Synthesis and redox properties of sterically crowded triarylphosphine and tetraaryldiphosphane bearing phenothiazinyl groups

Article abstract of DOI:10.1002/hc.20714

Sterically crowded triarylphosphines and tetraaryldiphosphane bearing phenothiazinyl groups as redox sites were synthesized by using 2-bromo-1,3-dialkyl-5-phenothiazinylbenzenes as key synthetic intermediates. Cyclic voltammograms of these compounds show the first redox wave corresponding to the oxidation of the phosphorus redox center and the following waves corresponding to the oxidation of the phenothiazinyl groups. Introduction of more sterically crowded triarylphosphine moiety leads to more reversible oxidation of the phosphine as well as the phenothiazine moieties. A crowded tetraaryldiphosphane bearing four phenothiazinyl groups are oxidized in two steps. Four phenothiazine moieties are oxidized nearly at once after oxidation of the diphosphane moiety.

Full text of DOI:10.1002/hc.20714

Heteroatom Chemistry
Volume 22, Number 3/4, 2011
Synthesis and Redox Properties of Sterically
Crowded Triarylphosphine and
Tetraaryldiphosphane Bearing Phenothiazinyl
Groups
Shigeru Sasaki, Midori Murakami, Fumiki Murakami,
and Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aoba,
Aoba-ku, Sendai 980-8578, Japan
Received 1 September 2010; revised 8 December 2010
INTRODUCTION
ABSTRACT: Sterically crowded triarylphosphines
and tetraaryldiphosphane bearing phenothiazinyl
groups as redox sites were synthesized by using
2-bromo-1,3-dialkyl-5-phenothiazinylbenzenes as key
synthetic intermediates. Cyclic voltammograms of
these compounds show the first redox wave cor-
responding to the oxidation of the phosphorus re-
dox center and the following waves corresponding
to the oxidation of the phenothiazinyl groups. In-
troduction of more sterically crowded triarylphos-
phine moiety leads to more reversible oxidation of the
phosphine as well as the phenothiazine moieties. A
crowded tetraaryldiphosphane bearing four phenoth-
iazinyl groups are oxidized in two steps. Four phe-
nothiazine moieties are oxidized nearly at once af-
Sterically crowded triarylphosphines and tetraaryl-
diphosphanes are known to be oxidized reversibly
to give persistent radical cations [1]. Representa-
tive compounds are 2,4,6-trimethylphenyl deriva-
tives 1a [2] and 2a [3], and 2,4,6-triisopropylphenyl
derivatives 1b [4] and 2b [5] (Chart 1). We syn-
thesized crowded triarylphosphines and arsines
having structure around pnictogen similar to 1a,
bearing amino and phenothiazinyl groups [6]. How-
ever, even with the help of the stability of phe-
nothiazinylbenzene redox cycle [7], triarylphos-
phines such as 3a were reversibly oxidized in two
steps only at low temperature (Chart 2). Tris(2,4,6-
triisopropylphenyl)phosphine (1b) and the related
compounds are reported to give more stable re-
dox systems as well as radical cations [4], and we
have synthesized crowded triarylphosphines struc-
turally similar to 1b, bearing various functional sites
such as donors [8], acceptors [9], and radicals [10].
The electrochemical measurement of these function-
alized crowded triarylphosphines showed that the
phosphine moieties worked as reversible redox sites
and the molecules as a whole became multistep re-
versible redox systems. Herein we report synthe-
sis and redox properties of sterically crowded tri-
arylphosphines 3b and 3c bearing phenothiazinyl
groups [11]. Enhanced stability of the phosphorus
ꢀ
C
ter oxidation of the diphosphane moiety.
2011 Wi-
ley Periodicals, Inc. Heteroatom Chem 22:506–513, 2011;
View this article online at wileyonlinelibrary.com. DOI
10.1002/hc.20714
Dedicated to Professor Kin-ya Akiba on the occasion of his 75th
birthday.
Correspondence to: Shigeru Sasaki; e-mail: ssasaki@
m.tohoku.ac.jp.
Contract grant sponsor: Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Contract grant numbers: 15550025, 17310063, and 22605001.
