−8). The reaction performed at 100 °C proved
optimal, affording the desired quinoline 1 in 73% NMR yield
and 70% isolated yield, a notable outcome for the three-
component A3 reaction (entry 6). The optimized conditions
gave similar outcomes in other halogenated solvents such as
chloro- or 1,2-dichlorobenzene (entries 9 and 10). Finally,
reducing the B(C6F5)3 loading to 10 mol % resulted in a slightly
lower conversion, while in the absence of B(C6F5)3 the reaction
was unsuccessful (entries 11 and 12, respectively).
To assess the feasibility of a mechanism involving N-(3-
phenylpropargyl)aniline formation and subsequent B(C6F5)3-
initiated cyclization/oxidation, N-(3-phenylpropargyl)aniline
was combined with an equimolar amount of B(C6F5)3 in wet
DCE. On mixing, the initial boron species formed was
[HOB(C6F5)3]− (by 11B and 19F NMR spectroscopy) from
deprotonation of H2O−B(C6F5)3 by the aniline as previously
reported.19 However, upon heating to 100 °C for 1 h, the
zwitterion 2 was produced as the major product (Scheme 3).
No significant quantity of reduction byproducts (e.g., di- or
tetrahydroquinolines) was observed by 1H NMR spectroscopy,
consistent with the evolution of gas observed. This indicates
that H2 is produced as the major byproduct from oxidation to
form 2. The 11B NMR spectrum of the zwitterion 2 showed a
sharp peak at −14 ppm, while 15 inequivalent signals were
observed in the 19F NMR spectrum due to the hindered
rotation in this 3,4-disubstituted quinoline.
RESULTS AND DISCUSSION
■
Our investigation started by assessing the feasibility of B(C6F5)3
to initiate the formation of quinolines, specifically, adding 1
equiv of p-tolylacetylene to N-benzylidene aniline (generated in
situ from the condensation of 1 equiv of benzaldehyde and 1.2
equiv of aniline), in the presence of 20 mol % B(C6F5)3, using
1,2-dichloroethane (DCE) as solvent (Table 1). The reaction
was performed under ambient conditions, without any
purification of starting materials, catalyst, or solvent. Upon
heating at 80 °C for 24 h, full imine consumption was observed,
but no propargylamine was detected. Instead, the two major
1
products formed in a 1:1 ratio (by H NMR spectroscopy)
were 4-tolyl-2-phenylquinoline 1 and N-benzylaniline (entry 1).
The use of other solvents (e.g., MeCN or toluene) under
otherwise identical conditions led to lower conversions to 1.
While quinoline 1 resulted from the desired three-
component reaction, N-benzylaniline derives from the reduc-
tion of the parent imine, presumably via a transfer hydro-
genation process where the imine acts as a hydrogen acceptor
in the oxidation step to form the quinoline as previously
observed.22 This prompted us to increase the amount of both
benzaldehyde and aniline (entries 2−4), which improved the
quinoline yield significantly (63%, entry 3) up to a certain
loading, beyond which no further improvement was observed.
In previous work, raised temperatures proved essential for
protonation of the inactive [HOB(C6F5)3]− to enable a critical
concentration of B(C6F5)3 to form post H2O dissociation;19
therefore the reaction was performed at higher temperatures
The zwitterion 2 was isolated in 45% yield by crystallization
from DCM/pentane. The solid-state structure (Scheme 3,
right) confirmed the formulation as 2, which contains all
trigonal planar carbons and nitrogen in the quinoline motif.
Steric crowding was indicated by the C3-phenyl ring being
almost orthogonal to the quinoline moiety (dihedral angles
∼85°) along with large C3−C2−B1 (126.8(2)°) and C2−C3−
C10 (123.1(2)°) angles. The formation of 2 is notable, as in the
absence of the phenyl alkyne substituent no cyclization was
B
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a
Scheme 3. Elemento-boration of N-(3-
Scheme 4. Deuterium Labeling Experiment
Phenylpropargyl)aniline with B(C6F5)3: (Right) Solid-State
Structure of 2, Thermal Ellipsoids at the 50% Probability
Level (a Cocrystallized Molecule of DCM Is Omitted for
Clarity); (Inset) Disparate Outcomes Reported by Stephan,
Paradies, and Co-workers on the Cyclization of Terminal-
and Phenyl-Substituted Alkynes
a
Identical reaction conditions to Table 1, entry 6.
present in nonpurified DCE and derived from the imine
formation. Therefore, based on these experiments a mechanism
proceeding by acetylide transfer and an N-propargylaniline
intermediate is disfavored.
