The Tetra(vinyl)phosphonium Cation [(CH2=CH)4P] +

Full text of DOI:10.1021/ja0399019

Published on Web 01/23/2004
The Tetra(vinyl)phosphonium Cation [(CH2dCH)4P]+
Uwe V. Monkowius, Stefan D. Nogai, and Hubert Schmidbaur*
Anorganisch-chemisches Institut, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4, 85747 Garching, Germany
Received December 2, 2003; E-mail: H.Schmidbaur@lrz.tum.de
Vinyl compounds CH2dCHX have a vast synthetic potential.1
This wealth has been exploited most extensively in the realm of
the simple olefins ethene, propene, or butadiene but also of the
vinyl-halides, -ethers, -sulfides, -amines, -silanes, and -boranes, and
of the large family of vinylmetallics.2 Surprisingly, however, the
chemistry of the vinyl compounds of phosphorus is not very well
developed, and even some of the simple prototypes of this class
have not yet been described.3,4 This is particularly true for the
elusive tetra(vinyl)phosphonium cation [(CH2dCH)4P]+. Early
reports of its synthesis have not been confirmed,5,6 and subsequent
preparative attempts apparently have not met with success. It is
also interesting to note that there is virtually no information on the
Figure 1. Molecular structure of the cation of 5.
synthesis, structure, and reactivity of tetra(vinyl)ammonium salts
[(CH2dCH)4N]+X- or the arsenic, antimony, and bismuth ana-
logues.
Scheme 1 a
We have recently shown that with tri(vinyl)phosphine (CH2d
CH)3P, the most obvious potential synthon for the preparation of
poly(vinyl)phosphonium cations, even simple quaternization, oxida-
tion, or complexation reactions can lead to immediate polymeri-
zation of the substrate or its products.7,8 Therefore, there was a
need for a synthetic strategy in which all sensitive vinyl groups
are generated only in the final preparative step from substituents
previously introduced at the common phosphonium center. After
several unsuccessful experiments with reactions suggested by the
literature,9-11 simple primary phosphines RPH2 were tested as model
substrates for hydrophosphination, before PH3 could finally be
considered as the de novo starting material.
a (Cy ) cyclohexyl, Vi ) vinyl)
Scheme 2
At the exploratory stage,12 cyclohexylphosphine (1) was first used
for photochemical hydrophosphination of vinyl acetate in hexane
with AIBN as a catalyst.13 Contrary to negative results reported in
the literature,9 the tertiary phosphine 2 was readily obtained. Its
quaternization with 2-iodo-ethanol afforded the quaternary salt 3
which upon acylation with acetic acid anhydride gave the sym-
metrical trifunctional derivative 4. Treatment of 4 with sodium
carbonate in refluxing dioxane led to conversion into cyclohexyl-
trivinylphosphonium iodide (5) (Scheme 1). The structure of this
salt has been confirmed in a single-crystal X-ray diffraction study
(Figure 1).14a
Encouraged by these results, phosphine gas was bubbled into a
solution of vinyl acetate and AIBN in toluene under UV irradiation.
After 4 h at 25 °C, tris(2-acetoxyethyl)phosphine (6) was obtained
in 60% yield. This product was subjected consecutively to the same
treatment as that described for the conversion of 3 into 4 and 5 to
give the quaternary salts 7, 8, and finally 9a (72% yield for the
last two reactions)(Scheme 2).15
symmetry. The target compound tetravinylphosphonium iodide (9a)
was also readily identified by conventional methods beyond doubt,
but all attempts to grow single crystals failed. Therefore, 9a was
converted into the chloride (9b) with AgCl, the picrate (9c) with
silver picrate, and the tetraphenylborate (9d) with NaBPh4, of which
the latter finally gave a suitable crystalline specimen (orthorhombic,
space group Pnma, Z ) 4).14c
In this salt, the smaller Vi4P+ cations occupy two distinct
positions, which could be accounted for by a 1:1 disorder model.
The structure of the cation is shown in Figure 3. The average P-C
and CdC bond lengths are 1.798 and 1.249 Å, indicating standard
single and double bonding, respectively, and the C-P-C and
P-CdC angles are all close to the 109° and 120° reference values.
The course of the crucial last step was easily followed by 31P
NMR spectroscopy, and all intermediates were detected (Supporting
Information). The most important ones of these were characterized
by their analytical and spectroscopic data, and the crystal structure
of the first homoleptic salt (8) was determined (Figure 2).14b The
crystals contain two independent cations, each of which has S4
-
The dimensions of the BPh4 anion show no anomalies.16
Tetraalkylphosphonium cations [R4P]+ are known to adopt
conformations of high symmetry, preferentially of point groups D2d
i
c
and S4, as observed for R ) Et, Pr, Pr,17-19 and for 8 (above).
