Noncovalent interactions between tetrazole and an N,N′-diethyl-substituted benzamidine

Article abstract of DOI:10.1021/jo005632i

Amidines have long been known to form strong noncovalent complexes with carboxylates and phosphates. However, their interaction with tetrazoles, which are acidic heterocycles and important bioisosteric replacements for carboxylic acids in medicinal chemistry, has remained unexplored so far. The binding of a tetrazole to an N,N′-diethyl-substituted benzamidine has been studied for the first time by X-ray crystallography, solution NMR methods, and electrospray mass spectrometry. The amidinium group of model complex 3 was found to prefer an E,Z configuration in the crystal. Benzamidinium and tetrazolate groups alternate along an infinite chain of hydrogen bonds and salt bridges between the amidine-NH groups and the two tetrazole-N atoms next to the ring carbon. In solution, a 1:1 complex was evident from Job's method of continuous variation, and an association constant of 4.0 × 10³ ± 1.6 × 10³ M^-1 (in CDCl₃/CD₃CN, 6:1) could be determined by ¹H NMR dilution experiments. Tetrazolate was not only found to be a weaker ligand than carboxylates but, surprisingly, the: binding mode also changed with concentration in neat CDCl₃. At low concentrations, the amidine group in complex 3 adapted an E,E configuration as it does in a related carboxylic acid complex 4. With increasing concentration, the E,Z isomer starts to predominate. A free activation enthalpy Δ₂₉₈? of 64 ± 1 kJ mol^-1 for the E,E to E,Z isomerization was determined by line shape analysis at different magnetic fields. Binding strength was further probed in a competition experiment between a bisamidine, a carboxylate, and a tetrazolate by electrospray mass spectrometry.

Full text of DOI:10.1021/jo005632i

J . Org. Chem. 2001, 66, 3291-3298
3291
Non cova len t In ter a ction s betw een Tetr a zole a n d a n
N,N′-Dieth yl-Su bstitu ted Ben za m id in e
Lars Peters,† Roland Fro¨hlich,‡ Alan S. F. Boyd,§ and Arno Kraft*,†,§
Institut fu¨r Organische Chemie und Makromolekulare Chemie II, Heinrich-Heine-Universita¨t Du¨sseldorf,
Universita¨tsstr. 1, D-40225 Du¨sseldorf, Germany, Organisch-Chemisches Institut der Universita¨t,
Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Corrensstr. 40, D-48149 Mu¨nster, Germany, and Department
of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom
a.kraft@hw.ac.uk
Received September 1, 2000 (Revised Manuscript Received March 9, 2001)
Amidines have long been known to form strong noncovalent complexes with carboxylates and
phosphates. However, their interaction with tetrazoles, which are acidic heterocycles and important
bioisosteric replacements for carboxylic acids in medicinal chemistry, has remained unexplored so
far. The binding of a tetrazole to an N,N′-diethyl-substituted benzamidine has been studied for the
first time by X-ray crystallography, solution NMR methods, and electrospray mass spectrometry.
The amidinium group of model complex 3 was found to prefer an E,Z configuration in the crystal.
Benzamidinium and tetrazolate groups alternate along an infinite chain of hydrogen bonds and
salt bridges between the amidine-NH groups and the two tetrazole-N atoms next to the ring carbon.
In solution, a 1:1 complex was evident from J ob’s method of continuous variation, and an association
1
constant of 4.0 × 103 ( 1.6 × 103 M-1 (in CDCl3/CD3CN, 6:1) could be determined by H NMR
dilution experiments. Tetrazolate was not only found to be a weaker ligand than carboxylates but,
surprisingly, the binding mode also changed with concentration in neat CDCl3. At low concentra-
tions, the amidine group in complex 3 adapted an E,E configuration as it does in a related carboxylic
acid complex 4. With increasing concentration, the E,Z isomer starts to predominate. A free
activation enthalpy ∆G298q of 64 ( 1 kJ mol-1 for the E,E to E,Z isomerization was determined by
line shape analysis at different magnetic fields. Binding strength was further probed in a competition
experiment between a bisamidine, a carboxylate, and a tetrazolate by electrospray mass
spectrometry.
In tr od u ction
to note that the recently reported X-ray crystal structure
of a tetrazole-containing inhibitor of HIV-1 integrase
shows close contacts between the tetrazole’s ring nitrogen
atoms N1 and N2 and two lysine residues within the
enzyme’s catalytic domain.5
The amidine group is another important pharmaco-
phore in modern drug design. Its complexation with
carboxylates and phosphates is well established. Several
X-ray crystal structures of amidinium carboxylates are
known, and all conform with a binding mode in which a
salt bridge is reinforced by two almost linear hydrogen
bonds between carboxylate-O and amidine-NH sites.6
Certain acidic heterocycles are used increasingly as
effective bioisosteric1 replacements for carboxylic acids
in medicinal chemistry. Tetrazoles2 are prominent ex-
amples of this class, and four out of six angiotensin II
receptor antagonists that are currently marketed for the
treatment of hypertension contain a tetrazole group.3
Mutagenesis studies have provided evidence that the
tetrazole of a nonpeptide antagonistslike the carboxy
terminus of angiotensin IIsinteracts with a protonated
lysine and a histidine at the recognition site of the
angiotensin II receptor.3a,4 In this context, it is interesting
* To whom correspondence should be addressed. Phone: +44-131-
4518040. Fax: +44-131-4513180.
† University of Du¨sseldorf
‡ University of Mu¨nster
§ Heriot-Watt University.
(1) Patani, G. A.; LaVoie, E. J . Chem. Rev. 1996, 96, 3147-3176.
(2) For general overviews of tetrazoles, see: (a) Butler, R. N. In
Comprehensive Heterocyclic Chemistry, A Review of the Literature
1982-1995; Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Storr, R.
C., Eds.; Pergamon: Exeter, 1996; Vol. 4, pp 621-678. (b) Meier, H.
R.; Heimgartner, H. In Methoden der Organischen Chemie (Houben-
Weyl), Hetarene III/ Teil 4; Schaumann, E., Ed.; Georg-Thieme-
Verlag: Stuttgart, 1994; Vol. E8d/4, pp 664-795. (c) Wittenberger, S.
J . Org. Prep. Proc. Int. 1994, 26, 499-531.
(3) (a) Wexler, R. R.; Greenlee, W. J .; Irvin, J . D.; Goldberg, M. R.;
Prendergast, K.; Smith, R. D.; Timmermans, P. B. M. W. M. J . Med.
Chem. 1996, 39, 625-656. (b) Naka, T.; Kubo, K. Curr. Pharm. Design
1999, 5, 453-472. (c) Burnier, M.; Brunner, H. R. Lancet 2000, 355,
637-645.
(4) Noda, K.; Saad, Y.; Kinoshita, A.; Boyle, T. P.; Graham, R. M.;
Husain, A.; Karnik, S. S. J . Biol. Chem. 1995, 270, 2284-2289.
