Ferrocenium salt induced ring expansion of a 2H-azaphosphirene complex: Synthesis of the first Δ3-1,4,2-oxazaphospholene complexes 1

Article abstract of DOI:10.1039/b004795n

Regioselective ring expansion of the 2H-azaphosphirene tungsten complex 1 in CH₂Cl₂ is performed using substoichiometric amounts of ferrocenium hexafluorophosphate and benzalaldehyde or cyclohexanone, thus yielding Δ³-l,4,2-oxazaphospholene complexes 2a,c; the molecular structure of complex 2c was established by a single crystal X-ray diffraction study. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b004795n

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Ferrocenium salt induced ring expansion of a 2H-azaphosphirene
complex: Synthesis of the first ꢀ3-1,4,2-oxazaphospholene
complexes†
Rainer Streubel,* Christoph Neumann and Peter G. Jones
Institut für Anorganische und Analytische Chemie der Technischen Universität Braunschweig,
Postfach 3329, 38023-Braunschweig, Germany. E-mail: r.streubel@tu-bs.de
Received 15th June 2000, Accepted 19th June 2000
Published on the Web 4th July 2000
Regioselective ring expansion of the 2H-azaphosphirene
tungsten complex 1 in CH2Cl2 is performed using sub-
stoichiometric amounts of ferrocenium hexafluoro-
phosphate and benzalaldehyde or cyclohexanone, thus
yielding ꢀ3-1,4,2-oxazaphospholene complexes 2a,c; the
molecular structure of complex 2c was established by a
single crystal X-ray diffraction study.
There is considerable current interest in the development of
sophisticated ligands including either sp2- and/or sp3-hybridised
phosphorus atoms in heterocyclic rings.1 In the last few years,
we have focussed our research on the synthesis of so-called
biomimetic P-heterocycles of the diazole- and triazole-type,
such as 2H-1,2-azaphospholes,2 2H-1,3,2-diazaphospholes3
and 2H-1,4,2-diazaphospholes.4 These were obtained as transi-
tion metal complexes using (i) selective P–C cleaving ring
Scheme 2 Reaction of complex 1 with FcϩPF6Ϫ, benzaldehyde and
expansions of the 2H-azaphosphirene ring system as a new syn-
thetic methodology (Scheme 1). Five-membered heterocycles of
cyclohexanone. Reagents and conditions: to a solution of 1 mmol of
complex 1, 2 mmol of benzaldehyde or cyclohexanone in 3 ml of
dichloromethane was added 0.2 mmol FcϩPF6Ϫ and stirred at ambient
temperature until 1 was consumed (31P NMR control); work-up by
column chromatography (SiO2) at low temperature and crystallization
from n-pentane yielded complexes 2a,c as pale yellow solids (2a: 45%,
mp 92 ЊC, decomp.; 2c: 44%, mp 140 ЊC, decomp.).
ca. 10% yield. If one equivalent of the ferrocenium salt was
added the reaction time decreased without affecting the selectiv-
ity of the reaction. In the case of benzaldehyde, 31P NMR
spectroscopic monitoring showed the formation of two reactive
intermediates (<5–10% of all phosphorus-containing products)
having resonances at δ 108 and 119. Because of the need for
Scheme 1 Ring expansion reactions of 2H-azaphosphirene complexes
᎐
(a᎐b denotes a π-system).
these or related types are especially interesting with regard to
the intrinsic electronic ambiguity of a tri-coordinate phos-
phorus centre, which depends significantly on the degree of
pyramidalisation.5 Recently, the first striking example of a
phosphole-type ring system was published, 1-[bis(trimethyl-
silyl)methyl]-3,5-bis(trimethylsilyl)-1,2,4-triphosphole,6 bearing
a phosphorus centre that is tri-coordinated and planar.
