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Modification of Hydrophobic and Polar Interactions by Charged Groups in Synthetic Host-Guest Complexes.

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Modification of Hydrophobic and Polar Interactions
by Charged Groups in Synthetic Host-Guest
Complexes**
By Hans-Jorg Schneider* and Thomas Blatter
The quantification of noncovalent interactions plays a n
important role in understanding biological processes at the
molecular level (for example, for the recognition between
substrate and receptor) as well as in planning modifications of enzymes by genetic engineering. Synthetic hostguest complexes, by allowing structurally defined changes
in a largely fixed environment, provide a way of obtaining
information about the individual interactions. Herein we
report on the complexation behavior of cyclobisdiphenylmethane compounds of types A and B in aqueous solution. In contrast to the earlier investigated,"] structurally
analogous macrocycle C, structures A and B do not have
positive charges o n the inner surface of the cavity, but instead negative charges that are distant from the binding
center."'
Q
I
so,"
C: X = @NMep
X-(CH,),-X
0.39 0.39
1
A:
B:
C:
L
A: - 14.7
8 : - 11.5
12.9
- 13.4
- 16.9
-
c : - 15.8
@ NMe3CIO
::3:
0.33
ms0'
0.59 0.34
0.83
/ 0.69
0.55 0.39
0.83 \
0.61 \
0.45 0.55
3
4
A : - 16.0
A : - 8.9
B: - 7.0
C: -22.6
B:
c: -
a)
4.8
of A and C should have similar geometries, also with respect to the sp3 hybridization of the N atoms. Indeed, similar results are obtained even for A and B, although somewhat different conformations are expected for the carboamide B. Measurements of the equilibrium constants K
were carried out using 'H-NMR-shift titrations with 1 and
its naphthalene derivatives 2-4 (Scheme 1) as well as computer fitting according to reported procedures.l4] For a positively charged substrate like trimethyldinaphthylammonium ion 3 in methanol/water (20:80 v/v), the K values
were, as expected, large compared with electroneutral
naphthalene 1 and larger still compared with the negatively charged guest molecule 4. The contributions of the
attractive and repulsive electrostatic interactions, ca. 3 kJ
mol-' (AGO comparison 311 for host A , Scheme 1) and ca.
4 kJ mol-' (114, host A), respectively, are relatively constant. Comparison of these values with the corresponding
AGO values for the complexation of the same substrate
with host C, which, while analogous in structure, carries
positive charges in the ring, reveals larger A G differences
of 12 (comparison 3/1) and 6 kJ mol-' (1/4), as expected
owing to the greater proximity of the host and guest charges.
The complexation-induced 'H NMR shifts obtained by
nonlinear curve fitting (CIS, see Scheme 1 for selected values), which show the expected shielding effects,I5] indicate
the pseudoequatorial inclusion of the substrates 1-4 in the
cavity of A and B, whereby the ring-current effect of both
the host and the guest molecule dominates. The considerably smaller CIS values obtained with the anionic host compounds A and B in comparison with the shifts observed
with the host C[''.51support our c o n ~ I u s i o n that
[ ~ ~ the effect of the electric field due to the charges cannot be neglected."bJ To compare the complexation behavior, it is important to note that the absence of N M R effects due to the
anionic phenyl side groups in A and B (CIS<O.Ol ppm)
rules out a significant involvement of these groups through
lipophilic interaction with the substrate.
Unexpected results were obtained upon comparing the
binding constants K of naphthalene 1 with A and C ; the
K value for A is five times smaller than that for compound
Cb'cl which contains four positively charged N atoms on
the surface of the cavity. The presence of these four ammonium centers in C - which, in addition, make the macrocycle water-soluble-should lead to a reduction instead
of an increase in the complexation constants, since the ammonium centers are much more hydrophilicf6I than the
neutral nitrogen atoms in A (and B) and since the binding
of hydrocarbons to 1 is mainly due to hydrophobic interactions. Moreover, dispersive lipophilic interactions
should be assisted rather than weakened by neutral nitrogen atoms. Investigations using other neutral substrates,
such as 2, confirm the unexpected order of stability
(Scheme l), although in these cases smaller differences are
possible owing to hydrogen bonds and polar groups. Aza
host compounds containing -NHT instead of -NMeT
units exhibit similar complexation constants. Apparently,
in contrast to previous assumptions, one should consider
the possibility of increased hydrophobic interactions, and
possibly special ion-n-system interactions,'2b1 instead of
hydrophilic interactions, in the vicinity of charged
groups.