An N,N′-diethyl substituted benzamidine, similar to
the one chosen for the study described in this paper, has
recently been introduced by Wulff et al. as the crucial
binding group and base in molecularly imprinted poly-
mers with esterase-like catalytic activity.7 Compared
(5) Goldgur, Y.; Craigie, R.; Cohen, G. H.; Fujiwara, T.; Yoshinaga,
T.; Fujishita, T.; Sugimoto, H.; Endo, T.; Murai, H.; Davies, D. R. Proc.
Natl. Acad. Sci. U.S.A. 1999, 96, 13040-13043.
(6) (a) Deng, Y.; Roberts, J . A.; Peng, S.-M.; Chang, C. K.; Nocera,
D. G. Angew. Chem., Int. Ed. Engl. 1997, 36, 2124-2127. (b) Papout-
sakis, D.; Kirby, J . P.; J ackson, J . E.; Nocera, D. G. Chem. Eur. J .
1999, 5, 1474-1480.
10.1021/jo005632i CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/14/2001

3292 J . Org. Chem., Vol. 66, No. 10, 2001
Peters et al.
Sch em e 1
with its parent benzamidine analogue, such an N,N′-
disubstituted amidine is more soluble in nonpolar organic
solvents, it has a higher stability toward hydrolysis, and
aggregation is usually suppressed owing to the additional
ethyl substituents. The substituted amidine derivative
offers advantages in binding studies, particularly in
nonpolar solvents or if weak interaction is expected.
Despite the importance of the arginine-carboxylate
and amidine-carboxylate (e.g., aspartate/glutamate) in-
teractions in protein folding and drug-receptor interac-
tions, there has hitherto been no indication that tetra-
zoles show a similar affinity to arginine. At the outset of
our study, it was unclear whether a tetrazole would, if
pushed, bind to an amidine base in the same way as its
corresponding carboxylic acid analogue. So far, little
structural information is available on the complexation
of a tetrazole with an amidine, a guanidine, or arginine.
Whereas Arg-Gly-Asp-based tripeptides are established
inhibitors of platelet aggregation, replacement of either
of the two carboxylic acid groups by a tetrazole has been
reported to lead to loss of activity.8 Theoretical predic-
tions by ab initio calculations have suggested that the
reason behind it is a less favorable electrostatic binding
between a guanidine and a tetrazolate compared to an
analogous guanidinium-carboxylate complex.9,10 The
answer to the question how tetrazoles interact with
amidines or guanidines has therefore implications for the
design of new inhibitors or antagonists to natural as well
as synthetic receptors. This paper describes how tetrazole
(2) binds to an N,N′-disubstituted benzamidine (1) both
in the crystal and in solution. There are important
differences from a related complex between 1 and benzoic
acid for which a crystal structure is also presented for
the first time.
Cr ysta l Str u ctu r es. Slow evaporation of a methanolic
solution of complex 3 gave crystals of sufficient quality
for X-ray crystallography. The crystal structure is de-
picted in Figure 1. The C-N bond lengths of 1.31 Å (1.34
Å) for the amidinium group and 1.31 Å (1.35 Å) for the
tetrazolate are characteristic of partial double bonds.
Benzamidinium and tetrazolate groups alternate along
an extended chain connected through almost linear (with
an N5*‚‚‚H-N15 angle of 172°) and slightly bent (with
an N12-H‚‚‚N2 angle of 147°) hydrogen bonds. The
involvement of nitrogens N2 and N5 of the tetrazole in
salt bridges correlates well with theoretical calculations
according to which the majority of the negative charge
around the tetrazolate anion resides on the nitrogen
atoms next to the ring carbon.9 The short N15‚‚‚N5* and
N12‚‚‚N2 distances of 2.85 Å each are consistent with
Resu lts
Syn th esis. Benzamidine 1 was obtained from 4-bro-
mobenzoyl chloride in three steps following a procedure
for a similar amidine derivative.11 Complex 3, a 1:1 salt,12
crystallized in analytical purity upon cooling of a solution
of 1 and tetrazole in warm acetonitrile (Scheme 1).
Benzoic acid complex 4 and complexes of terephthala-
midine 7 were prepared analogously.
(7) (a) Wulff, G.; Groâ, T.; Scho¨nfeld, R. Angew. Chem., Int. Ed. Engl.
1997, 36, 1962-1964. (b) Strikovsky, A. G.; Kasper, D.; Gru¨n, M.;
Green, B. S.; Hradil, J .; Wulff, G. J . Am. Chem. Soc. 2000, 122, 6295-
6296.
(8) Two crystal structures of compounds with a guanidinium and a
tetrazolate group lacked any evidence of hydrogen-bonding between
the two groups: (a) Duke, J . R. C. Chem. Commun. 1971, 2-3. (b)
Bracuti, A. J .; Troup, J . M.; Extine, M. W. Acta Crystallogr. C 1986,
42, 505-506. For reports of crystal structures of tetrazoles hydrogen-
bonded to an amine, see: (c) Chertanova, L. F.; Struchkov, Y. T.; Sopin,
V. F.; Kovalenko, V. I.; Timokhov, V. N.; Fronchek, E. V. Chem.
Heterocycl. Compd. 1995, 31, 655-657. (d) Parvez, M.; Unangst, P.
C.; Connor, D. T.; Mullican, M. D. Acta Crystallogr. 1991, C47, 608-
611.
(9) Zablocki, J . A.; Miyano, M.; Rao, S. N.; Panzer-Knodle, S.;
Nicholson, N.; Feigen, L. J . Med. Chem. 1992, 35, 4914-4917.
(10) Suvire, F.; Floridia, R.; Giannini, F.; Rodriguez, A.; Enriz, R.;
J auregui, E. Molecules 2000, 5, 583-584.
(11) (a) Wulff, G.; Scho¨nfeld, R. Adv. Mater. 1998, 10, 957-959. (b)
Scho¨nfeld, R. Ph.D. Thesis, University of Du¨sseldorf, 1998.
(12) Proton transfer was anticipated because of the two components’
difference in pKa. The pKa value for the related protonated 4-chloro-
N,N′-dimethylbenzamidine is 11.41 (in 50% ethanol), whereas tetrazole
has a pKa of 4.89 in water: (a) Piskov, V. B.; Kasperovich, V. P.;
Yakovleva, L. M. Chem. Heterocycl. Compd. 1976, 12, 917-923. (b)
Lieber, E.; Patinkin, S. H.; Tao, H. H. J . Am. Chem. Soc. 1951, 73,
1792-1795.
F igu r e 1. (a) Crystal structure of complex 3. (b) Extended
linear hydrogen-bonded chain of alternating molecules of 1 and
tetrazole, viewed along the crystallographic a axis.

Noncovalent Interactions between Tetrazole and a Benzamidine
J . Org. Chem., Vol. 66, No. 10, 2001 3293
F igu r e 2. Crystal structure of complex 4.
strong hydrogen bonds; the corresponding NH‚‚‚N dis-
tances are 1.65 and 2.07 Å, respectively. The only other
unusually close contact stems from an electrostatic
interaction between N3 of the tetrazolate anion and N15*
of a nearby cationic benzamidinium molecule at a dis-
tance of 3.38 Å.