2011 Wiley Periodicals, Inc.
c
ꢀ
506

Synthesis and Redox Properties of Triarylphosphine and Tetraaryldiphosphane 507
CHART 1
CHART 2
Triarylphosphines 3b and 3c show upfield 31P
NMR chemical shift [δ −46.2 (3b), −50.1 (3c),
−36 (1a), −52.4 (1b)] typical of this kind of
crowded triarylphosphines. Diphosphane 4 also
shows upfield 31P NMR chemical shift similar
to the related compounds [δ −35.7, −40.1 (2b)].
The substituents on the phosphorus of 3c exhibit
inversion-averaged 1H and 13C NMR spectra typ-
ical of the phosphines similar to 1b. As a re-
sult of the rapid inversion, triarylphosphine 3c, in
which one aryl group is different among three,
gives the three different aryl groups at 296 K.
The 1H and 13C NMR spectra of diphosphane
4 also reflect the crowded structure around the
phosphorus, where nonequivalent four methine and
eight methyl groups are observed. Diphosphane 4
shows downfield methine protons and AAꢁXXꢁ pat-
terns of 13C signals similar to 2b [5].
redox center contributes to the stability of the whole
redox systems. Tetraaryldiphosphane 4 bearing four
phenothiazinyl groups was also synthesized and re-
dox properties were revealed.
RESULTS AND DISCUSSION
Synthesis and Structure
Key synthetic intermediates, 2-bromo-1,3-dimethyl-
5-phenothiazinylbenzene (5a) and 2-bromo-1,3-
diisopropyl-5-phenothiazinylbenzene (5b), were
synthesized from 2-bromo-1,3-dialkyl-5-iodooben-
zenes [8] by Ullmann coupling (Scheme 1).
Bromobenzene 5a was converted to the corre-
sponding aryldichlorophosphine and was reacted
with 2,4,6-triisopropylphenylcopper(I) [12] to
give 3b. On the other hand, bromobenzene 5b
was converted to the corresponding arylcopper(I)
reagent and was coupled with chlorobis(2,4,6-
triisopropylphenyl)phosphine [13] to give tri-
arylphosphine 3c. The lithiation of 5b followed by
the reaction with a half equivalent of phosphorus
trichloride gave chlorophosphine 6. Attempted
synthesis of the triarylphosphine bearing three
phenothiazinylaryl groups was unsuccessful and
Redox Properties
The redox properties of 3b, 3c, and 4 were studied
by cyclic voltammetry. In spite of the introduction of
two 2,4,6-triisopropylphenyl groups, redox waves of
3b are irreversible even at 195 K (Fig. 1). Less ster-
ically crowded 3a exhibits nearly reversible waves
under similar conditions [6]. On the other hand, 3c
shows reversible two-step redox waves followed by
irreversible waves at 295 K (Fig. 2). The introduc-
tion of the phosphorus redox center similar to 1b
contributes to the stabilization of the whole redox
tetraaryldiphosphane
4 was obtained instead.
Diphosphane 4 was also synthesized by the reduc-
tive coupling of 6.
Heteroatom Chemistry DOI 10.1002/hc

508 Sasaki et al.
HN
S
Cu
/THF
/Cu/K2CO3
ii) PCl3
Br
I
3b
Br
Br
N
S
Cl2P
N
S
1,2-dichlorobenzene
reflux
86%
53% from 5a
5a
HN
S
/Cu/K2CO3
3c
N
S
Br
I
iii)
1,2,4-trichlorobenzene
31%
220°C
85%
5b
P
Cl
reflux
S
S
N
N
Mg
3
P Cl
4
5b
5b
0°C to reflux
19% from 5b
8%
6
SCHEME 1 Synthesis of sterically crowded triarylphosphines and tetraaryldiphosphane bearing phenothiazines.
systems as compared with 3a and 3b. Judging from
comparison with the oxidation potentials of each
subunit [1 E1/2 = 0.16 V (1b), 0.48 V (5b)], the first
oxidation takes place on the phosphorus followed by
the second oxidation on the nitrogen. The introduc-
tion of a phenothiazinyl group in place of an iso-
propyl group raises the oxidation potential of the
phosphines as seen in trimethylphenyl derivatives
[6]. Diphosphane 4 gave the first quasi-reversible
oxidation followed by one-step (unresolved) intense
reversible wave (Fig. 3). The first oxidation is de-
duced to take place on the diphosphane moiety, judg-
ing from comparison with the oxidation potential
of similar compounds [1 E1/2 = 0.04 (2b), 0.48 (5b)
V] and electron withdrawing effect of the phenoth-
iazinyl groups as compared with isopropyl groups as
shown above. Delocalization of the positive charge
to the two phosphorus atoms relieves positive charge
repulsion among charges on the phenothiazines and
the phosphorus atoms.
