1,8-Bis(tetramethylguanidino)naphthalene (TMGN): A new, superbasic and kinetically active "proton sponge"

Article abstract of DOI:10.1002/1521-3765(20020402)8:7<1682::AID-CHEM1682>3.0.CO;2-R

1,8-Bis(tetramethylguanidino)naphthalene (TMGN, 1) is a new, readily accessible, and stable "proton sponge" with an experimental pK_BH+ value of 25.1 in MeCN, which is nearly seven orders of magnitude higher in basicity than the classical proton sponge 1,8-bis(dimethylamino)-naphthalene (DMAN). Because of the sterically less crowded character of the proton-accepting sp²-nitrogen atoms, TMGN also has a higher kinetic basicity than DMAN, which is shown by time-resolved proton self-exchange reactions. TMGN is more resistant to hydrolysis and is a weaker nucleophile towards the alkylating agent EtI in comparison to the commercially available guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). Crystal structures of the free base, of the mono- and bisprotonated base were determined. The dynamic behavior of all three species in solution was investigated by variable-temperature ¹H NMR experiments. ΔG≠ values obtained by spectra simulation reveal a concerted mechanism of rotation about the C-N bonds of the protonated forms of TMGN.

Full text of DOI:10.1002/1521-3765(20020402)8:7<1682::AID-CHEM1682>3.0.CO;2-R

FULL PAPER
1
,8-Bis(tetramethylguanidino)naphthalene (TMGN): A New, Superbasic and
Kinetically Active ™Proton Sponge∫
Volker Raab, Jennifer Kipke, Ruth M. Gschwind, and Jˆrg Sundermeyer*[a]
Dedicated to Professor Rolf Gleiter on the occasion of his 65th birthday
Abstract: 1,8-Bis(tetramethylguanidi-
no)naphthalene (TMGN, 1) is a new,
readily accessible, and stable ™proton
which is shown by time-resolved proton
self-exchange reactions. TMGN is more
resistant to hydrolysis and is a weaker
nucleophile towards the alkylating agent
EtI in comparison to the commercially
available guanidine 7-methyl-1,5,7-tri-
Crystal structures of the free base, of
the mono- and bisprotonated base were
determined. The dynamic behavior of all
three species in solution was investigat-
sponge∫with an experimental p KBH
‡
1
value of 25.1 in MeCN, which is nearly
seven orders of magnitude higher in
basicity than the classical proton sponge
ed by variable-temperature H NMR
=
experiments. DG values obtained by
azabicyclo[4.4.0]dec-5-ene
(MTBD).
spectra simulation reveal a concerted
1
,8-bis(dimethylamino)-naphthalene
mechanism of rotation about the CÀN
(
DMAN). Because of the sterically less
bonds of the protonated forms of
TMGN.
Keywords: basicity ¥ NMR spectros-
copy ¥ peralkylated guanidines ¥
protonation ¥ proton sponges
crowded character of the proton-accept-
ing sp -nitrogen atoms, TMGN also has
a higher kinetic basicity than DMAN,
2
Introduction
i) destabilization of the base as a consequence of strong
repulsion of unshared electron pairs, ii) formation of an IHB
in the protonated form and iii) relief from steric strain upon
protonation. Two general concepts to raise the thermody-
namic basicity or proton affinity have been followed. One is to
replace the naphthalene skeleton by other aromatic spacers,
For over three decades, neutral organic bases with chelating
proton-acceptor functionalities have attracted particular in-
terest. On account of their high proton affinity, they are
named ™proton sponges∫according to the classical example
[8]
[9]
[10]
1
,8-bis(dimethylamino)naphthalene (DMAN) that was intro-
such as fluorene, heterofluorene, phenanthrene,
and
[
1, 2]
[11]
duced by Alder et al.
The field has been reviewed by Staab
biphenyl,
thus influencing the basicity by varying the
and Saupe,[ and more recently by Llamas-Saiz et al., as well
as in a limited manner on 1,8-diaminonaphthalene derivatives
by Pozharskii.[ Proton sponges are still the focus of current
research activity[ and are also the subject of vivid interest of
theoretical chemists.[ A general feature of all proton sponges
is the presence of two basic nitrogen centers in the molecule,
which have an orientation that allows the uptake of one
3]
[4]
nonbonding N ¥¥¥ N distance of the proton-acceptor pairs.
The other concept focuses on the variation of the basic
5]
[6d,e, 12, 13, 14]
nitrogen centers
tressing effect∫).
or its adjacent environment (™but-
6]
[2, 3, 5, 15, 16]
7]
‡
proton to yield a stabilized [N ¥¥¥ H ¥¥¥ N] intramolecular
hydrogen bond (IHB). Compared to ordinary alkyl and aryl
amines, amidines, and guanidines, such proton chelators
present a dramatic increase in basicity on account of
[
a] Prof. Dr. J. Sundermeyer, Dr. V. Raab, Dr. J. Kipke,
Dr. R. M. Gschwind
Fachbereich Chemie, Philipps-Universit‰t Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
Fax : (‡49)6421-28-28917
E-mail: jsu@chemie.uni-marburg.de
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/chemistry/ or from the author.
Scheme 1. Representative ™proton sponges∫in relation to TMGN ( 1).
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0947-6539/02/0807-1682 $ 20.00+.50/0 Chem. Eur. J. 2002, 8, No. 7
1682

1
682±1693
There is a trend that proton sponges with high thermody-
namic basicity typically have a low kinetic basicity: The
captured proton does not usually take part in rapid proton
exchange reactions, which would allow such neutral super-
bases to serve as catalysts in salt-free base-catalyzed reactions.
A successful strategy to overcome the kinetic inertness has
been presented by Schwesinger et al., who developed a
thermodynamically strong and at the same time kinetically
active superbase that incorporates the vinamidine structure
see above).[
17]
Scheme 2. Synthesis of TMGN (1).
(
However, its multistep synthesis, moderate
stability, and moderate solubility in aprotic solvents are some
limitations of this proton sponge, which takes up two protons
in the presence of excess acid.
be produced on a large scale from ureas and phosgene or
oxalyl chloride.
After an examination of the reported strategies to increase
the basicity of 1,8-diaminonaphthalene proton sponges and
being aware that peralkyl guanidines belong to the strongest
organic neutral bases known,[ we decided to use the
tetramethylguanidino group as an efficient and straightfor-
ward modification of the chelating proton acceptor. Peralkyl-
guanidines are several orders of magnitude higher in basicity
than tertiary amines because of the excellent stabilization of
the positive charge in their resonance-stabilized cations.[
Reactivity studies: TMGN (1) is cleanly monoprotonated by
an equimolar amount of NH PF in MeCN to yield
4
6
18]
[1‡H][PF ] (2a). Colorless single crystals are obtained by
6
slow diffusion of dry diethyl ether into the MeCN solu-
tion.
‡
Surprisingly, monoprotonated [1‡H] does not show the
kinetic inertness with respect to bisprotonation, that is typical
for many proton sponges. Similar to the kinetically active
vinamidine proton sponge of Schwesinger,[ it readily takes
up a second proton when treated with an excess of strong
acids, such as trifluormethanesulfonic acid, tetrafluoroboric
acid etherate, trifluoroacetic acid, aqueous hexafluorophos-
phoric acid, or gaseous HCl. However, complete bisprotona-
tion could not be achieved by excess NH PF in MeCN.
19]
17]
This trend may be demonstrated by the pKBH (MeCN) values
‡
of the 1,2,2,6,6-pentamethylpiperidinium cation (18.62), the
parent guanidinium cation (23.3), and the pentamethylguani-
dinium cation (25.00).[ It is interesting to note that pKBH
19c]
‡
values of a wide range of nonchelating N-aryl guanidines have
been determined[ and that one promising candidate for
proton chelation, 2,2'-bis(tetramethylguanidyl)-1,1'-biphenyl,
did not exhibit proton sponge properties, such as formation of
an IHB.[ Dissociation constants and kinetics of proton-
transfer reactions of nonchelating guanidine bases in acetoni-
20]
4
6
Colorless single crystals of [1‡2H][Cl, Cl H] (3a) were
2
obtained after excess hydrogen chloride was passed through a
21]
CH Cl solution of 1 and crystallization from MeCN. Bisproto-
2
2
nation is a rather unusual feature observed in only a small
trile[ as well as DMAN systems
22]
[15a,b, 23]
have been the subject
number of proton sponges, including those of high kinetic
basicity or those without a strong IHB.[
16a, 17, 26, 27]
In order to
of previous detailed studies.
