Fluoride ion abstraction of phosphonium cations from their counter anion: A novel synthetic method of fluorophosphoranes

Article abstract of DOI:10.1246/cl.2002.268

The phosphonium cations bearing the Martin ligand abstracted a fluoride ion from their counter anion, tetrafluoroborate, and the corresponding fluorophosphoranes were obtained in good yields.

Full text of DOI:10.1246/cl.2002.268

268
Chemistry Letters 2002
Fluoride Ion Abstraction of Phosphonium Cations from Their Counter Anion:
A Novel Synthetic Method of Fluorophosphoranes
Shohei Sase, Naokazu Kano, and Takayuki Kawashimaꢀ
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received November 12, 2001; CL-011134)
The phosphonium cations bearing the Martin ligand
borate was found to be controlled by the solvent. In order to obtain
the fluorophosphorane, N,N,N’,N’-tetramethylethylenediamine
(TMEDA) that is known to form its complex with trifluoroborane
was added to the ethereal solution.7 Removal of the precipitates
by filtration, evaporation of the filtrate, and further separation
with HPLC gave (ethylthio)fluorophosphorane 3 as colorless
solid in 90% yield. In 31P NMR spectrum (CDCl3) the isolated
phosphorane 3 showed a doublet at ꢀP ꢁ20:9 with 1JPF ¼ 801 Hz.
Its chemical shift is a typical value of pentacoordinate phosphorus
compounds, similarly to that in ether.8 The coupling constant of
the P–F bond is in the range of those of apical P–F bonds of
fluorophosphoranes.1b (Ethylthio)fluorophosphorane 3 is stable
at room temperature under argon atmosphere. On exposure in
open air for several days, however, it gradually hydrolyzed to give
the corresponding cyclic phosphinate 4.
Alkylfluorophosphoranes can also be synthesized by a
similar method (Scheme 2). The reactions of the corresponding
phosphine oxides 5a-d with trifluoromethanesulfonic acid gave
the phosphonium triflates 6a-d, respectively. Replacement of the
counter anion of 5a-d by tetrafluoroborate immediately afforded
the corresponding fluorophosphoranes 7a-d in good yields (53–
84%), respectively. In contrast to the ethylthio derivative 3, the
alkyl derivatives 7a-d are stable in open air.
abstracted a fluoride ion from their counter anion, tetrafluoro-
borate, and the corresponding fluorophosphoranes were obtained
in good yields.
Organofluorophosphoranes (RnPF5ꢁn, n ¼ 1{4) have long
been known as one of the most thermally stable phosphoranes
because of the stabilization effect of high electronegativity of
fluorine atoms.1 Organofluorophosphoranes bearing more than
two fluorine atoms (RnPF5ꢁn, n ¼ 1{3) have been synthesized by
various methods such as oxidative fluorination of trivalent
phosphorus compounds, fluorination of phosphine sulfides, and
halogen exchange of chlorophosphoranes, respectively.1b On the
other hand, only limited synthetic methods have traditionally
been used for the synthesis of monofluorophosphoranes, for
example, hydrofluorination of a phosphorus ylide,2 substitution
3;4
reactions of tri-or difluorophosphoranes,
and fluorination of
monochlorophosphoranes and phosphonium triflates,5 respec-
tively. In the last case, the addition of external tetramethylam-
monium fluoride or tris(diethylamino)difluorophosphorane is
essential to fluorination of the phosphorus compounds. In this
paper we report a novel synthetic method of monofluorophos-
phoranes by a fluoride ion abstraction of the corresponding
phosphonium cations from their counter anion. The Martin ligand
was used for the stabilization of phosphorane structure.6
S-Alkylation reaction of cyclic thiophosphinate 1 bearing the
Martin ligand with 2.0 equiv of triethyloxonium tetrafluoroborate
in CH2Cl2 at 50 ꢂC gave the corresponding phosphonium salt 2a
(Scheme 1). In 31P{1Hg NMR spectra, phosphonium salt 2a
showed a singlet at ꢀP 116.4, indicating no interaction between the
phosphorus atom and fluorine atoms of its counter anion,
tetrafluoroborate, in CH2Cl2. When the solvent was removed
and exchanged to ether, the signal of phosphonium salt 2a
completely disappeared with appearance of a new doublet (ꢀP
ꢁ20:9, 1JPF ¼ 801 Hz), suggesting quantitative conversion of 2a
to (ethylthio)fluorophosphorane 3. On the other hand, exchange
of the solvent from ether to CDCl3 resulted in the reverse reaction
back to the phosphonium tetrafluoroborate. Thus, interconversion
between the fluorophosphorane and the phosphonium tetrafluoro-
Scheme 2.
