Poly(propylenimine) dendrimers: large-scale synthesis via
heterogeneously catalyzed hydrogenation
Citation for published version (APA):
Berg, van den, E. M. M., & Meijer, E. W. (1993). Poly(propylenimine) dendrimers: large-scale synthesis via
heterogeneously catalyzed hydrogenation.
Angewandte Chemie - International Edition
,
32
(9), 1308-1311.
https://doi.org/10.1002/anie.199313081
DOI:
10.1002/anie.199313081
Document status and date:
Published: 01/01/1993
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GPC
analyses, coupling reactions
can
be excluded. Interest-
ingly, the solution behavior of poly(ethy1ene oxide), a typical
random-coil polymer, is quite different from that
of
the two
types of highly functional poly(trirnethy1ene imine) cascade
molecules prepared by
us.
As
apparent from Figure
5,
the
polyamine-functional cascade molecules require higher
elu-
tion volumes, which increase with increasing number of gen-
erations.
In
agreement with earlier observations,".
''I
it ap-
I
M
[g
mol-
12 14 16
18
20 22 24 26
v,
[mLI
-
Fig.
5.
Correlation of molecular weight
M
and elution volume
V,
in GPC
analyses
of
poly(ethy1ene oxide)
(o),
polynitrile-
(A)
and polyamine-functional
poly(trimethylene imines)
(*).
pears likely that cascade molecules when compared with
conventional random-coil polymers exhibit higher densities,
which increase with increasing molecular weight. The very
high elution volumes found for the polynitrile-functional
cascade molecules may result from adsorption phenomena,
which do not interfer with the
GPC
analysis of the molecular
weight distributions. In
all
three polymers there exists
a
lin-
ear correlation between molecular weight, determined by
vapor pressure osrnometry, and elution volume.
In
conclusion, the divergent synthesis involving cyano-
ethylation and catalytic hydrogenation efficiently doubles
the amino groups. With this reaction sequence poly(tri-
methylene imine) cascade molecules can be prepared in high
yields and purity without requiring extensive purifications
and excess reagent.
Experimental Procedure
Typical syntheses for the higher generations G4N. G4A, and GSN; the yields
are summarized in Table
I.
G4N:
To
a cold solution of G3A (2.56
g,
2.10
mmol)
in methanol
(20
mL) were
added acrylonitrile (20.0 mL.
305
mmol) (distilled over CaH, and stabilized
with 0.05 wt% hydroquinonemonomethyl ether) within
90
minutesat
5
-
10
C.
After stirring
1
h at 10
C
and 4 days at 60 'C. the quantitative bis(cyanoethyla-
tion) was obtained. For separation of the stabihzer. the reaction mixture was
passed through a neutral AI,O, column and eluted with methanol/ethyl acetate
(7:3).
and the methanol content was increascd slowly to 100%. The solvent was
evaporated in vacuo to obtain 5.02
g
(96%) of the highly viscous G4N. Correct
elemental analysis for C135HLZ2N+h (2489.55
gmol-I);
'HNMR (400 MHz,
CD,CI,):
/i
=
1.62
(q,
J
=
7
Hz.
42H; CH,CH,CH,). 2.56 (ni. 132H; CH,N),
2.83
(t.
J
=7 Hz, 48H; CH,CN),
"C
NMR (100 MHz, CD,CI,):
6
=17.3
(CH,CN), 23.9, 25.1 (CH,CH,CH,), 49.9 (CH,CH,CN), 51.7, 51.9, 52.2, 52.4
(CH,N), 119.5 (CN); IR (neat). i,[cm-']
=
2920,2800,2220, 1450,1410.1350,
1250, 1160, 1120, 1050.
G4A:
The hydrogenation was carried out in a
2
L stainless steel autoclave
(Biichi AG, UsteriSwitzerland). To
100
mL
of a 1.4
M
solution of NaOH
iu
EtOHjH,O (95:s). G4N (4.20g. 1.69mmol) was added. Under nitrogen the
mixture was treated with Raney-nickel(2.5
g)
(slurry in water. Aldrich). and the
mixture was hydrogenated at
1000
rpm
at
8
bar H, pressure for 4 days at 25
C.
