Novel chiral (salen)Mn(iii) complexes containing a calix[4]arene unit in 1,3-alternate conformation as catalysts for enantioselective epoxidation reactions of (Z)-aryl alkenes

Article abstract of DOI:10.1039/c3dt52550c

Two new chiral calix[4]arene-salen ligands 1a,b, based on calix[4]arene platforms in 1,3-alternate conformation, have been prepared by a new general synthetic pathway. Their Mn(iii) complexes, 3a,b, have shown fairly good efficiency in the asymmetric epoxidation of styrene and substituted styrenes, whereas excellent catalytic activity and selectivity were observed with rigid bicyclic alkenes, namely 1,2-dihydro-naphthalene and substituted 2,2′-dimethyl-chromene. The higher catalytic properties of 3a may be ascribed to the more rigid and inherently chiral structure as proved by molecular modelling, NMR spectroscopy and X-ray data of the similarly structured UO₂ complexes 2a,b.

Full text of DOI:10.1039/c3dt52550c

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PAPER
Novel chiral (salen)Mn(III) complexes containing a
calix[4]arene unit in 1,3-alternate conformation as
catalysts for enantioselective epoxidation
reactions of (Z)-aryl alkenes†
Cite this: Dalton Trans., 2014, 43,
2183
Carmela Bonaccorso,a Giovanna Brancatelli,b Francesco P. Ballistreri,a
Silvano Geremia,b Andrea Pappalardo,a Gaetano A. Tomaselli,*a Rosa M. Toscanoa
and Domenico Sciotto*a
Two new chiral calix[4]arene-salen ligands 1a,b, based on calix[4]arene platforms in 1,3-alternate confor-
mation, have been prepared by a new general synthetic pathway. Their Mn(III) complexes, 3a,b, have
shown fairly good efficiency in the asymmetric epoxidation of styrene and substituted styrenes, whereas
excellent catalytic activity and selectivity were observed with rigid bicyclic alkenes, namely 1,2-dihydro-
naphthalene and substituted 2,2’-dimethyl-chromene. The higher catalytic properties of 3a may be
ascribed to the more rigid and inherently chiral structure as proved by molecular modelling, NMR
spectroscopy and X-ray data of the similarly structured UO2 complexes 2a,b.
Received 16th September 2013,
Accepted 13th November 2013
DOI: 10.1039/c3dt52550c
www.rsc.org/dalton
generated in situ, supposed to be an oxo-Mn(V)-(salen) species
Introduction
(oxene), is considered to have a non-planar structure.2–4
Salen ligands (salen = N,N′-bis(salicylidene)ethylenediamine
The calix[4]arene platform provides peculiar arrangements
dianion) are structures of great value in homogeneous cata- of the binding/catalytic site in a complementary array for the
lysis;1 the pioneering work of Jacobsen and Katsuki paved the recognition/transformation of a potential substrate. Owing to
way to one of the most elegant protocols for the enantio- their capability of complexation and their selectivity, calix-
selective epoxidation of unfunctionalized alkenes catalyzed by arenes have been exploited as ion carriers,5,6 as analytical
chiral non-racemic manganese (salen) complexes.2
reagents,5,7 as catalytic systems8 and in particular as models
The control of both the substrate approach to the catalytic for mimicking the action of enzymes.9
site and its orientation upon approach is one of the central
In previous work we have reported the synthesis of (salen)-
problems to obtain high enantioselectivity with unfunctiona- Mn(III) and (salen)UO2 complexes containing a calix[4]arene
lized alkenes.2,3 Low energy non-bonding interactions and unit in the ligand framework. NMR studies and X-ray measure-
steric repulsion account for rate differences and enhanced ments of these (salen) UO2 complexes, assumed to be struc-
selectivity of the (salen) derivatives. The stereochemistry of the tural models of oxene, indicated a good correlation between
(salen) diimine bridge and the presence of suitable substitu- the conformations in solution and in the solid state and
ents in the ligand determine the preferred approaching allowed us to suggest a rationale of epoxidation enantio-
pathway of alkene. The two stereogenic centres of the bridge, selectivity, based on possible relevant conformations of oxene
close to the metal site, induce flexibility and the (salen) ligand itself and on the identities of alkenes.10 In the present paper
can adopt a non-planar structure; also the real oxidant species we report the synthesis and characterization of two new calix-
[4]arene-(salen) systems (1a,b), wherein the calix[4]arene
scaffold is fixed in 1,3-alternate conformation and is linked to
the salen-type moiety by spacers of different lengths. Further-
aDipartimento di Scienze Chimiche, Università di Catania, viale A. Doria 6, 95125
Catania, Italy
bCentro di Eccellenza in Biocristallografia, Dipartimento di Scienze Chimiche e
more we have prepared both the corresponding manganese
complexes (3a,b), which have been used as catalysts for asym-
metric epoxidation reactions of model unfunctionalised
alkenes, and their uranyl-(salen) derivatives (2a,b), which have
been employed as models of the oxo-Mn(V)-(salen) oxidant
species.
Farmaceutiche, Università di Trieste, via L. Giorgieri 1, 34127 Trieste, Italy.
E-mail: dsciotto@unict.it, gtomaselli@unict.it
†Electronic supplementary information (ESI) available: Further experimental
data, characterization data for all the new compounds and further crystallo-
graphic information, including CIF. CCDC 915083. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt52550c
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greater conformational flexibility of the ligand. Owing to the
1,3-alternate conformation of the calix[4]arene moiety, the
bridging methylene protons resonate as AB systems (at δ 3.78
Results and discussion
Synthesis of calix[4]arene-(salen) ligands
The calix[4]arene-(salen) ligands 1a,b were synthesized follow- and 3.73 ppm for 1a and 3.86 and 3.80 ppm for 1b) because of
ing the procedure described in Scheme 1. This synthetic strat- the small anisochrony of the axial and equatorial protons.
egy allows the modulation of (i) the conformation of the calix-
[4]arene moiety, (ii) the length of the methylene spacers, (iii)
the chiral diamine used for building the salen-type moiety and
Preparation of calix[4]arenes-(salen)-UO2 and calix[4]arenes-
(salen)Mn(III) complexes
(iv) the metal cation inserted in the salen moiety. Upon alkyl-
ation of dipropoxy-calix[4]arene 511 with 1,2-dibromoethane or
The complexation of the free ligands was carried out by
1,3-dibromopropane we obtained respectively compounds 6a,
b, blocked in 1,3 alternate conformation. Compounds 7a,b
were prepared by reaction of the protected salicylaldehyde 412
with 6a,b; the subsequent palladium catalysed deallylation13
afforded bisaldehydes 8a,b in quantitative yield. The reaction
of bisaldehydes with the chiral (1R,2R)-(+)-1,2-diphenylethylene-
diamine, under high dilution conditions, gives the calix[4]-
arene-(salen) ligands 1a,b.
