Oxidation of phosphanes with orthoquinones: An unusual decomposition of an unexpectedly stable zwitterion

Article abstract of DOI:10.1002/ejic.200600579

Oxidation of the ylide (Et₂N)₂P[=C(CN)CH(CF ₃)₂]CH₂P-(NEt₂)₂ (3) with tetrachloroorthobenzoquinone (TOB) does not give the expected 1,3,2-dioxaphospholane but stops at the format

Full text of DOI:10.1002/ejic.200600579

FULL PAPER
DOI: 10.1002/ejic.200600579
Oxidation of Phosphanes with Orthoquinones: An Unusual Decomposition of
an Unexpectedly Stable Zwitterion
Igor Shevchenko,*[a] Vasyl Andrushko,[a] Enno Lork,[b] and Gerd-Volker Röschenthaler*[b]
Keywords: Ylides / Phosphanes / Zwitterions / Carbodiphosphorane
Oxidation of the ylide (Et2N)2P[=C(CN)CH(CF3)2]CH2P-
(NEt2)2 (3) with tetrachloroorthobenzoquinone (TOB) does
not give the expected 1,3,2-dioxaphospholane but stops at
the formation of the unusually stable intermediate zwitterion
6. The molecular structure of 6 was established by X-ray
analysis. In solution at temperatures above 15 °C zwitterion
6 undergoes an unusual decomposition into symmetrical car-
bodiphosphorane 7 and hexafluoroisovaleronitrile 8.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Here we wish to report on the rather unusual reaction of
mono ylide 3 with tetrachloro-orthobenzoquinone (TOB).
The oxidation of phosphanes with orthobenzoquinones
normally leads to the formation of five-coordinate phos-
phoranes in which two oxygen atoms are connected to
phosphorus to form 1,3,2-dioxophospholane B. Such reac-
tions proceed in two steps via intermediate zwitterionic
compounds A, which are very unstable and can only be
detected by NMR spectroscopy at low temperatures.[3] The
increase of steric hindrance stabilizes the zwitterionic inter-
mediate. For example, the use of the more bulky phenan-
threnequinone instead of benzoquinone in the reaction with
triphenylphosphane gave the appropriate intermediate of
type A, which is stable at room temperature.[4] Unfortu-
nately, X-ray investigation of this compound could not be
performed.
Previously we have described the addition of 4,4,4-tri-
fluoro-3-(trifluoromethyl)-2-butenenitrile (2) to bis[bis(di-
methylamino)phosphanyl]methane (1) to give the monoyl-
ide 3. The reaction of 3 with a second equivalent of 2 does
not give the expected symmetrical diylide but leads unex-
pectedly to the almost quantitative formation of the fluori-
nated triene 4.[1,2] The second product of this reaction was
identified as the fluorine-substituted ylide 5. These two re-
actions can be combined in one and triene 4 can be easily
prepared by adding two equivalents of 2 to the starting
methylenediphosphane 1.
The most probable route for formation of the symmetric
molecule of triene 4 is the dimerization of the appropriate
carbene, and the only possible way for this to form is by
dissociation of the ylidic P=C bond. We reasoned that if
this is not a particular case, then an analogous cleavage may
also be observed if the trivalent phosphorus atom of 3 were
to react with other electrophilic reagents.
We have found that the reaction of compound 3 with
TOB does not lead to the expected phosphorane structure,
but stops at the formation of zwitterionic intermediate 6.
This compound crystallizes easily from the reaction mixture
in diethyl ether in 65% yield as colorless crystals that turned
out to be unusually stable. The detailed molecular structure
of 6 (Figure 1) was established by X-ray analysis.