In the crystal, the amidinium group favors an E,Z
configuration with one ethyl group being nearly coplanar
to the amidine whereas the CH3 residue of the other
points almost perpendicularly away from the binding
tetrazolate.13 The torsion angle of 66° between amidinium
group and adjacent phenyl ring is larger than for an
unsubstituted amidine (ca. 30-42°).6 This difference
clearly reflects the steric bulk of the N,N′-diethyl-
substituted amidine group.
F igu r e 3. J ob plot for benzamidinium salt 5 binding tetra-
butylammonium tetrazolate 6. The mole fraction x of 5 is
defined as [5]/([5] + [6]). The total concentration was main-
tained at 5 × 10-4 M in CDCl3/CD3CN (6:1), and the change
in the chemical shift ∆δ ) δobsd - δ0 of the tetrazolate CH
signal was determined for various compositions.
A pseudo-polymorph of complex 3 suitable for X-ray
crystallography could be grown from a concentrated
solution in CDCl3. One molecule of chloroform per
molecule of 3 was included in the crystal, causing slight
increases in torsion angles and the length of hydrogen
bonds. Despite this, the binding mode was essentially the
same as in Figure 1.14
The crystal structure of complex 4 is the first of a
complex between a carboxylic acid and an N,N′-diethyl-
substituted amidine (Figure 2). The most striking feature
is the E,E configuration of the amidine group, which
enables the benzoate ligand to bind to the amidine
through two strong H-bonds (with NH‚‚‚O angles of 160°
and 168°, NH‚‚‚O distances of 1.97 and 1.71 Å, and
N‚‚‚O distances of 2.75 and 2.70 Å). The N,N′-diethyl-
substituted amidinium group has an overall zigzag
conformation. The torsion angle between the NCN group
and the adjacent benzene ring is, not surprisingly, even
larger (81°) than in the aforementioned tetrazole complex
3 or in complexes of unsubstituted amidines with
carboxylates.6b
F igu r e 4. Concentration dependence of the tetrazolate CH
singlet’s NMR chemical shift of complex 3 in CDCl3/CD3OD
(97:3) at 20 °C.18 The curve represents the calculated isotherm
for 1:1 binding, whereas the filled squares are experimental
values.
estimate for the association constant Ka of 4.0 × 103 (
1.6 × 103 M-1 in CDCl3/CD3CN (6:1) and of 1.5 × 103 (
1.1 × 103 M-1 in CDCl3/CD3OD (97:3) for the binding of
tetrazolate to the amidinium ion.16 However, anomalies
occurred at higher concentration or in the absence of a
polar cosolvent, and for this reason, a Ka value could not
be determined in neat CDCl3 as will be discussed later.
By comparison, NMR dilution studies for benzamidinium
tetrazolate gave an association constant Ka of 38 ( 3 M-1
in DMSO-d6 for the binding of a tetrazolate to an
unsubstituted benzamidinium cation.17
Solu tion P r op er ties of Com p lex 3. A J ob plot
(Figure 3) supported a 1:1 stoichiometry in CDCl3/CD3-
CN (6:1).15
Figure 4 illustrates a typical NMR dilution curve of
complex 3. Nonlinear regression analysis revealed an
(13) For X-ray crystal structures of substituted amidines that
emphasize a preference for the E,syn isomer of the amidine base and
the E,Z isomer of the amidine hydrochloride, see: (a) Tinant, B.;
Dupont-Fenfau, J .; Declercq, J .-P.; Podlaha, J .; Exner, O. Collect. Czech.
Chem. Commun. 1989, 54, 3245-3251. (b) Trent, J . O.; Clark, G. R.;
Kumar, A.; Wilson, W. D.; Boykin, D. W.; Hall, J . E.; Tidwell, R. R.;
Blagburn, B. L.; Neidle, S. J . Med. Chem. 1996, 39, 4554-4562. (c)
Boere´, R. T.; Klassen, V.; Wolmersha¨user, G. J . Chem. Soc., Dalton
Trans. 1998, 4147-4154. (d) Hartmann, S.; Weckert, E.; Frahm, A.
W. Acta Crystallogr. 1999, C55, 806-808. (e) McNally, J . J .; Youngman,
M. A.; Lovenberg, T. W.; Nepomuceno, D. H.; Wilson, S. J .; Dax, S. L.
Bioorg. Med. Chem. Lett. 2000, 10, 213-216.
(14) NH‚‚‚N distances (angles): 1.84 Å (175°), 2.07 Å (156°); N‚‚‚N
distances: 2.87 and 2.85 Å. The torsion angle between NCN amidine
group and the adjacent benzene ring increased to 72°.
(15) (a) J ob, P. Ann. Chim. 1928, 9, 113-203. (b) Blanda, M. T.;
Horner, J . H.; Newcomb, M. J . Org. Chem. 1989, 54, 4626-4636.
Two pairs of triplets and quartets are observed for the
1
N-ethyl groups in the H NMR spectra of solutions of 3
(16) The hyperbolic shape of the dilution curve was fitted to 1:1
host-guest complexation by nonlinear regression analysis: (a)
Schneider, H. J .; Kramer, R.; Simova, S.; Schneider, U. J . Am. Chem.
Soc. 1988, 110, 6442-6448. (b) Wilcox, C. S. In Frontiers in Supramo-
lecular Chemistry and Photochemistry; Schneider, H.-J ., Du¨rr, H., Eds.;
VCH: Weinheim, 1991; pp 123-143. (c) Macomber, R. S. J . Chem.
Educ. 1992, 69, 375-378.
(17) Peters, L.; Kraft, A. Unpublished results.
(18) Association constants were determined by following the tetra-
zolate CH singlet. Its line shape remained largely unaffected by
dynamic processes of the amidine group.

3294 J . Org. Chem., Vol. 66, No. 10, 2001
Peters et al.
Sch em e 2
pecially when carried out in neat CDCl3, were hampered
by line-broadening of the amidine’s N-ethyl and aromatic
proton NMR signals. Since ligand exchange is expected
to be fast on the NMR time scale,22 a dependence of the
line shape on temperature and NMR frequency pointed
to a dynamic process that was independent of the
tetrazolate ligand, namely isomerization between the
E,E- and E,Z-configured amidine (Scheme 2).11,20 As
mentioned before, a complex between the E,E isomer and
tetrazole is observed in chloroform at 5 × 10-4 M (Figure
6a). The analysis of the NMR spectra is, however, further
complicated by the fact that additional signals of the E,Z
isomer emerged with increasing concentration (Figure
6b-d). Such a concentration-dependent change in isomer
ratio has been noted for other N,N′-disubstituted ami-
dinium salts before.11,13d It also interfered with the
determination of an association constant by a 1H NMR
titration experiment in chloroform since addition of
excess tetrazolate 6 similarly promoted isomerization to
the E,Z amidine. At high concentrations, the tetrazolate
singlet shifts even further upfield, which would be
expected if the tetrazole proton moved into the shielding
cone below the amidine’s benzene ring as in isomer 3A
(or an oligomeric analogue).