All new compounds (1a,b, 6a,b–8a,b) were characterized by
NMR and ESI-MS investigations (see ESI, Fig. S1–S8†). The
1H-NMR spectra of compounds 1a,b (see ESI, Fig. S7 and S8†)
exhibit: (i) the singlet of the azomethine CHvN protons
(δ 8.1–8.2 ppm), (ii) the broad singlet of OH protons, strongly
deshielded by the intramolecular hydrogen bonding
(δ 13.7 ppm and 14.0 ppm for 1a and 1b, respectively), and (iii)
the singlet of the diimine bridging protons (δ 4.8 and 5.0 ppm
for 1a and 1b, respectively). Three different resonances can be
identified for the two phenyl groups of the diimine bridge of
1a, while only a singlet is observed for the same groups in the
spectrum of 1b because of proton isochrony, likely due to a
addition of the appropriate metal acetate salt, according to
Scheme 2, in order to obtain the uranyl complexes 2a,b and
the manganese catalysts 3a,b.
a) Calix[4]arenes-(salen)-UO2 complexes. The complexation
with the UO22+ dication greatly influenced the 1H-NMR spectra
(see ESI, Fig. S9 and S12†). The signals of the hydroxyl protons
were not observed as expected because of the formation of the
metal complex, while the CHvN signals, which in the free
ligands were observed at δ 8.1–8.2 ppm, shifted to lower fields
(δ 9.1 ppm) in as good a manner as the proton signals of the
diimine bridge did, whereas the presence of the metal exerts a
slight influence both on the aromatic protons of the two
phenyl groups and on the protons of the polyether spacers
respectively. The ESI-MS spectra of the complexes show the
2+
molecular peaks of the deprotonated macrocycles plus UO2
and Na+ or K+ (see ESI, Fig. S11 and S14†). VT-NMR analysis of
2a showed no meaningful variation in the spectra, as indica-
tive of a rigid structure, whereas in the case of 2b the VT-NMR
analysis seems suggestive of some conformational flexibility
(see ESI, Fig. S15 and S16†).
Scheme 1 Synthesis of calix[4]arene-(salen) ligands 1a,b.
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Scheme 2 Synthesis of calix[4]arene-(salen) complexes 2a,b–3a,b.
2D NMR studies of the uranyl complexes allowed us to eluci- This is in accord with both the VT-NMR results that indicate a
date the structure of the calix[4]arene-(salen). The complete more rigid structure for 2a complex and the energy minimized
assignment of the signals, as well as the conformations of 2a,b, structures of Mn-oxo complexes 3a (see the next section).
were deduced on the basis of 2D-COSY (see ESI, Fig. S10 and
b) Calix[4]arenes-(salen)-Mn(III) complexes. The formation
S13†) and T-ROESY experiments (see ESI, Fig. S17 and S18†).
of calix[4]arene-(salen)-Mn(III) complexes was fully supported
Several NOE correlations were observed in the T-ROESY by the ESI-MS measurements (see ESI, Fig. S19 and S20†).
spectra of the complex 2a,b (Fig. 1). ESI-MS spectra of 3a,b show the signal of the deprotonated
Significant dipolar contacts were observed for the NvCH macrocycles plus manganese, thus indicating that the metal
imine proton with the ortho protons (Ar″) of the phenyl ring, atom is tightly bound to the calix[4]arene-(salen) framework in
the Hpara protons of the salicylidene moiety (Ar′) and the agreement with the formation of 1 : 1 species.
methynic hydrogen on the diimine bridge (red circles). The
Catalysis
signals of the polyether spacer show dipolar couplings with
the salicylidene units and the aromatic units of the calixarenic The epoxidation reactions were typically performed at 25 °C in
scaffold pointing upward (blue circles); other cross-peaks are CH2Cl2–H2O employing NaClO as an oxygen donor and
observed between the propyloxy pendants and the aromatic 4-phenyl-pyridine N-oxide (4-PPNO) as a coligand (substrate/
units of the calixarenic scaffold pointing downward.
oxidant/catalyst molar ratio of 1/1/0.05). In the case of cis-sub-
We have evidence of dipolar coupling (green circle) between stituted styrenes the corresponding cis epoxides together with
the ortho protons of the phenyl ring bonded on the bridge minor amounts of trans epoxides were obtained. ee values
stereocenter carbon (Ar″), the Hmeta of the aromatic units (Ar) were determined by capillary GLC analysis employing chiral
of the calixarene scaffold pointing upward and the Hpara columns for styrene, cis-substituted styrenes and 1,2-dihydro-
1
protons of the salicylidene moiety (Ar′) for complex 2a only. naphthalene epoxides, and by both GLC analysis and H-NMR
Fig. 1 Partial T-ROESY map of complexes 2a,b (500 MHz, CDCl3, 300 K).
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measurements in the presence of Eu(+)(hfc)3 shift reagent for catalysts.2,3,10 The observed ee values for the alkenes reported
chromene epoxides.14 The comparative catalytic activities of 3a in Table 1 as well as the structural data obtained by the X-ray
and 3b are reported in Table 1.
diffraction analysis (see the next section) are in line with this
Under the adopted conditions, some alkenes (entries 1–6) finding. Jacobsen reported that enantioselectivity is tied to the
are not very reactive and their conversion values within 24 h position of the transition state along the reaction coordinate.
remain under 50%. On the other hand good conversion values Therefore, electron withdrawing groups produce a more reac-
(70–90%) have been observed for dihydronaphthalene and tive oxene species which interacts with the alkene in an early
substituted chromenes (entries 7–12). Enantioselectivity values transition state affording lower enantioselectivity values,
depend on the identities of alkenes and of the catalysts. whereas electron donor groups attenuate the reactivity of
Usually cyclic alkenes (entries 7–12) undergo much more oxene species leading to a late transition state and then to
efficient enantioselective epoxidation reactions than acyclic higher enantioselectivity values. The data of substituted chro-
alkenes (entries 1–6). A general consideration is that for all the menes reported in Table 1 support this rationalization.
studied alkenes higher ee values are observed with catalyst 3a
rather than catalyst 3b.