In order to get further insight into the ox-
idation of the triarylphosphines bearing phe-
nothiazines, electron paramagnetic resonance
(EPR) study of the oxidation was carried out.
Phosphine 3b can be chemically oxidized by
tris(4-bromophenyl)aminium
perchlorate
in
dichloromethane to give yellow-brown solution.
Phosphine 3c can be oxidized by various reagents
such as tris(4-bromophenyl)aminium perchlorate
and silver(I) perchlorate in dichloromethane to give
brown solution. The EPR spectra of both solutions
FIGURE 1 Cyclic voltammogram of 3b at 195 K. Scan rate
50 mV s−1
.
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Redox Properties of Triarylphosphine and Tetraaryldiphosphane 509
amount of the oxidant did not give signals other
than those of the phosphorus radical cations.
Other signals such as the fine structures, ꢀm =
s
2 transitions typical of the triplet species, and
hyperfine structures of the nitrogen radicals were
not observed. The brown color of the solutions,
which is different from the typical purple color
of the radical cation of triarylphosphine, strongly
suggests the intramolecular interaction between the
phosphorus radical cation and the phenothiazine,
but it is probable that the highly oxidized species
are too reactive to allow direct observation rather
than the formation of the closed shell species such
as 6 (Scheme 2).
CONCLUSION
The redox study on a series of sterically crowded
triarylphosphines bearing phenothiazine moieties
FIGURE 2 Cyclic voltammogram of 3c at 295 K. Scan rate
30 mV s−1
showed that the development of the stable phospho-
rus redox centers greatly contributes to the construc-
tion of the stable multistep redox systems composed
of various redox sites. Although the triarylphosphine
redox centers still need marginal stability to over-
come instability of the highly oxidized state, this
work provides guideline for the construction of the
practical redox centers bearing main group elements
as key components.
.
consist of two lines typical of the radical cations
of the sterically crowded triarylphosphines and
the corresponding frozen solutions gave the axial
symmetrical powder patterns of the hyperfine
coupling tensor (Fig. 4). Isotropic and anisotropic
hyperfine coupling constants obtained by oxidation
of 3c are very similar to those of 1b (g = 2.007, a
= 23.7 mT, g// = 2.002, a// = 41.7 mT, g⊥ = 2.009,
a⊥ = 13.0 mT), suggesting generation of very similar
radical cations. However, the use of an excess
EXPERIMENTAL
1H, 13C, and 31P NMR spectra were measured on
a Bruker (Germany) AM600, AV400, or a JEOL
(Tokyo, Japan) JNM-A500 spectrometer. 1H and 13
C
NMR chemical shifts are expressed as δ downfield
from external tetramethylsilane and calibrated to the
residual proton of the deuterated solvents (δ 7.26
for chloroform-d, 7.27 for benzene-d6) or the carbon
of the deuterated solvent (δ 77.0 for chloroform-d,
128.0 for benzene-d6). 31P NMR chemical shifts are
expressed as δ downfield from external 85% H3PO4.
Mass spectra were measured on a Hitachi (Tokyo,
Japan) M-2500S with electron impact (EI) ionization
at 70 eV, or a JEOL HX-110 with fast atom bombard-
ment ionization using m-nitrobenzyl alcohol matrix.
Melting points were measured on a Yanagimoto
(Kyoto, Japan) MP-J3 apparatus, without correction.
Merck (Darmstadt, Germany) silica gel 60 and Sum-
itomo (Tokyo, Japan) basic alumina (KCG-30) were
used for the column chromatography. All reactions
were carried out under argon unless otherwise speci-
fied. Anhydrous tetrahydrofuran and ether were dis-
tilled from sodium diphenylketyl under argon just
prior to use. Cyclic voltammetry was performed
on a BAS (West Lafayette, IN) CV-50W controller
FIGURE 3 Cyclic voltammogram of 4 at 295 K. Scan rate
30 mV s−1
.
