Synthesis of phenylacetylene dendrimers having sterically congested diazo units and characterization of their photoproducts

Article abstract of DOI:10.1002/ejoc.200400123

The compound (4-tert-butyl-2,6-dimethylphenyl)(2,4,6-tribromophenyl) diazomethane (1a), a precursor for persistent triplet carbene, was found to be stable enough to survive Sonogashira coupling reaction conditions to give (4-tertbutyl-2,6-dimethylphenyl)(

Full text of DOI:10.1002/ejoc.200400123

FULL PAPER
Synthesis of Phenylacetylene Dendrimers Having Sterically Congested Diazo
Units and Characterization of Their Photoproducts
Tetsuji Itoh,[a] Toshiyuki Maemura,[a] Yusuke Ohtsuka,[a] Yoshiyuki Ikari,[a] Holger Wildt,[a]
Katsuyuki Hirai,[b] and Hideo Tomioka*[a]
Keywords: Carbenes / Diazo compounds / Steric hindrance / Photolysis / Magnetic properties
The compound (4-tert-butyl-2,6-dimethylphenyl)(2,4,6-tri-
states of carbenes generated by the photolysis of these den-
bromophenyl)diazomethane (1a), a precursor for persistent drimers, the magnetic susceptibility of the photoproducts
triplet carbene, was found to be stable enough to survive
Sonogashira coupling reaction conditions to give (4-tert-
butyl-2,6-dimethylphenyl)(2,6-dibromo-4-ethynylphenyl)-
diazomethane (2a). The diazomethane 2a was used as a start-
was measured. Although the data for the photoproduct of the
tris(diazo) compound were fitted with S = 2.5, those for the
photoproducts of hexakis and dodecakis(diazo) compounds
were fitted with S = 1.62, a value that is much smaller than
ing compound for synthesis of phenylacetylene dendritic the theoretically predicted value and even smaller than that
molecules having peripheral diazo groups by the convergent of triscarbene. The results are interpreted in terms of the dis-
method. Thus, tridendrons with peripheral groups bearing 3 joint-nondisjoint concept.
(3a), 6 (6a), and 12 (8a) diazo units were prepared, in which
diazo groups were introduced so as to generate polycarbenes
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
in a ferromagnetic fashion. In order to characterize the spin Germany, 2004
There has been an ever-increasing interest in molecular
However, those systems have two disadvantages that hin-
magnetism, in which spins of unpaired electrons in π-or- der their further use in magnetic materials. First, a triplet
bitals of light atoms, such as carbon, nitrogen, and oxygen, carbene unit is highly unstable for practical applications un-
are mainly responsible. Actually, many attempts have been der ambient conditions. In order to overcome these diffi-
made to prepare organic ferromagnetic materials.[1,2] Spin culties, we have made great efforts to stabilize and hopefully
sources used for such studies are mostly thermodynamically isolate the triplet carbene and have succeeded in preparing
stable radicals, such as galvinoxyls,[2g] triphenylmethyls,[3a] fairly stable carbenes that survive for hours in solution at
and nitroxides,[3b] mostly because of their ease of prep- room temperature.[6] The next step is to explore a way to
aration and application. Potential problems emerge for connect them while retaining a robust πϪspin polarization.
these radicals, for instance, the exchange coupling between The second disadvantage that arises is that diazo groups are
the neighboring nitroxides is weak. Triplet carbenes are re- also generally labile[7] and, hence, are usually introduced at
garded as one of the most effective spin sources since the the last step of synthesis.[5] In other words, the diazo com-
magnitude of the exchange coupling between the neighbor- pound cannot be used as a building block to prepare a more
ing centers is large.[4,5] Moreover, the photolytic production complicated poly(diazo) compound, and, thus, the synthesis
of polycarbenes is possible in solid solution at cryogenic of the precursor diazo compounds becomes more difficult
temperatures if poly(diazo) precursors are available. Iwa- and laborious as the number of diazo units increases. We
mura and co-workers have prepared a ‘‘starburst-’’type found, almost accidentally, that a diphenyldiazomethane
nona(diazo) compound and have demonstrated that nine prepared to generate a persistent triplet carbene is also per-
diazo groups are photolyzed at low temperature to give a sistent for the diazo compound and, hence, can be further
nonadecet ground state (S ϭ 9).[5]
modified with the diazo group intact into a more compli-
cated diazo compound. For instance, bis(2,4,6-tribromo-
phenyl)diazomethane is found to be stable enough to sur-
vive Sonogashira coupling reaction conditions and un-
dergoes a coupling reaction with trimethylsilylacetylene to
give bis(2,6-dibromo-4-trimethylsilylethynylphenyl)diazo-
methane. Three units of the diazo compound are then intro-
duced at the 1-, 3-, and 5-positions of the benzene ring
through the ethynyl group by employing a similar coupling
reaction to give a tris(diazo) compound, which eventually
[a]
Chemistry Department for Materials, Faculty of Engineering,
Mie University,
Tsu, Mie 514-8507 Japan
Fax: (internat.) ϩ59-231-9416
E-mail: tomioka@chem.mie-u.ac.jp
[b]
Instrumental Analysis Facilities, Life Science Research Center,
Mie University,
Tsu, Mie 514-8507 Japan
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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DOI: 10.1002/ejoc.200400123
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generates a fairly stable septet ground state triscarbene where n* and n are the number of starred and unstarred
upon irradiation.[8]
carbon atoms, respectively. When triplet carbenes that have
These results suggest that the diazo compound can be the spin quantum number S ϭ 1 are used as spin sources,
Equation (1) can be rewritten as follows [Equation (2)]:
used as a building block to prepare a poly(diazo) com-
pound, which then can generate a high-spin polycarbene
with considerable thermal stability. One way to realize this
is to prepare phenylacetylene dendrimers having sterically
congested diazo units. We report here our efforts to prepare
such dendrimers and to characterize the oligocarbenes gen-
erated from those precursor oligo(diazo) compounds.
S ϭ n* Ϫ n
(2)
If this method is applied to design dendrimers having di-
azo units so as to generate polycarbenes with a high-spin
ground state by using a Sonogashira coupling reaction, the
simplest way is to introduce the precursor diazo unit on a
benzene ring in a meta fashion. For instance, in dendritic
triscarbene A in Scheme 1, the numbers of starred and un-
starred carbon atoms are 18 and 15, respectively, and hence,
S ϭ 3 is predicted. Similarly, for dendrimers having 6 and
12 carbene units (B and C in Scheme 1, respectively) at the
terminal points, S ϭ 6 and 12 are predicted, respectively.
Thus, the preparation of the precursors for those carbenes
are executed.
Results and Discussion
Design of Precursor Dendrimers
There is a simple way to predict the ground-state multi-
plicity of alternant hydrocarbons. For instance, Ovchinni-
kov has derived a method of predicting the ground-state
multiplicity of alternant hydrocarbons based on valence
bond theory by counting starred and unstarred carbon
atoms.[9] His conclusion is expressed in terms of Equa-
tion (1):
Preparation of Phenylacetylene Dendrimers with Peripheral
Diazo Groups
Methods for synthesis of phenylacetylene dendritic
macromolecules have been developed.[10] The divergent
method is shown to be hampered by the poor solubility of
(1) the growing macromolecules,[10,11] and it also requires
S ϭ (n* Ϫ n)/2
Scheme 1
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Phenylacetylene Dendrimers Having Sterically Congested Diazo Units
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rather drastic conditions for the synthetic cycle to be com- butyl groups, on one side of phenyl ring in 1a are expected
pleted. For example, the iodine-promoted deprotection of to both protect the carbonic center and also improve solu-
1-aryl-3,3-dialkyltriazenes to the corresponding aryl iodide bility, which will become a serious problem at higher gener-
is carried out at 90Ϫ110 °C in a sealed tube.[12] The conver- ations.
gent method is shown to possess several advantages over
The desired diazomethane 1a was prepared starting from
the divergent method. Above all, the synthetic cycle can be 2,4,6-tribromobenzaldehyde by the procedures outlined in
completed under very mild conditions.[10,11] This is es- Scheme 2. It was easily purified either by silica gel column
pecially important for constructing a dendrimer carrying a chromatography at Ϫ5 °C or by repeated chromatography
chemically sensitive functional group such as a diazo group. on a gel permeation column to give a rather stable orange
The compound (4-tert-butyl-2,6-dimethylphenyl)(2,4,6- solid, which could be stored in a refrigerator at Ϫ10 °C for
tribromophenyl)diazomethane (1a) is selected as a starting several months without any appreciable decomposition.
diazo precursor for the following reasons. First, it is ex- The remarkable thermal stability of 1a was shown by its
pected to survive the chemical manipulations required to high melting point; it decomposed at 117 °C.
