Design of supramolecular metal complex catalytic systems for petrochemical and organic synthesis

Article abstract of DOI:10.1007/s11172-008-0117-5

Specific features of the behavior of the supramolecular metal complex catalysts based on calixarenes, cyclodextrins, and dendrimers in the reactions of hydroformylation, Wacker oxidation, hydroxylation of aromatics, 2-naphthol coupling, and oxidative coupling of styrenes and benzene were studied. The factors affecting the catalytic activity and selectivity are discussed.

Full text of DOI:10.1007/s11172-008-0117-5

Russian Chemical Bulletin, International Edition, Vol. 57, No. 4, pp. 780—792, April, 2008
780
Design of supramolecular metal complex catalytic systems
for petrochemical and organic synthesis
E. A. Karakhanov, A. L. Maksimov,★ E. A. Runova, Yu. S. Kardasheva, M. V. Terenina,
S. V. Kardashev, V. A. Skorkin, L. M. Karapetyan, and M. Yu. Talanova
Department of Chemistry, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 939 1951. Eꢀmail: max@petrol.chem.msu.ru
Specific features of the behavior of the supramolecular metal complex catalysts based on calixꢀ
arenes, cyclodextrins, and dendrimers in the reactions of hydroformylation, Wacker oxidation,
hydroxylation of aromatics, 2ꢀnaphthol coupling, and oxidative coupling of styrenes and benzene
were studied. The factors affecting the catalytic activity and selectivity are discussed.
Key words: supramolecular catalytic systems, molecular imprinting, cyclodextrins, calixarenes,
dendrimers, host—guest complexes, hydroformylation, Wacker oxidation, aromatics coupling, rhodꢀ
ium complexes, palladium complexes.
The control of selectivity in catalytic systems is one of
tures of the behavior of the immobilized homogeneous
systems.
Design of the catalysts on the basis of receptor moleꢀ
the main problems in petrochemical and organic syntheꢀ
ses. In the recent years researchers are interested in suꢀ
pramolecular catalysis, the phenomena based on the use
of substances capable of forming host—guest inclusion
complexes with various organic compounds as compoꢀ
nents of catalytic systems. This approach provides wide
possibilities for design of new highly active metal comꢀ
plex catalysts with high substrate, regioꢀ, and chemioseꢀ
lectivity.1—7
cules can be performed via two routes: (1) using the target
chemical modification of the receptor acting as a ligand;
(2) using the molecular imprinting method.3 The imꢀ
printing process proceeds as follows. At the first step
a receptor molecule interacts with a template molecule to
form a host—guest inclusion complex (Scheme 1).
The structure that formed is immobilized by binding
agents containing two and more functional groups. At the
Various classes of receptor molecules, particularly,
cyclodextrins, calixarenes, and dendrimers (Fig. 1), are
used for the design of supramolecular catalysts.2—4,8,9
Cyclodextrins are macrocyclic compounds formed by
αꢀDꢀglucopyranose. Calixarenes are the compounds synꢀ
thesized by the condensation of phenols and aldehydes of
diverse structure. The selective functionalization of
the upper and lower rims of these macrocycles by heteroꢀ
atomic groups can form a molecular system bearing sevꢀ
eral binding sites and in this way create catalysts capable
of molecular recognition. One more type of molecular
receptors is presented by dendrimers, which are polymer
molecules with regular branched structures and can be
considered as unique monomolecular globular micelles.
Due to noncovalent interactions between several fragꢀ
ments of the polymer chain and an organic molecule,
dendrimers can be selectively bonded to the substrate,
thus enhancing the efficiency and selectivity of the imꢀ
mobilized metal complex catalyst.3,9—18 Location of
the active sites at the ends of the dendrimer branches
can minimize diffusional restrictions, and the denꢀ
drimers become good models for studying specific feaꢀ
Scheme 1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 766—778, April, 2008.
1066ꢀ5285/08/5704ꢀ0780 © 2008 Springer Science+Business Media, Inc.