An alternative possible mechanism is imine activation by an
electrophile with the iminium species then reacting directly
with the terminal alkyne. To probe this, the reaction between
N-benzylidene aniline and p-tolylacetylene (3:1 equiv) was
explored in the presence of purified B(C6F5)3 (20 mol %) in
dichloromethane (DCM) (at 60 °C) or in DCE (at 100 °C)
under anhydrous conditions. In both cases upon heating for 24
h, quinoline 1 was a minor product, with the imine-borane
adduct and the iminium alkynylborate salt observed in situ as
major species (by 11B and 19F NMR spectroscopy). The
reaction was repeated using an equimolar mixture of imine/
alkyne/B(C6F5)3 under anhydrous conditions. Upon mixing,
11B/19F NMR analysis revealed that the main species formed at
room temperature were the imine-borane adduct (11B NMR
broad peak at −2.7 ppm), the iminium alkynylborate (11B
NMR at −20.8 ppm), and the 1,2-addition product (11B NMR
at −15.9 ppm, Scheme 5). After prolonged stirring (20 h at 25
observed by Stephan and co-workers on reacting B(C6F5)3 with
N-propargylaniline.15b This highlights the essential stabilizing
effect the phenyl provides to a vinyl cation-type intermediate.
Furthermore, the formation of 2 indicates that the 6-endo-dig
cyclization is favored over 5-exo-dig; in contrast many examples
of the latter are reported for the cyclization of propargyla-
mides16 or for the inter- and intramolecular hydroamination of
terminal alkynes with B(C6F5)3.15 Once again, the alkynyl
phenyl group plays a key role in favoring the formation of the
vinyl cation that leads to the 6-endo-dig cyclization over that
leading to 5-exo-dig cyclization. An analogous observation has
been recently reported by Paradies and colleagues in the
cyclization/dehydrogenation of benzyl amino alkyne with
stoichiometric B(C6F5)3 (Scheme 3, inset).15c
Scheme 5. 11B NMR δ of Imine-Borane Adduct (Red), the
1,2-Addition Product (Blue), and the Iminium
Alkynylborate (Green), Obtained from an Equimolar
Mixture of N-Benzylidene Aniline, p-Tolylacetylene, and
B(C6F5)3
Notably, prolonged heating did not result in subsequent
conversion of 2 to free B(C6F5)3 and quinoline. Furthermore, 2
proved stable to protodeboronation with exogenous acid (e.g.,
stable to HNTf2 addition) or bases, even on prolonged heating
(e.g., refluxing in neat AcOH). This indicates that the catalytic
reaction to form 1 does not proceed via N-propargylaniline
cyclization initiated by B(C6F5)3. This was confirmed by the
reaction of 0.2 equiv of unpurified B(C6F5)3 in “wet” 1,2-
dichloroethane with 1 equiv of N-(3-phenylpropargyl)aniline
with heating at 100 °C for 1 h, leading to formation of 2 as a
minor product, but even after prolonged heating (24 h)
significant N-(3-phenylpropargyl)aniline remained unreacted.
This latter reaction also indicates that an alternative mechanism
involving N-(phenylpropargyl)aniline cyclization on activation
by a Brønsted acid (e.g., H2O−B(C6F5)3) instead of B(C6F5)3
also was not proceeding. To confirm this using the conditions/
reagents that lead to 1, labeling experiments using a deuterated
alkyne were performed (Scheme 4). Using the optimized
reaction conditions (entry 6), replacing p-tolylacetylene with 1-
deuterium-p-tolylacetylene (>98% D-incorporation), a com-
parable yield of quinoline 1 was obtained. Importantly, the
isolated quinoline showed >90% deuterium incorporation at
position 3. This outcome is incompatible with the formation of
N-(1,3-diphenylpropargyl)aniline as an intermediate, as cycliza-
tion of this species by a Brønsted acid will produce a low level
of deuterium incorporation due to the large excess of H2O
°C), the imine-borane adduct and the iminium alkynylborate
were the two major boron species (by 11B/19F NMR
spectroscopy), with 4-tolyl-2-phenylquinoline 1 and 4-tolyl-2-
phenyl-1,2-dihydroquinoline observed as minor products by 1H
NMR and GC-MS analysis.
