Malonate crown ethers as building blocks for novel D-π-A chromophores

Article abstract of DOI:10.1021/ol016798o

(matrix presented) A series of new crown ethers, incorporating a malonate ester functionality, have been synthesized and derivatized with π-electron rich aldehydes to give, conjugated, extended "push-pull" compounds. Their ability to bind Lewis acid-like

Full text of DOI:10.1021/ol016798o

ORGANIC
LETTERS
2
002
Vol. 4, No. 1
3-26
Malonate Crown Ethers as Building
Blocks for Novel D-π-A Chromophores
Dario Pasini, Pier Paolo Righetti,* and Valerio Rossi
2
Department of Organic Chemistry, UniVersity of PaVia, Viale Taramelli,
10-27100 PaVia, Italy
pasini@chifis.unipV.it
Received September 22, 2001
ABSTRACT
A series of new crown ethers, incorporating a malonate ester functionality, have been synthesized and derivatized with π-electron rich aldehydes
to give, conjugated, extended “push−pull” compounds. Their ability to bind Lewis acid-like metal cations, such as Mg2 and Eu , has been
characterized, and the relative stability constants are presented. When the metal cation is bound to the malonate moiety within the crown
ether cavity, the D-π-A character of the molecular structure is greatly enhanced.
+
3+
5
“
Push-pull” compounds, composed by a conjugated delo-
and later used as phase-transfer catalysts by others, but their
derivatization via manipulation of the malonate functionality
has, to our knowledge, never been explored. It was recently
reported that malonate compounds such as 1 or the corre-
sponding crown ethers 2 are able to form weak complexes
calized system in which one termini is electron-donating and
the other is electron-withdrawing, are becoming increasingly
popular for a series of material properties such as the design
of second-order NLO organic materials.1 In this context,
organometallic systems are gaining attention because of
several useful characteristics, such as, for example, the
presence of a metal center able to undergo redox changes
leading to enhanced molecular polarizabilities.1
,2
2
+
with Mg salts in organic solvents (Figure 1). The com-
c,3
Malonate-containing crown ethers were synthesized and
4
characterized in the late 1970s by Bradshaw and co-workers,
(
1) (a) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994,
9
9
2
1
4, 31-75. (b) Kanis, D. R.; Marks, T. J.; Ratner, M. A. Chem. ReV. 1994,
4, 195-242. (c) Long, N. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 21-
8. (d) Marks, T. J.; Ratner, M. A. Angew. Chem., Int. Ed. Engl. 1995, 34,
55-173.
(2) For a list of recent reports in the field, see: (a) Kenis, P. J. A.;
Noordman, O. F. J.; Sch o¨ nherr, H.; Kerver, E. G.; Snellink-Ru e¨ l, B. H.
M.; van Hummel, G. J.; Harkema, S.; van der Vorst, C. P. J. M.; Hare, J.;
Picken, S. J.; Engbersen, J. F. J.; van Hulst, N. F.; Vancso, G. J.; Reinhoudt,
D. N. Chem. Eur. J. 1998, 7, 1225-1234. (b) Alain, V.; Blanchard-Desce,
M.; Ledoux-Rak, I.; Zyss, J. Chem. Commun. 2000, 353-354. (c) Lin, W.;
Ma, L.; Evans, O. R. Chem. Commun. 2000, 2263-2264. (d) Moore, A. J.;
Chesney, A.; Bryce, M. R.; Batsanov, A. S.; Kelly, J. F.; Howard, J. A. K.;
Perepichka, I. F.; Perepichka, D. F.; Meshulam, G.; Berkovic, G.; Kotler,
Z.; Mazor, R.; Khodorkovsky, V. Eur. J. Org. Chem. 2001, 2671-2687.
Figure 1. Model compound 1 and malonate crown ethers 2a-c.
plexation was inferred to involve directly the malonate
1
13
moiety by both H and C NMR spectroscopic studies and
also by an enhanced reactivity, in the presence of catalytic
amount of the salt, of the exocyclic double bond in Diels-
(
e) Gangopadhyay, P.; Radhakrishnan, T. P. Angew. Chem., Int. Ed. 2001,
0, 2451-2455. (f) Aqad, E.; Leriche, P.; Mabon, G.; Gorgues, A.;
Khodorovsky, V. Org. Lett. 2001, 3, 2329-2332.
4
6
Alder reactions.
1
0.1021/ol016798o CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/13/2001