Scheme I . Complexatlon energies AG" [kJ mol-'1 and selected CIS values
[-ppm] of host A in D20/CD30D (80:20). The complexat~onconstants of
3 / B were not determined, since the complex is not soluble in water.
The macrocyclic host compounds A and B are accessible by reaction of the corresponding tetraazacyclobisdiphenylmethane compounds with 1,3-benzenedisulfonyl dihalide and phthalic anhydride, respectively.[31 The cavities
[*I Prof. Dr. H.-J. Schneider, DipLChem. T. Blatter
Institut fur Organische Chernie der Universitat
D-6600 Saarbriicken 1 I (FRG)
I**]
Host-Guest Chemistry, Part 16. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. Part 15: H.-J. Schneider, D. Guttes, U. Schneider, J . A m . Chem.
Soc.. in press.
Angew. Chem. Int. Ed. Engl. 27 (1988) No. 9
Received: April 21, 1988 [Z 2716 IE]
German version: Angew. Chem I00 (1988) 121 1
[ I ] a) K. Odashima, K. Koga in P. M. Keehn, S. M. Rosenfeld (Eds.): Cyclophanes Vol. 2, Academic Press, New York 1983, p. 629; b) K. Odashima,
0 VCH Verlagsgeseiischafl mbH, 0-6940 Weinheirn, 1988
0570-0833/88/0909-1163 $ 02.50/0
1163
A Itai, Y. litaka, K. Koga, Tetrahedron Lett. 21 (1980) 4347; c) H:J.
Schneider, K. Philippi, J . Pohlmann, Angew. Chem. 96 (1984)907;Angew.
Chem. Int. Ed. Eng/. 23 (1984)908.
[2] We thank a referee for bringing to our attention recent publications on
other systematic investigations on the large effect of changes on hostguest complexes: a) F. Diederich, Angew. Chem. 100 (1988) 372 (especially p. 382); Angew. Chem. Int. Ed. Engl. 27 (1988) 362; b) T.J. Shepodd, M. A. Petti, D.A. Dougherty, J. Am. Chem. Soc llO(1988) 1983.
131 All new compounds gave satisfactory analytical data ( ' H and ''C NMR,
elemental analyses)
[4] H.-J. Schneider, R. Kramer, J. Pdhlmann, U. Schneider, J . Am. Chem.
Soc. 110 (1988), in press.
[5] H.-J. Schneider, R. Pohlmann, Bioorg. Chem. 15 (1987) 183.
161 Compare the hydrophobic increments of -NHCH3, whicheare roughly
five orders of magnitude larger compared with those of -NMe, substituents: C. Hansch, A. Leo: Substituent Constants for Correlation Analysis
in Chemistry and Biology, Wiley, New York 1979, p. 49ff.
+0.5 N2H,
(a)
1
+
0.5 N,
2
Transformation of ?*-Nitrosy1 to
q2-HydroxylaminylLigands in the Reduction of
IMO(NO)~(LS~')]
to [Mo(~~-NH~O)(NO)('S,')]:
A
Model Reaction for a Partial Step in the Enzymatic
[NO:-+ N H , ~Conversion**
By Dieter Sellmann,* Bernd Seubert, Matthias Moll, and
Falk Knoch
The transition-metal-catalyzed activation and reduction
of NO are important in both industrial''"] and enzymatic
reactions."'] In the treatment of exhaust gases, molecular
nitrogen is the desired end product; the biological
[NO:- NH3] conversion, however, avoids the formation of
the energetically favored N, molecule, and the reduction
of NO to the N H 2 0 H stage becomes the key step. The reduction of N O ligands to N H 2 0 H ligands in isolable complexes is therefore a n important model reaction for understanding the biological nitrogen cycle.