These results suggest the formation of the imine-borane
Lewis adduct is not the dominant pathway leading to 1 due to
the disparity observed in the degree of conversion to 1
performing the reaction with catalytic B(C6F5)3 under “wet”
conditions (as in the A3 reaction) versus that under anhydrous
conditions (isolated imine and alkyne with no Brønsted acid
present). This prompted us to search for an alternative
electrophile for imine activation. The H2O−B(C6F5)3 adduct
has been previously utilized as a Brønsted acid catalyst,21 and
we envisaged that the principal imine activation pathway could
be by protonation of the imine. To test this hypothesis, the
optimized reaction (aldehyde/aniline:alkyne, 3:3.2:1) was
repeated under “wet” conditions in DCE replacing B(C6F5)3
C
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with 20 mol % HCl, a Brønsted acid with comparable pKa to
H2O−B(C6F5)3 (pKa = 8.5 and 8.4, respectively, in MeCN).20
However, upon heating this reaction mixture at 100 °C for 24
h, minimal quinoline 1 was formed using HCl as catalyst. While
initially surprising we surmised that despite the similar pKa
values in MeCN, in DCE and with aniline/imine bases the
degree of association between the protonated imine and
chloride or [HOB(C6F5)3]− counteranions would be signifi-
cantly different. The degree of association between iminium
cations and the anion has been previously documented to affect
significantly the electrophilicity of the iminium cation.23 This is
supported by the observation that the N-H proton of N-
benzylidene anilinium is shifted 1.2 ppm further downfield
when the chloride counterion was replaced by [HOB(C6F5)3]−,
indicating a weaker association between the iminium cation and
[HOB(C6F5)3] anion (Scheme 6).
to the imine for proton transfer to occur directly). [HOB-
(C6F5)3]− is then a suitably weakly coordinating anion to allow
the iminium cation (I) to be sufficiently electrophilic to
undergo nucleophilic attack by the alkyne. The generated vinyl
cation intermediate II, stabilized by the phenyl substituent, is
then trapped by nucleophilic attack by the ortho-carbon of the
aniline, yielding the dihydroquinolinium species III. The latter
releases a proton (possibly by protonating the anion [HOB-
(C6F5)3]− or directly protonating another molecule of imine)
and forming IV (or tautomers thereof). Oxidation of the
dehydroquinoline furnishes the final quinoline, with an
equivalent of imine acting as a hydrogen acceptor. This transfer
hydrogenation could be catalyzed by B(C6F5)3, as reported in
previous studies.22
Combining the mechanistic investigations allows the key
requirements for the successful three-component reaction
initiated by B(C6F5)3 at raised temperatures in “wet” solvent
to be defined as (i) sufficient excess in situ imine to act as
hydrogen acceptor and (ii) a substituent adjacent to the C−C
triple bond of the alkyne able to stabilize the vinyl cation
intermediate. With the requirements for the A3 reaction in
hand, the scope of this process catalyzed by 20 mol % of
nonpurified B(C6F5)3 in “wet” DCE was explored (Table 2,
condition A). The same substrate screening was also tested in
1
Scheme 6. H NMR δ of N-Benzylidene Anilium Salts
Seeking a Brønsted acid that will lead to a more activated
iminium cation (than the Cl congener), HNTf2 was
investigated, as it is reported to be almost a factor of 108
more Brønsted acidic than H2O−B(C6F5)3, albeit in MeCN
(pKa HNTf2 = 0.3 in MeCN).24 N-Benzylidene aniline was
protonated with 1 equiv of HNTf2 in DCE, which led to an N-
H resonance shifted further downfield to 12.4 ppm. This
chemical shift suggests a more electrophilic iminium cation is
formed, and therefore the A3 reaction was repeated using 20
mol % of HNTf2. This led to the desired quinoline 1 being
obtained in 75% NMR yield under identical conditions to that
using B(C6F5)3 (entry 6, Table 1), confirming that a Brønsted
acid-initiated process is feasible provided a sufficiently weakly
coordinating anion is utilized.12d Based on these observations
feasible key processes occurring in situ in the tricomponent
reaction are summarized in Scheme 7.