The crystal structure of (ethylthio)fluorophosphorane 3 was
determined by X-ray crystallographic analysis (Figure 1).9 This is
the first example of the monoflurophosphorane with a P–S bond.
Compound 3 hasa pentacoordinate structure with an oxygenand a
fluorine atoms at the apical positions and two carbon and a sulfur
atoms at the equatorial positions, respectively. The bond angle
between two apical bonds (179.2(1) ꢂ) and the sum of three angles
between each two equatorial bonds (359.5 ꢂ) are very close to
those of the ideal TBP structure (%TBP ! SP ¼ 17:0).10 The P–
ꢀ
F bond length (1.647(3) A) is the almost same value as that
ꢀ
(1.659(1) A) of a monofluorophosphorane bearing the Martin
ꢀ
ligand and benzyl group and that (1.643(2) A) of 2-fluoro-2,2’-
diphenyl-1,3,2-naphthodioxa-ꢁ5-phosphole.11 The P–S bond
12a
ꢀ
length (2.092(2) A) is almost as same as those (2.098(2) A,
ꢀ
ꢀ 12b
2.0834(8) A ) of two phosphoranes bearing a sulfur atom at the
equatorial position.
Formation of fluorophosphoranes in these reactions clearly
shows that phosphonium cations abstracted a fluoride ion from its
Scheme 1.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
269
Dedicated to Prof. Teruaki Mukaiyama on the occasion of his
75th birthday.
References and Notes
1
a) R. Schmutzler, Inorg. Synth., 9, 63 (1963). b) R. Schmutzler, Angew.
Chem., Int. Ed. Engl., 4, 496 (1965). c) D. Hellwinkel, in ‘‘Organic
Phosphorus Compounds,’’ ed. by G. M. Kosolapoff and L. Mailer, John
Wiley & Sons, New York (1972), Vol. 3, Chap. 5B, p 185.
H. Schmidbaur, K.-H. Mitshke, and J. Weidlein, Angew. Chem., Int. Ed.
Engl., 11, 144 (1972).
2
3
J. Breker, P. G. Jones, and R. Schmutzler, Phosphorus, Sulfur Silicon Relat.
Elem., 62, 139 (1991).
4
5
K. I. The and R. G. Cavell, J. Chem. Soc., Chem. Commun., 1975, 279.
A. A. Kolomeitsev, Y. L. Yagupolskij, A. Gentzsch, E. Lork, and G.-V.
Figure 1. ORTEP drawing of 3 with thermal ellipsoid
ꢀ
plot (30% probability). Selected bond lengths (A) and
Roschenthaler, Phosphorus, Sulfur Silicon Relat. Elem., 92, 179 (1994).
¨
6
E. F. Perozzi, R. S. Michalak, G. D. Figuly, W. H. Stevenson, III, D. B.
Dess, M. R. Ross, and J. C. Martin, J. Org. Chem., 46, 1049 (1981).
H. C. Brown, B. Singaram, and J. R. Schwier, Inorg. Chem. 18, 51 (1979).
3: colorless crystals, mp 129.6–132.0 ꢂC (dec.); 1H NMR (500 MHz, CDCl3)
bond angles (deg): P(1)–F(1) 1.647(3), P1–O(1)
1.767(3), P(1)–C(1) 1.835(5), P(1)–C(2) 1.815(5),
P(1)–S(1) 2.092(2), O(1)–P(1)–F(1) 179.2(1), O(1)–
P(1)–C(1) 89.3(2), O(1)–P(1)–C(2) 87.4(2), O(1)–P(1)–
S(1) 86.0(1), C(1)–P(1)–S(1) 124.12(2), S(1)–P(1)–
C(2) 109.8(1), C(1)–P(1)–C(2) 125.6(2).