F igu r e 5. 1H NMR (500 MHz) spectra of (a) 3 in D2O, (b) 5
in CDCl3, (c) complex 3 in CDCl3 (0.005 M, 400 MHz), and (d)
complex 4 in CDCl3. Solvent and water signals are marked by
X.
in polar solvents, such as DMSO-d6, CD3OD, or D2O
(Figure 5a), indicating that the amidine exists exclusively
as the E,Z isomer; in all these polar solvents complex-
ation is negligible. Benzamidinium salt 5 with its non-
coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]-
borate counteranion gives rise to more than one set of
1H NMR signals for the amidine protons in chloroform
(Figure 5b). The major component is again identified as
the isomer with an E,Z stereochemistry at the partial
C-N double bonds of the amidine, whereas the minor
constituent is E,E-configured. A Z,Z isomer can be
excluded for steric reasons.19 As Figure 5c illustrates, a
solution of complex 3 in CDCl3 shows large downfield
shifts of the NH signal accompanied by significant
changes in isomer ratio. Although the amidine prefers
an E,Z configuration in the crystal, the NMR data
establish a predominance of the more symmetrical E,E
isomer under conditions that favor complex formation
(nonpolar solvent, coordinating counteranion). This as-
signment is supported by a single triplet at δH ) 1.19
and a quartet at δH ) 3.10 for the two N-ethyl groups,
We subsequently studied the temperature dependence
of the 1H NMR spectra at a 0.005 M concentration of
complex 3 in CDCl3. Under these conditions, the E,Z-
configured amidine was present as the minor component,
with a ratio of 9:1 for the mixture of E,E and E,Z isomers.
Whereas two of the CH3 signals overlapped, the chemical
shifts of the CH2 protons were more widely spread (δΗ )
3.10, 3.37, and 3.73) and could be easily extracted from
(21) In principle, tetrazolate can hydrogen-bond through either
nitrogen atom N1 or N2, and examples for both binding modes have
been found in a number of crystal structures. It is as yet unclear to
which extent binding through N2 may play a role in solution, but it is
of little importance for the subsequent discussion on amidine isomer-
ization. Further experiments with 15N-labeled tetrazoles will be
necessary to clarify this point.
1
and it thus closely resembles the H NMR spectrum of
benzoate complex 4 in chloroform (Figure 5d). An amidine
E,E configuration in the benzoate complex is evident from
both X-ray crystallography and solution studies.11,20 We
infer that complex 3 binds in chloroform, unlike in the
crystal, according to binding mode 3C or 3D (Scheme 1)
of which the latter should be more favorable since
coordination of the tetrazolate through N1 and N2 results
in a salt bridge that involves at least one of the more
negatively charged nitrogen atoms adjacent to the ring
carbon.9,21
(22) Apart from rare cases when major conformational reorganiza-
tion is necessary and dissociation constants are extremely small (i.e.,
in the nanomolar range), association rate constants tend to be diffusion
controlled, whereas dissociation rate constants equal the quotient of
the diffusion rate (around 108-109 M-1 s-1) and the association
constant Ka: Schneider, H.-J .; Yatsimirsky, A. In Principles and
Methods in Supramolecular Chemistry; Wiley-VCH: Weinheim, 2000;
pp 259-265.
Am id in e Isom er iza tion . Complexation studies, es-
(19) Hammond, G. S.; Neuman, R. C. J . Phys. Chem. 1963, 67,
1655-1659.
(20) Kraft, A. J . Chem. Soc., Perkin Trans. 1 1999, 705-714.

Noncovalent Interactions between Tetrazole and a Benzamidine
J . Org. Chem., Vol. 66, No. 10, 2001 3295
F igu r e 7. Experimental 1H NMR spectra of 3 in CDCl3 (0.087
M, 25 °C) at different NMR frequencies (left) and simulated
line shapes (right).
at room temperature in the 90 MHz NMR spectrum. The
chemical shifts were kept identical to those at 0.005 M,
and a ratio of 1:2.7 (extracted from integrating the NMR
spectrum at highest field) for the amidine’s E,E and E,Z
isomers was used in line shape simulations. A different
magnetic field consequently alters just the frequency of
the spin systems, whereas k must remain the same.
Again, line shape analysis reproduced the experimental
NMR spectra within a small error, thus allowing an
energy barrier ∆G298q of 64 ( 1 kJ mol-1 to be calculated.
The increase in line width for the amidine signals at
higher concentrations was consistent with the proposed
exchange process and a higher exchange rate.
Electr osp r a y Ma ss Sp ectr a . Electrospray ionization
(ESI) mass spectrometry has become increasingly popular
for the study of biological and synthetic noncovalent
macromolecular complexes.25 The mildness of the elec-
trospray technique has promoted its use for screening
combinatorial inhibitor libraries based on the notion that
a tightly bound enzyme-inhibitor complex gives rise to
a strong ion peak in the mass spectrum. Even solution
binding constants have been derived from the intensities
of mass ion peaks in certain cases.26 A good correlation
between the two has been reported for inhibitors of
carbonic anhydrase as well as for a number of synthetic
noncovalent complexes.27
F igu r e 6. 1H NMR spectra (400 MHz, 25 °C) of 3 in CDCl3 at
different concentrations: (a) 0.0005 M, (b) 0.005 M, (c) 0.02
M, and (d) 0.1 M. Solvent, water, and acetone impurity signals
are marked by X. Arrows indicate signals of the (E,Z) isomer.
the room-temperature spectrum in the slow exchange
1
regime (Figure 6b).23 The 400 MHz H NMR signals of
the CH2 signals coalesced upon raising the temperature
to 60 °C. Line shape simulations were carried out by
using the program DNMR5 to provide rates of exchange
k for the isomerization between the E,E and E,Z con-
figurations.24 We deduced that both ethyl groups in the
E,Z amidine change their environment when converting
to the E,E isomer. Interconversion of the two ethyl groups
in the E,Z-configured amidine was consistent with ex-
change cross-peaks in a two-dimensional NOESY/EXSY
experiment, and simulated line shapes suggested that
the rate of interconversion should be comparable to that
of E/Z isomerization in most cases. A free activation
enthalpy ∆Gq of 68 ( 1 kJ mol-1 thus derived was
independent of the temperature within the errors of the
∆Gq values.
