Synthesis of 2-alkenyl- and 2-alkynyl-benzo[ b ]phospholes by using palladium-catalyzed cross-coupling reactions

Article abstract of DOI:10.1021/ol401994e

Heck, Stille, and Sonogashira reactions of 2-bromobenzo[b]phosphole P-oxide afforded a series of 2-alkenyl- and 2-alkynyl-benzo[b]phosphole P-oxides. The charge-transfer character of the new benzo[b]phosphole π-systems in the excited state is enhanced by the terminal electron-donating substituents. Furthermore, the C-Sn cross-coupling of the bromide was applied to the facile synthesis of a new Stille-coupling precursor, 2-stannylbenzo[b]phosphole.

Full text of DOI:10.1021/ol401994e

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Synthesis of 2‑Alkenyl- and
2‑Alkynyl-benzo[b]phospholes
by Using Palladium-Catalyzed
Cross-Coupling Reactions
Yoshihiro Matano,*,† Yukiko Hayashi,‡ Kayo Suda,§ Yoshifumi Kimura, and
Hiroshi Imahori*,‡,#
Department of Chemistry, Faculty of Science, Niigata University, Nishi-ku, Niigata
950-2181, Japan, Department of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan, Department of
Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502,
Japan, Department of Molecular Chemistry and Biochemistry, Faculty of Science and
Engineering, Doshisha University, Kyotanabe 610-0394, Japan, and Institute for
Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku,
Kyoto 615-8510, Japan
matano@chem.sc.niigata-u.ac.jp
Received July 16, 2013
ABSTRACT
Heck, Stille, and Sonogashira reactions of 2-bromobenzo[b]phosphole P-oxide afforded a series of 2-alkenyl- and 2-alkynyl-benzo[b]phosphole
P-oxides. The charge-transfer character of the new benzo[b]phosphole π-systems in the excited state is enhanced by the terminal electron-
donating substituents. Furthermore, the CꢀSn cross-coupling of the bromide was applied to the facile synthesis of a new Stille-coupling
precursor, 2-stannylbenzo[b]phosphole.
Recently, much attention has been paid to phosphole-
containing π-systems, which are promising candidates for
a new class of phosphorus-based optical and electronic
materials.1 In particular, benzo[b]phospholes2 and related
compounds3ꢀ7 have been continuously investigated be-
cause of their versatility in the introduction of various
functional groups onto the R and β carbons of the phos-
phole unit to extend π-skeletons. Some π-extended and
π-fused benzo[b]phosphole derivatives are known to
(2) (a) Rausch, M. D.; Klemann, L. P. J. Am. Chem. Soc. 1967, 89,
€
5732. (b) Markl, G.; Jin, G. Y.; Berr, K.-P. Tetrahedron Lett. 1993, 34,
3103. (c) Trishin, Y. G.; Namestnikov, V. I.; Bel’skii, V. K. Russ. J. Gen.
Chem. 2004, 74, 189. (d) Tsuji, H.; Sato, K.; Ilies, L.; Itoh, Y.; Sato, Y.;
Nakamura, E. Org. Lett. 2008, 10, 2263. (e) Sanji, T.; Shiraishi, K.;
Kashiwabara, T.; Tanaka, M. Org. Lett. 2008, 10, 2689. (f) Fukazawa,
A.; Ichihashi, Y.; Kosaka, Y.; Yamaguchi, S. Chem.;Asian J. 2009, 4,
1729.
† Niigata University
‡ Graduate School of Engineering, Kyoto University
§ Graduate School of Science, Kyoto University
Doshisha University
(3) (a) Baumgartner, T.; Neumann, T.; Wirges, B. Angew. Chem., Int.
ꢀ
# Institute for Integrated Cell-Material Sciences, Kyoto University
Ed. 2004, 43, 6197. (b) Baumgartner, T.; Bergmans, W.; Karpati, T.;
ꢀ
ꢀ
Neumann, T.; Nieger, M.; Nyulaszi, L. Chem;Eur. J. 2005, 11, 4687. (c)
ꢀ
(1) For selected reviews, see: (a) Baumgartner, T.; Reau, R. Chem.
Durben, S.; Dienes, Y.; Baumgartner, T. Org. Lett. 2006, 8, 5893. (d)
Ren, Y.; Baumgartner, T. J. Am. Chem. Soc. 2011, 133, 1328. (e) Ren, Y.;
Baumgartner, T. Inorg. Chem. 2012, 51, 2669 and references therein.
