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Crown Ethers with a Lewis Acidic Center A New Class of Heterotopic Host Molecules.

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[5] S. Shambayati, W. E. Crowe, S. L. Schreiber. Trrruhedron Lert. 31 (1990)
[6] D. C. Billington, I. M. Helps, P. L. Pauson, W. Thomson. D. Willison, 1
Orgunomer. Chem. 354 (1988) 233.
[7] S. C. Brown, P. L. Pauson, 1 Chem. Sor. Perkin Trans. I 1990, 1205.
[S] N. E. Schore, M. C. Croudace. J; Org. Chem. 46 (1981) 5436.
[91 V. Rautenstrauch. P. Megdrd, J. Conesa. W Kuster. Angew. Chem. 102
(1990) 1441; .4ngew. Chem. Inr. Ed. Engl. 29 (1990) 1413.
[lo] W. D . Wulff, K. S. Chan. J. A m . Chem. SOC. I08 (1986) 5229.
[I I ] a) F. Camps, J. M. Moreto. S. Ricart. J. M. Vifias, E. Molins, C. Mirdvitlles, J; Chrm. So<.Chem. Commun. 1989.1560; b) F. Camps, A. Llebaria,
J. M. Moreto, S. Ricart, J. M. Visas, Tefruhedron Leu. 31 (1990) 2479;
c) K. L. Faron, W. D. Wulff, J. Am. Chem. Soc. 110 (1988) 8727; d) E. 0.
Fischer. H. J. Kalder. J. Organomer. Chem. 131 (1977) 57; e) F. Camps, A.
Llebaria, J. M. Moreto, S. Ricart. J. M. Vifias, J. Ros, R. Yafiez, ibid. 401
(1991) C 17; f ) H. Fischer. T. Meisner, J. Hofmann, Chem. Ber. 123 (1990)
[I21 K. H. Dotz, R. Noack, K . Harms, G. Muller, Tixruhedron46 (1990) 1235.
1131 M. E. Krafft, C. A. Juliano. 1. L. Scott. C. Wright, M. D. McEachin. J.
Am. Chem. SOC.113 (1991) 1693.
[14] The structural assignment has been made on basis of N M R data as well as
structural and mechanistic considerations. Particular significance has been
given to the different coupling constants for 2 d and 2e for the bridgehead
proton with the vicinal =-ketonic proton. These values ( J = 4.2 Hz for 2 d
and J = 8 Hz for 2e). although slightly higher than those reported by
Schore et al. for related organic structures (N. E. Schore, M. J. Kundsen,
J. Org. Client. 52 (1987) 569). are complementary and in the same order of
assignment (the larger for the cis-vicinal coupling constant and the smaller
for the corresponding fruns).Furthermore, the stereoisomers thus obtained
are those which are to be expected starting from a /ran.? olefin (E-crotylamine) and a c i . ~one (2-cyclohexenylamine) in a stereospecific process.
[IS] K. H . Dotz. Angew. Chem. 96 (1984) 573; Angew. Chrm. I n [ . Ed. Engl. 23
( I 984) 587.
[16] A. Wienandt. H. U. Reissig. Orgunomerullics 9 (1990) 3133. and references
Crown Ethers with a Lewis Acidic Center:
A New Class of Heterotopic Host Molecules**
By Manfred 7: Reetz,* Christof M . Miemeyer,
and Klaus Harms
Many cation-selective crown ethers and cryptands are
known,"' but only a few host compounds bind anions selectively. Naturally, the latter are electronically inverse macrocycles that usually contain suitably positioned protonated or
quaternary nitrogen functions.c21Here we report a new class
of anion-selective receptors, 2, which in addition to a conventional crown ether moiety for complexation of cations,
contain a o-bonded Lewis acidic metal center-in this case,
boron- for complexation of anions. Our concept is based
on the expectation that, for example, potassium salts, KX
(X = F, CI, Br, I, SCN, CN, OCH,) should be able to bind
to the host 2 in either a monotopic (3) or a heterotopic (4)
fashion. The selectivity would then be governed by quite
diverse factors, such as the strength of the B-X bond in the
ate complex 4, the lattice energy of the salt KX, Coulomb
and van der Waals interactions, and solvent effects.