introduce a proper functional group in order to modify it
The diazomethane 1a was also found to be stable enough
to a ‘‘monodendron’’, since the diazo carbon is well- to survive Sonogashira coupling reaction conditions in the
shielded by four relatively bulky groups at the ortho posi- presence of Pd0 and CuI (Scheme 3).[13] Thus, treatment of
tions. Secondly, it is also expected to generate a fairly per- 1a with trimethylsilylacetylene in the presence of
sistent triplet carbene upon photoexcitation as it has been (Ph3P)2PdCl2 and CuI in NEt3 for one day at 40 °C, fol-
shown that bis(2,4,6-tribromophenyl)diazomethane gener- lowed by gel permeation chromatography (GPC), gave (2,6-
ates the corresponding triplet carbene which survives min- dibromo-4-trimethylsilylethynylphenyl)(2,6-dimethyl-4-tert-
utes in solution and months in a crystalline state at room butylphenyl)diazomethane (2a-TMS) in a 50Ϫ60% yield.
temperature.[6,8] Finally, alkyl groups, i.e. methyl and tert- The presence of intact diazo functional groups in the prod-
Scheme 2
Scheme 3
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ucts was unequivocally shown by the presence of strong, the present system since the diazo functional groups could
sharp bands at 2053 cmϪ1 assigned to the diazo stretching not survive the ionization processes. One way to solve this
vibration in the IR spectra. These observations clearly indi- problem is to convert all the diazo groups in 3a to a stable
cate that the diazo group in 1a is remarkably resistant to- functional group and to determine the molecular weight of
ward metal halides, which usually catalyze the decompo- the resulting trimeric product. This sounded reasonable
sition of diazoalkanes.[14]
since there are several ways to quantitatively convert diazo
The deprotection[15] of the trimethylsilyl group in groups to other functional groups; these include the treat-
2aϪTMS proceeded equally smoothly by treating it with ment with hydrochloric acid to give the corresponding chlo-
NaOH and tBuOH at room temperature, and the diazo ride and photolysis in methanol to give the methyl ether as
group remained intact to give (2,6-dibromo-4-ethynyl- a result of OϪH insertion of the photogenerated carbene
phenyl)(2,6-dimethyl-4-tert-butylphenyl)diazomethane (2a). into the solvent.[14] However, all attempts to convert 3a
Before converting 2a into the zero-generation mono- cleanly into a tris compound with a stable functional group
dendron by reacting it with an appropriate difunctional were unsuccessful. This is obviously due to the highly con-
monomer, we coupled it with 1,3,5-triiodobenzene. The tri- gested nature of its reaction centers.
iodobenzene was chosen as a linker in which three carbene
Therefore, we decided to prepare an authentic sample.
precursor units are properly introduced so as to generate Tris(ketone) 3b was prepared following essentially the same
triscarbene connected in a ferromagnetic fashion. The reaction procedure as outlined in Scheme 2 with the use
coupling reaction proceeded smoothly under mild con- of (4-tert-butyl-2,6-dimethylphenyl)(2,4,6-tribromophenyl)-
ditions to form 1,3,5-tris{2-[3,5-dibromo-4-(4-tert-butyl-α- ketone (1b) as a starting compound. Matrix-assisted laser
diazo-2,6-dimethylbenzyl)phenyl]ethynyl}benzene (3a) in desorption ionization time-of-flight MS (MALDI-TOF
50% yield.
MS) of the ketone 3b affords a Mϩ ion peak at 1410 (1:1
The characterization of the tris(diazo) compound 3a was CH3OH/CHCl3, positive mode). The spectrum for the Mϩ
1
1
mainly based on H NMR and IR spectroscopy. The H ion cluster from the high-resolution spectrum exhibits iso-
NMR spectrum of 3a shows two singlets (δ ϭ 1.31 and topic distributions essentially identical to those simulated
2.18 ppm) corresponding to the tert-butyl and methyl for C69H60Br6O3 (Figure S1). The fully characterized tris-
groups, respectively, and three aromatic singlets (δ ϭ 7.10, ketone 3b was then used as ‘‘standard’’ in the analysis of
7.62, and 7.74 ppm) in the ratio of 3:2:2:1:2, in agreement the diazo counterpart by GPC. The elution volume of 3a
with the calculated value. A strong, sharp band at 2048 was actually found to be in complete agreement with that
cmϪ1 is observed in the IR spectrum. However, molecular of 3b (Figure S4).
weight determination by using MS, which is conventionally
Encouraged by these findings, we started to prepare a
employed to characterize dendrimers, could not be used in zero monodendron by treating 2 equivalents of 2a with a
Scheme 4
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Phenylacetylene Dendrimers Having Sterically Congested Diazo Units
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difunctional monomer. 2-(3,5-Diiodophenyl)-1-(trimethyl- dron was then deprotected under basic conditions to give a
silyl)acetylene (4)[16] was selected for the convergent syn- terminal alkyne 5a at the monodendron focal point. Two
thesis. This monomer is easily prepared[16] in good yield equivalents of 5a were then coupled with monomer 4 fol-
from 1,3,5-triiodobenzene. Scheme 4 and Scheme 5 illus- lowed by the deprotection to give the first-generation (n ϭ
trate the chemistry used in preparing phenylacetylene den- 1) product 7a bearing four peripheral diazo units in 72%
drimers containing 6 (6a) and 12 (8a) diazo units by the yield.
convergent approach. The synthesis begins with the prep-
Instead of continuing the repetitive process further, we
aration of a series of monodendrons (5a and 7a) using a coupled the zero- and first-generation monodendrons (5a
repetitive process. Thus, coupling of the aromatic diiodide and 7a, respectively) to a core, i.e. 1,3,5-triiodobenzene. The
4 with 2 equivalents of a terminal alkyne 2a carrying a per- coupling took place very smoothly to produce phenylacety-
sistent triplet carbene precursor resulted in the formation lene dendrimers having 6 (6a) and 12 (8a) diazo units in 79
of a zero-generation monodendron (5aϪTMS) in 86% and 60% yields, respectively, where six and twelve diazo
yield. The product was easily separated from the oxidative units are aligned so as to generate hexakis- and dodecakis-
dimer and the mono-substituted product by GPC. The den- carbene in a ferromagnetic fashion. The coupling reactions
Scheme 5
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T. Itoh, T. Maemura, Y. Ohtsuka, Y. Ikari, H. Wildt, K. Hirai, H. Tomioka
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were always accompanied by the formation of small tra because the line widths were broader than the simulated
amounts of partially substituted products and symmetrical line spacings. The signal spacing was so small that no fine
diacetylenes, but the desired dendrimers were easily sepa- structures were observed. This is consistent with the obser-
rated from those by-products by GPC.
vation that the higher the spin multiplicities are, the smaller
Proton NMR spectroscopy was found to be useful for the D values become.[18] Thus, it was very difficult to ident-
the characterization of the dendrimers to a first approxi- ify the spin states of triscarbene unequivocally from conven-
mation. The assignments are based on the integral ratio of tional X-band ESR spectroscopy.
the prominent protons, i.e. tert-butyl, methyl, and aromatic
In order to determine the spin states of the photoprod-
protons, which were well resolved to make relatively precise ucts from 3a, the magnetic susceptibility of the photoprod-
1
measurements possible. For instance, the H NMR spec- uct was measured. Thus, the 2-MTHF solution of the tris-
trum of 6a shows two singlets (δ ϭ 1.31 and 2.17 ppm) (diazo) compound 3a (0.5 m) was placed inside the sample
corresponding to the tert-butyl and methyl groups, respec- compartment of a Superconducting Quantum Interference
tively, with a ratio of 3:2, and five aromatic protons [δ ϭ Device (SQUID) magnet/susceptometer and irradiated with
7.10 (s), 7.63 (t), 7.65 (d), 7.68 (s), and 7.75 (s)] with the light (λ ϭ 488 nm) from an argon ion laser through an op-
ratio of 4:1:2:1:4. The presence of a sharp, strong band at tical fiber at 5Ϫ10 K. The magnetization for 3a at 5 K in a
2054 cmϪ1 in the IR spectrum of 6a and 8a again indicates constant field of 5 kOe was monitored in situ with respect
that the diazo functional group remains intact during the to irradiation time and is shown in Figure 2a. As the ir-
processes.
radiation time increased, the M values gradually increased
In order to determine the molecular weight of the dendri- as well and reached a plateau after several hours. The mag-
mers indirectly, again we prepared the corresponding ke- netization before and after irradiation, Mb and Ma, respec-
tone dendrimers as a standard for GPC analysis. The de- tively, were measured at 2 and 5 K in a field range of 0Ϫ50
sired dendrimers (6b and 8b) were prepared according to
essentially the same procedures as outlined in Schemes 4
and 5, except that the corresponding ketone 2b was used in
place of 2a. The molecular weight of the ketone dendrimers
determined by MS was found to be consistent with those
expected for the structure (Figures S2 and S3).[17] Those
fully characterized ‘‘authentic’’ dendrimers were then used
as a standard for GPC analysis, and excellent agreement of
the elution volume between the ketone and diazo dendri-
mers (Figure S4) was observed.