Since these compounds are composed of a π-electron-
donating aromatic unit and an electron-withdrawing group
diglycolyl dichloride afforded crown ether 5 in good yields
(25%).
(
the malonate ester functionality) which could be further
Functionalization of the malonate moiety via Knoevenagel
condensation was carried out with piperidinium acetate as a
catalyst, either with 4-dimethylaminobenzaldehyde or ferro-
cenecarboxaldehyde. Purification by column chromatography
afforded compounds 6-9 in 70-90% yield as orange (6, 7)
or deep red (8, 9) compounds. The UV-vis spectra of these
compounds showed an intramolecular charge-transfer band
polarized by the coordination of a metal cation, we set out
to investigate the possibility of assembling more stable
complexes between crown ethers containing a malonate
moiety and selected Lewis acid metal cations in order to
ascertain possible future applications as D-π-A chromophores
in functional devices.
Crown ether 3 was obtained in good yields (40%) by the
macrocyclization reaction of malonyl dichloride and hexa-
ethylene glycol in high dilution conditions. We also pursued
(
ICT) as direct evidence of their extended, dipolar nature,
centered at 380 nm for compounds 6 and 7, and at 480 nm
for compounds 8 and 9 (see also Figure 2).
7
the introduction of further “soft” carbonyl functionalities
while preserving the overall dimension of the crown ether
structure at 22 atoms. Reaction of malonyl dichloride
Upon addition of Mg2+ as a chelating metal cation into
MeCN solutions of 1 or 7, the ICT (centered at 380 nm) is
drastically shifted (ca. 100 nm), indicating an additional
polarization of the system upon coordination of the metal
(Scheme 1) with diethylene glycol monobenzyl ether, and
8
cation to the 1,3-dicarbonyl system. Albeit we explored the
qualitative behavior of other salts (such as Co(II), Cu(II),
and Fe(II) salts), which proved to be similar in terms of UV/
vis spectroscopic behavior, we set out to determine the
thermodynamic stability constants between the above-
mentioned crown ethers and magnesium perchlorate (MP)
and europium triflate (ET), since these salts have good
solubility in MeCN and their cations represent two opposites
Scheme 1a
9
in terms of ionic radii and polarizability.
The stability constants of the ligand-metal complexes
could be evaluated in MeCN at 25 °C by means of UV/vis
spectrophotometric titrations. Isosbestic points were found
in all titrations, strongly indicating the presence of a unique
1
0
complex in solution.
Two examples of such titrations are shown in Figure 2.
The values for the stability constants, together with the
observed λmax for the ICT band associated with the forming
complexes, are reported in Table 1. In the case of Mg-
1
1
(
ClO
detectable but too weak to be determined with sufficient
precision (log k < 1). A similar experiment between 1 and
ET revealed instead a much stronger binding (see Table 1,
4
), binding with model compound 1 was clearly
a
1
2
a
entry 1).
Reagents and conditions: (i) diethylene glycol monobenzyl
2
ether, PhH, ∆, then H , 10% Pd/C, EtOH, rt; (ii) hexaethylene
glycol, PhH, ∆; (iii) diglycolyl dichloride, PhH, ∆; (iv) 4-di-
methylaminobenzaldehyde or ferrocenecarboxaldehyde, piperi-
dinium acetate, PhH, ∆.
(
5) Anchisi, C.; Corda, L.; Fadda, A. M.; Maccioni, A. M.; Podda, G. J.
Heterocycl. Chem. 1988, 25, 735-737. See also: Ninagawa, A.; Maeda,
T.; Matsuda, H. Chem. Lett. 1984, 1985-1988.
(6) Desimoni, G.; Faita, G.; Ricci, M.; Righetti, P. P. Tetrahedron 1998,
5
4, 9581-9602.
7) Bradshaw, J. S.; Jolley, S. T.; Jones, B. A. J. Heterocycl. Chem.
1980, 17, 1317-8.
8) Coordination of the metal cation to the 1,3-dicarbonyl system is
further substantiated by C NMR studies of crown ether 7 in the presence
of increasing amounts of magnesium perchlorate or of europium triflate in
d3-MeCN, similarly to what reported in ref 6.
(
subsequent deprotection by hydrogenolysis, afforded com-
pound 4 in good yields, and high dilution cyclization with
(
13
(
3) For selected recent examples, see: (a) Barlow, S.; Marder, S. R.
Chem. Commun. 2000, 1555-1562. (b) Jayaprakash, K. N.; Ray, P. C.;
Matsuoka, I.; Bhadbhade, M. M.; Puranik, V. G.; Das, P. K.; Nishihara,
H.; Sarkar, A. Organometallics 1999, 18, 3851-3858. (c) M u¨ ller, T. J. J.;
Netz, A.; Ansorge, M.; Schm a¨ lzlin, E.; Br a¨ uchle, C.; Meerholz, K.
Organometallics 1999, 18, 5066-5074. (d) Di Bella, S.; Fragal a` , I.; Ledoux,
I.; Zyss, J. Chem. Eur. J. 2001, 7, 3738-3743.
(9) For determination of stability constants between 18-crown-6 and
lanthanide salts, see: (a) B u¨ nzli, J.-C.; Wessner, D. HelV. Chim. Acta 1981,
64, 582-596. (b) B u¨ nzli, J.-C.; Pilloud, F. Inorg. Chem. 1989, 28, 2638-
2642. (c) Piguet, C.; B u¨ nzli, J.-C. Chem. Soc. ReV. 1999, 28, 347-358.
See also: Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Brueniong, R. L. Chem.
ReV. 1991, 91, 1721-2085.
(
4) (a) Bradshaw, J. S.; Bishop, C. T.; Nielsen, S. F.; Asay, R. E.;
(10) Connors, K. A. Binding Constants; Wiley: New York, 1987.
(11) The possibility of using magnesium triflate was prevented by its
almost total insolubility in MeCN.
Masihdas, D. R. K.; Flanders, E. D.; Izatt, R. M.; Chrisensen, J. J. J. Chem.
Soc., Perkin Trans. 1 1975, 2505-2508. (b) Izatt, R. M.; Lamb, J. D.; Mass,
G. E.; Asay, R. E.; Bradshaw, J. S. Christensen, J. J. J. Am. Chem. Soc.
(12) Titration experiments were usually conducted adding increasing
amounts of metal salt to a solution at costant concentration of crown ether.
In the case of ligand 1, a titration was also conducted adding increasing
amounts of ligand 1 to a solution at costant concentration of ET, obtaining
essentially identical results in the calculation of the association constant.
1
977, 99, 2365-6. (c) Lamb, J. D.; Izatt, R. M.; Swain, C. S.; Bradshaw,
J. S.; Christensen, J. J. J. Am. Chem. Soc. 1980, 102, 479-482. (d) Izatt,
R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem.
ReV. 1985, 85, 271-339.
24
Org. Lett., Vol. 4, No. 1, 2002

