Convergent dendron approach using a phenol triallyl building block: Synthesis of a phenol dendron containing 27 allyl termini

Article abstract of DOI:10.1039/b210358n

A successful convergent dendron synthesis is achieved using a phenol triallyl (AB₃) building block by a sequence of reactions consisting of hydrosilylation-phenol protection. The key feature is the hydrosilylation using dimethylchloromethylsilane that avoids the side dehydrohalogenation previously found with linear branches containing β-hydrogens. The advantage of the AB₃ building block is the multiplication of the dendron branches by three at each generation, which allows the synthesis of the second-generation 27-allyl phenol dendron.

Full text of DOI:10.1039/b210358n

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Convergent dendron approach using a phenol triallyl building block:
synthesis of a phenol dendron containing 27 allyl termini
Sylvain Nlate,*a Jean-Claude Blaisb and Didier Astruc*a
a
ꢀ
´
Groupe Nanosciences Moleculaires et Catalyse, LCOO (UMR CNRS N 5802),
Universite Bordeaux I, 351 Cours de la Liberation, Talence Cedex, France.
´
´
E-mail: s.nlate@lcoo.u-bordeaux.fr; d.astruc@lcoo.u-bordeaux.fr
Laboratoire de Chimie Structurale Organique Biologique (EP CNRS Nꢀ 103),
b
´
Universite Paris VI, 4 Place Jussieu, 75252, Paris, France. E-mail: blaisjc@moka.ccr.jussieu.fr
Received (in Montpellier, France) 14th October 2002, Accepted 25th November 2002
First published as an Advance Article on the web 6th December 2002
A successful convergent dendron synthesis is achieved using a phenol triallyl (AB3) building block by a
sequence of reactions consisting of hydrosilylation-phenol protection. The key feature is the hydrosilylation
using dimethylchloromethylsilane that avoids the side dehydrohalogenation previously found with linear
branches containing b-hydrogens. The advantage of the AB3 building block is the multiplication of the dendron
branches by three at each generation, which allows the synthesis of the second-generation 27-allyl phenol
dendron.
Since Vo¨gtle’ seminal report of the first iteration 25 years ago,1
dendrimer syntheses have been achieved using a large variety of
strategies and frameworks, and numerous potential application
are forecast.2–4 The divergent synthetic method has been the
one used most because it is the easiest one to carry out, and also
because it is the only one that can lead to large dendrimers even
if defects cannot be discarded. The convergent method
appeared in the early 90’s with the reports of the groups of
Frechet,5 Miller6 and Moore7 and has been recently reviewed.8
It is elegant, and its great advantages are that it can lead to pure
dendrons that are bifunctional: they can be decorated on the
periphery for a given function or property, and they still bear
another functional group at the focal point for attachment
onto a dendritic core, nanoparticle, polymer, surface, etc.
Although dendrons have a limited size because their synthesis
cannot be extended due to steric bulk around the focal point
that needs be reactive, it is likely that they will play a key role
in future nanotechnology involving molecular components.
We have already shown that small dendrons bearing ferrocenyl-
silyl or amidoferrocenyl termini are efficient sensors for the
stirred for 48 h at ambient temperature. The solution was fil-
tered on Celite, and the solvent was removed under vacuum.
After column chromatography on silica gel with a mixture of
pentane : Et2O (95 : 5) as eluent, 2 was obtained as a colorless
oil (1.020 g, 1.74 mmol, 90%). Elemental analysis calcd for
C26H49Si3Cl3S: H 8.45, C 53.44; found: H 8.35, C 53.00%.
1H NMR (CDCl3): dppm 7.21 (s, C6H4 , 4H); 2.72 (s, ClCH2 ,
6H); 2.50 (s, SCH3 , 3H); 1.60 (m, CH2 , 6H); 1.06 (m, CH2 ,
6H); 0.57 (m, SiCH2 , 6H); 0.05 (s, SiCH3 , 18H). 13C NMR
(CDCl3): dppm 143.3 (Cq , ArS); 140.0 (Cq , Ar); 127.4 (CHAr);
114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2); 30.2 (ClCH2); 17.5
(CH2CH2Si); 15.9 (SCH3); 14.2 (CH2Si); ꢁ4.0 (SiMe).