SQUID Measurements of Photoproducts from Dendrimers
(3a, 6a, and 8a)
In order to characterize the spin states of carbenes gener-
ated by photolysis of the dendrimers, the following meas-
urements were carried out.
Photolysis of 3a in 2-methyltetrahydrofuran (2-MTHF)
at 77 K in an X-band ESR cavity resulted in the appearance
of single-line signals centered at around 330 mT (Figure 1).
Although the spectral features are very different from that
observed in the photolysis of the corresponding monodi-
azomethane (1a or 2a), it was difficult to simulate the spec-
Figure 2. (a) Plot of magnetization (M in emu) as a function of
irradiation time observed in the photolysis of tris(diazo) compound
3a in 0.5 m 2-methyltetrahydrofuran matrix measured at 5.0 K
and 5 kOe; (b) field dependence of the magnetization of the photo-
product from tris(diazo) compound 3a in 0.5 m 2-methyltetra-
hydrofuran matrix measured at 2.0 and 5.0 K; Mb and Ma refer to
Figure 1. ESR spectra obtained by irradiation of tris(diazo) com-
pound 3a in 2-methyltetrahydrofuran at 77 K
the magnetization value before and after irradiation, respectively,
and M ϭ Ma Ϫ Mb
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kOe. The paramagnetic contribution was then obtained by traced the theoretical curve with a very low S value. The
subtracting the corresponding values obtained before and experimental data (M/Ms vs. H/T) were fitted best with
after irradiation (Figure 2b). The effect of any paramag- Equation (3) with S ϭ 1.62 Ϯ 0.01, F ϭ 2.85 Ϯ 0.03 for the
netic impurities can be cancelled by this treatment. The hexakis(diazo) compound and S ϭ 1.62 Ϯ 0.01, F ϭ 3.35
plots of the magnetization (M ϭ Ma Ϫ Mb) versus the tem- Ϯ 0.04 for the dodecakis(diazo) compound. Thus, the val-
perature-normalized magnetic field (H/T) were analyzed in ues are disappointingly smaller than the theoretically pre-
terms of the Brillouin function as follows:[1c,5,19]
dicted values, S ϭ 6 and 12 for 6c and 8c, respectively, even
smaller than that of triscarbene 3c.
M ϭ Ma Ϫ Mb ϭ FNgJµBB(χ)
(3)
where B(χ) ϭ [(2J ϩ1)/2J]·coth{[(2J ϩ1)/2J]·χ} Ϫ [1/2J]·
coth(χ/2J) and χ ϭ JgµBH/kBT. F is the photolysis factor
of diazo compounds, N is the number of the molecule, J is
the quantum number for the total angular momentum, µB
´
is the Bohr magneton, g is the Lande g-factor, and kB is the
Boltzmann constant. Since these carbenes are constituted
of light elements, the orbital angular momentum should be
negligible, and J can be replaced with spin quantum num-
ber S.
The M/Ms versus H/T plots are shown in Figure 3, to-
gether with theoretical curves with S ϭ 1, 2, and 3.[20] The
observed data (M vs. H/T) for the photoproduct of the tris-
(diazo) compound 3a closely traced the theoretical curve
with S ϭ 3. The data were fitted with Equation (3) with
S ϭ 2.4 and F ϭ 1.05. The value is somewhat smaller than Figure 4. Plot of magnetization normalized by saturation magneti-
zation (Ms) obtained upon photolysis of hexakis(diazo) compound
the theoretically predicted value of 3, but indicates that the
6a measured at 2.0 and 5.0 K vs. temperature-normalized magnetic
field (H/T in kOe KϪ1); theoretical curves with S ϭ 1, 2 and 3 are
also indicated
three carbene centers interact ferromagnetically. Since the
magnetization data at two different temperatures were fitted
to the same Brillouin function, the sample is considered to
be free from ferro- or antiferromagnetic intermolecular in-
teractions and the septet state is considered to be a
ground state.
Figure 5. Plot of magnetization normalized by saturation magneti-
zation (Ms) obtained upon photolysis of dodecakis(diazo) com-
pound 8a measured at 2.0 and 5.0 K vs. temperature-normalized
magnetic field (H/T in kOe KϪ1); theoretical curves with S ϭ 1, 2
and 3 are also indicated
Figure 3. Plot of magnetization normalized by saturation magneti-
zation (Ms) obtained upon photolysis of tris(diazo) compound 3a
measured at 2.0 and 5.0 K vs. temperature-normalized magnetic
field (H/T in kOe KϪ1); theoretical curves with S ϭ 1, 2 and 3 are
also indicated
The reason for the apparent smaller values may be due
to the incomplete decomposition of the diazo functional
groups. To confirm this, the UV/V spectra of the diazo
compounds before and after the magnetic measurements
Similar measurements were made for the photoproducts were recorded. The rate of the photolysis could be moni-
from hexakis- (6a) and dodecakis(diazo) compounds (8a), tored by observing the nϪπ* absorption band. The degree
and the data are shown in Figure 4 and Figure 5, respec- of the photolysis was completed to be Ͼ95% on the whole
tively, with theoretical curves with S ϭ 1, 2 and 3. The data (Figure S5).
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Other factors that should be taken into account are the netic moment. The magnitude of the exchange coupling be-
inter- and/or intramolecular antiferromagnetic interactions, tween the spins is dependent upon the structure of the link-
as these interactions are known to reduce the effective mag- ers,[21] and the magnitude decreases as the number of
Scheme 6
Scheme 7
Scheme 8
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Conclusion
the intervening benzene rings increases, indicating that
polarization of the π-electrons on the benzene rings
gradually attenuates as the length between the radical sites
increases. If this value is not large, the antiferromagnetic
interaction becomes important, especially at low tempera-
ture.
The present research has revealed that the convergent
method for the synthesis of phenylacetylene dendritic mol-
ecules can be applied to prepare tridendrons having periph-
eral diazo functional groups. The poly(diazo) dendrimers
thus prepared generated polycarbenes cleanly and ef-
ficiently upon photoirradiation. Although triscarbene was
shown to have a septet ground state, as was expected, poly-
carbenes from higher-generation poly(diazo) compounds
were found to have a lower spin state of S ϭ 2 probably
because of the disjoint connectivity between exterior and
interior parts. This may mean that the present method is
not useful for the preparation of high-spin polycarbenes.
However, the connectivity will be improved by introducing
a spin unit on the core ring that disrupts the magnetic inter-
action between the spin units at the branches. A simple way
to realize this is to introduce a stable radical unit on a
core benzene.
Thus, it is potentially possible to extend the synthesis af-
ford larger dendritic molecules, which can generate high-
spin magnetic species upon photoirradiation. One of the
benefits from the high level of molecular precision has been
suggested to lie in the area of single-molecule electronic or
photonic devices.[2] The existence of large dentritic mol-
ecules with spin functionality will open a new window for
materials science.
The results can be more reasonably interpreted in terms
of the disjoint-nondisjoint concept based on MO the-
ory.[22,23] Borden and Davidson[23] noted that the localiz-
ability of the Hückel nonbonding molecular orbitals
(NBMO) is crucial in governing the ground states of non-
´
Kekule hydrocarbon diradicals. When the degenerate
Hückel nonbonding molecular orbitals have no atoms in
common, a situation that is known as ‘‘disjoint’’ arises, and
there is no probability that both electrons will simul-
taneously occupy the same AO. Thus, to a first approxi-
mation, the lowest singlet and the lowest triplet states have
the same energy. Since dynamic spin polarization[24] be-
tween the two electrons with an opposite spin is important
in the singlet state in this case, the singlet state becomes a
ground state. On the other hand, when NBMOs have atoms
in common, a situation that is described as ‘‘non-disjoint’’
arises, and a triplet ground state is expected since in this
state the Pauli principle prevents the two electrons in the
NBMOs from simultaneously appearing in the AOs that the
two NBMOs have in common.
By applying this theory to our molecules, it can be ex-
plained why triscarbene 3c has a triplet state as the Ovchin-
nikov rule predicts, while hexakis- and dodecakiscarbenes
have lower spin states than predicted. Thus, in the case of
the triscarbene, the three carbene units are connected to a
central benzene ring in a non-disjoint manner to afford a
high-spin ground state (Scheme 6). On the other hand,
hexakis 6c consists of a dendritic structure, which has a
biscarbene unit at the terminal branches. The connectivity
of two carbene units within this biscarbene unit is non-dis-
joint and, hence, they interact in a ferromagnetic fashion
with spin quantum number S ϭ 2. However, the connec-
tivity between three biscarbene units through the central
benzene ring is disjoint (Scheme 7). Consequently, each en-
ergy level of these disjoint systems is nearly degenerate.
