Synthesis of low-generation, aryl-/alkyl-type, nonpolar dendrons carrying protected hydroxyalkyl groups in the periphery

Article abstract of DOI:10.1021/jo025742k

An efficient convergent synthesis of first- and second-generation aryl-/alkyl-type nonpolar dendrons via Suzuki cross-coupling is described. The dendrons carry either one or two benzyl-protected hydroxyalkyl groups/terminus. Iododesilylation reactions of aryltrimethylsilanes with iodo chloride are used as a tool for the incorporation of iodo, an important functionality for transition-metal-catalyzed cross-coupling reactions. In the case of sensitive aromatics, the addition of some donor solvent like diethyl ether proved effective in suppressing side reactions through electrophilic aromatic iodination.

Full text of DOI:10.1021/jo025742k

Syn th esis of Low -Gen er a tion , Ar yl-/Alk yl-Typ e, Non p ola r
Den d r on s Ca r r yin g P r otected Hyd r oxya lk yl Gr ou p s in th e
P er ip h er y
Zhishan Bo and A. Dieter Schlu¨ter*
Institut fu¨r Chemie/ Organische Chemie, Freie Universita¨t Berlin,
Takustrasse 3, D-14195 Berlin, Germany
adschlue@chemie.fu-berlin.de
Received March 21, 2002
An efficient convergent synthesis of first- and second-generation aryl-/alkyl-type nonpolar dendrons
via Suzuki cross-coupling is described. The dendrons carry either one or two benzyl-protected
hydroxyalkyl groups/terminus. Iododesilylation reactions of aryltrimethylsilanes with iodo chloride
are used as a tool for the incorporation of iodo, an important functionality for transition-metal-
catalyzed cross-coupling reactions. In the case of sensitive aromatics, the addition of some donor
solvent like diethyl ether proved effective in suppressing side reactions through electrophilic
aromatic iodination.
In tr od u ction
Highly efficient and robust synthetic methods for CC-
coupling are limited. SCC is one of the few true excep-
tions, specifically as far as aryl/aryl couplings from aryl
boronic acids (and esters) and aryl bromides (or iodides)
are concerned. Aryl/alkyl couplings7 also work well, but
here occasionally problems with purification were en-
countered which can render their application in nonsolid-
phase iterative synthesis less attractive.8 We here report
on the synthesis of nonpolar dendrons containing both
aryl and alkyl subunits using aryl-/aryl-SCC as the
growth reaction.9 Consequently the dendrons synthesized
carry (m-) terphenylene units. The mode of synthesis is
convergent, and the peripheral branching units carry one
or two benzyl-protected hydroxyalkyl groups to system-
atically vary surface polarity in future target structures.
On route, conditions were discovered which allowed
suppressing undesired iodination side reactions in io-
dodesilylation steps. The reported dendrons are of inter-
est for the synthesis of globular dendrimers1 as well as
cylindrically shaped dendronized polymers.10 In particu-
lar, their combination with other low-generation den-
drimers opens the possibilities to design both the interior
and exterior of dendritic compounds as required either
using a modular or combinatorial type approach.
Due to their unique structure and properties, dendrim-
ers have attracted increasing attention in the past
decade, and an intensive effort has been expended in
their synthesis.1 The preparation of dendrimers with
heteroatom linkages such as aryl ether, aryl ester, aryl
amide, alkyl ether, alkyl ester, etc., has been well
documented. However, only few examples were described
with nonpolar hydrocarbon skeletons. The approaches to
this latter kind were described by Newkome et al. and
Miller and Neenan, who prepared an all-alkyl represen-
tative with peripheral carboxylic groups2 and a confor-
mationally less flexible hydrocarbon dendrimer based on
aryl-aryl linkage using the Suzuki cross-coupling (SCC)
reaction,3 respectively. A whole family of dendrimers
comprised of pentaphenylbenzene repeating units were
recently prepared by Mu¨llen et al. using an iterative
Diels-Alder reaction.4 Additionally, hydrocarbon den-
drimers based on benzene and acetylene5 or benzene-
ethylene6 were reported.
