Synthesis, structure, and properties of tris(2,6-diisopropyl-4- methoxyphenyl)phosphine

Article abstract of DOI:10.1080/10426500701735403

Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine (1) was synthesized to investigate substituent effect of methoxy groups on the properties of crowded triarylphosphines. Methoxy groups slightly lower the first oxidation potential of 1 and contribute blue-shi

Full text of DOI:10.1080/10426500701735403

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Phosphorus, Sulfur, and Silicon
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Synthesis, Structure,
and Properties of
Tris(2,6-diisopropyl-4-
methoxyphenyl)phosphine
Shigeru Sasaki a , Kohji Sasaki a & Masaaki Yoshifuji a
a Department of Chemistry, Graduate School
of Science , Tohoku University , Aoba, Sendai,
980-8578, Japan
Published online: 20 Jun 2008.
To cite this article: Shigeru Sasaki , Kohji Sasaki & Masaaki Yoshifuji (2008) Synthesis,
Structure, and Properties of Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine,
Phosphorus, Sulfur, and Silicon and the Related Elements, 183:2-3, 410-414, DOI:
10.1080/10426500701735403
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Phosphorus, Sulfur, and Silicon, 183:410–414, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701735403
Synthesis, Structure, and Properties of
Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine
Shigeru Sasaki, Kohji Sasaki, and Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku
University, Aoba, Sendai 980-8578, Japan
Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine (1) was synthesized to investigate
substituent effect of methoxy groups on the properties of crowded triarylphos-
phines. Methoxy groups slightly lower the first oxidation potential of 1 and con-
tribute blue-shift of UV-Vis absorption as compared with the corresponding 2,4,6-
triisopropylphenyl derivative.
Keywords Oxidation potential; Triarylphosphine; UV-Vis spectrum
INTRODUCTION
Unusual structure and physical properties of crowded triarylphos-
phines have attracted considerable attention for a long time.1 Typi-
cal crowded triarylphosphines such as trimesitylphosphine (2)2 and
tris(2,4,6-triisopropylphenyl)phosphine (3)3 are reversibly oxidized to
stable cation radicals, and they can be employed as redox centers of mul-
tistep redox systems.4 However, modification of properties of such redox
systems by π-conjugated substituents, which are general in the second-
row element systems such as triarylamines, has been still limited and
little is known for the effect of remote π-conjugated substituents on the
properties of such compounds. Herein, we report synthesis and proper-
ties of tris(2,6-diisopropyl-4-methoxyphenyl)phosphine (1). Substitent
effect of electron donating methoxy group on redox properties and elec-
tronic spectrum is discussed.
We thank Grants-in-Aid for Scientific Research from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan (Nos. 17310063, 15550025 and 13304049) for
financial support, Research and Analytical Center for Giant Molecules, Graduate School
of Science, Tohoku University, for taking mass spectra and elemental analysis.
Address correspondence to Shigeru Sasaki, Department of Chemistry, Graduate
School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan. E-mail: sasaki@
mail.tains.tohoku.ac.jp
410

Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine
411
RESULTS AND DISCUSSION
Triarylphosphine 1 was synthesized via 4 and 5 in a similar manner to
related compounds (Scheme 1).3,4 X-ray crystallography of 1 revealed
◦
˚
that average C–P–C bond angle (111.5 ) and P–C bond length (1.850 A)
◦
˚
are similar to those of 3 (111.5 , 1.845 A) (Figure 1).
SCHEME 1
Cyclic voltammogram of 1 exhibits reversible redox wave corre-
sponding to oxidation to the cation radical and the oxidation poten-
tial (E1/2 = 0.08 V vs Ag/Ag+) is lower than that of 3 (0.13 V) and
tris(2,6-diisopropylphenyl)phosphine (6) (0.27 V),5 probably because of
strong electron donating effect of methoxy group. UV-Vis spectrum of 1
in dichloromethane shows λmax at 321 (16300) nm, which is blue-shifted
as compared with 3 (327 (13500) nm), and 6 (328 (11000) nm) (Figure 2).
Apparent blue shift of 1 seems to be attributed to higher LUMO of 1
considering the oxidation potentials.
Replacement of isopropyl group at
4
position of 2,4,6-
triisopropylphenyl group with various substituents is expected to
modulate electronic properties of crowded triarylphosphines without
changing steric environment around the phosphorus. Because both
isopropyl and methoxy groups are good electron donating substituents,
effect of the replacement is limited for 1 and 3. However, simple
˚
FIGURE 1 Molecular structure of 1. Selected bond lengths (A) and angles
(◦): P1–C1 1.848(1), P1–C14 1.850(1), P1–C27 1.852(1), C1–P1–C14 107.56(6),
C1–P1–C27 116.71(6), C14–P1–C27 110.31(6).

