Neutral Pyrrole Nitrogen Atom as a π- And Mixed n,π-Donor in Hydrogen Bonding

Article abstract of DOI:10.1021/acs.joc.8b02562

9-Dimethylaminobenzo[g]indoles 3-6 and 1-dimethylamino-8-(pyrrolyl-1)naphthalene 7 were examined as possible models for establishing the ability of the pyrrole nitrogen atom to participate in [NHN]⁺ hydrogen bonding as a proton acceptor. Indole

Full text of DOI:10.1021/acs.joc.8b02562

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Article
Neutral Pyrrole Nitrogen Atom as a #- and
Mixed n,#-Donor in Hydrogen Bonding
Alexander F. Pozharskii, Valery A. Ozeryanskii, Ekaterina A. Filatova, Olga V. Dyablo,
Olga G. Pogosova, Gennady S. Borodkin, Aleksander Filarowski, and Dmitriy V. Steglenko
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02562 • Publication Date (Web): 21 Dec 2018
Downloaded from http://pubs.acs.org on December 21, 2018
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Neutral Pyrrole Nitrogen Atom as a - and Mixed n,-Donor in Hydrogen Bonding
a
a
a
Alexander F. Pozharskii,* Valery A. Ozeryanskii, Ekaterina A. Filatova,
a
a
b
Olga V. Dyablo, Olga G. Pogosova, Gennady S. Borodkin,
c
ab
Aleksander Filarowski, Dmitriy V. Steglenko
a
Department of Organic Chemistry, Southern Federal University, Zorge str. 7, 344090 Rostov-on-Don,
Russian Federation. E-mail: apozharskii@sfedu.ru
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b
Institute of Physical and Organic Chemistry, Southern Federal University, Stachki Ave. 194/2,
3
44090 Rostov-on-Don, Russian Federation
c
Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland and
Department of Physics, Industrial University of Tyumen, Volodarskogo 38, 625000 Tyumen, Russian
Federation
Pyrrole nitrogen in H-bonding
Me
Me
Me
Me
Me
R1
H
+
+ H
N
N
N
N
R2
Pure NH...N()
Mixed NH...N(n,)
Abstract: 9-Dimethylaminobenzo[g]indoles 3–6 and 1-dimethylamino-8-(pyrrolyl-1)naphthalene 7
were examined as possible models for establishing the ability of the pyrrole nitrogen atom to
+
participate in the [NHN] hydrogen bonding as a proton acceptor. Indoles 3–5 (to a lesser extent 6)
form rather stable tetrafluoroborates with the proton mostly located on the NMe group but
2
+
simultaneously engaged in the formation of charged intramolecular [NHN] hydrogen bond (IHB) with
+
–
+
–
the pyrrole N atom. The theoretically estimated energies of IHB in salts 3H BF –6H BF vary
4
4
between 7.0–10.7 and 6.2–7.0 kcal mol–1 in vapor and MeCN, respectively. The pyrrole N atom
undergoes a perceptible pyramidalization but still remains involved into the 6-electron aromatic
+
–
+
–
system, suggesting that the hydrogen bonding in salts 3H BF –6H BF represents a previously
4
4
unknown mixed NH…N(n,) interaction. Despite the favorable orientation of the N–H bond and the
+
–
pyrrole ring in salt 7H BF , no signs of NH…N(n) bonding in it were noticed and the existing
4
interaction was classified as pure NH…N(). The results obtained may be useful in studies of
secondary protein structures, especially those -helix sections which contain tryptophan residues.
Introduction
For a long time it was believed that the secondary structure of normal natural proteins, which is a right-
…
handed -helix, is maintained exclusively by multiple NH O=C hydrogen bonds between the amino
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acid residues within one coil of the helix. Similar interactions of more remote helix sites as well as
different protein subunits are responsible for the tertiary and quaternary protein structures. However,
over time, it came to be understood that such a picture is greatly simplified and many other non-
covalent forces are involved in the formation of the spiral structure, such as salt bridges, hydrophobic
2
–4
forces or XH... binding (X = N, O, S) (reviews ). Thus, diffraction studies of thousands of protein
molecules have shown that the share of such interactions reaches 15% of their total number. As -
donors, aromatic cores of phenylalanine, tyrosine and tryptophan participate in them with the 1 : 1.5 : 5
0
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0
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0
2a
relative proportion. The higher activity of the latter is ascribed to the larger electron donor ability of
the more expanded indole -system. X-ray data have disclosed that the X–H… bonding for
tryptophan residues is equally realized via the benzene and pyrrole ring. At the same time, various
5
a
quantum-chemical calculations of model systems, for example 1-methylindole–H O and 1-
2
methylpyrrole–alkanols5b unequivocally give preference to the pyrrole moiety. Normally, the
–1 2
uncharged X–H… bonds are considered to be rather soft and weak (< 4 kcal mol ) and directed
6
a
either toward the ring midpoint or the carbon atoms. X-Ray crystal structures for parent pyrrole 1 (see
6
b
7
also ref. ) and 1-phenyl-1-(1H-pyrrole-1-yl)ethanol 2 can serve as typical illustrations. The NH…C

and OH…C distances in these dimeric units are equal to 2.57 and 2.52 Å, respectively (for

comparison, the OH…N distance in 2 is of 2.95 Å). This is not surprising since the pyrrole nitrogen
atom being a -electron donor gains partial positive -charge while the ring carbon atoms receive
negative -charges, which are larger in -positions.8
Me
N
H
Ph
O H
N
N
H O
Ph
Me
N
H
2
1
To our knowledge, until now there were no reports on the X–H...N() or X–H...N(n) hydrogen
bonding centered on neutral pyrrole nitrogen atom.9–14 This raises the question: are there any
circumstances under which such kinds of binding might be favorable? To answer it, in the present work
a possibility of the formation of intramolecular hydrogen bonds (IHB) in protic salts of 1-methyl-9-
dimethylaminobenzo[g]indoles 3–6 and 1-dimethylamino-8-(pyrrolyl-1)naphthalene 7 was studied
(Scheme 1). Our choice was based on the following reasoning. First, in view of the importance of the
so-called “proximity effect”, peri-disubstituted naphthalene derivatives for a long time attract deep
15
attention as convenient models for studying various nonconventional interactions. One of the most
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striking examples is the observation by Lectka and his group that IHB in the protic salt of N-(2-
+
fluorobenzoyl)-N,N'N'-trimethyl-1,8-diaminonaphthalene 8H (Scheme 2) is realized via the amide
nitrogen atom, not the carbonyl group, as it normally occurs in amides (see above). Secondly, a
+
–
+
–
+
–
comparison of compounds 3H BF –6H BF and 7H BF looks rather intriguing since the pyrrole N
4
4
4
atom in the former is positioned in a way excluding competition from the ring carbon atoms while it
1
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0
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6
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9
0
1
2
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5
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9
0
1
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
–
can not be said about 7H BF due to relative flexibility of the 8-pyrrolyl substituent.
4
Scheme 1. 1-Methyl-9-dimethylaminobenzo[g]indoles and 1-Dimethylamino-8-(pyrrolyl-
1
)naphthalene along with Their Tetrafluoroborates
F3B
F
R1
3
Me
R1
Me
2
H
+
Me N
N
2
9
1
Me N
N1
2
2
HBF4
R2
R2
8
7
4
6
5
1
2
+
–
1
2
3 (R = R = H)
3H BF (R = R = H)
4
1
2
+
–
1
2
4 (R = 4-MeC H , R = Me) 4H BF (R = 4-MeC H , R = Me)
5 (R = 4-O NC H , R = Me) 5H BF (R = 4-O NC H , R = Me)
6
4
4
6 4
1
2
+
–
1
2
2
2
6
4
4
2
2
6 4
1
+
–
1
6 (R = H, R = CF )
6H BF (R = H, R = CF )
3
4
3
F3B
F
H
Me
N
+
Me N
2
CF3
+
–
6
H BF (a)
4
Me
Me
H
+
N
N
Me N
N
8
2
1
BF4–
8a
HBF4
2
3
7
6
4
5
+
–
7
7H BF
4
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Scheme 2. Some Auxiliary Compounds Mentioned or Studied in This Work
Me
Me
Me R
C
Me
Me
H
R
O
+
Me
N2 N1
C
N
N1
O
2
CF SO H
3
3
–
CF SO ·1/2H O
3
3
2
+
8
(R = 2-FC H )
8H (R = 2-FC H )
6 4
0
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3
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0
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9
0
6
4
Me
Ph
Me N
N1
2
N
N
Me N
N
N
CF3
2
Ph
OH
NMe2
NMe2
F3C
10
11
9
H
R N
2
NR2
R N + NR
2
2
H+
12 (NR = NMe )
12H+
13H
2
2
+
13 (NR = pyrrolidino)
2
+
H
NMe2
NMe
2
H+
R
R
14H+
5H
1
1
4 (R = H)
5 (R = pyrrolyl-1)
+
1
Results and Discussion
Synthesis
+
–
+
–
The required protic salts 3H BF –6H BF were obtained on treatment of recently described
4
4
17
benzo[g]indoles 3–6 with HBF (see Experimental Section). Previously unknown 1-dimethylamino-8-
4
1
8
(
pyrrolyl-1)naphthalene (7) was synthesized by methylation of amine 16 with dimethylsulfate in the
presence of Na CO (Scheme 3). This method gave also some amount of monomethylated product 17.
2
3
An attempt to synthesize 7 by reacting 1-dimethylamino-8-aminonaphthalene with 2,5-
dimethoxytetrahydrofuran in acetic acid failed. At the same time, isomeric to 7 compound 15 (Scheme
2) was easily obtained by this way from 1-dimethylamino-5-aminonaphthalene (see Experimental
Section).
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Scheme 3. Synthesis of 1-Dimethylamino-8-(pyrrolyl-1)naphthalene (7)
N
NH2
N
NR2
H N
2
NMe2
MeO O OMe
Me SO , Na CO
2
4
2
3
X
MeOH
AcOH, 
16
7 (NR = NMe )
2 2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
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2
2
2
2
2
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0
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0
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9
0
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3
4
5
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7
8
9
0
1
7 (NR = NHMe)
2
Protonated Benzo[g]indoles
We were managed to grow crystals of salts 3H BF –5H BF from anhydrous acetonitrile and
+
–
+
–
4
4
+
–
performed their X-ray investigation (ESI, pp. S3–S5). Salt 6H BF with CF group in the pyrrole ring
4
3
due to its caramel-like character and tendency to deprotonation could not be structurally examined. The
+
–
+
–
molecular structures of salts 3H BF –5H BF along with the key geometrical parameters are shown
4
4
in Figure 1 and Table 1. For comparison, those also include data for bases 4 and 5, Lectka’s amide 8
+
–
and its salt 8H CF SO , which is important because the amide N atom in 8, similar to pyrrole
3
3
counterparts 3–5 donates two electrons into the -system.