Molecular modeling studies were performed to give the opti-
mized conformations of the Mn-oxo derivatives by using semi-
Previously reported studies indicated that the dissymmetry empirical quantum-chemical calculations in the gas phase with
of diimine bridge should favour the approach of the si enantio- full geometry optimization. Molecular models were optimized
face of the alkene towards the Mn-oxo site of chiral salen with the semiempirical PM6 method as implemented in
Table 1 Enantioselective epoxidation reactions of alkenes catalyzed by 3a,b
Entry
1
Catalyst
Substrate
Conv.a (%)
44
Yielda (%)
85
eeb (%)
20
Config.
3a
R
2
3
4
5
6
7
8
9
3b
3a
3b
3a
3b
3a
3b
3a
23
30
67
90
89
92
85
96
93
87c
4
56
21
32
13
72
41
93d
R
1R,2S
1R,2S
n.d.
38
<20
<20
87
n.d.
1R,2S
1R,2S
3R,4R
81
72
10
11
12
3b
3a
3b
75
84c
93c
90c
66d
73d
50d
3R,4R
3R,4R
3R,4R
80b
77b
All reactions were carried out in CH2Cl2–H2O biphasic system at 25 °C. [Alkene] = 0.14 M, [Catalyst] = 0.007 M, [Coligand] = [4-PPNO] = 0.07 M,
[NaClO] = 0.14 M, [Na2HPO4] = 0.05 m at pH = 11.2 as a buffer. a Determined by GC on chiral columns. b In the case of cis-β-alkylstyrenes ee
1
values are referred to the major cis epoxide (eecis). c Yield by weight of the isolated product. d Determined by H-NMR in the presence of Eu(hfc)3
after isolation of desired epoxides.
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rotates around the axis of the calixarenic scaffold; the dihedral
angle between the phenyl rings linked to the diimine bridge is
nearly 180°. On the other hand catalyst 3b possesses a less dis-
torted structure: the two salicylidene units are almost coplanar
and the dihedral angle between the phenyl rings of the
diimine bridge is nearly 60°. These structural differences are
closely related to the length of the linker and, therefore, to the
distance between the calix[4]arene platform and the salen
moiety.
X-ray diffraction analysis
Good quality crystals of complex 2a, suitable for X-ray analysis,
were obtained by slow evaporation of a CHCl3–CH3CN mixture.
The molecular structure, determined using synchrotron radi-
ation, is shown in Fig. 3; selected bond lengths and angles are
listed in Table 2.
The salen ligand with absolute configuration R,R at stereo-
genic carbon atoms on the ethylene bridge acts as a tetra-
dentate ligand through its N2O2 donor moiety chelating the
uranyl ion at the equatorial plane. The coordination sphere of
the uranium center is completed by two trans-oxo axial groups
and a water molecule at the fifth equatorial site, leading to a
highly distorted pentagonal–bipyramidal geometry. The five-
member chelating ring adopts a twisted conformation, with
the pseudo-C2 symmetry axis passing through the uranium
Fig. 2 Energy minimized structures of Mn-oxo complexes 3a, left, and
3b, right (MOPAC2009, PM6).
MOPAC2009.15 Optimized structures (see Fig. 2) show that the
two complexes possess different structures.
For the complexes 3a, with the shorter polyether spacer, the
salen moiety adopts a stepped conformation and slightly
Fig. 3 Crystallographic structure of complex 2a: front (a) and side (b) view. Hydrogen atoms and co-crystallized acetonitrile molecules were
omitted for clarity. Atomic numbering scheme is shown for the atoms involved in the metal coordination sphere. The dihedral angles α and β (c) are
defined as those between the planes of the salicylidene rings and the mean equatorial plane N2O2U of the salen framework, whereas the dihedral
angle γ is defined as the angle between the plane bisecting the calix[4]arene and the mean equatorial plane N2O2U.
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Stereochemistry of calix[4]arene-(salen)-Mn(III) catalyzed
Table 2 Selected bond distances (Å) and angles (°) for complex 2a
oxidation for complex 3a
U(1)–O(1a)
U(1)–N(1)
U(1)–O(1)
U(1)–O(1w)
N(2)–C(2)
1.784(5)
2.591(9)
2.239(6)
2.504(8)
1.504(13)
U(1)–O(2a)
U(1)–N(2)
U(1)–O(2)
N(1)–C(1)
1.790(6)
2.567(8)
2.257(7)
1.453(13)
The NMR study of the uranyl-complex 2a, assumed to be a
structural model for the oxidant active species MnvO (oxene)
of complex 3a, shows a good correlation between the confor-
mation in solution and the structure in the solid state (X-ray
diffraction analysis). We propose that the oxidant active
species of complex 3a takes a similar conformation, and that
the alkenes’ approach from the upward benzene ring side (si
enantioface) is favoured with respect to the approach of the re
enantioface because of the steric crowding due to the down-
ward benzene ring and to the unfavourable folding of the cata-
lytic site. On the other hand, the longer spacer of the catalyst
3b generates a more flexible conformation making more
similar the activation energies respectively for the approach of
alkenes to the si and re enantiofaces, resulting in a less selec-
tive oxidation reaction.
O(1)–U(1)–O(1w)
O(2)–U(1)–N(2)
N(2)–U(1)–N(1)
77.7(4)
70.2(3)
65.2(3)
O(2)–U(1)–O(1w)
O(1)–U(1)–N(1)
78.3(3)
68.9(4)
centre and the middle point of the bond between the methine
carbon atoms (Fig. 3c). The conformation of the ethylenedia-
mino chelating bridge induces an anti-periplanar orientation
of the phenyl rings (Ph–C–C–Ph torsion angle of 169.2(8)°)
(Fig. 3b). Following this twisted arrangement of the chelating
ring, the salen ligand adopts an overall stepped conforma-
tion,1e with α and β dihedral angles between the planes of the
salicylidene rings and the equatorial plane of the uranium
centre (N2O2U) of 137.1(2)° and 144.5(2)°, respectively (Fig. 3c).