(1) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers
and Dendrons; Wiley-VCH: Weinheim, Germany, 2001. (b) Fre´chet,
J . M. J .; Tomalia, D. A. Dendrimers and other Dendritic Polymers;
Wiley-VCH: Weinheim, Germany, 2001.
Resu lts a n d Discu ssion
(2) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; J ohnson, A.
L.; Behera R. K. Angew. Chem., Int. Ed. Engl. 1991, 30, 1176.
(3) (a) Miller, T. M.; Neenan, T. X. Chem. Mater. 1990, 2, 346. (b)
Miller, T. M.; Neenan, T. X.; Zayas, R.; Bair, H. E. J . Am. Chem. Soc.
1992, 114, 1018.
Chart 1 contains the central branching unit 1, the
linear and branched peripheral building blocks 2 and 3,
(4) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen K. Chem. Rev. 2001,
101, 1267.
(5) (a) Xu, Z.; Moore, J . S. Angew. Chem., Int. Ed. Engl. 1993, 32,
246. (b) Xu, Z.; Kahr, M.; Walker, K. L.; Willins, C. L.; Moore, J . S. J .
Am. Chem. Soc. 1994, 116, 4537. (c) Liu, B.; Roy, R. Tetrahedron 2001,
57, 6909.
(6) (a) Deb, S. K.; Maddux, T. M.; Yu, L. J . Am. Chem. Soc. 1997,
1199, 9079. (b) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. Engl.
1964, 3, 7, 643. (c) Lehmann, M.; Schartel, B.; Hennecke, M.; Meier,
H. Tetrahedron 1999, 55, 13377. (d) Pillow, J . N. G.; Halim, M.; Lupton,
J . M.; Burn, P. L.; Samuel, I. D. W. Macromolecules 1970, 3, 2, 5985.
(7) Miyaura, N. Top. Curr. Chem. 2002, 219, 11.
(8) (a) Shu, L.; Schlu¨ter, A. D. Macromol. Chem. Phys. 2000, 201,
239. (b) Shu, L.; Scha¨fer, A.; Schlu¨ter, A. D. Macromolecules 2000, 33,
4321.
(9) For a related work using aryl/alkyl SCC, see: Beinhoff, M.;
Karakaya, B.; Schlu¨ter, A. D. Synthesis 2002, submitted for publica-
tion.
(10) (a) Schlu¨ter, A. D.; Rabe, J . P.; Angew. Chem., Int. Ed. 2000,
39, 864. (b) Shu, L., Schlu¨ter, A. D.; Ecker, C.; Severin, N., Rabe, J . P.
Angew. Chem., Int. Ed. 2001, 40, 4666.
10.1021/jo025742k CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/27/2002
J . Org. Chem. 2002, 67, 5327-5332
5327

Bo and Schlu¨ter
CHART 1
and the three dendrons 4-6 obtained (for X, see Schemes
1-3). Branching unit 1 is of the AB2-type and, besides
the two bromo coupling functionalities, carries a tri-
methylsilyl (TMS) group as a placeholder for its subse-
quent conversion into iodo function through iododesilyl-
ation chemistry with iodo chloride.11 Many examples of
this kind of reaction have been reported to be very high
yielding. Conversion of TMS aryls into aryl boronic acid
has also been reported.3a The conditions for this reaction,
however, were expected to be too harsh for cases such as
the present, where protected hydroxy functions are to be
considered. This conversion was therefore not applied
here. The dendrons carry either TMS or iodo functions
at the focal point. Dendron 4 was additionally prepared
bearing a boronic acid and the corresponding pinacol
ester. Coupling of 1 with either the linear or branched
building block 2 or 3 furnishes the dendrons 4 and 5,
respectively, whereas the coupling of 1 with 4 yields the
second generation (G2) dendron 6. The three main
reactions involved in these transformations are the
following: SCC for the growth step; iododesilylation
forthe incorporation of iodo functionality; halogen/lithium
exchange for the preparation of boronic acids.
