Dendrimers based on cyclophosphazene units and containing chiral ferrocenyl ligands for asymmetric catalysis

Article abstract of DOI:10.1039/a907931i

A simple and efficient synthesis and characterization of a series of first generation dendrimers based on cyclophosphazene cores and containing up to 16 peripheral chiral ferrocenyl ligands is described.

Full text of DOI:10.1039/a907931i

Dendrimers based on cyclophosphazene units and containing chiral ferrocenyl
ligands for asymmetric catalysis†
Raoul Schneider, Christoph Köllner, Immo Weber and Antonio Togni*
Laboratory of Inorganic Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zürich,
Switzerland. E mail: togni@inorg.chem.ethz.ch
Received (in Basel, Switzerland) 29th September 1999, Accepted 26th October 1999
A simple and efficient synthesis and characterization of a
series of first generation dendrimers based on cyclophospha-
zene cores and containing up to 16 peripheral chiral
ferrocenyl ligands is described.
Thus, the synthesis of the first-generation dendrimers
G112PN, G112PP and G116PP, containing 12, 12 and 16
ferrocenyl groups, respectively, was achieved from inter-
mediates 2 (Scheme 1), obtained by reacting 1 with 5-(tert-
butyldimethylsiloxy)isophthaloyl dichloride followed by de-
protection with Bu4NF. A slight excess of the sodium salt of 2
was allowed to react in dioxane with N3P3Cl6 and N4P4Cl8,
respectively, affording the desired compounds as orange
powders in good yields after purification by column chromatog-
raphy (Scheme 2).
Dendrimers,1 highly branched three-dimensional monodisperse
macromolecules, are in principle useful for applications in
catalysis, but have still rarely been prepared for this purpose.2
As an interface between heterogeneous and homogeneous
catalysis, such catalysts react in homogeneous solution but may
be recycled by means of nanofiltration.3 We have previously
shown that chiral ferrocenyl ligands may be successfully
applied to a variety of asymmetric catalytic reactions,4 and that
these ligands could be conveniently functionalised with a view
to attaching them to the periphery of dendrimers by a
convergent approach.5 It has been found that such macromole-
cules, when used in catalytic reactions, do not show relevant
differences in terms of stereoselectivity and catalytic activity, as
compared to their monomeric congeners and are completely
recovered by nanofiltration.5
In continuation of our studies, we aimed to improve and
simplify the preparation of our dendritic ligands. We report now
an efficient three-step synthesis of a new family of dendrimers
containing up to 16 chiral ferrocenyl ligand units with PP or PN
chelating systems. The synthesis starts from compounds of type
1 and utilises cyclotriphosphazene or tetraphosphazene units as
dendrimer cores. Although the N3P3 fragment has been used
previously in pioneering work by Majoral and co-workers,1g,h
the next larger oligomeric N4P4 phosphazene ring6 has not been
exploited before in dendrimer chemistry. The precursors
N3P3Cl6 and N4P4Cl8 offer more branching points, as compared
to more commonly used internal fragments, such as trisub-
stituted benzenes, or tetrasubstituted adamantanes, implying
that, for an equal number of synthetic steps, dendrimers
containing a higher number of peripheral units may be obtained.
Furthermore, because of the higher internal branching, dendritic
particles approaching a spherical shape may be obtained with a
lower number of generations.
All new multiple ferrocenyl ligands were characterised by
NMR (1H, 13C and 31P) and mass spectroscopy.† As expected,
the 31P NMR spectra of the products show one singlet for the
cyclophosphazene phosphorus atoms, indicating full substitu-
tion of the central core [Fig. 1(a)]. The peripheral phosphorus
atoms in G112PN display a singlet, whereas G112PP and
G116PP each show one pair of doublets with a typical long
4
range JPP coupling constant of ca. 40 Hz, indicating the
equivalence of the ferrocenyl units. The new dendrimers were
also characterised by MALDI-TOF mass spectroscopy and in
all cases the molecular peak could be detected. Several in-
source fragmentations at m/z values in good agreement with the
calculated ones were observed.† Furthermore, addition of 16
equiv. of [Rh(COD)2]BF4 to G116PP, for example, led to the
corresponding monodisperse species containing 16 equivalent
and non-interacting Rh complexes, as indicated by the ABX-
spin system in the 31P NMR spectrum of Fig. 1(b), displaying
2JPP and 1JPRh coupling constants of 30 and 142 Hz. Preliminary
experiments with G112PP and G116PP in the Rh-catalysed
hydrogenation of dimethyl itaconate, under the conditions
Scheme 1
† Experimental and spectral data for (R,S)-2b, G112PN, G112PP and
G116PP are available from the RSC web site, see http://www.rsc.org/
suppdata/cc/1999/2415/
Fig. 1 31P NMR (101.202 MHz, CDCl3) spectra of G1-dendrimers: (a) (i)
G112PN, (ii) G112PP and (iii) G116PP and (b) G116PP + 16 equiv. of
[Rh(COD)2]BF4.
Chem. Commun., 1999, 2415–2416
This journal is © The Royal Society of Chemistry 1999
2415