The catalyst was filtered and washed with EtOHjH,O
(95j5).
After diluting the
filtrate with H,O
(100
mL). EtOH was evaporated and the residue extracted
several times with CH,CI,. Thereby. the NaOH concentration in the aqueous
phase was increased in every extraction step. The organic layers were dried with
Na,SO, and the solvent evaporated
in
vacuo to give 3.01 g
(69%)
G4A. Correct
elemental analysis for C,,,H,,,N,, (2586.31
gmol-I);
'H NMR (250 MHz,
CDCI,):
/i
=1.54(m. 138H,CH,CH,CH,, NH,),
2.36(m.
84H; CH,N), 2.42
(t.
J=7Hz.48H;CHzCH,CH,NH,).
2.67(t, J=7Hz,48H;CH2NH,);
"C
NMR (100 MHz, CDCI,):
ii
=
24.6 (NCH,CH,CH,N), 30.9 (CH,CH,NH,).
40.7 (CH,NH,). 51.9. 52.3, 52.4, 52.6 (CH,N); IR (neat): i.[cm-']
=
3300,
3260,2910. 2780, 1570. 1450, 1360. 1170. 1070.960-770.
G5N:
The procedure was equivalent
to
that reported
for
the synthesis of G4N.
From G4A (600
mg.
0.232 mmol) was obtained 1.1 1
g
(93%) of the highly
viscous substance G5N Correct elemental analysis for C,,,H,,,N,,
(5133.35
gmol-'):
'HNMR (400MHz. CD,CI,):
6
=1.62
(m.
90H;
CH,CH,CH,), 2.52
(m.
276H; CH,N), 2.82 (t.
J
=7 Hz, 96H; CH,CN); I3C
NMR (100 MHz, CD,CI,):
6
=
17.2 (CH,CN), 23.8.25.1 (CH,CH,CH,), 49.8
(C'H,CH,CN), 51.6, 51.8, 52.1. 52.3 (CH,N).
119.5
(CN);
IR
(neat):
i'[cm-']
=
2920.2790, 2220. 1450, 1410. 1350, 1230, 1160, 1120, 1060.
The 'H NMR and "C NMR spectra were measured with a Bruker AC 400
spectrometer with tetramethylsilane as internal standard; CD,CI, was used for
the nitrile-functional and
CDCI,
for the amine-functional generations. End
group titration was performed in a solution of H,O/Et,O (1
:
1) with 0.1
N
HCI
with a Mettler
DC
25 titrator. Vapor pressure osmometry wiis carried out with
a Perkin-Elmer Molecular Weight Apparatus Type 115 at 30
C
with CHCI, as
solvent. The GPC analysis was performed on a Shimadzu LC-4A at 60JC with
a 0.05
M
K,HPO, buffer MeOH/H,O (70: 30) at pH
=
11 (HEMA 40 and HE-
MA
100
column (MZ Analysentechnik), flow rate 0.5 mLmin-', UV detector
228 nm, R1 detector. calibrated with the values obtained from vapour pressure
osmometry).
Received: April 1,
1993
[Z 5968 IE]
German version:
Angew.
Chem.
1993.
105.
1367
D.
A.
Tomalia.
A.
M. Naylor. W. A. Goddard
111,
Angew.
Chem.
1990.
102,
119-157;
Angw.
Chem. lnt.
Ed.
Engl.
1990,
29,
138-175.
E. Buhleier.
W
Wehner,
F.
Vogtle,
Sjn//zmir
1978,
155-
158.
H.-B. Mekelburger, W. Jaworek, F. Vogtle,
Angew.
Chem.
1992.
104.
1609-1614;
Angeu..
Cl7em. Inr.
Ed.
Engl.
1992,
31,
1571-1576.
R. G. Denkewalter.
J.
F.
Kolc.