Since potassium salts were to be investigated first, we
chose the 21-membered crown ether 2, which contains six
oxygen atoms. The aryl bromide 1 a was used for its synthes ~ s . ' The
~ ] corresponding aryllithium compound was borylated with B(OCH,), and the crude product was hydrolyzed to
Prof. Dr. M. T, Reetz ['I, DipLChem. C . M. Niemeyer, Dr. K. Harms
Fachbereich Chemie der Universitlt
Hans-Meerwein-Strasse, W-3550 Marburg (FRG)
[ '1 New address: Max-Planck-Institut fur Kohlenforschung
Kaiser-Wilhelm-Platz. W-4330 Mulheim a.d. Ruhr (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft (Leibniz-Program) and the Fonds der Cheniischen Industrie.
VCH Verlugsgesellschafi mbH, W-6940 Weinheim, 1991
1. tBuLi
la, R = Br
lb. R = H
CI; c, X = B r ; d, X = I
e, X = SCN; f, X = CN; g, X = O C H , ;
a, X = F; b,
give the arylboronic acid.151Treatment of the acid with catechol finally led to the target compound 2 (> 95 % overall
yield), which is stable and easy to handle under an inert
In the first complexation experiment, host 2 was added to
a suspension of dry K F (excess) in dichloromethane at room
temperature. A stoichiometric amount of the otherwise insoluble salt dissolved within 4 h with quantitative formation
of adduct 4a. This is noteworthy since potassium-specific
crown ethers such as [18]~rown-6[~]
or 1,3-xyly1[21]crown-6
(1 b)"] complex at most catalytic amounts of KF.I*] In the
case of 4a, the strength of the B-F bondcg1is decisive. The
usual interaction between K0 and the oxygen atom of the
crown ether acts synergistically, since a control experiment
with K F and the catechol ester of phenylboronic acid afforded no adduct."'. ' ' ]
The host-guest compound 4 a was investigated in solution
particularly by "B and I3C N M R spectroscopy. The "B
N M R spectrum (CD,CI,) at room temperature with BF,ether as external standard shows a single signal at 6 = 10.0,
which means a shift of A6 = 20 to higher field compared
with the signal of the uncomplexed host 2 (6 = 30). This is
characteristic of compounds with tetracoordinated boron.['21The participation of K Q as guest is shown by a comparison of the ',C N M R spectra of the free host 2 and the
adduct 4a (Fig. 1). The low-field shift of the signals of the
benzylic C atoms (C7 and C18) and the high-field shift of the
other ether C atoms are definite indications of cation binding.c'31The fact that the spectra of mixtures of compounds
2 and 4a exhibit sharp signals shows that, under these conditions, rapid exchange processes d o not occur on the N M R
time scale. The "F N M R (6 = - 124.4; CFCI, as external
standard) and the 'H N M R spectrum are also consistent
with structure 4a.
Figure 2 shows the result of an X-ray structure analysis of
4a.[I4]The inclusion of K 0 and F' is clearly evident (Fig. 2,
top), as are intermolecular interactions between the units of
the host-guest compound (Fig. 2, bottom). The "unsymmetrical" structure (Fig. 2, top) could be due to a crystal packing
effect, since the ',C N M R spectrum of 4a in solution does
not show a double set of signals in the range of 40°C to
- 80 "C.
0570-0833/91/llll-1472$ 3 . 5 0 + . 2 5 / 0
Angew. Chem. In[. Ed. Engl. 30 (1991) No. I1
The salts KCI and KBr do not undergo monotopic or
heterotopic interactions with 2, even after a reaction period
of two weeks. This behavior corresponds to that of crown
ether 1 b.[' We ascribe this to the fact that K@complexation
is not sufficiently favored energetically and to the weaker
B-CI and B-Br
By contrast, monotopic binding is
observed for KI and KSCN (formation of3d and 3e, respectively). After two days, the respective 13C NMR spectra
show the signal shifts characteristic of K@ inclusion,['3]
whereas the position of the "B NMR signal has hardly
changed (3d, 6 = 29.9; 3e, 6 = 29.4). The formation of the
host-guest compound 3 is energetically favored in these cases
by the usual K@complexation.[161This interpretation is consistent with the observation that the reaction of crown ether
1 b with KI or KSCN leads to inclusion of K@.In the reactions of host 2 with KCN (reaction time 2d) and KOCH, (1 h),
4f" '1 and 4g, respectively, are formed nearly quantitatively.