Triiodo-trisilicium aryl sulfide 3
A mixture of 2 (0.500 g, 0.855 mmol) and, NaI (1.77 g, 12.8
mmol), in 2-butanone, was stirred for 24 h at 80 ꢀC. After
removal of the solvent under vacuum, the residue was
extracted with Et2O (50 ꢂ 3 mL). This solution was washed
with 3 ꢂ 50 mL of a saturated aqueous solution of Na2S2O3 .
The solvent was removed under vacuum, which gave 3 as a
yellow oil (0.624 g, 0.726 mmol 85%). Elemental analysis calcd
for C26H49Si3I3S: H 5.75, C 36.37; found: H 5.55, C 35.82%.
1H NMR (CDCl3): dppm 7.21 (s, C6H4 , 4H); 2.50 (s, SCH3 ,
3H); 1.96 (s, ICH2); 1.60 (m, CH2 , 6H); 1.06 (m, CH2 , 6H);
0.60 (m, SiCH2 , 6H); 0.09 (s, SiCH3 , 18H). 13C NMR
(CDCl3): dppm 143.1 (Cq , ArS); 140.6 (Cq , Ar); 127.5 (CHAr);
114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2); 17.5 (CH2CH2Si);
16.00 (SCH3); 15.7 (CH2Si); ꢁ3.39 (SiMe); ꢁ13.60 (CH2I).
dihydrogenophosphate anion when, as part of thiolate ligands,
´
9
they are attached to gold nanoparticles. So far, Frechet’s den-
drons (AB2 units) have been widely used in the literature, and
AB3 units are relatively scarce.8 We now report another conver-
gent dendron synthesis based on a phenoltriallyl building block
up to the second generation, i.e. 27 allyl groups. This molecular
brick has already been used to synthesize dendrimers, but these
attempts of convergent syntheses of dendrons have so far been
frustrated by side reactions.10 Indeed the synthesis involved the
nucleophilic substitution of the iodo group of the iodoalkyl
branches by the phenol triallyl, but dehydrohalogenation
became dominant. The present strategy prevents this side reac-
tion by removing b-hydrogens on the termini.
9-allyl-trisilicium aryl sulfide 5
A mixture of triallylmethylphenol 4 (0.598 g, 2.620 mmol) and
K2CO3 (0.413g, 2.990 mmol), in 20 mL DMF, was stirred for
30 min at ambient temperature. The triiodo-trisilicium aryl sul-
fide 3 (0.500 g, 0.582 mmol) in solution in 5 mL DMF was
added, and the reaction mixture was stirred at 80 ꢀC 48 h.
The solvent was removed under vacuum and the product
was extracted with Et2O. The organic layer was washed several
times with water in order to remove DMF, then dried over
Experimental section
Trichloro-trisilicium aryl sulfide 2
A mixture of triallyl sulfide dendron 1 (0.50 g, 1.93 mmol) and
SiHMe2(CH2Cl) (0.945 g, 8.706 mmol, 1.1 mL), in Et2O, was
178
New J. Chem., 2003, 27, 178–183
DOI: 10.1039/b210358n
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sodium sulfate, filtered and evaporated under vacuum. After
column chromatography on silica gel using a mixture of pen-
tane : Et2O (9 : 1) as the eluent, 5 was obtained as a colorless
oil (0.410 g, 0.353 mmol, 60%). Elemental analysis calcd for
C74H106Si3O3S: H 9.21, C 76.62; found: H 9.23, C 76.52%.