a
Table 2. A3 Reaction Catalyzed by Triarylboranes
As reported in our previous work,19 H2O−B(C6F5)3 is
reversibly deprotonated by the weakly Brønsted basic anilines.
At raised temperatures, enough H2O−B(C6F5)3 is available to
protonate the imine (or the aniline is close enough in basicity
Scheme 7. Proposed A3 Reaction Pathway Initiated by H2O−
B(C6F5)3
a
Reactions run in sealed tubes under ambient atmosphere (aldehyde/
b
c
aniline/alkyne, 3:3.2:1). Isolated yield. 1H NMR yield using C6H12
as standard. 48 h.
d
D
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on an Agilent Technologies 7890A GC system equipped with an
Agilent Technologies 5975C inert XL EI/CI MSD with a triple-axis
detector. The column employed was an Agilent J&W HP-5 ms ((5%-
phenyl)methylpolysiloxane) with the following dimensions: length, 30
m; internal diameter, 0.250 mm; film, 0.25 μm. Mass spectra were
recorded on a Waters QTOF mass spectrometer. Purity was indicated
by multinuclear NMR spectroscopy in organic solvents (in which the
sample fully dissolved) and supported by MS or GC-MS analysis.
Synthesis of 2. A J. Young NMR tube equipped with a DMSO-d6
capillary was loaded with N-(3-phenylpropargyl)aniline (16 mg, 0.076
mmol, 1.00 equiv) and commercial B(C6F5)3 (39 mg, 0.076 mmol,
1.00 equiv) in 1,2-dichloroethane (0.5 mL). After heating at 100 °C
for 1 h, the main product detected by multinuclear NMR spectroscopy
was the zwitterion quinolinium borate 2 (slight effervescence was seen
upon opening the J. Young tube). The homogeneous reaction mixture
was dried under reduced pressure, washed with hexane, and dried,
obtaining the product 2 as a pale yellow solid (24 mg, 0.034 mmol,
the presence of catalytic B(3,5-Cl2-C6H3)3, synthesized in situ
from the protodeboronation of sodium tetrakis(3,5-
dichlorophenyl)borate (termed herein [Na][BArCl]), as
previously reported (Table 2, conditions B).25
This scoping revealed that different functionalized alkynes
were compatibile (5a−c), with in situ conversions and isolated
yields generally good. Furthermore, the alkyne substituent is
not limited to (hetero)aryl groups, as shown by 5b. However,
the importance of the cation stabilizing effect was highlighted
when 1-octyne and p-trifluoromethylphenylacetylene were used
as alkynes, with no desired quinolines observed in both cases.
The formation of quinoline 5b shows that the alkyne moiety
reacts in preference to the vinyl moiety26 and that the imine
acts as hydrogen acceptor in preference over the CC bond
(benzylaniline was again observed as a byproduct). Functional
groups on the benzaldehyde moiety were also tolerated (5d−f).
The lower yield of quinoline 5f derived from pentafluor-
obenzaldehyde could be due to the lower Brønsted basicity of
the fluorinated imine, disfavoring protonation. Replacing
benzaldehyde with cyclohexylcarboxyaldehyde was also possi-
ble, obtaining the desired quinoline 5g in good yield. Finally,
substitutions on the anilines component were also feasible
(5h−j), although when 2-methylaniline was employed, lower
yields were obtained (5k), possibly due to the increased steric
bulk around the imine nitrogen increasing the barrier to
nucleophilic attack by the alkyne.
