Synthesis of crowded triarylphosphines carrying functional sites

Article abstract of DOI:10.1016/j.jorganchem.2004.10.031

4,4′-Diphosphinobiaryls and 1,3- and 1,4-borylphosphinobenzenes carrying crowded triarylphosphine moieties were synthesized by reaction of the corresponding diarylchlorophosphine with an arylcopper(I) reagent. Intramolecular interaction of the phosphorus redox center with the other phosphorus or the boron redox center was investigated by cyclic voltammetry. 4,4′-Diphosphinobiaryls displayed two-step reversible redox waves with slight differences of the oxidation potentials due to weak interaction between two phosphorus redox centers across 4,4′-biarylene linkage. Borylphosphinobenzenes showed two step redox waves corresponding to oxidation at the phosphorus and reduction at the boron. Although significant interaction between the phosphorus and boron redox centers was not observed in the cyclic voltammograms due to large difference of the redox potentials between phosphorus and boron redox centers, an absorption due to weakly interacting phosphorus and boron was observed in the UV-Vis spectrum of the 1,4-borylphosphinobenzene.

Full text of DOI:10.1016/j.jorganchem.2004.10.031

Journal of Organometallic Chemistry 690 (2005) 2664–2672
www.elsevier.com/locate/jorganchem
Synthesis of crowded triarylphosphines carrying functional sites
*
Shigeru Sasaki , Fumiki Murakami, Midori Murakami, Mariko Watanabe,
*
Kiyotoshi Kato, Katsuhide Sutoh, Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
Received 18 August 2004; accepted 18 October 2004
Available online 18 November 2004
Abstract
4,40-Diphosphinobiaryls and 1,3- and 1,4-borylphosphinobenzenes carrying crowded triarylphosphine moieties were synthesized
by reaction of the corresponding diarylchlorophosphine with an arylcopper(I) reagent. Intramolecular interaction of the phosphorus
redox center with the other phosphorus or the boron redox center was investigated by cyclic voltammetry. 4,40-Diphosphinobiaryls
displayed two-step reversible redox waves with slight differences of the oxidation potentials due to weak interaction between two
phosphorus redox centers across 4,40-biarylene linkage. Borylphosphinobenzenes showed two step redox waves corresponding to
oxidation at the phosphorus and reduction at the boron. Although significant interaction between the phosphorus and boron redox
centers was not observed in the cyclic voltammograms due to large difference of the redox potentials between phosphorus and boron
redox centers, an absorption due to weakly interacting phosphorus and boron was observed in the UV–Vis spectrum of the 1,4-
borylphosphinobenzene.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Triarylphosphine; Biaryl; Triarylborane; Cyclic voltammetry
1. Introduction
composed of phosphorus redox centers and synthesized
compounds carrying both phosphorus and the other re-
dox sites since such investigations are expected to lead to
better understanding of newly developed structures as
well as to construction of practical functional molecules
[4]. Since crowded triarylphosphines are expected to be
good candidates for the redox site, we first attempted
Crowded triarylphosphines such as trimesitylphos-
phine (1) [1] have a unique structure represented by large
bond angles and lengths around phosphorus and some
of them are reversibly oxidized to persistent cation rad-
icals at low oxidation potentials since structural defor-
mation around the phosphorus raises the HOMO and
bulky aryl groups kinetically stabilize the cation radicals
(Scheme 1) [2]. We recently synthesized one of the most
crowded triarylphosphines known so far, tris(2,4,6-tri-
isopropylphenyl)phosphine (2), and determined its
structure and revealed redox properties [3]. We are also
interested in construction of the multistep redox systems
synthesis of aminophosphinobenzenes carrying
a
crowded triarylphosphine moiety similar to 1, and con-
siderable donor–donor, phosphorus–nitrogen, interac-
tion was observed although the cyclic voltammograms
were irreversible due to instability of the corresponding
dication diradical [5]. Herein we report the synthesis and
redox properties of 4,40-diphosphinobiaryls 3a, 3b and
borylphosphinobenzenes 4a, 4b. 4,40-Diphosphinobiar-
yls 3a and 3b were synthesized to study intramolecular
interaction of two crowded triarylphosphine moieties.
On the other hand, the crowded triarylborane moieties
were chosen to investigate donor–acceptor interaction
*
Corresponding authors. Tel.: +81 22 217 6558; fax: +81 22 217
6562.
E-mail addresses: yoshifj@mail.tains.tohoku.ac.jp (M. Yoshifuji),
sasaki@mail.tains.tohoku.ac.jp (S. Sasaki).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.10.031

S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
2665
Scheme 1. Crowded triarylphosphines.
with the crowded triarylphosphine, since trimesitylbo-
rane was reported to be a weak acceptor and to give
the corresponding anion radical as a stable molecule [6].
omerization of the propellers is slow as compared with
1
the time scale of 31P NMR. H NMR spectra of 6a, 6b
as well as 3a, 3b, 4a, 4b around 293 K also supported
this interpretation since two 2,4,6-triisopropylphenyl
groups attached to the same phosphorus atom were ob-
served as if they were inequivalent aromatic groups and
C2-symmetrical relative to P–C axis. On the other hand,
only one 2,4,6-triisopropylphenyl group which was C2-
symmetrical relative to P–C axis was observed for phos-
phine 2 [3].