Supramolecular catalysts for synthesis
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
781
Fig. 1. Macromolecular receptors for the design of the supramolecular catalysts: a, αꢀ and βꢀcyclodextrins (n = 0 and 1, respectively);
b, dendrimers; c, calix[n]arenes (n = 1, 3).
last step, the template is removed using diverse methods
(vacuum distillation, washing with various solvents,
and others).
The obtained macroligands are able to selectively
form inclusion complexes with the molecules that were
used as templates. Therefore, this process can be considꢀ
ered as a unique "tuning" of the material to a template
molecule.
The purpose of the present review is to consider and
analyze possibilities of using two approaches outlined
above to design supramolecular catalysts for petrochemꢀ
ical and organic syntheses (hydroformylation, Wacker
oxidation, and oxidation of aromatic compounds and
oxidative aromatics coupling).
formylation catalyst (activity, selectivity, and stability) deꢀ
pend substantially on the ligand nature, and its influence
on the ratio and yield of the reaction products is rather
complicated and determined by both electronic and steric
factors.19—22 An increase in the steric volume of the ligand
was found to increase the yield of aldehydes with the
normal carbon chain, whereas the enhancement of the
electronꢀaccepting properties favors the formation of the
catalytically active form of the rhodium complex due to
the compensation of an excessive negative charge on the
metal atom. Therefore, we synthesized the organoꢀ
phosphorus ligands from calix[4]ꢀ and calix[6]arenes
L1—L7 with different electronꢀdonating properties, macꢀ
rocycle size, and orientation of the complexing groups
relative to the calixarene platform.
Hydroformylation of unsaturated compounds
using calixareneꢀbased catalysts
OꢀPhosphorylated calixarenes L1—L5 (see Refs 5, 23,
and 24) were synthesized by the successive interaction of
the corresponding calixarenes with nꢀbutyllithium and
diphenylchlorophosphine (Scheme 2).
The NMR spectral data confirmed the structures of
the compounds and allowed one to determine their conꢀ
formational composition. The formation of a conformer
mixture was avoided only in the case of the L3 compound.
Hydroformylation of unsaturated hydrocarbons is
widely used in industry for the preparation of oxygenꢀ
containing compounds. The rhodium catalytic systems
including the phosphine and phosphite ligands are most
active in this process. The major properties of a hydroꢀ

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Karakhanov et al.
1
(δ 122.4), and the H NMR spectrum contains well reꢀ
solved symmetric doublets of protons of the methylene
bridges at δ 3.34—4.34, indicating the rigidly fixed cone
conformation.
It is known that phosphites are considerably less
sensitive to oxidation in air than phosphines and phosꢀ
phinites and their catalytic complexes are more stable.
pꢀtertꢀButylcalix[6]arenediphosphite L6 and calix[6]ꢀ
arenediphosphite L7 were synthesized by the reaction of
the corresponding calixarenes with phosphorus trichloꢀ
ride in toluene in the presence of triethylamine (Scheme 3).
Scheme 3
R = But (L6), H (L7)
Mass spectrometric data (ЕSI) indicated that the reacꢀ
tion of calixarene with PCl3 afforded dimeric compounds
along with diphosphites L6 and L7: the mass spectrum
contains two pairs of signals with equal intensity at m/z
1029, 2058 and 693, 1386 for L6 and L7, respectively.
Scheme 2
L
R
R´
PPh2
Me
Pr
n
2
2
2
L
L4
L5
R
But
H
R´
PPh2
PPh2
n
3
3
L1
L2
L3
But
But
But
R = H, But
Pure diphosphites L6 and L7 were isolated by preparaꢀ
tive column chromatography (eluent CH2Cl2). The L6
macrocycle was obtained in the cone conformation.23 In
The 31P NMR spectrum of dipropoxycalix[4]areneꢀ
phosphinite L3 exhibits the single signal as a singlet