Similarly, while the connectivity of two carbene units within
the biscarbene unit at the terminal branches in dodecakis-
carbene is disjoint to generate the S ϭ 2 spin state, the
connectivity between six biscarbene units through the core
part is disjoint. Thus, there are no magnetic interactions
between the biscarbene units through the core part in hexa-
kis- and dodecakiscarbene.
Experimental Section
General Information: IR spectra were measured on a JASCO FT/
IR-410 spectrometer, and UV/Vis spectra were recorded on a JA-
SCO V-560 spectrometer. The mass spectra were recorded on a
JEOL JMS-600H mass spectrometer and Voyger DE-PRO
MALDI-TOF mass spectrometer. 1H (300 MHz) and 13C
(75 MHz) NMR spectra were determined with JEOL JNM-AL300
FT/ NMR spectrometer in CDCl3 with Me4Si as an internal refer-
ence. The magnetic measurements were done on a 5-T Quantum
Design MPMS SQUID magnetometer. Thin-layer chromatography
was performed on Merck Kieselgel 60 PF254. Column chromatog-
raphy was carried out on a silica gel (Merck for dry column chro-
matography) or on an aluminum oxide, active neutral (activity I)
(ICN for column chromatography). GPC was performed with a
JASCO PU-986 chromatograph with a UV-970 UV/Vis detector
using a Shodex GPC H-2001 (20 mm ϫ 50 cm) or GPC H-2002.5
(20 mm ϫ 50 cm) column.
Preparation of (4-tert-Butyl-2,6-dimethylphenyl)(2,4,6-tribromo-
phenyl)diazomethane (1a): One-tenth of a solution of 2-bromo-5-
tert-butyl-1,3-dimethylbenzene[25] (7.2 g, 30 mmol) in anhydrous
Et2O (15 mL) was added to a mixture of magnesium ribbon (0.8 g,
33 mmol) and a crystal of iodine in anhydrous Et2O (10 mL), and
the mixture was sonicated for 1 h. The residual part of the bromide
solution was added dropwise and the mixture was refluxed over-
night. After cooling at 0 °C, a solution of 2,4,6-tribromobenzal-
dehyde[26] (10.3 g, 30 mmol) in anhydrous THF (25 mL) was added
to the Grignard reagent, and the mixture was refluxed for 5 h. A
saturated NH4Cl solution was added carefully and the mixture was
extracted with Et2O. The organic layer was dried over anhydrous
Na2SO4 and the solvents evaporated to leave a crude product,
On the other hand, the large F values, 2.85 and 3.35,
observed for the photoproducts from 6a and 8a, respec-
tively, indicate that more than one unit of the S ϭ 1.62 spin
states are formed from one mol of the diazo compounds.
In other words, hexakiscarbene 6c is regarded as a species
comprising of 2.85 units of S ϭ 1.62 spin state, while do-
decakiscarbene 8c is a species having 3.35 units of S ϭ 1.62
spin state. These are roughly in agreement with the spin
situation illustrated in Scheme 7 and Scheme 8.
Eur. J. Org. Chem. 2004, 2991Ϫ3003
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2999

T. Itoh, T. Maemura, Y. Ohtsuka, Y. Ikari, H. Wildt, K. Hirai, H. Tomioka
FULL PAPER
which was purified by silica gel column chromatography eluted
with CHCl3/n-hexane (1:1). (4-tert-Butyl-2,6-dimethylphenyl)-
in 53% yield. M.p. 117.3Ϫ118.0 °C dec. 1H NMR (300 MHz,
CDCl3): δ ϭ 1.30 [s, 9 H, C(CH3)3], 2.15 (s, 6 H, CH3), 7.08 (s, 2
H, ArH), 7.74 (s, 2 H, ArH) ppm. 13C NMR (75 MHz, CDCl3):
(2,4,6-tribromophenyl)methanol was obtained as a white solid in
1
36% yield. M.p. 91.5Ϫ93.4 °C. H NMR (300 MHz, CDCl3): δ ϭ δ ϭ 21.2 (CH3), 31.3 [C(CH3)3], 34.3 [C(CH3)3], 63.1 (ArCAr),
3
1.29 [s, 9 H, C(CH3)3], 2.26 (s, 6 H, CH3), 2.60 (d, J ϭ 8.82 Hz,
1 H, OH), 6.40 [d, 3J ϭ 8.82 Hz, 1 H, ArCH(OH)Ar], 6.97 (s, 2 H,
ArH), 7.72 (s, 2 H, ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ
21.7 (CH3), 31.2 [C(CH3)3], 34.1 [C(CH3)3], 76.3 [ArCH(OH)Ar],
121.3 (Ar), 124.8 (Ar), 126.9 (Ar), 133.9 (Ar), 136.1 (Ar), 136.6
(Ar), 139.6 (Ar), 150.6 (Ar) ppm. MS: m/z (%) ϭ 508 (M ϩ6, 5.0),
506 (M ϩ4, 14.1), 504 (M ϩ2, 14.6), 502 (4.8) [Mϩ], 343 (30.2), 57
(100). HRMS for C19H21OBr3: calcd. 501.9141; found 501.9136. A
solution of the methanol (5.5 g, 11 mmol) and SOCl2 (20 mL)
was stirred at room temperature for 2 days and the solvents were
evaporated under reduced pressure to give (4-tert-butyl-2,6-di-
121.1 (Ar), 124.5 (Ar), 125.7 (Ar), 126.0 (Ar), 131.6 (Ar), 135.6
(Ar), 137.3 (Ar), 150.9 (Ar) ppm. IR (NaCl): ν˜ ϭ 2054 cmϪ1
.
Preparation of (4-tert-Butyl-2,6-dimethylphenyl)(2,6-dibromo-4-
ethynylphenyl)diazomethane (2a): To a mixture of diazomethane 1a
(82.4 mg, 0.16 mmol), bis(triphenylphosphane)palladium dichlor-
ide (10 mg, 0.014 mmol), and copper() iodide(catalytic amount) in
anhydrous triethylamine (1 mL) was added trimethylsilylacetylene
(67.8 µL, 0.48 mmol) at room temperature under a nitrogen atmos-
phere. The mixture was stirred overnight at 40 °C, evaporated,
passed through a column (aluminum oxide deactivated with 5%
water, n-hexane), and purified by GPC (8 cycles, CHCl3). The di-
azomethane 2a-TMS was obtained as an orange solid in 50% yield.