Heteroatom Chemistry DOI 10.1002/hc

510 Sasaki et al.
FIGURE 4 EPR spectra obtained after oxidation of (a) 3b with tris(4-bromophenyl)aminium perchlorate in dichloromethane
at 293 K (g = 2.008, a = 22.9 mT), (b) at 77 K (g// = 2.003, a// = 38.9 mT, g = 2.009, a = 14.4 mT), (c) 3c with silver(I)
⊥
⊥
perchlorate in dichloromethane at 293 K (g = 2.008, a = 23.1 mT), and (d) at 77 K (g// = 2.002, a// = 40.2 mT, g = 2.010,
⊥
a
⊥
= 11.5 mT).
with a glassy carbon, Pt wire, and Ag/0.01 mol L−1,
AgNO3/0.1 mol L−1, n-Bu4NClO4/CH3CN as a work-
ing, counter, and reference electrode, respectively
(Ferrocene/Ferricinium = 0.18 V). A substrate (ca.
10−4 mol L−1) was dissolved in dichloromethane with
0.1 mol L−1 n-Bu4NClO4 as a supporting electrolyte,
and the solution was degassed by bubbling with ni-
trogen gas. X-band EPR spectra were measured on
a Bruker ESP300E spectrometer equipped with a
JEOL ES-UCD2X liquid nitrogen Dewar. Samples
SCHEME 2 Redox process of 3c.
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Redox Properties of Triarylphosphine and Tetraaryldiphosphane 511
were dissolved in dichloromethane, which was dis-
(2,6,-Dimethyl-4-phenothiazinylphenyl)bis(2,4,6-
triisopropylphenyl)phosphine (3b): To a solution
of 5a (403 mg, 1.05 mmol) in ether (80 mL),
t-butyllithium (1.47 mol L−1 in pentane, 1.50 mL,
2.21 mmol) was added at −78◦C. The solution was
stirred for 30 min and then phosphorus trichloride
(1.0 mL, 11.5 mmol) was added. The resultant mix-
ture was stirred for 30 min, warmed to room tem-
perature, stirred for 10 h, and concentrated under
reduced pressure to give crude dichloro(2,6-
dimethyl-4-phenothiazinylphenyl)phosphine. To a
solution of 2,4,6-triisopropylphenylmagnesium
tilled over calcium hydride, degassed by the freeze-
and-thaw cycles, and transferred to a sample by
bulb-to-bulb distillation. Oxidation was carried out
in an H-shaped sealed tube.
2-Bromo-1,3-dimethyl-5-phenothiazinylbenzene
(5a):
A
mixture of phenothiazine (100 mg,
0.504 mmol), 2-bromo-1,3-dimethyl-5-iodobenzene
(194 mg, 0.625 mmol), potassium carbonate
(275 mg, 1.99 mmol), copper powder (66.2 mg,
1.04 mmol), and 1,2-dichlorobenzene (3 mL) was
refluxed for 48 h. The mixture was cooled to 20◦C
and directly submitted to column chromatography
(SiO2, n-hexane) to give 5a (166 mg, 0.434 mmol,
bromide
prepared
from
2-bromo-1,3,5-
triisopropylbenzene (748 mg, 2.64 mmol), magne-
sium (68.3 mg, 2.81 mmol), and tetrahydrofuran
(5 mL), copper(I) chloride (302 mg, 3.05 mmol)
was added at −78◦C. The mixture was stirred for 5
min, warmed to 20◦C, stirred for 19 h, and cooled to
−78◦C. To the mixture, a solution of dichloro(2,6-
1
86%). 5a: colorless needles; mp 200.5–203.0◦C; H
NMR (500 MHz, CDCl3, 296 K): δ 7.12 (2H, s, Ar-4,6),
3
4
7.01 (2H, dd, JHH = 7.5 Hz, JHH = 1.5 Hz, pheno-
4,6), 6.90–6.70 (4H, brs, pheno-2,3,7,8), 6.25 (2H,
3
brd, JHH = 7.0 Hz, pheno-1,9), 2.49 (6H, s, CH3);
dimethyl-4-phenothiazinylphenyl)phosphine
in
13C{ H} NMR (126 MHz, CDCl3, 296 K): δ143.84
tetrahydrofuran (30 mL) was added in 10 min. The
mixture was stirred for 2 h, warmed, refluxed for
20 h, and cooled to room temperature. n-Hexane
was added and the resultant suspension was filtered
by Celite. The filtrate was concentrated under
reduced pressure and the residue was purified
by column chromatography (Al2O3, n-hexane) to
give 3b (412 mg, 0.557 mmol, 53%). 3b: colorless
1
(brs, pheno-9a,10a), 141.02 (s, Ar-1,3), 139.25
(brs, Ar-5), 130.13 (brs, Ar-4,6), 127.02 (s, Ar-2),
126.82 (s, pheno-2,8), 126.72 (brs, pheno-4,6),
122.55 (brs, pheno-3,7), 120.26 (brs, pheno-4a,5a),
116.04 (s, pheno-1,9), 24.01 (s, CH3); LRMS (70 eV,
EI) 383 (M+ + 2; 100), 381 (M+; 95), 302 (M+−Br).