7
8
3
ꢀ 0.59 (d, JHH ¼ 6:7 Hz, 3H, o-(CH3)(C’H3)CH of Tip), 1.21–1.28 (m,
12H, p-(CH3)2CH of Tip, CH3CH2S, o-(CH3)(C’H3)CH of Tip), 1.30 (d,
3JHH ¼ 6:7 Hz, 3H, o’-(CH3)(C’H3)CH of Tip), 1.39 (d, 3JHH ¼ 6:7 Hz, 3H,
o’-(CH3)(C’H3)CH of Tip), 2.79–2.95 (m, 3H, CH3CH2S, p-
(CH3)(C’H3)CH of Tip), 3.39–3.50 (m, 1H, o-(CH3)(C’H3)CH of Tip),
counter anion, tetrafluoroborate ion.13 There has been no report
on preparation of a fluorophosphorane by such a fluoride ion
abstraction of a phosphonium salt, although the reverse reaction,
that is, the conversion of a fluorophosphorane to a phosphonium
salt by trifluoroborane etherate has already been reported.14;15
The present reaction is formally similar to that of arylium ion in
Schiemann reaction, which affords a fluoroarene from an
aryldiazonium salt with tetrafluoroborate ion.16 Both Lewis
acidity of the phosphonium cation enhanced by strong electron-
withdrawing effect of the Martin ligand and coordination of ether
to trifluoroborane seem to have played an important role in
abstraction of a fluoride ion.
Fluorophosphoranes thus formed are easily defluorinated.
Treatment of 3 or 7b with 1.1 equiv of trimethylsilyl triflate
instead of trifluoroborane etherate in ether at room temperature
quantitatively gave the corresponding phosphonium triflate,
respectively (Scheme 3). Irreversibility of this reaction causes
the formation of fluorotrimethylsilane whose Si–F bond is very
strong and hard to dissociate.
3
4.91 (sept, JHH ¼ 6:7 Hz, 1H, o’-(CH3)(C’H3)CH of Tip), 6.93 (d,
4
4JPH ¼ 7:5 Hz, 1H, m-H of Tip), 7.11 (d, JPH ¼ 5:3 Hz, 1H, m’-H of
3
3
Tip), 7.65–7.76 (m, 3H), 8.31 (dd, JPH ¼ 11:5 Hz, JHH ¼ 7:8 Hz, 1H);
13C{1Hg NMR (126 MHz, CDCl3) ꢀ 15.6 (d, 3JPC ¼ 9 Hz), 23.6 (s), 23.7 (s),
2
3
23.8 (s), 24.5 (s), 25.7 (s), 27.6 (dd, JPC ¼ 18 Hz, JFC ¼ 6 Hz), 30.2 (d,
3JPC ¼ 5 Hz), 31.2 (dd, JPC ¼ 5 Hz, JFC ¼ 8 Hz), 33.9 (s), 82.4 (sept,
2JFC ¼ 31 Hz), 122.1 (q, 1JFC ¼ 289 Hz), 122.3 (d, 3JPC ¼ 17 Hz), 122.4 (q,
1JFC ¼ 288 Hz), 123.7 (d, 3JPC ¼ 17 Hz), 125.2 (d, 3JPC ¼ 16 Hz), 131.4 (d,
3
4
3JPC ¼ 15 Hz), 132.7 (dd, 1JPC ¼ 181 Hz, 2JFC ¼ 28 Hz), 133.0 (d, 4JPC
¼
2
3
2
3 Hz), 133.7 (dd, JPC ¼ 8 Hz, JFC ¼ 12 Hz), 135.5 (dd, JPC ¼ 21 Hz,
1
2
2
3JFC ¼ 4 Hz), 136.4 (dd, JPC ¼ 157 Hz, JFC ¼ 30 Hz), 148.8 (d, JPC
¼
16 Hz), 150.0 (d, 4JPC ¼ 4 Hz), 151.6 (dd, 2JPC ¼ 15 Hz, 3JFC ¼ 7 Hz); 19
F
1
NMR (254 MHz, CDCl3) ꢀ ꢁ25:4 (d, JPF ¼ 801:0 Hz, 1F), ꢁ74:8 (q,
4JFF ¼ 9:1 Hz, 3F), ꢁ75:4 (q, 4JFF ¼ 9:1 Hz, 3F); 31P{1Hg NMR (109 MHz,
1
CDCl3) ꢀ ꢁ20:9 (d, JPF ¼ 801:0 Hz). Anal. Calcd for C26H32F7OPS: C,
56.11; H, 5.86%. Found: C, 56.25; H, 5.60%.