The water molecule coordinated to the uranium center is
located almost symmetrically in the middle of the calix[4]arene
cavity, forming an intramolecular hydrogen bond (O(1w)⋯O(7)
2.76(1) Å) with the oxygen atom of a polyether spacer (see ESI,
Fig. S21†). The binding motif of water indicates the ability of
the 1,3-alternate calix[4]arene to lock small neutral molecules
via hydrogen bonding interactions.16 Furthermore the short
distances between the aqua ligand and the gravity centers of
the propoxy-substituted aromatic rings (O(1w)⋯Cg1 3.48(3) Å,
O(1w)⋯Cg2 3.47(3) Å) show that the phenyl rings, forming a
hydrophobic environment, protect the aqua ligand. On the
opposite side of calixarene, the ethoxy-substituted rings are
involved in hydrophobic CH⋯π interactions with an aceto-
nitrile molecule hosted in the cavity (distances between the
methyl group and gravity centers of B and D aromatic rings are
3.78(4) and 3.71(4) Å, respectively, see Fig. S21†).
The arrangement of dioxyethyl spacers causes a small twist
of 8.7(3)° of the calixarene scaffold with respect to the equa-
torial mean plane of the uranium centre (N2O2U) (Fig. 3c). The
mutual rotation of the calixarene moiety driven by the confor-
mation of dioxyethyl spacers produces an inherent axial chiral-
ity that adds up to the chirality of the stereogenic centers and
to the inherent helicity of the stepped conformation of salicyl-
idene groups. According to the structural parameters evi-
denced for compound 2a, one should expect that the longer
chain (dioxypropyl) of the polyether spacer in both compounds
2b and 3b increases the flexibility of the linker and the dis-
tance of the calixarene scaffold from the metal center. The
insertion of a methylene group in the linker, probably, also
affects the mutual rotation of the calixarene unit with respect
to the equatorial coordination plane. These structural features
can be related to the different catalytic behaviour of the
manganese species 3a,b in the epoxidation of alkenes. The
more rigid and compact complex 3a is more efficient in terms
of both catalytic activity and stereoinduction for the asym-
metric epoxidation of non-functionalized Z-alkenes.
Conclusion
Two new chiral calix[4]arene-salen ligands have been syn-
thesized and their Mn(III) complexes were employed as cata-
lysts for enantioselective epoxidation reactions. Complex 3a is
more efficient in terms of both catalytic activity and stereo-
induction for the asymmetric epoxidation of non-functiona-
lized Z-alkenes.
We were able to determine the structures of the model
chiral uranyl-salen complex 2a both in solution, through NMR
spectroscopy, and in solid state by means of X-ray crystallo-
graphic analysis. The central uranium atom in this complex
displays
a pentagonal bipyramidal geometry as already
observed for other complexes of the family. The salen moiety
adopts a stepped conformation, due to the twist conformation
of the five-membered chelate rings including the uranium ion
and the ethylenediamine unit, and is twisted with respect to
the calixarene scaffold, the whole structure retaining a pseudo-
C2 symmetry with an inherent axial chirality. Finally, the struc-
tural characterization allowed us to explain the different cata-
lytic activities of 3a and 3b catalysts in terms of a stepped
structure and rigidity; we were able to relate the conformation
of the model uranyl-salen complex 2a to the oxidant active
species generated by the calix(4)arene-(salen)-Mn(III) complex
3a and its appropriate control expands the scope of Mn-salen
catalyzed asymmetric epoxidations.
Experimental section
General
All solvents were purified by standard procedures; all other
chemicals were analytically pure and were used without
further purification. All reactions were carried out under a
nitrogen atmosphere unless stated otherwise.
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6-Cyano-2,2′-dimethylchromene
and
6-methoxy-2,2′- (64.80 g, 0.6 mmol), K2CO3 (83.45 g, 0.6 mmol) and 6a,b
dimethylchromene were prepared according to literature (0.15 mmol) was refluxed for 14 h in acetonitrile (15 ml). The
procedures.17
solvent was evaporated under reduced pressure and the
Thin layer chromatography (TLC) was carried out on silica residue was dissolved in CH2Cl2 and washed with HCl 3 N
gel plates (Merck 60, F254), column chromatography was (2 × 25 ml) and water (3 × 50 ml). Finally, the solvent was
carried out on silica gel 60 (Merck, 0.063–0.200 mm).
removed and the crude product was purified by column chrom-
NMR experiments were carried out in CDCl3 on a 500 MHz atography (SiO2, dichloromethane) to give 7a,b as white solids.
1
spectrometer (1H at 499.88 MHz, 13C at 125.7 MHz) equipped
7a: yield 43%; H-NMR: δH (500 MHz; CDCl3) 10.50 (s, 2H,
with a pulse field gradient module (Z axis) and a tunable Ar′COH), 7.46 (dd, 2H, J1 = 7.5 Hz, J2 = 1.5 Hz, Ar′Hortho), 7.08
5 mm Varian inverse detection probe (ID-PFG); chemical shifts (dd, 2H, J1 = 7.5 Hz, J2 = 1.5 Hz, Ar′Hpara), 7.08 (t, 2H, J =
(δ) are expressed in ppm and are referenced to residual deuter- 7.5 Hz, Ar′Hmeta), 7.06 (d, 4H, J = 7.5 Hz, ArHmeta), 7.04 (d, 4H,
ated solvent. NMR data were processed using the MestReC J = 7.5 Hz, ArHmeta), 6.78 (t, 2H, J = 7.5 Hz, ArHpara), 6.58 (t,
software. Two-dimensional gCOSY and T-ROESY experiments 2H, J = 7.5 Hz, ArHpara), 6.06 (m, 2H, Ar′OCH2CHvCH2), 5.35
were performed using Varian standard pulse sequences.
(dd, 2H, J1 = 17 Hz, J2 = 1.5 Hz, Ar′OCH2CHvCH2), 5.26 (dd,
MS-ESI spectra were recorded with a HP 1100 Series LC/ 2H, J1 = 10.5 Hz, J2 = 1.5 Hz, Ar′OCH2CHvCH2), 4.68 (d, 4H,
MSD spectrometer and on a Finnigan LCQ Deca XP ion trap J = 6.0 Hz, Ar′OCH2CHvCH2), 4.01 (t, 4H, J = 5.5 Hz,
(Thermo Fischer Scientific, USA).