The synthesis of nitrosyl complexes by use of hydroxylamine is well known[2a1and, in one case, the reversible intramolecular transformation of NO to N H 2 0 ligands has
been achieved by protonation-deprotonation;IZb1the electrons involved were derived from either the N H 2 0 ligand
or the metal center. The intermolecular reduction of N O to
N H 2 0 ligands, however, has not been reported previously.l3] We have now observed such a reaction in the reduction of 1 with hydrazine according to Equation (a) (THF,
20"C, 2 h ; see Experimental Procedure).
N2 evolves and two configurational isomers of the
they are
N H 2 0 complex 2 are formed in equal
obtained as 2 . D M F upon recrystallization from DMF/
Et20. The isomer shown schematically in Equation (a) was
characterized by an X-ray structure analysis.[']
Figure 1 shows that the molybdenum center is sevenfold
coordinated by one oxygen, two nitrogen, and four sulfur
atoms, forming a distorted pentagonal bipyramid with
S3-Mol -N2 as the main axis. The [Mo(NO)('S,')] fragment
of the educt 1 remains intact; the replacement of the q'NO ligand by a side-on-bonded q 2 - N H 2 0ligand leads to a
further reduction in the Sl-Mol-S4 angle from 160.1(3)'
Dr.D. Sellmann, DipLChem. B. Seubert, Dr. M. Moll,
Dr. F. Knoch
lnstitut fur Anorganische Chemie der
Universitat Erlangen-Niirnberg
Egerlandstrasse I , D-8520 Erlangen (FRG)
('Sd')20= 2,2'-(ethylenedithio)dibenzenethiolate.-Transition
Metals
with Sulfur Ligands, Part 39: This work was supported by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen 1ndustrie.Part 38: D. Sellmann, H. Kunstmann, M. Moll, F. Knoch, Inorg. Chim.
Acra, in press.
[*] Prof.
["*I
1 I64
0 VCH VerlagsgesellschaJi mbH, 0-6940 Weinheim, 1988
in 116]to 154.0(1)' in 2 . D M F . The distances in the
[Mo(NH,O)] unit clearly establish the q 2 bonding of the
N H 2 0 ligand; the N O distance of 137.5(5) pm corresponds
to a normal N O single bond.''' One of the two H atoms
forms a hydrogen bond to a D M F solvate molecule
(dN-H...o= 184.5 pm).
All
C13
Fig. I . Molecular structure of 2 . D M F [5].
The formation of two configurational isomers is revealed by NMR spectroscopy. The I3C N M R spectrum
(67.940 MHz) shows two sets of fourteen signals each for
the fourteen C atoms of the ('S:) ligand; they are also observed at higher temperatures (up to 60°C in [DJTHF and
u p to 75°C in [D6]DMSO), thus showing that the isomers
are configurationally stable u p to these temperatures. The
'H N M R spectrum (Fig. 2), too, is in agreement only with
the presence of two isomers: Four lines, arising from the
two different AB systems, are observed for the NH, protons. The temperature dependence of the signals indicates
exchange processes.
The 9sMo N M R spectrum (17.061 MHz) exhibits only a
broad 95Mosignal at 6= -395 (rel. (NH4)6M07024)with a
half-width of 760 Hz; on the one hand, the half-width presumably prevents resolution of the expected two Mo signals; on the other, it shows that the two Mo centers have
very similar chemical environments and differ only in the
position of the N H 2 0 ligand in relation to the
[Mo(NO)('S4')] fragment.
The 1 : 1 ratio of the two isomers is understandable in
terms of the route of formation of 2 : the reduction 1 2
-
0570-0833/88/0909-1164 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 27 11988) No. 9
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