1
At higher concentrations, the H NMR signals tended
We were able to obtain an ESI-MS of complex 3 from
a solution in acetonitrile. Although the neutral complex
3 eluded detection, the dominant mass ion peaks could
be identified as the singly charged (that is, protonated)
base 1 + H+; dimer 12 + H+; and a 2:1 complex 12‚2 +
H+. The last-mentioned cluster ion arose presumably
from an association process that led to a small chain
segment similar to that observed in the crystal depicted
in Figure 1b.
to broaden considerably even at room temperature. Since
a change in temperature would invariably affect both the
isomer ratio and the association constant, we conducted
a further dynamic NMR experiment that allowed us to
study the exchange process under conditions when these
parameters remained constant. A 0.087 M sample of
complex 3 in CDCl3 was hence probed at different
magnetic fields (Figure 7). The high concentration of
complex 3 was chosen because (i) the low-field spectrom-
eter was not very sensitive and (ii) coalescence occurred
We wondered whether it would be possible to compare
the binding strength of a tetrazolate with that of a
(23) Although the signals in the aromatic region were also affected
by the isomerization process, the higher order spin system proved to
be too complicated for line-shape analysis.
(24) (a) Binsch, G.; Kessler, H. Angew. Chem., Int. Ed. Engl. 1980,
19, 411-428. (b) Stephenson, D. S.; Binsch, G. J . Magn. Reson. 1978,
32, 145-152. (c) Stephenson, D. S.; Binsch, G. DNMR5: Iterative
Nuclear Magnetic Resonance Program for Unsaturated Exchange-
Broadened Band shapes, Program 365, Quantum Chemistry Program
Exchange, Indiana University, Bloomington, IN.
(26) J ørgensen, T. J . D.; Roepstorff, P.; Heck, A. J . R. Anal. Chem.
1998, 70, 4427-4432.
(27) (a) Cheng, X.; Chen, R.; Bruce, J . E.; Schwartz, B. L.; Anderson,
G. A.; Hofstadler, S. A.; Gale, D. C.; Smith, R. D. J . Am. Chem. Soc.
1995, 117, 8859-8860. (b) Gao, J .; Cheng, X.; Chen, R.; Sigal, G. B.;
Bruce, J . E.; Schwartz, B. L.; Hofstadler, S. A.; Anderson, G. A.; Smith,
R. D.; Whitesides, G. M. J . Med. Chem. 1996, 39, 1949-1955. For
examples with host-guest complexes of ammonium ions encapsulated
in dimeric capsules of calixarene tetraureas, see: (c) Schalley, C. A.;
Castellano, R. K.; Brody, M. S.; Rudkevich, D. M.; Siuzdak, G.; Rebek,
J . J . Am. Chem. Soc. 1999, 121, 4568-4579.
(25) Przybylski, M.; Glocker, M. O. Angew. Chem., Int. Ed. Engl.
1996, 35, 806-826.

3296 J . Org. Chem., Vol. 66, No. 10, 2001
Peters et al.
Discu ssion
Like carboxylic acids, tetrazoles are capable of forming
complexes with an amidine base, such as 1. The crystal
structure of our model 1:1 complex 3 reveals that the
protonated amidine prefers an E,Z configuration in the
solid state, no matter whether crystals were grown from
methanol or chloroform. The high charge density on N1
and N4 in the tetrazolate anion, predicted by theoretical
calculations,9,28 explains the observed preference of these
nitrogen atoms to form salt bridges with the benzami-
dinium cations. The situation is more complicated in
solution, and there it depends crucially on solvent, ligand,
temperature, and concentration. In polar solvents or in
the presence of noncoordinating counteranions the N,N′-
diethyl-substituted amidinium group exists preferably or
even exclusively in the E,Z form, which is the sterically
least hindered isomer.19 However, benzoate and tetra-
zolate interact strongly with the amidinium group in
chloroform solution. Since binding can occur through a
pair of hydrogen bonds to the E,E isomer only, this isomer
is consequently preferred.
The tetrazolate anion with its four N atoms is a flexible
hydrogen-bond acceptor which adapts quite easily to
different binding modes. Like a carboxylate ligand, the
tetrazolate forms a complex with the E,E-configured
amidine in chloroform at low concentration. But, as
concentration increases, it binds more and more to the
E,Z isomer instead. Tetrazolate is smaller in size and
apparently less effective in forcing the amidine into an
E,E configuration, which could be partly due to an
imperfect fit compared with a carboxylate ligand. The
observed change in binding mode is best explained by
an oligomerization process as intercomplex hydrogen
bonds are favored at higher concentrations and by a
sterically less demanding E,Z amidine isomer. We note
that an E,Z-configured amidine can only have a single
hydrogen bond between the amidine and tetrazole (like
in formulas 3A, 3B, or oligomeric analogues) that is
reminiscent of the binding in the crystal structure. In
addition, the NH signal shifts upfield again at high
concentrations (Figure 6d), suggesting that the hydrogen
bond weakens slightly, which conforms, too, with a
change in binding mode.
Such concentration-dependent changes in isomer ratios
make the determination of an association constant Ka
more difficult. However, we were able to determine a Ka
value of 38 ( 3 M-1 for the binding of a tetrazolate to an
unsubstituted benzamidine in DMSO-d6. The tetra-
zolate’s binding constant thus proves to be almost 2
orders of magnitude lower than for the formation of
benzamidinium-benzoate complexes (with a Ka of typi-
cally about 1550-2500 M-1) in the same solvent.6
The restricted rotation around the C-N partial double
bond in N-substituted amidines and amidinium salts has
been known for a long time.13d,19,29 The majority of
previous studies have been carried out with highly
substituted amidines since complications by tautomer-
ization and proton exchange could thus be avoided.
Rotational barriers were highly dependent on steric
effects and the type of substituent on the amidine group.
F igu r e 8. Electrospray mass spectrum of an equimolar
mixture of 7-9 (in MeCN/MeOH, 80:20).
carboxylate ligand by electrospray MS since competitive
solution binding studies proved to be fraught with
difficulties. Detection by mass spectrometry required a
1:1 complex with an additional positively charged group
that did not interfere with complexation. Our choice fell
to complexes of terephthalamidine 7 with a 4-dodecyloxy-
substituted phenyltetrazole 8 and an analogous benzoic
acid 9. The idea was that a protonated 1:1 complex would
be left behind once a 2:1 complex lost a single benzoate
or tetrazolate ligand. Figure 8a illustrates the tetra-
zolate-bisamidine 1:1 complex is indeed the base peak
in such a case. In contrast, a benzoate-terephthalami-
dine 1:1 adduct gives a weaker ion peak than protonated
7 or even its dimer (Figure 8b). When an equimolar
mixture of bisamidine 7, tetrazole 8, and carboxylic acid
9 was analyzed by ESI-MS (Figure 8c), the differences
in the molecular mass of the base (7, 274 Da) and the
two ligands (8, 330 Da; 9, 306 Da) ensured that individual
ions could be assigned without doubt. In this competition
experiment, the tetrazolate-bisamidine complex gave a
considerably larger peak than the corresponding ben-
zoate-bisamidine 1:1 complex.
(28) Arp, H. P. H.; Decken, A.; Passmore, J .; Wood, D. J . Inorg.
Chem. 2000, 39, 1840-1848.