ArOCH2CH2OAr′), 3.95 (t, 4H, J = 5.5 Hz, ArOCH2CH2OAr′),
7.5 Hz,
The elemental analyses were performed using a Carlo Erba 3.73 (AB, 8H, ArCH2Ar), 3.54 (t, 4H,
J
=
elemental analyzer EA 1108.
ArOCH2CH2CH3), 1.57 (m, 4H, J = 7.5 Hz, ArOCH2CH2CH3),
0.88 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3). 13C-NMR: δC
(125 MHz; CDCl3) 190.37, 156.82, 155.69, 152.27, 151.65,
Synthesis
General procedure for the preparation of dibromo deriva- 133.84, 133.53, 133.10, 130.40, 130.05, 129.60, 124.20, 122.28,
tives 6a,b. A mixture of calix[4]arene 5 (0.50 g, 0.98 mmol), 121.82, 120.00, 119.74, 118.89, 75.27, 73.31, 69.63, 68.54,
Cs2CO3 (1.28 g, 3.93 mmol) and, for 6a, 1,2-dibromoethane 36.72, 23.30, 10.35.
(1.11 g, 5.89 mmol) or, for 6b, 1,3 dibromopropane (1.18 g,
5.89 mmol) was refluxed for 48 h in acetonitrile (50 ml). The
MS-ESI: m/z 941.7 (M + Na)+; m/z 954.6 (M + K)+.
7b: yield 46%; H-NMR: δH (500 MHz; CDCl3) 10.46 (s, 2H,
1
solvent was evaporated under reduced pressure and Ar′COH), 7.43 (dd, 2H, J1 = 7.5 Hz, J2 = 1.5 Hz, Ar′Hortho), 7.13
the residue was dissolved in CH2Cl2 and washed with HCl 3 N (t, 2H, J = 7.5 Hz, Ar′Hmeta), 7.09 (dd, 2H, J1 = 7.5 Hz, J2
=
(2 × 50 ml) and water (3 × 100 ml). Finally, the solvent was 1.5 Hz, Ar′Hpara), 7.04 (d, 4H, J = 7.5 Hz, ArHmeta), 6.99 (d, 4H,
removed and the crude product was purified by column J = 7.5 Hz, ArHmeta), 6.78 (t, 2H, J = 7.5 Hz, ArHpara), 6.63 (t,
chromatography (SiO2, n-hexane/dichloromethane, 5/5) to give 2H, J = 7.5 Hz, ArHpara), 6.09 (m, 2H, Ar′OCH2CHvCH2), 5.37
6a,b as white solids.
(dd, 2H, J1 = 17 Hz, J2 = 1.5 Hz, Ar′OCH2CHvCH2), 5.27 (dd,
1
6a: yield 13%; H-NMR: δH (500 MHz; CDCl3) 7.06 (d, 4H, 2H, J1 = 10.5 Hz, J2 = 1.5 Hz, Ar′OCH2CHvCH2), 4.70 (d, 4H,
J = 7.5 Hz, ArHmeta), 7.03 (d, 4H, J = 7.5 Hz, ArHmeta), 6.86 (t, J = 6.0 Hz, Ar′OCH2CHvCH2), 3.91 (t, 4H, J = 6.5 Hz,
2H, J = 7.5 Hz, ArHpara), 6.82 (t, 2H, J = 7.5 Hz, ArHpara), 3.81 ArOCH2CH2CH2OAr′), 3.78 (t, 4H,
J
=
6.5 Hz,
(AM, 8H, ArCH2Ar), 3.65 (t, 4H, J = 6.5 Hz, ArOCH2CH2Br), ArOCH2CH2CH2OAr′), 3.77, 3.72 (AM, 8H, J = 15.0 Hz,
3.40 (t, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 2.75 (t, 4H, J = 6.5 Hz, ArCH2Ar), 3.46 (t, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 1.96 (m,
ArOCH2CH2Br), 1.28 (m, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 0.68 4H, J = 6.5 Hz, ArOCH2CH2CH2OAr′), 1.41 (m, 4H, J = 7.5 Hz,
(t, 6H, J = 7.5 Hz, ArOCH2CH2CH3). 13C-NMR: δC (125 MHz; ArOCH2CH2CH3), 0.81 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3).
CDCl3) 157.12, 155.31, 133.95, 133.64, 129.51, 129.05, 122.79, 13C-NMR: δC (125 MHz; CDCl3) 190.41, 156.90, 156.31, 152.27,
122.19, 72.21, 69.81, 37.67, 28.56, 22.91, 10.25.
151.30, 134.08, 133.58, 133.13, 130.29, 129.83, 129.39, 124.09,
122.22, 121.53, 120.03, 119.15, 118.81, 75.10, 72.75, 67.59,
MS-ESI: m/z 761.1 (M + K)+; m/z 745.1 (M + Na)+.
1
6b: yield 33%; H-NMR: δH (500 MHz; CDCl3) 7.06 (d, 4H, 66.03, 37.35, 29.90, 22.93, 10.20.
J = 7.5 Hz, ArHmeta), 7.02 (d, 4H, J = 7.5 Hz, ArHmeta), 6.82 (t,
4H, J = 7.5 Hz, ArHpara), 6.78 (t, 4H, J = 7.5 Hz, ArHpara), 3.78
MS-ESI: m/z 967.5 (M + Na)+; m/z 983.4 (M + K)+.
General procedure for the preparation of aldehydes 8a,b. A
(AB, 8H, ArCH2Ar), 3.63 (t, 4H, J = 6.5 Hz, ArOCH2CH2CH2Br), mixture of 7a,b (0.3 mmol), Pd(OAc)2 (2.0 mg, 0.01 mmol),
3.38 (t, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 3.16 (t, 4H, J = 6.5 Hz, PPh3 (12.5 mg, 0.05 mmol), Et3N (0.37 g, 3.7 mmol) and
ArOCH2CH2CH2Br), 1.88 (m, 4H,
J
=
6.5 Hz, HCOOH (0.16 g, 3.7 mmol) in 80% aqueous EtOH (6 ml) was
ArOCH2CH2CH2Br), 1.23 (m, 4H, J = 7.5 Hz, ArOCH2CH2CH3), refluxed for 3 h. The solvent was evaporated and the residue
0.71 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3). 13C-NMR: δC was dissolved in CH2Cl2 and washed with water (3 × 10 ml).