4 2 3
Figure 2. UV/vis spectrophotometric titrations between compound 7 and Mg(ClO ) (left) and compound 9 and Eu(OTf) (right).
1
0
A Job plot experiment between 1 and Eu(OTf)
3
in MeCN
In the case of ferrocene-derived compounds 8 and 9, the
ICT upon complexation undergo a more limited bathocromic
shift (ca. 50 nm). The association constants with ET also do
show a somewhat reduced stability when compared with 6
and 7, probably as a consequence of both steric and electronic
effects related to the different substituents. The maximum
of the ICT band upon complexation (530 nm) is well above
that of compounds 6 and 7, suggesting higher NLO responses
at 25 °C revealed a 1:1 stoichiometry, and a similar
experiment also confirmed a 1:1 stoichiometry for the
complex between 7 and ET, suggesting that the three triflate
counterions are directly involved in the coordination sphere
of the metal cation and are therefore able to statisfy the high
coordination number of Eu . Mass spectrometric analysis
of two solutions containing different ratios between 7 and
ET (1.5:1 and 1:3 of 7:ET in MeCN) showed that also in
the gas phase the 1:1 stoichiometry is the preferred one, as
revealed by the peak at 960 m/z, corresponding to the 1:1
complex with the loss of the triflate counterion (Figure 3).
The results displayed in Table 1 show how crown ethers
3+ 13
1c
for these compounds.
The introduction of further carbonyl functionalities (7 and
9
vs 6 and 8) do not seem to bring any additional
stabilization. There are, however, substantial differences
regarding the maximum wavelength for the ICT of the 1:1
complex, both in the case of ET and MP. Whereas with
crown ether 7 the above-mentioned values are very close to
6
and 7 do form more stable complexes (ca. 1 order of
magnitude) with ET than the corresponding, D-π-A open
ligand 1 (entries 3 and 5 vs 1 in Table 1). There is also a
marked selectivity in the binding of ET vs MP.
Table 1. Association Constants between Compounds 1 and
6
-9 and Europium Triflate (ET) and Magnesium Perchlorate
a
(MP) in MeCN at 25 °C
entry
ligand
log ka
λmax (nm)b
c
1
2
3
4
5
6
7
1
6
6
7
7
8
9
2.60 ( 0.05 (ET)
478
430
450
466
476
530
530
c
2.0 ( 0.2 (MP)
c
3.9 ( 0.1 (ET)
c
1.2 ( 0.05 (MP)
c
3.8 ( 0.1 (ET)
c
3.5 ( 0.1 (ET)
c
3.2 ( 0.1 (ET)
a
All values for the stability constants are in M-1. The values reported
are the average of at least two independent titrations, with all nonlinear
2
b
regression giving high confidence outputs (r > 0.995). Maximum for
c
the ICT band related to the metal cation:ligand complex. MP indicates a
value obtained by titration with magnesium perchlorate, whereas ET
indicates a value obtained by titration with europium triflate.
Figure 3. Mass spectrum (ESI) of a 1:3 solution of crown ether
7:ET in MeCN.
Org. Lett., Vol. 4, No. 1, 2002
25