In the present paper, we report on the synthesis as well as
the spectroscopic and structural properties of 1,8-bis(tetra-
methylguanidino)naphthalene (TMGN (1), see above) and its
mono- and bisprotonated forms. Surprisingly, this new proton
sponge, which has a basicity that is higher by the factor of
distinguish between both cases, we decided to determine the
crystal structures of the base and its protonated forms.
TMGN (1) is perfectly stable towards hydrolysis under
‡
acidic conditions (1m D O , 258C, 6 d). The hydrolysis of 1 in
3
1
basic media was monitored by H NMR (0.83m NaOD in
7
more than 10 than Alder×s classical DMAN, was successfully
[D ]DMSO/D O) and compared to the commercially avail-
6
2
prepared by a one-step synthesis in high yield. It is relatively
stable towards autoxidation, soluble in aprotic nonpolar
solvents and, in contrast to DMAN, it is kinetically active in
proton-exchange reactions.
able guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene
(MTBD). While 1 is stable at room temperature for 24 h,
hydrolysis of MTBD is observed. At elevated temperatures
(608C, 5 d), 44% of 1 and 73% of MTBD is hydrolyzed to the
corresponding urea.
In order to evaluate the nucleophilicity of 1, an alkylation
Results and Discussion
reaction of 1 and MTBD with C
2 5
H I was performed under
identical conditions (2.5 equiv C H I per guanidine function,
2
5
Synthesis: TMGN (1) is synthesized by a method previously
described by us for the synthesis of multidentate metal-
258C, CD Cl ). While no conversion of 1 was observed after
2 2
1
15 min by means of H NMR, ꢀ50% of MTBD was converted
into the ethyl guanidinium salt. At longer reaction times (3 d,
258C) even less nucleophilic 1 was converted into a mixture of
protonated and alkylated products, which however, could not
be quantified because of signal overlap.
[
24]
chelating oligoguanidines.
Tetramethylchlorformamidini-
um chloride[ is treated with 1,8-diaminonaphthalene in the
presence of triethylamine as an auxiliary base and in MeCN as
the solvent. After deprotonation of the guanidinium cation
with 50% aqueous KOH and extraction into hexane, 1 is
obtained in analytically pure form (Scheme 2). In comparison
to other proton sponges of similar basicity, this represents a
rather simple synthesis from convenient precursors, as the
Vilsmeyer salt [ClC(NMe ) ]Cl and related electrophiles may
25]
Structure of the base 1: Single crystals of TMGN suitable for
X-ray crystallography were obtained by crystallization from
hexane. The result of the structural analysis is shown in
Figure 1.
2
2
Chem. Eur. J. 2002, 8, No. 7
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1683

J. Sundermeyer et al.
FULL PAPER
brackets). The indicators are
the angle C(11)-C(16)-C(17)
1
1
22.68 (DMAN: 125.88; QQ:
25.48) of the naphthalene ring,
the
C(11) ¥¥¥C(17) 251.9 pm (DMAN:
56.2; QQ: 258.3), and the very
nonbonding
distance
2
short nonbonding distance be-
tween the acceptor atoms N(1)¥¥¥
N(4) 271.7 pm (DMAN: 279.2;
QQ: 272.7; 1,8-diaminonaphtha-
[5]
lene: 272 pm ). Additional evi-
dence for the extent of distortion
Figure 1. Molecular structure of TMGN (1). View perpendicular to the naphthalene ring plane (A) and
comes from the average anti-
coplanar torsion angles within
projection along the C
2
axis C(16) ± C(15) of the molecule (B).[
66]
the naphthalene skeleton C(11/
Selected bond lengths and angles are given in Table 1. The
17)-C(16)-C(15)-C(20/14) 173.08 (DMAN: 170.38; QQ:
178.98) and with respect to the twisted nitrogen-donor atoms
N(1/4)-C(11/17)-C(16)-C(15) 161.48 (DMAN: 168.2; QQ:
177.8). Because of the different hybridization of the N-atoms
(sp ) compared to typical proton sponges (sp ), the anti-
conformation of the unshared electron pairs is more easily
realized, as can be seen at the position of the imine nitrogen
atoms in Figure 1B. In addition, the average syn-coplanar
torsion angle C(1/6)-N(1/4)-C(11/17)-C(12/18): 55.58 (1) re-
veals that the degree of conjugation between the p-systems of
the naphthalene ring and the guanidine moiety is marginal.
molecular structure of 1 is close to C symmetric with only
2
small deviations from ideal symmetry as a consequence of the
strong repulsion of the two lone pairs centered at the proton-
2
2
3
acceptor sp -nitrogen atoms N(1) and N(4) in an anti-
orientation.
Both guanidine centers C(1) and C(6) are trigonal planar
(
Saˆ 359.98, Table 1). As found in other peralkyl oligogua-
[24a,b]
nidines,
the four dimethylamino groups in the periphery
deviate from ideal conjugation with the C(1)N and C(6)N
3
3
planes as indicated by torsion angles N-C-N-C of 8 ± 458. A
similar propeller-like distortion as a result of steric repulsions
has been found for the ground state of the hexamethylgua-
nidinium cation.[ The guanidine double bonds C(1/6) ± N(1/
Structure of monoprotonated 1: The result of the structural
analysis is shown in Figure 2; selected bond lengths and angles
are given in Table 1. In [1‡H][PF ] (2a), the C symmetry of
28]
4
) (128.2 Æ 0.1 pm) are shorter by a factor of 0.93 than the
6
2
average bonding lengths of C(1) and C(6) to the peripheral
the corresponding base structure is not preserved. The
naphthalene system is flattened and can be considered
virtually planar with the captured proton located between
the imine nitrogen atoms in the same plane. There are no
NMe nitrogen atoms (138.4 Æ 0.1 pm).
2
Some characteristic structural features reveal differences in
the steric constraint of TMGN and the sterically more
crowded DMAN[ and less crowded quinolino[7,8-h]quino-
line (QQ, see Introduction)[14a] (corresponding values in
29]
À
significant interactions between the PF6 anion and the cation
of 2a.
Table 1. Selected bonding and nonbonding distances[pm], angles, dihedral angles, and bond angle sums[8] in TMGN (1), [1‡H][PF
6
] (2a), and
[
a]
[
1‡2H][Cl, Cl
2
H] (3a).
TMGN (1)
[1‡H][PF
6
] (2a)
[1‡2H][Cl, Cl
2
H] (3a)
C(1)ÀN(1)
C(6)ÀN(4)
128.1(3)
128.3(3)
138.4 Æ 0.1
140.1(3)
139.6(3)
±
135.1(6)
132.6(6)
136.6(3)
136.5(3)
133.1 Æ 0.4
142.6(3)
143.4(3)
90(3)
C-NMe
2
(average)
134.2 Æ 2.0
C(11)ÀN(1)
140.9(6)
C(17)ÀN(4)
141.4(6)
91(6)
175(6)
259.3(5)
255.3(7)
124.6(4)
N(1)ÀH(1A)
N(4)ÀH(1A)/H(4)
N(1) ¥¥¥ N(4)
92(3)
271.7(3)
251.9(3)
122.6(2)
288.1(3)
258.2(4)
127.8(2)
C(11) ¥¥¥ C(17)
C(11)-C(16)-C(17)
C(11/17)-C(16)-C(15)-C(20/14)
N(1/4)-C(11/17)-C(16)-C(15)
C(16)-C(11/17)-N(1/4)-H(1A/4A)
C(1/6)-N(1/4)-C(11/17)-C(12/18)
SaN(1)
173.7(2)/172.3(2)
178.7(4)/178.9(4)
177.4(1)/ ± 171.0(4)
À 9(4)/ ±
À 10.3(8)/49.2(7)
360.0
178.2(2)/177.3(2)
À 172.5(2)/ ± 174.8(2)
62.58(1)/68.66(0)
31.4(3)/ ± 34.1(3)
350.6
À 161.6(2)/ ± 161.2(2)
±/ ±
57.7(3)/53.2(4)
±
SaN(4)
±
±
351.5
SaC(1)
SaC(6)
SaNMe2 (average)
359.9
359.9
353.2 Æ 3.9
360.0
359.9
359.0 Æ 1.1
360.0
360.0
359.7 Æ 0.3
[
a] Crystallographic standard deviations in parentheses, calculated average values are given with the corresponding standard deviation (Æ).
1684
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Proton Sponges
1682±1693
Figure 2. Molecular structure of [1‡H][PF
6
] (2a). View perpendicular to mean ring plane (A) and projection along C(15) ± C(16) (B), anion omitted for
[
66]
clarity.
In monoprotonated TMGN, the bond lengths between C(1/
) and N(1/4) become elongated, in bisprotonated species
Table 2. Comparison of NÀH bond lengths [pm] and angles [8] in various
proton sponges with [1‡H][PF ] (2a) and [1‡2H][Cl, Cl H] (3a).