The key branching unit 1 was synthesized according
to the sequence in Scheme 1. The TMS-substituted
compound 8,12 obtained from the commercial dibromide
7, contained a small amount of 1,4-bis(trimethylsilyl)-
benzene as an impurity that could not be removed even
after repeated vacuum distillation (approximately 3-4%
1
by H NMR). Compound 8 was therefore used without
further purification. For the allylation of aryl halides, the
known methods comprise the following: (a) Pd-catalyzed
reaction of aromatic tin compound with allyl bromide;13
(b) CuCN‚2LiCl-catalyzed reaction of arylmagnesium
with allyl bromide;14 (c) the same reaction like (b) but
with Pd(PPh3)4 as catalyst precursor.15 Because of the
hazardous potential of some of the reagents used in
(12) For example: Rehm, J . D. D.; Ziemer, B.; Szeimies, G. Eur. J .
Org. Chem. 1999, 2079.
(13) Sheffy, F. K.; Godschalx, J . P.; Stille, J . K. J . Am. Chem. Soc.
1984, 106, 4833.
(11) For example: (a) Hensel, V.; Schlu¨ter, A. D. Chem.sEur. J .
1999, 5, 421. (b) Hensel, V.; Schlu¨ter, A. D. Eur. J . Org. Chem. 1999,
451.
5328 J . Org. Chem., Vol. 67, No. 15, 2002

Low-Generation, Nonpolar Dendrons
SCHEME 1. Syn th esis of th e Br a n ch in g Un it 1
methods a and b, method c was chosen with a modifica-
tion: PdCl2 was used as catalyst precursor instead of Pd-
(PPh3)4. This proved to give slightly superior results
regarding achievable yield. Applied to the Grignard
derivative of compound 8 the reaction gave 9 in 82%
yield. Compound 9 still contained traces of 1,4-bis-
(trimethylsilyl)benzene as impurity, which came from the
starting material 8. No isomerization of the double bond
was observed (500 MHz 1H NMR of raw mixture).
Standard hydroboration of 9 with 9-BBN gave compound
10. The coupling partner of 10, the dibromoiodobenzene
13, was obtained from the commercially available tri-
bromobenzene 11, which was monosilylated to 12 fol-
lowed by iododesilylation.11 In the final step, SCC of 10
and 13 proceeded highly selectively at the iodinated
carbon of 13 and gave the pure key branching unit 1 free
of impurity 1,4-bis(trimethylsilyl)benzene in a yield of
77%. This entire route was scaled up to the degree that
compound 1 is now available on the 30 g scale.16
acid ester 3d . The G2 dendron 5a was obtained in 88%
yield by SCC of 1 and 3d under standard conditions using
Pd(PPh3)4 as catalyst precursor in the biphasic system
THF/aqueous NaHCO3. Flash chromatography afforded
also a side product, 16, which had formed in approxi-
mately 5% yield. This most likely is the result of ligand
scrambling occasionally observed in transition-metal-
catalyzed cross-coupling reactions.18 Its structure was
1
confirmed by H and 13C NMR spectroscopy as well as
elemental analysis. The next important step was the
transformation of 5a into its iodo analog 5b. This was
attempted under the same conditions that had been
proved successful when applied to 3b. This time, how-
ever, a side reaction was observed. Careful column
chromatography workup afforded, besides the desired
product, also some of the diiodo compound 17, where one
of the internal rings carrying benzyloxypropyl chains had
undergone iodination. The structure of 17 was proved by
all standard techniques. Obviously two alkyl substituents
on the outer phenyl rings of the terphenylene units in
5a and/or 5b activate these phenyls sufficiently strongly
to allow an electrophilic attack by iodochloride to occur.