(4) (a) Chen, R.-F.; Zhu, R.; Fan, Q.-L.; Huang, W. Org. Lett. 2008,
10, 2913. (b) Zhang, S.; Chen, R.; Yin, J.; Liu, F.; Jiang, H.; Shi, N.; An,
Z.; Ma, C.; Liu, B.; Huang, W. Org. Lett. 2010, 12, 3438.
ꢀ
Rev. 2006, 106, 4681. Correction: 2007, 107, 303. (b) Reau, R.; Dyer,
P. W. In Comprehensive Heterocyclic Chemistry III; Ramsden, C. A.,
Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsvier: Oxford, 2008; Chapter 3.15,
pp 1029ꢀ1048. (c) Matano, Y.; Imahori, H. Org. Biomol. Chem. 2009, 7,
1258. (d) Fukazawa, A.; Yamaguchi, S. Chem.;Asian J. 2009, 4, 1386.
(e) Ren, Y.; Baumgartner, T. Dalton Trans. 2012, 41, 7792.
r
10.1021/ol401994e
XXXX American Chemical Society

exhibit extremely high emitting abilities and/or high elec-
tron drift mobilities in the amorphous state.
2-alkynylbenzo[b]phosphole P-oxides 3aꢀd in 64ꢀ81%
yields (Scheme 2). The triisopropylsilyl group of 3d was
easily removed by treatment with Bu4NF, generating the
terminal free 2-ethynylbenzo[b]phosphole P-oxide 3e in
quantitative NMR yield.
Most of the benzo[b]phosphole derivatives have been
preparedby means of cycloaddition reactions of2-alkynyl-
phenyl(aryl)phosphines.3,8,9 In this type of transformation,
π-conjugative substituents should be incorporated into
starting materials; it is necessary to prepare a 2-alky-
nylphenyl(aryl)phosphine for each benzo[b]phosphole.
In 2012, we reported a new divergent method for the
R-functionalization of a benzo[b]phosphole skeleton,10
which was based on Stille coupling reactions of 2-bromo-
benzo[b]phosphole P-oxide11 (1, in Scheme 1) with
2-stannylbenzo[b]heteroles. It was found that the benzo-
[b]phospholeꢀindole hybrid was highly fluorescent with
large charge-transfer (CT) character in the excited state.10
To our knowledge, however, the kind of π-conjugative
substituents that have been introduced to the R position
of the benzo[b]phosphole skeleton is still limited. There-
fore, we have applied cross-coupling strategies to the
syntheses of new kinds of π-extended benzo[b]phosphole
derivatives. Herein, we report a series of 2-alkenyl- and
2-alkynyl-benzo[b]phospholes, which are available from
1 by using Stille, Heck, or Sonogashira reactions. The
electronic effects of the vinylene/acetylene spacers and
the para-substituents on the optical properties of the
newly constructed π-systems are discussed comparatively.
Furthermore, the CꢀSn cross-coupling of 1 was applied
to the facile synthesis of a new Stille-coupling precursor,
2-stannylbenzo[b]phosphole.
Scheme 1. Synthesis of 2-Alkenylbenzo[b]phosphole P-Oxides
Scheme 1 summarizes the synthesis of 2-alkenylbenzo-
[b]phosphole P-oxides 2. Treatment of the bromide 1 with
four kinds of vinylarenes under a standard Heck condition
[Pd(OAc)2, Bu4NCl, NaOAc, DMF at 120 °C] gave the
Scheme 2. Synthesis of 2-Alkynylbenzo[b]phosphole P-Oxides
corresponding
2-((E)-2-arylethenyl)benzo[b]phosphole
P-oxides 2aꢀd as the major products. The introduction
of vinyl groups was also achieved by Stille coupling reac-
tions of 1 with alkenyl(tributyl)stannanes, affording 2b
and 2e in 41ꢀ59% yields. The bromide 1 also reacted with
three kinds of arylacetylenes and triisopropylsilylacetylene
undera standard Sonogashiracondition [Pd, CuI, iPr2NH,
benzene at room temperature] to give the corresponding
(5) (a) Fukazawa, A.; Hara, M.; Okamoto, T.; Son, E.-C.; Xu, C.;
Tamao, K.; Yamaguchi, S. Org. Lett. 2008, 10, 913. (b) Fukazawa, A.;
Yamada, H.; Yamaguchi, S. Angew. Chem., Int. Ed. 2008, 47, 5582. (c)
Bruch, A.; Fukazawa, A.; Yamaguchi, E.; Yamaguchi, S.; Studer, A.
Angew. Chem., Int. Ed. 2011, 50, 12094. (d) Fukazawa, A.; Yamaguchi,
E.; Ito, E.; Yamada, H.; Wang, J.; Irle, S.; Yamaguchi, S. Organome-
tallics 2011, 30, 3870.