NÀH
6
6
2
they become equivalent to the peripheral CÀNR bonds
Compound N ¥¥¥ N H ¥¥¥ N N-H ¥¥¥ N
160.8(6) 153.3(5)
2
(
Table 1). This is in agreement with the existence of a true
[a]
[
DMANH]ClMH
264.4(2) 110.6(5)
258.3(2) 108(2)
256.7(5) 122(1)
[
b]
IHB in the monoprotonated form of TMGN (further evidence
is provided by the results of dynamic NMR spectroscopy, see
below).
[DMANH]HS
155(2)
139(1)
178(1)
±
157(2)
158
137.6
±
[
c]
[
[
[
[
[
TDMANH
2
]Br
2
[
d]
VAH](BPh
VAH ](ClO
1‡H][PF
1‡2H][Cl, Cl
4
)
254.1
284.5
92.0(3)
86.0(34)/86.0(49)
[
d]
2
4
)
2
The guanidine nitrogen atoms N(1) ¥¥¥ N(4) of 2a are found
at a closer distance of 259.3 pm compared with 1 (271.7 pm),
which is in the range of the average value of 258 pm in
6
] (2a)
259.3(5) 91(6)
175(6)
±
152(5)
±
2
H] (3a) 288.1(3) 90(3)/92(3)
[
a] 1-Dimethylamino-8-dimethylammonionaphthalene
1,2-dichloroma-
‡
[5, 30]
DMANH structures.
The torsion angles for the evalua-
[31]
leate, determined by means of neutron diffraction. [b] 1-Dimethylami-
no-8-dimethylammonionaphthalene hydrogen squarate.[30c] [c] 5,8-Bis(di-
methylamino)-1,4-bis(dimethylammonio)naphthalene
[
tion of planarity within the naphthalene ring indicate that it is
flattened to a large degree. On protonation, the angle C(11)-
C(16)-C(17) is stretched to 124.68 in 2a, while it remains
dibromide.[
59]
d] Vinamidine.[
17]
‡
[30c]
essentially unchanged in DMANH 125.68.
This trend is in
agreement with an elongation of the C(11) ¥¥¥ C(17) distance
to 255.3 pm (DMANH : 253.9). Furthermore, the average
H(1), H(4), and H(2) were found and isotropically refined. In
the dication 3a, the planarity of the naphthalene ring
observed in 2a is essentially maintained while the guanidi-
nium units are increasingly twisted with respect to each other.
The protons adopt a syn-conformation with hydrogen bond-
ing to a bridging chloride anion so that the inner core of the
‡
anti-coplanar torsion angles C(11/17)-C(16)-C(15)-C(20/14)
‡
1
78.88 (DMANH : 178.7) and the twisting of the donor atoms
N(1)-C(11)-C(16)-C(15) and N(4)-C(17)-C(16)-C(15) are
‡
1
1
77.48 and 171.08, respectively, (DMANH : 179.68 and
77.78) and indicate the trend of planarization. The proton
molecule has almost C symmetry. The chelating hydrogen
s
was found and isotropically refined. For confirmation of the
exact position of the proton or differentiation of the imine
bond lengths are 221(3) pm for N(1) ± H(1) ¥¥¥ Cl(1) and
229(3) pm for N(4) ± H(4) ¥¥¥ Cl(1). A solvate with hydrogen
chloride is formed by a second chloride anion. The resulting
[31]
nitrogen atoms, a neutron diffraction structure analysis and
[
32]
À
ESCA spectroscopy, respectively, would be the method of
choice. The proton is localized unsymmetrically in a nonlinear
hydrogen bridge N(1) ± H(1A) ¥¥¥ N(4), as indicated by the
short N(1) ± H(1A) bond length of 91 pm, a long distance
N(4) ¥¥¥ H(1A) of 175 pm, and an N(1)-H(1A) ¥¥¥ N(4) angle of
anion [Cl-H ¥¥¥ Cl] reveals an unsymmetrical linear hydrogen
bond similar to the structurally characterized example [H
O ¥
3
33]
18-crown-6][Cl
H][ within a variety of characteristic bond
2
lengths and angles that have been found for this anion
(Table 3).
1
528. A similar geometry has been found in other proton
As a result of the second protonation, the angle C(11)-
C(16)-C(17) is further enlarged to 127.88, the nonbonding
distances C(11) ¥¥¥ C(17) 258.2 pm and N(1) ¥¥¥ N(4) 288.1 pm
sponges (Table 2).
The syn-coplanar torsion angles, which indicate the prob-
ability of p-orbital overlap C(1/6)-N(1/4)-C(11/17)-C(12/18),
differ significantly for both guanidine groups of 2a. While the
guanidine group with the smaller NÀH bond length has a
Table 3. Comparison of Cl-H ¥¥¥ Cl bond lengths [pm] and angles [8] in
[1‡2H][Cl, Cl H] (3a) with structurally characterized reference com-
2
À
torsion angle of only 10.38, which permits conjugation, the
other group displays a torsion angle of almost 508, similar to 1.
pounds containing the [Cl
Compound
2
H] anion.
À
Cl H
Cl ¥¥¥ H
Cl-H ¥¥¥ Cl
[
[
[
[
1‡2H][Cl, Cl
2
H] (3a)
145(5)
154.6
147.1
138.4
170(5)
154.6
164.9
178.6
177(2)
180.0
168.2
163.2
4 2
][Cl
H][
60]
AsPh
O ¥ 18-crown-6][Cl
Structure of bisprotonated 1: The X-ray structure of
H
3
2
H][33]
[
1‡2H][Cl, Cl H] (3a) is shown in Figure 3; selected bond
2
[61]
3 2 2
(Me NH) Cl][Cl H]
lengths and angles are given in Table 1, the hydrogen atoms
Chem. Eur. J. 2002, 8, No. 7
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J. Sundermeyer et al.
FULL PAPER
are also elongated. These parameters reflect a dramatic
increase in steric strain as a result of bisprotonation: Both
protons point towards the bridging chloride ion, the guanidine
functions adopt a syn- and not an anti-conformation. As a
result, the guanidine N-atoms lie almost in the same plane as
the naphthalene ring, as indicated by the anti-coplanar torsion
angles N(1/4)-C(11/17)-C(16)-C(15) 172.5 and 174.88. This
sterically congested conformation is stabilized through hydro-
gen bonds to the chloride anion (Figure 3).
(Table 2). On the other hand the NH resonance of the
bisprotonated analogue [1‡2H][PF , BF ] (3b) is observed as
6
4
a sharp singlet at d ˆ 7.84 (CD CN) as a consequence of the
3
lack of N-H ¥¥¥ N hydrogen bonding. In this context, it is noted
that [1‡2H][Cl, Cl H] (3a) gives a relatively sharp singlet at
2
d ˆ 11.18, which is at lower field than [1‡2H][PF , BF ] (3b)
6
4
because of hydrogen bonding to the chloride anion. Further-
more, there is a broad singlet in 3a at d ˆ 4.33, which is
À
[35]
assigned to the proton of the [Cl H] anion. The resonances
2
in the aromatic region have a
typical ABX pattern.
For the determination of the
pKBH
‡
value,[
36, 37]
UV/Vis spec-
troscopy could not be used
because of the similarity of the
spectra for both 1 and 2a.
Therefore, the basicity of 1
1
was determined by H NMR
measurements of transprotona-
tion reactions.[
37±39]
The sterical-
ly hindered guanidine MTBD
proved to be the ideal base with
sufficiently slow proton-ex-
change rates in the low-temper-
Figure 3. Molecular structure of [1‡2H][Cl, Cl
2
H] (3a). View perpendicular to mean ring plane (A) and front
view along C(15) ± C(16) (B).[
66]
1
ature H NMR spectrum in
The planarity of the naphthalene ring is expressed by its
average anti-coplanar torsion angle C(11/17)-C(16)-C(15)-
C(20/14) 177.88. In 3a, the syn-coplanar torsion angles (C(1/
CD CN. Other bases, such as DBU and PMG (Table 4),
disclosed only coalesced signals as a result of fast proton-
3
exchange processes. The experimental pK
‡
value of 25.1 Æ
BH
0.2[ for TMGN (1) was obtained by integration of the
baseline-separated diagnostic resonances of the individual
species 1 and 2a (doublet at d ˆ 6.23 and 6.49, respectively). It
is in excellent agreement with the theoretically calculated
value for the absolute proton affinity (APA) in MeCN:
25.4.[ The comparison with tetramethylphenylguanidine
40]
6)-N(1/4)-C(11/17)-C(12/18)) relevant to evaluate a conjuga-
tion are approximately 338.