2-Bromo-1,3-diisopropyl-5-phenothiazinylbenzene
(5b): A mixture of phenothiazine (2.50g, 12.5 mmol),
2-bromo-1,3-diisopropyl-5-iodobenzene (3.52 g,
9.59 mmol), potassium carbonate (5.67 g,
41.0 mmol), copper powder (1.34 g, 21.1 mmol),
and 1,2,4-trichlorobenzene (9 mL) was stirred at
220◦C for 40 h. The mixture was cooled to 20◦C
and directly submitted to column chromatography
(SiO2, n-hexane, n-hexane/chloroform = 5/1) to give
5b (3.57 g, 8.15 mmol, 85%). 5b: pale yellow crys-
needles (EtOH-CH2Cl2); mp 192.0–193.5◦C; 1H NMR
4
(500 MHz, CDCl3, 296 K): δ7.00 (2H, d, JPH
=
3.2 Hz, Ar-3,5), 6.9763 (2H, dd, 3 JHH = 7.3 Hz, 4 JHH
=
4
1.7 Hz, pheno-4,6), 6.9758 (2H, dd, JHH = 3.0 Hz,
4 JPH = 3.3 Hz, Tip-3,5), 6.94 (2H, dd, JHH = 3.1 Hz,
4
4 JPH = 2.0 Hz, Tip-3,5), 6.83 (2H, ddd, 3 JHH = 7.5 Hz,
3 JHH = 8.1 Hz, JHH = 1.7 Hz, pheno-2,8), 6.78 (2H,
4
3
4
td, JHH = 7.4 Hz, JHH = 1.3 Hz, pheno-3,7), 6.25
3
4
(2H, dd, JHH = 8.1 Hz, JHH = 1.2 Hz, pheno-1,9),
3
4
3.56 [4H, septet of doublet, JHH = 6.1 Hz, JPH
=
=
1
3
tals; mp 175.0–178.0◦C; H NMR (400 MHz, C6D6,
6.1 Hz, CH(CH3)2-2, 6], 2.87 [2H, septet, JHH
3
4
296 K): δ7.17 (2H, s, Ar-4,6), 7.08 (2H, dd, JHH
=
6.9 Hz, CH(CH3)2-4], 2.25 (6H, d, JPH = 5.0 Hz,
4
3
7.5 Hz, JHH = 1.6 Hz, pheno-4,6), 6.78 (2H, td,
CH3), 1.24 [12H, d, JHH = 6.9 Hz, CH(CH3)2-4],
3 JHH = 7.8 Hz, JHH = 1.6 Hz, pheno-2,8), 6.70 (2H,
1.21 [6H, d, JHH = 6.7 Hz, CH(CH3)2-2,6], 1.19
4
3
3
4
3
td, JHH = 7.4 Hz, JHH = 1.3 Hz, pheno-3,7), 6.38
[6H, d, JHH = 6.7 Hz, CH(CH3)2-2,6], 0.81 [6H, d,
3
4
3
(2H, dd, JHH = 8.2 Hz, JHH = 1.2 Hz, pheno-1,9),
3 JHH = 6.6 Hz, CH(CH3)2-2,6], 0.66 [6H, d, JHH
=
3
1
3.64 [2H, sept, JHH = 6.8 Hz, CH(CH3)2], 1.11
6.7 Hz, CH(CH3)2-2,6]; 13C{ H} NMR (126 MHz,
CDCl3, 296 K): δ 153.33 (d, JPC = 17.7 Hz, Tip-
2,6), 153.06 (d, JPC = 17.7 Hz, Tip-2,6), 149.71 (s,
Tip-4), 145.07 (d, JPC = 19.4 Hz, Ar-2,6), 144.21 (s,
[12H, d, JHH = 6.9 Hz, CH(CH3)2]; 13C{ H} NMR
(101 MHz, C6D6, 296 K): δ 151.49 (s, Ar-1,3), 145.13
(s, pheno-9a,10a), 141.02 (s, Ar-5), 127.50 (s, Ar-2),
127.45 (s, pheno-2,8 or 4,6), 127.41 (s, pheno-2,8 or
4,6), 126.45 (s, Ar-4,6), 123.21 (s, pheno-3,7), 120.86
(s, pheno-4a,5a), 116.19 (s, pheno-1,9), 34.24 [s,
CH(CH3)2], 23.00 [s, CH(CH3)2]; LRMS (70 eV, EI)
m/z (rel intensity) 439 (M+; 100), 359 (M+−Br; 23),
198 (Tip; 14).