to its monomeric version, is not significantly detrimental to
enantioselectivity, at least up to the size of G116PP.
We are currently exploiting cyclophosphazenes for the
preparation of dendrimers of higher generations, as well as
using such dendrimers as catalysts in a continuous asymmetric
hydrogenation process. The results of such investigations shall
be disclosed in due course.
We are grateful to Walter Amrein (Laboratory of Organic
Chemistry, ETH-Zürich) for measuring MALDI-TOF mass
spectra.
Notes and references
† Selected data for G112PN: dH(250 MHZ, CDCl3)7.93 (s, 36H, C6H3),
7.61 (s, 12H, C6H3), 7.48 (m, 24 Ph-H), 7.33 (m, 36 Ph-H), 7.02 (m, 36 Ph-
H), 6.70 (m, 36H, 24 Ph-H and 12 NH), 5.56 (dq, 12H, 12 CHMeN), 5.05
(s, 12 Pz-H), 4.72 (m, 12 Cp-H), 4.36 (m, 12 Cp-H), 4.26 (m, 12 Cp-H), 4.12
(m, 12 Cp-H), 4.04 (m, 12 Cp-H), 3.73 (m, 12 Cp-H), 3.60 (m, 12 Cp-H),
3.21 (m, 24H, 12 CH2NH), 2.11 (s, 36H, 12 Pz C5-Me), 1.94 (s, 36H, 12 Pz
C3-Me), 1.78 (d, 36H, 12 CHMeN), 1.43 (m, 24H 12 CH2CH2CH2), 0.49
(m, 24H, 12 CH2Si), 0.12 (s, 36H, 36H of 12 SiMe2), 0.01 (s, 36H, 36H of
12 SiMe2); dc(62.86 MHz, CDCl3) 165.7 (CNO), 150.1 (Ar C of C6H3),
146.7 (PzC5), 138.4–127.0 (PPh2, PzC3, Ar C of C6H3), 123.0 and 121.7 (Ar
C of C6H3), 101.1 (Pz CH), 94.0 (d, Cp C), 75.2 (d, Cp C), 74.6–70.1 (Cp
CH and Cp C), 51.7 (d, CHMeN), 43.4 (CH2N), 24.0 (CH2CH2CH2), 20.6
(CHMeN), 14.0 (CH2Si), 13.6 (Pz C3-Me), 11.3 (d, Pz C5-Me), 22.4
(SiMe2), 22.5 (SiMe2); d3p(101.202 MHz, CDCl3) 8.5 (s, Pz), 224.2 (d,
PPh2); m/z (MALDI) 8321 ([M
+
Na]+), 7948 ([M
2
{C5H3(PPh2)(CH(Me)C5H7N2}]+), 6991 ([M 2 {2b}])+, 6616 ([6991 2
{C5H3(PPh2)(CH(Me)C5H7N2}]+), 5662 ([M 2 {2 32b}]+), 5291 (5662 2
{C5H3(PPh2)(CH(Me)C5H7N2}]+).
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Scheme 2 Reagents and conditions: i, Na, MeOH–1,4-dioxane, 60 °C, 1 h,
then N3P3Cl6, 1,4-dioxane, 16 h (G112PN: 54%, G112PP: 64,5%); ii, Na,
MeOH–1,4-dioxane, 60 °C, 1 h, then N4P4Cl8, dioxane, 16 h, 83%.
previously reported,5 afforded the product in ca. 98% ee. This
indicates that the dendritic structure of the catalyst, as compared
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