methylphenyl)(2,4,6-tribromophenyl)chloromethane as
a brown
semisolid in 99% yield: 1H NMR (300 MHz, CDCl3): δ ϭ 1.28
[s, 9 H, C(CH3)3], 2.38 (s, 6 H, CH3), 6.84 [s, 1 H, ArCH-
(Cl)Ar], 6.97 (s, 2 H, ArH), 7.74 (s, 2 H, ArH) ppm. 13C NMR
(75 MHz, CDCl3): δ ϭ 22.8 (CH3), 31.1 [C(CH3)3], 34.0 [C(CH3)3],
62.0 [ArCH(Cl)Ar], 122.1 (Ar), 125.7 (Ar), 127.2 (Ar), 130.9 (Ar),
136.4 (Ar), 137.2 (Ar), 137.6 (Ar), 150.6 (Ar) ppm. A mixture of
silver tetrafluoroborate (2.5 g, 13 mmol), ethyl carbamate (13.5 g,
152 mmol), and 1,4-dioxane (20 mL) was heated at 60 °C. A solu-
tion of the chloride (5.7 g, 11 mmol) in dioxane (30 mL) was added,
and the mixture was heated overnight at 100 °C. The mixture was
filtered, and water (30 mL) and CHCl3 (30 mL) were added to the
filtrate. The organic layer was washed well with water and dried
over anhydrous Na2SO4. A brown oil obtained after concentration
of the solution was chromatographed on a silica gel column eluted
with CH2Cl2/n-hexane (2:1). Ethyl N-[(4-tert-butyl-2,6-dimeth-
ylphenyl)(2,4,6-tribromophenyl)methyl]carbamate was obtained as
1
M.p. 48.5Ϫ49.5.0 °C. H NMR (300 MHz, CDCl3): δ ϭ 0.24 [s, 9
H, Si(CH3)3], 1.30 [s, 9 H, C(CH3)3], 2.14 (s, 6 H, CH3), 7.07 (s, 2
H, ArH), 7.66 (s, 2 H, ArH) ppm. 13C NMR (75 MHz, CDCl3):
δ ϭ Ϫ0.2 [Si(CH3)3], 21.1 (CH3), 31.3 [C(CH3)3], 34.3 [C(CH3)3],
63.7 (ArCAr), 97.9 (ArCCTMS), 101.5 (ArCCTMS), 124.3 (Ar),
124.6 (Ar), 124.7 (Ar), 125.9 (Ar), 132.6 (Ar), 136.2 (Ar), 137.4
(Ar), 150.9 (Ar) ppm. IR (NaCl): ν˜ ϭ 2162, 2053 cmϪ1. A mixture
of the latter diazomethane (158 mg, 0.30 mmol), tert-butyl alcohol
(10 mL), and 10% NaOH solution (4 mL) was stirred overnight at
room temperature. The mixture was poured onto water and ex-
tracted with Et2O. The ethereal layer was washed with water, dried
over anhydrous Na2SO4, and the solvents evaporated. The reaction
mixture was purified by GPC (3 cycles, CHCl3) to afford 2a as a
1
yellow semisolid in 85% yield. H NMR (300 MHz, CDCl3): δ ϭ
1.30 [s, 9 H, C(CH3)3], 2.15 (s, 6 H, CH3), 3.16 (s, 1 H, ArCCH),
7.08 (s, 2 H, ArH), 7.68 (s, 2 H, ArH) ppm. 13C NMR (75 MHz,
CDCl3): δ ϭ 21.1 (CH3), 31.3 [C(CH3)3], 34.3 [C(CH3)3], 63.7 (Ar-
CAr), 80.2 (ArCCH), 80.4 (ArCCH), 123.2, 124.6 (Ar), 125.9 (Ar),
133.1 (Ar), 136.4 (Ar), 137.5 (Ar), 151.0 (Ar) ppm. IR (NaCl): ν˜ ϭ
1
a brown semisolid in 66% yield: H NMR (300 MHz, CDCl3): δ ϭ
1.25 (t, 3J ϭ 7.17 Hz, 3 H, OCH2CH3), 1.28 [s, 9 H, C(CH3)3], 2.19
(s, 6 H, CH3), 4.16 (q, 3J ϭ 7.17 Hz, 2 H, OCH2CH3), 5.23 (d,
3J ϭ 8.82 Hz, 1 H, ArCHAr), 6.39 (d, 3J ϭ 8.82 Hz, 1 H, NH),
6.96 (s, 2 H, ArH), 7.71 (s, 2 H, ArH) ppm. 13C NMR (75 MHz,
CDCl3): δ ϭ 14.7 (OCH2CH3), 21.3 (CH3), 31.2 [C(CH3)3], 34.1
[C(CH3)3], 57.9 (OCH2CH3), 61.2 (ArCHAr), 120.9 (Ar), 125.0
(Ar), 126.9 (Ar), 132.3 (Ar), 136.3 (Ar), 136.8 (Ar), 137.5 (Ar),
150.6 (Ar), 154.9 (CϭO) ppm. MS: m/z (%) ϭ 579 (M ϩ6, 2.7),
577 (M ϩ4, 7.8), 575 (M ϩ2, 7.6), 573 (2.8) [Mϩ], 496 (82.6), 57
(100). HRMS for C22H26NO2Br3: calcd. 572.9512; found 572.9516.
To a vigorously stirred mixture of the carbamate (4.3 g, 7.5 mmol)
in acetic acid (30 mL) and acetic anhydride (40 mL) at 0 °C was
added a 1.0 g portion of sodium nitrite every 20 min for 3 h (total
10.0 g, 149 mmol). The mixture was stirred overnight at room tem-
perature, poured onto ice water(50 mL), and extracted with Et2O.
The ethereal layer was washed with 5% Na2CO3 solution and water,
dried over anhydrous Na2SO4, and the solvents evaporated to give
ethyl N-nitroso-N-[(4-tert-butyl-2,6-dimethylphenyl)(2,4,6-tribrom-
ophenyl)methyl]carbamate as a brown solid in 95% yield: 1H NMR
(300 MHz, CDCl3): δ ϭ 1.22 (t, 3J ϭ 7.17 Hz, 3 H, OCH2CH3),
3300, 2157, 2055 cmϪ1
.
Preparation of Tris(diazo) Compound 3a: To a mixture of diazo-
methane 2a (24.5 mg, 0.053 mmol), 1,3,5-triiodobenzene[27]
(8.1 mg, 0.018 mmol), bis(triphenylphosphane)palladium dichlor-
ide (10 mg, 0.014 mmol), and copper() iodide(catalytic amount)
was added anhydrous triethylamine (1 mL) at room temperature
under a nitrogen atmosphere. The mixture was stirred at room tem-
perature for 2 days, evaporated, passed through a column (alumi-
num oxide deactivated with 5% water, n-hexane), and purified by
GPC (3 cycles, CHCl3). Tris(diazo) compound 3a was obtained
from the first fraction as an orange semisolid in 50% yield. 1H
NMR (300 MHz, CDCl3): δ ϭ 1.31 [s, 27 H, C(CH3)3], 2.18 (s, 18
H, CH3), 7.10 (s, 6 H, ArH), 7.62 (s, 3 H, ArH), 7.74 (s, 6 H, ArH)
ppm. 13C NMR (75 MHz, CDCl3): δ ϭ 21.2 (CH3), 31.3 [C(CH3)3],
34.4 [C(CH3)3], 63.8 (ArCAr), 87.8 (ArCCAr), 90.2 (ArCCAr),
123.5 (Ar), 123.7 (Ar), 124.7 (Ar), 124.8 (Ar), 126.0 (Ar), 132.9
(Ar), 134.6 (Ar), 135.9 (Ar), 137.5 (Ar), 151.1 (Ar) ppm. IR (NaCl):
3
1.27 [s, 9 H, C(CH3)3], 1.96 (s, 6 H, CH3), 4.35 (q, J ϭ 7.17 Hz,
ν˜ ϭ 2048 cmϪ1
.
2 H, OCH2CH3), 6.91 (br. s, 3 H, ArCHAr and ArH), 7.69 (s, 2
H, ArH) ppm. Potassium tert-butoxide (1.39 g, 12.4 mmol) was ad-
ded to a solution of the nitroso compound (3.0 g, 5.0 mmol) in
anhydrous THF (50 mL) under an argon atmosphere. The mixture
was stirred overnight at room temperature, poured onto water, and
extracted with Et2O. The ethereal layer was washed with water,
dried over anhydrous Na2SO4, and the solvents evaporated. The
reaction mixture was passed through a column (aluminum oxide
Preparation of Hexakis(diazo) Compound 6a: To a mixture of diazo-
methane 2a (97.0 mg, 0.210 mmol), 2-(3,5-diiodophenyl)-1-(tri-
methylsilyl)acetylene (4)[16] (44.9 mg, 0.105 mmol), bis(triphenyl-
phosphane)palladium dichloride (10 mg, 0.014 mmol), and cop-
per() iodide(catalytic amount) was added anhydrous triethylamine
(3 mL) at room temperature under a nitrogen atmosphere. The mix-
ture was stirred at room temperature for 2 days. After workup of
deactivated with 5% water, n-hexane), and the first fraction was the reaction, the crude product was purified by GPC (3 cycles,
purified by GPC (5 cycles, CHCl3) to afford 1a as an orange solid CHCl3). The bis(diazo) compound 5a-TMS was obtained from the
3000
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 2991Ϫ3003

Phenylacetylene Dendrimers Having Sterically Congested Diazo Units
FULL PAPER
first fraction as an orange solid in 86% yield. Dec. 109.4Ϫ110.2
in 72% yield. 1H NMR (300 MHz, CDCl3): δ ϭ 1.31 [s, 36 H,
°C. 1H NMR (300 MHz, CDCl3): δ ϭ 0.26 [s, 9 H, Si(CH3)3], 1.31 C(CH3)3], 2.17 (s, 24 H, CH3), 3.15 (s, 1 H, ArCCH), 7.09 (s, 8 H,
[s, 18 H, C(CH3)3], 2.17 (s, 12 H, CH3), 7.09 (s, 4 H, ArH),
7.56Ϫ7.58 (m, 3 H, ArH), 7.72 (s, 4 H, ArH) ppm. 13C NMR NMR (75 MHz, CDCl3): δ ϭ 21.2 (CH3), 31.3 [C(CH3)3], 34.3
(75 MHz, CDCl3): Ϫ0.2 [Si(CH3)3], 21.2 (CH3), 31.3 [C(CH3)3], 63.8 (ArCAr), 78.8 (ArCCH), 81.7 (ArCCH), 87.8
[C(CH3)3], 34.3 [C(CH3)3], 63.8 (ArCAr), 87.5 (ArCCAr), 90.4 (ArCCAr), 88.8 (ArCCAr), 89.0 (ArCCAr), 90.3 (ArCCAr), 123.5
(ArCCAr), 96.5 (ArCCTMS), 102.7 (ArCCTMS), 123.1 (Ar), (Ar), 123.5 (Ar), 123.7 (Ar), 124.7 (Ar), 124.8 (Ar), 126.0 (Ar),
ArH), 7.62Ϫ7.65 (m, 9 H, ArH), 7.74 (s, 8 H, ArH) ppm. 13C
δ
ϭ
123.8 (Ar), 124.2 (Ar), 124.67 (Ar), 124.73 (Ar), 126.0 (Ar), 132.8 132.9 (Ar), 134.6 (Ar), 135.9 (Ar), 137.5 (Ar), 151.0 (Ar) ppm. IR
(Ar), 134.2 (Ar), 135.0 (Ar), 135.9 (Ar), 137.5 (Ar), 151.0 (Ar) ppm.