2. Results and discussion
2.1. Synthesis and characterization
4,40-Dibromobiaryls 5a [7], 5b were synthesized by
Pd-catalyzed coupling of the corresponding bromoiodo-
benzenes [8] (Scheme 2). Two bis(2,4,6-triisopropylphe-
nyl)phosphino groups were successively introduced by
the reaction of the corresponding arylcopper(I) reagent,
which was prepared in situ by lithiation of 5a, 5b or 6a,
6b followed by addition of CuCl, with chlorobis(2,4,6-
triisopropylphenyl)phosphine [9] and diphosphine 3a
and 3b were isolated as colorless solids. Borylphosphi-
nobenzenes 4a, 4b were also synthesized by the reaction
of the arylcopper(I) reagent, which were prepared from
borylbromobenzenes [10], with chlorobis(2,4,6-triiso-
propylphenyl)phosphine and isolated as yellow solids
2.2. Redox properties
Cyclic voltammograms of 3a and 3b consisted of two-
step reversible waves with slight differences of the oxida-
tion potentials (Fig. 1, Table 1). 4,40-Diphosphinobiaryl
3b, which has more crowded triarylphosphine moieties,
was oxidized at lower potential than 3a. Two phospho-
rus atoms separated by biarylene bridge still have com-
munication with each other and 3a and 3b can be
oxidized in a stepwise manner. Chemical oxidation of
3a and 3b with tris(4-bromophenyl)aminium perchlorate
and silver perchlorate in dichloromethane, respectively,
afforded brown solutions, which were different from
the pink to purple color typical of cation radicals such
as 1+Å or 2+Å (Fig. 2). However, EPR spectra of the solu-
tion (3a: g = 2.007, a(P) = 23.7 mT, 3b: g = 2.007,
a(P) = 23.7 mT) as well as the frozen solution (3a:
g^ = 2.010, a^(P) = 12.0 mT, gi = 2.003, ai(P) = 39.9
mT, 3b: g^ = 2.009, a^(P) = 11.3 mT, gi = 2.002,
ai(P) = 41.5 mT) were nearly identical with those of
cation radicals such as 2+Å [3] and a Dms = 2 transition
31
(Scheme 3). P NMR spectra of 4,40-diphosphinobiar-
yls 3a (dP ꢀ45.9, ꢀ46.2), 3b (dP ꢀ50.07, ꢀ51.12) and
borylphosphinobenzenes 4a (dP ꢀ46.6, ꢀ47.3), 4b (dP
ꢀ42.1, ꢀ42.5) consisted of two closely-spaced singlet
peaks in the high field region typical of crowded triaryl-
phosphines. Since diphosphines 3a, 3b as well as bor-
ylphosphinobenzenes 4a, 4b have two crowded
propellers consisting of three aromatic rings, there can
be diastereomers arising from the helicity of the propel-
lers and they can be separately observed if the enanti-

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S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
Scheme 2. Synthesis of 4,40-diphosphinobiphenyls.
Scheme 3. Synthesis of borylphosphinobenzenes.
typical of a triplet state was lacking. Since phosphorus
centered cation radicals such as 2+Å have considerable
unpaired electron density on the phosphorus atom,
intramolecular exchange as well as dipole–dipole inter-
action across 4,40-biarylene bridge could be very weak
and we could not conclude from the EPR spectra if we
observed a dication diradical or a cation radical gener-
ated by decomposition of 32(+Å)
Cyclic voltammograms of 4a and 4b consisted of a
reversible redox wave and a quasi-reversible redox wave
.

S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
2667
Fig. 1. Cyclic voltammograms of 4,40-diphosphinobiphenyls 3b in
dichloromethane.
Table 1
Redox potentials of 3a and 3b
Compounds
1E1/2/V
2E1/2/V
DE/V
3a
3b
0.24
0.19
0.36
0.35
0.12
0.16
Measured at 295 K in dichloromethane with 0.1 M n-Bu4NClO4 as a
support electrolyte. Working electrode: glassy carbon, counter elec-
trode: Pt wire, reference electrode: Ag/0.01 M AgNO3 in acetonitrile
with 0.1 M n-Bu4NClO4.
(1E1/2Ferrocene/Ferricinium) = 0.18 V). Scan rate = 30 mV sꢀ1
.
Fig. 2. EPR spectra obtained after oxidation of 3b with silver
perchlorate in dichloromethane.
with significant differences of the oxidation potentials
(Fig. 3, Table 2). Since crowded triarylboranes such as tri-
mesitylborane are weak electron acceptors with high
reduction potential though some of the corresponding
anion radical allowed isolation, clear evidence of interac-
tion between the phosphorus and boron redox centers
was not observed by cyclic voltammetry. Although there
was no significant difference between cyclic voltammo-
grams of the 1,3-derivative 4a and the 1,4-derivative 4b,
marked difference was observed between UV–Vis spectra
of 4a and 4b (Fig. 4). 1,4-Borylphosphinobenzene 4b
exhibited an absorption at kmax = 385 nm as well as at
324 nm; on the other hand, 4a showed a single absorption
maximum at 330 nm, which was close to kmax of the
crowded triarylphosphines and boranes (1: 314 nm, 2:
327 nm, Mes3B: 330 nm) [1,3,6]. Since only 1,4-derivative
4b showed absorption at longer wavelength, a quinoid-
type structure in the excited state (Scheme 4), which
was postulated to explain a similar red shift observed in
1,4-aminoborylbenzenes [8], was considered to contrib-
ute significantly. A downfield shift of the 31P resonance
of 4b was also consistent with this observation.
reagents with chlorophosphines. Two triarylphosphine
moieties of 3a and 3b were reversibly oxidized in two
steps with the slight difference of oxidation potentials.
However, dipole–dipole and exchange interaction typi-
cal of diradicals were probably too small to be detected
in EPR, although diphosphine 3b, which has two triaryl-
phosphine moieties as crowded as 2, was expected to
give a stable dication-diradical system. Investigation
on the diphosphines carrying crowded triarylphosphine
moieties separated by various linkages will provide fur-
ther insight into phosphorus–phosphorus interaction.
Phosphorus–boron intramolecular interaction observed
in cyclic voltammograms of 4a and 4b was weak since
the triarylborane moieties employed were too weak as
acceptors, however, UV–Vis spectrum of 4b suggested
considerable intramolecular interaction such as charge
transfer in crowded triarylphosphines connected to
stronger acceptors.
4. Experimental
3. Conclusion
4.1. General
4,40-Diphosphinobiaryls 3a, 3b and borylphosphino-
benzenes 4a, 4b carrying crowded triarylphosphine moi-
eties were synthesized by the reaction of arylcopper(I)
The 1H, 13C, and 31P NMR spectra were measured on
Bruker AC200P, AV400, or JEOL a-500 spectrometers.

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S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
Fig. 4. UV–Vis spectra of borylphosphinobenzenes 4a and 4b in
dichloromethane.
Fig. 3. Cyclic voltammograms of borylphosphinobenzenes 4a (top)
and 4b (bottom) in tetrahydrofuran.
Scheme 4.
ionization (ESI). Melting points were measured on a
Yanagimoto MP-J3 apparatus without correction.