(29) (a) Harris, D. L.; Wellman, K. M. Tetrahedron Lett. 1968, 5225-
5228. (b) Filleux, M. L.; Naulet, N.; Dorie, J . P.; Martin, G. J .; Pornet,
J .; Miginiac, L. Tetrahedron Lett. 1974, 1435-1438.

Noncovalent Interactions between Tetrazole and a Benzamidine
J . Org. Chem., Vol. 66, No. 10, 2001 3297
An N′-benzyl-N,N-dimethylbenzamidine, for example,
was found to show a free activation enthalpy of around
50 kJ mol-1 at the coalescence temperature, considerably
less than the corresponding amidine hydrochloride (84
kJ mol-1).30
binding in tetrazole-amidine complexes in the crystal,
in nonpolar (chloroform), and in the presence of more
competitive solvents. Although the acidic heterocycle
behaves like a carboxylic acid in many respects, the mode
of binding to amidine 1 surprisingly differed in the crystal
and in solution. Under certain conditions tetrazolate was
able to bind to an amidine through two hydrogen-bonds
in a manner similar to carboxylates. It is expected that
tetrazoles interact similarly with unsubstituted benz-
amidines and guanidines. The knowledge of possible
binding modes that can be adapted by tetrazoles should
be of considerable value for a better understanding of
tetrazole drugs in medicinal chemistry and in synthetic
enzyme mimetics.
Complex 3 exhibits temperature- and field-dependent
NMR spectra that are consistent with a similar dynamic
process. Coalescence of the three CH2 signals of the
N-ethyl groups (and also of the CH3 and aromatic proton
signals, although these were not used for analysis
because of considerable signal overlap) occurs at higher
temperatures. The Gibbs activation barrier ∆Gq of 68 (
1 kJ mol-1 derived from line shape analyses was com-
parable to that for other amidines with substituents that
were more sterically demanding than methyl groups.29
Because isomer ratios and association constants are
likely to be influenced by a rise in temperature, we then
decided to record a number of 1H NMR spectra at 25 °C,
but at different magnetic fields. In this way, chemical
shifts, isomer ratios, and exchange rates remain constant
from one spectrum to another. However, in the case of a
dynamic system, line shapes become dependent on the
spectrometer frequency as the difference in Hz between
the NMR signals varies. This is an unconventional, albeit
legitimate, approach to study a dynamic process. The free
activation enthalpy ∆G298q determined for a 0.087 molar
sample was thus found to be 64 ( 1 kJ mol-1. Although
∆Gq seemed not to depend on temperature, there was a
small but noticeable reduction with increasing concentra-
tion. The variable temperature and frequency NMR
measurements confirmed that an E,E to E,Z isomeriza-
tion was indeed responsible for the observed changes in
Exp er im en ta l Section
Gen er a l Meth od s. All solvents were dried and freshly
distilled before use. DSC: Mettler TC 11 with TA 4000
Processor. NMR: Bruker AC200, DPX 400, DRX 500. TMS
was used as chemical shift reference in the NMR measure-
ments. The multiplicities of 13C signals were determined by
DEPT experiments. IR: Bruker Vector 22 FT-IR. CI-MS:
Finnigan INCOS 50. ESI-MS: Thermoquest Automass bench-
top LC/GC MS. Elemental analyses: Pharmazeutisches In-
stitut der Heinrich-Heine-Universita¨t Du¨sseldorf.
4-Br om o-N-eth ylben za m id e. The compound was pre-
pared by a modified literature procedure.31 4-Bromobenzoyl
chloride (10.3 g, 46.7 mmol) was added portionwise over 10
min to an ice-cold aqueous solution of ethylamine (70%, 35
mL, 0.63 mol). The solution was heated to reflux, diluted with
water (50 mL), and cooled to room temperature. The precipi-
tate was collected by suction filtration, washed with water (2
× 30 mL), and dried. Yield: 9.96 g (93%), colorless solid.
Eth yl 4-Br om o-N-eth ylben zim id a te. A solution of 4-bro-
mo-N-ethylbenzamide (4.56 g, 20.0 mmol) and triethyloxonium
tetrafluoroborate (5.70 g, 30.0 mmol) in dry CH2Cl2 (30 mL)
was stirred for 24 h under argon at room temperature. The
mixture was concentrated in a vacuum, and the residue was
treated with ice-cold NaOH (3 M, 20 mL) and extracted with
ether (100 mL). The organic layer was dried over Na2SO4 and
concentrated again. The product was distilled (Kugelrohr, 170
°C/0.02 mbar) and, because of its moisture sensitivity, used
without further purification. Yield: 3.39 g (66%). 1H NMR (500
MHz, CDCl3): δ 1.06 (t, J ) 7.3 Hz, 3 H), 1.25 (t, J ) 7.1 Hz,
3 H), 3.19 (q, J ) 7.3 Hz, 2 H), 4.14 (q, J ) 7.1 Hz, 2 H), 7.14,
7.47 (AA′XX′, 2 × 2 H).
1
the H NMR line shape.
The electrospray MS results deserve further comment.
They indicate that binding between the amidine and
tetrazolate is stronger than formation of a benzoate-
amidine complex. To explain these results, we considered
that, according to 1H NMR evidence, the amidine favors
the E,Z-configuration in the polar solvent mixture (MeCN/
MeOH, 80:20) used for the ESI-MS experiments. Poten-
tial ligands will therefore have to interact with this
isomer under such conditions. Unlike our NMR binding
studies in chloroform-rich solvents, which are dominated
by hydrogen bonding and electrostatic interactions, ESI-
MS measures the relative amounts of ion pairs that are
not solvent-separated. Solvophobicity can consequently
have a crucial effect. It is well established that tetra-
zolates are more lipophilic than carboxylates,1 and this
fact contributes to the unexpectedly stronger binding of
the acidic heterocycle in MeCN/MeOH. However, an
unsubstituted amidine is more likely to bind to a car-
boxylate ligand in the usual way through two hydrogen
bonds, whereas our N,N′-disubstituted amidine 1 de-
scribes an example when the benzoate is forced to
interact in a less effective manner.
