Phase transfer of fluoride ion by phosphonioborins

Article abstract of DOI:10.1246/cl.2007.976

Two cationic triarylboranes based on phosphonioborin frameworks have been investigated. Electron-withdrawing phosphonio groups enhanced the Lewis acidity of these boranes which capture fluoride ion in aqueous media and transport it into organic phase. Cop

Full text of DOI:10.1246/cl.2007.976

976
Chemistry Letters Vol.36, No.8 (2007)
Phase Transfer of Fluoride Ion by Phosphonioborins
Tomohiro Agou,1 Junji Kobayashi,1 Youngmin Kim,2 Francꢀois P. Gabbaı,ꢀ2 and Takayuki Kawashimaꢀ1
¨
1Department of Chemistry, Graduate School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
2Department of Chemistry, Texas A&M University, College Station, Texas, U.S.A.
(Received May 11, 2007; CL-070510; E-mail: takayuki@chem.s.u-tokyo.ac.jp)
Two cationic triarylboranes based on phosphonioborin
frameworks have been investigated. Electron-withdrawing phos-
phonio groups enhanced the Lewis acidity of these boranes
which capture fluoride ion in aqueous media and transport it into
organic phase.
It is well known that tricoordinate boranes can form stable
complexes with hard Lewis bases, especially fluoride ion. Upon
formation of borane–fluoride complexes (fluoroborates), the
structure, electronic and optical properties of the boranes change
dramatically. For these reasons, optical sensors for fluoride ion
based on boranes have been studied extensively.1,2 Although
such molecular sensors can work effectively in organic solvents,
detection and capture of fluoride ion in aqueous media remain
a difficult task because of the strong hydration to fluoride ion
which hampers the formation of the complex. Nevertheless,
several triarylboranes with enhanced Lewis acidity have been
reported to detect fluoride ion in aqueous media or to promote
its transport from aqueous to organic phases.3
Scheme 2. Synthesis of phosphonioborin 1b.
In a previous report, we have described the phase transfer of
fluoride ion by phosphonioborin 1a (Scheme 1). Although 1a
showed a high affinity for the fluoride ion, the low stability of
the corresponding fluoroborate 2a hampered its isolation and
structural determination.4 In this communication, we report the
syntheses of new phosphonioborin 1b and fluoroborate 2b which
are both stable compounds. We also describe the utilization
of 1b for the phase transfer of fluoride ion and comparison with
that of 1a.
The synthesis of B-Tip derivative 1b was performed by an
adaptation of the method used previously for 1a (Scheme 2).4a
Dibromide 3 was prepared from o-bromoiodobenzene by
taking advantage of iodine–magnesium exchange reaction using
PriMgCl.5 Phosphaborin 4 was air-sensitive and showed a ten-
dency to oxidation. For this reason, the crude product containing
4 was treated with MeI to give phosphonioborin 1b as a yellow
solid.
Figure 1. ORTEP drawings of 1b and 2b (50% probability). (a)
1b. Solvent molecules and counter iodide ion were omitted for
clarity. (b) 2b. Solvent molecules were not shown.
no significant interactions between counter iodide ion and the
boron or phosphorus center of 1b. The boron atom has a trigonal
planar structure (ꢀCBC: 360ꢁ), and the dibenzophosphaborin
framework had a butterfly-like structure.
Fluoride ion capture by 1 in an organic solvent was investi-
gated. Reactions of 1a or 1b with an excess amount of solid KF
in CDCl3 were monitored by multinuclear NMR spectroscopy
to show quantitative conversion to the corresponding zwitterion
2 after 3 h. However, 2a could not be obtained as a pure material
because it easily decomposed under air by oxidation. On the
other hand, 2b was isolated as a colorless solid in 76% yield
and could be obtained as single crystals by recrystallization from
CHCl3–EtOH (Figure 1b).7,8 Due to the tetrahedral geometry
around the boron atom in 2b, rotation around the B–C(Tip) bond
was suppressed, and such hindered rotation was confirmed by
Single crystals of 1b were obtained by recrystallization
from CHCl3–EtOH, and the crystal structure was determined
by X-ray crystallographic analysis (Figure 1a).6,7 There were
1
VT H NMR spectroscopy.9
Since phosphonioborins 1a and 1b capture fluoride ion in or-
ganic media, we decided to investigate their ability to transport
fluoride ion from water into organic phases. CDCl3 solution of 1
and D2O solution of KF (1 equiv.) were placed in an NMR tube,
and the sample was shaken by hand. After shaking for 1 min, the
1
solution color changed from yellow to colorless. The H NMR
spectrum of the sample indicated the complete consumption of
Scheme 1. Complexation of phosphonioborin 1 with fluoride
ion under biphasic condition.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.8 (2007)
977
In conclusion, we have reported the synthesis and structure
of a new phosphonioborin as well as the structure of the
corresponding zwitterionic fluoroborate. We have used the