W.
J.
Lukasavage. US-A 4410688,
1983
(Clirm.
Absr.
1984,
100.
103907~).
A.M. Naylor. W. A. Goddard 111,
G.
E.
Kiefer, D. A. Tomalia.
J.
Am.
Cheni.
Soc.
1989.
111.
2339-2341.
D. A. Tomalia. H. Baker.
J.
Dewald,
M.
Hall. G. Kallos,
S.
Martin,
J.
Roeck.
J.
Ryder. P. Smith,
Po/pm.
J.
1985,
17.
117-132.
D. A. Tomaha. H. Baker.
.I.
Dewald. M. Hall,
G.
Kallos,
S.
Martin,
J.
Roeck,
J.
Ryder,
P.
Smith,
Mucromolecu1e.r
1986,
19,
2466-2468.
G. R. Newkome, G. R. Baker,
S.
Arai.
M.
J.
Saunders. P.
S.
Russo,
K.
J.
Theriot.
C.
N. Moorefield. L. E. Rogers.
J.
E.
Miller,
T.
R. Lieux, M.
E.
Murray. B. Phillips,
L.
Pascal,
J.
Am.
Clwm.
Soc.
1990,
112.
8458-8465.
X.
Lin. G. R. Newkome,
Mucr.omu1et.ule.s
1991.
24.
1443-1444.
P.
G.
de Gennes, H. Hervert. PI7js.
LP!~.
1983,
44.
351
-
360.
R.
L.
Lescanec, M. Muthukumar.
Mucromulecutes
1990.
23,
2280-2288.
C.
J.
Hawker.
J.
M.
1.
Frkchet,
J.
Am.
Clrem.
Sot..
1990.
112.
7638-7647.
K.
L.
Wooley. C.
J.
Hawker.
J.
M.
J.
Frechet,./.
Am.
Chrrn
Soc.
1991.
f13.
4252- 4261
R.
J.
Bergeron.
J.
R. Garlich. Synrhesrs
1984,
782-784.
0.
C. Dermer,
G.
E. Ham.
Ethdeneimrnr
und
other
uxridines.
Academic
Press, New
York,
1969,
p. 334.
S.
M. Aharoni. C. R. Crosby,
E.
K.
Walsh, Mat.romo1wrrks
1982,
IS,
1093
-
1098.
Poly(propy1ene imine) Dendrimers: Large-Scale
Synthesis by Hetereogeneously Catalyzed
Hydrogenations**
By
Ellen
M.
M.
de
Brabander-van
den
Berg*
and
E.
W
Meijer
Dendritic macromolecules are hyperbranched polymers
that emanate from a central core, have
a
defined number of
generations and functional end groups, and are synthesized
in a stepwise way by a repetitive reaction sequence."] The
syntheses described
so
far are either convergent, this
is
discrete
[*]
Dr.
E.
M. M. de Brabander-van den Berg, Prof.
Dr.
E.
W
Meijer
1''
DSM Research
P.O.
Box 18, NL-6160 MD Geleen (The Netherlands)
Laboratory of Organic Chemistry, Eindhoven University of Technology
P.O. Box 513, NL-5600
MB
Eiiidhoven (The Netherlands)
Telefax: Int. code
+
(31)40-451036
[**I
We
thank many colleagues at DSM Research, especially A. Nijenhuis,
J.
Keulen, M. Mure, B. Bosman,
F.
Vanderbooren, R. Reintjens. and
S.
van
der Wal
for
their vahxable input.
['I
Present address:
1308
TI
VCH
Verlug.~gesell.schufi
mhH,
0-69451
Weinherm,
IYY3
0570-oX33193jo9o9-130n
d
10.00+
2SiO
Angeic..
Chem.
In!.
Ed.
Engl.
1993,
32.
No.