MALDI TOF mass spectrum, m/z: 1160.04 [M]+ (calcd
2H); 2.59 (q, CH3CH2CO, 2H); 1.96 (s, ICH2 , 6H); 1.65 (m,
CH2 , 6H); 1.05 (m, CH2 , 6H); 0.60 (m, SiCH2 , 6H); 0.08
(s, SiCH3 , 18H). 13C NMR (CDCl3): dppm 175.8 (CO);
150.0 (Cq , ArO); 145.1 (Cq , Ar); 127.3 (CHAr); 120.8 (CHAr);
43.6 (Cq–CH2); 41.8 (CH2); 27.6 (CH2); 17.6 (CH2CH2Si); 15.7
(CH2Si); 9.1 (CH3CH2CO); ꢁ3.0 (SiMe); ꢁ13.1 (CH2I).
1
1159.98). H NMR (CDCl3): dppm 7.19 (d, C6H4 , 10H); 6.84
(d, C6H4 , 6H); 5.54 (m, CH2=CH, 9H); 5.00 (m, CH2=CH,
18H); 3.49 (s, CH2O, 6H); 2.45 (s, SCH3 , 3H); 2.43 (d,
CH2=CH–CH2 , 18H); 1.66 (m broad, CH2 , 6H); 1.20 (m
broad, CH2 , 6H); 0.61 (m broad, CH2 , 6H); 0.05 (s, SiMe,
18H). 13C NMR (CDCl3): dppm 159.41 (Cq , ArO); 145.00
(Cq , ArS); 137. 25 (Cq , Ar); 134.78 (CH2=CH); 134.62 (Cq ,
Ar); 127.46 (CHAr); 127.09 (CHAr); 117.43 (CH2=CH);
114.64 (CHAr); 113.55 (CHAr); 60.15 (CH2O); 43.53 (Cq–
CH2); 42.65 (Cq–CH2); 42.01 (CH2); 17.70 (CH2); 16.01
(SCH3); 14.64 (CH2); ꢁ4.56 (SiMe).
9-Allyl-trisilicium phenol dendron 9
A mixture of triallylmethylphenol 9 (0.680 g, 2.978 mmol) and,
K2CO3 (0.417 g, 2.978 mmol), in 20 mL DMF, was stirred
for 30 min at ambient temperature. The protected triiodo-
trisilicium phenol propionate 8 (0.440 g, 0.497 mmol) in solu-
tion in 5 mL DMF was added, and the reaction mixture was
stirred at 80 ꢀC for 48 h. Then 0.360 g K2CO3 and 0.600 mL
water were added, and the reaction mixture was stirred at
80 ꢀC for 24 h. The solvent was removed under vacuum and
the product extracted with Et2O. The organic layer was washed
several times with water in order to remove DMF, then dried
over sodium sulfate, filtered and evaporated under vacuum.
After column chromatography on silica gel using a mixture of
pentane : Et2O (9 : 1) as the eluent, 9 was obtained as a color-
less oil (0.380 g, 0.336 mmol, 68%). Elemental analysis calcd
for C73H104Si3O4 : H 9.28, C 77.60; found: H 9.31, C 77.09%.
MALDI TOF mass spectrum, m/z: 1151.77 [M + Na]+ (calcd
1152.87). 1H NMR (CDCl3): dppm 7.20 (d, C6H4 , 6H); 7.09
(d, C6H4 , 2H); 6.87 (d, C6H4 , 6H); 6.67 (d, C6H4 , 2H); 5.56
(m, CH2=CH, 9H); 5.03 (m, CH2=CH, 18H); 4.62 (s, OH,
1H); 3.52 (s, CH2O, 6H); 2.43 (d, CH2=CH–CH2 , 18H); 1.61
(m broad, CH2 , 6H); 1.14 (m broad, CH2 , 6H); 0.60 (m broad,
CH2 , 6H); 0.06 (s, SiMe, 18H). 13C NMR (CDCl3): dppm
159.38 (Cq , ArO); 152.84 (Cq , ArO); 139.77 (Cq , Ar); 137.17
(Cq , Ar); 134.74 (CH2=CH); 127.51 (CHAr); 127.40 (CHAr);
117.40 (CH2=CH); 114.65 (CHAr); 113.52 (CHAr); 60.08
(CH2O); 43.13 (Cq–CH2); 42.60 (Cq–CH2); 41.96 (CH2);
17.63 (CH2); 14.58 (CH2); ꢁ4.61 (SiMe).