3
·HBF4
4·HBF4
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3
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7
8
9
0
5
5
·HBF4
5
'·HBF4
5'
+
–
+
–
Figure 1. Molecular structures of tetrafluoroborates 3H BF –5H BF and base 5 (P = 50%, 120 K,
close NH proton–anion and NH proton–pyrrole nitrogen contacts are also given, Å). Two bottom
pictures are the view of salt 5·HBF and base 5 along the naphthalene ring plane with the peri-
substituents directed to the viewer; the small yellow-green ball in picture 5'·HBF denotes the nearest
4
4
4
4
fluorine atom of the counter-ion.
+
–
+
–
+
–
Table 1. Selected X-ray Diffraction Parameters for Salts 3H BF –5H BF , 8H CF SO and Bases
4
4
3
3
4
, 5 and 8
Compd.
Bond lengths and distances, Å
Angles (°) and out-of-plane
deviations, Å
Ref.
N(2)–H N(1)…H N(1)…N(2) H…F NHN N(1) N(2) Mea
+
–
b
b
b
3
H BF
0.91
0.90
0.89
–
2.43
2.33
2.34
–
3.175
3.074
3.046
3.051
3.016
2.17
2.25
2.40
–
139
140
136
–
353.4 334.2 0.597
345.6 334.3 0.843
342.9 334.2 0.943
354.0 341.8 0.562
351.7 347.3 0.676
4
4
4
+
–
–
4H BF
+
5
H BF
4
5
17a
17b
–
–
–
–
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8
H+
0.88
0.86
–
2.17
2.21
–
2.869
2.898
2.876
–
–
–
136
136
–
349.3 334.2
350.4 334.6
359.9 339.7
–
–
–
16
16
–
c
CF SO
3
3
8
a
Deviation of the N(1)-methyl group carbon atom from the average pyrrole ring plane. This work. c
Data for two independent cations are given.
b
1
1
1
1
1
1
1
1
1
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2
2
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2
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0
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0
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0
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0
1
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3
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8
9
0
1
2
3
4
5
6
7
8
9
0
Before discussing the data obtained, it is reasonable to ask a question: which of them could serve
as the most decisive argument in favor of the NHN hydrogen bonding? Obviously, such a parameter is
the pyramidalization of the pyrrole N atom that increases the stereo orientation of the unshared electron
+
19
20
pair orbital towards the proton of the Me NH group (see review and ref. for modern view on the
2
related hybridization phenomena). In its turn, the degree of the pyramidalization can be expressed via
the sum of the valence angles at the nitrogen atom, N; or alternatively, via the deviation of the pyrrole
NMe group from the average heterocyclic plane, Me. For example, in cations of proton sponges 12H+
+
–1 21
and 13H having especially strong IHB with the energy, E , estimated near 20 kcal mol , the N
value varies between 336–339°, which is 8–12° less than in the corresponding bases.22,23 For amide
HB
+
+
+
cation 8H , the pyramidalization is practically the same (N = 10°) and for cations 4H and 5H the
N’s are 8.4 and 8.6°, respectively. These values along with the Me parameters (Table 1) clearly
indicate the formation of IHB between the NH group and the pyrrole nitrogen atom in the solid

+
–
+
–
tetrafluoroborates 4H BF and 5H BF . It is noteworthy, that partial pyramidalization exists already
4
4
in bases 4 and 5 (Me = 0.562 and 0.676 Å, respectively). We attribute this to the steric pressure that
the 2-aryl- and partly the 9-NMe substituents exert on the pyrrole NMe group. Yet, as it is seen, the
2
+
+
transition to cations 4H and 5H results in considerable extra pyramidalization and is accompanied by
a decrease in the rotation angle of the 2-aryl ring relatively the pyrrole cycle (ESI, Table S5).
+
+
+
The structure of cation 3H considerably differ from that of 4H and 5H . Thus, the N(1) atom in
+
–
+
–
+
–
3
H BF is more flat [N(1) = 353.4°] as compared with 4H BF and 5H BF (345.6° and 342.9°)
4
4
4
+
and the deviation of pyrrolic NMe group for 3H is also smaller: Me = 0.597 against 0.843 and 0.943
+
+
Å for 4H and 5H , respectively (see also estimation of valent orbital contribution for pyrrole N atoms
in compounds under study, ESI, p. S100). We think that due to the absence of 2-aryl substituent in 3 its
N(1) atom is more involved into the aromatic -system and therefore extra energy is needed to form the
+ 24,25
NHN bond in 3H .
+
–
+
–
On the whole, the IHBs in the solid salts 3H BF –5H BF are strongly asymmetric with the NH
4
4
proton mostly located on the 9-NMe group and with the N(1)…N(2) distance in a range of 3.05–3.17
2
Å. Though this interval greatly exceeds that in a vast majority of proton sponge cations (2.55–2.60 Å)22
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3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
it is well below 3.2–3.8 Å what is commonly considered as the typical length for the X–H… hydrogen
2
+
–
+
–
bonds in proteins. A rather wide cleft between the nitrogen atoms in 3H BF –5H BF along with a
4
4
quite sharp NHN angle (136–140°) provokes close proximity of the NH proton and the nearest fluorine
–
atom of the BF anion with the inverse proportion between the H…F and N…N distances (Table 1).
4
+
–
+
–
Actually, in the solid salts 3H BF –5H BF we deal with the bifurcated hydrogen bonds and that is
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
why benzo[g]indoles 3–5 stay structurally much closer to the kinetically active pseudo-proton sponges
2
6
27
like 10 or hybrid sponges like 11 rather than to the genuine proton sponges 12 and 13 (Scheme 2).
+
–
Another important point, which was revealed during the diffraction studies of salts 3H BF –
4
+
–
5H BF , concerns a mode of changing N…N distance on protonation. Usually, in proton sponges and
4
their –N=/–N= and Me N/–N= counterparts a transition to the protonated form is accompanied by a
2
considerable contraction of the N…N distance (Figure 2a–c, NN parameter) due to the strain relief at
+
+
the formation of strong IHB. In contrast, such transition from 3–5 to cations 3H –5H leads to slight
elongation of the N…N parameter (Figure 2d). Apparently, the presence of both the NH proton and the
+
–
+
–
pyrrole NMe group in the internitrogen space of 3H BF –5H BF hampers the formation of more
4
4
effective NHN bond (Figure 2d). This conclusion is supported by the analysis of the naphthalene ring
deformation (twisting parameter) in the compounds under consideration (ESI, p. S99). Similar behavior
was previously noticed for some other proton sponge-like compounds21,28 and what is especially
indicative for Lectka’s amide 8.16
Me
Me
+
+
N
H
N
N
H
N
Me
Me

NN = –0.20...–0.25
–0.18
a
b
Meb
N
Mec
Me
Me
+
+
H
N H
N
N
Mea
–0.13
+0.025
c
d
Figure 2. Schematic representation of IHB geometry and typical changes of the average N…N
parameter (NN) in protonated peri-disubstituted naphthalenes relatively to the corresponding bases: a
+
+
–
1
classical proton sponge 14H ; b – symmetrical pseudo proton sponge of type 12H ; c – hybrid base
+
+
–
+
–
3H ; d – 9-dimethylamino-1-methylbenzo[g]indoles 3H BF –5H BF .
4 4
+
–
+
–
1
13
15
To elucidate structure of salts 3H BF –6H BF in solution, their H, C and N NMR spectra
4
4
were recorded in CD CN together with those of the corresponding bases (ESI, Figures S1–S11). The
3
chemical shift values for the NH, NMe and NMe groups are depicted in Table 2. Thus, in the spectrum
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of 6·HBF , the NH proton owing to the fast exchange with water admixture manifests itself at  4.65
4
+
ppm what testifies an absence of IHB in cation 6H in solution due to the electron-accepting CF group
3
+
–
[
see structure 6H BF (a), Scheme 1]. Unlike 6·HBF , the NH proton in salts 3·HBF –5·HBF under
4 4 4 4
similar conditions does not undergo visible exchange in the NMR time scale and resonates at  8.3–8.4
ppm as a markedly broaden signal (Table 2, see also ESI, Figures S1, S4). Such high field chemical
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
shifts contrasting with those in the H NMR spectra of cations 10H –13H ( = 17–20 ppm)22,23,25,27
1
+
+
NH
may result from a relative weakness of IHB in 3·HBF –5·HBF . At the same time, it should be
4
4
recalled (see above) that the NH proton in the latter is significantly deviated from the plane of the
aromatic system and thus can be affected by the paramagnetic component of the ring electron current.
+
16
This helps to understand why the NH values for amide cation 8H (11 ppm in MeCN) and even for
dimethyl(naphthyl-1)ammonium perchlorate 14·HClO (this work: 9.47 in MeCN; 10.45 ppm in
4
CDCl ) and 1-dimethyl(5-pyrrolylnaphthyl-1)ammonium picrate 15·PicOH (this work: 13.2 ppm,
3
CDCl ) are larger than those for 3·HBF –5·HBF .
3
4
4
Table 2. Selected Chemical Shift Values (, ppm, CD CN, 30 °C) for Benzo[g]indoles 3–6 and Their
3
Tetrafluoroborates 3·HBF –6·HBF
4
4
Compound
1H
13C
15N
NH
–
NMe2 NMe NMe2 NMe NMe2
NHeta
138.2
128.4
126.8
139.2
126.6
3
4
2.65
2.76
2.78
2.69
3.39
3.49d
3.52
4.02
3.61
3.63
4.06
3.88
3.43
3.46
4.00
42.7
42.4
42.5
43.3
50.1
50.0
50.2
50.4
39.4
38.5
39.0
40.3
38.0
39.1
39.3
38.4
42.9
43.4
43.2
–
5
–
6b
–
43.4
c
3
·HBF4
8.28
8.42
~8.4e
4.65f
4·HBF4
49.8
117.6
c
c
5
6
·HBF4
·HBF4
3.42
50.4
135.2
a
b
25 c
d
Heterocyclic nitrogen. In CDCl ; ref. Reliable signal could not be registered due to dynamics. A
3
3
e
f
doublet with J = 4.61 Hz. This peak partially overlaps with a doublet from 4-O NC H group.
2
6
4
Common peak for the NH/H O species due to a fast exchange.
2
The proton chemical shifts of the NMe and pyrrole NMe groups for bases 3–6 and
2
tetrafluoroborates 3·HBF –5·HBF are also instructive. Commonly, for the proton sponge, a transition
4
4
+
from the neutral base to the nearly symmetrical protonated form (12  12H ) is accompanied by a
2
2
low-field shift of the NMe peaks ( ~ +0.3 ppm). Unlike this, the protonation of 3–6 results in
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3
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4
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4
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4
5
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5
5
5
5
5
5
5
5
6
increasing the chemical shifts by 0.65–0.75 ppm which is twice as large as in 12. Clearly, this once
again evidences preferable localization of the NH proton in 3·HBF –5·HBF on the 9-NMe group.
4
4
2
Notably, that in the spectrum of 4·HBF , the signal of the NMe group resonates as a doublet with
4
2
3
+
22
J(NH,NMe) = 4.61 Hz (2.6 Hz for 12H ); for 3·HBF and 5·HBF it appears as a broaden singlet
4
4
likely due to enhanced dynamics. Such dynamics also accounts for the significant signal broadening of
0
1
2
3
4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
the aromatic protons in the spectra of 3·HBF and 5·HBF .
4
4
The behavior of the pyrrole NMe group on protonation of bases 3–6 is completely different. In this
case the hydrogen signal of the NMe fragment moves relatively the corresponding base to a high-field
region ( = –0.14…–0.18 ppm). We assume that this phenomenon results from a larger extent of sp3-
+
+
hybridization of the cyclic nitrogen atom in cations 3H –5H , which can be treated as another weighty
proof for the formation of IHB in salts 3·HBF –5·HBF . Nitrogen spectra in general confirm this
4
4
picture, while carbon spectra are less informative. From all of the above one can conclude that the IHB
strength in cations under consideration both in MeCN solution and in the solid changes in the sequence
+
+
+
+
5
H  4H > 3H > 6H .
Finally, we have estimated basicities of benzo[g]indoles 3 and 4 by measuring their pK values in
a
MeCN solution via competitive NMR monitoring (ESI, p. S104). We proceeded from the fact that,
when protonation is accompanied by the formation of chelated cation this commonly results in
increasing pK , i.e. basicity, relatively a standard base whose cation is not stabilized by IHB. Proton
a
29
30
sponge 12 (pK = 18.6, MeCN) and 1-dimethylaminonaphthalene 14 (pK = 11.3, MeCN) are
a
a
textbook examples of this pattern. The pK values obtained by us for 3 and 4 (12.8 and 13.5,
a
respectively) follow this rule thus leaving no doubts in the existence of weaker but quite pronounce
IHB in their cations in solution. We could not measure pK for 4-nitrophenyl derivative 5 in MeCN due
a
to solubility restrictions.
Protonated 1-Dimethylamino-8-(pyrrolyl-1)- and 8-phenylnaphthalenes
3
1
Regrettably, our attempt to conduct the X-ray diffraction study of salt 7·HBF failed. However, in the
4
case of base 7 we were more successful and the results obtained, along with theoretical calculations,
turned out to be helpful. Rather strained X-ray structure of 7 (ESI, Figure S37a) closely reminds that of
diamine 12. This includes noticeably twisted naphthalene framework, nitrogen atoms diverging in
different sides from the averaged naphthalene ring plane, and the N-methyl groups turned away from
the internitrogen space. Besides, the pyrrole ring in 7 faces the amine nitrogen free electron pair as if
the molecule were already pre-organized to form NH… or NH…N(n) hydrogen bond. To our
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satisfaction, the solid structure of 7 was surprisingly close to the calculated one for the gas phase (ESI,
Figure S37b, Table S11) what allowed to be treated with confidence in the theoretical characteristics of
7
·HBF (Figure 3).
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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4
4
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4
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4
4
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4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(a)
(b)
+
Figure 3. DFT calculated structures of "naked" cation 7H (a) and tetrafluoroborate 7·HBF (b) (in
4
vapor). Both structures are shown with the peri-substituents directed to the viewer.
+
In the "naked" cation 7H (Figure 3a, Table S12) the pyrrole ring is perfectly flat (N = 359.8°)
1
and strictly perpendicular to the naphthalene plane ( = 90°). Accordingly, the N–H bond points the
pyrrole ring face with the NH proton being closer to the ring N atom (1.907 Å) rather than to C (2.324

Å) or to the ring centroid (2.168 Å). In salt 7·HBF (Figure 3b, Table S12), the cation structure
4
–
undergoes significant changes owing to the attractive interaction between the anion BF and the NH
4
proton. This is evidenced by the very short NH...F distance (1.783 Å against 2.457 Å for NH…N), the
+
increase in the sharpness of the NHN angle (112.1° against 146.6° in 7H ), the larger asymmetry of the
IHB and, particularly, by marked deviation of the proton NH away from the naphthalene ring plane.
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6
Following the last change, the pyrrole ring also rotates to some angle ( = 73.1°), as if trying to
preserve the interaction with the NH proton. At the same time, the pyrrole N atom in 7·HBF remains
4
essentially flat undergoing only subtle pyramidalization (N = 356.8°). From this, one can conclude
1
that in 7·HBF we deal with the almost pure NH...N() interaction where the pyrrole N atom acts just
4
as a component of the aromatic -system. Clearly, the closeness of the NH proton in 7·HBF to the
4
0
1
2
3
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5
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8
9
0
1
2
3
4
5
6
7
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9
0
1
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3
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5
6
7
8
9
0
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
pyrrole N atom results from the molecular arrangement although a minor contribution of the NH...N(n)
binding should not be fully rejected.
1
Such view is generally consisted with H NMR spectroscopy. Salt 7·HBF was generated directly
4
in the NMR ampoule by adding an equivalent amount of HBF to a solution of 7 in MeCN. In the
4
protic spectrum of 7·HBF the signals of all protons are expectedly shifted to a low field relatively the
4
base while keeping unchanged general spectral pattern (ESI, compare Figures S13 and S20). The only
exception is the highly broaden NH peak which appears at  6.41 ppm, i.e. in unusually strong field for
the chelated proton (cf. with  ~8.4 and 4.65 ppm for benzo[g]indole salts 3·HBF –5·HBF and
4
4
6
·HBF , respectively, Table 2). In our opinion, this fact can be attributed both to the influence of the
4
paramagnetic component of the pyrrole ring current in 7·HBF and some weakening of IHB due to the
4
considerable bifurcation. Indeed, the NH value for molecular complex 15·PicOH (13.2 ppm) with the
ammonium and pyrrole fragments on the opposite sides of the naphthalene core, is much larger than
those for 7·HBF (ESI, Figures S28 and S38).
4
In the above context, it seemed rather interesting to perform similar studies for previously unknown
1
-dimethylamino-8-phenylnaphthalene (18) and its salt 18·HBF . This model compound was prepared
4
via standard Suzuki procedure on treatment of 1-bromo-8-acetylaminonaphthalene with phenylboronic
acid, followed by hydrolysis, N,N-dimethylation, and protonation steps (see Experimental Section). The
X-ray diffraction study of 18·HBF (Figure 4) has revealed the formation of the perfect NH...
4
+
–
1
hydrogen bond in cation 18H and the absence of any bifurcation involving the anion BF . In the H
4
NMR spectrum of the tetrafluoroborate in MeCN, the proton NH appears as a broad signal at  8.1 ppm
(
Figure S34). From this, one can suggest that the NH… interaction in 18·HBF in solution is stronger
4
than in 7·HBF and is commensurable with that in benzo[g]indole salts 3·HBF –5·HBF (see also
4
4
4
discussion in the last section).
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. X-ray structure of salt 18·HBF (P = 50%, 120 K, close cation–anion and NH proton–phenyl
4
ring contacts are given, Å). Key parameters: N…C = 2.858 Å, N...H = 0.91 Å, NHC = 146°,
1
1
11
NH…centroid = 173°; Ph rotation angle,  = 83.7°.
Hydrogen Bond Energy
In the above discussion, we qualitatively ranged the salts under study for the strength of their IHB,
based on the solid X-ray and solution NMR data. To achieve more accurate estimate we then
performed the DFT calculations of the IHB energies, E , using the B3LYP method with the 6-
HB
3
11++G** basis set. The calculations were conducted both for "naked" cations and tetrafluoroborate
salts in the gas phase and acetonitrile solution. Three previously reported approaches were tested for
2
1
this. The first one known as isodesmic gave the results badly consistent with the experiments (for
details see ESI, pp. S100–S102). In contrast, the second approach, which can be called "rotational",32
turned out to be more realistic. In accordance with it, the EHB values were obtained as a difference
between the total energies for the computationally optimized chelated (close) and non-chelated (open)
forms (Scheme 4, see also ESI, Table S10).
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2
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2
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3
3
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3
3
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3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Scheme 4. Structures with Close and Open IHBs Used for Calculation of EHB Values by "Rotational"
Method
Me
Me
Me
Me
Me
N
R1
R1
Me
+
N
H
H
N
+
N
R2
R2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
+
E (H ) (close)
E (H ) (open)
1
2
+
1
2
3H (R = R = H)
+
1
2
4H (R = p-Tol, R = Me)
+
1
2
5H (R = p-O NC H , R = Me)
2
2
6 4
+
1
6H (R = H, R = CF )
3
F
B
F
F
F
F
Me
Me
Me
R1
Me Me
N +
N
Me
B
R1
F
F
H
+
F
H
N
N
R2
R2
E (HBF ) (close)
E (HBF ) (open)
2 4
1
4
+
–
1
2
3H BF (R = R = H)
4
+
–
1
2
4
5
6
H BF (R = p-Tol, R = Me)
4
+
–
1
2
H BF (R = p-O NC H , R = Me)
4
2
2
6 4
+
–
1
H BF (R = H, R = CF )
4
3
EHB = E – E2
1
Table 3. Calculated Hydrogen Bond Energies, E , in "Naked" Cations (B3LYP/6-311++G**)
HB
Cation Medium Calculation
approach
EHB
kcal/mol
–10.5
–8.8
EHB(ZPE)
kcal/mol
–10.7
–8.8
3H+
3H+
Vapor
MeCN
Vapor
Vapor
MeCN
Vapor
Vapor
MeCN
Vapor
Vapor
Rotational
Rotational
NBO
3
4
4
H+
H+
H+
–7.3
–
Rotational
Rotational
NBO
–11.2
–9.4
–11.4
–9.7
4H+
–10.7
–10.9
–9.2
–
5
5
5
H+
H+
H+
Rotational
Rotational
NBO
–11.1
–9.8
–9.2
–
6H+
Rotational
–10.0
–10.2
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H+
7H+
H+
MeCN
Vapor
MeCN
Vapor
MeCN
Rotational
Rotational
Rotational
Rotational
Rotational
–8.4
–13.3
–11.4
–13.7
–11.7
–8.8
–13.9
–11.4
–13.8
–11.8
7
1
1
8H+
8H+
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 4. Calculated Hydrogen Bond Energies, E , in Tetrafluoroborates (B3LYP/6-311++G**,
HB
"
Rotational" Approach)
Salt
Medium
EHB
kcal/mol
–7.1
EHB(ZPE)
kcal/mol
–7.0
Salt
Medium
EHB
kcal/mol
–7.6
EHB(ZPE)
kcal/mol
–7.4
+
–
–
–
–
–
–
+
–
–
–
–
3
3
4
H BF
Vapor
MeCN
Vapor
MeCN
Vapor
MeCN
6H BF
Vapor
MeCN
Vapor
MeCN
Vapor
MeCN
4
4
4
4
4
4
4
4
4
4
+
+
H BF
–6.6
–6.2
6H BF
–6.6
–6.5
+
+
H BF
–9.9
–9.6
7H BF
–1.9
–1.8
+
+
4H BF
–6.9
–6.5
7H BF
–6.2
–6.5
+
+
–
5
5
H BF
–11.0
–7.2
–10.7
–7.0
18H BF
–6.6
–6.2
4
+
+
–
H BF
18H BF
–8.6
–8.5
4
The results obtained (Tables 3 and 4) lead to the following conclusions.
) The first thing that attracts attention is seemingly inflated EHB values. This can be attributed to
two peculiarities of the studied models: participation in the binding of a positively charged proton-
1
2
+
donor functionality, Me NH , and the widely known specifics of peri-interactions providing the close
2
3
3
proximity of interacting groups and their fairly rigid spatial fixation. Nevertheless, we are not inclined
to discount the probability that the EHB values obtained may indeed be somewhat overestimated due to
differences in the strain energies, E , of the close and open forms shown in Scheme 4. Thus, in case of
SE
the benzo[g]indole cations, the strain energy in the close form can result from the repulsion of the
nearby located NH proton and pyrrolic NMe group. In the open form, such repulsion should be even
greater, since it involves already three N-methyl groups. Unfortunately, when calculating the EHB
values by "rotational" method, it is difficult to obtain a sufficiently accurate correction for the
difference between spatial destabilization of the close and open forms since the EHB itself, at such
approach, actually consists of two components: hydrogen bonding and strain energies. Yet, we tried to
estimate it, at least roughly, by calculating the energies of similar rotational conformers for non-
protonated bases 3 and 4 and taking the difference between them: E (base) = E – E (Scheme 5). The
SE
1
2
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ESE(base) values obtained for the gas phase and the acetonitrile solution for 3 and 4 range between 5.8
–
1
and 6.7 kcal mol . Subtracting them from the E given in Table 3, one can get the E (corr) values,
HB
HB
–
1
which lie in the range 3–5 kcal mol . However, despite the fact that they look being in harmony with
the energies accepted for the NH... interactions in natural proteins, it is difficult to consider them as
realistic because of the neglect of the strain energies in the cations. We then reasoned that a possible
way to improve the situation would be to calculate EHB by means of the so-called natural bond orbital
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
4
method (NBO). The point is that the NBO does not consider two forms, close and open, and involves
calculating the electron transfer energy (it is usually designated as E(2)DA interaction) from the n-
orbital occupied by an unshared electron pair of a donor atom onto the antibonding -orbital of the X–
+
+
H bond (N–H in our case). We tested this method on "naked" cations 3H –5H , obtaining the gas-
–
1
phase hydrogen bond energies for them equal to –7.3, –10.7 and –9.2 kcal mol , respectively (Table
). As it is seen, the course of these changes qualitatively coincides with the X-ray diffraction and
NMR spectral data but the EHB themselves are in absolute values smaller than those obtained by the
3
–
1
uncorrected (Table 3) "rotational" method by 3.2, 0.5 and 1.7 kcal mol , respectively. From this, one
can conclude that NBO values adequately reflect the necessary corrections for the strain energies,
which should be taken into account when considering the EHB obtained by the "rotational" method.
Nevertheless, without having a complete set of NBO data, we left in Table 3 the uncorrected
"rotational" values, bearing in mind that their absolute values should be on average 1.5–2 kcal less.
Scheme 5. Estimation of Strain Energy in "Open" Forms of Bases 3 and 4
Me
N
R1
Me
Me
R1
Me
Me
Me
N
N
N
R2
R2
E ("close")
1
E ("open")
2
1
2
3
4
(R = R = H)
1
2
(R = p-Tol, R = Me)
ESE(base) = E – E
2
1
+
+
2
) The NHN hydrogen bond energies for the "naked" benzo[g]indole cations 3H –6H are larger
–1
than those for tetrafluoroborates by 3.7, 1.8, 0.4 and 2.8 kcal mol , respectively (vapor, ZPE).
Obviously, this phenomenon is due to the bifurcation of the hydrogen bond in the case of
tetrafluoroborates (see above and below the additional discussion on this topic).
–
1
3
) In the series of benzo[g]indole cations, the IHB strength decreases by 1–2 kcal mol upon
transition from the gas phase to the acetonitrile solution. This applies to both "naked" cations and
tetrafluoroborates.
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4) It is known that the hydrogen bond of general form X–H…Y has a predominantly electrostatic
3
5
nature with a certain contribution of the covalent term. Visually, this can be expressed through the
principal possibility of proton transfer (PT) from X to Y: X–H...Y ⇄ X...H–Y. A two-well potential
corresponds to such proton motion. The ease of the PT (the height of the barrier) and therefore the
strength of the hydrogen bonding increase as the X…Y distance becomes shorter. To date, cations of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
+
proton sponges, for example 12H and 13H , are recognized as one of the best models for the study of
PT due to importance of this question for mechanism of enzyme catalysis.15c,36 Usually, the barrier
height for proton sponges does not exceed 4.5 kcal mol–1,36c but in case of species with strongly
compressed IHB, e.g. the cation of 1,8-bis(dimethylamino)-2,7-bis(trimethylsilyl)naphthalene, the
–1 37
barrier drops to 0.7 kcal mol .
Importantly, that the very fact of realizing two-well profile can be interpreted in favor of the
existence of IHB, whereas both the barrier height and the energy of the PT structure indicate the ease of
the proton motion and relative hydrogen bond strength. Taking these circumstances into account, we
+
+
performed gas phase calculations of PT for "naked" cations 3H –7H (Figure 5). The points on the
potential energy curves represent energy changing as the NH proton moves from the 9-NMe group
2
towards the pyrrole nitrogen atom. The results obtained demonstrate that, despite the considerable
+
+
localization of the proton in 3H –6H onto the NMe group, its transfer to the pyrrole N atom is
2
+
+
allowed, since all 3H (PT)–6H (PT) structures lie in the potential minima. Judging from their energy
and barrier heights, the benzo[g]indole cations are arranged in the following sequence, characterizing
+
+
+
+
the decrease in the NHN strength: 4H > 5H > 3H > 6H . As can be seen, this row corresponds
exactly to the estimates obtained from other data (see above). In contrast, no minimum has been found
+
for the 7H (PT) form testifying the absence of the proton transfer, even in spite of the shorter inter-
+
+
+
nitrogen distance in 7H (2.82 Å) than in 3H and 4H (2.89 and 2.95 Å, respectively). We assume that
this stems from the higher aromaticity of the pyrrole ring in 7 as compared with that in benzo[g]indoles
+
3–6; as a consequence, pyrrolium cation 7H (PT) differs by smaller stability. On the other hand,
simple calculations demonstrates that the losses in the aromatic resonance energy associated with the
+
+
pyramidalization of the pyrrole nitrogen atom in cations 3H –5H are rather insignificant and are
compensated by the energy gain from the intramolecular NHN binding (ESI, pp. S102–S103).
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1
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1
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2
2
2
2
2
2
2
2
2
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3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
5
5
6
Me
N
R1
Me
Me
Me
Me
R1
H
H
+
Me
N
N
+N
R2
R2
+
1
2
+
3
H (R = R = H)
3H (PT)
+
1
2
+
4
5
6
H (R = 4-MeC H , R = Me) 4H (PT)
H (R = 4-O NC H , R = Me) 5H (PT)
H (R = H, R = CF )
6 4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
1
2
+
2
2
6 4
+
1
+
6H (PT)
3
Me
Me
Me
Me
H
H
+
N
N
N
+N
H+
+
7
7H (PT)
Figure 5. Calculated [B3LYP/6-311++G(d,p)] potential energy curves for the gas phase proton transfer
+
+
in cations 3H –7H [d(NH) – distance between the proton and the nitrogen atom of NMe group].
2
5
) In accordance with theoretical calculations, the “naked” cations both in vapor and MeCN form
the identical sequences in the order of decreasing the IHB strength. As can be seen, the relative position
of the benzo[g]indole cations is generally in accord with that observed in experimental studies. The
+
+
priority of cations 7H and 18H can be explained by the flexibility of the pyrrolyl and phenyl rings
providing the most favorable orientation for pure NH… interaction.
“Naked” cations:
+
+
+
+
+
+
Gas phase: 18H  7H > 4H > 5H > 3H > 6H
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+
+
+
+
+
+
MeCN solution: 18H > 7H > 4H  5H > 3H  6H
The situation with tetrafluoroborates is not so unambiguous. In MeCN media, salt 18H BF
+
–
4
+
–
remains first but 7H BF moves to the end of the row. The permutations are even more dramatic in the
4
+
–
+
–
gas phase where IHB’s in 7H BF and 18H BF become the weakest dropping to –1.8 and –6.2 kcal
4
4
–
1
mol (ZPE correction), respectively. At the same time, relative arrangement of the benzo[g]indole
tetrafluoroborates is practically unchanged.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Tetrafluoroborate salts:
+
+
+
+
+
+
Gas phase: 5H > 4H > 6H > 3H > 18H >> 7H
+
+
+
+
+
+
MeCN solution: 18H > 5H > 4H  6H  7H > 3H
Consideration of these discrepancies suggests that the use of "rotational" method for calculating the
IHB energies of tetrafluoroborates is somewhat questionable. Strictly speaking, the EHB values obtained
in these cases characterize not so much the energy of the intramolecular NHN hydrogen bond in the
salts as the difference between total energy of bifurcated hydrogen bonds in the close form and energy
of the external F…H bond in the corresponding open form. Such an approach would be acceptable
providing that the approximate equality of the energies EF...H in the open and closed forms is obtained.