9
organic compounds are synthesized, or divergent, that is the
construction
of
dendrimers can be regarded as a step poly-
merization with polydispersities of almost
1
.[2-41
Detailed
studies show that only
a
limited number of reaction schemes
can be used for the synthesis of these dendrimers;['*5-71 all
known methods to date do not deliver pure dendrimers in
large quantities, which
is
due to the use of sophisticated
building blocks and/or (large excesses of) expensive reagents
and/or complicated purifications.[61 In this paper we present
a very convenient reaction sequence for the large-scale pro-
duction
of
pure poly(propy1ene imine) dendrimers.
The reaction sequence presented here (Scheme
1)
is
based
on
the first approaches to dendrimeric structures by Vogtle
et al. in which the low yields in the reduction hampered the
synthesis
of
higher generations."] Our sequence
is
a repeti-
Scheme
1.
Synthetic scheme
for
poly(propy1ene imine) dendrirners with di-
aminobutane
as
core.
tion of
a
double Michael addition of acrylonitrile to primary
amines, followed by the heterogeneously catalyzed hydro-
genation of the nitriles, resulting in
a
doubling of the number
of primary amines. In this sequence 1,4-diaminobutane has
been used as the dendrimer core; but
a
variety of molecules
with primary or secondary amine groups can also be used.
We have optimized conditions for both reaction steps in the
sequence
so
that this occurs almost quantitatively and with
optimal selectivity.[''
All Michael reactions were performed similarly; 2.5-
4 equivalents of acrylonitrile per primary amine are used at
a
concentration of
0.1
M
in water; the first equivalent
of
acrylonitrile is added at room temperature and the second
equivalent at 80 "C. The reaction time for complete conver-
sion increases with every generation: 1 h for generation 0.5
(DAB-&ndr-(CN),),
3
h for generation 4.5 (DAB-dendr-
(CN)64).f101 The excess of acrylonitrile
is
distilled off as a
water azeotrope, leaving
a
clear two-phase system. from
which the pure dendrimers with nitrile end groups can be
isolated by pouring off the water layer. If necessary the wa-
ter-soluble side products for example HOCH,CH,CN (the
Michael addition adduct of H,O to acrylonitrile) or incom-
pletely cyanoethylated products can be separated by washing
the residue with water.
The hydrogenations of the cyanoethylated structures with
H, (30-70 bar) and Raney/Cobalt as catalyst are also per-
formed preferably in water. The reaction time required for
complete hydrogenation increases at higher generations, but
even at the higher generations this heterogeneous hydro-
genation is quantitative and selective. Dendrimers with NH,
end groups are isolated by evaporating the water from the
filtered reaction mixture. The process window for a quantita-
tive hydrogenation
is
small and side reactions easily occur.
Three major side reactions which lead to dendrimers with
structural defects have been observed, and the corresponding
side products have been characterized
:
a) the occurrence of
the retro-Michael addition during the hydrogenation, yield-
ing secondary amines,"
']
b) incomplete cyanoethylation in
the Michael addition, resulting in dendrimers missing one
ethyl cyanide side chain, and c) the formation of cyclic di-
amines and NH, by intramolecular addition of amines to the
intermediate imines during the hydrogenation reactions. Un-
der the optimized conditions these side products are not
detectable and poly(propy1ene imine) dendrimers can be syn-
thesized up to generation 4.5 with 64 terminal nitrile groups,
a molecular weight of 6912 gmol- and in quantities of sev-
eral grams up
to
several kilograms. Apart from the first
generation
(0.5)
which is a white crystalline solid, all other
generations are colourless
oils,
which are readily soluble: the
DAB-dendr-(NH,), in
H,O
and methanol, the DAB-dendr-
(CN), in common organic solvents.