(6) (a) Nakano, K.; Oyama, H.; Nishimura, Y.; Nakasako, S.;
Nozaki, K. Angew. Chem., Int. Ed. 2012, 51, 695. (b) Yavari, K.;
Moussa, S.; Hassine, B. B.; Retailleau, P.; Voituriez, A.; Marinetti, A.
Angew. Chem., Int. Ed. 2012, 51, 6748.
(7) Weymiens, W.; Zaal, M.; Slootweg, J. C.; Ehlers, A. W.;
Lammertsma, K. Inorg. Chem. 2011, 50, 8516.
(8) Aitken, R. A. In Science of Synthesis; Maas, G., Ed.; Georg Thieme
Verlag: Stuttgart, 2001; Vol. 10, Chapter 10.17ꢀ19, pp 789ꢀ838.
(9) (a) Winter, W. Tetrahedron Lett. 1975, 16, 3913. (b) Winter, W.
Chem. Ber. 1977, 110, 2168. (c) Butters, T.; Winter, W. Chem. Ber. 1984,
117, 990.
The 2-substituted benzo[b]phosphole P-oxides 2 and 3
were characterized by using conventional spectroscopic
techniques (NMR, HRMS, IR). The 31P NMR peaks of 2
and 3 appeared at δP = 31.5ꢀ33.3 and 34.5ꢀ35.5 ppm,
respectively. The structures of 2bꢀd and 3b,c were unam-
biguously elucidated by X-ray crystallography (Figures 1
and S1 in the Supporting Information).12 The styryl
derivatives 2b,c possess highly planar frameworks with
torsion angles at the inter-ring linkage (C1ꢀC9ꢀC10ꢀ
C11) of ca. 179°, whereas the 2-(1-naphthyl)ethenyl deri-
vative 2d and the arylethynyl derivatives 3b,c display
twisted π-networks probably due to the crystal packing
effects. Inthecrystalline statesof 2bꢀd, the vinylene-linked
(10) Hayashi, Y.; Matano, Y.; Suda, K.; Kimura, Y.; Nakao, Y.;
Imahori, H. Chem.;Eur. J. 2012, 18, 15972.
(11) Nief, F.; Charrier, C.; Mathey, F.; Simalty, M. Phosphorus,
Sulfur Silicon Relat. Elem. 1982, 13, 259.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Optical Data and Redox Potentials of 2 and 3 in CH2Cl2
compd
λab/nm (log ε)
λ
em/nma (ΦF)b
Eox/Vc
Ered/Vc
2a
2b
2c
2d
2e
3a
3b
3c
3d
383 (4.31)
367 (4.30)
369 (4.40)
376 (4.63)
339 (3.79)
374 (4.39)
362 (4.25)
361 (4.30)
347 (3.97)
469 (0.73)
446 (0.66)
447 (0.60)
468 (0.70)
408 (0.81)
455 (0.65)
434 (0.77)
437 (0.71)
418 (0.69)
þ0.77
þ1.04
þ1.12
þ0.97
n.d.
ꢀ2.12
ꢀ2.07
ꢀ2.03
ꢀ2.05
ꢀ2.23
ꢀ2.02
ꢀ1.96
ꢀ1.95
ꢀ2.06
þ1.08
n.d.
n.d.
n.d.
a Excited at λab b Absolute fluorescence quantum yields. c First
.
oxidation (Eox) and reduction (Ered) potentials (vs Fc/Fcþ) determined
by DPV (0.1 M Bu4NþPF6ꢀ; Ag/Agþ). n.d. = Not determined.
Figure 1. Crystal structures of (a) 2b and (b) 3b: Top views
˚
(upper) and side views (lower). Selected bond lengths (A), bond
angles (deg), and torsion angles (deg): 2b: PꢀO, 1.4859(10);
C1ꢀC2, 1.3510(19); C1ꢀC9, 1.4414(19); C9ꢀC10, 1.3421(19);
C10ꢀC11, 1.466(2); C1ꢀC9ꢀC10, 125.43(12); C9ꢀC10ꢀC11,
125.79(13); C2ꢀC1ꢀC9ꢀC10, 177.03(13); C1ꢀC9ꢀC10ꢀC11,
178.81(12). 3b: PꢀO, 1.4827(8); C1ꢀC2, 1.3499(16); C1ꢀC9,
1.4196(16); C9ꢀC10, 1.2013(17); C10ꢀC11, 1.4375(17);
C1ꢀC9ꢀC10, 175.93(12); C9ꢀC10ꢀC11, 178.07(13).