UV–Vis spectra were measured on a Hitachi U-3210
spectrometer. Microanalyses were performed at the
Instrumental Analysis Center for Chemistry, Graduate
School of Science, Tohoku University. Merck silica gel
60 and Sumitomo basic alumina (KCG-30) were used
for column chromatography. Gel permeation chroma-
tography (GPC) was performed on a Japan Analytical
Industry LC-908 recycling preparative HPLC equipped
with JAIGEL 1H + 2H columns. All reactions were car-
ried out under argon. Tetrahydrofuran was distilled
from sodium benzophenone ketyl under argon just prior
to use. Cyclic voltammetry was performed with a BAS
CV-50W controller with glassy carbon, Pt wire, and
Ag/0.01 M AgNO3/0.1 M n-Bu4NClO4/CH3CN as a
working, counter, and reference electrodes, respectively.
A substrate ca. 10ꢀ4 M was dissolved in dichlorometh-
ane or tetrahydrofuran with 0.1 M n-Bu4NClO4 as a
supporting electrolyte, and the solution was degassed
by bubbling with nitrogen gas. X-band EPR spectra
were measured on a Bruker ESP300E spectrometer at
a room temperature and 77 K. A JEOL ES-UCD2X liq-
uid nitrogen Dewar was used for measurement at 77 K.
Table 2
Redox potentials of 4a and 4b
ox
Red
Compounds
E
/V
1/2
E
/V
1/2
DE/V
4a
4b
0.42
0.38
ꢀ2.58
ꢀ2.55
3.00
2.93
Measured at 293 K in tetrahydrofuran with 0.1 M n-Bu4NClO4 as a
support electrolyte. Working electrode: glassy carbon, counter elec-
trode: Pt wire, reference electrode: Ag/0.01 M AgNO3 in acetonitrile
with 0.1 M n-Bu4NClO4.
(1E1/2Ferrocene/Ferricinium) = 0.17 V). Scan rate = 30 mV sꢀ1
.
The 1H and 13C NMR chemical shifts are expressed as d
downfield from external tetramethylsilane. The chemical
shifts were calibrated to the residual proton of the deu-
terated solvent (d 7.25 for chloroform-d) or the carbon
of the deuterated solvent (d 77.0 for chloroform-d).
The 31P NMR chemical shifts are expressed as d down-
field from external 85% H3PO4. The mass spectra were
measured on a Hitachi M-2500S with electron impact
(EI) ionization at 70 eV or a JEOL HX-110 with fast
atom bombardment (FAB) ionization using the m-nitro-
benzyl alcohol matrix. FT-ICR-MS spectra were meas-
ured on
a Bruker APEX3 with electron spray

S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
2669
Samples were dissolved in dichloromethane, which was
distilled over calcium hydride, degassed by freeze-and-
thaw cycles and transferred to a sample by bulb-to-bulb
distillation. Oxidation was carried out in an H-shaped
sealed tube and tris(4-bromophenyl)aminium perchlor-
ate and silver perchlorate were used as oxidant for 3a
and 3b, respectively.
vacuo, and purified by column chromatography (SiO2,
n-hexane) to afford 2-bromo-5-iodo-1,3-diisopropylben-
zene (7.76 g, 21.1 mmol, 77%).
2-Bromo-5-iodo-1,3-diisopropylbenzene: colorless liq-
uid; 1H NMR (400 MHz, CDCl3, 293 K) d 7.38 (s,
2H, arom.), 3.41 (sept, 2H, JHH = 6.83 Hz, CH(CH3)2-
1,3), 1.22 (d, 12H, JHH = 6.62 Hz, CH(CH3)2-1,3); 13C
NMR (101 MHz, CDCl3, 293 K) d 150.52 (s, arom-
1,3), 133.82 (s, arom-2), 127.04 (s, arom-4,6), 94.15 (s,
arom-2), 33.92 (s, CH(CH3)2-1,3), 23.36 (s, CH(CH3)2-
1,3).; LRMS (EI, 70 eV) m/z (rel. intensity) 368
(M+ + 2; 99), 366 (M+; 100), 353 (M+ + 2 ꢀ CH3; 65),
351 (M+ ꢀ CH3; 67), 226 (M+ + 2 ꢀ CH3 ꢀ I; 5), 224
(M+ ꢀ CH3 ꢀ I; 6), 211 (M+ + 2 ꢀ 2 · CH3 ꢀ I; 5),
209 (M+ ꢀ 2 · CH3 ꢀ I; 5); HRMS (EI, 70 eV). Found:
m/z 365.9481. Calc. for C12H16BrI: M, 365.9480.
4.2. Synthesis
4.2.1. Chlorobis(2,4,6-triisopropylphenyl)phosphine
To a solution of 2-bromo-1,3,5-triisopropylbenzene
(17.7 g, 62.5 mmol) in tetrahydrofuran (100 mL) was
added n-butyllithium (41.0 mL, 1.56 M in n-hexane,
64.0 mmol) at ꢀ78 ꢁC in 5 min. After being stirred for
30 min at ꢀ78 ꢁC, phosphorus trichloride (2.7 mL,
30.9 mmol) was added to the reaction mixture at ꢀ78
ꢁC. The mixture was stirred for 30 min at ꢀ78 ꢁC and
gradually warmed to room temperature and stirred for
2 h. After removal of the solvent, the residue was sub-
mitted to column chromatography (SiO2, n-hexane) to
give 13.4 g (28.4 mmol, 92%) of chlorobis(2,4,6-
triisopropylphenyl)phosphine.
4.2.3. 4,40-Dibromo-3,30,5,50-tetramethylbiphenyl (5a)
A mixture of 2-bromo-5-iodo-1,3-dimethylbenzene
[11] (2.50 g, 8.05 mmol), tetra-n-butylammonium bro-
mide (1.33 g, 4.11 mmol), palladium(II) acetate (91.2
mg, 0.406 mmol), potassium carbonate (1.10 g, 7.99
mmol) in DMF (0.9 mL), water (0.35 mL), and isopro-
pyl alcohol (1.25 mmol) was heated at 115 ꢁC for 12 h.
The reaction mixture was extracted with ether, washed
with saturated NaCl solution, and dried over anhydrous
magnesium sulfate. After removal of the drying agent
and the solvent, the residue was submitted to column
chromatography (silica gel, n-hexane) to give 5a (1.12
g, 3.05 mmol,76%).
Chlorobis(2,4,6-triisopropylphenyl)phosphine: color-
1
less prisms; m.p. 52.0–54.0 ꢁC; H NMR (400 MHz,
4
CDCl3, 296 K) d 6.94 (4H, d, JPH = 2.40 Hz, arom.-
3,5), 3.72 (4H, brm, CH(CH3)2-2,6), 2.82 (2H, septet,
3JHH = 6.80 Hz, CH(CH3)2-4), 1.19 (12H, d,
3
3JHH = 6.81 Hz, CH(CH3)2-4), 1.01 (12H, d, JHH
=
3
6.79 Hz, CH(CH3)2-2,6), 0.98 (6H, d, JHH = 6.81 Hz,
CH(CH3)2-2,6); 13C NMR (101 MHz, CDCl3, 296 K)
d151.48 (d, JPC = 16.0 Hz, arom.-2,6), 150.24 (s,
arom.-4), 134.17 (d, JPC = 50.6 Hz, arom.-1), 122.44
(d, JPC = 1.8 Hz, arom.-3,5), 34.12 (s, CH(CH3)2-4),
31.30 (d, JPC = 18.4 Hz, CH(CH3)2-2,6), 24.56 (s,
CH(CH3)2-4), 23.84 (s, CH(CH3)2-2,6), 23.80 (s,
CH(CH3)2-2,6), 23.68 (s, CH(CH3)2-2,6); 31P NMR
(162 MHz, CDCl3, 296 K) d 89.9(s); LRMS (70 eV,
EI) m/z (rel. intensity) 474 (M+ + 2; 12), 472 (M+; 33),
438 (Tip2P+ + 1; 38), 429 (M+ ꢀ i-Pr; 100), 268 (Tip-
PCl+ + 1; 14).
1
5a: colorless needles; m.p. 150.0–152.0 ꢁC; H NMR
(200 MHz, CDCl3) d 7.25 (4H, s, arom.), 2.47 (12H, s,
Me); 13C NMR (50 MHz, CDCl3) d 138.65 (s, arom.-
1,10), 138.53 (s, arom.-3,30,5,50), 126.77 (s, arom.-4,40),
126.61 (s, arom.-2,20,6,60), 23.97 (s, Me-3,305,50); LRMS
(70 eV, EI) m/z (rel. intensity) 370 (M+ + 4; 50), 368
(M+ + 2; 100), 366 (M+; 51), 289 (M+ + 2 ꢀ Br; 7), 287
(M+ ꢀ Br; 6), 208 (M+ ꢀ 2Br; 4); HRMS (70 eV, EI).
Found: m/z 365.9643. Calc. for C16H16Br2: M, 365.9646.
4.2.4. 4,40-Dibromo-3,30,5,50-tetraisopropylbiphenyl (5b)
A mixture of 2-bromo-5-iodo-1,3-diisopropylbenzene
(2.97 g, 8.14 mmol), tetra-n-butylammonium bromide
(1.32 g, 4.09 mmol), potassium carbonate (1.14 g, 8.25
mmol), palladium acetate (0.13 g, 0.41 mmol), water
(1.5 mL), isopropyl alcohol (6 mL), and N,N-dimethyl-
formamide (4 mL) was stirred for 40 h at 120 ꢁC simi-
larly to 5a to afford 5b (1.15 g, 2.40 mmol, 59%).
4.2.2. Bromo-5-iodo-1,3-diisopropylbenzene
To a mixture of 2,6-diisopropyl-4-iodoaniline (8.27 g,
27.3 mmol) in acetic acid (65 mL) and conc. sulfuric acid
(30 mL) was added isopentyl nitrite (6.8 mL, 50.6 mmol)
in 10 min at 0 ꢁC and the mixture was stirred for 25 min
at 0 ꢁC. To a solution of copper(I) bromide (5.52 g, 38.5
mmol) in hydrobromic acid (47–49%, 100 mL) was
added the mixture of the diazonium salt in 30 min at 0
ꢁC and the mixture was gradually warmed to 20 ꢁC, stir-
red for 14 h, and further stirred at 80 ꢁC for 15 min. The
reaction mixture was poured onto ice-water, extracted
with n-hexane, washed with NaHSO3 solution and satu-
rated NaCl solution, and dried over anhydrous magne-
sium sulfate. The mixture was filtered, concentrated in
1
5b: colorless crystals; m.p. 96.0–98.0 ꢁC; H NMR
(400 MHz, CDCl3, 296 K) d 7.26 (4H, s, arom.), 3.77
3
(4H, sept, JHH = 6.84 Hz, CH(CH3)2), 1.32 (24H, d,
3JHH = 6.87 Hz, CH(CH3)2); 13C NMR (101 MHz,
CDCl3, 296 K) d 148.57 (s, arom.-3,30,5,50), 140.90 (s,
arom.-1,10), 126.35 (s, arom.-4,40), 123.57 (s, arom-
2,206,60), 34.09 (s, CH(CH3)2), 23.52 (s, CH(CH3)2);

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S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
1
Anal. Calc. for C24H32Br2: C, 60.02; H, 6.71; Br, 33.27.
Found: C, 59.494; H, 6.503; Br, 32.68%.
3a: pale yellow solid; m.p. 200.0–202.0 ꢁC; H NMR
(500 MHz, CDCl3, 295 K) d 7.23 (4H, brs, arom.-
2,20,6,60), 6.94 (4H, brs, arom.-300,500), 6.89 (4H, brs,
arom.-300,500), 3.51 (4H, septet, 3JHH = 6.6 Hz,
4.2.5. Bis(2,4,6-triisopropylphenyl)(40-bromo-3,30,50,50-
tetramethyl-4-biphenylyl)phosphine (6a)
3
CH(CH3)2-200,600), 3.47 (4H, septet, JHH = 7.0 Hz,
3
CH(CH3)2-200,600), 2.84 (4H, septet, JHH = 6.9 Hz,
To a solution of 5a in tetrahydrofuran (30 mL) was
added n-butyllitium (1.8 mL, 1.52 M in n-hexane) at
ꢀ78 ꢁC. After being stirred for 15 min, copper(I) chlo-
ride (312 mg, 3.16 mmol) was added to the reaction mix-
ture at ꢀ78 ꢁC. The mixture was gradually warmed to
room temperature and stirred for 12 h. Chloro-
bis(2,4,6-triisopropylphenyl)phosphine (1.27 g, 2.69
mmol) was dissolved in tetrahydrofuran (15 mL) and
the solution was added to the reaction mixture and ref-
luxed for 24 h. After cooling to room temperature, n-
hexane was added and the suspension was filtrated by
Celite. The mixture was purified by column chromatog-
raphy (silica gel, n-hexane to n-hexane/ethyl acetate
(50:1, v/v) to give 6a (1.31 g, 1.80 mmol, 67%).