The bonding situation around phosphorus P(2) does not
correspond to the trigonal-bipyramidal coordination geom-
etry typical of a λ5P5 configuration but displays the tetrahe-
dral geometry of the phosphonium structure λ4P5. The ap-
pearance of a positive charge at phosphorus P(2) causes
some shortening of the P(2)–C(1) bond in comparison with
[a] Institute of Bioorganic Chemistry,
Murmanskaya Street 1, 02094 Kiev, Ukraine
Fax: +38-044-573-2552
E-mail: ishev@bpci.kiev.ua
[b] Institute of Inorganic and Physical Chemistry, University of
Bremen,
NW2, C2350, Leobener Str, 28334 Bremen, Germany
Fax: +49-421-218-4267
E-mail: gvr@chemie.uni-bremen.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2007, 259–262
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
259

I. Shevchenko, V. Andrushko, E. Lork, G.-V. Röschenthaler
FULL PAPER
at δ = 44.51 and 57.20 ppm (2JP,P = 11.2 Hz). These values
are typical for ylidic and positively charged tetracoordinate
phosphonium structures, respectively. If phosphorus P(2)
were connected to the second oxygen O(2) it would have a
five-coordinate phosphorane structure and a chemical shift
of about δ = –50 ppm. The methylene group between the
two electron-withdrawing phosphonium centers displays a
1
doublet of doublets at δ = 4.74 ppm in the H NMR spec-
trum. This value suggests that the acidity of the CH2 pro-
tons is comparable with that of CH2Cl2. In the presence of
the adjacent anionic centers in the molecule this may lead
to the appearance of other prototropic forms. Indeed, the
1H NMR spectrum of compound 6 in chloroform displays
a temperature-dependent equilibrium between zwitterionic
form 6 and other possible forms 6a,b resulting from the
migration of one of the protons to the negatively charged
oxygen O(2).
the starting compound 3 (179.5 and 187.6 pm, respectively).
The negatively charged oxygen O(2) is situated between the
two positively charged phosphorus atoms. The P(1)–O(2)
and P(2)–O(2) distances (415.9 and 320.1 pm, respectively)
are substantially longer than the P(2)–O(1) bond
[157.93(19)], which excludes bonding between the atoms.
The structure around the ylidic phosphorus P(1) does not
differ substantially from that of the starting compound 3.
The two adjacent nitrogen atoms N(1) and N(2) have
slightly different configuration. The N(1) atom is almost
flat, whereas the N(2) atom has a more pronounced tetrahe-
dral geometry [the sums of angles are 358.5(6)° and
348.0(1)°, respectively]. In solution, however, both dieth-
ylamino groups are equivalent and display common signals
At +15 °C the position of the equilibrium is completely
shifted to the zwitterionic form 6 and the P–CH2–P methyl-
ene protons display a broad doublet of doublets at δ =
4.74 ppm. At 0 °C two additional very broad signals at δ =
9.46 and 2.46 ppm appear in the spectrum. The low-field
signal belongs to the OH proton and the chemical shift
value of the second one is more in agreement with the
P=CH–P proton of isomer 6a. This proton is connected to
the negatively charged ylidic carbon atom and is thus
shifted to higher field than that of structure 6b. The inten-
sity of the signals of the OH-containing isomers increases
upon lowering the temperature. The contribution of struc-
ture 6b probably increases at low temperature because the
broad signal of the CH proton shifts gradually to lower
field (δH = 2.94 ppm at –40 °C). The ratio between isomers
6 and 6a,b at this temperature reaches 40:60. We did not
find signals for the fourth possible prototropic form 6c in
1
in the H NMR spectrum.
The ylidic carbon C(10) has an ideal flat conformation
accounting for the delocalization of the negative charge on
the nitrile group. That is why the P(1)–C(10) bond
(171.4 pm) is somewhat longer than a normal ylidic bond
(168 pm),[5] whereas the C(1)–C(2) distance is considerably
shorter (139.7 pm) than the average single C–C bond. These
data are consistent with the structure of the other nitrile-
substituted phosphorus ylides.[6–8]
Considering that the ylidic P(1)–C(10) bond is highly po-
larized, the molecule contains two positively charged phos-
phorus atoms and negatively charged oxygen and carbon
atoms. The 31P NMR spectrum of 6 displays two doublets
1
the H NMR spectrum.
Figure 1. Perspective view and labeling scheme for 6. Selected interatomic distances [pm] and bond angles [°]: P(1)–C(10) 171.4(3), P(1)–
C(1) 183.0(3), C(1)–P(2) 179.5(2), P(1)–N(1) 165.3(2), P(1)–N(2) 165.5(2), P(2)–N(4) 161.2(2), P(2)–N(5) 162.3(2), C(10)–C(11) 139.7(4),
C(11)–N(3) 116.1(4), C(10)–C(12) 151.3(3), P(2)–O(1) 157.93(19), O(2)–P(1) 415.9, O(2)–P(2) 320.1; C(10)–P(1)–C(1) 115.05(12), P(2)–
C(1)–P(1) 122.90(14), C(11)–C(10)–C(12) 118.6(2), C(11)–C(10)–P(1) 122.0(2), C(12)–C(10)–P(1) 119.37(19), N(3)–C(11)–C(10) 178.8(3),
O(1)–P(2)–C(1) 107.57(11), N(4)–P(2)–C(1) 106.08(12).