However, from Tables S13 and S14 [parameter RF...H (close)/RF...H (open)] it follows that this is far from
+
–
the case, especially for salt 18H BF . The relatively satisfactory results when using the "rotational"
4
method for tetrafluoroborates in acetonitrile can be explained by the significant polarity of this solvent,
which contributes to the strong separation of ion pairs.38 In particular, this is evidenced by the
considerable increase in the RF...H distance in closed forms of the tetrafluoroborates in MeCN compared
to the gas phase (ESI, Tables S13 and S14). In fact, the structure of tetrafluoroborates in acetonitrile
can be regarded as approaching that in the “naked” cations. Judging by the sharp increase in the F...Н
+
–
+
–
distance this is especially true for salts 18H BF and 7H BF (Table S13). Notably, that, judging by
4
4
+
–
the X-ray experimental distances F...H (given below in brackets), the IHB strength in salt 18H BF
4
also far exceeds that in the benzo[g]indole tetrafluoroborates. We believe that the absence of any
+
–
bifurcation in the solid 18H BF mainly results from sterics since the orthogonal to the naphthalene
4
–
+
system 8-phenyl group actually acts as a “wall” preventing the approach of the BF anion to the NH
4
proton (see also Figure 4). Apparently, to a lesser extent this effect should be also manifested in salt
+
–
7
H BF .
4
+
–
+
–
+
–
+
–
1
8H BF (3.67 Å) > 5H BF (2.40 Å) > 4H BF (2.25 Å) > 3H BF (2.17 Å)
4
4
4
4
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4
4
4
4
4
4
4
4
4
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5
5
6
Conclusions
In summary, two types of unique pre-organized model compounds, namely protonated 9-
dimethylamino-1-methylbenzo[g]indoles 3·HBF –6·HBF and 1-dimethylamino-8-(pyrrolyl-
)naphthalene 7·HBF , in which a neutral pyrrole nitrogen atom could serve as an n- or -donor at the
4
4
1
4
formation of intramolecular NHN hydrogen bond, were prepared and studied. It has been found that in
both cases the NH proton, being strongly shifted to the NMe group, closes the IHB which is centered
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
–
1
on the pyrrole N atom. The average energy of such binding lies in the range of 7–10 kcal mol ,
according to the DFT calculated data. This is the first example of such kind of hydrogen bonding in
pyrroles; in all previously reported cases, the X–H bond was pointed to annular carbons or ring
centroid. At the same time, the principal difference between the two above model systems is that while
+
in the benzoindole cations the pyrrole N atom is markedly pyramidalized, in the 7H cation it remains
+
practically flat. On this basis, IHB in 7H was attributed to the NH...N() type since the pyrrole
heteroatom actually appears here as a part of the aromatic system, whereas the NHN binding in
benzoindole cations seems to be reasonably classified as mixed NH…N(n,) interaction.
Experimental Section
1
13
General. If not indicated otherwise, solution H and C NMR experiments were performed with a 600
1
5
MHz spectrometer using the solvent residual peaks as the internal reference. N Chemical shifts were
referenced relative to nitromethane. All reagents and starting materials were obtained from commercial
sources and used without further purification. 9-Dimethylaminobenzo[g]indoles 3 and 4 were
synthesized as described in ref.17a Benzo[g]indole 6 was synthesized according to the published
2
5
17b
procedure while the synthesis of compound 5 was described in ref.
1
N,N,1-Trimethyl-1H-benzo[g]indol-9-amine (3). H NMR (CD CN, 600 MHz, 30 °C)  2.65 (s, 6H),
3
4
.02 (s, 3H), 6.59 (d, J = 3.04 Hz, 1H), 7.14 (dd, J = 7.93, 1.18 Hz, 1H), 7.18 (d, J = 3.04 Hz, 1H), 7.29
(t, J = 7.83 Hz, 1H), 7.40 (d, J = 8.39 Hz, 1H), 7.50 (dd, J = 7.93, 1.18 Hz, 1H), 7.59 (d, J = 8.39 Hz,
1
3
1
1
1
H); C{ H} NMR (CD CN, 150 MHz, 30 °C)  39.4, 42.7, 103.0, 114.1, 118.6, 121.2, 121.3, 122.9,
3
24.0, 127.2, 131.3, 132.2, 134.0, 148.8.
1
N,N,1,3-Tetramethyl-2-p-tolyl-1H-benzo[g]indol-9-amine (4). H NMR (CD CN, 600 MHz, 30 °C)
3
 2.36 (s, 3H), 2.43 (s, 3H), 2.76 (s, 6H), 3.61 (s, 3H), 7.13 (dd, J = 7.92, 1.03 Hz, 1H), 7.30 (t, J = 7.84
Hz, 1H), 7.36 (d, J = 8.29 Hz, 2H), 7.45 (d, J = 8.29 Hz, 2H), 7.48 (d, J = 8.41 Hz, 1H), 7.52 (dd, J =
1
3
1
7
.92, 1.03 Hz, 1H), 7.63 (d, J = 8.41 Hz, 1H); C{ H} NMR (CD CN, 150 MHz, 30 °C)  9.1, 20.5,
3
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38.5, 42.4, 110.2, 113.9, 118.1, 119.0, 121.3, 122.6, 124.1, 127.4, 129.4, 129.9, 130.6, 134.2, 134.8,
1
37.7, 140.8, 148.8.
Proton Complexes with Tetrafluoroboric Acid (Hydrogen Tetrafluoroborates) were prepared by
mixing equimolar amounts of bases 3–6 (usually 0.05 mmol) and 40% aqueous HBF in a minimum
4
volume of EtOAc (1.5 to 2 mL) followed by 3-fold dilution with Et O. The residue thus formed was
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
washed with Et O (except for 6·HBF ) and vacuum dried to give the desired salts in high yield. In case
2
4
of 6·HBF , the solvents from clear solution were deleted in vacuum and the non-crystalline residue
4
thus formed was vacuum dried to give 6·HBF as clear caramel.
4
N,N,1-Trimethyl-1H-benzo[g]indol-9-amine Hydrogen Tetrafluoroborate (3·HBF ): 58% (18 mg
4
1
from 22 mg of 3); pale-yellow plates with mp 136–138 °C (decomp.). H NMR (CD CN, 600 MHz, 30
3
°
C):  3.39 (br. s, 6H), 3.88 (s, 3H), 6.86 (br. s, 1H), 7.41 (br. s, 1H), 7.61–7.64 (m, 2H), 7.82–7.83 (m,
13
1
1
H), 7.94–7.95 (m, 1H), 8.13–8.15 (m, 1H), 8.28 (br. s, 1H, NH); C{ H} NMR (CD CN, 150 MHz,
3
30 °C)  38.0, 50.1, 106.6, 113.6, 118.4, 122.0, 123.0, 124.3, 129.9, 130.1, 131.4, 133.7, 134.9, 135.9.
Anal. Calcd for C H BF N (312.12): C, 57.72; H, 5.49; N, 8.98. Found: C, 57.41; H, 5.62; N, 9.07.
15 17
4
2
N,N,1,3-Tetramethyl-2-p-tolyl-1H-benzo[g]indol-9-amine Hydrogen Tetrafluoroborate (4·HBF ):
4
1
8
0% (12 mg from 12 mg of 4); light-beige prisms with mp 203–205 °C (darken above 175 °C). H
NMR (CD CN, 600 MHz, 30 °C)  2.41 (s, 3H), 2.45 (s, 3H), 3.43 (s, 3H), 3.49 (d, J = 4.61 Hz, 6H),
3
7
.42 (d, J = 7.87 Hz, 2H), 7.63–7.67 (m, 3H), 7.77 (d, J = 8.41 Hz, 1H), 7.87 (d, J = 8.41 Hz, 1H), 7.99
1
3
1
(
br. d, J = 7.72 Hz, 1H), 8.19 (br. d, J = 7.72 Hz, 1H), 8.42 (br. s, 1H, NH); C{ H} NMR (CD CN,
3
1
50 MHz, 30 °C)  9.2, 20.6, 39.1, 50.0, 113.9, 114.3, 118.8, 121.0, 122.8, 124.1, 128.5, 129.6, 130.7,
131.5, 131.7, 133.4, 133.7, 136.0, 138.8, 146.0. Anal. Calcd for C H BF N (416.27): C, 66.36; H,
2
3
25
4
2
6
.05; N, 6.73. Found: C, 66.52; H, 6.12; N, 6.49.
N,N,1,3-Tetramethyl-2-p-nitrophenyl-1H-benzo[g]indol-9-amine Hydrogen Tetrafluoroborate
1
(
5·HBF ): 74% (14 mg from 15 mg of 5); yellow-brown needles with mp 216–217 °C (decomp.). H
4
NMR (CD CN, 600 MHz, 30 °C)  2.49 (s, 3H), 3.46 (s, 3H), 3.52 (br. s, 6H), 7.38–7.44 (m, 1H),
3
7
8
1
.68–7.70 (m, 1H), 7.80–7.81 (m, 1H), 7.90–7.91 (m, 1H), 8.02–8.03 (m, 2H), 8.21–8.22 (m, 1H),
1
3
1
.40–8.41 (br. m, 2H + NH); C{ H} NMR (CD CN, 150 MHz, 30 °C)  9.4, 39.3, 50.2, 113.8, 119.1,
3
21.2, 123.3, 123.8, 124.0, 124.9, 131.5, 131.8, 132.3, 134.3, 134.7, 136.3, 137.8, 143.6, 147.6. Anal.
Calcd for C H BF N O (447.24): C, 59.08; H, 4.96; N, 9.40. Found: C, 58.91; H, 5.07; N, 9.43.
22
22
4
3
2
N,N,1-Trimethyl-3-trifluoromethyl-1H-benzo[g]indol-9-amine
Hydrogen
Tetrafluoroborate
1
(
6·HBF ): 96% (25 mg from 20 mg of 6); clear hygroscopic caramel soluble in EtOAc and Et O. H
4
2
+
NMR (CD CN, 600 MHz, 30 °C):  3.42 (s, 6H), 4.00 (s, 3H), 4.65 (br. s, H O + H ), 7.74 (t, J = 7.95
3
2
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Hz, 1H), 7.79 (d, J = 8.64 Hz, 1H), 7.88 (q, J = 1.20 Hz, 1H), 7.90 (dd, J = 8.64, 0.88 Hz, 1H), 8.05
1
3
1
(br. d, J = 7.96 Hz, 1H), 8.21 (br. d, J = 7.96 Hz, 1H); C{ H} NMR (CD CN, 150 MHz, 30 °C) 
3
3
8.4, 50.4, 109.3 (q, J = 36.5 Hz), 113.5, 119.7, 120.3, 123.8, 124.5 (q, J = 218.3 Hz), 125.6, 130.0,
31.6, 133.4 (q, J = 5.0 Hz), 134.0, 139.2, 142.9. Anal. Calcd for C H BF N (380.11): C, 50.56; H,
1
1
6
16
7
2
4.24; N, 7.37. Found: C, 50.42; H, 4.19; N, 7.48.