The reaction of 1 with aqueous hexafluorophosphoric acid
gave light brown crystals, which were subjected to an X-ray
analysis that identified the product as [1‡2H][PF , BF ] (3b).
6
4
41]
This surprising result arises from an HBF impurity in the
4
HPF introduced in the manufacturing process, as has been
(TMPhG, pKBH
‡
ˆ 20.6, Table 4) states unquestionably that
6
[
34]
À
previously reported. Further evidence for the BF anion is
there is a cooperative effect of both chelating guanidine
functions which overcompensates inductive effects of the
naphthalene system. This value is almost in the range of
monoiminophosphoranes, for example IPNMe (Table 4), and
even considerably higher than most ordinary guani-
dines.[
4
provided by NMR ( F, P, 11B) as well as ESI mass
spectrometry (see the Experimental Section). Even though
the R values of its structure resolution were rather poor
because of twinning and disorder of the anions, the propor-
tions in the naphthalene cation can be considered accurate
according to the temperature factors of cation versus anion.
Without going into a detailed discussion of structural param-
eters, one specific difference between 3a and 3b should be
pointed out: While the two protons are in a syn-conformation
in 3a, in 3b an anti-conformation is realized with essentially
non-coordinating anions. Thus, the energetic difference
between the syn- and anti-conformers cannot be very high.
This is also documented by the small difference in the
populations revealed by dynamic NMR studies of solutions of
bisprotonated 3c (see below).
1
9
31
18a, 19c, 42]
From the pKBH
contrast to classical proton sponges,[
‡
determination it became clear that, in
15a,b, 23, 27]
1 allows rather
Table 4. Relative basicity values.
Base
pKBH‡ (MeCN)
31.94[
27.58[
17]
vinamidine (VA)
19c, 62]
(
Me
MTBD
TMGN (1)
2
N)
3
PˆNMe (IPNMe)
25.43[18b, 19c]
25.1[
a]
1
NMR spectra and pKBH
‡
: The 400 MHz H NMR spectrum of
25.00[
24.32
19c]
(
Me
DBU
Me N)
2
N)
2
CˆNMe (PMG)
[
1‡H][PF ] (2a) in CD CN shows a broad signal at d ˆ 14.28
6
3
[19c]
for the NH proton. This relatively moderate downfield shift of
[19c]
(
2
2
CˆNH (TMG)
23.3
20.6[
18a, 20a]
the NH proton in 2a compared to d ꢀ 18.6 (CD CN) in
(Me N) CˆNPh (TMPhG)
3
2
2
DMAN and other naphthalene-based proton sponges,[5] or
DMAN
18.18[5, 63]
12.8[ (DMSO)
14b]
quinolino[7,8-h]quinoline (QQ)
even d ˆ 19.38 ([D ]DMSO) for QQ, is assigned to the
6
formation of a weaker, unsymmetrical hydrogen bridge
[a] Present work.
1686
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Proton Sponges
1682±1693
fast rates of proton exchange which are required for
applications as auxiliary bases in base-catalyzed reactions.
This will be demonstrated in particular by comparing the
proton self-exchange rates between the free base (1) and the
‡
monoprotonated form (2a) relative to the DMAN/DMANH
system[ as a criterion for its kinetic activity. Equimolar
amounts of 1 and 2a were dissolved in CD CN, and H NMR
43]
1
3
spectra of the mixture were recorded at temperatures ranging
from 344 to 225 K with a coalescence signal at 300 K
(
500 MHz). The free energy of proton self-exchange was
À1
determined to be 59.3 kJmol from an Eyring plot that
resulted from a lineshape analysis of the variable-temperature
1
[44]
H NMR spectra (Figure 4).
However, no coalescence
‡
could be observed for DMAN/DMANH , either in CD CN or
3
[
ly.
D ]DMSO up to temperatures of 336 and 371 K, respective-
6
45]
[
Therefore, it can be estimated that DMAN should
À1
exhibit a free energy of proton exchange of >72.6 kJmol
1
(
[D ]DMSO) which is in the range of the activation enthalpy
Figure 5. Temperature-dependent H NMR spectra at 400 MHz for the N-
6
‡
‡ [46]
2 2
methyl singlet of TMGN (1) in CD Cl (experimental spectra: left,
estimated for the exchange of D in DMAN/DMAND .
simulated by iterative fitting:[ right).
49]
Figure 6. Rotation about the CˆN double bond in TMGN (1).
between the methyl groups, cannot be present. Within the
guanidine base 1, the CÀNR single bonds rotate rapidly with
2
[
51]
respect to the NMR timescale, even at low temperatures.
=
From an Eyring plot, the free energy of activation DG for
was determined to be 49.0 kJmol (256 K), which corre-
sponds to 49.3 kJmol at 238 K. This value is in agreement
À1
1
À1
À1
with the 50.7 kJmol (at T ˆ 238 K, 60 MHz) measured with
c
TMPhG.[
49a]
Of course, these values vary from alkyl-substi-
Figure 4. Eyring plot from proton self-exchange experiments of 1/2a.
tuted guanidines, which exhibit a reasonably higher barrier of
=
À1
activation (PMG: DG ˆ 78.7 kJmol
at T ˆ 351 K,
c
100 MHz[ ) because the CˆN double bond is not weakened
48d]
NMR spectra and molecular dynamics: The intrinsic dynamic
behavior of the guanidine group, combined with the change in
structure as a result of protonation, leads to complex intra-
molecular exchange processes which were investigated by
by inductive effects of the aromatic system. In 1, however, a
considerable weakening of the CˆN bond order is realized by
inductive effects of the naphthalene ring.
1
dynamic H NMR spectroscopy.
1
Monoprotonated 1: The low-temperature H NMR spectrum
Free base 1: Compound 1 exhibits one signal for the methyl
groups at room temperature. At lower temperatures, this
signal splits into two signals of equal intensity (Figure 5), with
of 2a reveals four separate resonances (1:1:1:1) at 190 K
(Figure 7) and is markedly more complicated than that of the
free base 1. It also differs significantly from the known spectra
‡
‡
[47]
a coalescence point at T ˆ 253 K (400 MHz), which is
of [HTMPhG] and [PMPhG] .
c
attributed to hindered rotation about the CˆN bond (Fig-
ure 6). This is caused by the typical syn ± anti isomerization of
guanidines, which is already documented in the literature for
The fact that 2a also exhibits real proton sponge properties
in solution with a rapidly exchanging proton in an IHB
(Scheme 3), is documented in the number of aromatic proton
signals, along with the number of methyl signals in the range
of fast exchange. Unsymmetrical protonation would result in
six resonances in the aromatic region and two for the methyl
signals. In 2a only three aromatic signals and one methyl
signal is observed, indicating the equivalence of both basic
centers. Now two exchange phenomena are possible, leading
to the four methyl resonances in the low-temperature
spectrum. A single concerted process with all four of the
other guanidine[
24, 47, 48]
and TMG systems.
[49]
Generally, this isomerization can be caused by rotation or
inversion, but in the case of 1, only the rotation has to be
considered for steric reasons and inversion is unlikely. At
ambient temperature, the protons of the two N-dimethylami-
[50]
no units are equivalent. Thus, a flip mode with two separate
ground-state conformers,[ such as that observed in DMAN,
where free rotation is impossible because of the repulsion
7c]
Chem. Eur. J. 2002, 8, No. 7
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1687

J. Sundermeyer et al.
FULL PAPER
with DG= (249 K) ˆ 48.7, DG (231 K) ˆ 48.6, and DG
=
=
1
2
3
À1
(
224 K) ˆ 48.5 kJmol (all recorded at 400 MHz). Further-
more, the lineshape analysis on the basis of a single concerted
process as the exchange mechanism leads to an excellent
agreement with the obtained experimental spectra. In addi-
=
tion, the DG values determined from the resulting Eyring
plot are in good agreement with those received from the
calculations based on the experimental coalescence points.
They are in the range of the activation barriers found in
‡
[47c]
=
=
[
HTMPhG]
(DG (248 K) ˆ 52.3, DG (248 K) ˆ 54.0,
1
2
=
À1
and DG (225 K) ˆ 46.9 kJmol , all recorded at 60 MHz)
3
‡
[47a]
=
and quite contradictory to [PMPhG]
(only DG
1
=
À1
(
301 K) ˆ 64.9 and DG (227 K) ˆ 46.5 kJmol were deter-
2
mined at 60 MHz). These findings are in good agreement with
the values of benzyl-substituted guanidinium salts which show
‡
considerably higher free energies of activation ([Bz TMG] :
2
=
À1[48b]
DG (at T ˆ 276 K, 60 MHz) ˆ 61.1 kJmol
) since there
c
is no weakening of the CˆN bond by the adjacent aromatic
system resulting in lower energy barriers. In summary, the
described behavior is a convincing argument that a true IHB
with a rapidly equilibrating proton is formed, both guanidine
groups are indiscriminate within the observed temperature
‡
[46]
region (as is reported for the NMe units of DMANH
). It
2
1
is assumed that [1‡H][PF ] (2a) possesses a symmetrical
Figure 7. Temperature-dependent H NMR spectra at 400 MHz for the N-
6
methyl singlet of [1‡H][PF
6
] (2a) in CD
simulated by iterative fitting:[ right).