3
1
pheno-9a,10a), 139.52 (s, Ar-4), 138.62 (d, JPC
=
26.3 Hz, Ar-1), 130.61 (d, JPC = 2.9 Hz, Ar-3,5),
130.37 (d, JPC = 19.4 Hz, Tip-1), 126.66 (s, pheno-2,8
or 4,6), 126.53 (s, pheno-2,8 or 4,6), 122.31 (d, JPC
=
4.6 Hz, Tip-3,5), 122.16 (s, pheno-3,7), 122.15 (d,
JPC = 4.6 Hz, Tip-3,5), 119.62 (s, pheno-4a,5a),
Heteroatom Chemistry DOI 10.1002/hc

512 Sasaki et al.
115.70 (s, pheno-1,9), 34.05 [s, CH(CH3)2-4], 32.11
[s, Tip-CH(CH3)2-2,6], 24.44 [s, Tip-CH(CH3)2-
2,6], 23.92 [s, Tip-CH(CH3)2-4,4], 23.90 [s, Tip-
CH(CH3)2-4,4], 23.19 [s, Ar-CH(CH3)2-2,6], 23.16 [s,
Tip-CH(CH3)2-2,6], 22.51 [s, Tip-CH(CH3)2-2,6]; 31P
NMR (162 MHz, CDCl3, 296 K): δ −50.1 (s); LRMS
(70 eV, EI) m/z (rel intensity) 795 (M+; 100), 438
(M+−Ar; 6), 359 (M+−Tip2P; 3), 198 (Tip; 15).
[d, JPC = 11.4 Hz, CH(CH3)2-2,6], 31.95 [d, JPC
=
12.6 Hz, CH(CH3)2-2,6], 24.43 [s, CH(CH3)2-2,6],
24.36 [s, CH(CH3)2-2,6], 23.88 [s, CH(CH3)2-4], 23.56
(d, JPC = 16.0 Hz, CH3), 23.55 [s, CH(CH3)2-2,6],
23.04 [s, CH(CH3)2-2,6]; 31P NMR (81 MHz, CDCl3,
296 K): δ −46.2 (s); FABMS m/z (rel intensity) 739
(M+; 27), 303 (M+–Tip2P + 1; 34), 84 (M+–655; 100).
Chlorobis(2,6-diisopropyl-4-phenothiazinylphe-
nyl)phosphine (6): To a solution of 5b (1.26 g,
2.88 mmol) in tetrahydrofuran (40 mL), butyl-
lithium (1.57 mol L−1 in n-hexane, 2.20 mL,
3.45 mmol) was added at −78◦C. The solution was
stirred for 60 min and then phosphorus trichloride
(0.15 mL, 1.72 mmol) was added. The resultant
mixture was warmed to room temperature, stirred
for 12 h, and concentrated under reduced pressure.