The "B NMR spectra (4f, 6 = 10.7; 4g, 6 = 10.6), as well as
3C NMR signal shifts characteristic of K@binding, confirm
a heterotopic behavior.
The selectivity described here for the series of potassium
salts KX has not been previously observed for any other
receptor." - 3l Selectivity in competition experiments is also
possible. For example, if 2 is allowed to react with a mixture
of KF, KCI, KBr, and KI the K F adduct 4a is formed exclusively within two days. Host 2 is also capable of distinguish-
Fig. 2. Top: Asymmetric unit of 4 a in the crystal (H atoms omitted). Bottom:
Section of the packing of 4 a in the crystal (H atoms omitted).
ing between potassium salts leading to heterotopic hostguest compounds 4. Thus, upon reacting the host 2 with a
only 4a
KF/KCN mixture for a sufficient length of
is formed. The suitability of host compounds of type 2 with
respect to synergistic effects in molecular recognition of organic compounds is the subject of further investigation^.*'^^
Received: June 10, 1991 [Z 4689 IE]
German version: Angew. Chem. 103 (1991) 1515
CAS Registry numbers:
2, 136426-4; 3d, 136577-16-7; 3% 136577-18-9;4a. 136577.15-6: 4f. 13657719-0; 4g, 136577-20-3.
[l] a) D. J. Cram, Angew. Chem. lOO(1988) 1041;Angew. Chem. I n / . Ed. Engl.
27(1988) 1009; b) C. J. Pedersen, ibid. lOO(1988) 1053and 28(1988) 1021;
c) J.-M. Lehn, ibid. 100 (1988) 91 and 27(1988) 89; ibid. 102 (1990) 1347
and 29 (1990) 1304; d) E. Weber, F. Vogtle, Top. Curr. Chem. 98 (1981) 1;
e) Y. Takeda, ibid. 121 (1984) 1.
[2] a) Ammonium or guanidinium hosts: J.-M. Lehn, E. Sonveaux, A. K.
Willard, J. Am. Chem. Sor. 100(1978)4914; F. P. Schmidtchen, G. Muller.
J Chem. Soc. Chem. Commun. 1984, 1115; Y. Murakami, J. Kikuchi. T.
Ohno, 0.Hayashida, M. Kojima, J. Am. Chem. Sor. 112 (1990) 7672; J.-M.
Fig. I . "C NMR spectroscopic investigation of the reaction of host 2 with K F
l o give adduct 4 a : a) Host 2 ; b) after about 35% uptake of KF; c) after about
65% uptake of KF; d) host-guest compound 4a.
Angm. Chiw. hi.Ed. EngI. 30 (1991) N o . 11
Lehn, R. Meric, J.-P. Vigneron, I. Bkouche-Waksman, C . Pascard. J.
Chem. Soc. Chem. Commun. 1991,62; A. Galan, E. Pueyo, A. Salmeron.
J. de Mendoza, Tetrahedron Leu. 32 (1991) 1827; b) macrocylic Lewis
acids: J. D. Wuest, B. Zacharie, J. Am. Chem. Soe. 109 (1987) 4714; M. T.
Blanda, J. H. Homer, M. Newcomb, J. Org. Chem. 54 (1989) 4626, M. E.
Jung, H. Xia, Tetrahedron Lett. 29 (1988) 297; A. R. Van Doorn, M. Bos,
S . Harkema, J. van Eerden, W. Verboom, D. N. Reinhoudt, J. Org. Chem.
56 (1991) 2371.
