Self-assembled Dendrimer-bound Pd(II) Complexes via Acid-base Interactions and their Catalysis for Allylic Amination

Article abstract of DOI:10.1246/cl.2003.692

The Pd complexes were noncovalently attached on the periphery of poly(propyleneimine) dendrimers based on acid-base interactions. The dendritic Pd complexes were found to possess catalytic activities for allylic amination, and to be able to mimick the cat

Full text of DOI:10.1246/cl.2003.692

692
Chemistry Letters Vol.32, No.8 (2003)
Self-assembled Dendrimer-bound Pd(II) Complexes via Acid-base Interactions and their Catalysis
for Allylic Amination
Masahiko Ooe, Makoto Murata, Atsushi Takahama, Tomoo Mizugaki, Kohki Ebitani, and Kiyotomi KanedaÃ
Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received May 12, 2003; CL-030405)
The Pd complexes were noncovalently attached on the pe-
riphery of poly(propyleneimine) dendrimers based on acid-base
interactions. The dendritic Pd complexes were found to possess
catalytic activities for allylic amination, and to be able to mim-
ick the catalytic activity of the analogous monomeric Pd com-
plex catalyst.
Dendrimers are hyperbranched macromolecules with mon-
odispersed molecular weights, precisely determined cascade
structures, and a specific number of end groups.1 One of the
promising application of metallodendrimers is in the field of
catalysis.2 Immobilization of metal complex catalysts onto solu-
ble or insoluble polymers by covalent binding have often en-
countered the difficulties in mimicking the structures of parent
complexes and mass transfer limitations, which results in de-
creases of the catalytic activities.3 Organometallic dendrimers,
however, because of their structurally well-defined and specific
number of active sites as well as their congested surface regard-
ed as solid supports, offer the potential for bridging the gap be-
tween homogeneous and heterogeneous catalysis.4 Recently, we
have reported the preparation of covalently phosphinated
poly(propyleneimine) (PPI) dendrimer-bound PdCl2 complexes
and Pd(0) complexes, and their unique catalyses in the selective
hydrogenation of dienes to monoenes5 and the stereoselective
allylic amination.6 We wish to report here the alternative ap-
proach for the preparation of dendritic catalysts using ionic in-
teractions between the carboxyl groups of phosphine ligands
and the terminal amino groups on the PPI dendrimers,7 and their
catalysis in allylic amination.8
Figure 1. Preparation of self-assembled dendrimer-bound Pd
complexes.
complexes were bound on the dendrimers through ionic interac-
tions between the carboxyl group of DPPBA and the peripheral
amino groups of 1 (Figure 1).
The catalytic performance of dendrimer-bound Pd com-
plexes 3 for the allylic amination of cinnamyl acetate with mor-
pholine are summarized in Table 1.11 The allylic amination
smoothly proceeded with the dendritic catalysts 3a and 3b, in
which the reaction rates strongly dependent on the P/Pd ratio;
the highest activity was obtained at P/Pd = 2 (Entries 2 and
8). Above P/Pd = 2, the catalytic activities decreased with in-
creasing P/Pd ratios. Similar trend was also observed for the re-
actions in the absence of the dendrimers (Entries 12-16). Al-
though the present dendrimer-bound Pd complexes were
insoluble in DMF solvent, the allylic amination proceeded
smoothly under such heterogeneous conditions (Entry 4). Gen-
erally, for many organic reactions, immobilization of metal
complexes using organic polymers tends to decreases their cat-
alytic activity as compared to the monomeric analogs,3 which is
attributable to the mass transfer limitation of substrates to the
active sites within the polymer matrices.12 It can be said that
all the active Pd species attached to the periphery of the den-
drimers act as active catalytic centers without the diffusion con-
trol even under heterogeneous conditions using DMF solvent.5
The stereoselectivity of the amination12 was investigated
using cis-3-acetoxy-5-carbomethoxy-1-cyclohexene with mor-
pholine (Scheme 1). A 60% selectivity of the cis product was
observed with the dendritic catalyst 3b, whereas a 1:1 mixture
of cis- and trans products was observed using 3a and the den-
drimer-free catalyst. Interestingly, in DMF solvent, the cis-se-
lectivity was 62% in the case of 3a. In our previous paper, high-
er stereoselectivity for this amination was obtained, when
increasing the generation of the phosphinated dendrimer-bound
Based on the well-defined cascade structures and the specif-
ic number of primary amino groups on the periphery that are
available for catalyst binding sites, the third and fifth generation
PPI dendrimers (1a, 1b) were chosen as the host supports. The
dendrimer-bound Pd complexes (3a and 3b) were prepared via
two steps as shown in Figure 1.9,10 Treatment of the PPI den-
drimers 1 with 4-diphenylphosphinobenzoic acid (DPPBA)
gave phosphinated dendrimers 2 (step i), followed by the addi-
tion of [PdCl(p-C3H5)]2 to afford the self-assembled dendritic
Pd complexes 3 (step ii).