clearly observed for 2aꢀc and 3aꢀc, in which Stokes shifts
increase with the increase of solvent polarity (Figures S2
and S3, Supporting Information). It should be noted that
the changes in Stokes shifts of the para-methoxy deriva-
tives (2a and 3a) are larger than those of the para-chloro
derivatives (2cand 3c). Apparently, the CT character in the
S1 states of 2-alkenyl- and 2-alkynyl-benzo[b]phosphole
P-oxides are appreciably enhanced when combined with
the electron-donating para-methoxyphenyl groups; the
benzo[b]phosphole unit behaves as the electron-accepting
function. The λab and λem values as well as the degree of
solvatochromism (Δλem = 1250 cmꢀ1) observed for 2a are
comparable to those for 3a (Δλem = 1250 cmꢀ1), suggest-
ing that the linkage effects of the vinylene and acetylene
spacers on the π-conjugation are close to each other. The
electrochemical redox processes of 2 and 3 were measured
by cyclic voltammetry (CV) and differential pulse voltam-
metry (DPV). In the CV measurements, irreversible CV
waves were observed for both oxidation and reduction
processes (Figure S4, Supporting Information). As shown
in Table 1, the para-substituents affect the oxidation
potetials (Eox) more efficiently than the reduction poten-
tials (Ered). This is consistent with theoretical prediction
(B3LYP/6-31G*); the orbital coefficient at the para carbon
in HOMO is more distinct than that in LUMO (Figure S7,
Supporting Information). The differences in the redox
potentials (Eox ꢀ Ered) increase in the order, 2a (2.89 V)
< 2b (3.11 V) < 2c (3.15 V) for the vinylene-linked series,
which agrees well with the order of their HOMOꢀLUMO
gaps obtained by the DFT calculations.
To attain some insight into excited-state dynamics of the
newly constructed π-systems, we measured fluorescence
lifetimes (τf) of 2aꢀe and 3aꢀc in CH2Cl2 at room temp-
erature. The τf values of the styryl derivatives 2aꢀd
(2.7ꢀ3.7 ns) are comparable to those of the arylethynyl
derivatives 3aꢀc (3.0ꢀ3.9 ns). The radiative and non-
radiative decay rate constants (kr and knr) calculated
from τf and Φf values are summarized in Table S2 in the
Supporting Information. Among 2aꢀc and 3aꢀc, the
effects of para substituents and π-spacers on the kr values
(1.9ꢀ2.4 ꢁ 108 sꢀ1) are negligible compared to those on the
knr values(0.6ꢀ1.5ꢁ 108 sꢀ1), although the absolutevalues
π-networks are partially stacked with the πꢀπ distances of
˚
3.4ꢀ3.5 A.
To understand the effects of the para-substituents and
the π-spacers (vinylene, acetylene) on the optical and
electrochemical properties of benzo[b]phosphole P-oxides,
we measured absorption/emission spectra and redox po-
tentialsof2 and 3 in CH2Cl2. Theresultsaresummarizedin
Figure 2 and Table 1. All the compounds examined are
moderately to highly fluorescent in solution. In each series
of the para-substituted derivatives (2aꢀc and 3aꢀc), the
absorption maxima (λab) and emission maxima (λem) shift
to longer wavelength with increasing the electron donating
ability of the para substituents on the terminal benzene
ring. In each series, the red shifts of the emission maxima
are larger than those of the absorption maxima, suggesting
that the excited singlet (S1) states are more sensitive than
the ground states to the para-substituent effects. In addi-
tion, the solvatochromism of the fluorescence spectra was
(12) 2b: C22H17OP, MW = 328.33, 0.30 ꢁ 0.25 ꢁ 0.10 mm, mono-
˚
˚
˚
clinic, P21/c, a = 9.308(2) A, b = 11.131(2) A, c = 17.312(4) A, β =
3
103.927(3)°, V = 1741.0(6) A , Z = 4, Fcalcd = 1.253 g cmꢀ3, μ = 1.62
˚
cmꢀ1, collected 13447, independent 3943, parameters 217, Rw = 0.0914,
R = 0.0395 (I > 2σ(I)), GOF = 1.053. 2c: C22H16ClOP, MW = 362.77,
˚
0.40 ꢁ 0.15 ꢁ 0.10 mm, monoclinic, P21/n, a = 6.3504(2) A, b =
3
˚
˚
˚