260
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 259–262

Oxidation of Phosphanes with Orthoquinones
FULL PAPER
certed. Taking into account our previous results,[1,2] as well
as the recently reported P=C bond dissociation of strained
four-membered cyclic ylides,[9] the homolytic cleavage of the
P(1)–C(10) ylidic bond of 6 with the intermediate formation
of the reactive carbene in this reaction cannot be excluded.
Another possible mechanism of the decomposition of 6
may include, for example, an intermediate formation of
compound 6d which, because of steric hindrance around
phosphorus P(1), may eliminate 8 to give carbodiphosphor-
ane 7.
In the early stages of the decomposition of zwitterion 6
carbodiphosphorane 7 is the only product and no other sig-
nals except for those of the starting material and the prod-
uct are observed in the 31P and 19F NMR spectra. However,
after completion of the process the content of numerous by-
products in the 31P NMR spectrum reaches 30–40%. Car-
bodiphosphorane 7 is rather stable in the reaction solution
and its signal decreases only gradually on heating to 70 °C
during 10 h. However, our attempts to isolate the product
in crystalline form failed. The structure of 7 was confirmed
It is interesting that the 31P NMR spectrum does not by its reaction with water. This process includes the forma-
reflect all these changes of the molecule and shows two tion of several intermediate compounds which finally give
well-resolved doublets with constant chemical shifts at all the product 9. Unlike carbodiphosphorane 7, this com-
temperatures below 20 °C.
In crystalline form zwitterion 6 is stable at room tem- inequivalent phosphorus atoms that appear as two doublets
perature under an inert atmosphere. For example, after 12 h of an AB spin system at δ = 18.00 and 30.72 ppm (2JP,P
pound is not symmetrical and contains two magnetically
=
we did not find additional signals in the NMR spectra. In 5.5 Hz) in the 31P NMR spectrum. The signal at δ =
chloroform solution compound 6 is stable up to 15 °C, and 18 ppm, when recorded without proton decoupling, appears
at 20 °C irreversible decomposition begins. The two dou- as a characteristic doublet of triplets. It belongs to the
blets of 6 in the 31P NMR spectrum become broad and phosphorus atom of the 1,3,2-dioxaphospholane system be-
unresolved and a new, sharp singlet appears at δ = cause of the interaction with only two P–CH2–P protons.
56.8 ppm. After about 25–30 h the spectrum contains only Compound 9 was isolated as colorless crystals and its struc-
traces of the starting material. The conversion of two dou- ture was additionally confirmed by 1H NMR and HR mass
blets due to the two inequivalent phosphorus nuclei into spectrometry.
one singlet, and its chemical shift value, indicate that com-
pound 6 rearranges into the symmetrical carbodiphosphor-
ane 7, which contains magnetically equivalent phosphorus
nuclei. In the 19F NMR spectrum the decomposition of 6
is shown by the disappearance of the doublet at δ =
–66.42 ppm (3JH,F = 8 Hz) and the growth of a new doublet
at δ = –68.04 ppm (3JH,F = 6.5 Hz), which belongs to six
equivalent fluorine atoms of the second decomposition
product [hexafluoroisovaleronitrile (8)]. This compound is
a colorless, volatile liquid that can be easily distilled from
Experimental Section
General Remarks: All operations were performed under nitrogen in
a dry box. The solvents were dried by the usual procedures. The
NMR spectra were recorded with Varian Gemini-400 and JEOL
FX-90Q spectrometers. The 1H and 13C chemical shifts were refer-
enced to tetramethylsilane (TMS). The 19F and 31P chemical shifts
were measured using CFCl3 and 85% aqueous H3PO4 as standards,
respectively. The digital resolutions were 0.25, 0.5, 1.0, and 1.25 Hz
the reaction mixture together with the solvent (chloroform).