Supramolecular catalysts for synthesis
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
783
the 1H NMR spectrum of the L7 calixarene, the group
of doublet signals of the bridging methylene protons
(δ 3.52—4.78) indicates that they are nonequivalent and,
hence, the macrocycle molecule is not symmetric, which
is confirmed by two singlets in the 31P NMR spectrum
(103.4 and 103.9 ppm, JP,P = 93 Hz). The mass spectra
(ESI) of compounds L6 and L7 contain only the [M]+
signals (m/z 1029 and 693, respectively).
Yield (%)
a
Isomeric alkenes
Aldehydes
100
80
60
40
20
1
The NMR and mass spectrometry studies of the reacꢀ
tions of diphosphites L6 and L7 with Rh(acac)(CO)2 and
[Rh(COD)Cl]2 (COD is cyclooctaꢀ1,5ꢀdiene) in soluꢀ
tion at different calix/metal ratios8,23 show that at
P/Rh = 2 (equimolar amounts of rhodium and calixareꢀ
ne) the rhodium atom is bound to one macrocyclic ligand.
The further increase in the calixarene content in a soluꢀ
tion (twofold and higher molar excess of cyclophane)
results in the predominant formation of the RhL2 comꢀ
plex in which two bulky macrocycles are coordinated to
the metal center.
1
2
4
6
8
P/Rh
Yield (%)
100
b
80
60
40
20
0.5
1
2
4
P/Rh
Fig. 2. Dependence of the yield of aldehydes and isomeric alkenes
in nonꢀ1ꢀene hydroformylation in the presence of the L6 (a) and
L7 (b) ligands on the P/Rh ratio ([substrate]/[catalyst] = 150,
50 °C, 2 h, 0.5 MPa).
crease in the ratio P/Rh > 2. In our opinion, this is due to
the fact that for the formation of a sterically hindered
macrocomplex the access of substrate molecules and even
carbon monoxide to the metal center is restricted, thus
impeding the catalytic process.
Since phosphinites L1 and L4 were obtained as a mixtuꢀ
re of conformers, the geometry of the complexes formed
from them cannot distinctly be determined. As shown by
the data in Fig. 3, the decrease in the reaction rate with an
increase in the concentration of the phosphorusꢀcontaining
ligand is not so pronounced as for the L6 and L7 ligands.
In these cases, most likely, even at high P/Rh ratios, only
one ligand can be coordinated to the metal center.
No formation of a complex with two macroligands was
observed by the NMR study of the ligand properties of pꢀ
tertꢀbutyldipropoxycalix[4]arenediphosphite L3, being in
the rigidly fixed cone conformation. Unlike the singlet
signal in the spectrum of the starting calixarene L3, a
doublet at δ 130 appears in the 13P NMR spectrum of the
mixture formed upon mixing of solutions of diphosphinite
L3 and Rh(COD)2BF4 in CDCl3 in a molar ratio of 2 : 1
(P/Rh = 1) (Fig. 4). The further increase in the concenꢀ
tration of the macrocyclic component results only in the
appearance of an additional intense singlet caused by unꢀ
bound phosphinite L3. Based on the data of mass specꢀ
trometry (m/z 1311 [M – BF4], the calculated and obꢀ
These data help to explain the change in the catalytic
activity of the complexes based on ligands L6 and L7 at
different P/Rh ratios (Fig. 2). The synthesized macrocyꢀ
cles were studied as components of the catalytic systems
in the hydroformylation of unsaturated compounds of
different structure. The process was carried out in a steel
temperatureꢀcontrolled 30ꢀmL autoclave equipped with
a magnetic stirrer at temperatures from 25 to 100 °C in
a toluene solution under the synthesis gas pressure from
0.5 to 5.0 MPa. As a result, the corresponding aldehydes
with the normal and branched carbon chains are formed
along with some amount of the isomerization products.
The rhodium complexes with the calixareneꢀcontaining
ligands were synthesized in situ from Rh(acac)(CO)2 and
the corresponding calixarene. The data presented in Fig. 2
show that the yield of aldehydes decreases with an inꢀ

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Karakhanov et al.
Yield (%)
100
a
Isomeric alkenes
Aldehydes
a
b
c
80
60
40
20
1
2
4
6
8
P/Rh
Yield (%)
100
b
80
60
40
20
d
2
3
4
6
P/Rh
140
130
120
δ
Fig. 3. Dependence of the yield of aldehydes and isomeric alkenes in
nonꢀ1ꢀene hydroformylation in the presence of the L1 (a) and L4 (b)
ligands on the P/Rh ratio ([substrate]/[catalyst] = 150, 50 °C, 2 h,
0.5 MPa).
Fig. 4. 31Р NMR spectra (in CDCl3) of solutions of the L3 ligand
(a) and an L3 + Rh(COD)2BF4 mixture at P : Rh = 1 : 1 (b), 2 : 1 (c),
and 4 : 1 (d).
Scheme 4
served isotopeꢀmass distributions virtually coincide), the
structure can be proposed for the formed complex in
which the rhodium atom is bonded to one cyclooctadiene
fragment and two phosphorus atoms of the calixarene
ligand.
[Rh(COD)Cl]2 + 2 L3
2 Rh(COD)L3BF4
The decrease in the activity of nonꢀ1ꢀene hydroformyꢀ
lation at P/Rh > 2 was observed for the catalytic system
consisting of Rh(acac)(CO)2 and L3 (Fig. 5). It is possiꢀ
ble that the formation of acyl intermediate is hindered in
the presence of the free macroligand preventing the coorꢀ
Yield (%)
Isomeric alkenes
Aldehydes
100
80
60
40
20
The complex of precisely this composition was isolated
in the pure state (85% yield) upon the reaction of
bis{[chloro(cyclooctaꢀ1,5ꢀdiene)]rhodium} with diꢀ
phosphinitedipropoxycalix[4]arene L3 in the presence of
AgBF4 (Scheme 4).
1
2
4
P/Rh
Fig. 5. Dependence of the yield of aldehydes and isomeric alkenes in
nonꢀ1ꢀene hydroformylation in the presence of the L3 ligand on
the P/Rh ratio ([substrate]/[catalyst] = 150, 50 °C, 2 h, 0.5 MPa).