(NaCl): ν˜ ϭ 2253, 2055 cmϪ1. To a mixture of tetrakis(diazo) com-
pound 7a (20.0 mg, 9.4 µmol), 1,3,5-triiodobenzene (1.4 mg,
IR (NaCl): ν˜ ϭ 2155, 2055 cmϪ1. A mixture of the bis(diazo) com-
pound 5a-TMS (74.6 mg, 0.068 mmol), MeOH/THF (10 mL), and 3.1 µmol), bis(triphenylphosphane)palladium dichloride (10 mg,
10% K2CO3 solution (4 mL) was stirred overnight at room tem-
perature. After workup of the reaction, the crude product was puri-
fied by GPC (3 cycles, CHCl3) to afford the bis(diazo) compound
14 µmol), and copper() iodide (catalytic amount) was added anhy-
drous triethylamine (1.5 mL) at room temperature under a nitrogen
atmosphere. The mixture was stirred at room temperature for 2
days. After workup of the reaction, the crude product was purified
1
5a as an orange solid in 64% yield. H NMR (300 MHz, CDCl3):
δ ϭ 1.31 [s, 18 H, C(CH3)3], 2.17 (s, 12 H, CH3), 3.14 (s, 1 H, by GPC (5 cycles, CHCl3) to afford dodecakis(diazo) compound
1
ArCCH), 7.09 (s, 4 H, ArH), 7.59Ϫ7.62 (m, 3 H, ArH), 7.73 (s, 4
H, ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ 21.2 (CH3), 31.3 δ ϭ 1.31 (s, 108 H, C(CH3)3], 2.17 (s, 72 H, CH3), 7.08 (s, 24 H,
[C(CH3)3], 34.3 [C(CH3)3], 63.8 (ArCAr), 78.9 (ArCCH), 81.5 ArH), 7.58Ϫ7.71 (m, 30 H, ArH), 7.71 (s, 12 H, ArH), 7.73 (s, 12
(ArCCH), 87.7 (ArCCAr), 90.2 (ArCCAr), 123.2 (Ar), 123.3 (Ar), H, ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ 21.2 (CH3), 31.3
8a as an orange solid in 60% yield. H NMR (300 MHz, CDCl3):
123.7 (Ar), 124.66 (Ar), 124.74 (Ar), 126.0 (Ar), 132.9 (Ar), 134.7
(Ar), 135.1 (Ar), 135.9 (Ar), 137.5 (Ar), 151.0 (Ar) ppm. IR (NaCl):
[C(CH3)3], 34.3 [C(CH3)3], 63.8 (ArCAr), 87.9 (ArCCAr), 88.8
(ArCCAr), 89.0 (ArCCAr), 89.2 (ArCCAr), 90.3 (ArCCAr), 90.3
ν˜ ϭ 2154, 2055 cmϪ1. To a mixture of the bis(diazo) compound 5a (ArCCAr), 123.5 (Ar), 123.5 (Ar), 123.7 (Ar), 124.7 (Ar), 124.8
(44.4 mg, 0.044 mmol), 1,3,5-triiodobenzene (6.6 mg, 0.015 mmol),
bis(triphenylphosphane)palladium dichloride (10 mg, 0.014 mmol),
and copper() iodide(catalytic amount) was added anhydrous tri-
ethylamine (1 mL) at room temperature under a nitrogen atmos-
phere. The mixture was stirred at room temperature for 2 days.
After workup of the reaction, the crude product was purified by
GPC (5 cycles, CHCl3) to afford the hexakis(diazo) compound 6a
(Ar), 126.0 (Ar), 132.9 (Ar), 134.6 (Ar), 135.9 (Ar), 137.5 (Ar),
151.1 (Ar) ppm. IR (NaCl): ν˜ ϭ 2052 cmϪ1
.
Preparation of (4-tert-Butyl-2,6-dimethylphenyl)(2,4,6-tribromo-
phenyl)methanone (1b): A mixture of chromium() oxide (7.9 g,
79 mmol), concentrated sulfuric acid (8 mL), and water (15 mL)
was added dropwise over 30 minutes at room temperature to a solu-
tion of (4-tert-butyl-2,6-dimethylphenyl)(2,4,6-tribromophenyl)me-
thanol (2.0 g, 4.0 mmol) in acetone (40 mL). The mixture was
stirred for 3 h. After adding 2-propanol (6 mL), the solvent was
removed by rotary evaporator, and the mixture was extracted with
diethyl ether. The organic layer was washed with Na2CO3 and
water, dried over anhydrous Na2SO4, and the solvents evaporated
1
as an orange solid in 79% yield. Dec. 128.9Ϫ132.0 °C. H NMR
(300 MHz, CDCl3): δ ϭ 1.31 [s, 54 H, C(CH3)3], 2.17 (s, 36 H,
4
CH3), 7.10 (s, 12 H, ArH), 7.63 (t, J ϭ 1.53 Hz, 3 H, ArH), 7.65
(d, 4J ϭ 1.53 Hz, 6 H, ArH), 7.68 (s, 3 H, ArH), 7.75 (s, 12 H,
ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ 21.2 (CH3), 31.3
[C(CH3)3], 34.3 [C(CH3)3], 63.8 (ArCAr), 87.8 (ArCCAr), 90.2
(ArCCAr), 123.4 (Ar), 123.7 (Ar), 124.6 (Ar), 124.6 (Ar), 126.0
(Ar), 132.9 (Ar), 134.6 (Ar), 135.9 (Ar), 137.5 (Ar), 151.0 (Ar) ppm.
1
to give 1b as white needles in 71% yield. M.p. 107.5Ϫ107.8 °C. H
NMR (300 MHz, CDCl3): δ ϭ 1.31 [s, 9 H, C(CH3)3], 2.28 (s, 6 H,
CH3), 7.05 (s, 2 H, ArH), 7.78 (s, 2 H, ArH) ppm. 13C NMR
(75 MHz, CDCl3): δ ϭ 22.0 (CH3), 31.0 [C(CH3)3], 34.6 [C(CH3)3],
121.8 (Ar), 123.9 (Ar), 126.7 (Ar), 134.3 (Ar), 135.2 (Ar), 138.5
(Ar), 141.8 (Ar), 154.8 (Ar), 195.6 (ArCOAr) ppm. IR (KBr): ν˜ ϭ
1666 cmϪ1. MS: m/z (%) ϭ 506 (M ϩ6, 1.1), 504 (M ϩ4, 2.7), 502
(M ϩ2, 2.8), 500 (1.1) [Mϩ], 423 (100). HRMS for C19H19OBr3:
calcd. 499.8985; found 499.9058.
IR (NaCl): ν˜ ϭ 2213, 2054 cmϪ1
.
Preparation of Dodecakis(diazo) Compound 8a: To a mixture of the
bis(diazo) compound 5a (65.0 mg, 0.064 mmol), 3,5-diiodotrimeth-
ylsilylethynylbenzene 4 (13.6 mg, 0.032 mmol), bis(triphenylphos-
phane)palladium dichloride (10 mg, 0.014 mmol), and copper()
iodide(catalytic amount) was added anhydrous triethylamine
(4 mL) at room temperature under a nitrogen atmosphere. The mix-
ture was stirred at room temperature for 2 days. After workup of Preparation of Tris(ketone) 3b: (4-tert-Butyl-2,6-dimethylphe-
the reaction, the crude product was purified by GPC (7 cycles,
CHCl3). The tetrakis(diazo) compound 7a-TMS was obtained from
the first fraction as an orange semisolid in 67% yield. 1H NMR
(300 MHz, CDCl3): δ ϭ 0.27 [s, 9 H, Si(CH3)3], 1.31 [s, 36 H,
nyl)[2,6-dibromo-4-(trimethylsilylethynyl)phenyl]methanone (2b-
TMS) was obtained by the reaction of benzophenone 1b with tri-
methylsilylacethylene, followed by column chromatography and
GPC, in 56% yield as a white solid. M.p. 136.3Ϫ136.8 °C. 1H NMR
C(CH3)3], 2.17 (s, 24 H, CH3), 7.09 (s, 8 H, ArH), 7.64Ϫ7.62 (m, (300 MHz, CDCl3): δ ϭ 0.25 [s, 9 H, Si(CH3)3], 1.31 [s, 9 H,
9 H, ArH), 7.74 (s, 8 H, ArH) ppm. 13C NMR (75 MHz, CDCl3): C(CH3)3], 2.28 (s, 6 H, CH3), 7.04 (s, 2 H, ArH), 7.65 (s, 2 H,
δ ϭ Ϫ0.2 [Si(CH3)3], 21.3 (CH3), 31.5 [C(CH3)3], 34.5 [C(CH3)3],
64.0 (ArCAr), 87.9 (ArCCAr), 88.8 (ArCCAr), 89.4 (ArCCAr), (CH3), 31.0 [C(CH3)3]), 34.6 [C(CH3)3], 99.2 (ArCCSi), 101.0
90.5 (ArCCAr), 96.0 (ArCCTMS), 103.1 (ArCCTMS), 123.6 (Ar), (ArCCSi), 120.9 (Ar), 126.7 (Ar), 126.7 (Ar), 134.4 (Ar), 135.6
124.4 (Ar), 123.9 (Ar), 124.9 (Ar), 124.9 (Ar), 126.1 (Ar), 133.1 (Ar), 138.6 (Ar), 142.6 (Ar), 154.7 (Ar), 195.9 (ArCOAr) ppm. IR
ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ Ϫ0.3 [Si(CH3)3], 22.0
(Ar), 134.4 (Ar), 135.5 (Ar), 136.1 (Ar), 137.7 (Ar), 151.2 (Ar) ppm.