2
Cl
2
(experimental spectra: left,
double minimum energy profile with a low barrier of
49]
[5, 53]
transition.
Bisprotonated 1: On account of the insolubility of hydro-
chloride 3a in CD Cl at low temperatures, the triflate 3c was
employed for the kinetic NMR studies (Figure 8). The low-
guanidine units exchanging at the same time. Otherwise, one
rotation would be preferably restricted if the energy content
of the partial double bonds would significantly differ from
each other (probably as a result of conjugation of the former
CˆN double bond with the aromatic ring system). The latter
2
2
1
temperature H NMR spectra of [1‡2H][OTf] (3c) resemble
2
those of [1‡H][PF ] (2a) down to a temperature of 222 K.
6
The split of the methyl resonances follow the same interpre-
tation, and it becomes clear that the proton exchange in the
should first cause a symmetrical split (1:1) into two singlets,
followed by a second split of the two singlets at even lower
temperatures into another set of singlets, which can be seen in
IHB of 1‡HPF (2a) does not affect the splitting of the signal,
6
‡
[47]
otherwise the spectra of mono- and bisprotonated 1 should
part in [RTMPhG] (R ˆ H, Me).
differ.
2‡
Scheme 3. Rotation about partial CˆN double bonds in [1‡H][PF
6
] (2a)
Figure 8. Rotation about partial C .
ˆ
N double bonds in [TMGNH2]
with equilibration of the proton.
The free energies of activation for the first three coales-
There are several indications of a single concerted process
cences of 3c (similar to 2a) are calculated from the
1
=
in the variable-temperature spectra of 2a. In the H NMR
coalescence points in the experimental spectra: DG ˆ 48.2/
Tc
1
À1
spectra of 2a recorded below 249 K (T ), firstly an unsym-
47.3/45.4 kJmol (T ˆ 256/235/226 K). In the TMGN system
c
c
metrical separation of the singlet for the N-methyl protons
with an intensity of 3:1 becomes visible; this excludes a
sequential process. Thereafter, the downfield signal gradually
splits into three singlets (1:1:1) that exhibit a similar chemical
shift which is significantly different from the signal at higher
field (for studies on mesomeric cations, see ref. [52]). More-
over, the concerted process is confirmed by the free energies
of activation of the three observable coalescence phenomena
that are obtained from an Eyring plot and are very similar
and its protonated species, contrary steric and electronic
effects seem to interfere. Taking only the electronic effect into
account, the CˆN bond order and barrier to rotation is
expected to decrease with the extent of protonation. Whereas
=
=
the DG values do not reflect this prediction, the DH values,
although hampered with a large experimental error,[ are
much better at demonstrating the expected trend of lowering
the activation barriers of the base and its corresponding
mono- and bisprotonated acids (Table 5).
54]
1688
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Proton Sponges
1682±1693
Table 5. Free activation energies and enthalpies of rotation, syn ± anti
conformation and self-exchange in the bis(guanidine) proton sponge
[
a]
system.
=
=
DH=
Compound
DG298
[
DG256
kJmol ][64]
À1
[kJmol ][64]
À1
[kJmol ][64]
À1
TMGN (1)[b]
48.3 Æ 0.1
49.2 Æ 0.1
51.9 Æ 1.0
47.1 Æ 0.1
49.0 Æ 0.1
48.8 Æ 0.1
53.4 Æ 0.3
46.5 Æ 0.3
27.0 Æ 3.0
36.1 Æ 0.3
[
[
[
TMGNH]PF
6
(2a)[b]
[
[
c]
[d]
TMGNH
TMGNH
2
]OTf
]OTf
2
(3c)
(3c)
48.2 Æ 0.1
b]
2
2
45.6 Æ 0.1
58.2 Æ 0.1
syn/anti-NÀH
‡
[b]
TMGN/[TMGNH]
self-exchange
59.3 Æ 0.1
51.6 Æ 0.3
[
a] Activation entropy is omitted because of the large error in its
[
65]
determination. [b] Calculated from graphical analysis of rate constants
obtained from simulated spectra.[66] [c] Values obtained by extrapolation on
the basis of the three experimentally determined individual DG
=
[54]
T
c
values.
[
d] Value obtained from coalescence point (T
c
ˆ 256 K) of the experimen-
[
67]
tal spectrum.
Figure 10. Graphical analysis of rate constants determined from spectra
simulation for the syn ± anti conformation equilibrium of [1‡2H][OTf]
2
Below 222 K further coalescence phenomena are observed,
the exchange rates between the syn- and the anti-conforma-
tion become detectable. Finally, at 190 K (400 MHz) no less
than seven resonances are recorded for the guanidine methyl
groups with two additional signals for the NH protons
3c).[
60]
(
bone of DMAN. To date it is the most basic compound in the
class of naphthalene-based ™proton sponges∫. TMGN (1) has
(
Figure 9).
an experimentally determined pKBH
‡
value of 25.1 Æ 0.2
(
MeCN) revealing a thermodynamic basicity nearly seven
orders of magnitude higher than the parent DMAN. Fur-
thermore, TMGN (1) also has a much higher kinetic basicity
than DMAN, which is demonstrated by the kinetics of its
=
À1
proton self-exchange (DG2 ˆ 59.3 Æ 0.1 kJmol ). On ac-
98
count of this high kinetic activity, the monoprotonated
‡
[
1‡H] may take up a second proton if treated with an excess
of strong acid. TMGN is more stable to hydrolysis in
comparison to the commercially available guanidine MTBD
and less nucleophilic towards alkylating agents, such as C H I.
2
5
1
Dynamic H NMR studies on mono- and bisprotonated
TMGN reveal a concerted mechanism of rotation about three
almost equivalent CÀN bonds. Furthermore, it is shown that
there is an equilibrium between a syn- and an anti-conforma-
tion with respect to both guanidine functionalities of the
bisprotonated 3c. The base and corresponding acids are
spectroscopically and structurally characterized. It is antici-
pated that a high thermodynamic basicity combined with a
high kinetic activity is interesting for base-catalyzed applica-
tions.
Figure 9. Low-temperature (190 K) 1H NMR spectrum (400 MHz) of
1 ‡ 2H][OTf] in CD Cl
[
2
2
2
.
Integration of the signals indicates a syn/anti population
ratio of 42:58. In agreement with our preliminary crystal
structure determination on 3b and theoretical calculations on
[41]
the gas-phase structure of bisprotonated 1, it is assumed
that the anti conformer is predominant in solution, especially
if non-coordinating counteranions are employed. From the
Experimental Section
Materials and methods: All experiments were carried out in glassware
assembled while hot in an inert atmosphere of argon4.8 dried with P O
4 10
granulate. Solvents and triethylamine were purified according to literature
corresponding Eyring plot, the free energy of activation for
=
the syn ± anti equilibrium is calculated to be DG
ˆ
2
98
À1
4
7.1 kJmol (Figure 10). It is interesting to note that in the
bisprotonated 3c, the DG2 value for the free energy of the
syn ± anti process (47.1 kJmol ) is lower than the barrier to
procedures and also kept under an inert atmosphere. 1,8-Diaminonaph-
=
[55]
4 6
thalene (Merck) was purified by distillation from zinc dust. NH PF ,
98
À1
4 4 6
NH ClO , trifluoromethanesulfonic acid (Aldrich), aqueous HPF (60 ±
À1
65%, Strem Chemicals), HCl gas (MERCK), and ethyl iodide (Fluka)
for protonation and alkylation, respectively, were used as purchased.
Substances sensitive to moisture and air were kept in a nitrogen-flushed
glove-box (Braun, TypeMB150BG-I). Spectra were recorded on the
following spectrometers: NMR: Bruker DRX500, DRX400 and
AMX300; IR: Bruker IFS88FT; UV/Vis: Hitachi U-3410; MS(EI,
rotation (51.9 kJmol ) about the CˆN bonds.
Conclusion
7
0 eV): Varian MATCH-7a; MS(FD): Finnigan MAT95S; MS (ESI):
TMGN (1) is a readily accessible and extremely basic
guanidine derivative with the classical proton sponge back-
Hewlett Packard HP5989B; elemental analysis: Heraeus CHN-Rapid;
melting points: B¸chi MPB-540 (uncorrected).