All products were characterized by
'H,
13C NMR and IR
spectroscopy, mass spectrometry, HPLC, gel permeation
chromatography (GPC), differential scanning calorimetry
(DSC), thermogravimetric analysis (TGA), and intrinsic vis-
cosity. All data are consistent with the proposed dendrimer
structures. NMR spectroscopy appears to be a very suitable
technique to detect and assign structural failures
in
the out-
ermost dendrimer generation. In the
'H
NMR spectra of the
dendrimers with CN end groups the shape and intensity of
the most downfield triplet of the NCH,CH,CN group at
6
=
2.85
is
characteristic for defects in the outer generation,
whilst the growth of the dendrimers
is
clearly indicated by
the ratio of the integrals of the signal at
6
=
1.40
(NCH,CH,CH,CH,N protons of the dendrimer core) to the
integral of the signal at
6
=
1.58 (NCH,CH,CH,N branch
protons). Incompletely cyanoethylated dendrimers are
most readily concluded from additional signals in the
I3C
NMR spectra at
6
=
45.1 (NHCH,CH,CN) and 18.7
(NHCH,CH,CN) as well as the products of retro-Michael
additions of dendrimers with NH, end groups with signals at
6
=
47.7,
46.7,
39.1, 32.0, and
25.6.
Dendrimers with CN end groups can be separated and
analyzed with HPLC. In Figure
1
a typical chromatogram of
DAB-dendr-(CN), from a kg-scale run
is
shown. From mass
spectrometric experiments it has been concluded that the
small peak corresponds with the fully cyanoethylated
product of DAB-dendr-(NH,),
.
If equal absorption coeffi-
cients are assumed for both compounds, the average selectiv-
ity per conversion in the first three steps is 99.8
%.
Figure 2 shows a typical gel permeation chromatogram
of
various dendrimers with NH, end groups taken from
a
large-
scale synthesis. The absence
of
detectable impurities is evi-
An,qiw.
Uwm.
In!.
Ed
Engl
1993,
32.
No.
Y
VCH
Verlu,qsgesell.~chufi
mhH.
0-69451
Wcinheim,
lY93 0570-0833/93/0909-1309
S
10
00+
.25/0
1309
and again increases with increasing molecular weight as
determined by TGA (For
DAB-dendr-(NH,),, DAB-den&-
are
330, 378. 424,
and
470"C,
respectively. For the
DAB-
tiendr-(NH,),
less than
1.0%
weight
loss
is
observed at
T=
310°C
and scan speed
20°C
min-'.) Dendrimers with
CN
end groups are less stable, although also in this case the
stability increases at higher generations. With TGA-MS, a
thermally induced retro-Michael addition is observed for
DAB-dencir-(CN),,.
followed by a degradation similar to
DAB-tlencir-(NH2)32.
A
molecular interpretation of the lat-
ter is not possible with the data available.
(NH,),,DAB-dend~-(NH,),,.DAB-de~dr-(NH,),,,TGA,,,
1.34~
1.42-
1.50
--lg
[r71
0
5
10
15
20
t
[min]
---
Fig.
1.
Typical
HPLC
chromatograph
of
DAB-dmdr-(CN),.
8
3.5
4.0 4.5
5.0
5.5
t
[minl
-
Fig.
2
Typical GPC trace
of
DAB-~~.~I~~-(NH~)~
(1).
DAB-[/CW/T-(NH,)~
(2).
DAB-[/)Iir/r-(NH2)lh (3). and DAB-~c~/~-(NH,),~ (4).
dent from the chromatograms. Furthermore, the character-
istic relation between the molecular weight
M,
and the in-
trinsic viscosity
y~
is presented in Figure
3.
As
reported for
two other classes of dendrimers, the intrinsic viscosity of the
poly(propy1ene imine) dendrimers with
CN
end groups de-
creases with increasing molecular weight at a certain genera-
tion;'"
"]
in this case after the fourth generation.
1.261
*.
1.581..
,
I
'
'
'
2.4
2.6
2.8
3.0
3.2 3.4
3.6 3.8
4.0
lg
M"
-
Fig. 3. Plot of
-
lg
q
vs.
Ig
M,,
of the various poly(propy1ene imine) dendrimers
Nith CN end groups.
Thermal analysis of the dendrimers synthesized shows a
number
of
interesting features. The glass transition tempera-
ture Tg has been recorded with
DSC
(Fig.