Crystallographic data for 3: C26H32F7OPS, FW ¼ 556:56, T ¼ 173 K,
9
ꢀ
monoclinic, P21/n, a ¼ 8:640ð4Þ, b ¼ 18:800ð1Þ, c ¼ 16:724ð7Þ A, ꢂ ¼
90:730ð3Þ , V ¼ 2713:1ð2Þ A , Z ¼ 4, Dcalc ¼ 1:362 g cmꢁ3. The final
cycle of full-matrix least squares refinement was based on 4069 observed
reflections (I > 3:00ꢃðIÞ) and 325 variable parameters and converged at
RðRwÞ ¼ 0:054 (0.058). Crystallographic data reported in this paper have
been deposited with Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC-176386. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
ꢂ
ꢀ 3
10 R. R. Holmes and J. A. Deiters, J. Am. Chem. Soc., 99, 3318 (1977).
11 a) Y. Yamamoto, K. Nakao, and K.-y. Akiba, the 23rd Symposium on
Heteroatom Chemistry, Okayama, Dec., 1996, Abstr., No. 28. b) M. Well,
A. Fischer, P. G. Jones, and R. Schmutzler, Phosphorus, Sulfur Silicon
Relat. Elem., 69, 231 (1992).
12 a) R. R. Holmes, K. C. K. Swamy, J. M. Holmes, and R. O. Day, Inorg.
Chem., 30, 1052 (1991). b) V. A. Pinchuk, I. Neda, C. Muller, A. Fisher,
Scheme 3.
¨
P. G. Jones, Y. G. Shermolovich, and R. Schmutzler, Chem. Ber., 127, 1395
(1994).
13 Decomposition of [Ph3Bi(Ph3AsO)2(BF4)2] into Ph3BiF2 and Ph3AsOBF3
was reported, see: R. E. Beaumont, R. G. Gouel, and H. S. Prasad, Inorg.
Chem. 12, 944 (1973).
14 Ligand exchanges of ꢁantimony(V) and phosphorus(V) porphyrins by a
fluoride ion of the PF6 counteranion were reported, see: a) K. M. Kadish,
M. Autret, Z. Ou, K.-y. Akiba, S. Matsumoto, R. Wada, and Y. Yamamoto,
Inorg. Chem., 35, 5564 (1996). b) K.-y. Akiba, R. Nadano, W. Satoh, Y.
Yamamoto, S. Nagase, Z. Ou, X. Tan, and K. M. Kadish, Inorg. Chem., 40,
5553 (2001).
In summary, we developed the novel synthetic method of
fluorophosphoranes. The key in this method seems to be the
cooperative action of high Lewis acidity of the phosphorus atom
with coordination of ether to trifluoroborane. And we demon-
strated that the phosphonium triflates are readily obtained from
the fluorophosphoranes.
This work was partially supported by Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science, Technology of Japan. We are grateful to Central
Glass and Tosoh Finechem Corporation for the gifts of
organofluorine compounds and alkyllithiums, respectively.
15 For conversion of
trifluoroborane etherate, see: a) U. von Allworden and G.-V. Roschenthaler,
a fluorophosphorane to a phosphonium salt by
¨
¨
Chem.-Ztg., 109, 352 (1985). b) E. Fluck, P. Kuhm, and H. Riffel, Z. Anorg.
Allg. Chem., 567, 39 (1988).
16 C. M. Sharts, J. Chem. Edu., 45, 185 (1968).