4-Br om o-N,N′-d ieth ylben za m id in e (1). A solution of
ethyl 4-bromo-N-ethylbenzimidate (3.39 g, 13.2 mmol) and
ethylamine hydrochloride (2.81 g, 34.4 mmol) in dry ethanol
(30 mL) was stirred for 5 days at room temperature. The
mixture was concentrated in a vacuum, and the residue was
suspended in ethyl acetate (100 mL) and washed with 6 N
NaOH (20 mL). The organic layer was dried over Na2SO4 and
concentrated in a vacuum. The crude product was then purified
by gradient sublimation at 110 °C/10-4 mbar. Yield: 3.08 g
(91%), colorless solid. Mp (DSC): 86 °C (∆H 95 J g-1). 1H NMR
(200 MHz, CD3OD): δ 1.11 (br t, J ≈ 7.1 Hz, 6 H), 3.13 (br q,
J ≈ 7.1 Hz, 4 H), 7.23, 7.60 (AA′XX′, 2 × 2 H). 13C NMR (50
MHz, CD3OD): δ 16.3 (broad), 130.9, 132.7 (CH, CH3), 41.3
(broad, CH2), 124.1, 136.4, 161.4 (ipso-C, CdN). MS (CI,
NH3): m/z 257, 255 (M+H+). IR (KBr, cm-1): ν 3200, 3011,
2955, 2920, 1617, 1546, 1482, 1311, 1012, 826. Anal. Calcd
for C11H15BrN2 (255.15): C, 51.78; H, 5.93; N, 10.98. Found:
C, 51.68; H, 5.99; N, 10.84. Hydrochloride 1‚HCl was obtained
as a colorless solid by freeze-drying a solution of 1 in aqueous
HCl. Mp (DSC): 193 °C (∆H 83 J g-1). 1H NMR (500 MHz,
DMSO-d6): δ 1.08 (t, J ) 7.1 Hz, 3 H), 1.22 (t, J ) 6.1 Hz, 3
H), 3.12 (q, J ) 6.1 Hz, 2 H), 3.40 (q, J ) 7.1 Hz, 2 H), 7.55,
7.84 (AA′XX′, 2 × 2 H), 9.46 (br s, 2 H). 13C NMR (50 MHz,
Con clu sion
The present study has demonstrated that tetrazole
indeed forms a noncovalent complex with an N,N′-
disubstituted amidine, such as 1, although association
constants were found to be considerably weaker than for
carboxylates. We were able to study different aspects of
(30) McKennis, J . S.; Smith, P. A. S. J . Org. Chem. 1972, 37, 4173-
4178.
(31) Beak, P.; Musick, T. J .; Chen, C.-W. J . Am. Chem. Soc. 1988,
110, 3538-3542.

3298 J . Org. Chem., Vol. 66, No. 10, 2001
Peters et al.
CD3OD): δ 13.2, 15.8, 131.2, 133.8 (CH, CH3), 39.4, 41.9 (CH2),
127.9, 128.7, 164.7 (ipso-C, CdN). IR (KBr, cm-1): ν 2982,
3100-2500 (broad), 1637 (broad), 1495, 1439, 1146, 1012, 839.
Anal. Calcd for C11H16BrClN2 (291.61): C, 45.31; H, 5.53; N,
9.61. Found: C, 45.11; H, 5.41; N, 9.38.
H), 1.40-1.50 (m, 4 H), 1.70-1.90 (m, 4 H), 3.12 (br s, 4 H),
3.48 (br s, 4 H), 4.00 (t, J ) 6.8 Hz, 4 H), 6.96, 7.93 (AA′XX′,
2 × 4 H), 7.48 (s, 4 H). IR (KBr, cm-1): ν 2923, 2851, 1638,
1536, 1443, 1249, 838. Anal. Calcd for C50H78N12O2‚H2O: C,
66.93; H, 8.99; N, 18.73. Found: C, 66.86; H, 9.19; N, 18.42.
Com p lex betw een 733 a n d p-Dod ecyloxyben zoic Acid
(9).35 Yield: 70% (MeCN). Mp (DSC): 136 °C (∆H 130 J g-1).
1H NMR (500 MHz, CDCl3/CD3OD, 6:1): δ 0.80 (t, J ) 6.7 Hz,
6 H), 1.00-1.32 (m, 44 H), 1.35-1.45 (m, 4 H), 1.65-1.75 (m,
4 H), 3.92 (t, J ) 6.5 Hz, 4 H), 6.80, 7.86 (AA′XX′, 2 × 4 H),
7.54 (br s, 4 H). IR (KBr, cm-1): ν 2922, 2852, 1681, 1645,
1606, 1255. Anal. Calcd for C54H86N4O6‚3 H2O: C, 68.90; H,
9.85; N, 5.84. Found: C, 69.10; H, 9.64; N, 5.84.
Com p lex 3. 4-Bromo-N,N′-diethylbenzamidine 1 (400 mg,
1.57 mmol) and 2 (110 mg, 1.57 mmol) were dissolved in hot
acetonitrile (20 mL). Upon cooling to room temperature, the
complex crystallized and was isolated by suction filtration.
Yield: 408 mg (80%). DSC: 87 °C/Tg/132 (∆H 15 J g-1)/K/174
1
°C (∆H 96 J g-1)/I. H NMR (500 MHz, D2O): δ 1.14 (t, J )
7.3 Hz, 3 H), 1.32 (t, J ) 7.3 Hz, 3 H), 3.29 (q, J ) 7.3 Hz, 2
H), 3.42 (q, J ) 7.3 Hz, 2 H), 7.47, 7.80 (AA′XX′, 2 × 2 H),
8.58 (s, 1 H). 13C NMR (125 MHz, CD3OD): δ 13.4, 15.9, 131.3,
134.2, 150.2 (CH, CH3), 39.5, 42.2 (CH2), 128.3, 129.1, 165.2
(ipso-C, CdN). IR (KBr, cm-1): ν 2978, 2792, 1637, 1495, 1439,
1147, 1011, 837. Anal. Calcd for C12H17BrN6 (325.21): C, 44.32;
H, 5.27; N, 25.84. Found: C, 44.26; H, 5.50; N, 25.71.
Deter m in a tion of Associa tion Con sta n ts. NMR dilution
experiments were carried out with complex 3, which contained
exactly equimolar amounts of amidinium and tetrazolate ions.
Nonlinear regression analysis [with Kaleidagraph version 3.5
(Synergy Software)] was used to fit the concentration C of
1
complex 3 and the experimental H NMR chemical shifts δ of
Com p lex 4. Prepared from 1 and benzoic acid. Yield: 95%
(from MeCN). Mp (DSC): 159 °C (∆H 75 J g-1). 1H NMR (500
MHz, CDCl3): δ 1.18 (t, J ) 7.3 Hz, 6 H), 3.04 (q, J ) 7.3 Hz,
4 H), 7.22, 7.74 (AA′XX′, 2 × 2 H), 7.35-7.45 (m, 3 H), 8.05-
8.15 (m, 2 H), 13.2 (br s, 2 H). IR (KBr, cm-1): ν 2980, 2938,
1676, 1380, 721. Anal. Calcd for C18H21BrN2O2 (377.28): C,
57.30; H, 5.61; N, 7.43. Found: C, 57.03; H, 5.76; N, 7.14.