Trichloro-trisilicium phenol dendron 6
A mixture of triallyl dendron 4 (1 g, 4.380 mmol) and SiH-
Me2(CH2Cl) (2.279 g, 24.091 mmol, 1.2 mL), in Et2O, was stir-
red for 48 h at ambient temperature. The solution was filtered
on Celite, and the solvent was removed under vacuum. After
column chromatography on silica gel with a mixture of pen-
tane : Et2O (95 : 5) as eluent, 6 was obtained as a colorless
oil (2.209 g, 3.985 mmol, 91%). Elemental analysis calcd for
C25H47Si3Cl3O: H 8.55, C 54.18; found: H 8.35, C 54.62%.
1H NMR (CDCl3): dppm 7.12 (d, C6H4 , 2H); 6.76 (d, C6H4 ,
2H); 4.65 (s, OH, 1H); 2.72 (s, ClCH2 , 6H); 1.60 (m, CH2 ,
6H); 1.06 (m, CH2 , 6H); 0.57 (m, SiCH2 , 6H); 0.05 (s, SiCH3 ,
18H). 13C NMR (CDCl3): dppm 155.0 (Cq , ArO); 140.0 (Cq ,
Ar); 127.5 (CHAr); 114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2);
30.5 (ClCH2); 17.5 (CH2CH2Si); 14.5 (CH2Si); ꢁ4.1 (SiMe).
Triiodo-trisilicium phenol dendron 7
A mixture of 6 (2.178 g, 3.930 mmol) and, NaI (2.279 g, 58.945
mmol), in 2-butanone, was stirred for 24 h at 80 ꢀC. After
removal of the solvent under vacuum, the residue was
extracted with Et2O (50 ꢂ 3 mL). This solution was washed
with 3 ꢂ 50 mL of a saturated aqueous solution of Na2S2O3 .
The solvent was removed under vacuum, which gave 7 as a yel-
low oil (3.234 g, 3.903 mmol, 99%). Elemental analysis calcd
for C25H47Si3I3O: H 5.72, C 36.24; found: H 5.64, C 37.01%.
1H NMR (CDCl3): dppm 7.12 (d, C6H4 , 2H); 6.76 (d, C6H4 ,
2H); 4.76 (s, OH, 1H); 1.94 (s, ICH2 , 6H); 1.60 (m, CH2 ,
6H); 1.05 (m, CH2 , 6H); 0.58 (m, SiCH2 , 6H); 0.07 (s, SiCH3 ,
18H). 13C NMR (CDCl3): dppm 155.1 (Cq , ArO); 140.6 (Cq ,
Ar); 127.5 (CHAr); 114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2);
17.5 (CH2CH2Si); 15.7 (CH2Si); ꢁ2.95 (SiMe); ꢁ13.1 (CH2I).
27-Allyl-9-siliciumphenol dendron 10
A mixture of 9-allyl-3-silicium phenol dendron 9 (0.500 g,
0.442 mmol) and, K2CO3 (0.070 g, 0.500 mmol), in 10 mL
DMF, was stirred for 30 min. at ambient temperature. The
protected tri-iododendron 8 (0.087 g, 0.098 mmol) in solution
in 3 mL DMF was added, and the reaction mixture was stirred
at 80 ꢀC 72 h. Then, 0.052 g K2CO3 and 0.090 mL water were
added, and the reaction mixture was stirred at 80 ꢀC for 48 h.