IR (NaCl): ν˜ ϭ 2213, 2158, 2056 cmϪ1. A mixture of the tetrakis-
(diazo) compound 7a (47.3 mg, 0.022 mmol), tert-butyl alcohol
(15 mL), benzene (10 mL), and 10% NaOH solution (5 mL) was
stirred overnight at room temperature. After workup of the reac-
tion mixture, the crude product was purified by GPC (2 cycles,
CHCl3) to afford tetrakis(diazo) compound 7a as an orange solid
(KBr): ν˜ ϭ 1670 cmϪ1. (4-tert-Butyl-2,6-dimethylphenyl)(2,6-di-
bromo-4-ethynylphenyl)methanone (2b) was obtained by the reac-
tion of 2b-TMS with K2CO3, followed by GPC, in 98% yield as a
white solid. M.p. 103.0Ϫ104.0 °C. 1H NMR (300 MHz, CDCl3):
δ ϭ 1.30 [s, 9 H, C(CH3)3], 2.29 (s, 6 H, CH3), 3.23 (s, 1 H,
ArCCH), 7.05 (s, 2 H, ArH), 7.67 (s, 2 H, ArH) ppm. 13C NMR
(75 MHz, CDCl3):
δ
ϭ
22.0 (CH3), 31.0 [C(CH3)3]), 34.6
Eur. J. Org. Chem. 2004, 2991Ϫ3003
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3001

T. Itoh, T. Maemura, Y. Ohtsuka, Y. Ikari, H. Wildt, K. Hirai, H. Tomioka
[C(CH3)3], 80.0 (ArCCH), 81.0 (ArCCH), 121.0 (Ar), 125.65 (Ar), NMR(75 MHz, CDCl3): δ ϭ Ϫ0.2 [Si(CH3)3], 22.0 (CH3), 31.0
FULL PAPER
126.7 (Ar), 134.3 (Ar), 135.9 (Ar), 138.6 (Ar), 143.2 (Ar), 154.8 [C(CH3)3]), 34.6 [C(CH3)3], 87.4 (ArCCTMS), 87.6 (ArCCTMS),
(Ar), 195.8 (ArCOAr) ppm. IR (KBr): ν˜ ϭ 1667 cmϪ1. Tris(ketone)
3b was obtained by the reaction of 2b with 1,3,5-triiodobenzene,
88.4 (ArCCAr), 89.4 (ArCCAr), 90.6 (ArCCAr), 91.0 (ArCCAr),
121.1 (Ar), 123.2 (Ar), 123.3 (Ar), 123.4 (Ar), 124.0 (Ar), (Ar),
followed by GPC, in 67% yield as a white solid. M.p. 136.3Ϫ136.8 124.3 (Ar), 126.0 (Ar), 126.1 (Ar), 126.7 (Ar), 134.3 (Ar), 134.6
°C. 1H NMR (300 MHz, CDCl3): δ ϭ 1.31 [s, 27 H, C(CH3)3], 2.31 (Ar), 135.0 (Ar), 135.4 (Ar), 138.6 (Ar), 142.9 (Ar), 154.8 (Ar),
(s, 18 H, CH3), 7.06 (s, 6 H, ArH), 7.67 (s, 3 H, ArH), 7.73 (s, 6 195.8 (ArCOAr) ppm. IR (NaCl): ν˜ ϭ 2212, 1668 cmϪ1. MS
H, ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ 22.0 (CH3), 31.0 (MALDI-TOF) for
C
111H94Br8O4Si: calcd. 2159.035; found
[C(CH3)3]), 34.6 [C(CH3)3], 87.5 (ArCCAr), 90.7 (ArCCAr), 121.1 2159.425. A mixture of 7b-TMS (73.1 mg, 33.9 µmol), tert-butyl
(Ar), 123.3 (Ar), 126.0 (Ar), 126.7 (Ar), 134.3 (Ar), 135.0 (Ar),
135.3 (Ar), 138.5 (Ar), 142.8 (Ar), 154.8 (Ar), 195.8 (ArCOAr)
alcohol (20 mL), benzene (7 mL), and 5% NaOH aqueous solution
(10 mL) was stirred overnight at 35 °C. After workup of the reac-
ppm. IR (KBr): ν˜ ϭ 1668 cmϪ1. HRMS(ESI) for C69H60Br6O3: tion, the crude product was purified by GPC (2 cycles, CHCl3) to
calcd. 1409.9643; found 1409.9633.
afford tetrakisketone compound 7b as an brown solid in 51% yield.
M.p. 107.4Ϫ107.6 °C. H NMR (300 MHz, CDCl3): δ ϭ 1.31 (s,
1
Preparation of Hexakis(ketone) 6b: Trimethylsilylethynyl-bis(ke-
tone) 5b-TMS was obtained by the reaction of ethynyl-benzo-
phenone 2b with 3,5-diiodotrimethylsilylethynylbenzene (4), fol-
lowed by GPC, in 88% yield as a brown solid. M.p. 138 °C. 1H
NMR (300 MHz, CDCl3): δ ϭ 0.26 (s, 9 H), 1.31 [s, 9 H, C(CH3)3],
18 H, C(CH3)3], 2.31 (s,12 H, CH3), 3.16 (s,1 H, ArCCH), 7.06 (s,
4 H, ArH), 7.64Ϫ7.67 (m, 9 H, ArH), 7.75 (s, 4 H, ArH) ppm. 13
C
NMR (75 MHz, CDCl3): δ ϭ 22.0 (CH3), 31.0 [C(CH3)3]), 34.6
[C(CH3)3], 78.9 (ArCCH), 81.6 (ArCCH), 87.4 (ArCCAr), 89.2
(ArCCAr), 90.6 (ArCCAr), 90.9 (ArCCAr), 121.1 (Ar), 123.2 (Ar),
123.3 (Ar), 123.4 (Ar), 123.5 (Ar), 123.9 (Ar), 126.1 (Ar), 126.7
(Ar), 134.3 (Ar), 134.7 (Ar), 135.0 (Ar), 135.2 (Ar), 135.4 (Ar),
138.6 (Ar), 142.9 (Ar), 154.8 (Ar), 195.8 (ArCOAr) ppm. IR
(NaCl): ν˜ ϭ 2213, 1668 cmϪ1. To a mixture of tetrakisketone com-
pound 7b (36.0 mg, 17.4 µmol), 1,3,5-triiodobenzene (2.6 mg, 5.8
µmol), bis(triphenylphosphane)palladium dichloride (1.0 mg, 1
µmol), and copper() iodide(catalytic amount) was added anhy-
drous triethylamine (2 mL) at 35 °C under an argon atmosphere.
The mixture was stirred overnight at 35 °C. After workup of the
reaction, the crude product was purified by GPC (7 cycles, CHCl3)
to afford 8b as a brown solid in 16% yield. M.p. 187.6Ϫ189.0 °C.
1H NMR (300 MHz, CDCl3): δ ϭ 1.31 (s, 108 H, C(CH3)3], 2.17
(s, 72 H, CH3), 7.08 (s, 24 H, ArH), 7.58Ϫ7.71 (m, 30 H, ArH),
7.71 (s, 12 H, ArH), 7.73 (s, 12 H, ArH) ppm. 13C NMR (75 MHz,
CDCl3): δ ϭ 22.0 (CH3), 31.0 [C(CH3)3]), 34.6 [C(CH3)3], 87.4
(ArCCAr), 88.9 (ArCCAr), 89.2 (ArCCAr), 90.88 (ArCCAr), 90.94
(ArCCAr), 93.9 (ArCCAr), 121.1 (Ar), 123.2 (Ar), 123.8 (Ar),
126.1 (Ar), 126.2 (Ar), 126.7 (Ar), 126.9 (Ar), 134.3 (Ar), 134.7
(Ar), 134.8 (Ar), 135.0 (Ar), 135.3 (Ar), 136.1(Ar), 136.6 (Ar),
138.6 (Ar), 139.0 (Ar), 142.9 (Ar), 154.8 (Ar), 195.8 (ArCOAr)
4
2.31 (s, 6 H, CH3), 7.06 (s, 4 H, ArH), 7.60 (t, J ϭ 1.47 Hz, 1 H,
4
ArH), 7.62 (d, J ϭ 1.47 Hz, 2 H, ArH), 7.71 (s, 4 H, ArH) ppm.