Its structure was confirmed by NMR spectroscopy and
mass spectrometry. The decomposition rate depends on the
properties of the solvent used. In chlorobenzene it is twice
as fast as in chloroform.
Unlike other known unstable zwitterions of this type, the
negatively charged oxygen atom O(2) of the former ortho-
quinone unit does not form the expected 1,3,2-dioxaphos-
pholane cycle with phosphorus P(2), but attacks the other
positively charged phosphorus P(1). The mechanism of the
decomposition should include the dissociation of the P(1)–
C(10) ylidic bond and the elimination of two hydrogen
atoms from the methylene unit. There is no clue as to
1
for H, 13C, 19F, and 31P NMR spectra, respectively.
Zwitterionic Compound 6: A solution of tetrachlororthobenzoqui-
none (37 mg, 0.150 mmol) in 2 mL of dry diethyl ether was added
in portions of 5–8 drops to a solution of compound 3 (85 mg,
0.153 mmol) in 3 mL of diethyl ether at –10 °C. Each portion of
TOB was added at an interval of 2–3 min. The reaction solution
was left for 12 h at –10 °C and then for 48 h at –30 °C. The color-
whether these processes proceed consecutively or are con- less crystals of 6 formed over this time were separated from the
Eur. J. Inorg. Chem. 2007, 259–262
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
261

I. Shevchenko, V. Andrushko, E. Lork, G.-V. Röschenthaler
FULL PAPER
mother liquor, washed with cold diethyl ether, and dried in vacuo
0.0745 and wR2 (all data) = 0.1465 {w = 1/[σ2(Fo2) + (0.0698P)2 +
P]}, where P = (2Fc2 + Fo2)/3. All non-hydrogen atoms were refined
anisotropically and the position of the hydrogen atoms was calcu-
(0.05 Torr). Yield: 73 mg (59%). 1H NMR (CDCl3, 15 °C): δ =
3
3
1.12 (t, JH,H = 7.1 Hz, 12 H, NCH2CH3), 1.13 (t, JH,H = 7.1 Hz,
2
2
12 H, NCH2CH3), 2.62 (dd, JP,H = 16.5, JP,H = 21.1 Hz, 1 H, P– lated as a riding model. The final residual Fourier positive and
CH–P), 3.02–3.29 (m, 16 H, NCH2CH3), 3.47 [sept, 3JF,H = 6.9 Hz
negative peaks were equal to 1.024 and –1.034 eÅ–3. One ethyl
group of one NEt2 moiety is disordered. The terminal C(18) atom
is split over two positions with an occupancy of 70% for C(18) and
2
2
1 H, CH(CF3)2], 4.74 (br. dd, JP,H = 18.4, JP,H = 21.8 Hz, 1 H,
P–CH2–P) ppm. 19F NMR (CDCl3): δ = –66.4 ppm (d, JH,F
=
3
7.5 Hz, 6 F). 31P NMR (CDCl3): δ = 44.5 (d, JP,P = 11.2 Hz), 30% for C(18a). The distances C(17)–C(18) and C(17)–C(18a) were
2
2
57.0 ppm (d, JP,P = 11.2 Hz).
set to be equal. The Cl atoms of one solvent molecule are disor-
dered over two positions (50% occupancy). The distances C(30)–
Cl(8), C(30)–Cl(9), C(30)–Cl(8a) and C(30)–Cl(9a) were set to be
equal.
Decomposition of 6: A solution of freshly crystallized zwitterionic
compound 6 in CDCl3 was left at 20 °C for 24 h.
Carbodiphosphorane 7: 31P NMR (CDCl3): δ = 56.8 ppm (s).
CCDC-610625 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif.
1
3
Hexafluoroisovaleronitrile 8: H NMR (CDCl3): δ = 2.93 (d, JH,H
= 6.5 Hz, 2 H, CH2CN), 3.37 ppm [m, JH,H = 6.5, 3JF,H = 7.0 Hz,
3
1 H, (CF3)2CH]. 19F NMR (CDCl3): δ = –68.0 ppm (d, JH,F
=
3
7.0 Hz, 6 F). MS (EI, 70 eV, 250 °C): m/z (%) 191 (2) [M+], 172
(36) [M – F]+, 122 (100) [M – CF3]+. HRMS [M – F]: calcd.