The solvent was removed under vacuum and the product was
extracted with Et2O. The organic layer was washed several
times with water in order to remove DMF, then dried over
sodium sulfate, filtered and evaporated under vacuum. After
chromatography on silica gel using a mixture of pentane :
Et2O (9 : 1) as the eluent, 10 was obtained as a colorless oil
(0.169 g, 0.044 mmol, 45%). Elemental analysis calcd for
Triiodo-trisilicium phenol propionate 8
13.35 mL (1.776 g, 9.655 mmol) of a fresh solution of
C2H5COI, prepared by slowly adding 2.4 mL C2H5COCl to
5 g of Me3SiI and 22.6 mL CH2Cl2 , was added to a solution
of 7 (2 g, 2.414 mmol) in 30 mL CH2Cl2 . Then, NEt2Pri
(0.030 g, 0.214 mmol) was also added. The reaction mixture
was stirred for 16 h at ambient temperature. After removal
of the solvent under vacuum, the residue was extracted with
CH2Cl2 (3 ꢂ 40 ml). The organic solution was washed with a
saturated aqueous solution of sodium bicarbonate, then with
an aqueous solution saturated with Na2S2O3 and dried over
sodium sulfate. After removing the solvent under vacuum,
the protected phenol dendron 8 was obtained as an orange–
yellow oil. Elemental analysis calcd for C28H51Si3I3O: H 5.92,
C 36.72; found: H 5.72, C 36.21%. (1.943 g, 2.196 mmol, 91%).
1H NMR (CDCl3): dppm 7.24 (d, C6H4 , 2H); 7.03 (d, C6H4 ,
Scheme 1
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Scheme 2
C244H356Si12O13 : H 9.36, C 76.43; found: H 9.54, C 75.72.
MALDI TOF mass spectrum, m/z: 3856.37 [M + Na]+ (calcd
3857.51). H NMR (CDCl3): dppm 7.20 (d, C6H4 , 24H); 7.01
(d, C6H4 , 2H); 6.88 (d, C6H4 , 24H); 6.67 (d, C6H4 , 2H);
5.55 (m, CH2=CH, 27H); 5.02 (m, CH2=CH, 54H); 4.67 (s,
OH, 1H); 3.51 (s, CH2O, 6H); 2.43 (d, CH2=CH–CH2 ,
54H); 1.63 (m broad, CH2 , 18H); 1.15 (m broad, CH2 ,
18H); 0.61 (m broad, CH2 , 18H); 0.06 (s, SiMe, 54H). 13C
NMR (CDCl3): dppm 159.27 (Cq , ArO); 159.00 (Cq , ArO);
139.08 (Cq , Ar); 137.07 (Cq , Ar); 134.63 (CH2=CH); 127.30
1
Scheme 3
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Scheme 4
(CHAr); 127.05 (CHAr); 117.32 (CH2=CH); 113.40 (CHAr);
113.31 (CHAr); 60.02 (CH2O); 42.92 (Cq–CH2); 42.49 (Cq–
CH2); 41.86 (CH2); 17.54 (CH2) 14.43 (CH2); ꢁ4.69 (SiMe).
already been successfully used by Seyferth for dendrimer synth-
esis.12 The chloromethyl termini are transformed into iodo-
methyl by reaction with sodium iodide in refluxing butanone,
then nucleophilic substitution by phenol triallyl leads to the
next generation. With 1, these reactions can be carried out in
a straightforward way without any protection (Scheme 2).
With the phenol triallyl brick, it is of course necessary to
protect the phenol group during the dendron construction,
i.e. this synthesis is now a real convergent strategy for the con-
struction of a dendron that is ultimately deprotected either to
react on a protected dendron or onto another framework.
Thus, after introduction of the iodo groups on the branches,
the phenol function is protected by the propionate before
synthesis of the first generation nona-allyl dendron (Scheme
3). The protection allows the condensation of the phenol
triallyl AB3 unit, and later, of the phenol nona-allyl AB9
Results and discussion
Dendritic growth has been carried out in parallel using the
arylsulfide triallyl 1 and the phenol triallyl 4 that is an AB3 unit
for dendritic or hyperbranched polymer construction. The
model 1 serves to test the divergent reaction of the allyl termini
since it does not need protection. The compounds 1 and 4 have
been reported, and are accessible by perallylation of the CpFe+
complexes of the aryl ether and aryl sulfide respectively10b,11
(Scheme 1).