9
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J. AM. CHEM. SOC. 2004, 126, 1632-1633
10.1021/ja0399019 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Supporting Information Available: Crystallographic data of 5,
8, 9d, and [Ph3PMe][BVi4] (CIF) and syntheses and characterization
of 2-5 and 9a-9d (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry, 2nd ed.; J. Wiley &
Sons: Hoboken, NJ, 2003.
(2) Seyferth, D. In Progress in Inorganic Chemistry; Cotton, F. A., Ed.;
Interscience: New York, 1961; Vol. 3.
(3) Maier, L. In Organic Phosphorus Compounds; Kosolapoff, G. M., Maier,
L., Eds.; Wiley-Interscience: New York, 1972; Vol. 1.
(4) Cristau, H. J.; Plenat, F. In The Chemistry of Organophosphorus
Compounds; Hartley, F. R., Ed.; John Wiley & Sons: Chichester, 1994;
Vol. 3, p 73 ff.
(5) Maier, L.; Seyferth, D.; Stone, F. G. A.; Rochow, E. G. J. Am. Chem.
Soc. 1957, 79, 5884.
(6) Kaesz, H. D.; Stone, F. G. A. J. Org. Chem. 1959, 24, 635.
(7) Monkowius, U.; Nogai, S.; Schmidbaur, H. Organometallics 2003, 22,
145.
Figure 2. Molecular structure of the cation of 8.
(8) Monkowius, U.; Nogai, S.; Schmidbaur, H. Dalton Trans. 2003, 987.
(9) Jia, G.; Drouin, S. D.; Jessop, P. G.; Lough, A. J.; Morris, R. H.
Organometallics 1993, 12, 906.
(10) Grim, S. O.; Barth, R. C. J. Organomet. Chem. 1975, 94, 327.
(11) Lindner, E.; Meyer, S.; Wegner, P.; Karle, B.; Sickinger, A.; Steger, B.
J. Organomet. Chem. 1987, 335, 59.
(12) The sequence of reactions leading from 1 to 5 are described in full in the
Supporting Information.
(13) Hechenbleikner, I.; Enlow, W. P. Preparation of 2-Hydroxyalkylphos-
phines. German Patent 26 01 520, 1976.
(14) Crystal structures: (a) [CyVi3P]+I-, 5 (143 K): monoclinic, space group
P21/n, Z ) 4; a ) 7.1098(1) Å, b ) 24.8571(6) Å, c ) 8.1218(2) Å, â
) 90.253(1)°. R1 ) 0.0264, wR2 ) 0.0574. (b) {P[CH2CH2OC(O)-
Me]4}+I-, 8 (143 K): tetragonal, space group I4c2, Z ) 8; a ) 10.5930-
(10) Å, c ) 37.5286(6) Å. R1 ) 0.0240, wR2 ) 0.0684. (c) [Vi4P]+[BPh4]-,
9d (143 K): orthorhombic, space group Pnma, Z ) 4; a ) 19.3966(3) Å,
b ) 13.7062(2) Å, c ) 9.9160(1) Å; R1 ) 0.0496, wR2 ) 0.1229. (d)
[MePh3P]+[BVi4]- (143 K): monoclinic, space group P21/n, Z ) 4; a )
10.3108(1) Å, b ) 14.5850(2) Å, c ) 15.6994(3) Å, â ) 100.1099(6)°.
R1 ) 0.042, wR2 ) 0.103. For details see Supporting Information.
(15) 6-9a: vinyl acetate (72.7 g, 0.84 mol) is dissolved in toluene (250 mL),
10 mg of AIBN is added, and phosphine gas PH3 is bubbled into the
solution at room temperature with stirring and irradiation with a UV lamp.
After 4 h all volatile components are removed in a vacuum to leave a
yellow oily product (6, 49.7 g, 60.7% yield), which should be kept under
nitrogen below -32 °C. NMR (C6D6, 20 °C), 31P: -41.5 (s). 13C: 169.9
(s, CO2); 61.9 (d, J ) 20.8), 26.9 (d, J ) 15.6) for CH2; 20.5 (s, Me). 6
(15 g, 51.3 mmol) is treated with 2-iodo-ethanol (12.0 g, 70 mmol) in
tetrahydrofuran (30 mL) at 20 °C with stirring for 12 h. Removal of all
volatiles in a vacuum gives a residue of compound 7 (17.9 g, 75% yield),
which also needs to be kept below -32 °C to prevent decomposition.