The above dendrimers were characterized using IR and 31
P
NMR. In IR spectra, addition of DPPBA(16 eq.) to 1a showed a
new band at 1534 cmÀ1 due to the carboxylate of DPPBA and
the band for free DPPBA at 1694 cmÀ1 disappeared (step i).
The 31P{1H} NMR spectrum exhibited a singlet signal for free
DPPBA gave at À4:8 ppm, which shifted slightly upfield to
À5:2 ppm for the dendritic hosts 2. Furthermore, addition of
[PdCl(p-C3H5)]2 to 2 resulted in a shift of the DPPBA signal
from À5:2 to 23.4 ppm. From these results, the Pd phosphine
Copyright Ó 2003The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
693
Table 1. Pd-catalyzed allylic amination of cinnamyl acetate
with morpholinea
the self-assembled dendrimer Pd catalysts can mimic low mo-
lecular weight Pd catalysts in terms of catalytic activity and se-
lectivity.
In conclusion, we present the preparation of self-assembled
dendritic Pd catalysts using noncovalent binding via acid–base
interactions between dendritic hosts and phosphine ligands.
Since the active Pd species are attached on the periphery of
PPI dendrimers, high catalytic performance, which is compara-
ble to that of the parent Pd complex for allylic amination, were
observed under both homogeneous and heterogeneous condi-
tions.
References and Notes
1
For reviews and books on dendrimers, see: a) D. A. Tomalia, H.
Baker, J. R. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J.
Ryder, and P. Smith, Polym. J., 17, 117 (1985). b) A. W. Bosman,
H. M. Janssen, and E. W. Meijer, Chem. Rev., 99, 1665 (1999). c)
´
‘‘Dendrimers and Other Dendritic Polymers,’’ ed. by J. M. J. Frechet
and D. A. Tomalia, J. Wiley & Sons, New York (2001). d) G. R.
Newkome, C. N. Moorefield, and F. Vogtle, ‘‘Dendrimers and Den-
¨
drons: Concepts, Syntheses, Applications,’’ Wiley-VCH, Weinheim
(2001).
2
For reviews on dendritic catalysts, see: a) G. E. Oosterom, J. N. H.
Reek, P. C. J. Kamer, and P. W. N. M. van Leeuwen, Angew. Chem.,
Int. Ed., 40, 1828 (2001). b) D. Astruc and F. Chardac, Chem. Rev.,
101, 2991 (2001). c) R. van Heerbeek, P. C. J. Kamer, P. W. N. M.
van Leeuwen, and J. N. H. Reek, Chem. Rev., 102, 3717 (2002).
3For recent reviews on polymer-bound metal complex catalysts, see:
a) N. E. Leadbeater and M. Marco, Chem. Rev., 102, 3217 (2002). b)
D. E. Bergbreiter, Chem. Rev., 102, 3345 (2002).
4
a) J. W. J. Knapen, A. W. van der Made, J. C. de Wilde, P. W. M. N.
van Leeuwen, P. Wijkens, D. M. Grove, and G. van Koten, Nature,
372, 659 (1994). b) M. T. Reetz, G. Lohmer, and R. Schwickardi,
Angew. Chem., Int. Ed. Engl., 36, 1526 (1997). c) D. de Groot, E.