those recorded with model compound 1, in the case of 6
these values are somewhat distant. We speculate this is an
indication of a more or less optimal interaction with the 1,3-
dicarbonyl chromphore moiety by the metal cation, probably
properties of lanthanides, their possible utilization as light
harvesting or energy converting devices.
1
5
Acknowledgment. We thank the University of Pavia
(FAR 2000-2001) for funding and Dr. F. Corana (Centro
Grandi Strumenti, University of Pavia) for the mass spetro-
metric analyses described above.
14
as a result of subtle differences in complexation geometry.
In conclusion, we have synthesized and characterized novel
functionalized, “push-pull” crown ethers which are able to
form thermodinamically stable 1:1 complexes with ET in
MeCN solutions. Their structures are however crucial in
order to obtain and maximize interaction between the metal
cation and the cromophore moieties of the compounds.
We are currently evaluating the NLO responses of these
complexes and, given the noteworthy absorption/emission
Supporting Information Available: Full experimental
procedures for compounds 5-9, together with detailed
experimental procedure for the determination of the binding
constants and selected examples of titration curves. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(
13) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry;
Wiley: New York, 1988.
14) Albeit compound 7 is crystalline, all attempts to grow good quality
single crystals of the complex with ET failed.
OL016798O
(
(15) Piguet, C.; B u¨ nzli, J.-C. Chem. Soc. ReV. 1999, 28, 347-358.
26
Org. Lett., Vol. 4, No. 1, 2002