4-Br om o-N,N′-d ieth ylben za m id in iu m Tetr a k is[3,5-bis-
(tr iflu or om eth yl)p h en yl]bor a te (5). A mixture of 1‚HCl
(130 mg, 0.45 mmol) and potassium tetrakis[3,5-bis(trifluo-
romethyl)phenyl]borate32 (402 mg, 0.45 mmol) were dissolved
in methanol (25 mL) and concentrated in a vacuum. The
residue was then suspended in a 1:1 mixture of CHCl3/MeCN
and passed through a membrane filter. The filtrate was
concentrated in a vacuum, and the remaining colorless amor-
phous solid was dried. Yield: 480 mg (96%). 1H NMR (500
MHz, CDCl3) major (E,Z) isomer: δ 1.17 (t, J ) 7.2 Hz, 3 H),
1.26 (t, J ) 7.2 Hz, 3 H), 3.27 (qd, J ) 7.2, 5.3 Hz, 2 H), 3.32
(qd, J ) 7.2, 6.3 Hz, 2 H), 6.42 (br s, 1 H), 6.50 (br s, 1 H),
7.22, 7.74 (AA′XX′, 2 × 2 H), 7.54 (br s, 4 H), 7.69 (br s, 8 H);
a set of additional signals could be assigned to the minor (E,E)
isomer: δ 1.12 (t, J ) 7.3 Hz, 6 H), 3.05 (qd, J ) 7.3, 6.3 Hz,
4 H), 7.17, 7.78 (AA′XX′, 2 × 2 H), 8.96 (br s, 2 H). IR (KBr,
cm-1) ν 2997, 1639, 1356, 1279, 1121, 887, 838, 712, 682, 671.
Anal. Calcd for C43H29BBrF24N2 (1119.37): C, 46.14; H, 2.52;
N, 2.50. Found: C, 46.12; H, 2.51; N, 2.63.
the tetrazole CH singlet in the unbound (δ0) and the complexed
form (δc) to the equation16
δ0 - δ
2KaC
δ ) δ0 -
c ‚ [2K C + 1 - 4K C + 1]
x
a
a
Sin gle-cr ysta l X-r a y d iffr a ction a n a lysis of 3: formula
12H17BrN6, M ) 325.23, colorless needle 0.35 × 0.10 × 0.05
C
mm, a ) 11.225(5) Å, b ) 14.226(5) Å, c ) 9.112(2) Å, V )
1455.1(9) Å3, Fcalcd ) 1.485 g cm-3, µ ) 38.29 cm-1, empirical
absorption correction via ψ scan data (0.348 e T e 0.832), Z
) 4, orthorhombic, space group Pna21 (No. 33), λ ) 1.54178
Å, T ) 223 K, ω/2θ scans, 1566 reflections collected (+h, +k,
+l), [(sin θ)/λ] ) 0.62 Å-1, 1566 independent and 1351 observed
reflections [I g 2σ(I)], 183 refined parameters, R ) 0.038, wR2
) 0.106, maximum residual electron density 0.43 (-0.78) e
Å-3, Flack parameter -0.01(4). Hydrogens were found at the
nitrogen atoms from difference Fourier calculations, others
were calculated and refined as riding atoms.
Sin gle-cr ysta l X-r a y d iffr a ction a n a lysis of 4: formula
C
18H21BrN2O2‚1/8H2O, M ) 379.53, colorless block 0.20 × 0.20
× 0.15 mm, a ) 19.663(2) Å, b ) 11.879(1) Å, c ) 18.265(1) Å,
â ) 119.25(1)°, V ) 3722.3(5) Å3, Fcalcd ) 1.354 g cm-3, µ )
31.00 cm-1, empirical absorption correction via ψ scan data
(0.576 e T e 0.654), Z ) 8, monoclinic, space group C2/c (No.
15), λ ) 1.54178 Å, T ) 223 K, ω/2θ scans, 3932 reflections
collected ((h, -k, -l), [(sin θ)/λ] ) 0.62 Å-1, 3807 independent
and 2261 observed reflections [I g 2σ(I)], 222 refined param-
eters, R ) 0.050, wR2 ) 0.149, maximum residual electron
density 0.42 (-0.38) e Å-3. Hydrogens were found at the
nitrogen atoms from difference Fourier calculations, others
were calculated and refined as riding atoms. Data sets were
collected with an Enraf Nonius CAD4 diffractometer. Pro-
grams used: Express, MolEN, SHELXs-97, SHELXL-97,
SCHAKAL.
Tetr a bu tyla m m on iu m Tetr a zola te (6). A solution of
tetrazole (2.10 g, 30.0 mmol), aqueous tetrabutylammonium
hydroxide (40%, 19.8 mL, 30.0 mmol), and NaOH (6 M, 10 mL)
was extracted with CHCl3 (200 mL). The organic extract was
then concentrated in a vacuum to afford 6 (9.00 g, 97%) as a
colorless amorphous solid that, owing to its hygroscopicity, was
1
stored under argon. H NMR (500 MHz, CDCl3): δ 0.97 (t, J
) 7.6 Hz, 12 H), 1.38 (tq, J ) 6.9, 7.6 Hz, 8 H), 1.49-1.56 (m,
8 H), 3.10 (m, 8 H), 8.35 (s, 1 H). 13C NMR (125 MHz, CDCl3):
δ 13.6, 19.7, 23.9, 58.6, 149.8. IR (KBr, cm-1): ν 2962, 2875,
1487, 1470, 1421, 1383, 1140, 1109, 882, 748. Anal. Calcd for
Ack n ow led gm en t. We thank the Deutsche Fors-
chungsgemeinschaft, Heriot-Watt University, and Prof.
Dr. G. Wulff for support, Ms. S. J ohann for assistance
in the preparation of starting materials, and Dr. R.
Ferguson for the electrospray mass spectra.
C
17H37N5‚0.5 H2O: C, 63.71; H, 11.95; N, 21.84. Found: C,
63.73; H, 11.66; N, 21.76.
Com p lex betw een N,N′,N′′,N′′′-Tetr a eth ylter ep h th a l-
a m id in e (7)33 a n d 5-(p-Dod ecyloxyp h en yl)tetr a zole (8).34
Yield: 83% (MeCN/MeOH). DSC: K1/86 °C (∆H 35 J g-1)/K2/
133 °C (∆H 14 J /g)/I. 1H NMR (500 MHz, CDCl3/CD3OD, 6:1):
δ 0.89 (t, J ) 6.8 Hz, 6 H), 1.10 (br s, 12 H), 1.20-1.40 (m, 24
Su ppor tin g In for m ation Available: Detailed X-ray struc-
tural information on complex 3, complex 3‚CDCl3, and complex
4; concentration dependence of the amidine isomer ratio; time-,
(32) Nishida, H.; Takada, N.; Yoshimura, M. Bull. Chem. Soc. J pn.
1984, 57, 2600-2604.
1
frequency- and temperature-dependent H NMR and NOESY/
EXSY spectra of 3 in CDCl3; additional line shape analysis
results; ESI-MS of 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
(33) Gru¨n, M. Diplomarbeit, University of Du¨sseldorf, 1996.
(34) Kraft, A.; Osterod, F.; Fro¨hlich, R. J . Org. Chem. 1999, 64,
6425-6433.
(35) Parker, R. A.; Kariya, T.; Grisar, J . M.; Petrow, V. J . Med.
Chem. 1977, 20, 781-791.
J O005632I