412
S. Sasaki et al.
FIGURE 2 UV-Vis spectrum of 1 and related compounds in dichloromethane.
molecular design and synthetic strategy shown in this study is promis-
ing way to tune, even if limited, properties of crowded triarylphosphines
and related compounds.
EXPERIMENTAL
Synthesis
2-Bromo-1,3-diisopropyl-methoxybromobenzene (4)
A mixture of 2-bromo-5-iodo-1,3-diisopropylbenzene (6.02 g, 16.4
mmol), CuI (3.05 g, 16.0 mmol), sodium methoxide (prepared from
methanol (13.5 mL) and sodium (0.38 g, 16.4 mmol)) in pyridine (60 mL)
was refluxed for 12 h. The resultant mixture was poured onto ice-
water, extracted with ether, washed with 4 M HCl solution, saturated
NaHCO3 solution, and saturated NaCl solution, and dried over anhy-
drous MgSO4. Drying agent was removed by filtration and the filtrate
was concentrated under reduced pressure and purified by column chro-
matography (SiO2, hexane) to give 4 (3.54 g, 13.1 mmol, 80%) as col-
1
orless oil. H NMR (400 MHz, CDCl3, 296 K) δ 6.68 (2H, s, arom.),
3.80 (3H, s, OCH3), 3.47 (2H, sept, J = 6.66 Hz, CH), 1.20 (12H, d, J =
6.66 Hz, CH3).
Chlorobis(2,6-diisopropyl-4-methoxyphenyl)phosphine (5)
Butyllithium (5.70 mmol, 1.58 molL−1 in hexane) was added to
a solution of 4 (1.53 g, 5.65 mmol) in tetrahydrofuran (20 mL) at

Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine
413
−78◦C, and the solution was stirred for 30 min. Phosphorus trichlo-
ride (0.23 mL, 2.64 mmol) was added and the mixture was allowed to
warm to room temperature and stirred for 12 h. The mixture was con-
centrated under reduced pressure and purified by column chromatog-
raphy (SiO2, hexane) to give 5 (710 mg, 3.16 mmol, 56%) as colorless
crystals.
M.p. 112.0–114.0◦C. 1H NMR (400 MHz, CDCl3, 296 K) δ 6.65 (4H, d,
J = 2.70 Hz, arom.), 3.79 (6H, s, OCH3), 3.76 (4H, m, CH), 1.03 (12H, d,
J = 6.68 Hz, CH3), 1.02 (12H, d, J = 6.63 Hz, CH3). 13C NMR (101 MHz,
CDCl3, 296 K) δ 160.86 (s, p-arom.), 153.87 (d, J = 16.9 Hz, o-arom.),
PC
128.06 (d, J = 50.6 Hz, ipso-arom.), 110.06 (d, J = 1.90 Hz, m-arom),
PC
PC
54.92 (s, OCH3), 31.22 (d, J = 18.9 Hz, CH), 24.28 (s, CH3), 23.95 (s,
PC
CH3). 31P NMR (162 MHz, CDCl3,296 K) δ 89.9 (s). IR (KBr) 3005, 2970,
2960, 2935, 2929, 2866, 2837, 1431, 1383, 1363, 1325, 1269, 1248, 1236,
1196, 1176, 1165, 1105, 1047, 1028, 937, 870, 849, 746, 625, 517, 484
and 463 cm−1. Anal. calcd. for C26H38ClO2P: C, 69.55; H, 8.53; Found:
C, 69.84; H, 8.33.
Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine (1)
–1
Butyllithium (1.80 mmol, 1.58 molL in hexane) was added to a so-
lution of 4 (0.50 g, 1.84 mmol) in tetrahydrofuran (10 mL) at −78◦C,
and the solution was stirred for 30 min. CuCl (183 mg, 1.85 mmol)
was added and the mixture was warmed to 20◦C and stirred for 3.5 h.
A solution of 5 (710 mg, 1.58 mmol) in tetrahydrofuran (3 mL) was
◦
added at −78 C, and the mixture was warmed to room temperature
and refluxed for 60 h. The mixture was concentrated under reduced
pressure and purified by column chromatography (Al2O3, hexane, hex-
ane:chloroform = 4:1) to give 1 (490 mg, 0.81 mmol, 44%) as colorless
crystals.
1
M.p. 171.0–172.5◦C. H NMR (400 MHz, CDCl3, 296 K) δ 6.61 (6H,
d, J = 2.71 Hz, arom.), 3.79 (9H, s, OCH3), 3.50 (6H, m, CH), 1.14
(18H, d, J = 6.68 Hz, CH3), 0.70 (18H, d, J = 6.63 Hz, CH3). 13C
NMR (101 MHz, CDCl3, 296 K) δ 160.18 (s, p-arom.), 155.07 (d, J
=
PC
19.2 Hz, o-arom.), 126.56 (d, J = 22.7 Hz, ipso-arom.), 109.61 (d,
PC
J
= 4.69 Hz, m−arom), 54.83 (s, OCH3), 32.03 (d, J = 18.0 Hz,
PC
PC
CH), 24.41 (s, CH3), 23.21 (s, CH3). 31P NMR (162 MHz, CDCl3,296
K) δ –53.5 (s). UV-Vis (CH2Cl2, c = 3.29 × 10−5 molL−1) λmax (ε)/nm
321 (16300). IR (KBr) 2964, 2933, 2866, 2837, 1566, 1462, 1429, 1383,
1360, 1321, 1268, 1234, 1196, 1174, 1128, 1105, 1051, 1028, 937, 867,
849, 752, 652, 627, 580, 521, 492, 453 and 437 cm–1. FT-ICR-MS (ESI)
Calcd. for [C39H57O3P+H]+: 605.4118, Found : 605.4118. Anal. calcd.
for C39H57ClO3P: C, 77.44; H, 9.50; Found: C, 77.04; H, 9.53.