13C NMR (75 MHz, CDCl3): δ ϭ Ϫ0.2 [Si(CH3)3], 22.0 (CH3), 31.0
[C(CH3)3]), 34.6 [C(CH3)3], 87.1 (ArCCTMS), 91.1 (ArCCTMS),
96.8 (ArCCAr), 102.5 (ArCCAr), 121.1 (Ar), 122.9 (Ar), 124.4
(Ar), 126.2 (Ar), 126.7 (Ar), 126.8 (Ar), 134.3 (Ar), 135.3 (Ar),
135.5 (Ar), 138.6 (Ar), 142.8 (Ar), 154.8 (Ar), 195.9 (ArCOAr)
ppm. IR (KBr): ν˜
ϭ . MS (MALDI-TOF) for
1668 cmϪ1
C53H50Br4O2Si: calcd. 1066.027; found 1066.146. Ethynyl-bis(ke-
tone) 5b was obtained by the reaction of the ketone 5b-TMS with
K2CO3, followed by GPC, in 66% yield as a brown solid. M.p.
92.3Ϫ94.0 °C. 1H NMR (300 MHz, CDCl3): δ ϭ 1.31 [s, 9 H,
C(CH3)3], 2.31 (s, 6 H, CH3), 3.16 (s, 1 H, ArCCH), 7.06 (s, 4 H,
4
ArH), 7.63 (d, J ϭ 1.47 Hz, 2 H, ArH), 7.64 (t, 4J ϭ 1.47 Hz, 1
H, ArH), 7.72 (s, 4 H, ArH) ppm. 13C NMR (75 MHz, CDCl3):
δ ϭ 22.0 (CH3), 31.0 [C(CH3)3]), 34.6 [C(CH3)3], 79.2 (ArCCH),
81.4 (ArCCH), 87.3 (ArCCAr), 90.9 (ArCCAr), 121.1 (Ar), 123.1
(Ar), 123.4 (Ar), 126.1 (Ar), 126.7 (Ar), 134.3 (Ar), 134.8 (Ar),
135.4 (Ar), 135.4 (Ar), 138.6 (Ar), 142.9 (Ar), 154.8 (Ar), 195.8
(ArCOAr) ppm. IR (KBr): ν˜ ϭ 1667 cmϪ1. Hexakis(ketone) 6b was
obtained by the reaction of the ketone 5b with 1,3,5-triiodoben-
zene, followed by GPC, in 71% yield as a brown solid. M.p.
ppm. IR (NaCl): ν˜ ϭ 2213, 1668 cmϪ1
.
1
107.5Ϫ107.8 °C. H NMR (300 MHz, CDCl3): δ ϭ 1.31 [s, 54 H,
EPR Measurements: The diazo compound was dissolved in 2-meth-
yltetrahydrofuran (10Ϫ3 ) and the solution was degassed in a
C(CH3)3], 2.31 (s, 36 H, CH3), 7.06 (s, 12 H), 7.67 (t, 4J ϭ 1.47 Hz,
4
3 H, ArH), 7.69 (d, J ϭ 1.47 Hz, 6 H, ArH), 7.69 (s, 3 H, ArH), quartz cell by three freeze-degas-thaw cycles. The sample was co-
7.73 (s, 12 H, ArH) ppm. 13C NMR (75 MHz, CDCl3): δ ϭ 22.0 oled in an optical transmission EPR cavity usually at 77 K and
(CH3), 31.0 [C(CH3)3]), 34.6 [C(CH3)3], 87.1 (ArCCAr), 88.7 irradiated with a Wacom 500 W Xe lamp using a Pyrex filter. EPR
(ArCCAr), 89.2 (ArCCAr), 90.9 (ArCCAr), 121.1 (Ar), 123.2 (Ar), spectra were measured on a JEOL JES TE 200 spectrometer (X-
123.7 (Ar), 123.9 (Ar), 126.1 (Ar), 126.7 (Ar), 126.9 (Ar), 134.3
(Ar), 134.7 (Ar), 135.4 (Ar), 138.6 (Ar), 142.9 (Ar), 154.8 (Ar),
band microwave unit, 100 kHz field modulation). The signal posi-
tions were read with the use of a gaussmeter. A JEOL liquid helium
195.8 (ArCOAr) ppm. IR (KBr): ν˜ ϭ 1668cmϪ1. HRMS (FAB) for transfer system was attached for the low-temperature measurement.
C156H126Br12O6: calcd. 3041.9755; found 3041.9755.
The temperature was controlled by a 9650 Microprocessor-based
Digital Temperature Indicator/Controller, and the precision of the
measurements is within Ϯ0.1 K and the control ability within
Ϯ0.2 K. Errors in the measurements of component amplitudes did
not exceed 5%, and the accuracy of the resonance fields determi-
nation was within Ϯ0.5 mT.
Preparation of Dodecakis(ketone) Compound 8b: To a mixture of
ethynyl-bis(ketone) 5b (90 mg, 90.5 mmol), 3,5-diiodotrimethyl-
silylethynylbenzene 4 (19 mg, 45.2 µmol), bis(triphenylphosphane)-
palladium dichloride (4.5 mg, 6.3 µmol), and copper() iodide
(catalytic amount) was added anhydrous triethylamine (3 mL) at 35
°C under an argon atmosphere. The mixture was stirred overnight
at 35 °C. After workup of the reaction mixture, the crude product
Low-Temperature UV/Vis Spectra: Low-temperature spectra at
77 K were obtained by using an Oxford variable-temperature
was purified by GPC (7 cycles, CHCl3) to afford the tetrakisketone liquid-nitrogen cryostat (DN, 1704) equipped with a quartz outer
compound 7b-TMS as brown solid in 46% yield. M.p. window and a sapphire inner window. The sample was dissolved in
158.7Ϫ159.1 °C. 1H NMR (300 MHz, CDCl3): δ ϭ 0.26 [s, 9 H, dry 2-MTHF, placed in a long-necked quartz cuvette of 1-mm path
Si(CH3)3], 1.31 [s, 36 H, C(CH3)3], 2.31 (s, 24 H, CH3), 7.06 (s, 8 length, and degassed thoroughly by repeated freeze-degas-thaw
H, ArH), 7.64Ϫ7.67 (m, 9 H, ArH), 7.74 (s, 8 H, ArH) ppm. 13C cycles at a pressure near 10Ϫ5 Torr. The cuvette was flame-sealed,
a
3002
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2004, 2991Ϫ3003

Phenylacetylene Dendrimers Having Sterically Congested Diazo Units
FULL PAPER
rials (Ed.: P. Lahti), Marcel Dekker Inc., New York, 1999,
pp. 629Ϫ659.
under reduced pressure, placed in the cryostat, and cooled to 77 K.
The sample was irradiated for several minutes in the spectrometer
with a Halos 500-W high-pressure mercury lamp using a Pyrex
filter, and the spectral changes were recorded at appropriate time
intervals. The spectral changes upon thawing were also monitored
by carefully controlling the matrix temperature with an Oxford In-
strument Intelligent Temperature Controller (ITC, 4).
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terference Device (SQUID) magnetometer/susceptometer. Ir-
radiation with light from an argon ion laser (488 nm, Omnichrome
543-150BS) through a flexible optical fiber which passes through
the inside of the SQUID sample holder was performed inside the
sample room of the SQUID apparatus at 5Ϫ10 K. One end of the
optical fiber was located 40 mm above the sample cell (capsule),
and the other was attached to a coupler for the laser. The bottom
part of the capsule (6 mm ϫ 10 mm) without a cap was used as a
sample cell. A sample of the solution (60 µL, 0.125Ϫ0.5 m) in 2-
MTHF was placed in the cell which was held by a straw. Irradiation
was continued until there was no further change in the magnetiz-
ation monitored at 5 K in a constant field of 5 kOe. The magnetiz-
ation before and after irradiation, Mb and Ma, was measured at
5 K in a field range 0Ϫ50 kOe. The plots of the magnetization
[M ϭ F(Ma Ϫ Mb); F is the photolysis factor for diazo compounds]
versus the magnetic field were analyzed in terms of the Burillouin
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The authors are grateful to the Ministry of Education, Culture,
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through a Grant-in-Aid for Scientific Research for Specially Pro-
moted Research (No. 12002007). The support from the Mitsubishi
Foundation and the Nagase Science and Technology Foundation
are also appreciated. We also acknowledge JSPS for an award of a
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Received February 24, 2004
Early View Article
Published Online June 1, 2004
[4c]
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3003