The growing strategy is based on the hydrosilylation of the
allyl branches using chloromethyldimethylsilane that has
Fig. 1 MALDI-TOF mass spectrum of the 27-allyl dendron 10 (molecular peaks [M + Na]+ at 3856.37).
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Scheme 5
first-generation dendron. This convergent synthesis involving
protection–deprotection has been carried out until the synth-
esis of the 27-allyl phenol that has been obtained in its depro-
tected form, i.e. as the second-generation dendron (Scheme 4).
The first generation 9-allyl and second generation 27-allyl den-
drons have been characterized by their standard spectroscopic
and analytical data, but also especially by the prominent mole-
cular peak in the MALDI TOF mass spectrum. In these mass
spectra (see Fig. 1), impurities due to unfinished reactions are
detected but are present only in relatively small amounts. The
side peak at 3814.32 Dalton that is close to the molecular peak
results from a small impurity corresponding to the phenol-
triallyl AB3 unit 4 in which one allyl group is missing (unfin-
ished synthesis during the CpFe+-induced allylation
reaction). The second side peak at 3557.27 Dalton corresponds
to a dendron in which the ninth tripod is missing (again
uncompleted introduction of the phenoltriallyl group 4). The
third side peak at 3515.36 Dalton corresponds to the combina-
tion of both impurities (unfinished triallylation and unfinished
introduction of the phenoltriallyl group 4). The peak at
1277.77 Dalton corresponds to exactly one third of the mole-
cular peak, and is thus attributed to this molecular peak with
a tricationic charge (m/z with z ¼ 3).
nucleophilic substitution was considerably slowed down due
to the steric bulk of the two other nearby covalently attached
dendrons. Thus, dehydrohalogenation of the third iodoalkyl
branch proceeded quantitatively (Scheme 5). With the new
triiodoalkyl dendron, the lack of b positions inhibits the dehy-
drohalogenation reaction.
Conclusion
The arylsulfide 1 and phenol derivative 4 are easily available in
relatively good yields in only three steps from p-chlorotoluene
via CpFe+-mediated synthesis. Thus, the chemistry reported
here is easily carried out and should prove useful. The aryl-
sulfide is a good model for the convergent synthesis from 4.
It is also a good model for the synthesis of aryl thiol dendrons
that could bind gold nanoparticles and surfaces. The conver-
gent synthesis of the 27-allyl phenol dendron from 4 was
successful and proceeds in relatively good yields. Its advantage
is that it multiplies the number of branches by three at each
generation, allowing a very rapid growth up to 27 allyl
branches in the second-generation dendron.
This successful synthesis of the 27-allyl dendron phenol con-
trasts with the preceding attempt using a triiodoalkyl phenol
bearing hydrogens in the position b relative to the iodo group.
In that case, nucleophilic substitution proceeded correctly
for two of the branches because it was faster than
dehydrohalogenation. A rather pure 19-allyl phenol dendron
was obtained (as shown in Scheme 5), however, the third
Acknowledgements
We are grateful to the Institut Universitaire de France (IUF),
the Centre National de la Recherche Scientifique (CNRS) and
the Universities Bordeaux I and Paris VI for financial support.
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´
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For recent books, see: (a) G. R. Newkome, C. N. Moorefield and
F. Vo¨gtle, Dendrimers and Dendrons. Concepts, Syntheses, Appli-
cation, Wiley-VCH, Weinheim, 2001; (b) Dendrimers and Other
´
Dendritic Polymers, eds. J. M. J. Frechet and D. A. Tomalia,
Wiley, Chichester, 2001.
the organic derivatives 1 and 210b
New J. Chem., 2003, 27, 178–183
183