In summary, B(C6F5)3 is an effective initiator for the
aldehyde−aniline−alkyne reaction at raised temperature to
form quinolines. This is an operationally simple methodology
where all the components are mixed at the start under ambient
conditions and without the need for purification of solvent,
borane, or reagents. Excess aldehyde and aniline has to be used
since the imine acts as a hydrogen acceptor, facilitating the
oxidation of the dihydroquinoline. Mechanistic insights
revealed that the reaction does not proceed via alkyne
activation by B(C6F5)3. Instead imine protonation by H2O−
B(C6F5)3 appears to be the main imine activation mode,
although direct imine complexation to the borane may still be
operative as a minor pathway. Finally, this study shows that the
catalytic activity of H2O−B(C6F5)3 as a Brønsted acid has to be
carefully assessed alongside that of B(C6F5)3 when utilizing this
borane in “wet” conditions; furthermore, despite their
comparable pKa values (in MeCN) the use of HCl as a test
for Brønsted acid catalysis by H2O−B(C6F5)3 is not necessarily
sufficient, as the different coordinating ability of Cl− and
[HOB(C6F5)3]− can result in disparate outcomes in low-
nucleophilicity media (e.g., dichloroethane).
1
45%). H NMR (CD2Cl2, 400 MHz, 298 K): δ 11.93 (brs, 1H, NH),
8.85 (s, 1H), 7.98 (d, J = 8.1 Hz, 1H), 7.94 (t, J = 8.1 Hz, 1H), 7.61 (t,
J = 7.6 Hz, 1H), 7.37 (t, J = 7.5 Hz, 1H), 7.28 (d, J = 8.7 Hz, 1H),
1
6.80−7.80 (brm, 3H), 5.70−6.60 (brs, 1H) ppm. H NMR (CD2Cl2,
400 MHz, 243 K): δ 13.10 (brs, 1H, NH), 8.78 (s, 1H), 8.02 (d, J =
8.5 Hz, 1H), 7.91 (t, J = 7.5 Hz, 1H), 7.56 (t, J = 7.5 Hz, 1H), 7.38−
7.46 (m, 1H), 7.28−7.38 (m, 2H), 7.19 (d, J = 8.5 Hz, 1H), 6.99−7.10
(m, 1H), 5.89 (d, J = 7.2 Hz, 1H) ppm. 11B NMR (CD2Cl2, 128 MHz,
298 K): δ −14.0 ppm. 19F NMR (CD2Cl2, 376 MHz, 298 K): δ
−126.2 (d, J = 24.2 Hz, 1Fo), −127.5 (brs, 1Fo), −129.3 (brs, 1Fo),
−132.0 (brs, 1Fo), −132.5 (d, J = 21.5 Hz, 2Fo), −160.1 (t, J = 20.1
Hz, 1Fp), −161.9 (m, 2Fp), −163.7 (t, J = 18.4 Hz, 1Fm), −164.9 (bs,
1Fm), −166.7 (m, 3Fm), −167.9 (brs, 1Fm) ppm. 13C{1H} NMR
(CD2Cl2, 100 MHz, 298 K): δ 167.6, 148.1, 136.5, 135.3, 134.3, 131.1,
129.6, 129.1, 128.9, 128.5, 128.2, 119.1 ppm (C−B and C−F carbons
were clearly resolved). MS: m/z calcd for C33H11BF15N 717.1, found
ES− 716.2 [M − H+]−, found ES+ 550.1 [M − (C6F5)−]+. Accurate
mass for [M − H+]− 716.0672, found 716.0688.
General Procedure for the A3 Reaction Initiated by B(C6F5)3
or [Na][BArCl]. A J. Young NMR tube equipped with a DMSO-d6
capillary was loaded with unpurified B(C6F5)3 (0.2 equiv) or
[Na][BArCl] (0.1 equiv) in nonpurified 1,2-dichloroethane, followed
by the addition of aldehyde (3.0 equiv), aniline (3.2 equiv), and alkyne
(1.0 equiv). After monitoring the initial reaction mixture by
multinuclear NMR spectroscopy, the J. Young NMR tube was heated
at 80 °C for 24 h. After cooling to room temperature, the system was
monitored again by multinuclear NMR spectroscopy. The reaction
mixture was then concentrated under reduced pressure, and the crude
analyzed by GC-MS analysis and purified by flash column
chromatography on silica gel.