The residue was purified by column chromatogra-
phy (SiO2,n-hexane/chloroform = 5/1) to give crude
(2,6-Diisopropyl-4-phenothiazinylphenyl)bis(2,4,6-
triisopropylphenyl)phosphine (3c): To a solution
of 5b (500 mg, 1.15 mmol) in tetrahydrofuran
(20 mL), t-butyllithium (1.51 mol L−1 in pentane,
1.70 mL, 2.57 mmol) was added at −78◦C. The
solution was stirred for 60 min and then copper(I)
chloride (250 mg, 2.50 mmol) was added. The re-
sultant mixture was warmed to room temperature,
stirred for 2 h, and cooled to −78◦C. A solution
of
chlorobis(2,4,6-triisopropylphenyl)phosphine
1
(270 mg, 1.14 mmol) in tetrahydrofuran (5 mL)
was added and the resultant mixture was warmed
to room temperature, refluxed for 20 h, and con-
centrated under reduced pressure. The residue
was purified by column chromatography (Al2O3/n-
hexane, n-hexane/chloroform = 5/1) and GPC (Jaigel
1H + 3H/chloroform) to give 3c (280 mg, 0.35 mmol,
31%). 3c: pale yellow solid; mp 81.0–82.0◦C; 1H
6 (290 mg, 0.370 mmol, 26%). 6: colorless solid; H
4
NMR (400 MHz, C6D6, 296 K):δ 7.24 (4H, d, JPH
=
3
4
2.8 Hz, Ar-3,5), 7.10 (4H, dd, JHH = 7.5 Hz, JHH
=
1.5 Hz, pheno-4,6), 6.83 (4H, td, 3 JHH = 7.8 Hz, 4 JHH
3
= 1.6 Hz, pheno-2,8), 6.73 (4H, td, JHH = 7.4 Hz,
3
4 JHH = 1.1 Hz, pheno-3,7), 6.50 (4H, dd, JHH
=
8.2 Hz, 4 JHH = 1.2 Hz, pheno-1,9), 4.15–4.05 [4H, m,
3
CH(CH3)2], 1.11 [12H, d, JHH = 6.7 Hz, CH(CH3)2],
3
NMR (600 MHz, CDCl3, 296 K): δ 7.06 (2H, d,
1.07 [12H, d, JHH = 6.7 Hz, CH(CH3)2]; 31P NMR
3
4 JPH = 2.8 Hz, Ar-3,5), 6.99 (2H, dd, JHH
=
(162 MHz, CDCl3, 296 K): δ 85.0 (s).
4
7.4 Hz, JHH = 1.6 Hz, pheno-4,6), 6.95 (4H, d,
4 JPH = 3.3 Hz, Tip-3,5), 6.83–6.75 (4H, m, pheno-
Attempted Synthesis of Tris(2,6-diisopropyl-4-
phenothiazinylphenyl)phosphine: To a solution of 5b
(210 mg, 0.48 mmol) in tetrahydrofuran (10 mL),
t-butyllithium (1.60 mol L−1 in pentane, 0.70 mL,
1.12 mmol) was added at −78◦C. The solution was
stirred for 60 min and then copper(I) chloride (110
mg, 1.10 mmol) was added. The resultant mixture
was warmed to room temperature, stirred for 12 h,
and cooled to −78◦C. A solution of 6 (290 mg,
0.37 mmol) in tetrahydrofuran (5 mL) was added
and the resultant mixture was warmed to room tem-
perature, refluxed for 12 h, and concentrated under
reduced pressure. The residue was purified by col-
umn chromatography (SiO2, n-hexane/chloroform =
5/1) and GPC (Jaigel 1H + 3H/chloroform) to give 4
(140 mg, 0.09 mmol, 8%). 4: pale yellow solid; mp
61.0–61.5◦C (decomp); 1H NMR (400 MHz, C6D6, 296
K): δ 7.20 (2H, brs, Ar-3,5), 7.15 (4H, brs, Ar-3,5),