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N,N-Dimethyl-8-(1H-pyrrol-1-yl)naphthalene-1-amine (7).
A
mixture of 8-(1Н-pyrrol-1-
1
8
yl)naphthalene-1-amine (16) (0.208 g, 1.0 mmol), Me SO (0.38 mL, 4.0 mmol), Na CO ·10H O
2
4
2
3
2
(1.15 g, 4.0 mmol) and МеОН (5 mL) was stirred for 96 h at r.t. The reaction mixture was poured in
H O (5 mL) and extracted with CHCl (3 х 3 mL). Evaporation of the solvent gave 0.198 g (84%) of a
2
3
mixture of amine 7 and N-methyl-8-(1H-pyrrol-1-yl)naphthalene-1-amine (17). The ratio of
1
compounds 7 and 17 in the mixture based on the H NMR data was 2:1. The mixture was further
separated by column chromatography on Al O using hexane as eluent. The first fraction with R 0.9
2
3
f
gave dimethylaminopyrrole 7 as colorless crystals with mp 50–52 °C. The second fraction with R 0.8
f
1
allowed to isolate monomethylated amine 17 as colorless solid with mp 54–56 °C. Compound 7: Н
NMR (250 MHz, CDCl ) δ 2.42 (s, 6H), 6.23–6.24 (m, 2H), 6.88–6.90 (m, 2H), 7.05 (dd, J = 7.4, 1.2
3
1
Hz, 1H), 7.38–7.54 (m, 4H), 7.79 (dd, J = 7.9, 1.6 Hz, 1H). H NMR (600 MHz, CD CN, 30 °C) δ 2.35
3
(s, 6H), 6.16 (t, J = 2.11 Hz, 2H), 6.85 (t, J = 2.11 Hz, 2H), 7.07 (d, J = 7.51 Hz, 1H), 7.33 (dd, J =
7
.28, 1.10 Hz, 1H), 7.40 (t, J = 7.80 Hz, 1H), 7.44 (t, J = 7.72 Hz, 1Н), 7.52 (d, J = 8.02 Hz, 1Н), 7.80
1
(
dd, J = 8.15, 0.95 Hz, 1Н). Н NMR (250 MHz, DMSO-d ) δ 2.34 (s, 6Н), 6.13–6.15 (m, 2Н), 6.90–
6
6.92 (m, 2Н), 7.08 (dd, J = 7.5, 0.9 Hz, 1Н), 7.37 (dd, J = 7.3, 1.3 Hz, 1Н), 7.39–7.54 (m, 2Н), 7.58
1
3
1
(
dd, J = 8.1, 0.9 Hz, 1Н), 7.87 (dd, J = 8.1, 1.2 Hz, 1Н). C{ H} NMR (150 MHz, CD CN, 30 °C) δ
3
1
3
1
4
3.2, 107.5, 115.4, 117.4, 122.2, 124.0, 124.8, 125.5, 126.6, 128.1, 137.5, 138.0, 150.0. C{ H} NMR
(
62.9 MHz, DMSO-d ) δ 43.8, 107.8, 115.5, 121.2, 122.3, 124.1, 125.0, 125.9, 126.8, 128.3, 137.3,
6
137.5, 149.7. Anal. Calcd for С H N (236.31): С, 81.32; Н, 6.82; N, 11.85. Found: С, 81.41; Н,
1
6
16
2
1
6
.93; N, 11.94. N-Methyl-8-(1H-pyrrol-1-yl)naphthalene-1-amine (17): Н NMR (250 MHz,
DMSO-d ) δ 2.55 (d, J = 4.9 Hz, 3Н), 3.48 (br. q, J = 4.5 Hz, 1Н), 6.35–6.37 (m, 2Н), 6.49 (d, J = 7.5
6
Hz, 1Н), 6.95–6.96 (m, 2Н), 7.21–7.27 (m, 2Н), 7.35–7.48 (m, 2Н), 7.90 (dd, J = 8.2, 1.2 Hz, 1Н).
1
3
1
C{ H} NMR (62.9 MHz, DMSO-d ) δ 31.4, 105.2, 109.9, 116.4, 119.1, 124.2, 125.2, 125.7, 127.8,
6
–
1
1
30.0, 136.4, 136.5, 145.0. IR (nujol) νmax 3470 cm . Anal. Calcd for С H N (222.28): С, 81.05; Н,
15 14 2
6
.35; N, 12.60. Found: С, 81.14; Н, 6.45; N, 12.70.
N,N-Dimethyl-8-(1H-pyrrol-1-yl)naphthalene-1-amine Hydrogen Tetrafluoroborate (7·HBF4)
was generated on addition of equimolar amount of 48% tetrafluoroboric acid to a solution of compound
1
7
in CD CN in an NMR ampoule. H NMR (600 MHz, CD CN, 30 °C) δ 3.17 (d, J = 5.12 Hz, 6H),
3
3
2
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6.40 (br. s, 1H), 6.66 (t, J = 2.05 Hz, 2Н), 7.19 (t, J = 2.05 Hz, 2Н), 7.73–7.80 (m, 3Н), 7.99 (d, J =
1
3
1
7
.83 Hz, 1Н), 8.24 (dd, J = 8.09, 1.37 Hz, 1Н), 8.27 (d, J = 8.24 Hz, 1Н). C{ H} NMR (150 MHz,
CD CN, 30 °C) δ 50.1, 113.2, 117.6, 121.9, 123.9, 126.9, 127.2, 130.2, 131.5, 132.4, 133.2, 135.9,
3
1
38.1.
Synthesis of N,N-dimethyl-5-(1H-pyrrol-1-yl)naphthalene-1-amine (15) (see ESI, p. S2 for general
1
1
1
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2
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scheme).
1
-(5-Nitronaphthalene-1-yl)-1H-pyrrole (B). A mixture of 5-nitronaphthalene-1-amine (A) (0.376 g,
2
.0 mmol), 2,5-dimethoxytetrahydrofuran (1 mL, 7.6 mmol) and AcOH (10.0 mL) was refluxed for 2 h.
The reaction mass was evaporated followed by column chromatography of the residue (Al O , CHCl ,
2
3
3
R 0.5) to yield 0.376 g (79%) of the title compound as yellow crystals with mp 102–103 °С (MeOH).
f
1
Н NMR (250 MHz, CDCl ) δ 6.39–6.43 (m, 2Н), 6.94–6.98 (m, 2Н), 7.53 (dd, J = 8.5, 7.7 Hz, 1H),
3
7.60 (dd, J = 7.3, 1.0 Hz, 1Н), 7.74 (dd, J = 8.8, 7.0 Hz, 1Н), 7.96 (d, J = 8.6 Hz, 1H), 8.22 (dd, J = 7.6,
1
3
1
1
1
4
.0 Hz, 1H), 8.54 (d, J = 8.8 Hz, 1H). C{ H} NMR (62.9 MHz, CDCl ) δ 109.8, 122.9, 123.5, 124.4,
3
24.9, 125.2, 126.9, 128.8, 129.8, 131.3, 138.7, 147.1. Calcd for С H N O (238.24): С, 70.58; Н,
1
4
10
2
2
.23; N, 11.76. Found: С, 70.65; Н, 4.34; N, 11.67.
5-(1H-Pyrrole-1-yl)naphthalene-1-amine (C). A mixture of pyrrole B (0.357 g, 1.5 mmol), Fe
powder (0.504 g, 9.0 mmol), FeSO ·7H O (0.081 g, 0.3 mmol) and H O (5.0 mL) was refluxed for 3.5
4
2
2
h. Then EtOH (3.0 mL) and charcoal were added to the mixture, boiled for 30 min, filtered and
extracted with CHCl (3 х 5 mL). The organic fraction was purified by column chromatography on
3
Al O (eluent – CHCl –light petroleum ether, 1:1), collecting a fraction with R 0.6. This yielded 0.24 g
2
3
3
f
1
(
77%) of the title compound as a red-brown oil. Н NMR (250 MHz, CDCl ) δ 4.21 (br. s, 2Н), 6.36–
3
6
.40 (m, 2Н), 6.81 (dd, J = 7.3, 1.0 Hz, 1H), 6.95–6.99 (m, 2Н), 7.15 (d, J = 8.5 Hz, 1H), 7.26–7.30
(m, 1Н), 7.40–7.49 (m, 2Н), 7.83–7.87 (m, 1H). Anal. Calcd for С H N (208.26): С, 80.74; Н, 5.81;
1
4
12
2
N, 13.45. Found: С, 80.67; Н, 5.90; N, 13.36.
N,N-Dimethyl-5-(1H-pyrrol-1-yl)naphthalene-1-amine (15). A mixture of amine C (0.208 g, 1.0
mmol), Me SO (0.38 mL, 4.0 mmol), Na CO ·10H O (1.15 g, 4.0 mmol) and МеОН (5 mL) was
2
4
2
3
2
stirred for 96 h at rt. The reaction mixture was poured in H O (5 mL) and extracted with CHCl (3 х 3
2
3
mL). Column chromatography of the residue (Al O , CHCl , R 0.9) yielded 0.179 g (76%) of the title
2
3
3
f
1
compound as light brown oil. Н NMR (250 MHz, CDCl ) δ 2.92 (s, 6Н), 6.38–6.40 (m, 2Н), 6.97–
3
1
3
1
6.98 (m, 2Н), 7.09–7.13 (m, 1Н), 7.36–7.53 (m, 4Н), 8.29 (dd, J = 8.2, 1.1 Hz, 1H). C{ H} NMR
(
62.9 MHz, CDCl ) δ 45.3, 108.8, 114.6, 118.0, 123.3, 123.4, 124.2, 124.4, 126.8, 129.8, 131.5, 138.6,
3
+
+
+
1
51.0. EI MS m/z (%): 236 (23) [М] , 221 (5) [М – Ме] , 205 (17) [М – 2Ме – Н] , 97 (41), 85 (42),
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57 (100). Anal. Calcd for С H N (236.31): С, 81.32; Н, 6.82; N, 11.85. Found: С, 81.35; Н, 6.83; N,
1
6
16
2
1
1.82.