During our current study of oxidation and reduction reac-
tions of 2H-azaphosphirene metal complexes, we have observed
that such complexes undergo selective P–N cleaving ring expan-
Ϫ
FcϩPF6 in these reactions or other oxidizing agents such as
Cu() salts or elemental sulfur, in the absence of oxidizing
agents no reactions occurred at ambient temperature, we
propose that the ring expansion proceeds via electron transfer
catalysis,9 which, to the best of our knowledge, would be
unprecedented in the chemistry of phosphorus-containing
heterocycles.10 At higher temperature the reactions yielded
selectively 2H-1,3,2-oxazaphospholecomplexes via insertion of
the C–O π-system into the P–C bond.3b
The complexes 2a,c were isolated by low-temperature
chromatography and crystallisation; the constitutions of the
complexes 2a,c are unambigously established by their NMR
spectroscopic data‡ (13C, 31P) in solution and their MS
data.‡ Furthermore, the ring constitution of complex 2c was
confirmed by X-ray structure analysis (Fig. 1).§
Complexes 2a,c show resonances for the imino carbon atoms
at δ 173.6 and 169.7 with coupling constant magnitudes
|J(31P,13C)| of ca. 13–16 Hz, and for the sp3-hybridized carbon
atoms at δ 108.1 and 113.1 with coupling constant magnitudes
|J(31P,13C)| of ca. 4–7 Hz. 31P resonances are observed in a small
range [δ 135.3 (2a), 141.5 (2b) and 136.4 (2c)] with characteristic
coupling constants |J(183W, 31P)| of ca. 270–280 Hz. The molec-
ular structure of complex 2c (Fig. 1) confirms the constitution
of the heterocyclic ring system, which is almost planar (mean
sion reactions (ii) at ambient temperature with various
Ϫ
π-systems, when ferrocenium hexafluorophosphate (FcϩPF6
)
is present (Scheme 1). We now report the synthesis of the first
∆3-1,4,2-oxazaphospholene complexes under very mild condi-
tions using FcϩPF6Ϫ, benzaldehyde and cyclohexanone and a
2H-azaphosphirene tungsten complex.
The reaction of 2H-azaphosphirene tungsten complex 17
with 2 equivalents of either benzaldehyde or cyclohexanone
Ϫ 8
and 0.2 equivalents of FcϩPF6 in dichloromethane at ambi-
ent temperature afforded regioselectively the ∆3-1,4,2-oxaza-
phospholene complexes 2a (together with 2b, most probably a
diastereoisomer) and 2c (Scheme 2) in reasonable yields (45 and
44%, respectively); ferrocene was isolated in both reactions in
† Dedicated to Professor Herbert W. Roesky on the occasion of his
65th birthday.
DOI: 10.1039/b004795n
J. Chem. Soc., Dalton Trans., 2000, 2495–2496
This journal is © The Royal Society of Chemistry 2000
2495

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130.6 (s, Ph), 130.7 (s, Ph), 132.7 (d, JPC 25.2, Ph), 137.8 (d, JPC 3.5, Ph)
173.6 (d, JPC 13.3, PCN), 197.1 (d, 3JPC 7.4, cis-CO), 198.9 (d, 3JPC 28.2,
trans-CO); 31P NMR: δ 135.3 (s, 1JPW 280.2); m/z (EI) 723 (Mϩ, 28). 2c:
13C NMR: δ 1.9 [d, 3JPC 1.6, Si(CH3)3], 2.2 [d, 3JPC 2.6, Si(CH3)3], 21.8 (s,
CH2), 22.0 (s, CH2), 23.9 (s, CH2), 35.4 (d, 1JPC 13.3, CH), 36.1 (s, CH2),
2
36.3 (s, CH2), 113.1 (d, JPC 4.7 POC), 127.5 (s, Ph), 129.3 (s, Ph),
130.2 (s, Ph), 132.7 (d, JPC 25.2, Ph), 169.7 (d, JPC 15.4, PCN), 198.2
3
3
(d, JPC 7.2, cis-CO), 199.4 (d, JPC 26.3, trans-CO); 31P NMR: δ 136.4
(s, 1JPW 273.4); m/z (EI) 715 (Mϩ, 8).
§ Crystal structure analysis of 2cؒ0.5C5H12: empirical formula C27.5H40-
NO6PSi2W, M = 751.6; monoclinic, space group C2/c; a = 36.058(4),
b = 10.4819(12), c = 19.602(2) Å, β = 119.330(6)Њ, V = 6458.9(13) Å3,
Z = 8, Dc = 1.546 Mg mϪ3; λ = 0.71073 Å, T = 143(2) K. The crystal
(0.27 × 0.24 × 0.17 mm) was mounted in inert oil. 38692 intensities
were measured (ω and θ scans, 2θ 3–60Њ) using using Mo-Kα radiation
on a Bruker SMART 1000 CCD diffractometer. After absorption cor-
rection (multiple scans) 9462 were unique (Rint = 0.0454) and used for
all calculations (SHELXL-9714). All hydrogen atoms were refined with
a riding model or as rigid methyl groups. Final wR(F2) was 0.0589,
with conventional R(F) 0.0247, for 186 parameters and 351 restraints;
highest peak 1.479, hole Ϫ0.861 e ÅϪ3
CCDC reference number 186/2042.
See http://www.rsc.org/suppdata/dt/b0/b004795n/ for crystallo-
graphic files in .cif format.
.
Fig. 1 Molecular structure of complex 2c (ellipsoids represent 30%
probability levels; solvent and hydrogen atoms are omitted for clarity).