NMR (CDCl3, 20 °C), 31P: 33.0 (s). 13C: 170.1 (s, CO2); 57.4 (d, J )
6.9), 21.9 (d, J ) 70.7) CH2 acetoxyethyl; 55.0 (d, J 8.5), 24.3 (d, J 24.0)
CH2 hydroxyethyl; 21.2 (s, Me). 7 (15 g, 32 mmol) is dissolved in acetic
acid anhydride (50 mL) and refluxed for 10 h. Most of the solvent is
evaporated and the residue treated again with fresh Ac2O (40 mL) for 6
h under reflux. Excess reagent is removed in a vacuum and the residue
washed with diethyl ether (3 × 50 mL). The remaining brown oil is
extracted with chloroform to give a colorless solid which can be
crystallized from acetone (8, mp 146 °C, 88% yield). NMR (CD3CN, 20
°C), 31P: 33.4 (s). 13C: 170.9 (s, CO2); 58.3 (d, J ) 5.7), 21.7 (d, J )
48.3) CH2; 21.1 (s, Me). 8 (15 g, 28 mmol) is refluxed with 15 g of soda
in 80 mL of dioxane for 6 h. The reaction mixture is filtered, and the
solids are extracted with hot acetonitrile. The product precipitates on
cooling and addition of diethyl ether and can be crystallized from acetone/
diethyl ether (9a, 6.13 g, 72% yield, mp 80 °C with decomposition). NMR
(CD3CN, 25 °C), 31P: 9.7 (s); 13C: 145.04 (s, Câ), 118.1 (d, J ) 83.6
CR). MS (FAB): m/z 139 (100%) [Vi4P]+.
Figure 3. Molecular structure of the tetra(vinyl)phosphonium cation
[(CH2dCH)4P]+ in the tetraphenylborate 9d.
According to quantum chemical calculations, tetra(vinyl)element
species (CH2dCH)4E (E ) C, Si, Ge, Sn) can adopt many
conformations with very similar energies.20,21 Calculations carried
out in the course of the present study have shown that the cations
[Vi4P]+ with point groups of maximum attainable symmetry (D2d
and S4) are higher in energy and that only a model of point group
C1 represents a pronounced minimum (Supporting Information).
The cations of 9d have no crystallographically imposed symmetry,
but an analysis shows that three of the vinyl groups (with C3, C5,
C7) form a propeller of approximate local C3 symmetry, which is
almost superimposable with the PVi3 part of the [CyPVi3]+ cation
(Figure 1). The array of the four vinyl groups in the [Vi4P]+ cation
is thus in agreement with the quantum chemical calculations and
is moreover exactly analogous to the molecular structures of tetra-
(vinyl)methane and -silane, which were determined only very
recently by X-ray diffraction.22,23 In our own supplementary
investigations, the tetra(vinyl)borate anion [B(CHdCH2)4]- was
also found to have a similar conformation in its methyltriph-
enylphosphonium salt24 (Supporting Information). Taken together,
these results indicate that the structure shown in Figure 3 is probably
not greatly influenced by packing forces but represents the intrinsic
configuration and conformation of (CH2dCH)4E species.
(16) Westerhaus, W. J.; Knop, O.; Falk, M. Can. J. Chem. 1980, 58, 1355.
(17) Schmidbaur, H.; Frazao, C. M. F.; Reber, G.; Mu¨ller, G. Chem. Ber. 1989,
122, 259.
(18) Anderson, D. G.; Rankin, D. W. H.; Robertson, H. E.; Frazao, C. M. F.;
Schmidbaur, H. Chem. Ber. 1989, 122, 2213.
(19) Schmidbaur, H.; Schier, A.; Frazao, C. M. F.; Mu¨ller, G. J. Am. Chem.
Soc. 1986, 108, 976.
Tetravinylphosphonium salts are very reactive toward nucleo-
philes and undergo a large variety of clean reactions, but the
conditions have to be carefully chosen to avoid decomposition
through random polymerization. Attempts to use 9a-d for the
preparation of the unknown pentavinylphosphorane, Vi5P, have been
unsuccessful.
(20) Rustad, S.; Beabley, B. J. Mol. Struct. 1978, 48, 381.
(21) Schultz, G.; Hargittai, I. J. Mol. Struct. 1998, 445, 47.
(22) Bartkowska, B.; Kru¨ger, C. Acta Crystallogr. 1997, 53C, 1066.
(23) Benet-Buchholz, B.; Boese, R.; Haumann, T. In The Chemistry of Dienes
and Polyenes; Rapoport, Z., Ed; John Wiley & Sons: Chichester, 1997,
Vol. 1, p 25.
(24) Seyferth, D.; Weiner, M. A. J. Am. Chem. Soc. 1961, 83, 3583.
Acknowledgment. Dedicated to Professor O. E. Scherer on the
occasion of his 70th birthday.
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