Schmidtchen has synthesized heterotopic compounds that contain a crown
ether moiety and an ammonium macrocycle: F. P. Schmidtchen, Tetrahedron Lea. 25 (1984)4361; F. P. Schmidtchen,J Org. Chem. 51 (1986) 5161.
M. Newcomb, S. S . Moore, D. J. Cram, J. Am. Chem. Soc. 99 (1977) 6405.
Reinhoudt et al. have carried out a similar borylation and have oxidized the
resulting, nonisolated boronic acid to the corresponding phenol: M.
Skowronska-Ptasinska. V. M. L. J. Aarts, R. J. M. Egberink, J. van
Eerden, S . Harkema, D. N.Reinhoudt, J. Org. Chem. 53 (1988) 5484.
According to Liotra et al., [18]crown-6 binds potassium fluoride (as a
suspension in benzene or acetonitrile) only to the extent of 2-5%. which
Verlagsgeselischufi mbH. W-6940 Weinheim, 1991
is nonetheless sufficient to accelerate S,2 reactions: C. L. Liotta, H. P.
Harris, J. Am. Chem. SOC.96 (1974) 2250.
[7] D. N. Reinhoudt, R. T. Gray, Tetrahedron Let/. 197S, 2105.
[8] When the suspension of K F in CH,CI, is allowed to react with 1,3-xylyl[2l]crown-6 1 b for several days under ultrasonication, the ‘H and I3C
NMR spectra of the solution show no evidence of binding of K@ions.
191 Since dative bonds are formed in the ate complexes, the usual values of the
bonding energies, which refer to diatomic molecules, cannot be used (cf. A.
Haaland, Angew. Chem. 101 (1989) 1017; Angew. Chem. In/. Ed. Engl. 28
(1989) 1010). The usual trend toward weaker bonding in the order
B-F > B-CI > B-Br > B-I probably still holds, however.
[lo] a) Tris(dimethy1amino)sulfonium fluoride reacts with 1 ,g-naphthalenediylbis(dimethy1borane) to form an adduct: H. E. Katz, J. Org. Chem. 50
(1985) 5027; b) [18]crown-6 binds the counterion of pentacoordinated i l icon compounds: J. L. Brefort, R. J. P. Corriu, B. J. L. Henner, W. W. C. W.
Chi Man, Organometallics 9 (1990) 2080.
[Ill The mixture containing the catechol ester of phenylboronic acid and 1,3xylyl[2l]crown-6 (1 b) also effects the dissolution of KF. The synergistic
effect, however, is smaller. For example, if adduct 4 a is allowed to react
with a 1:1 mixture containing 1b and the catechol ester of phenylhoronic
acid, an equilibrium is established in favor of 4 a (z70:30).
[I21 H. Noth, B. Wrackmeyer, NMR: Basic Princ. Prog. 14 (1978).
[I31 M. C. Fedarko, J Magn. Reson. 12 (1973) 30; D. Live. S. 1. Chan, J Am.
Chem. Soc. 98 (1976) 3769.
[14] Crystallographic data for 4a: (C,,H,,BFKO,, M,= 516.4): monoclinic,
space group P2,/n, a = 1022.8(2), b = 2075.7(4), c = 1238(2)pm, p =
106.05(3)”, Z = 4,
= 1.357 g ~ r n - ~
= 23.05 cm-’. Measurement carried out on an Enraf-Nonius CAD4 diffractometer (Cu,.
radiation, 1= 1.54184 A, graphite monochromator, room temperature);
2379 measured reflections, 2164 of which were independent (R,,,=
0.0355). 1893 with F > 3u(F) were regarded as observed. Solution by direct
methods and refinement with Semen’s-SHELXTL-PLUS(VMS) program
package, R = 0.0479, R, = 0.0355 (o= 1/u2(F)),all non-hydrogen atoms
anisotropically, H atoms “riding” with groupwise common isotropic temperature factors. Empirical correction of the measurement data with the
program DIFABS (N. Walker, D. Stuart, Acia Crystallogr. Sect. A 39
(1983) 158). Further details of the crystal structure investigation are available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technischeInformation mbH, W-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD320264, the names of the authors, and the journal citation.