Chem. Eur. J. 2002, 8, No. 7
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0947-6539/02/0807-1689 $ 20.00+.50/0
1689

J. Sundermeyer et al.
FULL PAPER
‡
2 31 6 6
] ; elemental analysis calcd (%) for C20H N PF
Lineshape analysis: The lineshape analysis of the variable-temperature
m/z (%): 355 [(1)H
(500.5): C 48.00, H 6.24, N 16.79; found: C 47.92, H 6.08, N 16.16.
,8-Bis(1,1,3,3-tetramethylguanidinium)naphthalene perchlorate (2b,
1‡H][ClO ]): The monoprotonated perchlorate salt was obtained by
stirring 1 (354 mg, 1 mmol) and NH ClO (115 mg, 0.98 mmol) in MeCN
1
H NMR spectra were analyzed with the dynamic NMR simulation
[
44]
program WIN-DYNA.
Kessler.
Errors are quoted as defined by Binsch and
1
[
[
65]
4
Caution! Phosgene is a severe toxic agent that can cause pulmonary
embolism and in case of heavy exposition may be lethal. Use only in a well-
ventilated fume hood.
Tetramethylchloroformamidinium chloride:[25] Phosgene was passed
4
4
(15 mL) for 1 h at 508C. After evaporation of the solvent, the solid material
was redissolved in dry MeCN, stirred over activated charcoal and filtered
through Celite. The volatiles were removed in vacuo and 2b was crystal-
lized from MeCN/Et
M.p. 2298C (decomp.); H NMR (400.1 MHz, [D
2
O (428 mg, 0.94 mmol, 96%) as colorless crystals.
through a solution of tetramethylurea (50 g) in toluene (200 mL) kept at
1
]DMSO, 258C): d ˆ 14.19
0
2
8C in a flask equipped with a reflux condenser cryostated to À308C for
6
3
(
brs, 1H; NH), 7.41 ± 7.33 (m, 4H; H3 ± 6), 6.50 (dd, J(H2,H3) ˆ 7.1 Hz,
h. Subsequently, the phosgene inlet was closed and the solution was
4
1
J(H2,H4) ˆ 1.0 Hz, 2H; H2,7), 2.87 (s, 24H; CH
3
); H NMR (400.1 MHz,
allowed to warm to room temperature. The mixture was stirred for another
3
CD
2
Cl
2
, 288C): d ˆ 14.76 (brs, 1H; NH), 7.40 (dd, J(H4,H3) ˆ 8.3 Hz,
2
4 h, while the reflux condenser was maintained at À308C. The white
4
3
3
J(H4,H2) ˆ 1.1 Hz, 2H; H4,5), 7.33 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.8 Hz,
precipitate was filtered off, washed three times with dry diethyl ether, and
3
4
2
2
1
H; H3,6), 6.43 (dd, J(H2,H3) ˆ 7.4 Hz, J(H2,H4) ˆ 1.2 Hz, 2H; H2,7),
dried in vacuo. Yield: ꢀ95%.
1
.93 (s, 24H; CH
3
); H NMR (400.1 MHz, CD
2
Cl
2
, À838C): d ˆ 14.63 (brs,
1,8-Bis(1,1,3,3-tetramethylguanidino)naphthalene (1, TMGN): A solution
3
4
H; NH), 7.32 (dd, J(H4,H3) ˆ 8.2 Hz, J(H4,H2) ˆ 1.0 Hz, 2H; H4,5),
of the Vilsmeyer salt [(Me
30 mL) was added under cooling on an ice bath to a solution of 1,8-
diaminonaphthalene (4.8 g, 30.0 mmol) and triethylamine (6.1 g, 8.5 mL,
0.0 mmol) in acetonitrile (50 mL). After the exothermic reaction was
2 2
N) CCl]Cl (10.26 g, 60.0 mmol) in acetonitrile
3
3
7
.28 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.6 Hz, 2H; H3,6), 6.30 (dd,
(
3
4
J(H2,H3) ˆ 7.2 Hz, J(H2,H4) ˆ 1.0 Hz, 2H; H2,7), 3.08 (s, 6H; CH
3
),
1
3
3
3 3 3
.03 (s, 6H; CH ), 2.93 (s, 6H; CH ), 2.40 (s, 6H; CH ); C NMR
6
(
100.6 MHz, [D
6
]DMSO, 258C): d ˆ 158.5 (CN
17.6, 113.5 (aromat. C), 39.5 (CH
); IR (KBr): nÄ ˆ 2917m, 1617m, 1559s,
467m, 1405s, 1371m, 1352m, 1235w, 1163m, 1088s, 1015w, 830m, 769m,
3
), 142.0, 136.0, 126.1, 121.4,
complete the mixture was refluxed for 3 h and a clear solution developed.
Subsequently, NaOH (2.4 g, 60.0 mmol) in water (15 mL) was added under
1
1
6
3
3
vigorous stirring in order to deprotonate the HNEt
3
Cl. After removal of
À1
À5
À1
22m cm ; UV/Vis (MeCN, c ˆ 2  10 molL ): lmax (e) ˆ 348.2 (14400),
the solvent as well as excess NEt , the precipitate was washed three times
3
‡
06.7 (6800), 233.3 nm (50500); MS (FD, MeCN): m/z (%): 355 [(1)H
2
] ;
with dry diethyl ether to remove unreacted amine, and was then dried in
vacuo. TMGN (1) was obtained by complete deprotonation of the
bis(hydrochloride) with 50% KOH (50 mL) and extraction of the aqueous
phase with MeCN (3 Â 50 mL). The combined filtrates were evaporated to
dryness and taken up in warm hexane (100 mL). The solution was dried
over MgSO , stirred with activated charcoal to eliminate impurities, and
4
filtered warm through Celite. Recrystallization from hexane and drying in
vacuo gave 1 as weakly beige crystals (9.03 g, 25.5 mmol, 85%). M.p.
elemental analysis calcd (%) for C20
8.47; found: C 52.53, H 6.88, N 17.83.
,8-Bis(1,1,3,3-tetramethylguanidinium)naphthalene
1‡2H][Cl, Cl H]): Gaseous HCl was bubbled into a solution of 1
354 mg, 1 mmol) in CH Cl (10 mL) for 5 minutes. The clear, light yellow
31 6 4
H N ClO (455.0): C 52.80, H 6.87, N
1
1
dichloride
(3a,
[
2
(
2
2
solution was stirred for 1 h. The precipitate, which formed after adding
Et O, was washed with dry diethyl ether. Crystallization from MeCN gave
3a as colorless crystals (495 mg, 0.98 mmol, 98%). The product analyzed as
2
238C; 1H NMR (400.1 MHz, CD
3
1
8
6
3
CN, 258C): d ˆ 7.19 (d, J(H4,H3) ˆ
3
3
an adduct with 1 molecule HCl and 1 molecule CH
3
CN. M.p. 2338C;
.3 Hz, 2H; H4,5), 7.13 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.5 Hz, 2H; H3,6),
1
.23 (d, 3J(H2,H3) ˆ 6.8 Hz, 2H; H2,7), 2.65 (s, 24H; CH
1
H NMR (400.1 MHz, CD
3
CN, 258C): d ˆ 11.20 (s, 2H; NH), 7.92 (d,
3
); H NMR
3
3
3
J(H4,H3) ˆ 8.1 Hz, 2H; H4,5), 7.54 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.9 Hz,
(
400.1 MHz, [D
6
]DMSO, 258C): d ˆ 7.16 ± 7.09 (m, 4H; H3 ± H6), 6.16 (dd,
4
3
3
1
4
2H; H3,6), 7.02 (d, J(H2,H3) ˆ 7.2 Hz, 2H; H2,7), 4.33 (brs, 1H; Cl
2
H),
),
); IR (KBr):
J(H2,H3) ˆ 6.7 Hz, J(H2,H4) ˆ 1.6 Hz, 2H; H2,7), 2.62 (s, 24H; CH
3
);
1
3
3
2.94 (s, 24H; CH
3
); C NMR (100.6 MHz, CD
3
CN, 258C): d ˆ 161.3 (CN
3
2 2
H NMR (400.1 MHz, CD Cl
, 288C): d ˆ 7.21 (dd, J(H4,H3) ˆ 8.3 Hz,
3
3
137.5, 133.8, 128.6, 127.1, 123.9, 123.1 (aromat. C), 41.0 (CH
3
J(H4,H2) ˆ 1.3 Hz, 2H, H4,5), 7.15 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.5 Hz,
3
4
nÄ ˆ 3417m, 3033m, 2916m, 1636s, 1540s, 1467m, 1429m, 1406m, 1371m,
2
2
H; H3,6), 6.27 (dd, J(H2,H3) ˆ 7.1 Hz, J(H2,H4) ˆ 1.5 Hz, 2H; H2,7),
À1
1
1337m, 1300m, 1172m, 1066m, 1019m, 796m, 772m cm
;
UV/Vis
.66 (s, 24H; CH
3
); H NMR (400.1 MHz, CD
2
Cl
2
, À738C): d ˆ 7.18 (d,
3
À5
À1
3
3
(MeCN, c ˆ 2  10 molL ): lmax (e) ˆ 346.7 (11800), 232.8 nm (50200);
J(H4,H3) ˆ 8.0 Hz, 2H; H4,5), 7.12 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.5 Hz,
‡
‡
3
MS (FD, MeCN): m/z (%): 389 [[(1)H
analysis calcd (%) for Cl ¥ HCl ¥ CH
(504.9)): C 52.33, H 7.19, N 19.42; found: C 53.14, H 7.12, N 18.98.