17 only formed to a few percent, and both products, 5b
and 17, could be cleanly separated by column chroma-
tography. This side reaction was nevertheless considered
detrimental in regard to all potentially interesting higher
generation compounds because of both increasing puri-
fication difficulty with increasing molar mass and the
statistical factor, which increasingly works in favor of the
side reaction. We therefore sought for a way out of this
dilemma. The presence of donors should influence the
electrophilicity of iodochloride. If the iododesilylation was
performed not in pure methylene chloride (as above) but
rather in a mixture with a donor solvent, its reactivity
Using 1 as the branching unit, dendrons 5 (Scheme 2)
as well as 4 and 6 were synthesized (Scheme 3). Hy-
droboration of 1417 gave 15, which was subjected to SCC
with compound 12 to furnish the first generation (G1)
dendron 3a in 79% yield. Its iododesilylation to 3b was
done in virtually quantitative yield by treatment with
ICl in CH2Cl2 at 0 °C without noticeable side reaction.
Iodo/lithium exchange on 3b with BuLi at -78 °C in ether
and subsequent quenching of the anion with trimethyl
borate gave the boronic acid 3c (87%) whose esterification
with pinacol cleanly furnished the corresponding boronic
(14) (a) Knochel, P.; Yeh, M. C. P.; Berk, S. C.; Talbert, J . J . Org.
Chem. 1988, 53, 2390. (b) Boymond, L.; Rottla¨nder, M.; Cahiez, G.;
Knochel, P. Angew. Chem. 1998, 110, 1801. (c) Kitagawa, K.; Inoue,
A.; Shinokubo, H.; Oshima, K. Angew. Chem., Int. Ed. 2000, 39, 2481.
(15) Bumagin, N. A.; Kasatkin, A. N.; Beletskaya, I. P. Bull. Acad.
Sci. USSR Div. Chem. Sci. (Engl. Transl.) 1984, 33, 1696.
(18) (a) Kong, K. C.; Cheng, C. H. J . Am. Chem. Soc. 1991, 113,
6313. (b) O’Keefe, D. F.; Marcuccio, S. M. Tetrahedron Lett. 1992, 33,
6679. (c) Koch, F.; Heitz, W. Macromol. Chem. Phys. 1997, 198, 1531.
(16) Bo, Z. S., Schlu¨ter, A. D. Manuscript in preparation.
(17) Cookson, R. C.; Wallis, S. R. J . Chem. Soc. B 1966, 1245.
J . Org. Chem, Vol. 67, No. 15, 2002 5329

Bo and Schlu¨ter
SCHEME 2. Syn th esis of th e Den d r on s 3 a n d 5
could possibly be fine-tuned. The same reaction was
therefore tried in a 20:1 mixture of methylene chloride
and diethyl ether, and in fact, the conversion of 5a to 5b
in this mixture went absolutely cleanly. No side product
was observed as judged by visual inspection of a high-
SCC of 8 and 15 afforded the linear peripheral unit
2a , which was easily converted to the iodide 2b and the
boronic acid 2c. Coupling of 2c with the key branching
unit 1 afforded the G1 dendron 4a in a yield of 88%. The
iodo desilylation of 4a did not require the above-described
precautions, which is not surprising because its terphen-
ylene carries only one alkyl chain on each side. 4b
wasobtained in a yield of 99%. Its subsequent conversion
to 4c using BuLi and B(O-iso-Pr)3 did not proceed so well
and gave the product in a yield of 41%. Pinacolization
proceeded cleanly. Pure ester 4d was obtained by flash
chromatography on silica gel, though the separation has
1
field H NMR spectrum with an excellent signal-to-noise
ratio of the raw mixture. The methylene protons of the
alkyls ortho to the iodo function appear as an easy to
detect, baseline-separated singlet at δ ) 4.53 ppm. Along
the same line, if the reaction was tried in ether as the
only solvent, no reaction was observed; not even the
sensitive TMS function was replaced.