CH(CH3)2-400), 2.27 (6H, s, CH3-3,30 or 5,50), 2.27
3
(6H, s, CH3-3,30 or 5,50), 1.22 (24H, d, JHH = 6.9 Hz,
CH(CH3)2-400), 1.16 (12H, d, 3JHH = 6.9 Hz, CH(CH3)2-
3
200,600), 1.15 (12H, d, JHH = 6.9 Hz, CH(CH3)2-200,600),
3
0.75 (12H, d, JHH = 6.9 Hz, CH(CH3)2-200,600), 0.55
(12H, d, JHH = 6.9 Hz, CH(CH3)2-200,600); 13C NMR
3
(126 MHz, CDCl3) d 153.75 (d, JPC = 18.3 Hz, arom.-
200,600), 152.86 (d, JPC = 17.7 Hz, arom.-200,600), 149.35
(s, arom.-400), 142.18 (d, JPC = 18.9 Hz, arom.-
3,30,5,50), 139.25 (s, arom.-1,10), 139.09 (s, arom.-1,10),
136.26 (d, (JPC = 26.3 Hz, arom.-4,40), 131.30 (d,
JPC = 19.4 Hz, arom.-100), 126.77 (s, arom.-2,20,6,60),
122.18 (d, JPC = 4.0 Hz, arom.-300,500), 121.95 (d,
JPC = 4.0 Hz, arom.-300,500), 34.04 (s, CH(CH3)2-400),
32.16 (s, CH(CH3)2-200,600), 32.00 (s, CH(CH3)2-200,600),
24.53 (s, CH(CH3)2-200,600), 24.22 (s, CH(CH3)2-200,600),
23.90 (s, CH(CH3)2-400), 23.72 (s, CH3), 23.59 (s,
CH(CH3)2-200,600), 22.77 (s, CH(CH3)2-200,600); 31P NMR
(81 MHz, CDCl3, 295 K) d ꢀ45.9 (s), ꢀ46.2 (s); FABMS
m/z (rel. intensity) 1083 (M+ + 1; 100), 233 (Tip+; 35).
1
6a: pale yellow oil; H NMR (200 MHz, CDCl3, 295
4
K) d 7.32 (2H, s, arom.-20,60), 7.16 (2H, d, JPH = 3.2
Hz, arom.-2,6), 6.93 (2H, d, JPH = 2.8 Hz, arom.-
4
4
300,500), 6.93 (2H, d, JPH = 2.8 Hz, arom.-300,500), 3.49
3
(4H, septet, JHH 6.9 Hz, CH(CH3)2-200,600), 2.84 (2H,
3
septet, JHH = 6.9 Hz, CH(CH3)2-400), 2.47 (6H, s,
CH3-30-50), 2.26 (3H, s, CH3-3-5), 2.25 (3H, s, CH3-3-
3
5), 1.25 (6H, d, JHH = 6.2 Hz, CH(CH3)2-200,600), 1.22
4.2.7. (40-Bromo-3,30,5,50-tetraisopropyl-4-biphenylyl)-
bis(2,4,6-triisopropylphenyl)phosphine (6b)
3
(12H, d, JHH = 6.9 Hz, CH(CH3)2-400), 1.15 (6H, d,
3JHH = 6.7 Hz, CH(CH3)2-200,600), 0.72 (6H, d,
3JHH = 6.7 Hz, CH(CH3)2-200,600), 0.56 (6H, d,
3JHH = 6.6 Hz, CH(CH3)2-200,600); 31P NMR (81 MHz,
CDCl3) d = ꢀ46.3 (s); FABMS m/z (rel. intensity) 726
(M+ + 2; 18), 724 (M+; 16), 646 (M+ ꢀ Br + 1; 23), 542
(M+ ꢀ 4Me ꢀ i-Pr ꢀ Br; 100), 499 (M+ ꢀ 4Me ꢀ
2i-Pr ꢀ Br; 24); HRMS (70 eV, EI). Found: m/z
724.3799. Calc. for C46H62PBr: M, 724.3772.
To a solution of 5b (1.07 g, 2.24 mmol) in tetra-
hydrofuran (20 mL) was added t-butyllithium (4.23
mmol, 1.41 molLꢀ1 in n-pentane) at ꢀ78 ꢁC and the
mixture was stirred for 30 min. Copper(I) chloride
(270 mg, 2.71 mmol) was added and the mixture was
gradually warmed and stirred for 5 h at 20 ꢁC. A solu-
tion of chlorobis(2,4,6-triisopropylphenyl)phosphine
(0.96 g, 2.04 mmol) in tetrahydrofuran (5 mL) was
added at ꢀ78 ꢁC and the mixture was gradually warmed
and refluxed for 12 h. The mixture was concentrated
under reduced pressure and purified by column
chromatography (SiO2/n-hexane: chloroform = 5:1,
chloroform) to afford 6b (320 mg, 0.38 mmol, 17%).
4.2.6. 4,40-Bis[bis(2,4,6-triisoprophylphenyl)phosphino]-
3,30,5,50-tetramethylbiphenyl (3a)
To a solution of bis(2,4,6-triisopropylphenyl)(40-bro-
mo-3,30,50,50-tetramethyl-4-biphenylyl)phosphine
in
1
tetrahydrofuran (15 mL) was added t-butyllitium (1.2
mL, 1.64 M in n-pentane) at ꢀ78 ꢁC. After being stirred
for 30 min, copper(I) chloride (120 mg, 1.21 mmol) was
added to the reaction mixture at ꢀ78 ꢁC. The mixture
was gradually warmed to room temperature and stirred
for 12 h. A solution of chlorobis(2,4,6-triisopropylphe-
nyl)phosphine (451 mg, 0.954 mmol) in tetrahydrofuran
(10 mL) was added to the reaction mixture at ꢀ78 ꢁC
and the mixture was refluxed for 24 h. After cooling to
room temperature, n-hexane was added and the suspen-
sion was filtrated by Celite. The mixture was purified by
column chromatography (SiO2, n-hexane to n-hexane/
ethyl acetate (20:1, v/v) and GPC (JAIGEL 1H + 2H,
chloroform) to give 3a (270 mg, 0.250 mmol, 27%).
6b: yellow solid; m.p. 87.0–97.0 ꢁC; H NMR (400
MHz, CDCl3, 296 K) d 7.51 (2H, s, arom.-20 or 60),
7.24 (2H, d, JPH = 4.40 Hz, arom.-2,6), 6.91 (4H, d,
4
4JPH = 1.76 Hz, arom.-300,500), 6.90 (4H, d, JPH = 2.76
4
Hz, arom.-300,500), 3.52 (8H, sept, 3J = 6.73 Hz,
CH(CH3)2-3,5,30,50,200, 600), 2.82 (2H, sept,3JHH = 6.73
Hz, CH(CH3)2-400), 1.33–1.28 (18H, m, CH(CH3)2),
3
1.20 (12H, d, JHH = 6.80 Hz, CH(CH3)2), 1.16–1.14
3
(12H, m, CH(CH3)2), 0.76 (6H, d, JHH = 7.24 Hz,
3
CH(CH3)2), 0.72 (6H, d, JHH = 7.20 Hz, CH(CH3)2),
0.65 (6H, d, JHH = 6.60 Hz, CH(CH3)2); 31P NMR
3
(162 MHz, CDCl3, 296 K) d –1.2(s); FT-ICR-MS
(ESI, positive). Found: m/z 836.5023. Calc. for
C54H78PBrH+: 836.5019 ([M + H]+).

S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
2671
4.2.8. 4,40-Bis[bis(2,4,6-triisopropylphenyl)phosphino]-
3,30,5,50-tetraisopropylbiphenyl (3b)
tetrahydrofuran (5 mL) was added at ꢀ78 ꢁC and the
mixture was warmed to room temperature and refluxed
for 12 h. The mixture was concentrated under reduced
pressure and purified by column chromatography
(Al2O3/n-hexane) and GPC (JAIGEL 1H + 2H/toluene)
to afford 4a (243 mg, 0.30 mmol, 26%).
To a solution of 6b (0.35 g, 0.41 mmol) in tetra-
hydrofuran (10 mL) was added t-butyllithium (0.77
mmol, 1.40 mol Lꢀ1 in n-pentane) at ꢀ78 ꢁC and the
mixture was stirred for 30 min. Copper(I) chloride (40
mg, 0.37 mmol) was added and the mixture was gradu-
ally warmed and stirred for 20 h at 20 ꢁC. A solution of
chlorobis(2,4,6-triisopropylphenyl)phosphine (0.16 g,
0.34 mmol) in tetrahydrofuran (5 mL) was added at
ꢀ78 ꢁC and the mixture was gradually warmed and ref-
luxed for 12 h. The mixture was concentrated under re-
duced pressure and purified by column chromatography
(SiO2/n-hexane: chloroform = 5:1, chloroform, ethyl
acetate) and GPC (JAIGEL 1H + 2H/chloroform) to af-
ford 3b (60 mg, 0.53 mmol, 13%).
1
4a: bright yellow solid; m.p. 195–197 ꢁC; H NMR
(400 MHz, CDCl3, 296 K) d 6.91 (s, 1H, arom.), 6.86
(s, 2H, arom.), 6.80 (s, 1H, arom.), 6.77 (s, 1H, arom.),
6.73 (s, 2H, arom.), 6.66 (s, 1H, arom.), 6.64 (s, 1H,
arom.), 3.56–3.33 (4H, m, CH(CH3)2-2,6), 2.82 (1H,
sept, J = 6.86 Hz, CH(CH3)2-4), 2.81 (1H, sept,
J = 6.86 Hz, CH(CH3)2-4), 2.27 (3H, CH3), 2.24 (3H,
CH3), 2.20 (3H, CH3), 2.19 (3H, CH3), 2.14 (3H,
CH3), 1.99 (3H, CH3), 1.97 (3H, CH3), 1.92 (3H,
CH3), 1.91 (3H, CH3), 1.21 (12H, d, J = 6.89 Hz,
CH(CH3)2-4), 1.18 (3H, d, J = 6.91 Hz, CH(CH3)2-
2,6), 1.11 (3H, d, J = 6.33 Hz, CH(CH3)2-2,6), 1.10
(3H, d, J = 6.37 Hz, CH(CH3)2-2,6), 1.02 (3H, d,
J = 6.62 Hz, CH(CH3)2-2,6), 0.80 (3H, d, J = 6.56 Hz,
CH(CH3)2-2,6), 0.72 (3H, d, J = 6.51 Hz, CH(CH3)2-
2,6), 0.47 (3H, d, J = 6.56 Hz, CH(CH3)2-2,6), 0.42
(3H, d, J = 6.35 Hz, CH(CH3)2-2,6); 13C NMR (101
MHz, CDCl3, 296 K) d 153.98 (d, J = 22.14 Hz, arom.
(Tip)-2,6), 153.53 (d, J = 14.83 Hz, arom.(Tip)-2,6),
152.86 (d, J = 20.52 Hz, arom.(Tip)-2,6), 152.14 (d,
J = 13.77 Hz, arom.(Tip)-2,6), 149.10 (s, arom.(Tip)-4),
149.04 (s, arom.(Tip)-4), 146.69 (s, arom.-3), 144.41
(brs, arom.(Mes)-1), 144.24 (brs, arom.(Mes)-1),
143.09 (d, J = 14.34 Hz, arom.(Mes)-2,6), 143.04 (d,
J = 22.90 Hz, arom.(Mes)-2,6), 140.91 (s, arom.(MeC)),
140.78 (s, arom.(MeC)), 140.60 (s, arom.(MeC)), 140.34
(s, arom.(MeC)), 139.40 (s, arom.(MeC)), 139.23 (s, ar-
om.(MeC)), 138.91 (s, arom.(MeC)), 135.26 (d,
J = 27.03 Hz, arom.-1), 132.33 (d, J = 20.61 Hz, arom.