172.01857; found 172.01787 (–4.0 ppm, –0.7 mmu).
Acknowledgments
Compound 9: A solution of 6 (100 mg, 0.18 mmol) in CH2Cl2
(2 mL) was left at 20 °C for 24 h, then water (50 mg) was added
and the mixture was stirred for 48 h at 20 °C. The solvent was
evaporated, the residue was dried in vacuo (0.05 Torr), washed with
diethyl ether and crystallized from toluene/hexane (at –20 °C) to
give 15 mg of a colorless solid. 1H NMR (CDCl3): δ = 1.09 (t,
We thank the Deutsche Forschungsgemeinschaft (DFG, 436 UKR
113/58/0-1) for financial support. We are grateful to Dr. Thomas
Duelks and Dr. Dorit Kaempke for mass spectrometric measure-
ments.
2
2
3JH,H = 7.3 Hz, 12 H, NCH2CH3), 2.40 (dd, JP,H = 16.5, JP,H
=
20.8 Hz, 2 H, P–CH2–P), 3.04 (m, 8 H, NCH2CH3) ppm. 31P NMR
[1] I. Shevchenko, V. Andruschko, E. Lork, G.-V. Röschenthaler,
Eur. J. Inorg. Chem. 2003, 54–56.
[2] I. Shevchenko, V. Andruschko, E. Lork, G.-V. Röschenthaler,
Eur. J. Inorg. Chem. 2002, 2985–2990.
2
2
(CDCl3): δ = 18.0 (d, JP,P = 5.6 Hz), 30.7 (d, JP,P = 5.6 Hz) ppm.
MS (CI, negative, NH3, 200 °C): m/z (%) 496 (90) [M]; (EI, 70 eV,
207 °C): m/z (%) 496 (10) [M+], 424 , (35) [M – Et2N]+. HRMS
[M+]: calcd. 495.98090; found 495.97677 (–8.3 ppm, –4.1 mmu);
calcd. for [M – Et2N]: 423.89957; found 423.89897 (–1.4 ppm,
–0.6 mmu).
[3] N. Dubau-Assibat, A. Baceiredo, G. Bertrand, J. Am. Chem.
Soc. 1996, 118, 5216–5220.
[4] F. Ramirez, A. S. Gulati, C. P. Smith, J. Am. Chem. Soc. 1967,
89, 6276–6282.
[5] O. I. Kolodiazhnyi, in Phosphorus Ylides, Wiley-VCH,
Weinheim, New York, Chichester, Brisbane, Singapore, To-
ronto, 1999.
[6] I. Kawamoto, T. Hata, Y. Kishida, C. Tamura, Tetrahedron
Lett. 1971, 12, 2417.
[7] H. J. Hecht, G. Ruban, Cryst. Struct. Commun. 1981, 495.
[8] H. Grützmacher, H. Pritzkow, Chem. Ber. 1989, 122, 1411–
1416.
[9] Y. Canac, S. Conejero, M. Soleilhavoup, B. Donnadieu, G. Ber-
trand, J. Am. Chem. Soc. 2006, 128, 459–464.
[10] G. M. Sheldrick, SHELX-97, University of Göttingen, Ger-
many, 1997.
X-ray Crystallographic Study: The single-crystal X-ray structure de-
termination was performed at 173(2) K on a Siemens P4 dif-
fractometer using graphite-monochromated Mo-Kα radiation (λ =
71.073 pm) and an LT2 low temperature device. The structure was
solved by direct methods and refined by full-matrix least-squares
on F2 using the SHELX-97 program system.[10]
Crystal data for 6 (C28H43Cl4F6N5O2P2): Mw = 799.46, triclinic,
¯
space group P1,
a = 1199.2(2), b = 1279.93(15), c =
1428.87(17) pm, α = 82.875(8)°, β = 89.202(12)°, γ = 85.148(11)°,
V = 2.1685(6) nm3, Z = 2, Dc = 1.484 Mgm–3, µ = 0.654 mm–1;
11563 reflections collected, 256 parameters refined using 9956
unique reflections (Rint = 0.0334) to final indices R1 [I Ͼ 2σ(I)] =
Received: June 20, 2006
Published Online: November 17, 2006
262
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 259–262