1,8-Bis(1,1,3,3-tetramethylguanidinium)naphthalene hexafluorophos-
phate-tetrafluoroborate (3b, [1‡2H][PF , BF ]): An impure, HBF -con-
taining sample of aqueous hexafluorophosphoric acid (0.35 mL, 60 ± 65%)
was added dropwise to 1 (354 mg, 1 mmol) in CH Cl (10 mL). Within
2
]Cl] , 354 [(1)H
2
] ; elemental
2
H; H3,6), 6.29 (d, J(H2,H3) ˆ 7.2 Hz, 2H; H2,7), 2.71 (s, 12H; CH
3
), 2.32
),
); C NMR
), 150.1, 136.7, 125.9,
); IR (KBr): nÄ ˆ 3440w (br),
002w, 2937m, 1630vs, 1593s, 1558s, 1493s, 1452m, 1431m, 1371s, 1233m,
1
3
C
20
H
32
N
6
2
3
CN (427.4‡36.5‡41.1
(
s, 12H; CH
1
100.6 MHz, [D
1
3
1
3
); C NMR (100.6 MHz, CD
50.7, 137.4, 126.1, 119.8, 115.7 (aromat. C), 39.4 (CH
]DMSO, 258C): d ˆ 154.0 (CN
22.7, 119.6, 115.2 (aromat. C), 39.7 (CH
3
CN, 258C): d ˆ 155.0 (CN
3
1
3
3
(
6
3
3
6
4
4
À1
À5
À1
135s, 985m, 830m, 760m cm ; UV/Vis (MeCN, c ˆ 2  10 molL ): lmax
2
2
(
(
[
e) ˆ 349.0 (15600), 235.0 (46000), 213 nm (36300); MS (FD, MeCN): m/z
seconds, a white precipitate developed which was washed with dry diethyl
and dried in vacuo. The precipitate was recrystallized from MeOH to yield
‡
‡
%): 354 [M] ; MS (EI, 70 eV): m/z (%): 354.0 (86.5) [M] , 310.0 (7.7)
‡
‡
‡
1
M À NMe
2
] , 253.0 (26.1) [M À C(NMe
2
)
2
] , 100.0 (55.9) [C(NMe
2
)
2
] ,
(554 mg, 0.92 mmol, 92%) 3b as light brown crystals. M.p. 2278C; H NMR
‡
3
8
6
5.0 (100) [C
4
H
9
N
2
] ; elemental analysis calcd (%) for C20
H
30
N
6
(354.50): C
(400.1 MHz, CD
3
CN, 258C): d ˆ 7.94 (d, J(H4,H3) ˆ 8.3 Hz, 2H; H4,5),
3
3
7.76, H 8.53, N 23.71; found: C 67.55, H 8.53, N 23.53.
,8-Bis(1,1,3,3-tetramethylguanidinium)naphthalene hexafluorophosphate
2a, [1‡H][PF ]): The monoprotonated hexafluorophosphate salt was
obtained by treatment of the free guanidine base 1 (354 mg, 1.00 mmol)
with a solution of NH PF (1 equiv, 160 mg, 0.98 mmol) dissolved in MeCN
15 mL) and stirring for 1 h at 508C. The solvent was evaporated and the
7.84 (s, 2H; NH), 7.58 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.8 Hz, 2H; H3,6), 7.04
3
13
(
d, J(H2,H3) ˆ 7.3 Hz, 2H; H2,7), 2.95 (s, 24H; CH
3
);
C NMR
1
(
100.6 MHz, CD
3
CN, 258C): d ˆ 160.6 (CN
3
), 137.2, 132.8, 128.6, 127.3,
(
6
1
9
1
23.1, 121.7 (aromat. C), 41.6 (CH
3
); F NMR (188.3 MHz, CD
3
CN, 258C):
2
31
d ˆ À70.9 (d, J(F,P) ˆ 706.6 Hz, PF
6
), À149.6 (s, BF
4
);
P
NMR
);
4
6
1
(
162.0 MHz, CD
3
CN, 258C): d ˆ À142.9 (sept., J(P,F) ˆ 706.5 Hz, PF
6
(
1
1
B NMR (96.3 MHz, [D
6
]DMSO, 258C): d ˆ À1.2 (s, BF ); IR (KBr): nÄ ˆ
4
solid was redissolved in MeCN. The solution was stirred over activated
charcoal and passed through Celite. Removal of the solvent and
3
1
383m, 2950m, 1637s, 1544s, 1473m, 1434m, 1409m, 1344m, 1299m,
À1
281m, 1167m, 1065m, 1022m, 841s, 762m, 558s cm ; MS (ESI pos,
crystallization from MeCN/Et
quantitative yield (476 mg, 0.95 mmol, 97%). M.p. 2558C; H NMR
2
O gave 2a as colorless crystals in almost
‡
‡
‡
2
] ; MS
1
MeCN): m/z (%): 501 [[(1)H
2
]PF
6
] , 441 [(1)H
2
]BF
4
] , 355 [(1)H
] ; elemental analysis calcd
32 6
H N BF10P (588.3): C 40.83, H 5.48, N 14.29; found: C 40.54, H
À À
3
(ESI neg, MeCN): m/z (%): 145 [PF
(%) for C20
5.29, N 13.90.
1,8-Bis(1,1,3,3-tetramethylguanidinium)naphthalene
2
): Compound 1 (354 mg, 1 mmol) diluted in dry Et O
] , 87 [BF
6 4
(
8
6
400.1 MHz, CD
3
CN, 258C): d ˆ 14.28 (brs, 1H; NH), 7.40 (d, J(H4,H3) ˆ
3
3
.3 Hz, 2H; H4,5), 7.34 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.9 Hz, 2H; H3,6),
.49 (d, 3J(H2,H3) ˆ 7.5 Hz, 2H; H2,7), 2.87 (s, 24H; CH
13
3
); C NMR
(
100.6 MHz, CD
3
CN, 258C): d ˆ 159.8 (CN
3
), 142.9, 136.9, 126.7, 122.0,
bistriflate
(3c,
1
1
1
1
14.2 (aromat. C), 39.9 (CH
473m, 1431m, 1410m, 1371m, 1278vs, 1247vs, 1171s, 1153s, 1068m,
3
); IR (KBr): nÄ ˆ 3325m, 2922m, 1646vs, 1547s,
[1‡2H][OTf]
2
(30 mL) was added dropwise to trifluoromethanesulfonic acid (0.9 mL,
10 mmol), which resulted in the instant precipitation of the bisprotonated
triflate salt. Subsequently, the suspension was stirred for another 2 h, and
À1
034vs, 848m, 766m, 639s, 573m, 517 m cm ; UV/Vis (MeCN, c ˆ 2 Â
À5
À1
0
molL ): lmax (e) ˆ 348.0 (14300), 233.3 nm (50800); MS (FD, MeCN):
1690
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0807-1690 $ 20.00+.50/0
Chem. Eur. J. 2002, 8, No. 7

Proton Sponges
1682±1693
1
À5
the precipitate was filtered and washed with dry Et
2
O three times.
H NMR basic hydrolysis experiment: TMGN (1, 21.3 mg, 6 Â 10 mol)
À5
Recrystallization from MeCN/Et
2
O gave 3c as colorless crystals (602 mg,
and MTBD (9.0 mg, 6 Â 10 mol) were each dissolved in dry [D
6
]DMSO
1
1
0
8
.92 mmol, 92%). M.p. 2428C; H NMR (400.1 MHz, CD
3
CN, 258C): d ˆ
2
(0.5 mL). After the addition of NaOD in D O (0.1 mL, 5m) their H NMR
3
.08 (s, 2H; NH), 7.93 (d, J(H4,H3) ˆ 8.3 Hz, 2H; H4,5), 7.57 (dd,
spectra were recorded at various times (t ˆ 0 ! 1 h/RT! 1 d/RT! 3 h/
3
3
3
J(H3,H4) ꢀ J'(H3,H2) ꢀ 7.9 Hz, 2H; H3,6), 7.04 (d, J(H2,H3) ˆ 7.5 Hz,
608C ! 1 d/608C ! 5 d/608C).