Supramolecular catalysts for synthesis
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
785
Table 1. Results of nonꢀ1ꢀene hydroformylation in the presence of
the L1—L7 ligands*
Table 3. Activity of the catalytic systems based on the L1 and L4
ligands in arylalkene hydroformylation in the absence of a solventa
Liꢀ
gand
Conversion
Yield of aldehydes
%
Regioselectivity
Substrate
Turnover frequency of reaction/h–1
b
c
L1
L4
L1
L2**
L3
L4
L5
42
67
90
90
85
94
90
20
48
74
75
68
75
63
70
62
54
55
60
62
70
120
200
130
180
800
740
L6
L7
* [Substrate]/[catalyst] = 150, P/Rh = 2, 50 °C, 2 h, 0.5 MPa.
** 1.0 MPa.
650
470
a [Substrate]/[catalyst] = 1000, 50 °C, 1 h, 2.5 MPa.
b P/Rh = 4.
c P/Rh = 3.
dination of alkene and carbon monoxide to the metꢀ
al center.
The rhodium complexes based on all synthesized phosꢀ
phorusꢀcontaining calixarenes manifested high activity in
the hydroformylation of linear alkenes (С7—С12) and sevꢀ
eral aromatic substrates.8,23
The data on nonꢀ1ꢀene hydroformylation in the presꢀ
ence of ligands L1—L7 under identical conditions are
presented in Table 1. It is seen that the catalytic systems
based on calix[6]arenes are characterized by the highest
activity. They have larger hydrophobic cavities than
calix[4]arenes.
For the hydroformylation of substituted styrenes (Taꢀ
ble 2), the difference in sizes of the macrocyclic ligand
cavity exerts virtually no effect on the reaction rate. Howꢀ
ever, the rate increases noticeably with an increase in the
steric volume of the substituent in the paraꢀposition of
the benzene ring.
Since the solubility of pꢀtertꢀbutylcalix[4]areneꢀ
tetraphosphinite L1 and pꢀtertꢀbutylcalix[6]arenehexaꢀ
phosphinite L4 in aromatic compounds is high, the reacꢀ
tions of arylalkenes with the synthesis gas can be carꢀ
ried out without a solvent (the substrate to catalyst ratio
reaches 1000) with high rates (Table 3), which consiꢀ
derably enhances the efficiency of the use of the metal
complex.
The proposed phosphiniteꢀ and phosphitecalixareꢀ
nes, especially the systems based on the sixꢀmembered
macrocycle, can successfully be used in hydroformylation.
Oxidative coupling of aromatic compounds
using cyclodextrinꢀbased catalysts
Comparing the molecular models of the inclusion
complexes of the catalysts based on pꢀtertꢀbutylcalix[6]ꢀ
arenehexaphosphinite L4 with arylalkenes (Fig. 6),
one can suggest that the acceleration of the catalytic proꢀ
cess is favored by an additional immobilization of
the bulky substrate in the calixarene cavity, due to
which the transition state of the reaction is more easiꢀ
ly achieved.
One of the possible directions of design of the cataꢀ
lysts based on the molecular receptors (cyclodextrins) is
the use of the molecular imprinting method.3,25—31 In
this case, the macroligand is synthesized in the presence
of a specially chosen template molecule, whose presence
predetermines the direction of modification and the strucꢀ
ture of the ligand formed.
Table 2. Activity of the catalytic systems based on the L1 and L4 ligands in the hydroformylation of substituted styrenes and vinylnaphꢀ
thalene*
Ligand
P : Rh
Turnover frequence of reaction/h–1
L1
L4
4
3
35
36
42
37
53
57
140
141
147
146
086
123
* [Substrate]/[catalyst] = 150, 50 °C, 1 h, 2.5 MPa.