(Tip)-1), 131.46 (d, J = 19.38 Hz, arom.(Tip)-1), 131.11
(d, J = 4.07 Hz, arom.(Mes)-5), 128.73 (s, arom.(Mes)-
3,5), 128.69 (s, arom.(Mes)-3,5), 128.54 (s, ar-
om.(Mes)-3,5), 122.38 (d, J = 3.68 Hz, arom.(Tip)-3,5),
122.14 (d, J = 4.68 Hz, arom.(Tip)-3,5), 121.92 (d,
J = 2.76 Hz, arom.(Tip)-3,5), 121.46 (d, J = 5.23 Hz, ar-
om.(Tip)-3,5), 34.03 (s, CH(CH3)2-4), 33.96 (s,
CH(CH3)2-4), 32.41 (d, J = 15.63 Hz, CH(CH3)2-2,6),
3b: yellow solid; m.p. 138–140 ꢁC; 1H NMR (400
4
MHz, CDCl3, 296 K) d 7.27 (4H, d, JPH = 2.19 Hz
4
arom.-2,20,6,60), 6.91 (4H, d, JPH = 3.04 Hz, arom.-
4
300,500), 6.90 (4H, d, JPH = 2.80 Hz, arom.-300,500), 3.50
(12H, m, CH(CH3)2-3,305,50,200, 600), 2.82 (4H, sept,
3JHH = 6.85 Hz, CH(CH3)2-400), 1.21 (36H, d,
3JHH = 6.85 Hz, CH(CH3)2-3,305,50,200, 600), 1.17 (12H,
3
d, JHH = 6.65 Hz, CH(CH3)2-400), 1.14 (12H, d,
3JHH = 6.60 Hz, CH(CH3)2-400), 0.77 (24H, d,
3JHH = 6.00 Hz, CH(CH3)2-3,305,50,200, 600), 0.63 (12H,
3
d, JHH = 6.50 Hz, CH(CH3)2-3,305,50,200, 600); 13C
NMR (101 MHz, CDCl3, 296 K) d 153.59 (d,
JPC = 17.6 Hz, arom.-200,600), 153.58 (d, JPC = 17.4 Hz,
arom.-200,600), 153.37 (d, JPC = 18.3 Hz, arom.-3,5),
149.66 (s, arom.-400), 141.5 (s, arom.-1,10), 134.93 (dd,
JPC = 26.7 Hz, J = 4.5 Hz, arom.-4,40), 132.40 (d,
JPC = 33.2 Hz, arom.-100), 122.59 (d, JPC = 3.6 Hz,
arom.-2,20,6,60), 122.45 (d, JPC = 4.4 Hz, arom.-300,500),
122.35 (d, JPC = 5.1 Hz, arom.-300,5000), 34.49 (s,
CH(CH3)2-400), 32.57 (d, JPC = 12.0 Hz, CH(CH3)2-
3,30,5,50), 32.43 (d, JPC = 20.0 Hz, CH(CH3)2-200,600),
32.33 (d, JPC = 24.0 Hz, CH(CH3)2-200,600), 25.07 (s,
CH(CH3)2-400), 24.94 (s, CH(CH3)2-400), 24.48 (s,
CH(CH3)2-3,30,5,50), 24.46 (s, CH(CH3)2-200,600), 23.90
(s, CH(CH3)2-400), 23.48 (s, CH(CH3)2-3,30,5,50), 23.32
(s, CH(CH3)2-200,600); 31P NMR (162 MHz, CDCl3, 296
K) d ꢀ50.07 (s), ꢀ51.12 (s); FT-ICR-MS (ESI, positive).
Found: m/z 1194.9185. Calc. for C84H124Pþ2 : 1194.9173
([M]+).
31.97-31.66
(d · 3,
CH(CH3)2-2,6),
24.69–21.16
(CH(CH3)2 and CH3); 31P NMR (162 MHz, CDCl3,
296 K) d ꢀ46.6 (s), ꢀ47.4 (s); UV–Vis (CH2Cl2,
c = 4.15 · 10ꢀ5 mol Lꢀ1) kmax(log e)/nm 330 (4.22);
FT-ICR-MS (ESI, positive). Found: 805.6017. Calc.
for C57H78BPH+: 805.6007 ([M + H]+).
4.2.9. 3-(Dimesitylboryl)mesityl]bis(2,4,6-
triisopropylphenyl)phosphine (4a).
To a solution of bromo-3-(dimesitylboryl)mesitylene
(521 mg, 1.17 mmol) in tetrahydrofuran (15 mL) was
added t-butyllithium (2.33 mmol, 1.50 mol Lꢀ1 in n-pen-
tane) at ꢀ78 ꢁC and the mixture was stirred for 30 min.
Copper(I) chloride (115 mg, 1.17 mmol) was added at
ꢀ78 ꢁC and the mixture was warmed to 15 ꢁC and stir-
red for 3.5 h. A solution of chlorobis(2,4,6-triisopropyl-
4.2.10. [4-(Dimesitylboryl)duryl]bis(2,4,6-
triisopropylphenyl)phosphine (4b)
To a solution of bromo-4-(dimesitylboryl)durene
(502 mg, 1.09 mmol) in tetrahydrofuran (15 mL) was
added t-butyllithium (2.16 mmol, 1.50 mol Lꢀ1 in n-pen-
tane) at ꢀ78 ꢁC and the mixture was stirred for 30 min.
phenyl)phosphine
(554
mg,
1.17
mmol)
in

2672
S. Sasaki et al. / Journal of Organometallic Chemistry 690 (2005) 2664–2672
Copper(I) chloride (107 mg, 1.08 mmol) was added at
ꢀ78 ꢁC and the mixture was warmed to 15 ꢁC and stir-
red for 3.5 h. A solution of chlorobis(2,4,6-triisopropyl-
Acknowledgements
Financial support by Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports,
Science and Technology (Nos. 15550025 and 1330409)
is gratefully acknowledged. The authors also thank the
Instrumental Analysis Center for Chemistry, Graduate
School of Science, Tohoku University, for measurement
of 600 MHz NMR, mass spectra, and elemental analysis.
phenyl)phosphine
(518
mg,
1.09
mmol)
in
tetrahydrofuran (5 mL) was added at ꢀ78 ꢁC and the
mixture was warmed to room temperature and refluxed
for 36 h. The mixture was concentrated under reduced
pressure and purified by column chromatography
(Al2O3/n-hexane) and GPC (JAIGEL 1H + 2H/toluene)
to afford 4b (217 mg, 0.26 mmol, 24%).