(125 MHz; CDCl3) 156.88, 156.37, 134.06, 133.72, 129.59, The solvent was removed, under reduced pressure, to give 8a,b
129.31, 122.32, 121.93, 72.16, 68.27, 37.81, 33.13, 31.08, 22.61, as dark solids which were used without further purification.
1
10.10.
MS-ESI: m/z 773 (M + Na)+; m/z 789.1 (M + K)+.
8a: yield 84%; H-NMR: δH (500 MHz; CDCl3)10.96 (s, 2H,
Ar′OH), 9.98 (s, 2H, Ar′COH), 7.22 (dd, 2H, J1 = 8.0 Hz, J2 = 1.5
General procedure for the preparation of protected alde- Hz, Ar′Hortho), 7.10 (dd, 2H, J1 = 7.5 Hz, J2 = 1.5 Hz, Ar′Hpara),
hydes 7a,b. A mixture of 2-(2-allyloxy)-3-hydroxy-benzaldehyde 7.07 (d, 4H, J = 7.5 Hz, ArHmeta), 7.05 (d, 4H, J = 7.5 Hz,
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ArHmeta), 6.92 (t, 2H, J = 8.0 Hz, Ar′Hmeta), 6.77 (t, 2H, J =
Calix[4]arene-salen 1b: yield 85%; 1H-NMR: δH (500 MHz;
7.5 Hz, ArHpara), 6.58 (t, 2H, J = 7.5 Hz, ArHpara), 4.07 (t, 4H, CDCl3) 13.70 (s, 2H, Ar′OH), 8.23 (s, 2H, –NvCH–Ar′), 7.27 (s,
J = 5.5 Hz, ArOCH2CH2OAr′), 3.96 (t, 4H, J = 5.5 Hz, broad, 10H, Ar″H), 7.09 (d, 2H, J = 7.5 Hz, ArHmeta), 7.03 (d,
ArOCH2CH2OAr′), 3.73 (AM, 8H, ArCH2Ar), 3.52 (t, 4H, J = 2H, J = 7.5 Hz, ArHmeta), 6.98 (d, 2H, J = 7.5 Hz, Ar′Hmeta), 6.92
7.5 Hz, ArOCH2CH2CH3), 1.56 (m, 4H,
J = 7.5 Hz, (d, 2H, J = 7.5 Hz, ArHmeta), 6.86 (d, 2H, J = 7.0 Hz, Ar′Hpara),
ArOCH2CH2CH3), 0.88 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3). 6.81 (t, 2H, J = 7.5 Hz, Ar′Hmeta), 6.80 (t, 2H, J = 7.5 Hz,
13C-NMR: δC (125 MHz; CDCl3) 190.91, 151.56, 150.47, ArHpara), 6.78 (t, 2H, J = 7.5 Hz, ArHpara), 4.82 (s, 2H, Ar″–CH–
147.08, 142.33, 128.63, 128.52, 124.95, 124.66, 119.77, 117.03, NvCH), 3.87 (m, 4H, ArOCH2CH2CH2OAr′), 3.86 (AB, 8H, J =
116.85, 116.07, 115.61, 114.37, 67.98, 64.43, 63.53, 31.62, 16.5 Hz, ArCH2Ar), 3.80 (AB, 8H, J = 16.5 Hz, ArCH2Ar), 3.53
18.07, 5.16.
(m, 4H, ArOCH2CH2CH2OAr′), 3.34 (t 4H,
ArOCH2CH2CH3), 1.64 (m, 4H, ArOCH2CH2CH2OAr′), 1.21 (m,
8b: yield 80%; H-NMR: δH (500 MHz; CDCl3) 11.06 (s, 2H, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 0.69 (t, 6H, J = 7.5 Hz,
Ar′OH), 9.93 (s, 2H, Ar′COH), 7.18 (dd, 2H, J1 = 8.0 Hz, J2
ArOCH2CH2CH3). 13C-NMR: δC (125 MHz; CDCl3) 166.19,
J = 7.5 Hz,
MS-ESI: m/z 861.2 (M + Na)+.
1
=
1.5 Hz, Ar′Hortho), 7.09 (dd, 2H, J1 = 7.5 Hz, J2 = 1.5 Hz, Ar′ 156.86, 156.59, 153.02, 151.23, 146.97, 139.86, 134.17, 134.10,
Hpara), 7.03 (d, 4H, J = 7.5 Hz, ArHmeta), 6.99 (d, 4H, J = 7.5 Hz, 134.09, 134.06, 129.68, 129.58, 129.39, 128.58, 127.78, 127.73,
ArHmeta), 6.97 (t, 2H, J = 8.0 Hz, Ar′Hmeta), 6.78 (t, 2H, J = 124.79, 122.33, 122.30, 122.15, 120.59, 119.24, 117.96, 117.83,
7.5 Hz, ArHpara), 6.60 (t, 2H, J = 7.5 Hz, ArHpara), 3.95 (t, 4H, 79.60, 71.99, 67.75, 67.15, 38.26, 38.23, 29.47, 22.53, 10.03.
J = 6.5 Hz, ArOCH2CH2CH2OAr′), 3.74 (t, 4H, J = 6.5 Hz,
Anal. Calcd for C68H68N2O8: C, 78.43; H, 6.58; N, 2.69.
ArOCH2CH2CH2OAr′), 3.74 (AB, 8H, ArCH2Ar), 3.45 (t, 4H, J = Found: C, 78.45; H, 6.54; N, 2.70.
7.5 Hz, ArOCH2CH2CH3), 1.95 (m, 4H,
ArOCH2CH2CH2OAr′), 1.40 (m, 4H,
J
=
6.5 Hz,
7.5 Hz,
MS-ESI: m/z 1064.3 (M + Na)+; m/z 1080.5 (M + K)+.