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
a
Karakhanov et al.
b
c
Rh
Rh
Rh
Fig. 6. Molecular models of the complexes of pꢀtertꢀbutylcalix[6]arenehexaphosphinite L4 with styrene (a), 2ꢀvinylnaphthalene (b) and
4ꢀvinylbiphenyl (c).
Molecular imprinting can be applied to the design of
new catalysts using two methods. The first method is
"tuning" of a catalytic system to a substrate. For this
approach, we chose the reactions involving "nonrigid"
molecules, viz., alkꢀ1ꢀenes and aldehydes, as model proꢀ
cesses. As an example we can address Wacker oxidation
of alkꢀ1ꢀenes in the biphase systems using the palladiumꢀ
and copperꢀcontaining macrocomplexes and aldehyde
hydrogenation to alcohols. The biphase conditions and
the ruthenium metal complexes with sulfated tripheꢀ
nylphosphine were used. It was shown that the cataꢀ
lyst activity increases considerably in the presence of
the ligands synthesized by molecular imprinting. The
reason may lie in the increase in the stability constants
of a complex of the substrate with the macroligand.3,32
The second method is performed by "tuning" to
the compound, which can be considered according to its
structure as an analog of the transition state of the reacꢀ
tion. As a result, the reaction rate can increase due to
a decrease in the activation energy caused by an optimum
arrangement of the macroligand fragments for the bindꢀ
ing and mutual orientation of the reactants in the transiꢀ
tion state. This approach seems to be the most promising
for design of the catalysts for C—C bonding, in particuꢀ
lar, the catalysts of oxidative coupling or dimerization.
The reaction product, viz., dimer of the corresponding
aromatic compound, can be used as a model of the analog
of the transition state and, hence, template.26,27,33—35
The oxidative coupling of 2ꢀnaphthol and its various
derivatives by iron(III) chloride under uniꢀ and biꢀ
phase conditions (Scheme 5) and the oxidative coupling
of benzene with styrene and its paraꢀsubstituted derivaꢀ
tives catalyzed by the palladium complexes were chosen as
models.
The oxidative coupling of 2ꢀnaphthol and its derivaꢀ
tives was carried out by the action of iron trichloride
under the biphase conditions (water—dichloroethane).
The components of the catalytic system were the macroꢀ
ligands synthesized by molecular imprinting, where
the template was 1,1´ꢀbis(2ꢀnaphthol) and epichlorohyꢀ
drin served as the binding agent. To estimate the influꢀ
ence of imprinting on the catalyst activity, we used
the coefficient that reflects an increase in the reaction
rate. The coefficient is defined by the TOF1/TOF2 ratio
(TOF is turnover frequency). TOF1 and TOF2 is the numꢀ
ber of moles of the product that are formed per 1 mole of
the catalyst per hour when the macroligands synthesized
in the presence and absence of the template are used,
respectively.
For the oxidation of some 2ꢀnaphthols, the oxidative
coupling rate using the macroligand synthesized by moꢀ
lecular imprinting is significantly higher than that for
analogous compounds formed in the absence of a temꢀ
plate molecule (Fig. 7). The arrangement of substituents
in a substrate molecule during the formation of an incluꢀ
sion complex with a macroligand should not prevent
K
1
2
3
2.0
1.5
1.0
0.5
5
4
Scheme 5
Fig. 7. Coefficient of increasing the oxidative coupling rate (K) of
2ꢀnaphthol (1), 6ꢀtertꢀbutylꢀ2ꢀnaphthol (2), 2,6ꢀdinaphthol (3),
1,3ꢀnaphthalenediol (4), and 2ꢀnaphtholꢀ3ꢀcarboxylic acid (5) in
the presence of the macroligand obtained by molecular imprinting
(2ꢀnaphthol : FeCl3 : macroligand = 20 : 40 : 1, water : dichloroetꢀ
hane = 1 : 1, 60 °С).

Supramolecular catalysts for synthesis
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
787
Yield (%)
the С(1)—С(1´) bond formation upon bringing together
two molecules of the 2ꢀnaphthol derivative. Therefore,
for the naphthalene derivatives with the substituents in
positions 2 and 6, this increase in the rate is substantial,
unlike the derivatives with the substituents in posiꢀ
tions 1 and 3.
3
50
40
3
30
1
The results of 2ꢀnaphthol oxidation with iron trichloꢀ
ride in air in dichloromethane using the iron(III) comꢀ
plexes with βꢀcyclodextrin as heterogeneous catalysts are
presented in Fig. 8. The best results were achieved for the
macroligand synthesized by molecular imprinting and
used as the component of the catalytic system. The initial
activity of the complex of the macroligand synthesized in
the absence of a template molecule was virtually the same
as the activity of pure FeCl3.
The iron complexes with cyclodextrins can easily be
separated from the reaction products and used repeatedly.
For the crossꢀcoupling of benzene with styrene and its
paraꢀsubstituted derivatives, transꢀstilbene was chosen
as the template during cyclodextrin modification by epiꢀ
chlorohydrin. The reaction was carried out in excess benꢀ
zene. The heterogeneous catalyst was prepared by the
interaction of palladium(II) acetate with the correspondꢀ
ing macroligand. The complex with the ligand syntheꢀ
sized using the molecular imprinting method considerꢀ
ably increases the yield of stilbenes for styrene and
pꢀmethylstyrene compared to other complexes (Fig. 9).
In the absence of a template, the hydroxy groups of
cyclodextrin are substituted chaotically. Accordingly, the
structure of the inclusion complexes formed with the
substrate molecules prevents the reacting components
from approaching in space to form the C—C bond. Moreꢀ
over, the chaotically arranged groups decrease the reacꢀ
tion rate, if the latter is compared with the reaction rate in
the presence of unsubstituted cyclodextrin.
20
10
1
2
2
1
2
3
Styrene
pꢀMethylꢀ
styrene
pꢀtertꢀButylꢀ
styrene
Fig. 9. Yield of the products of the oxidative coupling of benzene
with various styrenes in the presence of the heterogeneous catalyst
based on βꢀcyclodextrin (1) and βꢀcyclodextrin modified in
the absence (2) and presence (3) of the template (72 h, 100 °С,
PO = 0.5 MPa, benzene, [substrate] : [PdII] : [ligand] = 30 : 1 : 3,
[su2bstrate] = 0.66 mol L–1).
the complex with nonmodified βꢀcyclodextrin. A tertꢀbuꢀ
tylstyrene molecule is too large for the formation of a macꢀ
roligand—benzene—tertꢀbutylstyrene complex.
Wacker oxidation of unsaturated compounds
using dendrimerꢀbased catalysts
One of the most interesting properties of the denꢀ
drimerꢀbased catalysts is the change in their activity with
an increase in the dendrimer generation. The cases are
known12—14 when an increase in the generation can either
increase or decrease the reaction rate.
Using Wacker oxidation as an example, we were able
to demonstrate that the negative dendrite effect can be
used to increase the selectivity of the reaction. Polyproꢀ
pyleneimine dendrimers were used as macromolecules.
The presence of the internal amino groups capable of
coordinating one type of metals and the external active
groups that bind other metals makes it possible to preꢀ
pare a bimetallic catalyst with unique properties.15 Diꢀ
aminobutane and diaminohexane served as the dendrimꢀ
er core (Fig. 10). It is shown that, under the reaction
conditions, the copper ions are bound by the dendrimer
branches and are localized mainly inside the macromolꢀ
ecule and the palladium ions are coordinated predomiꢀ
nantly with the nitryl groups of the dendrimer.15
For the bulky substrate, viz., pꢀtertꢀbutylstyrene,
the rate of stilbene formation is maximum in the case of
Yield (%)
3
80
2
60
1
40
Wacker oxidation was carried out in a water—ethanol
(1 : 4) medium at temperatures from 60 to 90 °С in a steel
autoclave under an oxygen pressure of 0.5 MPa. Linear
terminal alkenes С6—С12, heptꢀ3ꢀene, cyclohexene, and
allylbenzene were used as substrates.
Under these conditions, the reversible isomerization
of the substrate occurs rapidly in the reaction mixture
(Fig. 11), and already 5 min after octꢀ1ꢀene is virtually
absent in the system. A wide range of products is observed
20
20 40 60 80 100 120 140 160 t/min
Fig. 8. Yields of dimers in 2ꢀnaphthol oxidative coupling with the
use of the heterogeneous catalysts FeCl3 (1) and FeCl3 complexes
with βꢀcyclodextrin modified in the absence (2) and presence (3) of
the template (CH2Cl2, 60 °C, [2ꢀnaphthol] = 0.35 mol L–1, 2ꢀnaphꢀ
thol : FeCl3 : macroligand = 20 : 40 : 1).