5330 J . Org. Chem., Vol. 67, No. 15, 2002

Low-Generation, Nonpolar Dendrons
layer was separated, the aqueous one extracted with CH2Cl2,
and the combined organic layer dried over MgSO4 and evapo-
rated to dryness. The residue was redissolved in hexane and
filtered through a short silica gel pad. After removal of solvent,
recrystallization from hexane afforded 13 as white needles
(37.0 g, 85%): mp 124.4-124.7 °C; 1H NMR δ 7.77 (s, 1H),
7.61 (s, 2H); 13C NMR δ 138.44, 133.59, 123.34, 94.47. Anal.
Calcd for C6H3Br2I: C, 19.92; H, 0.84. Found: C, 19.77; H,
0.68.
SCHEME 3. Syn th esis of Den d r on s 4 a n d 6
{4-[3-(3,5-Dibr om op h en yl)p r op yl]p h en yl}tr im eth ylsi-
la n e (1). To a stirred solution of 9 (3.01 g, 15.8 mmol) in THF
(40 mL) was added a solution of 9-BBN (31.5 mL, 15.8 mmol,
0.5 M in THF) at room temperature under N2 over 0.5 h. The
mixture was stirred for further 3 d at room temperature. The
obtained adduct (10) was used for next reaction without
purification.
To this solution were added 13 (9.05 g, 25.0 mmol), NaOH
solution (4.0 g in 30 mL of water), and Pd(PPh3)4 (0.48 g, 0.42
mmol). The mixture was carefully degassed, recharged with
N2, and stirred at room temperature for 1 d and then at 35 °C
for 6 h, and finally refluxed for 16 h. The organic layer was
separated, the aqueous phase extracted with ether, and the
combined organic layer dried over MgSO4 and evaporated to
dryness. Chromatography on silica gel using hexane as an
eluent gave the product as a colorless oil (5.2 g, 77%): 1H NMR
δ 7.48 (s, 1H), 7.30 (AB system, 4H), 7.23 (s, 2H), 2.62 (t, 2H),
2.57 (t, 2H), 1.91 (p, 2H), 0.25 (s, 9H); 13C NMR δ 146.23,
142.17, 137.65, 133.49, 131.45, 130.29, 127.86, 122.74, 35.19,
34.77, 32.29, -1.08. Anal. Calcd for C18H22Br2Si: C, 50.72; H,
5.20. Found: C, 50.40; H 5.07.
[3,5-Bis(3-(b en zyloxy)p r op yl)p h en yl]t r im et h ylsila n e
(3a ). To a stirred solution of 14 (15.6 g, 106 mmol) in THF
(50 mL) was added a solution of 9-BBN in THF (0.5 M, 211
mL, 106 mmol) at room temperature under N2, and the
mixture was stirred for 3 d. The formed borane adduct (15)
was used without further purification.
to be carried out quickly because of partial decomposition
on the column (hydrolysis of ester). Finally, SCC of the
G1 dendron 4d with the branching unit 1 afforded the
G2 dendron 6a in 75% yield. Iododesilylation of 6a gave
6b in 99% yield without any side reaction.
Compounds 1 and 4-6 will be used for synthesis of
hyperbranched polymers (115), dendrimers (4, 5), and
dendronized polymers (5, 6).
To the solution of 15 were added 12 (15.9 g, 51.7 mmol), an
aqueous NaOH solution (15 g in 100 mL water), and Pd(PPh3)4
(2.46 g, 2.13 mmol). The mixture was carefully degassed,
recharged with N2, and refluxed for 2 d. The organic layer was
separated, the aqueous layer extracted with ether three times,
and the combined organic layer dried over MgSO4 and evapo-
rated to dryness. Chromatography on silica gel eluting with
hexane/ethyl acetate (100:4) gives 3a as a colorless oil (18.2 g,
79.2%): 1H NMR δ 7.36-7.28 (m, 10H), 7.15 (s, 2H), 6.98 (s,
1H), 4.50 (s, 4H), 3.50 (t, 4H), 2.65 (t, 4H), 1.90 (p, 2H), 0.22
(s, 9H); 13C NMR δ 141.20, 140.35, 138.61, 130.95, 129.25,
128.36, 127.63, 127.51, 72.95, 69.75, 32.47, 31.54, -1.03. Anal.
Calcd for C29H38O2Si: C, 77.97; H, 8.57. Found: C, 77.75; H,
8.50.
Exp er im en ta l Section
Compounds 8,12 12,12 and 1417 were prepared according to
literature procedures. Compounds 7 and 11 were purchased
from Acros. The catalyst precursor Pd(PPh3)4 was prepared
according to the literature19 and stored under nitrogen in a
high-quality glovebox. All solvents were purified or dried by
1
standard methods. H and 13C NMR spectra were recorded at
270 and 68 MHz in CDCl3, respectively. For the boronic acids,
2c, 3c, and 4c, combustion analysis and NMR measurements
were not performed, because boronic acids always contain
water or form boroxines.
(4-Allylp h en yl)tr im eth ylsila n e (9). A mixture of Mg (14.0
g, 0.57 mol), iodine (50 mg), and THF (700 mL) was refluxed
until it was colorless. Approximately 20 g of 8 was then added,
and the mixture was heated to reflux until the reaction kept
refluxing without heating; the rest of 8 (total amount: 114.5
g, 0.5 mol) was added dropwise to keep the reaction mixture
refluxing smoothly. After the addition was completed, the
mixture was heated to reflux for 6 h, cooled to room temper-
ature, and the transferred to a flask which was already
charged with allyl bromide (66.54 g, 0.55 mol) and Pd(PPh3)4
(5.13 g, 4.43 mmol). The resulting mixture was stirred at room
temperature for 2 h. Removal of the solvent and vacuum
distillation of the residue (65 °C, 4.2 × 10-2 mbar) afforded 9
as a colorless liquid (78.3 g, 82%). 1H NMR: δ 7.30 (AB system,
4H), 5.85-6.05 (m, 1H), 3.38 (d, 2H), 0.22 (s, 9H). 13C NMR:
δ 140.67, 137.71, 137.24, 133.47, 127.98, 115.83, 40.19, -1.10.
1,3-Dibr om o-5-iod oben zen e (13). A solution of 12 (37.10
g, 120.4 mmol) in 300 mL of CH2Cl2 was cooled to 0 °C under
stirring, and a solution of ICl in CH2Cl2 (1.0 M, 132 mL) was
added dropwise over 10 min. After the addition, the reaction
was kept at 0 °C for 2 h. A concentrated aqueous solution of
NaOH was then added to quench the reaction. The organic
1,3-Bis(3-(ben zyloxy)p r op yl)-5-iod oben zen e (3b). To a
solution of 3a (15 g, 33.5 mmol) in CH2Cl2 (150 mL) was added
dropwise a solution of ICl in CH2Cl2 (1 M, 50 mL) at 0 °C over
5 min. The reaction was kept stirring for 0.5 h at this
temperature. A concentrated aqueous solution of NaOH was
added. The organic layer was separated, the aqueous one
extracted with CH2Cl2, and the combined organic layer dried
over MgSO4 and evaporated to dryness. Filtration through a
short silica gel pad eluting with CH2Cl2 gave the product as a
colorless oil (15.8 g, 94%): 1H NMR δ 7.35-7.25 (m, 12H), 6.90
(s, 1H), 4.46 (s, 4H), 3.45 (t, 4H), 2.60 (t, 4H), 1.85 (p, 4H); 13
C
NMR δ 144.27, 138.47, 134.93, 128.36, 128.18, 127.62, 127.56,
94.46, 72.92, 69.22, 31.89, 31.17. Anal. Calcd for C26H29IO2:
C, 62.40; H, 5.84. Found: C, 61.96; H, 5.67.
3,5-Bis(3-(ben zyloxy)p r op yl)p h en ylbor on ic a cid (3c).
A solution of 3b in ether (250 mL) was cooled to -78 °C under
N2 and a solution of BuLi in hexane (1.6 M, 18.75 mL) added
dropwise over 10 min. The reaction was stirred for further 50
min at -78 °C, and then B(O-isopropyl)3 (7.5 g, 40 mmol) was
added. The reaction was stirred overnight and allowed to warm
to room temperature gradually. Dilute HCl was added, the
organic layer separated, the aqueous one extracted with ether
(19) Clouson, D. Inorg. Synth. 1972, 13, 121.