B. Eggeling, J. C. de Wilde, H. Kooijman, R. J. van Haaren, A.
W. van der Made, A. L. Spek, D. Vogt, J. N. H. Reek, P. C. J.
Kamer, and P. W. N. M. van Leeuwen, Chem. Commun., 1999,
1623. d) N. J. Hovestad, E. B. Eggeling, H. J. Heidbuchel, J. T.
B. H. Jastrzebski, U. Kragl, W. Keim, D. Vogt, and G. van Koten,
Angew. Chem., Int. Ed., 38, 1655 (1999). e) E. B. Eggeling, N. J.
Hovestad, J. T. B. H. Jastrzebsky, D. Vogt, and G. van Koten, J.
Org. Chem., 65, 8857 (2000).
aReaction conditions: cinnamyl acetate 3.0 mmol, morpholine
4.5 mmol, [PdCl(p-C3H5)]2, 0.01 mmol, DPPBA 0.02-
0.12 mmol, 1a 1.25-7.50 mmol, 1b 0.31-1.88 mmol, MeOH
16 mL, 5 ꢀC.
c
bDMF was used as a solvent at 10 ꢀC. Without the dendrim-
ers.
Pd(0) complex catalyst, in which the active sites were covalent-
ly attached to the periphery of the dendrimers.6 It seems that,
compared to the immobilization by covalent binding, the pres-
ent immobilization of Pd complexes on the dendrimers by ionic
interactions may afford active sites with high mobility on the
surfaces,7 even under the heterogeneous conditions. That is,
5
6
7
T. Mizugaki, M. Ooe, K. Ebitani, and K. Kaneda, J. Mol. Catal. A:
Chem., 145, 329 (1999).
T. Mizugaki, M. Murata, M. Ooe, K. Ebitani, and K. Kaneda, Chem.
Commun., 2002, 52.
a) D. de Groot, B. F. M. de Waal, J. N. H. Reek, A. P. H. J.
Schenning, P. C. J. Kamer, E. W. Meijer, and P. W. M. N. van
Leeuwen, J. Am. Chem. Soc., 123, 8453(2001). b) R. van de
Coevering, M. Kuil, R. J. M. K. Gebink, and G. van Koten, Chem.
Commun., 2002, 1636.
8
9
M. Johannsen and K. A. Jorgensen, Chem. Rev., 98, 1689 (1998).
Recent report on heterogeneous Pd catalyst using polystyrene-based
support functionalized with DPPBA, see: Y. Uozumi, H. Danjo, and
T. Hayashi, Tetrahedron Lett., 38, 3557 (1997).
10 Typical procedures for the preparation of 3a (P/Pd = 2) are as fol-
lows: 1a (0.0025 mmol) and DPPBA (0.04 mmol) were dissolved in
methanol (16 mL), and stirred for 30 min under an argon atmos-
phere. [PdCl(p-C3H5)]2 (0.01 mmol) was added to the above meth-
anol solution and stirred at room temperature for 30 min under an
argon atmosphere. 31P NMR (109 MHz, CD3OD, 30 ꢀC): 23.4 ppm
from H3PO4 as an external reference.
11 Typical procedures for the allylic amination of cinnamyl acetate
with morpholine catalyzed by dendrimer-bound Pd complex 3a
are as follows: to a catalyst solution prepared according to reference
10, was added cinnamyl acetate (3.0 mmol) and morpholine
(4.5 mmol). After vigorous stirring of the reaction mixture at 5 ꢀC,
the yields of products were determined by GC using an internal
standard method.
Scheme 1. Generation dependence of stereoselectivity in allyl-
ic amination of cis-3-acetoxy-5-carbomethoxy-1-cyclohexene
with morpholine.
12 B. M. Trost and E. Keinan, J. Am. Chem. Soc., 100, 7779 (1978).
Published on the web (Advance View) July 7, 2003; DOI 10.1246/cl.2003.692