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
Karakhanov et al.
Fig. 10. Polypropyleneimine dendrimers based on diaminobutane (a) and diaminohexane (b).
when the Wacker oxidation is carried out without a macꢀ
roligand (Scheme 6).
drimer molecule was introduced into the system. The
yields of ketones in the reaction with the third halfꢀgenerꢀ
ations of dendrimers containing the diaminobutane and
diaminohexane cores are given in Table 4. For all the
substrates, the selectivity by methyl ketone for the use of
a dendrite molecule is 1.5—4 times higher than that in the
absence of the molecule. The introduction of the macroꢀ
molecule exerts no effect on the rate and equilibrium of
substrate isomerization, which occurs in the reaction
mixture within several minutes. The difference in the
yields of isomeric ketones increases in time with an inꢀ
crease in the temperature and substrate conversion. The
selectivity increases with an increase in the dendrimer
generation regardless of the nature of its core (Fig. 12).
The determining role of the dendrimer in the imꢀ
provement of the process selectivity is confirmed by the data
obtained when an excess of the dendrimer ligand over palꢀ
ladium was used. We have shown that a noticeable inꢀ
crease in the selectivity by methyl ketone is observed for
the catalytic system with two nitryl groups per one pallaꢀ
dium ion, whereas the selectivity by methyl ketone reachꢀ
es 95% for four nitryl groups per one palladium ion.3
Since the equilibrium in the alkene isomerization
reaction is established very rapidly and the oxidation rate
of terminal alkene is substantially higher compared to the
isomers with the internal double bond, both methyl keꢀ
tone and other isomeric ketones are formed in the reacꢀ
tion. The situation changed considerably after a denꢀ
Yield (%)
70
1
2
60
50
40
30
20
10
1
2
1
2
1
2
Octꢀ1ꢀene
Octꢀ2ꢀene
Octꢀ3ꢀene
Octꢀ4ꢀene
Fig. 11. Composition of the products of octꢀ1ꢀene isomerizaꢀ
tion 5 min after the beginning of the Wacker oxidation in
the presence (1) and absence (2) of the dendrimer (PO = 0.5 MPa,
2
[Pd] : [CN] : [Сu] : [substrate] = 1 : 2 : 10 : 180).

Supramolecular catalysts for synthesis
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
789
Scheme 6
In the case of heptꢀ3ꢀene, the addition of the dendrimꢀ
er substantially decreases the yield of ketones, especially
heptanꢀ3ꢀone and heptanꢀ4ꢀone. The oxidation rate deꢀ
creases sharply with an increase in the dendriꢀ
mer/palladium ratio. A similar effect was also observed
for cyclohexene oxidation: the yield of ketone decreased
5 times. For the oxidation of allylbenzene in the absence
of the dendrimer, the predominant product is ethyl pheꢀ
nyl ketone, which is formed, most likely, from the prodꢀ
uct of allylphenol isomerization, namely, propenylpheꢀ
nol. Only methyl phenyl ketone was found in the reaction
products when the dendrimer was used (Table 5).
The high selectivity of alkꢀ1ꢀene oxidation to methyl
ketone can be explained by the "negative" dendrite effect
toward alkenes containing internal double bonds. The rate
of their oxidation with an increase in the size of the
dendrimer molecule decreases much more rapidly than
the oxidation rate of terminal alkenes.
Under the reaction conditions, the dendrimer remains
in the aqueousꢀalcohol phase and, therefore, the catalyst
can completely be separated from the nonpolar products
and used repeatedly without additional regeneration. To
increase the volume of the nonpolar phase, an organic
solvent (hexane), which with other solvents forms a biꢀ
phase system, is added along with the substrate to the
reaction mixture. Under these conditions, the selectivity
of the process increases significantly in the reaction sysꢀ
tem (in the case of octene, from 80 to 95%) and the
catalytic system can be used multiply without an activity
loss (Fig. 13).
[Methyl ketone]/[Isomeric ketones]
a
4
4
5
4
3
2
1
4
4
3
3
3
2
3
Yield (%)
70
60
50
40
30
20
10
a
1
2
2
1
2
1
1
Heptꢀ1ꢀene Octꢀ1ꢀene Nonꢀ1ꢀene Decꢀ1ꢀene
[Methyl ketone]/[Isomeric ketones]
b
3
4
1
2
3
4 Oxidation cycle
8
4
Yield (%)
100
b
4
6
4
90
80
70
60
50
40
30
20
10
3
4
3
2
4
2
3
3
2
2
2
2
1
1
1
1
1
Hexꢀ1ꢀene Heptꢀ1ꢀene Octꢀ1ꢀene Nonꢀ1ꢀene Decꢀ1ꢀene
1
2
3
4 Oxidation cycle
Fig. 12. Increase in the selectivity of the Wacker oxidation using the
catalysts containing dendrimers DAB(CN)n (a) and DAH(CN)n (b):
blank entry (1), n = 4 (2), 8 (3), and 16 (4) (80 °С, PO = 0.5 MPa,
[Pd] : [CN] : [Сu] : [substrate] = 1 : 2 : 10 : 180, 1 h)2.
Fig. 13. Yield of ketones (a) and selectivity by methyl ketone (b)
during four cycles of octꢀ1ꢀene oxidation (90 °С, РО2 = 0.5 MPa,
[Pd] : [CN] : [Сu] : [octꢀ1ꢀene] = 1 : 2 : 10 : 180, 1 h).

790
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
Karakhanov et al.
Table 4. Yield of ketone (Y) and selectivity by methyl ketone (S)* in the Wacker oxidation of alkꢀ1ꢀenes in the
absence of a ligand and in the presence of the DAB(СN)16 and DAH(CN)16 ligands**
Substrate
T/°C
Without ligand
Y (%)
DAB(СN)16
Y (%)
DAH(CN)16
S
S
Y (%)
S
Hexꢀ1ꢀene
Heptꢀ1ꢀene
Octꢀ1ꢀene
Nonꢀ1ꢀene
Decꢀ1ꢀene
Dodecꢀ1ꢀene
Hexꢀ1ꢀene
Heptꢀ1ꢀene
Octꢀ1ꢀene
60
60
60
60
60
60
80
80
80
80
80
80
45
44
32
30
29
13
74
71
64
60
57
25
3.1
3.4
3.7
3.9
4.7
7.7
1.8
2.0
2.1
2.1
2.7
4.2
27
37
34
27
23
12
39
40
44
57
35
18
4.7
8.8
8.9
8.7
9.0
25.0
3.6
4.4
4.5
4.6
5.4
16.0
24
35
25
23
22
10
23
29
33
51
39
18
6.0
7.0
6.5
7.2
11.0
14.0
5.3
7.0
7.2
7.3
11.0
20.0
Nonꢀ1ꢀene
Decꢀ1ꢀene
Dodecꢀ1ꢀene
* S = [methyl ketone]/[isomeric ketones].
** PO = 0.5 MPa, [Pd] : [CN] : [Сu] : [substrate] = 1 : 2 : 10 : 180, 1 h.
2
1
The hydroxylation of aromatics by hydrogen peroxide
by the data of H and 13C NMR spectroscopy and mass
provides another example of the dendrimerꢀbased cataꢀ
lysts. In this case, the immobilized metal complex is
a unique analog of oxidation enzymes. In the latter
the active centers are complexes of such metals as copper
and iron, whose environment is formed by various cheꢀ
late polydentate groups. To create similar centers,
we modified the dendrimers of the first, second, and third
generations by various aromatic aldehydes (salicylic,
2ꢀhydroxypyridinic, and 2,4ꢀdihydroxybenzaldehyde)
(Scheme 7).
The reaction was carried out according to a standard
procedure: twofold excess (based on the terminal group)
of a solution of aldehyde in methanol was added to
a solution of the dendrimer in methanol.6 The structure
and purity of the synthesized compounds were confirmed
spectrometry.
To study the catalytic properties of the dendrimers,
we used the hydroxylation of phenol and benzene by
a dilute solution of hydrogen peroxide as models. The
reaction was carried out under both homogeneous (waꢀ
ter) and biphase conditions (water—dichloroethane)
at 55 °С and the substrate to oxidant ratio equal to 1 : 1.
The major products of phenol oxidation turned out to
be hydroquinone, pyrocatechol, and benzoquinone
(Scheme 8), and the selectivity of the process depended
substantially on the nature of the ligand and metal used.
When the copper complexes were used, only pyrocateꢀ
chol and benzoquinone were formed and hydroquinone
was virtually absent. The latter was immediately transꢀ
formed, most likely, into benzoquinone.
Table 5. Results of the Wacker oxidation of nonꢀterminal alkenes*
Substrate
Ligand
Products
Yield (%)
Heptꢀ3ꢀene
—
—
Heptanꢀ2ꢀone
Isomeric ketones
Heptanꢀ2ꢀone
Isomeric ketones
Cyclohexanone
13
28
6
5
35
7
35
10
50
74
8
DAB(СN)16
DAB(СN)16
—
DAB(СN)16
—
Cyclohexene
Allylbenzene
Cyclohexanone
Propenylbenzene
Benzyl methyl ketone
Ethyl phenyl ketone
Propenylbenzene
Benzyl methyl ketone
Ethyl phenyl ketone
—
—
DAB(СN)16
DAB(СN)16
DAB(СN)16
<1
* PO = 0.5 MPa, [Pd] : [CN] : [Сu] : [substrate] = 1 : 2 : 10 : 180, 1 h.
2