J . Org. Chem, Vol. 67, No. 15, 2002 5331

Bo and Schlu¨ter
three times, and the combined organic layer dried over Na2-
SO4 and evaporated to dryness. Chromatography on silica gel
eluting with CH2Cl2 increasing to CH2Cl2/ethyl acetate (5:1)
gave the product as a colorless oil (9.2 g, 87%): 1H NMR δ
8.12 (AB system, 2H), 7.39-7.25 (m, 11H), 4.50 (s, 2H), 3.50
(t, 2H), 2.80 (t, 2H), 1.97 (p, 2H).
3,5-Bis(3-(ben zyloxy)p r op yl)p h en ylbor on ic Acid P i-
n a col Ester (3d ). A mixture of 3c (9.0 g, 21.5 mmol), pinacol
(2.7 g, 23.0 mmol), and dry CH2Cl2 (500 mL) was refluxed for
2 h. Removal of the solvent and flash chromatography of the
remaining mass on silica gel eluting with CH2Cl2 gave the
product as a colorless oil (7.3 g, 68%). Further eluting with
ethyl acetate recovered the hydrolyzed boronic ester as the
corresponding boronic acid (2.3 g): 1H NMR δ 7.50 (S, 2H),
7.36-7.28 (m, 10H), 7.12 (s, 1H), 4.51 (s, 4H), 3.49 (t, 4H),
2.70 (t, 4H), 1.94 (p, 4H), 1.35 (s, 12H); 13C NMR δ 141.30,
138.59, 132.34, 131.77, 128.30, 127.61, 127.44, 83.62, 72.84,
69.65, 32.21, 31.43, 24.83. Anal. Calcd for C32H41BO4: C, 76.80;
H, 8.26. Found: C, 76.60; H, 8.12.
3,5,3′′,5′′-Tet r a k is(3-(b en zyloxy)p r op yl)-5′-[3-(4-iod o-
p h en yl)p r op yl][1,1′:3′,1′′]ter p h en yl (5b). To a solution of
5a (3.40 g, 3.35 mmol) in CH2Cl2 (50 mL) was added dropwise
ICl (7.0 mL) at 0 °C. The reaction was stirred at 0 °C for 50
min. An aqueous solution of NaHSO3 was added, and the color
disappeared. The organic layer was separated, the aqueous
one extracted with CH2Cl2, and the combined organic layer
dried over MgSO4 and evaporated to dryness. Filtration
through a short silica gel pad eluting with CH2Cl2 gave 5b as
a mixture with 17 (total amount: 3.40 g, 95%). Pure 5b was
obtained as a colorless oil by careful chromatography on silica
1
gel eluting with CH2Cl2: Rf ) 0.61 (CH2Cl2); H NMR δ 7.62
(s, 1H), 7.59 (s, 2H), 7.36-7.28 (m, 26H), 7.03 (s, 2H), 6.96
(one set of AB system, 2H), 4.53 (s, 8H), 3.54 (t, 8H), 2.81-
2.65 (three sets of signals superimpose each other, 12H), 2.04-
1.97 (two sets of signals superimpose each other, 10H); 13C
NMR δ 142.72, 142.50, 142.04, 141.79, 141.42, 138.53, 137.30,
130.59, 128.33, 127.75, 127.63, 127.50, 126.25, 125.05, 123.91,
72.94, 69.61, 35.53, 35.02, 32.81, 32.46, 31.50. Anal. Calcd for
Tr im eth yl(4-{3-[3,5,3′′,5′′-tetr akis(3-(ben zyloxy)pr opyl)-
[1,1′:3′,1′′]t er p h en yl-5′-yl]p r op yl}p h en yl)sila n e (5a ). A
mixture of 3d (5.48 g, 10.9 mmol), 1 (2.33 g, 5.47 mmol),
NaHCO3 (4.2 g, 50 mmol), and Pd(PPh3)4 (250.6 mg, 0.217
mmol) in THF (50 mL) and H2O (30 mL) was carefully
degassed and recharged with N2. The reaction was refluxed
for 3 d. The mixture was extracted with CH2Cl2 three times,
and the combined organic layer was dried over MgSO4 and
evaporated to dryness. Chromatography on silica gel eluting
with hexane/ethyl acetate (5:1) gives 5a as a colorless oil (4.89
C
67H71IO4: C, 75.41; H, 6.71. Found: C, 74.33; H, 6.62.
Side product 17 was obtained as a colorless oil from the
same silica gel column eluting with CH2Cl2: Rf ) 0.74 (CH2-
Cl2); 1H NMR δ 7.61 (s, 1H), 7.59 (s, 2H), 7.35-7.28 (m, 26H),
7.05 (s, 1H), 6.95 (one set of a AB system, 2H), 4.55 (s, 4H),
5.52 (s, 4H), 3.58 (t, 4H), 3.53 (t, 4H), 3.00 (t, 4H), 2.77 (two
sets of signals superimpose each other, 6H), 2.68 (t, 2H), 2.05-
1.95 (m, 10H); 13C NMR δ 145.79, 142.93, 142.73, 142.54,
142.29, 141.66, 141.23, 140.96, 140.74, 138.49, 137.29, 132.79,
130.53, 129.52, 129.48, 128.29, 127.83, 127.57, 127.45, 126.65,
125.95, 125.32, 125.03, 123.70, 72.89, 69.55, 38.88, 35.47,
34.98, 32.73, 32.43, 31.47, 30.19, 29.30. Anal. Calcd for
1
g, 88%): Rf ) 0.61 (5:1 hexane/ethyl acetate 5:1); H NMR δ
7.55 (s, 1H), 7.45-7.18 (m, 30H), 7.00 (s, 2H), 4.51 (s, 8H),
3.55 (t, 8H), 2.80-2.70 (three sets of signals superimpose each
other, 12H), 2.12-1.92 (two sets of signals superimpose each
other, 10H), 0.25 (s, 9H); 13C NMR δ 143.03, 142.88, 142.48,
142.00, 141.50, 138.55, 133.40, 128.34, 127.94, 127.64, 127.50,
126.28, 125.08, 123.84, 72.94, 69.63, 35.76, 35.60, 32.92, 32.47,
31.50, -1.07. Anal. Calcd for C70H80O4Si: C, 82.96; H, 7.96.
Found: C, 82.77; H, 7.92.
C
67H70I2O4: C, 67.45; H, 5.91. Found: C, 67.42; H, 5.87.
Ack n ow led gm en t. This work was financially sup-
ported by the Deutsche Forschungsgemeinschaft (Sfb
448, TP A1) and the Fonds der Chemischen Industrie,
which is gratefully acknowledged.
The side product 16 was separated from the same silica gel
column eluting with hexane/ethyl acetate (5:1) as a colorless
oil (0.25 g, 5%): Rf ) 0.71 (hexane/ethyl acetate 5:1); 1H NMR
δ 7.60-7.22 (m, 17H), 7.05 (s, 1H), 4.55 (s, 4H), 3.55 (t, 4H),
2.78 (t, 4H), 2.00 (p, 4H); 13C NMR δ 142.45, 141.39, 141.25,
138.55, 128.60, 128.33, 127.61, 127.49, 127.14, 127.06, 124.91,
72.90, 69.55, 32.41, 31.42. Anal. Calcd for C32H34O2: C, 85.29;
H, 7.61. Found: C, 85.07; H, 7.64.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for the preparation of 2a -c, 4a -d , and 6a ,b, spectra
and physical data for 2a -c, 4a -d , and 6a ,b, and copies of 1H
NMR spectra for key compounds 3d , 4d , 5a ,b, 17, and 6a ,b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
J O025742K
5332 J . Org. Chem., Vol. 67, No. 15, 2002