phosphonioborin for the phase transfer of fluoride ion but
observed that the rate of the transfer declines due to the bulk
and lipophilicity of the Tip group. These results also show that
the phoshonioborin could be used as a solid-phase scavenger
for fluoride ion.
(a)
This work was supported by Grants-in-Aid for the 21st COE
program for Frontiers in Fundamental Chemistry and for Scien-
tific Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan (T. K.); the Research Fellow-
ships of the Japan Society for the Promotion of Science
for Young Scientists (T. A.); the U. S. Army Medical Research
Institute of Chemical Defense (F.P.G.), the Welch Foundation
(Grant A-1423, F.P.G.). We thank Tosoh Finechem Corp. for
the generous gifts of alkyllithiums.
(b)
References and Notes
1
a) X. Y. Liu, D. R. Bai, S. Wang, Angew. Chem., Int. Ed.
2006, 45, 5475. b) S. Yamaguchi, T. Shirasaka, S. Akiyama,
K. Tamao, J. Am. Chem. Soc. 2002, 124, 8816. c) S.
Yamaguchi, S. Akiyama, K. Tamao, J. Am. Chem. Soc.
2001, 123, 11372. d) S. Yamaguchi, S. Akiyama, K. Tamao,
J. Am. Chem. Soc. 2000, 122, 6335. e) Z.-Q. Liu, Q. Fang,
D.-X. Cao, D. Wang, G.-B. Xu, Org. Lett. 2004, 6, 2933.
f) M. Miyata, Y. Chujo, Polym. J. 2002, 34, 967. g) A.
Figure 2. 1H NMR spectral change upon mixing CDCl3 solu-
tion of the phosphonioborins with D2O solution of KF. (a) 1a.
(b) 1b.
Sundararaman, M. Victor, R. Varughese, F. Jakle, J. Am.
Chem. Soc. 2005, 127, 13748.
´
a) S. Sole, F. P. Gabbaı, Chem. Commun. 2004, 1284. b) M.
¨
¨
phosphonioborin 1a or 1b and the formation of the correspond-
ing zwitterion 2a or 2b (Figures 2a and 2b). The yields of 2a
and 2b were estimated to be close to 95% based on the use of
an internal standard.10
2
Melaımi, F. P. Gabbaı, Adv. Organomet. Chem. 2005, 53, 61.
¨
c) M. Melaımi, S. Sole, C.-W. Chiu, H. Wang, F. P. Gabbaı,
¨
´
Inorg. Chem. 2006, 45, 8136. d) T. W. Hudnall, M. Melaımi,
¨
¨
¨
The fluoride ion transfer was also monitored by UV–vis
spectroscopy. The reaction was performed in a quartz cell con-
taining a H2O solution of KF and a CHCl3 solution of 1a or
1b (1:0 ꢂ 10ꢃ4 M). The absorbance was monitored at 367 nm
without stirring. Under these conditions, 1a completely disap-
peared after 21 h. On the other hand, the spectral change of 1b
was much slower, and 94% conversion was observed after
36 h. Such decline in the reactivity against aqueous fluoride
ion may originate from the steric bulk of Tip group, which de-
creases the reactivity of the boron center. Another contributing
factor may be the increased lipophilicity of 1b which could
decrease its ability to transport fluoride ion from water into
organic phases.
Because the phosphonioborins function as phase-transfer
reagent against fluoride ion under biphasic conditions, we next
tried to scavenge aqueous fluoride ion using 1b as a solid. A
D2O solution of KF was mixed with solid 1b (1 equiv.) in an
NMR tube. The sample was sonicated and the concentration of
fluoride was monitored by 19F NMR spectroscopy. After the son-
ication for 1 h, the signal corresponding to aqueous fluoride ion
had completely disappeared. Moreover, 2b was not detected in
solution indicating that 1b may serve as an irreversible solid-
phase scavenger for fluoride ion. Analogous experiments were
carried out with solid [p-(Mes2B)C6H4(PMePh2)]I.4a Despite
extended sonication times, fluoride capture was not observed.
The increased steric crowding of the dimesitylboryl moiety
may be responsible for this observed lack of reactivity.
F. P. Gabbaı, Org. Lett. 2006, 8, 2747.
a) M. Melaımi, F. P. Gabbaı, J. Am. Chem. Soc. 2005, 127,
¨ ¨
¨
3
4
9680. b) C.-W. Chiu, F. P. Gabbaı, J. Am. Chem. Soc.
¨
2006, 128, 14248.
a) M. H. Lee, T. Agou, J. Kobayashi, T. Kawashima,
F. P. Gabbaı, Chem. Commun. 2007, 1133. b) T. Agou,
¨
J. Kobayashi, T. Kawashima, Inorg. Chem. 2006, 45,
9137.
P. Knochel, W. Dohle, N. Gommerann, F. F. Kneisel, F.
Kopp, T. Korn, I. Sapountzis, V. A. Vu, Angew. Chem.,
Int. Ed. 2003, 42, 4302.
5
6
.
Crystal data for 1b 2CHCl3 (CCDC-643630): triclinic,
˚
ꢁ
P1, a ¼ 8:429ð4Þ, b ¼ 13:259ð6Þ, c ¼ 17:080ð8Þ A, ꢀ ¼
105:475ð5Þꢁ, ꢁ ¼ 94:91ð5Þꢁ, ꢂ ¼ 102:703ð5Þꢁ, V ¼
3
˚
1774ð1Þ A , Z ¼ 2, R1 ¼ 0:0241 (I > 2ꢃðIÞ), wR2 ¼
0:0698 (all data).
G. M. Sheldrick, SHELXL 97, Program for crystal structure
7
8
determination, University of Gottingen, Germany, 1997.
.
¨
.
Crystal data for 2b 0.5CHCl3 0.5EtOH (CCDC-643629):
monoclinic, P21=c, a ¼ 12:223ð2Þ, b ¼ 9:611ð2Þ, c ¼
ꢁ
3
˚
˚
25:351ð8Þ A, ꢁ ¼ 101:271ð5Þ , V ¼ 2920:7ð1Þ A , Z ¼ 4,
R1 ¼ 0:0472 (I > 2ꢃðIÞ), wR2 ¼ 0:1269 (all data).
See Supporting Information for detail, which is available
electronically on the CSJ-Journal web site, http://www.
csj.jp/journals/chem-lett/.
9
10 Remaining 5% of the products might be decomposed
compounds via oxidation of zwitterion 2a or 2b.
Published on the web (Advance View) July 14, 2007; doi:10.1246/cl.2007.976