Supramolecular catalysts for synthesis
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
791
Scheme 7
TONdend /TON0
2
2.0
1.5
1.0
0.5
2
3
1
1
3
2
1
3
2
2
3
1
1
3
DAB(NH2)n
DAB(CN)n
DAB(salim)n
DAB(pyim)n DAB(4ꢀOH(sal))n
Fig. 14. Catalytic activity of the copper(II) complexes with
the dendrimers of the first (1), second (2), and third (3) generations
containing various terminal groups in phenol hydroxylation (55 °С,
[Cu] : [L] : [phenol] : [Н2О2] = 1 : 2 : 80 : 80, [H2O] : [С2H4Cl2] =
5 : 1, 30 min). Here and in Figs 15 and 16 TONdend and TON0 are
the turnover numbers of the reaction in the presence and absence
of the dendrimer, respectively.
poorly dissolved in water. Under the reaction conditions,
they were in the organic phase and, most likely, acted as uniꢀ
que phaseꢀtransfer oxidant carriers. Concentrating the
catalyst in dichloroethane was confirmed by mass spectromꢀ
etry, according to which almost the whole dendrimerꢀ
containing complex was in dichloroethane and its compoꢀ
sition remained almost unchanged during the reaction.
For the use of the iron complexes under the biphase
conditions, the increase in the reaction rate is somewhat loꢀ
wer than that in the case of the copper compounds (Fig. 15).
The results of benzene hydroxylation by hydrogen perꢀ
oxide catalyzed by the copper and iron complexes with the
synthesized dendrimers in a water—acetonitrile medium
are presented in Fig. 16. Only the iron complexes with
TONdend /TON0
1
1.8
Scheme 8
1
3
3
2 3
1
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2
2
2 3
1
It is shown that with the use of the copper complexes
with the dendrimers containing the terminal amine, nitrꢀ
yl, and salicylamine groups the turnover number (TON)
of the reaction is 1.3—2 times higher than that of the
system without a ligand (Fig. 14). The most active is the
system containing the DAB(NH2)8 dendrimer. The proꢀ
nounced dendrite effect was observed for DAB(salim)n:
the hydroxylation rate increased substantially on going
from the first to third generation. All the dendrimerꢀ
containing complexes (except for DAB(NH2)n) were
DAB(CN)n
DAB(salim)n
DAB(pyim)n DAB(4ꢀOH(sal))n
Fig. 15. Catalytic activity of the iron(III) complexes with the denꢀ
drimers of the first (1), second (2), and third (3) generations conꢀ
taining various terminal groups in phenol hydroxylation (55 °С,
[Fe] : [L] : [phenol] : [H2O2] = 1 : 2 : 80 : 80, [H2O] : [С2H4Cl2] =
5 : 1, 30 min).

792
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
Karakhanov et al.
TONdend /TON0
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1.6
1.4
1.2
1.0
0.8
0.6
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0.2
3
1
2
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This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 06ꢀ03ꢀ32889a
and 07ꢀ03ꢀ00221).
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Received January 18, 2008;
in revised form April 7, 2008