4);
in both series
of dendrimers with
NH,
and
CN
end groups the
Tg's
ob-
served are low and an increase in Tg
is
observed by increas-
ing molecular weight.
In
all cases the
CN
dendrimers possess
the highest Tg. which is expected on basis of the dipole-
dipole interaction of the
CN
groups. The thermal stability of
the dendrimers with
NH,
end groups is unexpectably high
-1001
A
,
0.0
1.0
2.0 3.0 4.0
5.0
G--
Fig 4. Plot of the g'lass transition temperatures
(c)
of the poly(propy1ene
imine) dendrimers with nitrile
(0)
and amino groups
(0).
G
=
generation.
As
a consequence of the extensive purifications and low
yields in the first approaches,[*] it was assumed until recently
that this
acrylonitrile-hydrogenation
sequence is not suit-
able for dendrimer synthesis. However, by the correct choice
of reagents and reaction conditions we have shown that this
reaction sequence is very efficient. The main advantages of
our synthetic procedures are
1)
the simple reaction and isola-
tion procedures which can readily be scaled up;
2)
the use
of
one solvent for all reaction steps,
so
that the intermediates
must not be isolated;
3)
the use of readily available reagents
which do not have to be protected;
4)
the high yields and
selectivities obtained;
5)
a simple purification method which
can easily be applied to large-scale quantities;
6)
the versitil-
ity
of
the reaction sequence which allows for the introduc-
tion of a variety of end and core groups.
The dendrimers with
NH,
end groups offer remarkable
thermal and good hydrolytic stability, and have a low
5.
The drop in intrinsic viscosity after the fourth generation
indicates that a spherical shape with a sterically hindered
shell is formed at the fifth generation. The low
T,
indicates
that the dendrimers possess a large degree of conformationa1
freedom, despite their hyperbranching and well-defined
chemical and geometrical structure. Research into the syn-
thesis of higher generations and the functionalization of the
dendrimers is in progress.
E.uprrimental
Procedure
DAB-r/mr/r-(CN),: Acrylonitrile (8.35
mol.
443
g)
was added to
a
solution of
diaminobutane
(1.67
mol.
147
g)
in
1.176
kg
water. The exothermic reaction
caused the temperature to rise to
38
-C.
After this exothermiceffect the reaction
mixture was heated
:it
80-C
for
1
h to complete the addition. Then the excess
of
acrylonitrile was removed as a water azeotrope by vacuum distillation
(I6
mbar. bottom temperature 40
C).
Phase separation ofthe reaction mixture
afforded 499
g
(Y9%)
HPLC-pure DAB-dendr-(CN),, which was recrystallized
from methanol.
(NCH,CH,CN. 24.9 (NCH2CH,CH,CH,N),
16.9
(CH,CN);
'H
NMR
(200
MHz,
CDCI,):
<>
=
2.85
(t.
8H, NCH,CH,CN). 2.55
(m..
4H,
NCH,CHiCH,CH2N), 2.48 (t, XH. CH,CN). 1.55
(m.
4H.
NCH2CH,CH,CH2N):
IR
(KBr):
i.
=
2245cm-l
(CN).
"CNMR(50
MHz.
CDCI,):
d
=119(CN).
53.1 (NCH,CH,CH,CH,N).49.4
DAB-(/[,ii//,.-(NH,),:
To
a hydrogenation vessel. filled with Raney Cobalt cata-
lyst (Cr promoted. Grace 2724. pretreated with hydroxide.
YO0
g) and wafer
(22.5
L)
was iidded DAB-r/mdr-(CN), (450 g) dissolved in methanol. Subse-
quentl! the mixture was hydrogenated at 40 atm hydrogen pressure at
70
C
for
1
h. The cooled reaction mixture was then filtered and the solvents were evap-
orated at reduced pressure. The residue contained 450
g
(95%) NMR spectro-
scopically pure DAB-r/wx/r-(NH,), as a colorless oil.
l3C
NMR (50 MH7,
D,O).
6
=
53.4 (NCH,CH,CH,CH,N), 51.1
(NCH,CH,CH2NH,). 39.5 (CH,NH,). 28.8 (CH2CH2NHL). 23.9
(NCH,C'H,CH,CH,N); 'H NMR (200 MHz.
CDCI,):
6
=
2.70
(t.
8H.
Cfl,NH,). 2.44
(I.
8H. NCH,CHJH,NH,). 2.40 (m.
4H.
NCfI,C'HICH,CIf,N). 1.58 (quin. 8H. CHLCH,NHL), 1.42 (m. 12H.
NCH,C'/I,CH,CH,N. NH,):
IR
(film):
i.
=
3284. 3355 cm-'
(NH?).
Received: April
28,
1993
[Z
6041
IEJ
German version:
Angeii.
C/im.
1993.
105,
1370
[I] D.
A.
Tomalia,
A.
M. Naylor. W.
A.
Goddard
111,
Angru,.
Chwi.
1990.
102.
119.
.4ngeu
Chem.
Inr.
€d.
€ng/.
1990,
29,
138-175.
[2]
D.
A.
Tomalia.
H.
Baker, J. Dewald. M. Hall,
G.
Kallos,
S.
Martin.
J.
Roeck.
J.
Ryder.
P.
Smith.
Poh.777.
J.
Tokjo
1985.
17,
117-132.
[3]
c'.
J. Hawker,
J.
M.
J.
Frechet.
J.
Am.
Chem.
Sw.
1990.
f12,
7638.
[4]
G
R.
Neukome.
X.
Lin,
Mucromulecules
1991.24.1443;
G.
R. Newkome,
A
Nayak. R.
K.
Behera,
C.
N. Moorefieid.
G.
R.
Baker.
J.
Org.
Chun.
1992.
57.
358.
[5]
C.
J.
Hawker.
J.
M.
J.
Frkhet,
Poljino.
1992.
33.
1507.
[h]
H
-B. Mekelburger. W. Jaworek, F. Vogtle.
Ang~iv.
Chem.
1992.
104.
1609:
.4iig<w.
<'/lcv71.
Inl.
Ed.
Ei7gl.
1992,
31,
1571.
[7]
T.
M. Miller,
E.
W. Kwock. T.
X.
Neenan.
Mucroi77olecrrles
1992.25,
3143:
A
W
van der Made.
P.
W. N. M. van Leeuwen.
J.
Cliern.
Soc. CIiem.
Com-
imm
1992.
1400:
A.
Morikawa. M. Kakimato,
Y.
Imai.
Mucromo/wu/e.\
1991.
24.
3469.
[XI
E.
Buhleier. W. Wehner,
F.
Vogtle.
S?nlhrc.i.c
1978.
155- 158.
[Y]
Independently
C.
Worner
and R. Mulhaupt
(Angew.
Cliem.
1993.
105.
1367
1370;
Angen..
Chew.
Inl.
Ed.
Engl.
1993,
32.
1306
~
1308) have dis-
closed
a
similar reaction scheme.
[lo] DAB-hir/r-(CN), means a dendrimer with DAB (DiAminoButane) as
core and
s
nitrile end groups: DAB-dendr-(NH,), one with
.r
primary
iimine end groups. A proposal
for
the systematic naming
of
dendrimers is
given by
G.
R.
Newkome,
G.
R. Baker. J.
K.
Young,
3.
G.
Traynhdm.
J.
P(i/i~iii.
Sci.
Purl
.4
1993.
31.
641
-
651.
[l
1
I
Retro-Michael addition during the hydrogenations is strongly base-cata-
lyrcd by dendrimers with NH, end groups.
[12]
T. H. Mourey.
S.
R. Turner, M. Rubinstein.
3.
M.
J.
Trechet.
C.
J.
Hawker.
K.
L.
Wooley.
Mucron7o/rcu/e.s
1992.
26.
2401
Intramolecular Base Stabilization
of
Silicenium
Ions:
A
New Route to Siliconium
Ions
By
Ckiude Chuii,
Robert
J.
P. Corriu,*
Ahmad Mehdi,
and
Curherine ReyG
The possible existence of trisubstituted silylium "siliceni-
um" ions (R,Si') has intrigued chemists for a long time.[',21
Nevertheless claims to have generated such species in
solu-
tionI3' have been disputedJ4' and the question whether trior-
ganylsilyl perchlorates undergo ionization in solvents has
been reviewed by EabornrS1 and Lickiss.16] Recently silyl
cations have been prepared in nonnucleophilic solvents with
weakly coordinating
anion^.[^-*^
However, the structural
analysis of one
of
these derivatives showed a strong interac-
tion between anion and cation.[']
Some
silyl
cations that can be prepared are spabilked by
the
rr-pentamethylcyclopentadienyl
ligand,"] intermolecular
coordination,[',
lo
-
31
or intramolecular coordination.[14-
'1
[*J
Prof:
Dr.
R
J
P.
Corriu. Dr.
C.
Chuit, A. Mehdi, Dr.
C.
Reye
Universite de Montpellier
I1
Sciences et Techniques du Languedoc, URA
1097
Place Eugene Batdillon, case
007.
F-34095 Montpellier Ckdex 5 (France)
Telehx: In[. code
+
(67)1438-88
However, these examples do not constitute
a
general route to
cationic silicon species with
a
functional group at silicon that
enables reactivity studies.
Some years ago, we showed that pentacoordinate neutral
and anionic silicon species have unusual, often unexpected
reactivity.[161 That led
us
to prepare cations containing pen-
tacoordinate silicon centers (siliconium ions) in order to
study their reactivity.
Since intramolecular coordination of the amino group sta-
bilizes silanethione,['
71
silaphosphene.[lS1 silanimine,['
'I
and
transition metal silanediyl complexes very efficiently,['8,
we decided
to
extend this intramolecular stabilization to the
silylium ions. We report here a novel and general route to
functional and nonfunctional siliconium ions using the po-
tentially bischelating iigdnd A,["]
Me,
The reaction of the silane
1
with half an equivalent of
iodine in ether at room temperature results in the formation
of
a precipitate and
loss
of half an equivalent of H, in a redox
reaction. The precipitate was identified as the ionic species
2a
(Scheme
1).
0.5
I?
PhCOCl
\
Mez
\
cF~s0,OsiMq
/
/
@yh
-
F
/-\
PhCOBr
[q-/;j+x-
\
Ph&?
BFC
Me,
2
a:
X=I(loO%)
b:
X=Br(90%)
C:
X=C1(92%)
e
:
X
=
CF3S03
(91%)
1
d
:
X
=
BF4
(86%)
Scheme 1.
The 'H NMR spectrum of
2a
shows a singlet at
6
=
5.3,
shifted to low field with respect to the resonance signal of the
SiH,
protons of
1
and assigned to the SiH proton. The signal
of the methylene protons in
2a
has an
AB
pattern. The two
NMe, groups are magnetically equivalent as
a
result of the
coordination of both nitrogen atoms to the silicon center and
give rise to two singlets of equal intensity.
In
the coupled 'H
29Si NMR spectrum
of
2a,
a doublet is observed at
6
=
-
29.7
('JS,,"
=
280
Hz),
whereas the 29Si resonance
of
the
pentacoordinate silane
1["1
appears as a triplet at
6
=
-
51.5
('&,
=
200
Hz).
Interestingly, the 29Si NMR spec-
trum of
2a
in the solid state shows a sharp signal with the
same chemical shift
(6
=
-
28.8)
as
in
solution. which indi-
cates that
2a
remains ionic even in the solid state.
The IR spectrum of
2a
in CHCI, shows
a
Si-H stretch at
i,
=
2202 cm-
',
substantially higher than for
1
(21
11
cm-
I).
Moreover this frequency is very close to that observed by
Jutzi et aLr91 for the silyl cation [(rr-Me,C,),SiH]+.