+
–
N,N-Dimethyl-5-(1H-pyrrol-1-yl)naphthalene-1-ammonium Picrate (15H PicO ). A solution of
dimethylaminopyrrole 15 (28 mg, 0.12 mmol) in EtOH (1 mL) was added to a solution of PicOH (33
о
mg, 0.12 mmol) in EtOH (1 mL). The mixture was kept at –10 С for 1 h, the precipitate thus formed
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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7
8
9
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2
3
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5
6
7
8
9
0
was filtered off and washed with cold Et O (1 mL). Yield 50 mg (90%). Yellow-green crystals with mp
2
1
1
6
07–108 °С (decomp.). Н NMR (250 MHz, CDCl , 21 °C) δ 3.40 (s, 6Н), 6.40–6.41 (m, 2Н), 6.94–
3
1
.96 (m, 2Н), 7.49–7.64 (m, 4Н), 7.77 (d, J = 7.9 Hz, 1Н), 8.37 (d, J = 8.3 Hz, 1Н), 8.99 (s, 2Н). Н
NMR (250 MHz, CDCl , –56 °C) δ 3.54 (s, 6Н), 6.42 (br. s, 2Н), 6.98 (br. s, 2Н), 7.55–7.63 (m, 3Н),
3
7
.75 (d, J = 7.2 Hz, 1H), 7.92 (d, J = 8.3 Hz, 1H), 8.31 (d, J = 7.9 Hz, 1H), 9.04 (s, 2Н), 13.19 (s, 1Н).
1
Н NMR (250 MHz, DMSO-d , 50 °C) δ 3.00 (s, 6Н), 6.28–6.30 (m, 2Н), 6.99–7.01 (m, 2Н), 7.33 (d, J
6
= 8.2 Hz, 1Н), 7.62 (dd, J = 8.5, 7.3 Hz, 1H), 8.23 (d, J = 8.5 Hz, 1Н), 8.53 (s, 2Н). Anal. Calcd for
С H N O (465.41): С, 56.77; Н, 4.11; N, 15.05. Found: С, 56.87; Н, 4.20; N, 15.13.
22 19
5
7
3
9
N,N-Dimethyl-8-phenylnaphthalene-1-amine (18). A mixture of 8-phenylnaphthalene-1-amine (0.1
g, 0.46 mmol), Me SO (1.5 mL, 15.83 mmol), Na CO ·10H O (1.31 g, 4.58 mmol) and H O (0.5 mL)
2
4
2
3
2
2
was stirred for 24 h at rt. Next, the reaction mass was poured in H O (5 mL), extracted with CH Cl (3
2
2
2
х 3 mL), evaporated to dryness and subjected to column chromatography (Al O , CH Cl , R 0.9) to
2
3
2
2
f
1
obtain 0.058 g (51%) of the title compound as yellow-brown oil. Н NMR (250 MHz, CDCl ) δ 2.24 (s,
3
6
1
6
8
H), 7.06 (d, J = 7.4 Hz, 1H), 7.27–7.49 (m, 8Н), 7.57 (d, J = 8.0 Hz, 1H), 7.80 (dd, J = 8.0, 1.1 Hz,
1
Н). Н NMR (250 MHz, CD CN) δ 2.22 (d, J = 0.8 Hz, 6H), 7.13 (d, J = 7.4 Hz, 1H), 7.29–7.38 (m,
3
Н), 7.45 (dd, J = 7.3, 4.4 Hz, 1Н), 7.54 (dd, J = 9.3, 2.2 Hz, 1H), 7.61 (d, J = 8.1 Hz, 1Н) 7.86 (d, J =
1
.1 Hz, 1Н). Н NMR (250 MHz, DMSO-d ) δ 2.17 (s, 6H), 7.09 (dd, J = 7.4, 1.0 Hz, 1H), 7.27–7.32
6
1
3
1
(m, 6Н), 7.42–7.52 (m, 2Н), 7.61 (dd, J = 8.1, 0.9 Hz, 1H), 7.86 (dd, J = 8.1, 1.2 Hz, 1Н). C{ H}
NMR (62.9 MHz, CDCl ) δ 43.5, 115.6, 123.1, 125.0, 125.6, 125.7, 126.0, 128.2, 129.2, 129.7, 136.5,
3
+
+
+
1
39.7, 144.2, 150.9. EI MS m/z (%): 247 (99) [M] , 232 (10) [M – CH ] , 217 (17) [M – 2CH ] . Anal.
3 3
Calcd for С H N (247.33): С, 87.41; Н, 6.93; N, 5.66. Found: С, 87.49; Н, 6.84; N, 5.73.
18
17
N,N-Dimethyl-8-phenylnaphthalene-1-amine Hydrogen Tetrafluoroborate (18·HBF4) was
obtained by a procedure analogous to the synthesis of protic complexes 3·HBF –5·HBF . Yield 40%
4
4
1
(
15 mg from 28 mg of 18); colorless crystals with mp 131–132 °C. Н NMR (600 MHz, CD CN, 30
3
°
C) δ 3.02 (d, J = 5.28 Hz, 6H), 7.52 (dd, J = 7.07, 1.09 Hz, 1Н), 7.66–7.68 (m, 2H), 7.72–7.76 (m,
5
1
4
H), 7.89 (d, J = 7.57 Hz, 1Н), 8.09 (br. s, 1H), 8.15 (dd, J = 8.28, 0.85 Hz, 1H), 8.23 (d, J = 8.12 Hz,
H). Anal. Calcd for С H BF N (335.15): С, 64.51; Н, 5.41; N, 4.18. Found: С, 64.60; Н, 5.48; N,
1
8
18
4
.27.
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Supporting Information
Electronic supplementary information (ESI) available: X-ray crystallographic data, spectral data,
additional comments and computational details (PDF). Atomic coordinate file of all calculated
structures (XYZ). CCDC 1837903–1837905 and 1867330–1867332. For ESI and crystallographic data
in CIF or other electronic format see DOI: ...
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Acknowledgements
This work was supported by the Russian Foundation for Basic Research (Project No. 17-03-00035). X-
ray studies were performed by Dr. Kyrill Y. Suponitsky (A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences, Moscow). We acknowledge the Wrocław
Centre for Networking and Supercomputing (WCSS) for generous grants of computer time.
References and Notes
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stabilizing local 3D structures. J. Mol. Biol. 2001, 305, 535–557. (b) Fernandez-Recio, J.; Romero,
A.; Sancho, J. Energetics of a hydrogen bond (charged and neutral) and of a cation-π interaction in
apoflavodoxin. J. Mol. Biol. 1999, 290, 319–330.
(
3) Ma, J. C.; Dougherty, D. A. The Cation−π Interaction. Chem. Rev. 1997, 97, 1303–1324.
4) Mahadevi, A. S.; Sastry, G. N. Cation−π Interaction: Its Role and Relevance in Chemistry, Biology,
and Material Science. Chem. Rev. 2013, 113, 2100–2138.
(
(5) (a) Zhang, R. B.; Somers, K. R. F.; Kryachko, E. S.; Nguen, M. T.; Zeegers-Huyskens, T.;
Ceulemans, A. Hydrogen Bonding to π-Systems of Indole and 1-Methylindole:ꢀ Is There Any
OH···Phenyl Bond? J. Phys. Chem. A 2005, 109, 8028–8034. (b) Muñoz, M. A.; Galán, M.;
Gómez, I.; Carmona, C.; Guardado, P.; Balón, M. The pyrrole ring as hydrogen-bonding π-donor
base: an experimental and theoretical study of the interactions of N-methylpyrrole with alcohols.
Chem. Phys. 2003, 290, 69–77.
(
6) (a) Goddard, R.; Heinemann, O.; Krüger, C. Pyrrole and a Co-crystal of 1H- and 2H-1,2,3-Triazole.
Acta Cryst. 1997, C53, 1846–1850. (b) Stefov, V.; Pejov, L.; Šoptrajanov, B. Experimental and
quantum chemical study of pyrrole self-association through N–H···π hydrogen bonding. J. Mol.
Struct. 2003, 649, 231–243.
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(7) Evans, D. A.; Borg, G.; Scheidt, K. A. Remarkably Stable Tetrahedral Intermediates: Carbinols
from Nucleophilic Additions to N–Acylpyrroles. Angew. Chem. Int. Ed. 2002, 41, 3188–3191.
(8) Katritzky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic Chemistry; 2nd ed.; Pergamon:
Amsterdam, 2000, p. 58.
1
0
(9) There is a single communication on protonation of pyrrole nitrogen atom in solution. Thus, on
0
1
2
3
4
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7
8
9
0
1
2
3
4
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8
9
0
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
o
treatment of carbazole and N-methylcarbazole with fluorosulphuric acid at 30 C cations A and B
1
were registered by H NMR spectroscopy. Their pK values were estimated as equal to –6.0 and –
a
8
.0, respectively. Indole and N-methylindole are protonated exclusively at the C-3 atom with pK –
a
1
1
3.5 and –2.32, respectively, while the protonation of pyrrole in solution occurs kinetically at the
-position with fast rearrangement of the primarily formed C adduct into more stable C . For



1
2
pyrrole and 1-methylpyrrole, pK ’s of –3.80 and –2.90 were reported. For the protonation of
a
10
pyrrole and indole at the nitrogen atom, extrapolation value of pK about –10 was suggested.
a
3
3
+
Accordingly to Speranza, pyrrole in the gas phase is tritiated with the He H electrophilic reagent
1
3
at the N, C and C positions with the ratio 18, 37 and 45%, respectively. However, theoretical


calculations of the gas-phase protonation energies showed that the -protonation of pyrrole (PA =
–1
–1 14
2
08.9 kcal mol ) is more preferable than  (205.2 kcal mol ).
+
N
R
H
A (R = H)
B (R = Me)
(10) Chen, H. J.; Hakka, L. E.; Hinman, R. L.; Kresge, A. J.; Whipple, E. B. Basic strength of
carbazole. Estimate of the nitrogen basicity of pyrrole and indole. J. Am. Chem. Soc. 1971, 93,
5
102–5107.
(11) Indoles; ed. Houlihan, W. J.; Wiley-Intersc.: New York, 1972, part 1, 587 pp.
(12) Jones, R. A.; Bean, J. P. The Chemistry of Pyrroles; Academic Press: New York, 1977, 526 pp.
(
13) (a) Speranza, M. First example of predominant β-orientation in electrophilic substitution of
pyrrole. Chem. Commun. 1981, 1177–1178. (b) Angelini, G.; Lagazzi, G.; Sparapani, C.; Speranza,
M. Gas-phase heteroaromatic substitutions. 5. Gas-phase protonation of five-membered
3
3
+
heteroaromatic rings by hydro-t-helium(1+)- He ( HeT ) ions. A model reaction for a theoretical
approach to heteroaromatic reactivity. J. Am. Chem. Soc. 1984, 106, 37–41. (c) Speranza, M. Gas-
phase reactivity of five-membered heteroaromatics toward electrophiles, an experimental check on
theoretical predictions. Pure & Appl. Chem. 1991, 63, 243–254.