414
S. Sasaki et al.
Cyclic Voltammetry
Cyclic voltammetry was performed on a BAS CV-50W controller
with a glassy carbon, Pt wire, and Ag/0.01 M AgNO3/0.1 M n-
Bu4NClO4/CH3CN as a working, counter, and reference electrode, re-
spectively (Ferrocene/Ferricinium = 0.18 V). A substrate ca. 10−4 M was
dissolved in dichloromethane with 0.1 M n-Bu4NClO4 as a supporting
electrolyte and the solution was degassed by bubbling with nitrogen
gas.
Crystallographic Analysis of 1
Crystal data of 1: C39H57O3P, M = 604.85, colorless chunk grown from
mixed solvent of hexane and ethanol (1:3), crystal dimensions 0.50 ×
0.25 × 0.25 mm3, monoclinic, space group P21/n (no. 14), a = 11.9747(3),
◦
3
˚
˚
b = 22.2050(4), c = 13.6296(2) A, β = 93.6097(6) , V = 3616.9(1) A ,
Dc = 1.111 gcm−3, µ = 0.110 mm−1, T = 140 K, Z = 4, F(000) = 1320.00.
Of 28322 reflections measured (2θmax = 51.0◦), 6762 were observed [I >
0.0σ(I)] (R = 0.032). Variable parameters 389. Goodness of fit S =
int
1.23 for observed reflections, and R = 0.040 for I > 2.0σ(I), R = 0.046,
1
Rw = 0.067 for all reflections. The maximum and minimum peaks on
−3,
˚
the final difference Fourier map corresponded to 0.37 and −0.26 eA
respectively. CCDC 643412.
REFERENCES AND NOTES
[1] S. Sasaki and M. Yoshifuji, Curr. Org, Chem., 11, 17 (2007), and references therein.
[2] (a) J. F. Blount, C. A. Maryanoff, and K. Mislow, Tetrahedron Lett., 48, 913, (1975);
(b) K. Mislow, Acc. Chem. Res., 9, 26 (1976); (c) A. J. Bellamy, R. O. Gould, and M. D.
Walkinshaw, J. Chem. Soc., Perkin Trans., 2, 1981, 1099; (d) M. Culcasi, Y. Berchad-
sky, G. Gronchi, and P. Tordo, J. Org. Chem., 56, 3537 (1991); (e) C. Palau, Y. Berchad-
sky, F. Chalier, J.-P. Finet, G. Gronchi, and P. Tordo, J. Phys. Chem., 99, 158 (1995); (f)
A. V. Il’yasov, Yu. M. Kargin, and A. A.Vafina, Russ. J. Gen. Chem., 63, 1833 (1994).
[3] S. Sasaki, K. Sutoh, F. Murakami, and M. Yoshifuji, J. Am. Chem. Soc., 124, 14830
(2002).
[4] (a) S. Sasaki, F. Murakami, M. Murakami, M. Watanabe, K. Kato, K. Sutoh, and M.
Yoshifuji, J. Organomet. Chem., 690, 2664 (2005); (b) S. Sasaki, F. Murakami, and M.
Yoshifuji, Organometallics, 25, 140 (2006); (c) K. Sutoh, S. Sasaki, and M. Yoshifuji,
Inorg. Chem., 45, 992 (2006).
[5] R. T. Boere´, Y. Zhang, J. Organomet. Chem., 690, 2651 (2005).