No complexation could be observed by NMR spectroscopy. This also
holds for samples heated under ultrasonification.
Why the soft anions do not bind to the hard boron in 3d-e is explained,
among other things, by the weak B-I- and B-S bonds as well as by possible
steric solvent effects.
In the case of KCN, complex 4 1 was formed in about 95% yield; in
addition, 5 % of a further, unidentified ate complex was obtained.
According to the NMR spectra, different adducts are formed during the
first hours of reaction. ‘H, I3C and IIB NMR spectra recorded after 10 d
show only the presence of 4a.
M. T. Reetz, C. M. Niemeyer, K. Harms, Angew. Chem. 103(1991) 1517;
Angew. Chem. Inr. Ed. Engl. 30 (1991) 1474.
work was motivated by the hope that compounds with one
acceptor site (e.g., a metal centerr3])and several donor sites
(e.g., ether functions) might be potential hosts for alcohols
and amines, because, after proton transfer, the alkoxide anion
and the ammonium cation could bond through reversible dative and hydrogen bonds, respectively (Scheme 1). Such bind-
Scheme 1. Simultaneous molecular recognition of amines and alcohols (M
metal center with empty orbital; D =donor site with occupied orbital).
ing would therefore involve heterotopic host molecules whose
Lewis acid/base property affects the equilibrium of the
Bronsted acid/base pair (alcohol and amine; see Scheme 1).
In an initial experiment the boron-containing crown ether
lr4I was allowed to react with a 1: 1 mixture of methanol and
benzylamine in dichloromethane. A spontaneous reaction
ensued, leading to the formation of 2. The assumption that
methoxide bonds to boron and the ammonium ion to the
crown ether moiety was supported by B, ‘H, and 13CNMR
PhCH&Tl& + C b O H
Heterotopic Host Molecules for Binding
Two Different Guests **
By Manfred T. Reetz,* Christof M . Niemeyer,
and Klaus Harms
An important aspect of host/guest chemistry is the simulation of biological systems.“] It is all the more surprising,
therefore, that practically no host molecules capable of
recognizing and binding two different organic guest molecules have been synthesized.[‘-31 Here we describe a new
concept of molecular recognition in which alcohols and
amines are recognized simultaneously and selectively. This
[*] Prof. Dr. M. T.Reetz [‘I, Dip1.-Chem. C. M. Niemeyer, Dr. K. Harms
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, W-3550 Marburg (FRG)
[ ‘1 New address: Max-Planck-Institut fur Kohlenforschung
Kaiser-Wilhelm-Platz, W-4330 Mulheim a. d. Ruhr (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft (Leibniz Program) and the Fonds der Chemischen Industrie.
Verlagsgeselkchafl mbH, W-6940 Weinheim, 1991
The X-ray structure analysisr6]provided final confirmation and also brought to light some structural details. The
ammonium ion is completely embedded in the crown ether
moiety, as a space-filling picture reveals (Fig. 1b[’]). This
structural element is unique, since ammonium ions are usually situated “outside” the crown ether and form hydrogen
bonds to the ether oxygen
In this case, however,
the ammonium ion is drawn deeply into the host owing to
the formation of a hydrogen bond to the methoxide. The N
atom is located exactly in the average plane of the crown
ether atoms, the maximum deviation of the ring atoms being
f 0.8
This type of communication between the two
guests leads to a rotaxane-type structure.
To determine whether host 1 can distinguish
between different alcohols and amines, we first carried out competition
experiments involving benzylamine and, in each case, two
alcohol^.^^^ The results summarized in Table 1 establish that
steric factors are mainly responsible for the molecular recognition.“’] Thus, host can distinguish between
and ethanol (73 YOin favor of the methanol adduct). Control
0 5 7 0 - 0 8 3 3 ~ 9 l / l t l l - l 4 7 48 3.50+ .25/0
Angew. Chem. Ini. Ed. Engl. 30 (1991) No. 11
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