4-(4-Methoxyphenyl)-2-phenylquinoline (5a). Following the gen-
eral procedure, a J. Young NMR tube was loaded with B(C6F5)3 (14
mg, 0.027 mmol, 0.20 equiv) in 1,2-dichloroethane (0.5 mL), followed
by the addition of benzaldehyde (42 μL, 0.401 mmol, 3.00 equiv),
aniline (39 μL, 0.431 mmol, 3.20 equiv), and 4-ethynylanisole (18 μL,
0.135 mmol, 1.00 equiv). After heating at 100 °C for 24 h, the reaction
mixture was cooled to room temperature and then concentrated in
vacuo. The crude was purified by flash chromatography (petr. ether/
Et2O, 95:5), obtaining 4-(4-methoxyphenyl)-2-phenylquinoline as a
pale brown solid (38 mg, 0.122 mmol, 90%). Rf = 0.15. The NMR data
were in agreement with those reported in the literature.27
4-(Cyclohex-1-en-1-yl)-2-phenylquinoline (5b). Following the
general procedure, a J. Young NMR tube was loaded with B(C6F5)3
(14 mg, 0.027 mmol, 0.20 equiv) in 1,2-dichloroethane (0.5 mL),
followed by the addition of benzaldehyde (42 μL, 0.401 mmol, 3.00
equiv), aniline (39 μL, 0.431 mmol, 3.20 equiv), and 1-
ethynylcyclohexene (16 μL, 0.135 mmol, 1.00 equiv). After heating
at 100 °C for 24 h, the reaction mixture was cooled to room
temperature and then concentrated in vacuo. The crude was purified
by flash chromatography (petr. ether/Et2O, 95:5), obtaining 4-
(cyclohex-1-en-1-yl)-2-phenylquinoline as a pale yellow oil (28 mg,
0.098 mmol, 73%). Rf = 0.25. The NMR data were in agreement with
those reported in the literature.27
EXPERIMENTAL SECTION
■
General Comments. Unless otherwise indicated, all manipulations
were conducted under ambient conditions and all compounds and
solvents were purchased from commercial sources and used as
received. When required, commercial B(C6F5)3 was dried by stirring a
suspension in pentane with Et3SiH (5 equiv for 1 equiv of borane) for
24 h at room temperature, followed by in vacuo solvent removal and
subsequent sublimation. Solvents for column chromatography were of
technical grade and used without further purification. Column
chromatography was performed on silica gel (230−400 mesh).
NMR spectra were recorded with a Bruker AV-400 spectrometer.
1H and 13C{1H} NMR chemical shifts are reported in ppm relative to
residual protio impurities and the deuterated solvent resonance,
2
respectively, while H, 11B, and 19F NMR spectra were referenced to
external Si(CD3)4, BF3·Et2O, and Cl3CF, respectively. Coupling
constants J are given in hertz (Hz). GC-MS analysis was performed
E
DOI: 10.1021/acs.organomet.7b00174
Organometallics XXXX, XXX, XXX−XXX

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Article
2-Phenyl-4-(thiophen-2-yl)quinoline (5c). Following the general
procedure, a J. Young NMR tube was loaded with B(C6F5)3 (15 mg,
0.029 mmol, 0.20 equiv) in 1,2-dichloroethane (0.5 mL), followed by
the addition of benzaldehyde (45 μL, 0.438 mmol, 3.00 equiv), aniline
(43 μL, 0.468 mmol, 3.20 equiv), and 3-ethynylthiophene (15 μL,
0.146 mmol, 1.00 equiv). After heating at 100 °C for 24 h, the reaction
mixture was cooled to room temperature and then concentrated in
vacuo. The crude was purified by flash chromatography (petr. ether/
Et2O, 95:5), obtaining 2-phenyl-4-(thiophen-2-yl)quinoline as a pale
yellow solid (31 mg, 0.108 mmol, 74%). Rf = 0.20. The NMR data
were in agreement with those reported in the literature.27
2,4-Ditolylquinoline (5d). Following the general procedure, a J.