General procedure for the synthesis of the UO2-calix[4]arene-
J
=
ArOCH2CH2CH3), 0.80 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3). salens 2a,b. Warning! Care should be taken when handling
13C-NMR: δC (125 MHz; CDCl3) 196.50, 156.85, 156.40, 151.98, uranyl-containing compounds because of their toxicity and
147.60, 134.05, 133.68, 129.80, 129.48, 124.62, 122.02, 121.64, residual radioactivity.
120.94, 119.53, 119.46, 72.70, 67.45, 66.34, 37.38, 29.81, 22.92,
10.20.
A solution of calix[4]arene-salens 1a,b (0.013 mmol, 6 ml
EtOH) was stirred, UO2(OAc)2·2H2O (9.5 mg, 0.019 mmol) was
added, as a solid, and stirring was continued for 2 h. The
MS-ESI: m/z 887.8 (M + Na)+; m/z 904.3 (M + K)+.
General procedure for the synthesis of the calix[4]arene- solvent was evaporated to give the crude products, which were
salens 1a,b. Solutions of bisaldehydes 8a,b (0.03 mmol, dissolved with CH2Cl2 and were used without further
100 ml EtOH) and (1R,2R)-(+)-1,2-diphenylethylenediamine purification.
1
(6.6 mg, 0.03 mmol, 100 ml EtOH) were added separately to
UO2-calix[4]arene-salen 2a: yield 83%; H-NMR (CDCl3): δH
100 ml of refluxing EtOH, for 3 h, whereupon refluxing was (500 MHz; CDCl3) 9.13 (s, 2H, –NvCH–Ar′), 7.56 (d, 4H, J =
continued for 8 h. After the solution was cooled, the solvent 8.0 Hz, Ar″Hortho), 7.44 (d, 2H, J = 7.5 Hz, Ar′Hortho), 7.36 (d,
was evaporated, under reduced pressure, to give, after recrystal- 2H, J = 7.5 Hz, ArHmeta), 7.30 (t, 4H, J = 8.0 Hz, Ar″Hmeta), 7.25
lization from MeOH, 1a,b as yellow solids.
(d, 2H, J = 7.5 Hz, ArHmeta), 7.25 (t, 2H, J = 7.5 Hz, Ar″Hpara),
Calix[4]arene-salen 1a: yield 83%; 1H-NMR: δH (500 MHz; 7.15 (d, 2H, J = 7.5 Hz, Ar′Hpara), 7.15 (d, 2H, J = 7.5 Hz,
CDCl3) 13.98 (s, 2H, Ar′OH), 8.15 (s, 2H, –NvCH–Ar′), 7.40 (d, ArHmeta), 6.86 (d, 2H, J = 7.0 Hz, ArHpara), 6.64 (t, 2H, J =
4H, J = 8.0 Hz, Ar″Hortho), 7.31 (t, 4H, J = 8.0 Hz, Ar″Hmeta), 8.0 Hz, Ar′Hmeta), 6.48 (t, 2H, J = 7.5 Hz, ArHpara), 6.02 (s, 2H,
7.24 (t, 2H, J = 8.0 Hz, Ar″Hpara), 7.06 (d, 2H, J = 7.5 Hz, Ar′ Ar″–CH–NvCH), 4.86–4.46 (m, 4H, ArOCH2CH2OAr′),
Hortho), 7.03 (d, 4H, J = 7.5 Hz, ArHmeta), 7.01 (d, 2H, J = 7.5 Hz, 4.41–4.25 (m, 4H, ArOCH2CH2OAr′), 3.92 (AM, 4H, J = 14.5 Hz,
ArHmeta), 6.89 (d, 2H, J = 7.5 Hz, ArHmeta), 6.86 (d, 2H, J = ArCH2Ar), 3.77 (AM, 4H, J = 14.5 Hz, ArCH2Ar), 3.63 (t, 4H, J =
7.0 Hz, Ar′Hpara), 6.84 (t, 2H, J = 7.5 Hz, Ar′Hmeta), 6.81 (t, 2H, 7.5 Hz, ArOCH2CH2CH3), 1.77 (m, 4H,
J = 7.5 Hz,
J = 7.5 Hz, ArHpara), 6.75 (t, 2H, J = 7.5 Hz, ArHpara), 4.94 (s, ArOCH2CH2CH3), 0.96 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3).
2H, Ar″–CH–NvCH), 4.01 (m, 4H, ArOCH2CH2OAr′), 3.85 (m, 13C-NMR: δC (125 MHz; CDCl3) 170.90, 162.88, 156.76, 155.89,
4H, ArOCH2CH2CH2OAr′), 3.78 (AB, 8H, J = 15.5 Hz, ArCH2Ar), 150.48, 140.23, 134.27, 134.23, 134.15, 131.25, 131.22, 130.81,
3.73 (AB, 8H, J = 15.5 Hz, ArCH2Ar), 3.36 (t, 4H, J = 7.5 Hz, 130.71, 130.04, 128.92, 128.71, 128.68, 128.64, 128.31, 127.99,
ArOCH2CH2CH3), 1.31 (m, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 126.91, 124.98, 123.50, 122.12, 116.80, 81.44, 72.99, 72.82,
0.69 (t, 6H, J = 7.5 Hz, ArOCH2CH2CH3). 13C-NMR: δC 71.91, 37.29, 37.16, 23.47, 10.25.
(125 MHz; CDCl3) 166.74, 156.41, 156.28, 154.12, 146.92,
Anal. Calcd for C66H64N2O11U: C, 61.54; H, 4.97; N, 2.16.
139.97, 134.20, 134.09, 133.68, 133.51, 132.13, 132.05, 131.93, Found: C, 61.49; H, 4.98; N, 2.12.
129.74, 129.48, 128.71, 128.55, 128.45, 127.74, 126.36, 124.00,
122.70, 122.38, 119.80, 117.87, 78.81, 72.17, 68.76, 67.34,
37.83, 37.78, 22.96, 10.20.
MS-ESI: m/z 1303.6 (M + Na)+.
UO2-calix[4]arene-salen 2b: 1H-NMR (CDCl3, 27 °C): δH
(500 MHz; CDCl3) 9.08 (s, 2H, –NvCH–Ar′), 7.62 (d, 4H, J =
Anal. Calcd for C66H64N2O8: C, 78.24; H, 6.37; N, 2.77. 8.0 Hz, Ar″Hortho), 7.31 (d, 4H, J = 7.5 Hz, Ar′Hortho), 7.23 (d,
Found: C, 78.25; H, 6.48; N, 2.72.