7.9511(3) A, c = 35.8818(12) A, β = 94.457(2)°, V = 1806.29(11) A ,
Z = 4, Fcalcd = 1.334 g cmꢀ3, μ = 3.06 cmꢀ1, collected 14141,
independent 4142, parameters 226, Rw = 0.0956, R = 0.0381 (I >
2σ(I)), GOF = 1.058. 2d: C26H19OP, MW = 378.38, 0.35 ꢁ 0.20 ꢁ 0.15
˚
˚
mm, monoclinic, P21/c, a = 8.3951(13) A, b = 8.9874(13) A, c =
3
˚
˚
25.691(4) A, β = 94.065(3)°, V = 1933.5(5) A , Z = 4, Fcalcd = 1.300 g
cmꢀ3, μ = 1.56 cmꢀ1, collected 22777, independent 4418, parameters
253, Rw = 0.0917, R = 0.0368 (I > 2σ(I)), GOF = 1.090. 3b: C22H15OP,
MW = 326.31, 0.30 ꢁ 0.25 ꢁ 0.15 mm, monoclinic, P21/c, a = 7.9918(8)
˚
˚
˚
A, b = 18.1517(17) A, c = 11.7749(12) A, β = 96.3610(10)°, V =
1697.6(3) A , Z = 4, Fcalcd = 1.277 g cmꢀ3, μ = 1.66 cmꢀ1, collected
3
˚
13548, independent 3851, parameters 217, Rw = 0.0929, R = 0.0340
(I > 2σ(I)), GOF = 1.039. 3c: C22H14ClOP, MW = 360.75, 0.30 ꢁ
˚
˚
0.20 ꢁ 0.10 mm, monoclinic, P21/c, a = 9.961(2) A, b = 8.7326(17) A,
3
˚
˚
c = 21.055(4) A, β = 102.507(3)°, V = 1788.0(6) A , Z = 4, Fcalcd
=
1.340 g cmꢀ3, μ = 3.09 cmꢀ1, collected 13927, independent 4054,
parameters 226, Rw = 0.0912, R = 0.0344 (I > 2σ(I)), GOF = 1.044.
Org. Lett., Vol. XX, No. XX, XXXX
C

Scheme 3. Synthesis of 2-(Tributylstannyl)benzo[b]phosphole 4
and Its Conversion to 3b and 5
bondformation startingfrom 2-stannylbenzo[b]phosphole
derivative.
In summary, we have established convenient and diver-
gent methods for the synthesis of 2-alkenyl-, 2-alkynyl-,
and 2-stannyl-benzo[b]phosphole P-oxides by using four
kinds of cross-coupling reactions of 2-bromobenzo[b]
phosphole P-oxide. The CT character of the vinylene-
and acetylene-bridged benzo[b]phosphole π-systems in
the excited state has proven to vary widely depending on
the terminal para-substituents. With the present results in
hand, a variety of π-conjugated benzo[b]phosphole deri-
vatives are conceivableasfunctional dyes for use in organic
electronic devices, and studies along this line are now in
progress.
Figure 2. UVꢀvis absorption (dashed line) and emission spectra
(dotted line, excited at the absorption maxima) of 2aꢀd (a) and
3aꢀc (b) in CH2Cl2.
of their differences are small. It is worth noting that the
attachment of the phenyl group onto the vinyl function
(2a vs 2e) appreciably enhances the knr value (9.7 ꢁ 107 vs
2.0 ꢁ 107 sꢀ1). More efficient internal conversion and/or
intersystem crossing with the increased vibrational mode
as well as photochemical reactions may account for the
acceleration of nonradiative decay from the S1 state of
2aꢀd.13
Furthermore, we conducted a Pd-catalyzed CꢀSn cross-
coupling reaction of 1 with hexabutyldistannane to obtain
2-stannylbenzo[b]phosphole P-oxide 4 as a new common
precursor for Stille coupling reactions (Scheme 3). The
desired stannylation occurredslowly under the Pd catalysis
at reflux in toluene to give 4 in 59% yield. Subsequently,
the 2-stannyl derivative 4 underwent Stille coupling with
β-bromostyrene and 2-bromobenzothiazole to yield 3b
and benzo[b]phospholeꢀbenzothiazole hybrid 5,10 res-
pectively. These are the first examples of the CꢀC
Acknowledgment. This work was supported by a Grant-
in-Aid (No. 25288020) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan
and Ogasawara foundation. Y.M. and Y.H. deeply thank
Prof. Hiroko Yamada (NAIST) for her kind support and
valuable suggestions on this research project.
Supporting Information Available. Experimentaldetails,
crystal structures, spectral data, CIF files, and DFT cal-
culation results. This material is available free of charge via
the Internet at http://pubs.acs.org.
(13) When irradiated in toluene with a high-pressure Hg arc lamp
(>360 nm) for 1 h in an argon atmosphere, 2b decomposed to some
extent (judged by 1H and 31P NMR spectra).
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX