First example of dendrons as topological amplifiers

Article abstract of DOI:10.1002/ejic.200600217

A phosphorus-containing dendron was synthesized and grafted to a tetraphosphorus macrocycle. The presence of such bulky substituents allowed for the first time the amplification of topological differences in the macrocycle. These differences become detect

Full text of DOI:10.1002/ejic.200600217

SHORT COMMUNICATION
DOI: 10.1002/ejic.200600217
First Example of Dendrons as Topological Amplifiers
Cédric-Olivier Turrin,[a] Alexandrine Maraval,[a] Germinal Magro,[a] Valérie Maraval,[a]
Anne-Marie Caminade,*[a] and Jean-Pierre Majoral*[a]
Dedicated to the memory of our friend, the late Professor Marcial Moreno-Mañas
Keywords: Dendrons / Dendrimers / Macrocycles / Phosphorus / NMR spectroscopy
A
phosphorus-containing dendron was synthesized and
the appearance of several signals for the phosphorus atoms
included in the macrocycle.
grafted to a tetraphosphorus macrocycle. The presence of
such bulky substituents allowed for the first time the amplifi-
cation of topological differences in the macrocycle. These dif-
ferences become detectable by 31P NMR spectroscopy, with
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
amplifiers. More precisely, we used macrocycle 1,[17] consti-
tuted of 4 tetracoordinate phosphorus atoms (PIV), two of
them being included in O–P–O linkages, the two others in
N–P–N linkages. The four PIV atoms are functionalized by
an azide group as noncyclic substituent. This macrocycle
possesses 4 asymmetric phosphorus centres, which can be
easily functionalized through the reactivity of the azide
groups (Figure 1).
Obviously, the presence of diastereoisomers in macro-
cycle 1 can be anticipated, and the zero value found for
its optical rotation strongly suggests a 50:50 ratio for the
stereoisomers on each phosphorus atom. If the macrocycle
is relatively planar or if it is flexible, five diasteroisomers
should be present; if the macrocycle exists in stable bent
forms, as shown by the X-ray diffraction structure of a re-
lated macrocycle,[18] nine diasteroisomers should exist.
However, the existence of such diastereoisomers was never
detected previously. Even 31P NMR spectroscopy, which is
very useful for the detection of topological differences in
constraint small cycles,[19] is inefficient with unconstraint
large cycles such as 1. The 31P NMR spectrum of this 36-
membered ring is shown in Figure 1; the 4 phosphorus
atoms give two sharp singlets, one for the O–P–O linkages
(Pc) and one for the N–P–N linkages (PЈc).
Introduction
Dendrons,[1] also called dendritic wedges,[2] play an im-
portant role in dendritic macromolecular science, generally
as large building blocks for the convergent synthesis of den-
drimers[3] or dendronized polymers,[4] and for the elabora-
tion of complex dendritic architectures.[5] Besides these clas-
sical uses of dendrons, their very particular topological fea-
tures have already found more original applications, for in-
stance in steric protection responsible for site isolation,[6] in
new modes of self-organization,[7] in enantiomer separation
of dendronized molecular knots,[8] as light harvesting ante-
nae,[9] or as rotaxane stoppers.[10] However, to the best of
our knowledge, dendrons were never used as amplifiers of
local topological phenomena. We thought that such big,
ramified, and conical substituents could facilitate the detec-
tion of previously undetectable phenomena, by amplifying
the local difference.
Being interested since a long time in the synthesis and
applications of dendrimers[11] and dendrons,[5,12] generally
based on phosphorus,[13,14] we decided to test this assump-
tion by grafting dendrons to compounds in which stereoiso-
mers must exist but were never detected. We previously re-
ported the synthesis of a series of phosphorus macro-
cycles,[15] obtained quantitatively by condensation reactions
between 2 equiv. of phosphorylated dialdehydes and
2 equiv. of phosphodihydrazides.[16] These compounds ap-
peared as the most suitable objects to confirm or invalidate
our hypothesis about the role of dendrons as topological
In a first attempt to detect the presence of diastereoiso-
mers, a small but relatively hindered compound was grafted
to the macrocycle. We used a Staudinger reaction between
the azide groups and the functionalized phosphane 2.[20]
The creation of the P=N double bond occurs easily with
the azide groups connected to the O–P–O linkages, and
more slowly with the azide groups connected to the N–P–
N linkages, as shown by 31P NMR spectroscopy after 1 h.
Indeed, the singlet corresponding to the O–P–O linkage has
[a] Laboratoire de Chimie de Coordination du CNRS,
205 route de Narbonne, 31077 Toulouse Cedex 4, France
Fax: + 33-5-61-55-30-03
E-mail: caminade@lcc-toulouse.fr
majoral@lcc-toulouse.fr
2556
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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First Example of Dendrons as Topological Amplifiers
SHORT COMMUNICATION
Figure 1. Synthesis of the macrocycle substituted by 4 dendrons and detection of diastereoisomers by 31P NMR spectroscopy. Only the
parts of the spectrum corresponding to Pc–N=P0 and PЈc–N=PЈ0 are shown for compound 5. The spectra were recorded at 25 °C for 1
and 3, at 80 °C for 5.
disappeared on behalf of two doublets at δ = 16.2 ppm linkages as expected.[22] No trace of diastereoisomers could
(P=N) and 55.7 ppm (P=S), whereas the singlet corre- be detected even with these relatively bulky substituents
sponding to the N–P–N linkages remains unchanged. This (Figure 1).
difference in the reactivity of both types of azide groups
In order to graft dendrons on the 4 aldehyde groups of
could be ascribed to a higher electron-withdrawing influ- 3 (one for each phosphorus atom included in the macro-
ence of the O–Aryl groups compared to the N–Me substitu- cycle), it was necessary to build a dendron possessing one
ents, favouring the reaction.[21] However, the macrocycle 3 NH2 group at the core, preferably pertaining to an hydra-
is finally isolated in nearly quantitative yield after reaction zine or hydrazide linkage to ensure the stability of the fu-
overnight. The 31P NMR spectrum gives two sets of two ture CH=N bond. The size of the dendron is also of crucial
sharp doublets corresponding to two types of P=N–P=S importance, because it must be large enough to induce de-
Eur. J. Inorg. Chem. 2006, 2556–2560
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C.-O. Turrin, A. Maraval, G. Magro, V. Maraval, A.-M. Caminade, J.-P. Majoral
SHORT COMMUNICATION
tectable differences, but not too large to allow the signals of angles occurs for the S=PЈc–N=PЈ0 linkage in form B,
of the macrocycle to be still detectable. For this purpose, and for the S=Pc–N=P0 linkage in form C. Dendrons hav-
we synthesized the second-generation dendron 4, obtained ing a single interaction should induce a moderate modifica-
by
a
Staudinger reaction between H2NNMeP(S)- tion of angles, for the S=PЈc–N=PЈ0 linkage in forms C and
(OC6H4PPh2)2[23] and N3P(S)(OC6H4CO2Me)2, synthesized D, and for the S=Pc–N=P0 linkage in forms B and D. Fi-
according to
(Scheme 1).
a
method we reported previously[24] nally, no interaction should occur for dendrons linked to
the S=PЈc–N=PЈ0 linkage in forms B and E, and for den-
drons linked to the S=Pc–N=P0 linkage in forms C and E.
It is known that 31P NMR chemical shifts are sensitive to
modification of the angles around the phosphorus atom.[19]
The minimization of interactions between the end groups
of the dendrons linked to some diastereoisomers of the
macrocycle necessitates modification of the angles at the
level of their “root” (the Pc–N=P0 and PЈc–N=PЈ0 linkages),
which is translated into variation of the chemical shift.
Thus, one may attribute the phenomenon observed here by
31P NMR spectroscopy to the existence of diastereoisomers.
It demonstrates without any ambiguity that our assumption
concerning the role of dendrons as amplifiers of local phe-
Scheme 1.
In the last step, 4 equiv. of the dendron 4 are condensed nomena was correct, and could be usefully applied to other
with the aldehyde groups of macrocycle 3, to afford the sec- systems.
ond-generation dendrimer 5, (Figure 1) as shown by the dis-
appearance of the signal corresponding to the CHO group
by 1H, 13C NMR, and IR spectroscopy. The 31P NMR
spectrum of compound 5 at room temperature gives rela-
tively sharp signals as expected for the phosphorus atoms
pertaining to the dendrons (a single singlet for P1 and PЈ1,
a single doublet for P2 and PЈ2, and a single doublet for P
and PЈ, not shown in Figure 1). Indeed, no particular steric
hindrance or lock of bonds is foreseeable around these
phosphorus atoms. However, broad signals are observed for
the P=N–P=S linkages in which the P=S group is part of
the macrocycle, in contrast to the 4 sharp doublets obtained
for the same atoms in macrocycle 3. Such behaviour is gen-
erally characteristic of a “frozen” internal structure; we
have observed previously such a phenomenon for high-gen-
eration polycationic phosphorus dendrimers,[25] but never
for such small generations. Variable-temperature 31P NMR
experiments carried out in DMSO up to 80 °C improved
only slightly the definition of the signals. A slightly better
resolution is observed for the P=N groups linked to the
macrocycle (P0 and PЈ0), compared to the P=S groups in-
cluded in the macrocycle (Pc and PЈc), whose degree of free-
dom is smaller. The better resolution observed for P0 com-
pared to PЈ0 is presumably due to the higher degree of free-
Figure 2. Schematization of the five diastereoisomers of compound
dom of Pc compared to PЈc. Indeed, all the X ray structure
determinations carried out up to now for PcN(Me)-
NC=CHC6H4OPЈc linkages have shown that all atoms from
Pc to O lie in a same plane, whereas PЈc is out of this
plane.[26] At least 3 sharp doublets can be distinguished for
P0, which might correspond to the three main topological
features expected; indeed, the dendron connected to P0 can
be surrounded with either zero, one or two dendrons, in the
half space to which it belongs, defined by the average plane
of the macrocycle (Figure 2). In form A, the dendrons
linked to Pc and PЈc have two interactions, thus inducing a
large modification of angles at the level of the S=Pc–N=P0
5, showing the interactions between dendrons, and the movement
to avoid these interactions, which modify the angles around the Pc
(and P0), and PЈc (and PЈ0) phosphorus atoms.
Experimental Section
General: All manipulations were carried out with standard high-
vacuum and dry-argon techniques. The solvents were freshly dried
and distilled (THF and diethyl ether with sodium/benzophenone,
pentane and CH2Cl2 with phosphorus pentoxide). Classical 1H,
13C, 31P NMR spectra were recorded with Bruker AC 200, AC
250, DPX 300, or AMX 400 spectrometers. References for NMR
and S=PЈc–N=PЈ0 linkages. The same level of modification chemical shifts are 85% H3PO4 for 31P NMR, SiMe4 for 1H and
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First Example of Dendrons as Topological Amplifiers
SHORT COMMUNICATION
13C NMR spectroscopy. The attribution of 13C NMR signals has
been done using Jmod, two-dimensional HMBC, and HMQC,
broad-band or CW 31P decoupling experiments when necessary.
The numbering used for NMR assignments is depicted in Figure 3.
IR analyses were recorded with Perkin–Elmer FT 1725x. Elemental
analyses were performed by the Service d’Analyse du LCC, Toul-
4.4 Hz, C2 ), 128.80 (d, 2JCP = 12.3 Hz, C2m), 131.06 (s, C2 ), 132.68
i
3
2
4
(d, JCP = 11.7 Hz, C2o), 132.85 (s, C2p), 132.91 (s, C2 ), 134.75 (d,
2JCP = 12.0 Hz, C1 ), 154.16 (d, JCP = 7.9 Hz, C1 ), 155.75 (d,
3
2
1
1
2JCP = 8.2 Hz, C ), 166.61 (s, C=O) ppm. IR (KBr): ν = 1718 cm–1
˜
2
(CO). C69H61N4O14P5S3 (1421.31): calcd. C 58.31, H 4.33, N 3.94;
found C 58.42, H 4.51, N 3.80.
ouse, France. Compounds 1,[17] 2,[20] and H2NNMeP(S)-
Synthesis and Characterization of N3P(S)(OC6H4CO2Me)2: To a
solution containing 4.000 mL (39.3 mmol) of PSCl3 in THF
(100 mL) at –95 °C was added dropwise a solution of 15.584 g
(82.7 mmol, slight excess) of methyl 4-hydroxybenzoate and
16.5 mL (82.7 mmol, slight excess) of triethylamine in THF
(150 mL). The reaction mixture was allowed to reach room tem-
perature overnight and the white precipitate was filtered off. After
solvent removal under reduced pressure, the crude oil was dissolved
in acetone (50 mL) and treated with 5.850 g (90 mmol, slight ex-
cess) of sodium azide at room temperature for 2 d. The precipitate
was removed by filtration, and after solvent removal under reduced
pressure, the azide was extracted from the crude oily product with
pentane. Evaporation of pentane afforded the expected compound
[23]
(OC6H4PPh2)2
were synthesized according to methods pre-
viously reported.
as
a
white powder in 75% yield (12.00 g). 31P{1H} NMR
1
(81.015 MHz, CDCl3): δ = 61.3 (s) ppm. H NMR (250.133 MHz,
3
CDCl3): δ = 3.89 (s, 6 H, OMe), 7.27 (d, JHH = 7.6 Hz, 4 H,
3
Harom), 8.06 (d, JHH = 7.6 Hz, 4 H, Harom) ppm. 13C{1H } NMR
3
Figure 3. Numbering scheme used for NMR assignments.
(62.896 MHz, CDCl3): δ = 52.33 (s, OMe), 121.17 (d, JCP
=
4.4 Hz, Cb), 128.23 (s, Cd), 131.66 (s, Cc), 153.22 (d, 2JCP = 8.0 Hz,
Ca), 165.96 (s, C=O) ppm. C16H14N3O6PS (407.34): calcd. C 47.18,
H 3.46, N 10.32; found C 47.19, H 3.52, N 10.19.
Synthesis and Characterization of 3: To a solution containing 51 mg
(0.050 mmol) of 1 in dichloromethane (5 mL) at room temp. was
added 67 mg (0.23 mmol, slight excess) of Ph2P(C6H4CHO). The
reaction mixture was stirred at room temp. overnight and concen-
trated to dryness. The crude residue was washed with a THF/pen-
tane mixture to afford 3 as a white powder in 92% yield (95 mg).
Synthesis and Characterization of 5: To a solution containing 65 mg
(0.032 mmol) of 3 in dichloromethane (5 mL) at room temp. was
added 182 mg (0.128 mmol) of 4. The reaction mixture was stirred
at room temp. overnight and the residue was precipitated with di-
ethyl ether, filtered and washed twice with 20 mL of diethyl ether to
2
31P{1H}NMR (81.015 MHz, CDCl3): δ = 13.4 (d, JPP = 27 Hz,
2
2
PЈc = N), 15.8 (d, JPP = 32 Hz, Pc=N), 55.9 (d, JPP = 32 Hz,
afford 5 as a white powder in 95% yield (230 mg). 31P{1H} NMR
2
P0=S), 60.8 (d, JPP
=
27 Hz, PЈ0=S) ppm. 1H NMR
2
(161.975 MHz, CDCl3): δ = 14–15 (m, PЈ0=N), 16.3 (d, JPP
=
(200.132 MHz, CDCl3): δ = 3.14 (br. d, 3JHP = 7.5 Hz, 12 H, NMe),
6.94–7.91 (m, 76 H, CH=N, Harom), 9.91 (s, 2 H, CHO), 9.95 (s, 2
H, CHO) ppm. 13C{1H} NMR (62.896 MHz, CDCl3): δ = 32.65
31 Hz, P0=N), 16.5 (d, 2JPP = 31 Hz, P0=N), 16.7 (d, 2JPP = 31 Hz,
2
2
P0=N), 17.05 (d, JPP = 30 Hz, P=N, PЈ=N), 53.2 (d, JPP
=
2
30.0 Hz, P2=S, PЈ2=S), 54.6 (d, JPP = 31 Hz, Pc=S), 54.9 (m,
Pc=S), 59.3–60.0 (m, PЈc=S), 63.2 (s, P1, PЈ1) ppm. 1H NMR
(400.130 MHz, CDCl3): δ = 3.21 (br. s, 12 H, PЈ0-NMe), 3.37 (br.
s, 12 H, P1-NMe and PЈ1-NMe), 3.84 (s, 48 H, CO2Me), 7.02–7.98
(m, 256 H, CH=N, Harom) ppm. 13C{1H} NMR (100.613 MHz,
CDCl3): δ = 33.23–33.86 (m, NMe), 52.53 (s, OMe), 121.78 (d, 3JCP
2
2
3
(d, JCP = 9.4 Hz, NMe), 121.57 (br. s, Cc ), 126.96 (s, Cc ), 127.66
(d, JCP = 108.5 Hz, C0 ), 128.60 (d, JCP = 12.0 Hz, C0m), 128.81
1
i
3
(d, JCP = 12.3 Hz, C0Јm), 129.07 (d, JCP = 12.0 Hz, C0Ј3), 129.11
3
3
1
3
3
(d, JCP = 108.4 Hz, C0Јi), 129.25 (d, JCP = 11.6 Hz, C0 ), 131.19
(s, C0 , C0Јp), 132.62 (d, JCP = 11.0 Hz, C0o), 132.80 (d, JCP
=
p
2
2
12.0 Hz, C0Јo), 132.90 (s, Cc ), 133.33 (d, JCP = 11.8 Hz, C0 ),
4
2
2
= 4.8 Hz, C2 , C2Ј2, Cc ), 122.05 (dd, 3JCP = 5.2 Hz, 3JCP = 12.9 Hz,
2
2
2
4
133.52 (d, JCP = 11.8 Hz, C0Ј2), 134.21 (s, C0 ), 134.44 (s, C0Ј4),
C1 , C1Ј2), 125.85 (d, JCP = 108.2 Hz, C1Ј4, C1 ), 128.35 (d, JCP
2
1
4
1
135.21 (d, 1JCP = 102.2 Hz, C0 ), 136.47 (d, 1JCP = 101.8 Hz, C0Ј1),
1
= 108.1, C2 , C2Јi), 128.53 (s, Cc ), 128.96 (s, C2 , C2Ј4), 129.24 (br.
i
3
4
3
2
1
138.51 (d, JCP = 10.0 Hz, CH=N), 151.57 (d, JCP = 9.6 Hz, Cc ),
191.70 (s, CHO) ppm. C108H92N12O8P8S4 (2062.02): calcd. C 62.91,
H 4.50, N 8.15; found C 63.02, H 4.61, N 7.92.
d, 3JCP = 13.1 Hz, C0m, C2m, C0 , C0Јm, C2Јm, C0Ј3), 131.48 (s, C2 ,
3
3
C0 , C2Ј3, C0Јp), 133.05 (br. d, 2JCP = 10.8 Hz, C0 , C2 , C0Јo, C2Јo),
p
o
o
133.34 (br. s, C0 , C2 , C0Ј4, C2Јp), 133.80 (br. s, C0 , C0Ј2), 135.18
4
p
2
(d, JCP = 11.9 Hz, C1 , C1Ј3), 138.81–139.68 (m, CH=N), 152.17
2
3
Synthesis and Characterization of 4: To a solution containing
663 mg (1.1 mmol, slight excess) of H2NNMeP(S)(OC6H4PPh2)2 in
dichloromethane (10 mL) at room temp. was added 815 mg
(2 mmol) of N3P(S)(OC6H4CO2Me)2. The reaction mixture was
stirred at room temp. overnight and concentrated to dryness. The
crude residue was flash-chromatographed on silica gel (dichloro-
(br. s, Cc ), 154.25 (br. s, C1 , C1Ј1), 156.15 (d, JCP = 8.9 Hz, C2 ,
1
1
2
1
C Ј1), 166.92 (s, C=O) ppm. IR (KBr): ν = 1718 cm–1 (CO).
˜
2
C384H328N28O60P28S16 (7675.2): calcd. C 60.09, H 4.31, N 5.11;
found C 60.01, H 4.40, N 5.01.
methane as eluent) to afford 4 as a white powder in 90% yield
2
(1.28 g). 31P{1H}NMR (81.015 MHz, CDCl3): δ = 16.1 (d, JPP
=
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C.-O. Turrin, A. Maraval, G. Magro, V. Maraval, A.-M. Caminade, J.-P. Majoral
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Published Online: May 18, 2006
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