4H, J = 8.0 Hz, Ar″Hmeta), 7.18 (d, 2H, J = 7.5 Hz, ArHmeta), 7.17
(d, 2H, J = 8.0 Hz, Ar″Hpara), 7.11 (d, 4H, J = 7.5 Hz, ArHmeta),
MS-ESI: m/z 1051.4 (M + K)+.
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7.03 (d, 2H, J = 7.5 Hz, ArHmeta), 7.00 (d, 2H, J = 7.5 Hz, Ar′ β-methylstyrene the chiral column DMeTButiSililBETA (25 m ×
Hpara), 6.96 (t, 2H, J = 7.5 Hz, Ar′Hmeta), 6.86 (t, 2H, J = 7.5 Hz, 0.25 mm ID × 0.25 μm film) was used (column conditions:
ArHmeta), 6.60 (t, 2H, J = 7.5 Hz, ArHpara), 6.08 (s, 2H, Ar″– 50 °C (0 min) to 120 °C (1 min) at 2 °C min−1). The injector
CH–NvCH), 4.41–4.28 (m, 4H, ArOCH2CH2CH2OAr′), and detector temperatures were maintained at 250 °C for both
4.17–4.09 (m, 4H, ArOCH2CH2CH2OAr′), 3.91 (AB, 4H, J = columns. n-Decane was used as an internal standard
15.5 Hz, ArCH2Ar), 3.82 (AB, 4H, J = 15.5 Hz, ArCH2Ar), 3.45 throughout.
(t 4H, J = 7.5 Hz, ArOCH2CH2CH3), 2.42–2.38 (m, 4H,
Enantiomeric excess values for the formation of epoxides
ArOCH2CH2CH2OAr′), 1.64 (m, 4H, ArOCH2CH2CH2OAr′), 1.36 were determined by 1H-NMR in the presence of Eu(+)(hfc)3
(m, 4H, J = 7.5 Hz, ArOCH2CH2CH3), 0.76 (t, 6H, J = 7.5 Hz, reagent shift for chromene derivatives epoxides.
ArOCH2CH2CH3). 13C-NMR: δC (125 MHz; CDCl3) 170.83,
161.07, 156.52, 155.73, 155.71, 150.80, 134.22, 134.16, 133.89,
Crystal data collection and refinement
133.86, 130.87, 130.67, 130.59, 130.46, 129.88, 128.76, 128.22,
Orange crystals of complex 2a suitable for X-ray analysis were
127.77, 127.23, 124.38, 123.72, 122.67, 120.26, 120.24, 116.56,
obtained by slow evaporation of a CHCl3–CH3CN mixture at
81.85, 71.78, 69.96, 67.37, 38.23, 31.39, 29.70, 22.48, 9.94.
room temperature. Data collection was carried out at the X-ray
Anal. Calcd for C68H68N2O11U: C, 61.53; H, 5.16; N, 2.11.
diffraction beam-line (XRD1) of Elettra Synchrotron, Trieste
Found: C, 61.50; H, 5.12; N, 2.08.
(Italy), with the rotating crystal method and a monochromatic
MS-ESI: m/z 1348.6 (M + K)+.
wavelength of 0.700 Å. Routinely, the crystal dipped in Para-
tone, as a cryoprotectant, was mounted in a loop and flash
frozen to 100 K with liquid nitrogen. Diffraction data were
General procedure for the synthesis of the Mn-calix[4]arene-
salens 3a,b. A solution of calix[4]arene-salens 1a,b
(0.032 mmol,
6 ml EtOH) was stirred, Mn(OAc)3·2H2O
indexed and integrated using MOSFLM20 and scaled with SCA-
LEPACK.21 The structure was solved by direct methods using
SIR200822 and non-hydrogen atoms were anisotropically
refined (H atoms at the calculated positions) with bond length
and angle restraints by full-matrix least-squares methods on F2
using SHELXL-97.23 Empirical absorption corrections were
applied by the XABS2 program.24 Essential crystal data and
refinement details are reported in Table S1 (see ESI†).
(10.52 mg, 0.032 mmol) was added, as a solid, and stirring was
continued for 2 h. The solvent was evaporated to give the
crude products, which were dissolved with CH2Cl2 and were
used without further purification.
Mn(III)-calix[4]arene-salen 3a: yield 86%;
Anal. Calcd for C68H65N2O10Mn: C, 72.59; H, 5.82; N, 2.49.
Found: C, 72.54; H, 5.79; N, 2.42.
MS-ESI: m/z 1065.4 (M)+.
The CCDC database code number 915083 contains the sup-
plementary crystallographic data for this paper.
Mn(III)-calix[4]arene-salen 3b: yield 87%;
Anal. Calcd for C70H69N2O10Mn: C, 72.90; H, 6.03; N, 2.43.
Found: C, 72.85; H, 5.99; N, 2.39.
MS-ESI: m/z 1093.4 (M)+.
Acknowledgements
General procedures for the epoxidation reactions
University of Catania (Progetto d’Ateneo) and MIUR
(PRIN-2009A5Y3N and PRIN-2008AZT7RK project) are grate-
fully acknowledged for partial support.
To
a stirred solution of alkene (0.35 mmol), catalyst
(0.0175 mmol) and 4-phenylpyridine-N-oxide (4-PPNO,
0.175 mmol) in CH2Cl2 (2.5 mL), maintained at 25 °C in a
thermostatic bath, buffered bleach (0.35 mmol, buffered to pH
= 11.2 with 0.05 M Na2HPO4) was added. The course of the
reaction was monitored by GC. After 24 h, the phases were sep-
arated, and the aqueous layer was extracted with CH2Cl2. The
combined organic phases were dried over Na2SO4 and concen-
trated. The crude product was purified by PLC (SiO2).
Standard epoxides were synthesized by the reaction of the
corresponding olefins with m-chloroperbenzoic acid in
CH2Cl2, isolated from the reaction mixture by preparative PLC
(SiO2) and characterized by comparison of their 1H NMR
spectra with those reported in the literature.18 Absolute con-
figurations were assigned by comparison of the signs of
measured [α]2D0 to the literature values.19
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