Young NMR tube was loaded with B(C6F5)3 (14 mg, 0.026 mmol,
0.20 equiv) in 1,2-dichloroethane (0.5 mL), followed by the addition
of 4-methylbenzaldehyde (47 μL, 0.390 mmol, 3.00 equiv), aniline (38
μL, 0.416 mmol, 3.20 equiv), and 4-ethynyltoluene (17 μL, 0.130
mmol, 1.00 equiv). After heating at 100 °C for 48 h, the reaction
mixture was cooled to room temperature and then concentrated in
vacuo. The crude was purified by flash chromatography (petr. ether/
Et2O, 95:5), obtaining 2,4-ditolylquinoline as a pale brown solid (26
mg, 0.084 mmol, 65%). Rf = 0.20. The NMR data were in agreement
with those reported in the literature.28
2-(4-Chlorophenyl)-4-(4-methoxyphenyl)quinoline (5e). Follow-
ing the general procedure, a J. Young NMR tube was loaded with
B(C6F5)3 (13 mg, 0.024 mmol, 0.20 equiv) in 1,2-dichloroethane (0.5
mL), followed by the addition of 4-chlorobenzaldehyde (52 mg, 0.359
mmol, 3.00 equiv), aniline (35 μL, 0.383 mmol, 3.20 equiv), and 4-
ethynylanisole (16 μL, 0.120 mmol, 1.00 equiv). After heating at 100
°C for 24 h, the reaction mixture was cooled to room temperature and
then concentrated in vacuo. The crude was purified by flash
chromatography (petr. ether/Et2O, 95:5), obtaining 2-(4-chlorophen-
yl)-4-(4-methoxyphenyl)quinoline as a pale yellow solid (33 mg, 0.097
mmol, 81%). Rf = 0.15. The NMR data were in agreement with those
reported in the literature.29
6-Chloro-4-(4-methoxyphenyl)-2-phenylquinoline (5i). Following
the general procedure, a J. Young NMR tube was loaded with
[Na][BArCl] (8 mg, 0.013 mmol, 0.10 equiv) in 1,2-dichloroethane
(0.5 mL), followed by the addition of benzaldehyde (42 μL, 0.401
mmol, 3.00 equiv), 4-chloroaniline (56 mg, 0.428 mmol, 3.20 equiv),
and 4-ethynyltoluene (18 μL, 0.135 mmol, 1.00 equiv). After heating
at 100 °C for 18 h, the reaction mixture was cooled to room
temperature and then concentrated in vacuo. The crude was purified
by flash chromatography (petr. ether/Et2O, 95:5), obtaining 6-chloro-
4-(4-methoxyphenyl)-2-phenylquinoline as a pale brown solid (38 mg,
0.110 mmol, 82%). Rf = 0.15. The data were in agreement with those
reported in the literature.32
4-(4-Methoxyphenyl)-6-methyl-2-phenylquinoline (5j). Following
the general procedure, a J. Young NMR tube was loaded with
[Na][BArCl] (8 mg, 0.013 mmol, 0.10 equiv) in 1,2-dichloroethane
(0.5 mL), followed by the addition of benzaldehyde (42 μL, 0.404
mmol, 3.00 equiv), 4-methylaniline (47 mg, 0.431 mmol, 3.20 equiv),
and 4-ethynylanisole (18 μL, 0.135 mmol, 1.00 equiv). After heating at
100 °C for 18 h, the reaction mixture was cooled to room temperature
and then concentrated in vacuo. The crude was purified by flash
chromatography (petr. ether/Et2O, 95:5), obtaining 4-(4-methox-
yphenyl)-6-methyl-2-phenylquinoline as a yellow solid (41 mg, 0.126
mmol, 93%). Rf = 0.10. The NMR data were in agreement with those
reported in the literature.33
8-Methyl-2,4-diphenylquinoline (5k). Following the general
procedure, a J. Young NMR tube was loaded with [Na][BArCl] (8
mg, 0.013 mmol, 0.10 equiv) in 1,2-dichloroethane (0.5 mL), followed
by the addition of benzaldehyde (42 μL, 0.401 mmol, 3.00 equiv), 2-
methylaniline (46 μL, 0.428 mmol, 3.20 equiv), and phenylacetylene
(15 μL, 0.134 mmol, 1.00 equiv). After heating at 100 °C for 18 h, the
reaction mixture was cooled to room temperature and then