7.15–7.08 (2H + 8H, m, Ar-3,5, pheno-4,6), 6.95–
6.86 (8H, m, pheno-2,8), 6.80–6.71 (8H, m, pheno-
4
3
2,3,7,8), 6.13 (2H, dd, JHH= 7.8 Hz, JHH = 1.3 Hz,
pheno-1,9), 3.66–3.58 [2H, m, Ar-CH(CH3)2-2,6],
3.57–3.45 [4H, m, Tip-CH(CH3)2-2,6], 2.85 [2H, sept,
4 JHH = 6.9 Hz, Tip-CH(CH3)2-4], 1.22 [12H, d,
3 JHH = 6.9 Hz, Tip-CH(CH3)2-4], 1.20 [6H, d,
3 JHH = 6.8 Hz, Ar-CH(CH3)2-2,6], 1.18 [6H, d,
3 JHH = 6.8 Hz, Tip-CH(CH3)2-2,6], 1.16 [6H, d,
3 JHH = 6.7 Hz, Tip-CH(CH3)2-2,6], 0.82 [6H, d,
3 JHH = 6.7 Hz, Ar-CH(CH3)2-2,6], 0.74 [6H, d,
3 JHH = 6.6 Hz, Tip-CH(CH3)2-2,6], 0.67 [6H, d,
1
3 JHH = 6.5 Hz, TipCH(CH3)2-2,6]; 13C{ H} NMR
(151 MHz, CDCl3, 296 K): δ 156.49 (d, JPC = 19.0 Hz,
Ar-2,6), 153.24 (d, JPC = 18.3 Hz, Tip-2,6), 152.75
(d, JPC = 18.0 Hz, Tip-2,6), 149.69 (s, Tip-4), 144.42
(s, pheno-9a,10a), 141.04 (s, Ar-4), 136.76 (d, JPC
=
27.2 Hz, Ar-1), 131.01 (d, JPC = 23.1 Hz, Tip-1),
126.79 (s, pheno-2,8), 126.54 (s, pheno-4,6), 126.36
(d, JPC = 3.5 Hz, Ar-3,5), 122.20 (d, JPC = 4.2 Hz,
Tip-3,5), 122.17 (d, JPC = 3.9 Hz, Tip-3,5), 122.11
(s, pheno-3,7), 119.24 (s, pheno-4a,5a), 115.32 (s,
pheno-1,9), 34.10 [s, Tip-CH(CH3)2-4], 32.34 [d,
3
3,7), 6.46 (4H, d, JHH = 8.4 Hz, pheno-1,9), 5.35–
5.18 [2H, m, CH(CH3)2-2,6], 4.20–4.10 [2H + 2H,
3
JPC = 16.7 Hz, Tip-CH(CH3)2-2,6], 32.11 [d, JPC
=
m, CH(CH3)2-2,6], 3.90 [2H, sept, JHH = 6.4 Hz,
17.7 Hz, Tip-CH(CH3)2-2,6], 31.93 [d, JPC = 18.6 Hz,
CH(CH3)2-2,6], 1.29 [6H, d, 3 JHH= 6.0 Hz, CH(CH3)2-
2,6], 1.25 [6H + 6H, d, 3 JHH = 7.2 Hz, CH(CH3)2-2,6],
Ar-CH(CH3)2-2,6], 24.58 [s, Ar-CH(CH3)2-2,6], 24.53
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Redox Properties of Triarylphosphine and Tetraaryldiphosphane 513
3
Tohoku University, for taking 500 and 600 MHz
1.23 [6H, d, JHH = 7.2 Hz, CH(CH3)2-2,6], 0.81
3
NMR spectra, mass spectra, and elemental analysis.
This work was partially carried out in the Advanced
Instrumental Laboratory for Graduate Research of
Department of Chemistry, Graduate School of Sci-
ence, Tohoku University.
[6H, d, JHH = 6.4 Hz, CH(CH3)2-2,6], 0.75 [6H, d,
3 JHH = 6.8 Hz, CH(CH3)2-2,6], 0.48 [6H, d, JHH
=
3
3
6.8 Hz, CH(CH3)2-2,6], 0.47 [6H, d, JHH = 7.2 Hz,
CH(CH3)2-2,6]; 13C{ H} NMR (101 MHz, C6D6, 296
1
ꢁ
K): δ 158.93 (pseudo t, JPC + JP C = 39.6 Hz, Ar-
ꢁ
2,6), 158.39 (pseudo t, JPC + JP C = 8.9 Hz, Ar-
ꢁ
2,6), 158.08 (pseudo t, JPC + JP C = 35.2 Hz, Ar-2,6),
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ACKNOWLEDGMENT
We thank the Research and Analytical Center for
Giant Molecules, Graduate School of Science,
Heteroatom Chemistry DOI 10.1002/hc