1
2
H; H2,7), 2.95 (s, 24H; CH
3
); H NMR (400.1 MHz, CD
2
Cl
2
, 288C): d ˆ
1
À4
H NMR nucleophilicity experiment: TMGN (1, 46.3 mg, 1.3 Â 10 mol)
3
8
.51 (s, 2H; NH), 7.86 (d, J(H4,H3) ˆ 7.7 Hz, 2H; H4,5), 7.52 (dd,
À4
and MTBD (20.0 mg, 1.3 Â 10 mol) were each dissolved in dry CD
2 2
Cl
3
3
3
J(H3,H4) ꢀ J'(H3,H2) ꢀ 8.1 Hz, 2H; H3,6), 6.91 (d, J(H2,H3) ˆ 7.5 Hz,
À4
(
5
0.6 mL) and C
2
H
5
I (TMGN: 102 mg, 0.053 mL, 6.5 Â 10 mol; MTBD:
À4
1
2
H; H2,7), 2.96 (s, 24H; CH
3
); H NMR (400.1 MHz, CD
2
Cl
2
, À928C):
1 mg, 0.026 mL, 3.25 Â 10 mol) was added, 2.5 equiv of the alkylating
1
d ˆ 8.54 (syn population 42%  2H each NH), 8.34 (anti population
agent per guanidine function, respectively. The H NMR spectra were
recorded at various times (t ˆ 0 ! 15 min/RT! 1 h/RT! 1 d/RT! 3 d/
RT).
5
6
8% Â 2H each NH), 7.87 ± 7.75 (m, 2H; H4,5), 7.53 ± 7.40 (m, 2H; H3,6),
.93 ± 6.75 (m, 2H; H2,7), 3.23, 3.05, 2.87, 2.17 (anti population 58% Â 6H
each CH
3
), 3.18, 3.06, 2.94, 2.28 (syn population 42% Â 6H each CH
3
);
X-ray structure analysis: Crystal data and experimental conditions are
listed in Table 6. The molecular structures are illustrated as Schakal[ plots
in Figures 1 ± 4. Selected bond lengths and angles with standard deviations
in parentheses are presented in Table 1. The collected reflections were
corrected for Lorentz and polarization effects. All structures were solved
by direct methods and refined by full-matrix least-squares methods on
F . Hydrogen atoms were calculated and isotropically refined, except for
H(1A) (2a) and H(1)/H(2)/H(4) (3a) which were found and isotropically
refined.[ The correctness of the absolute structure of 2a was confirmed by
the Flack parameter refined to 0.01(5).
1
H NMR (400.1 MHz, CD
2
Cl
2
, 258C): d ˆ 8.51 (s, 2H; NH), 7.86 (d,
3 3
56]
3
J(H4,H3) ˆ 7.7 Hz, 2H; H4,5), 7.52 (dd, J(H3,H4) ꢀ J'(H3,H2) ꢀ 8.1 Hz,
2
H; H3,6), 6.91 (d, 3J(H2,H3) ˆ 7.5 Hz, 2H; H2,7), 2.96 (s, 24H; CH
3
3
);
1
C NMR (100.6 MHz, CD
27.2, 123.2 (aromat. C), 40.6 (CH
3
CN, 258C): d ˆ 160.7 (CN
3
), 137.2, 133.0, 128.6,
); IR (KBr): nÄ ˆ 3436w (br), 2809w,
1
1
1
3
631m, 1590m, 1562s, 1539s, 1512s, 1469m, 1450m, 1415s, 1368m, 1350m,
159m, 1067m, 1016m, 837vs, 767m, 557m cm ; MS (FD, MeCN): m/z
2
[57]
À1
‡
‡
2
] ; elemental analysis calcd (%) for
(
%): 505 [[(1)H
2
]OTf] , 355 [(1)H
58]
C
22 32 6 6 6 2
H N F O S (654.7): C 40.36, H 4.93, N 12.84; found: C 39.98, H 4.89, N
1
2.35.
1
H NMR self-exchange experiment: Equimolar amounts of TMGN (1)
À6
À6
(
1.81 mg, 5  10 mol) and [1‡H][PF
6
] (2a, 2.50 mg, 5 Â 10 mol) were
1
dissolved together in dry CD
3
CN (0.5 mL), and H NMR spectra were
Acknowledgement
recorded at various temperatures ranging from 344 to 225 K. An analogous
À5
experiment was carried out with DMAN (4.28 mg, 2 Â 10 mol) and
This work was supported by the DFG, SFB260, and by the Fonds der
Chemischen Industrie. We would also like to thank Professor Z. B. Maksi c¬ ,
Zagreb, for stimulating discussions.
‡
À5
DMANH (7.20 mg, 2 Â 10 mol) in dry CD
3
CN (0.9 mL) and dry
[
6
D ]DMSO (0.8 mL), respectively.
Table 6. Crystal data and structure refinement for 1, 2a, and 3a.
Complex
TMGN (1)
[1‡H][PF
6
] (2a)
[1‡2H][Cl, Cl
2
H] (3a)
empirical formula
C
354.5
20
H
30
N
6
C
500.5
20
H
31
N
6
F
6
P
C
504.9
20
H
32
N
6
Cl
2
3
 HCl, CH CN
À1
F
w
[gmol
]
T [K]
183(2)
213(2)
213(2)
crystal system
space group
a [pm]
b [pm]
c [pm]
monoclinic
P2 /n
1313.7(1)
1165.1(2)
1480.1(1)
90
monoclinic
P2
873.8(1)
1151.5(1)
1240.9(1)
90
monoclinic
P2 /c
1083.0(1)
3374.5(1)
743.5(1)
90
1
1
1
a [8]
b [8]
g [8]
V [ä ]
Z
113.786(6)
90
2072.9(4)
4
1.136
0.071
107.003(5)
90
1194.1(1)
2
1.392
1.629
101.311(4)
90
2664.3(2)
4
1.259
3.292
3
À3
1
[Mgm
]
À1
m [mm
F(000)
crystal size[mm ]
diffractometer
l [pm]
scan technique
q range for data collection[8]
index ranges
]
768
524
1072
3
0.50 Â 0.30 Â 0.30
Enraf Nonius CAD4
MoKa/71.073
w scan
0.40 Â 0.40 Â 0.20
Enraf Nonius CAD4
CuKa/154.178
w scan
3.72 ± 59.94
À 9 ꢁ h ꢁ 9
À 12 ꢁ k ꢁ 0
0 ꢁ l ꢁ 13
1967
0.45 Â 0.27 Â 0.12
Enraf Nonius CAD4
CuKa/154.178
w scan
2.62 ± 59.90
0 ꢁ h ꢁ 12
0 ꢁ k ꢁ 37
À 8 ꢁ l ꢁ 8
4166
2.43 ± 24.87
À 14 ꢁ h ꢁ 13
À 13 ꢁ k ꢁ 0
0
3725
ꢁ l ꢁ 17
reflns collected
independent reflns
3315
1875
3942
R
int
0.0354
1970
3315/0/243
0.992
0.0156
1868
1875/0/307
1.070
0.0627
0.0478
3468
3942/0/310
1.054
0.0573
obs. reflns [F ꢂ 4s(F)]
data/restraints/parameters
GoF on F2
R
wR
[
a]
1
[F
0
ꢂ 4s(F)]
0.0552
[
a]
2
(all data)
0.1404
0.1681
0
.1575
transmission (max/min)
largest diff. peak and hole [eä
0.9791/0.9655
0.147/ À 0.214
0.7365/0.5619
0.952/ À 0.754
0.6934/0.3189
0.676/ À 0.400
À3
]
2
2
2
2†2]}1/2.
[
a] R
1
ˆSjjF
0
jÀjF
c
jj/SjF
0
j ; wR
2
ˆ{S[w(F
o
ÀF
c
† ]/S[w(F
o
Chem. Eur. J. 2002, 8, No. 7
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0807-1691 $ 20.00+.50/0
1691

J. Sundermeyer et al.
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1692
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
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[
9
9.
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=
(3c) were obtained by extrapolation
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[
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c
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A
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B
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M. Hesse, H. Meier, B. Zeeh in Spektoskopische Methoden in der
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Received: September 18, 2001 [F3560]
Chem. Eur. J. 2002, 8, No. 7
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0807-1693 $ 20.00+.50/0
1693