Proton-Sponge-Like Superbases Built on the Benzo[h]quinoline Platform

Article abstract of DOI:10.1002/ejoc.201800652

6,10-Bis(dimethylamino)benzo[h]quinoline (6) and its “reverse” counterpart, the 6,7-isomer 7 (a “pyridine-extended” proton sponge), have been prepared in a three-step synthetic protocol starting from 1,5-bis- and 1,8-bis(dimethylamino)naphthalenes, respectively, through azo coupling and Skraup cyclisation reactions. These novel polyfunctional nitrogen bases demonstrate excellent chelation ability towards protons and even Pd^II, and behave as kinetically active (6) and kinetically inactive (7) compounds, as demonstrated by NMR transprotonation experiments in DMSO. Single-crystal X-ray diffractometry was used to characterise rare types of chelated [NHN]⁺ hydrogen bonds and to reveal the location of the proton in protonated 6 (H⁺ between NMe₂ and the pyridine nitrogen) and its zwitterionic azo-dye precursor 8Z (H⁺ between NMe₂ and the azo group). Experimental pK_a values measured in DMSO show that 6 and 7 are strong bases, and the strength of the basicity of the 6,10-isomer 6 (pK_a = 8.1) places this benzo[h]quinoline in the category of heterocyclic superbases.

Full text of DOI:10.1002/ejoc.201800652

A Journal of
Accepted Article
Title: Proton Sponge-Like Superbases Built on the Benzo[h]quinoline
Platform
Authors: Valery A. Ozeryanskii and Alexander F. Pozharskii
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800652
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
1
Proton Sponge-Like Superbases Built on the Benzo[h]quinoline Platform
[
a]
[a]
Prof. Valery A. Ozeryanskii* and Prof. Alexander F. Pozharskii
[
a]
Department of Organic Chemistry, Southern Federal University, Zorge 7, 344090 Rostov-on-
Don, Russian Federation. E-mail: vv_ozer2@sfedu.ru
Abstract. 6,10-Bis(dimethylamino)benzo[h]quinoline (6) and its 'reverse' counterpart, 6,7-isomer 7
a 'pyridine-extended' proton sponge), were prepared in a three-step synthetic protocol starting from
,5-bis- and 1,8-bis(dimethylamino)naphthalenes and utilising azo-coupling and Skraup cyclisation
(
1
reactions. These novel polyfunctional nitrogen bases demonstrate excellent chelation ability to
protons or even Pd(II), and behave as kinetically active (6) and kinetically inactive (7) compounds
by means of NMR transprotonation experiments in DMSO. Single-crystal X-ray diffraction was
+
used to characterise rare types of chelated [NHN] hydrogen bonds and to reveal proton location in
+
protonated 6 (H between NMe
2
and pyridine nitrogen) and its zwitterionic azo-dye precursor 8Z
values measured in DMSO show that 6 and 7
+
(
H between NMe
2
and azo-group). Experimental pK
a
a
are rather strong bases, and the basicity strength of 6,10-isomer 6 (pK = 8.1) allows to put this
benzo[h]quinoline into a category of heterocyclic superbases.
Introduction
Recently we have shown that 1,5-bis(dimethylamino)- (1) and 1,8-bis(dimethylamino)naphthalenes
2), being closely related and the most available regioisomers of the corresponding
naphthylenediamines, differ in basicity by 4.1–5.7 log units depending on the media used for
(
[1]
measurements. While both diamines are widely utilised for modelling steric hindrance, peri-
interaction, and their consequences on molecular structure and reactivity of functional groups (for
[2,3]
representative examples, see refs. ), only the second compound is known in practical chemistry as
Proton Sponge and occupies its niche as a mild nitrogen base due to enhanced basicity [pK (2) =
a
[4]
[5]
1
2.1 (H
2
O) , 7.5 (DMSO) ] and reduced kinetic activity (relatively low rate of protonation-
[
3a]
deprotonation). Interestingly, replacement of one of the NMe
2
groups in 2 by a pyridine ring as in
a 2
benzo[h]quinoline 3 rises the basic strength of the compound even higher [pK (3) = 12.3 (H O), 7.7
+
(
DMSO)] despite the asymmetry of the intramolecular [NHN] hydrogen bridge in the
corresponding monoprotonated form originated from the different hybrid state of the two nitrogen
[
6]
atoms.
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
2
NMe2
Me N
NMe2
Me N
N
2
2
Me N
2
1
2
3
There exist two general synthetic approaches leading to quinoline 3. Their principal difference
is in the key fragment that appears first: either it is a pyridine ring (as in amine 4) or a
dimethylamino group (as in peri-diamine 5, Scheme 1). Both approaches start from 1-
[
6,7]
aminonaphthalene and require 5–6 synthetic steps to get to molecule 3.
molecules with the opposed NAlk and Py fragments quite rare in spite of interesting photophysical,
charge transfer and proton transfer properties.
synthesis of their rather simple precursors, e.g. naphthalenes with peri-located tertiary and primary
This obstacle makes
2
[
6,7]
For this reason, new shorter methods for the
[
8]
amino groups as in compound 5 are generally sought.
H N
2
N
Me N
NH2
2
Skraup
cyclization
N-Alkylation
3
4
5
Scheme 1. General routes to compound 3.
With this in mind, it seemed justified to use 1,5-diamine 1 as a platform to construct new
quinoline superbase via addition of an amino group to one of the two equivalent peri-positions.
[2d]
Although, except direct lithiation, compound 1 could not be selectively functionalised at position
[9]
4
(for example, nitration in different media gave either 3-nitro- or 2,4,6,8-tetranitro derivative ),
we envisioned that less reactive electrophile such as a diazonium cation would result in preferential
α-substitution. Indeed, this was found to be the case, and here we introduce preparation of
bis(dimethylamino)benzo[h]quinoline 6 based on successive azo-coupling, reduction, and Skraup
cyclisation steps.
Me N
N
Me N
NMe2
2
2
NMe2
N
6
7
It was also of principal importance to synthesise the 'reverse' counterpart, isomer 7, by
annealing a pyridine ring to the proton sponge molecule using the same strategy. We reasoned that
high basicity of the parent proton sponge may stay generally unaffected (a light drop was expected
due to electron-withdrawing nature of a pyridine fragment) while add some new features to the
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
3
protonation behaviour, ligand properties, and reactivity of proton sponge 2.
Results and Discussion
Synthesis of 6,10-Bis(dimethylamino)benzo[h]quinoline
The target 6,10-disubstituted benzo[h]quinoline
6
was synthesized by using 1,5-
bis(dimethylamino)naphthalene (1) as the starting material (Scheme 2). Initially, compound 1 was
converted into 4-amino-1,5-bis(dimethylamino)naphthalene (9) by first reacting it with 4-
diazobenzene sulfonate in aqueous hydrochloric acid, which gave azo-dye 8, followed by reduction
of the azo-group with stannous chloride. The cyclisation of the amino group of 9 into the annealed
pyridine ring was accomplished with the help of Skraup reaction in excess of glycerol for smoother
performance to give the desired product 6 as the only isolable compound. Although the yield on the
last step is quite low, the transition 9 → 6 is synthetically and theoretically meaningful, as this
2
demonstrates that two NMe groups, both being in sterically unfavourable positions, are tolerant and
could survive the harsh reaction conditions. It should be noted also that synthesis of quinolines with
even one unhindered dimethylamino group (e.g. 7-dimethylaminoquinoline) usually is quite a
[10,11]
challenge.
N2
SO3
NMe2
1)
0.5 M HCl, 2 h
1
2) NaOH
64%
Me N
N
N
2
8
SO Na
3
SO Na
3
1
) SnCl , HCl
2
56%
NO2
OH
2
) KOH
HO
HO
Me N
NH2
2
H BO FeSO ·
3
7
3,
4
4
H O, H SO
2 2
6
o
1
45 C/1 h
14%
NMe2
9
Scheme 2. Synthesis of bis(dimethylamino)benzo[h]quinoline 6.
We noticed further that azobenzenesulfonate 8, being a close analogue of famous organic
indicator methyl orange, is the first compound in naphthalene series with peri-situated
dimethylamino and azo-groups. Compounds of this type have not been investigated to date in terms
of their structure and protolytic properties. It is known that methyl orange is protonated at a
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
4
2
nitrogen atom distant from the NMe group to form resonance-stabilized cation 10 (zwitterionic
[12]
structure 10 corresponds to the crimson-red or magenta form of the azo-dye).
In contrast,
protonation of sulfonate 8 is not so unambiguous. Here, it is possible to form an intramolecular
hydrogen bond of the chelate type with participation of the azo-group nitrogen far from the
sulfonated benzene ring (structure 8Z), rather than the near nitrogen atom as in alternative structure
8
Z' with a seven-membered chelate ring.
H
N+
SO3–
Me N
N
2
1
0
NMe2
NMe2
N
H N
Me N + N
Me N +
2
2
N
H
SO3–
SO3–
8
Z
8Z'
We found that acidification of brown sodium salt 8 instantly produces a crimson-purple colour,
indicating a strong delocalization of the positive charge in the protonated form (see Supporting
Information, Figure S2 for colour changes, λmax 505 nm in MeOH–HCl; cf. acidified methyl orange
[
12]
λ
max 506 nm in H
2
O–HCl or 510 nm in MeOH for 10). At the same time, the relatively small
+
value of the [NHN] proton chemical shift (δ
H
11.7 ppm in CD
3
CN, 12.9 ppm in DMSO-d
6
) did not
allow us to unambiguously assign the protonated form to type 8Z or 8Z' in solution. Some certainty
was brought by the results of XRD analysis (Figures 1 and S3) that confirmed the formation of the
six-membered chelate cycle as in structure 8Z. This is the first structural study succeeded for
compounds of this kind. Moreover, since the structure is formed by two independent molecules, we
+
were able to investigate the effect of weak bases (water) on the hydrogen bond [NHN] between the
amino and azo-groups.
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
5
(
a)
Figure 1. Molecular structure of zwitterionic peri-azo-dye 8Z crystallized from aqueous acetonitrile
120 K, P = 50%) and showing dislocation of the solvate water molecule next to the sulfonic group
(b)
(
+
(
a) and the [NHN] hydrogen bridge (b).
+
It turned out that in zwitterion 8Z the [NHN] proton is localised mainly on the 8-NMe group,
2
forming a highly asymmetric hydrogen bond with the parameters: N...N 2.625, N(2)–H 0.90,
o
H···N(3) 1.80 Å, ∠NHN 151 (Figure 1a). On the other hand, the water molecule as a base
successfully competes with the azo-group for the proton, which in the second independent molecule
of the azo-dye forms a bifurcated hydrogen bond with the next properties: N...N 2.712, N(2)–H
o
o
0
.90, H···N(3) 2.11, H···O(W) 2.10 Å, ∠NHN 123 , ∠NHO 143 (Figure 1b). In general, the N(4)
nitrogen atom of the azo-group does not participate in coordination with the proton (Figure S3), and
the azo-group itself has a length of 1.292 Å in the first molecule and 1.262 Å in the second (cf.
[
12]
1
.308 Å in zwitterionic methyl orange 10). Thus, the structural preorganisation of the molecule
of polynitrogen base 8 leads to unusual direction of its protonation, i.e. against the resonance
delocalization of the positive charge with the participation of electron-donating groups, the
traditional way of stabilisation of such systems.
Benzo[h]quinoline 6 is a non-crystalline oily compound whose structure was confirmed by a
set of spectral data and XRD analysis of its crystalline tetrafluoroborate (see below).
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European Journal of Organic Chemistry
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6
Synthesis of 6,7-Bis(dimethylamino)benzo[h]quinoline
A similar approach was realised for the preparation of isomeric benzo[h]quinoline 7 using
commercially available diamine 2 as the starting compound. For this, proton sponge 2 was
converted into azo-dye 11 followed by reduction with SnCl
2
to give amine 12 (yield 40–50% over
two steps) or, alternatively, via selective mono-nitration–catalytic hydrogenation steps to get to the
[
13]
same amine 12 (overall yield 60–80%).
4
Interestingly, on addition of equimolar amount of 60% HClO to a dark purple DMSO solution
of azo-dye 11, a strong discolouration takes place and a fine precipitate of the greenish-brown
zwitterion starts to form (see Figure S4 for colour changes and UV/Vis spectra, hypsochromic shift
1
of the longwave absorption band is 145 nm). Unlike compound 8, the H NMR spectrum of the
zwitterion is quite informative and is in full agreement with structure 11Z (Scheme 3). Although the
solubility of the compound is low, the proton spectrum recorded in DMSO-d
6
contains two six-
proton signals belonging to the protonated NMe groups (3.15 and 3.23 ppm) and a strongly
2
deshielded broad singlet due to the NH proton chelated by a proton sponge fragment (18.45 ppm),
and leaving no doubts that the proton sponge residue (not the azo-group) of 11Z is mono-protonated
[3a]
in solution. In the EI-MS spectrum of compound 11Z, the molecular ion peak with mass m/z =
98 is absent, and the main peak m/z = 229 (100%) corresponds to the loss of the C H NO S
3
6
4
3
fragment. Probably the possible mechanism for the decay of zwitterion 11Z in the gas phase is as
shown in Scheme 3 and mostly reflects the fragmentation of the neutral rather than zwitterionic
form of the substance, in line with the gas phase behaviour of many amino acids. The prototropic
equilibrium leads to neutral form 11N, in which the through conjugation is possible, the bond
between the azo-group nitrogen atoms decreases its multiplicity and, hence, it is more easily broken
(
Scheme 3).
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European Journal of Organic Chemistry
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7
Scheme 3. Prototropic equilibrium and gas phase fragmentation of azobenzenesulfonate 11.
The Skraup cyclisation with amine 12 furnished the desired 6,7-bis(dimethylamino) derivative
(19%) along with quinolinol 13 (24%). The latter compound is an indicator of lability of the 6-
7
2
NMe group in diaminobenzoquinoline 7 under reaction conditions (the mechanism of its formation
will be discussed below) and is stabilised by rather strong intramolecular hydrogen bond (δOH 15.03
ppm, the substance does not dissolve in aqueous alkali), which is hardly observed in its IR
[14]
spectra.
H
Me N
2
O
N
13
The structure of 6,7-bis(dimethylamino)benzo[h]quinoline (7) was confirmed by spectral data
and comparison with its 6,10-isomer 6. Both regioisomers, 6 and 7, display similar sets of signals in
their NMR spectra, but the protons and carbons of 6 are deshielded more in total than those of
isomer 7 (e.g. NMe
2
signals are at 2.91 and 2.97 ppm for 6 and 2.83 and 2.84 ppm for 7, CDCl
group is directly conjugated with the 1-aza-group of the same molecule
cf. compound 3), while in 6,7-isomer 7 both amino groups are not conjugated with the pyridine
3
). In
6
,10-isomer 6, the 10-NMe
2
[
6]
(
nitrogen atom. In spite of this, the brown-yellow colouration of isomeric benzo[h]quinolines 6 and 7
is due to the tail absorption, as their longwave absorption bands are located in the UV-region (330
nm for 6 and 332 nm for 7, MeOH).
Protonation, Basicity and Reactivity of Isomeric Bis(dimethylamino)benzo[h]quinolines
On action of mineral acids, benzoquinoline 6 forms stable crystalline monoprotonated salts with the
+
asymmetric [NHN] hydrogen bridge. For example, in tetrafluoroborate 6·HBF , the NH proton
4
resides on the 10-dimethylamino group by more than 80% in solution (MeCN, DMSO) and by 60%
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
8
+
in the solid state (Figure 2). The non-protonated 6-NMe
2
group in cation 6·H is still in good
o
conjugation with the benzo[h]quinoline system making a 37 angle with the mean aromatic plane.
This may indirectly enhance the basicity of the pyridine nitrogen atom (see below).
4
Figure 2. Molecular structure of salt 6·HBF showing close interaction (Å) between the proton and
the tetrafluoroborate counter-ion (120 K, P = 50%). Key parameters: N(1)...N(2) 2.603, N(2)–H(2)
о
1
.01, H(2)...N(1) 1.67 Å, ∠N(2)–H(2)...N(1) 151 .
1
The protonation of benzoquinoline 7, as evidenced by the H NMR spectra of its cationic forms
3 4
in CD CN prepared by step-by-step addition of HBF , affected first the peri-dimethylamino groups
+
+
to give the [NHN] hydrogen bridge (cation 7·H ), after that the protonation of the aza-group
+
becomes possible (cation 7·2H in Scheme 4, see also Figures S20 and S21, SI). Benzo[h]quinoline
7
demonstrates excellent ligand properties and on action of palladium acetate gives after 2 h of
∩
stirring the dimeric C N-complex 14 in high yield (Scheme 4). This is much faster than in the case
[15]
of unsubstituted benzo[h]quinoline (4–8 h depending on the solvent used) and can be explained
by an enhanced coordination activity of the pyridine nitrogen atom in 7 owing to a strong electron-
donating effect of the two dimethylamino groups towards the aromatic π-system. Subsequent
substitution of the H(10) hydrogen atom during the cyclopalladation is evidenced by its absence in
1
the H NMR spectrum while the two multiplets of protons H(8) and H(9) in 7 are changed by two
2
closely situated doublets in 14 (cf. Figures S18 and S25, SI). As seen, the NMe groups are not
affected by coordination.
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
9
Scheme 4. Protonation sequence of 6,7-bis(dimethylamino) derivative 7 and its ligation by Pd(II).
Using model experiments, we were able to determine at which stage of the Skraup reaction the
6
-dimethylamino group of 6,7-bis(dimethylamino)benzo[h]quinoline (7) is replaced by a hydroxyl.
The essence of chemical transformations and the reagents used are clear from Scheme 5.
Scheme 5. Model experiments to determine the conversion path 7 → 13.
Thus, it was found that water in a solution of sulphuric acid acts as a nucleophile, which attacks
+
the electrophilic center (ipso-carbon atom) of dication 7·2H , formed by protonation of the
dimethylamino groups and quinoline nitrogen. The conversion of quinoline 7 to phenol 13 proceeds
without the formation of by-products and thus in the course of the Skraup reaction, when a
sufficient amount of water (nucleophile) appears and the acidity of the medium is still high. Further
action of potassium hydroxide during standard work-up no longer affects the ratio of reaction
products 7 and 13. For comparison, similar conversions were attempted with 1,8-
bis(dimethylamino)naphthalene (2) and its 4-nitro derivative 15. The model experiments with
proton sponges 2 and 15 do not yield any results (the starting materials remain unchanged both on
reflux in 2% as well as in 15% H SO ) although the mononitrated proton sponge 15 is able to
2
4
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European Journal of Organic Chemistry
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1
0
[
16]
2
exchange one of its NMe groups by a hydroxyl to give naphthol 16 under the action of alkali.
Thus, the data received clearly demonstrates that transformation aminoquinoline 7 → quinolinol 13
is governed not only by the amount of positive charge on the ipso-carbon atom and the presence of
additional electrophilic centres. Other factors may be at play, for example, the reduced aromaticity
of the central ring and the enhanced reactivity of the so-called K-region in phenanthrene and its
derivatives (e.g. elevated double bond character of the C –C bond in 7) leading to a more stable σ-
5
6
[
17]
adduct.
Next, we have measured the pK
a
values of 6 and 7 in DMSO-d
The transprotonation experiments with equimolar amounts of proton
sponge 2 as the reference compound and monoprotonated bases 6·HBF and 7·HBF , respectively,
were successfully completed to give the data depicted in Figures S27 and S28. It has been found
that diamine 6 with pK = 8.1 is notably stronger as a base than the classical proton sponge, while
the basicity of benzo[h]quinoline 7 with pK = 6.9 in DMSO-d is by the same amount less than that
of compound 2 (but still is much higher than that of nitro compound 15, pK = 3.5, or quinazoline
In general, benzoquinolines 6 and 7 turned out to be the extreme
6
by the competitive method
[
18]
using NMR monitoring.
4
4
a
a
6
a
[
6,19]
1
a
7, pK = 5.9 in DMSO).
representatives when the basic strength of the parent compounds, 2 and 3, is concerned (Scheme 6),
and the strategy of annealing the pyridine ring to 1 proved to be quite successful in terms of basicity
growth (transition 1 → 6 gains 6.3 pK
3 → 6). At the same time, the isolated pyridine ring exhibits moderate acceptor properties with
regard to the naphthalene π-system dropping down the basicity of the starting diamine (2 → 7).
a 2
units!) as well as the introduction of additional NMe groups
(
Scheme 6. Relative basic strength of proton sponge 2 and benzo[h]quinolines 3, 6, 7.
1
It is noteworthy that in the H NMR spectrum of the mixture shown in Figure S27, most peaks
+
are considerably broadened and those belonging to cation 6·H and its neutral form 6 are common
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
1
1
+
(
see red and green marks) unlike those of 2 and 2·H species (blue and black marks are located in
different spectral regions). Apparently, this reflects kinetic activity of base 6 in the NMR time scale
[
6]
and thus put it into a category of so-called pseudo-proton sponges. This opinion is confirmed with
the X-ray structure of 6·HBF
4
(Figure 2) revealing quite a short distance between the NH proton
–
4
[6,20]
and the BF
anion in 6·HBF
4
(2.58 Å), notably shorter than in 2·HBF
4
(2.91 Å).
At the same
time, all four components of the mixture depicted in Figure S28 are observed in their own spectral
regions as though four spectra were superimposed. Hence, benzoquinoline 7 is kinetically inactive
and, similar to the parent diamine 2, obviously belongs to the proton sponge family.
Conclusions
In summary, 1,5-bis- and 1,8-bis(dimethylamino)naphthalenes were azo-coupled with 4-
diazobenzene sulfonate in α-position to give new azo-dyes 8 and 11. Both azo-dyes, being the
extended variants of methyl orange, demonstrate non-conventional direction of protonation
governed by the formation of intramolecular N–H...N bridges, either between azo- and NMe
between two NMe groups. We have proved that both azo-dyes can serve as convenient precursors
for transforming them into the corresponding bis(dimethylamino)benzo[h]quinolines 6 and 7 via
Skraup cyclisation. The 6,7-bis(dimethylamino) isomer is able to exchange 6-NMe group by OH in
2
or just
2
2
acidic media (K-region amino group) and easily coordinate with Pd(II) (bay region pyridine
nitrogen atom). Both bis(dimethylamino)benzo[h]quinolines are excellent proton chelators, which is
confirmed by basicity measurements. Although 6,7-regioisomer 7 as a base was found to be weaker
than the proton sponge 2 (this compound is usually considered as a threshold of superbasicity),
6
,10-bis(dimethylamino)benzo[h]quinoline with pK
a
= 8.1 (DMSO) can well be classified as a
[
21]
heterocyclic superbase.
Experimental Section
о
Sodium 4-[(4,8-Bis(dimethylamino)naphth-1-yl]azobenzenesulfonate (8): To a cooled to 0–5 C
solution of diamine 1 (1.05 g, 4.90 mmol) in 0.5 M HCl (25 mL) 4-diazobenzene sulfonate (for
preparation and isolation, see SI; 0.95 g, 5.16 mmol) was gradually added in small portions for 30
min with stirring. The mixture soon turned dark crimson; the stirring was continued for another 2 h
without cooling followed by addition of NaOH (1 g, 25 mmol). The thick and dark brown mixture
was dried in warm air, the solid residue was washed with EtOAc (3×10 mL) to remove minor
quantities of the starting diamine followed by repeated drying. The compound was recrystallised
from minimal amount of water to give azo-dye 8 (1.32 g, 64%) as brown crystals with metallic
о
lustre; decomp. above 300 C. The solutions of the dye in 10% HCl are crimson in colour, while in
1
1
0% KOH the colour is yellowish-brown. H NMR (250 MHz, DMSO-d
6
): δ = 2.65 (s, 6H, 8-
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
1
2
NMe
J = 8.2, 7.5 Hz, 1H, 6-H), 7.81 (br. s, 4H, C
MHz, DMSO-d ): δ = 45.1 (NMe ), 45.6 (NMe
27.2, 130.7, 146.7, 149.8, 151.1, 153.0, 153.2. IR (KBr), ν/cm : 2977, 2935, 2859, 2830, 2784
2 2
), 2.87 (s, 6H, 4-NMe ), 7.09 (br. s, 2H, 2-H, 3-H), 7.22 (br. d, J = 7.5 Hz, 1H, 7-H), 7.47 (dd,
1
3
6
H
4
), 7.86 (br. d, J = 8.2 Hz, 1H, 5-H). C NMR (62.9
6
2
2
), 113.7, 113.8, 116.8, 119.3, 122.4, 125.3, 126.1,
–
1
1
–
(
3 21 4 3
C–H), 1626, 1582, 1510 (arom.), 1205, 1190, 1125, 1035 (SO ). C20H N NaO S (420.47): calcd.
C 57.13, H 5.03, N 13.33; found C 57.24, H 5.19, N 13.22.
4
The compound turns into zwitterionic form 8Z on addition of 1 equiv of aqueous 60% HClO to a
solution of sodium salt 8 in 50% aqueous acetonitrile. The zwitterionic form crystallises from the
o
H
2
O/MeCN mixture at 20 C as monohydrate 8Z·H
2
O and the crystals (black-maroon plates) were
): δ = 3.09 (s, 6H, 4-NMe ),
1
found to be suitable for XRD studies. H NMR (250 MHz, DMSO-d
6
2
3
8
.49 (s, 6H, 8-NMe
2
), 7.33 (d, J = 9.0 Hz, 1H, 3-H), 7.71–7.79 (m, 3H, 3'-H, 5'-H, 6-H), 7.90 (d, J =
1
.4 Hz, 2H, 2'-H, 6'-H), 8.30–8.39 (m, 3H, 2-H, 5-H, 7-H), 12.87 (br. s, 1H, NH). H NMR (250
MHz, CD CN, low solubility): δ = 3.49 (d, J = 4.7 Hz, 6H, 8-NMe ), 3.79 (s, 6H, 4-NMe ), 7.33
3
2
2
(
br. d, J = 8.7 Hz, 2H), 7.66 (br. d, J = 10.6 Hz, 1H), 7.91 (t, J = 8.3 Hz, 1H, 6-H), 8.00 (br. d, J =
8
.5 Hz, 2H), 8.16 (br. d, J = 10.8 Hz, 1H), 8.25 (br. d, J = 8.2 Hz, 2H), 11.72 (br. s, 1H, NH).
-Amino-1,5-bis(dimethylamino)naphthalene (9): To a suspension of azo-dye 8 (0.60 g, 1.43
4
2 2
mmol) in 20% aqueous HCl (6 mL) was added SnCl ·2H O (0.50 g, 2.22 mmol) in several portions
until disappearing of distinct crimson colour. Then, the mixture was made strongly basic by
addition of 10% aqueous KOH with stirring to pH 10. After cooling the resulting mixture to room
temperature, the product was extracted with benzene (3×10 mL), the benzene layer was dried with
Na
darken in air; m.p. 87–88 C. H NMR (250 MHz, CDCl
.75 (br. s, 2H, NH ), 6.54 (d, J = 8.2 Hz, 1H, 3-H), 6.98 (d, J = 8.2 Hz, 1H, 2-H), 7.16 (d, J = 7.3
2 4
SO , and the solvent was evaporated. This gave amine 9 (0.18 g, 56%) as colourless crystals
о
1
3
2 2
): δ = 2.75 (br. s, 12H, 1-NMe , 5-NMe ),
5
2
1
3
Hz, 1H, 6-H), 7.35 (t, J = 7.9 Hz, 1H, 7-H), 8.07 (d, J = 8.5 Hz, 1H, 8-H). C NMR (62.9 MHz,
CDCl ): δ = 45.8 (NMe ), 46.2 (NMe ), 109.2, 115.3, 116.0, 119.8, 120.9, 125.0, 132.4, 141.2,
41.6, 152.2. IR (nujol), ν/cm : 3370, 3264 (NH ), 1588, 1533 (arom.). C14 (229.33): calcd.
C 73.33, H 8.35, N 18.32; found C 73.32, H 8.41, N 18.23.
,10-Bis(dimethylamino)benzo[h]quinoline (6): Amine 9 (0.36 g, 1.60 mmol) was thoroughly
mixed with sodium m-nitrobenzenesulfonate (0.16 g, 0.71 mmol), boric acid (0.04 g, 0.64 mmol),
FeSO ·7H O (0.035 g, 0.13 mmol), and glycerol (0.70 mL, 9.40 mmol). Then conc. H SO (0.31
mL, 5.80 mmol) was gradually added to the mixture with stirring (warming-up!) and heated in an
3
2
2
–
1
1
2
19 3
H N
6
4
2
2
4
o
o
open flask for 1 h at 145 C. After cooling to 50 C, the mixture was poured into water (10 mL), the
flask was washed with acetone (4 mL) and the combined water-acetone phase was basified to pH 12
with several chips of solid KOH. After stirring and grinding, the resulted mixture was evaporated
3
almost to dryness, the reaction products were extracted with CHCl (100 mL, Soxhlet apparatus),
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European Journal of Organic Chemistry
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1
3
concentrated to a minimal volume and separated chromatographically. The main brownish fraction
of average mobility (Al
2
O
3
/CHCl
3
, yellow-green luminescence under UV-light) gave quinoline 6
1
(
0.06 g, 14%) as brown-yellow oil. H NMR (250 MHz, CDCl
3
): δ = 2.91 (s, 6H, 6-NMe
2
), 2.97 (s,
6H, 10-NMe
2
), 7.08 (s, 1H, 5-H), 7.26 (br. d, J = 6.1 Hz, 1H, 9-H), 7.32 (dd, J = 8.0, 4.3 Hz, 1H, 3-
H), 7.55 (t, J = 8.0 Hz, 1H, 8-H), 7.92–7.99 (m, 2H, 4-H, 7-H), 8.94 (dd, J = 4.3, 1.9 Hz, 1H, 2-H).
1
3
C NMR (62.9 MHz, CDCl
3
): δ = 44.9 (NMe
2
), 45.5 (NMe
2
), 111.6, 114.9, 117.0, 120.2, 123.6,
+
1
2
8
27.2, 127.6, 134.2, 134.4, 144.8, 146.0, 149.9, 152.3. MS, m/z (I (%)): 265 [M] (71), 251 (23),
50 (100), 236 (36), 235 (33), 234 (50), 221 (25), 219 (15), 206 (16), 205 (25), 110 (22), 103 (17),
9 (21), 44 (63), 43 (26), 42 (82). UV/Vis (MeOH) λmax (lgε): 240 (4.64), 276 (sh., 4.20), 330
(
19 3
3.88), end absorption up to 507 nm. C17H N (265.36): calcd. C 76.95, H 7.22, N 15.83; found C
7
7.09, H 7.15, N 15.74.
-Dimethylamino-10-(dimethylammonium)benzo[h]quinoline Tetrafluoroborate (6·HBF
6
4
): To
an orange-brown solution of quinoline 6 (0.040 g, 0.15 mmol) in EtOAc (1 mL), 1 equiv. of
aqueous 40% HBF (0.026 mL, 0.15 mmol) was added. The resulting salt was immediately
4
precipitated as beige flocculent solid, which was additionally phased-out on addition of Et
mL). The product was decanted and dried in air to give salt 6·HBF (0.051 g, 96%) as beige crystals
with m.p. 192–194 C (decomp., from MeCN). H NMR (250 MHz, CD CN): δ = 2.95 (s, 6H, 6-
NMe ), 3.30 (d, J = 4.3 Hz, 6H, 10-NMe ), 7.51 (s, 1H, 5-H), 7.82 (dd, J = 8.4, 4.8 Hz, 1H, 3-H),
.96 (t, J = 8.2 Hz, 1H, 8-H), 8.11 (dd, J = 7.9, 1.0 Hz, 1H, 9-H), 8.48–8.53 (m, 2H, 4-H, 7-H),
2
O (5
4
o
1
3
2
2
7
8
6
8
+
1
.87–8.90 (m, 1H, 2-H), 18.10 (br. s, 1H, NHN ). H NMR (250 MHz, DMSO-d
-NMe ), 3.39 (d, J = 4.5 Hz, 6H, 10-NMe ), 7.62 (s, 1H, 5-H), 7.92 (dd, J = 8.2, 4.7 Hz, 1H, 3-H),
.04 (t, J = 8.2 Hz, 1H, 8-H), 8.41–8.49 (m, 2H, 4-H, 9-H), 8.65 (dd, J = 8.4, 1.8 Hz, 1H, 7-H), 8.93
6
): δ = 2.92 (s, 6H,
2
2
+
13
(
dd, J = 4.7, 1.5 Hz, 1H, 2-H), 17.37 (br. sept, J = 4.3 Hz, 1H, NHN ). C NMR (62.9 MHz,
CD CN): δ = 44.5 (NMe ), 45.3 (NMe ), 113.2, 120.0, 121.5, 123.8, 126.3, 128.5, 129.4, 131.7,
(353.17): calcd. C 57.81, H 5.71, N 11.90; found C
3
2
2
1
38.2, 142.1, 144.3, 144.8, 149.2. C17
7.69, H 5.76, N 11.83.
4 3
H20BF N
5
Although the compound crystallises well from acetonitrile, we found that the resulting crystals are
unstable and quickly degrade into powder due to apparently loss of the captured solvent. The
o
solvent-free crystalline sample was prepared at 20 C as yellow-brown plates on slow crystallisation
from acetone and turned out to be suitable for XRD studies.
о
Sodium 4-[4,5-Bis(dimethylamino)naphth-1-yl]azobenzenesulfonate (11): To a cooled to 0–5 C
solution of diamine 2 (0.43 g, 2.00 mmol) in 75% aqueous MeCN (16 mL) 4-diazobenzene
sulfonate (0.18 g, 1.00 mmol) was gradually added in small portions for 30 min with stirring. The
о
mixture soon turned dark wine-red; the stirring was continued for another 45 min at 0–5 C
followed by addition of NaOH (0.08 g, 2.00 mmol). The reaction mixture was dried in warm air, the
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European Journal of Organic Chemistry
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4
solid residue was washed with EtOAc (3×10 mL) to remove the starting diamine followed by
repeated drying. The resulting solid was chromatographed on Al O using MeOH as eluent. After
2
3
two hours of elution, the first crimson fraction was cut-off, the product was washed out of the
adsorbent with water, and the resulting solution was evaporated to dryness. This gave azo-dye 11
о
(
0.47 g, 56%) as almost black crystals with metallic lustre; decomp. above 300 C. The solutions of
1
the dye in 10% HCl are dull yellow in colour, while in 10% KOH the colour is dark crimson. H
NMR (250 MHz, DMSO-d
6
): δ = 2.79 (s, 6H, 5-NMe
2
), 2.96 (s, 6H, 4-NMe
), 8.50 (d, J = 7.9 Hz, 1H, 8-H).
6 2 2
C NMR (62.9 MHz, DMSO-d ): δ = 43.6 (NMe ), 43.7 (NMe ), 111.3, 112.7, 113.4, 115.0, 117.7,
2
), 7.01–7.04 (m, 2H, 3-
H, 6-H), 7.50 (t, J = 7.6 Hz, 1H, 7-H), 7.80–7.91 (m, 5H, 2-H, C
6
H
4
1
3
1
4
22.0, 127.2, 127.7, 136.7, 140.2, 149.3, 151.1, 153.4, 154.8. UV/Vis (MeOH) λmax (lgε): 226 (sh.,
.54), 294 (4.29), 353 (sh., 4.02), 502 (4.15), end absorption up to 685 nm. UV/Vis (MeOH + HCl)
λ
max (lgε): 260 (4.13), 303 (4.12), 357 (4.25), end absorption up to 545 nm. C20
21 4 3
H N NaO S
(420.47): calcd. C 57.13, H 5.03, N 13.33; found C 57.29, H 5.06, N 13.35.
4-[4,5-Bis(dimethylamino)naphth-1-yl]azobenzenesulfonic Acid Zwitterion (11Z): To a
solution of azo-dye 11 (0.032 g, 0.076 mmol) in DMSO (0.7 mL) 1 equiv. of aqueous 60% HClO
0.0083 mL, 0.076 mmol) was added. The resulting zwitter-ion was precipitated, the solid was
filtered off, washed with CH Cl (2 mL), acetone (2 mL), and dried in air to give compound 11Z
0.027 g, 90%) as dull greenish-brown powder soluble in aqueous KOH, but badly soluble in water,
4
(
2
2
(
1
alcohols, MeCN or DMF. H NMR (250 MHz, DMSO-d
6
): δ = 3.15 (s, 6H, 4-NMe
), 7.87 (d, J = 8.1 Hz, 2H, 3'-H, 5'-H), 7.92–7.99 (m, 2H, 2-H, 7-H), 8.05 (d, J = 8.1 Hz, 2H,
'-H, 6'-H), 8.21–8.29 (m, 2H, 3-H, 6-H), 9.06 (br. d, J = 8.8 Hz, 1H, 8-H), 18.45 (br. s, 1H,
2
), 3.23 (s, 6H,
5
2
-NMe
2
+
13
NHN ). C NMR data were not collected due to low solubility. MS, m/z (I (%)): 230 (17), 229
(
100), 214 (31), 199 (30), 198 (21), 197 (17), 184 (20), 183 (52), 182 (27), 168 (24). C20
398.49): calcd. C 60.28, H 5.57, N 14.06; found C 60.32, H 5.44, N 13.98.
22 4 3
H N O S
(
Skraup Reaction with 1-Amino-4,5-bis(dimethylamino)naphthalene was conducted by analogy
with the procedure described above for amine 9. After the corresponding work-up, basification to
pH 8–9, drying, CHCl
3 3
-extraction, and column chromatography (silica gel, EtOAc/CHCl , 1:1)
amine 12 (0.458 g, 2.00 mmol) gave the next compounds in the order of elution.
7
-Dimethylaminobenzo[h]quinolin-6-ol (13): R
f
= 0.85. Compound 13 (0.114 g, 24%) was
о
collected as brownish-yellow waxy crystals with m.p. 115–116 C (from CHCl
3
); darkens in air and
): δ = 2.88
1
has yellow-green luminescence under UV-light in solutions. H NMR (250 MHz, CDCl
3
(s, 6H, NMe
2
), 6.93 (s, 1H, 5-H), 7.42 (dd, J = 8.2, 4.5 Hz, 1H, 3-H), 7.63–7.73 (m, 2H, 8-H, 9-H),
7
1
1
.97 (dd, J = 8.2, 1.8 Hz, 1H, 4-H), 8.75 (dd, J = 4.5, 1.8 Hz, 1H, 2-H), 9.25 (dd, J = 7.6, 1.9 Hz,
1
3
3 2
H, 10-H), 15.03 (s, 1H, OH). C NMR (62.9 MHz, CDCl ): δ = 46.4 (NMe ), 104.7, 120.7, 121.4,
–
1
22.4, 123.6, 127.1, 128.6, 133.5, 134.1, 141.5, 145.4, 149.4, 155.1. IR (CCl ), ν/cm : 3054, 3039,
4
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European Journal of Organic Chemistry
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5
2
1
990, 2956, 2875, 2796, 2671 (C–H), 3000–2500 (continuous background O–H band), 1632, 1560,
–
1
521, 1465, 1437 (arom.). IR (nujol) ν/cm : 1626, 1602, 1571, 1517, 1329 (arom.). MS, m/z (I
+
(
%)): 239 (19), 238 [M] (100), 223 (25), 166 (15). C15
H
14
2
N O (238.29): calcd. C 75.61, H 5.92, N
11.76; found C 75.49, H 5.84, N 11.70.
6
f
,7-Bis(dimethylamino)benzo[h]quinoline (7): R = 0.10. Due to low mobility, the fraction
containing the diamine may be cut-off and washed out with acetone. This gave compound 7 (0.103
1
g, 19%) as brown oil. H NMR (250 MHz, CDCl
3
): δ = 2.83 (s, 6H, 7-NMe
2
), 2.84 (s, 6H, 6-NMe
2
),
6
7
.95 (s, 1H, 5-H), 7.20 (br. d, J = 7.6 Hz, 1H, 8-H), 7.36 (dd, J = 7.9, 4.4 Hz, 1H, 3-H), 7.56 (t, J =
.9 Hz, 1H, 9-H), 7.95 (dd, J = 8.0, 1.6 Hz, 1H, 4-H), 8.73 (dd, J = 4.4, 1.6 Hz, 1H, 2-H), 8.92 (br.
1
3
d, J = 8.2 Hz, 1H, 10-H). C NMR (62.9 MHz, CDCl
3 2
): δ = 43.7 and 44.3 (NMe ), 107.4, 116.0,
117.4, 121.5, 121.8, 126.8, 127.4, 133.6, 135.6, 143.9, 145.7, 149.1, 150.8. MS, m/z (I (%)): 265
+
[
M] (58), 250 (30), 235 (34), 234 (74), 233 (66), 221 (60), 220 (57), 219 (100), 206 (21), 205 (42),
1
1
91 (16), 180 (16), 179 (22), 178 (50), 177 (32), 152 (18), 151 (29), 150 (21), 132 (17), 124 (19),
18 (37), 116 (36), 110 (24), 103 (19), 89 (19), 45 (32), 44 (85), 43 (32), 42 (83). UV/Vis (MeOH)
λ
max (lgε): 234 (4.50), 260 (4.50), 298 (3.85), 332 (3.96), 382 (sh., 3.58), end absorption up to 508
(265.36): calcd. C 76.95, H 7.22, N 15.83; found C 76.91, H 7.11, N 15.89.
-Dimethylamino-7-(dimethylammonium)benzo[h]quinoline Tetrafluoroborate (7·HBF
solution of quinoline 7 (0.040 g, 0.15 mmol) in EtOAc (1 mL), 1 equiv. of aqueous 40% HBF
19 3
nm. C17H N
6
4
): To a
4
(
0.026 mL, 0.15 mmol) was added. The resulting salt was immediately precipitated as beige
2
flocculent solid, which was additionally phased-out on addition of Et O (5 mL). Filtration, drying,
and recrystallisation from EtOH (9 mL) afforded salt 7·HBF
4
(0.042 g, 80%) as dark beige needles
with m.p. 280 C and decomp. at 295−296 C. H NMR (250 MHz, CD CN): δ = 3.21–3.23 (m,
2H, 6-NMe , 7-NMe ), 7.80 (dd, J = 8.2, 4.5 Hz, 1H, 3-H), 8.01 (t, J = 8.1 Hz, 1H, 9-H), 8.16 (dd,
J = 7.9, 1.1 Hz, 1H, 8-H), 8.30 (s, 1H, 5-H), 8.45 (dd, J = 8.2, 1.8 Hz, 1H, 4-H), 9.13 (dd, J = 4.5,
o
о
1
3
1
2
2
+
1
1
.8 Hz, 1H, 2-H), 9.52 (dd, J = 8.2, 1.1 Hz, 1H, 10-H), 18.96 (br. s, 1H, NHN ). H NMR (250
MHz, CD CN + 1 equiv. HBF : bis-protonated salt 7·2HBF ): δ = 3.19 (d, J = 2.0 Hz, 6H, 6-
NMe ), 3.27 (d, J = 3.2 Hz, 6H, 7-NMe ), 8.27 (t, J = 8.3 Hz, 1H, 9-H), 8.38 (dd, J = 8.3, 5.7 Hz,
H, 3-H), 8.44 (dd, J = 8.1, 0.9 Hz, 1H, 8-H), 8.59 (s, 1H, 5-H), 9.04 (dd, J = 8.4, 0.9 Hz, 1H, 10-
H), 9.22 (dd, J = 5.7, 1.5 Hz, 1H, 2-H), 9.27 (dd, J = 8.3, 1.5 Hz, 1H, 4-H), 18.71 (br. s, 1H,
3
4
4
2
2
1
+
1
NHN ). H NMR (250 MHz, DMSO-d
6
): δ = 3.18 (d, J = 1.9 Hz, 6H, 6-NMe
), 7.84 (dd, J = 8.1, 4.5 Hz, 1H, 3-H), 8.01 (t, J = 8.2 Hz, 1H, 9-H), 8.34 (dd, J = 7.7,
.1 Hz, 1H, 8-H), 8.48–8.52 (m, 2H, 4-H, 5-H), 9.11 (dd, J = 4.5, 1.8 Hz, 1H, 2-H), 9.42 (dd, J =
2
), 3.22 (d, J = 2.7 Hz,
6
1
8
4
1
H, 7-NMe
2
+
13
.4, 1.1 Hz, 1H, 10-H), 18.58 (br. s, 1H, NHN ). C NMR (62.9 MHz, CD CN): δ = 45.7 (NMe ),
3
2
2
5.9 (NMe ), 119.7, 121.9, 123.1, 123.7, 125.5, 125.6, 128.6, 133.7, 136.3, 143.6, 144.3, 144.6,
50.9. C17
H20BF
4
N
3
(353.17): calcd. C 57.81, H 5.71, N 11.90; found C 57.74, H 5.89, N 11.72.
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European Journal of Organic Chemistry
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6
Synthesis of Quinolinol 13 from Diaminoquinoline 7: A solution of diamine 7 (27 mg, 0.10
mmol) in 2% aqueous H SO (6 mL) was refluxed for 7 h. Next, the solution was cooled to room
temperature, carefully neutralised with Na CO ·10H O until pH 7–8, and extracted with EtOAc
3×5 mL). Evaporation of the solvent gave quinolinol 13 (20 mg, 83%) as brownish-yellow crystals
with the spectral and analytical properties identical to those for the sample described above.
,7-Bis(dimethylamino)benzo[h]quinolinyl Palladium Acetate Dimer (14): To a solution of
diamine 7 (0.040 g, 0.15 mmol) in CH Cl (2 mL) was added Pd(OAc) (0.034 g, 0.15 mmol). After
stirring for 2 h at room temperature, the brown-yellow solution was concentrated in vacuum and the
2
4
2
3
2
(
6
2
2
2
2
solid residue was quickly (as the product is rather soluble in diethyl ether) triturated with Et O (2
1
mL) to afford 0.058 g (89%) of the title compound as a brown-yellow powder. H NMR (250 MHz,
CDCl
3 2
): δ = 2.41 (s, 6H, Ac), 2.75 and 2.83 (br. s and s, 24H, NMe ), 6.35 (s, 2H, 5-H), 6.56 (dd, J
=
8.1, 5.3 Hz, 2H, 3-H), 6.87 (d, J = 7.9 Hz, 2H, 9-H), 7.03 (d, J = 7.9 Hz, 2H, 8-H), 7.34 (dd, J =
1
3
8
.1, 1.1 Hz, 2H, 4-H), 7.83 (dd, J = 5.3, 1.1 Hz, 2H, 2-H). C NMR (62.9 MHz, CDCl
3
): δ = 24.9
(
Ac), 44.4 and 44.5 (NMe ), 106.3, 116.7, 119.3, 121.7, 125.3, 129.2, 132.9, 141.7, 142.4, 145.9,
2
1
42 6 4 2
46.4, 150.4, 150.9, 181.7 (Ac). C38H N O Pd (859.63): calcd. C 53.10, H 4.92, N 9.78; found C
53.24, H 4.87, N 9.64.
Supporting Information: Experimental section (general considerations and preparation of starting
1
materials); details of XRD analyses; UV/Vis spectra and colour changes of azo-dyes 8 and 11; H
1
3
and C NMR spectra of new compounds.
2
CCDC 1839919 (for 6·HBF ) and 1839920 (for 8Z·H O) contain the supplementary
4
crystallographic data for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
Acknowledgements
Financial support from the Russian Foundation for Basic Research (project Nos. 14-03-00010 &
17-03-00035) is gratefully acknowledged. NMR spectra were recorded at the Scientific and
Educational Laboratory of Resonance Spectroscopy, Department of Natural and High Molecular
Compounds Chemistry of Southern Federal University. X-Ray studies were performed by Dr.
Kyrill Y. Suponitsky (A. N. Nesmeyanov Institute of Organoelement Compounds, Moscow).
Keywords: Azo compounds, Benzo[h]quinolines, Basicity, Hydrogen bonds, Protonation.
References and Notes
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European Journal of Organic Chemistry
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European Journal of Organic Chemistry
10.1002/ejoc.201800652
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Graphical Abstract
Azo coupling and Skraup reactions were employed to get to the previously unknown
bis(dimethylamino)benzo[h]quinolines. Proper molecular organisation accounts for excellent
ligation of protons and Pd(II), while measurements of basicity, which can be higher than that of
DMAN and 10-DBhQ (see picture), put some of them into a category of heterocyclic superbases.
Key Topic
Benzo[h]quinolines as bases
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