1
4b: bright yellow solid; m.p. 98–101 ꢁC; H NMR
References
(400 MHz, CDCl3, 296 K) d 6.92 (2H, brs, Tip-3,5),
6.90 (2H, d, J = 2.39 Hz, Tip-3,5), 6.74 (4H, brs, Mes-
3,5), 3.54 (4H, brs, CH(CH3)2-2,6), 2.84 (2H, sept,
J = 6.88 Hz, CH(CH3)2-4), 2.27 (6H, s, CH3), 2.12
(3H, brs, CH3), 2.09 (3H, brs, CH3), 2.10 (6H, s,
CH3), 1.96 (6H, s, CH3), 1.94 (6H, s, CH3), 1.27 (6H,
[1] (a) J.F. Blount, C.A. Maryanoff, K. Mislow, Tetrahedron Lett.
48 (1975) 913;
(b) B.I. Stepanov, E.N. Karpova, A.I. Bokanov, J. Gen. Chem.
USSR 39 (1969) 1514.
[2] (a) M. Culcasi, Y. Berchadsky, G. Gronchi, P. Tordo, J. Org.
Chem. 56 (1991) 3537;
3
brs, CH(CH3)2-2,6), 1.23 (12H, d, JHH = 6.91 Hz,
CH(CH3)2-4), 1.14 (6H, brs, CH(CH3)2-2,6), 0.70 (6H,
(b) C. Palau, Y. Berchadsky, F. Chalier, J.-P. Finet, G. Gronchi,
P. Tordo, J. Phys. Chem. 99 (1995) 158.
3
d, JHH = 6.33 Hz, CH(CH3)2-2,6), 0.64 ((6H, d,
[3] S. Sasaki, K. Sutoh, F. Murakami, M. Yoshifuji, J. Am. Chem.
Soc. 124 (2002) 14830.
3JHH = 6.05 Hz, CH(CH3)2-2,6); 13C NMR (101 MHz,
CDCl3, 296 K) d 153.46 (d, JPC = 17.8 Hz, arom.(Tip)-
2,6), 152.45 (d, JPC = 17.4 Hz, arom.-(Tip)-2,6), 149.05
(s, arom.(Tip)-4), 148.40 (brs, arom.-4), 144.65 (brs, ar-
om.(Mes)-1), 140.97 (s, arom.(Mes)-2,6), 140.85 (s, ar-
om.(Mes)-2,6), 140.74 (s, arom.(Mes)-2,6), 139.67 (d,
J = 25.23 Hz, arom.-1), 137.41 (d, J = 19.2 Hz,
arom.-2,6), 135.36 (d, J = 3.89 Hz, arom.-3,5), 135.14
(d, J = 3.48 Hz, arom.-3,5), 132.11 (d, J = 21.2 Hz, ar-
om.(Tip)-1), 128.80 (s, arom.(Mes)-3,5), 128.69 (s, ar-
om.(Mes)-3,5), 122.19 (d, J = 4.3 Hz, arom.(Tip)-3,5),
121.83 (d, J = 4.0 Hz, arom.(Tip)-3,5), 34.04 (s,
CH(CH3)2-4), 32.00 (d, J = 18.2 Hz, CH(CH3)2-2,6),
31.50 (d, J = 17.0 Hz, CH(CH3)2-2,6), 24.42 (d,
J = 21.4 Hz, CH3-2,6), 23.91 (s, CH(CH3)2-4), 23.65
(s), 23.23 (s), 22.76 (s), 21.20 (s), 20.59 (s), 19.87 (s),
19.69 (s); 31P NMR (162 MHz, CDCl3, 296 K) d
ꢀ42.1 (s), ꢀ42.5 (s); UV–Vis (CH2Cl2, c = 4.15 · 10ꢀ5
mol Lꢀ1) kmax (log e)/nm 324 (3.96), 385 (3.29);
FT-ICR-MS (ESI). Found: 819.6182. Calc. for
C58H80BPH+: 819.6163 ([M + H]+).
[4] (a) K. Tsuji, S. Sasaki, M. Yoshifuji, Tetrahedron Lett. 40 (1999)
3203;
(b) S. Sasaki, H. Aoki, K. Sutoh, S. Hakiri, K. Tsuji, M.
Yoshifuji, Helv. Chim. Acta 85 (2002) 3842.
[5] (a) S. Sasaki, F. Murakami, M. Yoshifuji, Tetrahedron Lett. 38
(1997) 7095;
(b) S. Sasaki, F. Murakami, M. Murakami, R. Chowdhury, K.
Sutoh, M. Yoshifuji, Phosphorus, Sulfur Silicon 177 (2002) 1477.
[6] (a) A. Schultz, W. Kaim, Chem. Ber. 122 (1989) 1863;
(b) M.M. Olmstead, P.P. Power, J. Am. Chem. Soc. 108 (1986)
4235;
(c) K. Okada, T. Sugawa, M. Oda, J. Chem. Soc. (1992) 74.
[7] G. Alberti, C.U. Costantino, F. Marmottini, R. Vivani, P.
Zappelli, Angew. Chem., Int. Ed. Engl. 32 (1993) 1357.
[8] V. Penalva, J. Hassan, L. Lavenot, C. Gozzi, M. Lemaire,
Tetrahedron Lett. 39 (1998) 2559.
[9] (a) S. Freeman, M.J. Harger, J. Chem. Soc., Parkin Trans. 1
(1987) 1399;
(b) D.J. Brauer, F. Bitterer, F. Do¨rrenbach, G. Hessler, O.
Stelzer, C. Kruger, F. Lutz, Z. Naturforsch., B: Chem. Sci. 51
¨
(1996) 1183.
[10] S. Yamaguchi, T. Shirasaka, K. Tamao, Org. Lett. 2 (2000) 4129.
[11] C.H.M. Amijs, G.P.M. van Klink, G. van Koten, Green Chem. 5
(2003) 470.