concentrated in vacuo. The crude was purified by flash chromatography
(petr. ether/Et2O, 95:5), obtaining 8-methyl-2,4-diphenylquinoline as
a pale gray solid (21 mg, 0.071 mmol, 53%). Rf = 0.20. The NMR data
were in agreement with those reported in the literature.34
2-(Perfluorophenyl)-4-phenylquinoline (5f). Following the general
procedure, a J. Young NMR tube was loaded with [Na][BArCl] (8 mg,
0.013 mmol, 0.10 equiv) in 1,2-dichloroethane (0.5 mL), followed by
the addition of 2,3,4,5,6-pentafluorobenzaldehyde (50 μL, 0.401
mmol, 3.00 equiv), aniline (39 μL, 0.428 mmol, 3.20 equiv), and
phenylacetylene (15 μL, 0.134 mmol, 1.00 equiv). After heating at 100
°C for 18 h, the reaction mixture was cooled to room temperature and
then concentrated in vacuo. The crude was purified by flash
chromatography (petr. ether/Et2O, 95:5), obtaining 2-(perfluoro-
phenyl)-4-phenylquinoline as a white solid (20 mg, 0.054 mmol, 40%).
Rf = 0.15. The NMR data were in agreement with those reported in
the literature.30
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00174.
Analytical data and crystallographic information (PDF)
Crystallographic data (CIF)
2-Cyclohexyl-6-methoxy-4-phenylquinoline (5g). Following the
general procedure, a J. Young NMR tube was loaded with
[Na][BArCl] (8 mg, 0.013 mmol, 0.10 equiv) in 1,2-dichloroethane
(0.5 mL), followed by the addition of cyclohexane carboxyaldehyde
(50 μL, 0.401 mmol, 3.00 equiv), p-methoxyaniline (53 mg, 0.428
mmol, 3.20 equiv), and phenylacetylene (15 μL, 0.134 mmol, 1.00
equiv). After heating at 100 °C for 18 h, the reaction mixture was
cooled to room temperature and then concentrated in vacuo. The
crude was purified by flash chromatography (petr. ether/Et2O, 80:20),
obtaining 2-cyclohexyl-6-methoxy-4-phenylquinoline as a pale gray
solid (34 mg, 0.107 mmol, 80%). Rf = 0.50. The NMR data were in
agreement with those reported in the literature.31
6-Methoxy-2,4-diphenylquinoline (5h). Following the general
procedure, a J. Young NMR tube was loaded with B(C6F5)3 (14 mg,
0.027 mmol, 0.20 equiv) in 1,2-dichloroethane (0.5 mL), followed by
the addition of benzaldehyde (42 μL, 0.401 mmol, 3.00 equiv), p-
methoxyaniline (53 mg, 0.428 mmol, 3.20 equiv), and phenylacetylene
(15 μL, 0.134 mmol, 1.00 equiv). After heating at 100 °C for 24 h, the
reaction mixture was cooled to room temperature and then
concentrated in vacuo. The crude was purified by flash chromatography
(petr. ether/Et2O, 95:5), obtaining 6-methoxy-2,4-diphenylquinoline
as a white solid (29 mg, 0.093 mmol, 70%). Rf = 0.20. The NMR data
were in agreement with those reported in the literature.27
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: michael.ingleson@manchester.ac.uk.
ORCID
Michael J. Ingleson: 0000-0001-9975-8302
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was made possible by financial support from the
EPSRC (EP/K03099X/1 and EP/M023346/1) and the Royal
Society (for the award of a University Research Fellowship).
Additional research data supporting this publication are
available as Supporting Information accompanying this
publication.
F
DOI: 10.1021/acs.organomet.7b00174
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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DOI: 10.1021/acs.organomet.7b00174
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