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14) Catalan, J.; Yanez, M. The  vs.  protonation of pyrrole and indole. J. Am. Chem. Soc. 1984, 106,
421–422.
(
15) (a) Kirby, A. J.; O’Carroll, F. Highly efficient intramolecular general acid catalysis of enol ether
hydrolysis, with rapid proton transfer to carbon. J. Chem. Soc, Perkin Trans. 2 1994, 649–655. (b)
Kirby, A. J.; Dutta-Roy, N.; da Silva, D.; Goodman, J. M.; Lima, M. F.; Roussev, C. D.; Nome, F.
Intramolecular General Acid Catalysis of Phosphate Transfer. Nucleophilic Attack by Oxyanions
on the PO32– Group. J. Am. Chem. Soc. 2005, 127, 7033–7040. (c) Perrin, C. L.; Ohta, B. K.
Symmetry of NHN hydrogen bonds in solution. J. Mol. Struct. 2003, 644, 1–12.
1
1
1
1
1
1
1
1
1
1
2
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2
2
2
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0
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8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
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5
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7
8
9
0
1
2
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4
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(16) Cox, C.; Wack, H.; Lectka, T. Strong Hydrogen Bonding to the Amide Nitrogen Atom in an
Amide Proton Sponge”: Consequences for Structure and Reactivity. Angew. Chem. Int. Ed. 1999,
8, 798–800.
“
3
(
17) (a) Filatova, E. A.; Pozharskii, A. F.; Gulevskaya, A. V.; Ozeryanskii, V. A. Multiple
transformations of 2-alkynyl-1,8-bis(dimethylamino)naphthalenes into benzo[g]indoles. Pd/Cu-
Dependent switching of the electrophilic and nucleophilic sites in acetylenic bond and a puzzle of
porcelain catalysis. J. Org. Chem. 2015, 80, 872–881. (b) Filatova, E. A.; Gulevskaya, A. V.;
Pozharskii, A. F. Synthesis of 2-aryl-3-methylbenzo[g]indoles from 2-(arylethynyl)-1,8-
bis(dimethylamino)naphthalenes: new examples of [1,3]-migration involving N-methyl group.
Chem. Heterocycl. Compd. 2018, 54, 38–42.
(
(
(
18) Cheeseman, G. W. H.; Eccleshall, S. A. Synthesis of Pyrrolo[1,2-a]naphtho[1,8-
ef][1,4]diazepines. J. Heterocyclic Chem. 1986, 23, 65–67.
19) Alabugin, I. V.; Bresch, S.; Gomes, G. P. Orbital hybridization: a key electronic factor in control
of structure and reactivity. J. Phys. Org. Chem. 2015, 28, 147–162.
20) Alabugin, I. V.; Manoharan, M.; Buck, M.; Clark, R. J. Substituted anilines: The tug-of-war
between pyramidalization and resonance inside and outside of crystal cavities. J. Mol. Struct.
THEOCHEM 2007, 813, 21–27.
(
(
(
21) Howard, S. T. Relationship between Basicity, Strain, and Intramolecular Hydrogen-Bond Energy
in Proton Sponges. J. Am. Chem. Soc. 2000, 122, 8238–8244.
22) Pozharskii, A. F.; Ozeryanskii, V. A. in The Chemistry of Anilines, ed. Rappoport, Z.; J. Wiley &
Sons: Chichester, 2007, Part 2, Ch. 17, pp. 931–1026.
23) Ozeryanskii, V. A.; Shevchuk, D. A.; Pozharskii, A. F.; Kazheva, O. N.; Chekhlov, A. N.;
Dyachenko, O. A. Protonation of naphthalene proton sponges containing higher N-alkyl groups.
Structural consequences on proton accepting properties and intramolecular hydrogen bonding. J.
Mol. Struct. 2008, 892, 63–67.
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(24) Although we do not yet have the X-ray data for 3, one can assume that configuration of the pyrrole
N atom in it should be strongly flatten. A weighty, albeit indirect proof of this suggestion is the
earlier reported X-ray data for related compound 9 (Scheme 2) in which the pyrrole ring is nearly
o
25
planar [N(1) = 358.7 , Me = 0.266 Å].
(25) Kachalkina, S. G.; Borodkin, G. S.; Pozharskii, A. F.; Antonov, A. S.; Borodkina, I. G.; Maltsev,
Y. F.; Filatova, E. A.; Filarowski, A.; Ozeryanskii, V. A. Base-promoted transformation of 2-
C(O)R-1,8-bis(dimethylamino)naphthalenes into benzo[g]indole derivatives. Mendeleev Commun.
0
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9
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3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
015, 25, 182–184.
(26) Pozharskii, A. F.; Ozeryanskii, V. A.; Mikshiev, V. Y.; Antonov, A. S.; Chernyshev, A. V.;
Metelitsa, A. V.; Borodkin, G. S.; Fedik, N. S.; Dyablo, O. V. 10-Dimethylamino derivatives of
benzo[h]quinoline and benzo[h]quinazolines: fluorescent proton sponge analogues with opposed
peri-NMe /–N= groups. How to distinguish between proton sponges and pseudo-proton sponges. J.
2
Org. Chem. 2016, 81, 5574–5587.
+
–
+
–
(
27) Notably, the hydrogen bond bifurcation in salt 3H BF significantly exceeds that in 4H BF and
4
4
+
–
–
5
H BF . Probably in the last two salts an approach of BF to the NH proton is sterically hampered
4 4
+
–
by the 2-aryl groups. This may be an additional factor weakening the IHB in 3H BF .
4
(
28) Staab, H. A.; Diehm, M.; Krieger, C. Synthesis, structure and ‘proton sponge’ properties of 1,12-
bis(dimethylamino)benzo[c]phenanthrene. Tetrahedron Lett. 1995, 36, 2967–2970.
29) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. Extension of
the Self-Consistent Spectrophotometric Basicity Scale in Acetonitrile to a Full Span of 28 pKa
Units:ꢀ Unification of Different Basicity Scales. J. Org. Chem. 2005, 70, 1019–1028.
30) Kurasov, L. A.; Pozharskii, A. F.; Kuz’menko, V. V.; Klyuev, N. A.; Chernyshev, A. I.; Goryaev,
S. S.; Chikina, N. L. Nitration of 1,8-bis(dialkylamino)naphthalenes. Zh. Org. Khim. 1983, 19,
590–597.
(
(
(
+
–
31) Problems arising during growing crystals of salt 7H BF are caused by its tendency to transform
4
into another compound; unusual chemistry of this conversion will be reported soon.
(32) Burov, O. N.; Kletskii, M. E.; Fedik, N. S.; Kurbatov, S. V.; Lisovin, A. V. Experimental and
quantum-chemical study of nucleophilic substitution mechanism in berberine. Chem. Heterocycl.
Compd. 2015, 51, 997–1007.
–1
(
33) It is noteworthy, that similar scale of theoretically EHB values (10–22 kcal mol ) was reported for
+
+
+
+
the NH … complexes of benzene and ethylene with NH , Me NH and MeNH ions: (a) Lee, J.
4
2
2
3
Y.; Lee, S. J.; Choi, H. S.; Cho, S. J.; Kim, K. S.; Ha, T.-K. Ab initio study of the complexation of
benzene with ammonium cations. Chem. Phys. Lett. 1995, 232, 67–71. (b) Deakyne, C. A.; Meot-
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+
Ner (Mautner) M. Unconventional ionic hydrogen bonds. 2. NH ···π Complexes of onium ions
with olefins and benzene derivatives. J. Am. Chem. Soc. 1985, 107, 474–479.
(
34) (a) Reed, A. E.; Weinhold, F.; Curtiss, L. A.; Pochatko, D. J. Natural bond orbital analysis of
molecular interactions: Theoretical studies of binary complexes of HF, H O, NH , N , O , F , CO,
2
3
2
2
2
and CO with HF, H O, and NH . J. Chem. Phys. 1986, 84, 5687–5705. (b) Stone, A. J. Natural
2
2
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
3
3
3
4
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4
4
4
4
5
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5
5
5
5
5
5
5
5
6
0
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4
5
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7
8
9
0
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2
3
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5
6
7
8
9
0
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7
8
9
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9
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2
3
4
5
6
7
8
9
0
Bond Orbitals and the Nature of the Hydrogen Bond. J. Phys. Chem. A. 2017, 121, 1531–1534.
35) (a) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.;
Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.;
Nesbitt, D. J. Definition of the hydrogen bond (IUPAC Recommendations, 2011), Pure App. Chem.
(
2
011, 83, 1637–1641. (b) Parthasarathi, R.; Subramanian, V. in Hydrogen Bonding – New Insights;
ed. Grabowski, S.; Springer, 2006, pp. 1–50.
(36) (a) Perrin, C. L.; Ohta, B. K. Symmetry of N−H−N Hydrogen Bonds in 1,8-
+
+
Bis(dimethylamino)naphthalene·H and 2,7-Dimethoxy-1,8-bis(dimethylamino)naphthalene·H . J.
Am. Chem. Soc. 2001, 123, 6520–6526. (b) Anslyn, E. V.; Dougherty, D. A. Modern Physical
Organic Chemistry; University Science Books: Sausalito, CA, 2006, p 179. (c) Llamas-Saiz, A. L.;
Foces-Foces, C.; Elguero, J. Proton sponges. J. Mol. Struct. 1994, 328, 297–323. (d) Pozharskii, A.
F.; Ozeryanskii, V. A. Proton sponges and hydrogen transfer phenomena. Mendeleev Commun.
2
012, 22, 117–124.
(
37) Degtyarev, A. V.; Ryabtsova, O. V.; Pozharskii, A. F.; Ozeryanskii, V. A.; Starikova, Z. A.;
Sobczyk, L.; Filarowski, A. 2,7-Disubstituted proton sponges as borderline systems for
investigating barrier-free intramolecular hydrogen bonds. Protonated 2,7-bis(trimethylsilyl)- and
2
,7-di(hydroxymethyl)-1,8-bis(dimethylamino)naphthalenes. Tetrahedron 2008, 64, 6209–6214.
(38) Kütt, A.; Leito, I.; Kaljurand, I.; Sooväli, L.; Vlasov, V. M.; Yagupolskii, L. M.; Koppel, I. A. A
Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Brønsted Acids in
Acetonitrile. J. Org. Chem. 2006, 71, 2829–2838.
(
39) Jurok, R.; Cibulka, R.; Dvořáková, H.; Hampl, F.; Hodačová, J. Planar Chiral Flavinium Salts –
Prospective Catalysts for Enantioselective Sulfoxidation Reactions. Eur. J. Org. Chem. 2010,
5
217–5224.
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