Selected bond lengths (Å) and angles (Њ): W–C1 2.011(3), W–P
2.5209(6), P–C8 1.821(2), P–O6 1.6309(17), P–C6 1.877(2), N–C6
1.273(3), N–C7 1.454(3), C7–O6 1.465(3); C8–P–W 117.45(8), O6–P–
C6 88.92(9), P–C6–N 111.51(17), C6–N–C7 115.21(9), N–C7–O6
108.02(18).
1 K. B. Dillon, F. Mathey and J. F. Nixon, Phosphorus: The Carbon
Copy, Wiley, Chichester, 1998, p. 203.
2 R. Streubel, H. Wilkens, A. Ostrowski, C. Neumann, F. Ruthe and
P. G. Jones, Angew. Chem., Int. Ed. Engl., 1997, 36, 1492; U. Rohde,
F. Ruthe, P. G. Jones and R. Streubel, Angew. Chem., Int. Ed., 1999,
38, 215; H. Wilkens, A. Ostrowski, J. Jeske, F. Ruthe, P. G. Jones and
R. Streubel, Organometallics, 1999, 18, 5627.
3 (a) H. Wilkens, J. Jeske, P. G. Jones and R. Streubel, Chem.
Commun., 1997, 2317; (b) H. Wilkens, F. Ruthe, P. G. Jones and
R. Streubel, Chem. Eur. J., 1998, 4, 1542.
deviation 0.042 Å) and shows, in comparison to {[2-bis(tri-
methylsilyl)methyl-5-phenylbenz[c]-1,2-oxaphospholane]penta-
carbonyltungsten(0)}11 3 and 1,1-diphenyl-3,3,5,5,8,8-hexakis-
(trifluoromethyl)-2,4,9-trioxa-7-aza-1λ5-phosphabicyclo[4.3.0]-
non-6-ene12 4, C–O and P–O bond distances [C7–O6 1.465(3)
and P–O6 1.6309(17) Å], which are very similar to those in 3
[C–O 1.463(4) and P–O 1.641(2)11 Å], but differ significantly
from those in 4 [C–O 1.374(3) and P–O 1.732(2)12 Å]; the C–N
double bond distances of 2c and 4 are also slightly different [2c:
N–C6 1.273(3) and 4: N–C 1.291(4)12 Å].
Currently, electrochemical and ESR studies to elucidate the
reaction mechanism are in progress, furthermore, we are
exploiting this new synthetic methodology by employing
nitriles instead of carbonyl derivatives.13
4 R. Streubel, H. Wilkens, F. Ruthe and P. G. Jones, Chem. Commun.,
1999, 2127.
5 For
a more recent work on aromaticity in phospholes and
polyphosphaphospholes, see: A. Dransfeld, L. Nyulászi and P. v. R.
Schleyer, Inorg. Chem., 1998, 37, 4413.
6 F. G. N. Cloke, P. B. Hitchcock, P. Hunnable, J. F. Nixon, L. Nyul-
ászi, E. Niecke and V. Thelen, Angew. Chem., Int. Ed., 1998, 37,
1083.
7 R. Streubel, U. Rohde, J. Jeske, F. Ruthe and P. G. Jones, Eur. J. Inorg.
Chem., 1998, 2005.
8 N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877.
9 Reviews: H. Taube, J. Chem. Ed., 1968, 45, 452; M. Chanon,
Acc. Chem. Res., 1987, 20, 214; R. A. Marcus, Angew. Chem.,
Int. Ed. Engl., 1993, 32, 1111; D. Astruc, Electron-Transfer Processes
in Transition Metal Chemistry, VCH Publishers, New York, 1995.
10 For ET-catalyzed ring expansions of oxiranes and aziridines in
organic synthesis, see: L. Eberson, Electron Transfer Reactions in
Organic Chemistry, Springer Verlag, Berlin, 1987.
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for financial support and
Mr Andreas Weinkauf for the crystallographic measurement.
11 R. Streubel, A. Ostrowski, H. Wilkens, F. Ruthe, J. Jeske and
P. G. Jones, Angew. Chem., Int. Ed. Engl., 1997, 36, 378.
12 H. W. Roesky, V. W. Pogatzki, K. S. Dhathathreyan, A. Thiel,
H. G. Schmidt, M. Dyrbusch, M. Noltemeyer and G. M. Sheldrick,
Chem. Ber., 1986, 119, 2687.
13 R. Streubel, H. Wilkens, C. Neumann, F. Ruthe and P. G. Jones,
unpublished work.
Notes and references
‡ Satisfactory elemental analyses were obtained for complexes 2a,c.
NMR data were recorded in CDCl3 solutions at 50.3 MHz (13C) and
81.0 MHz (31P), using SiMe4 and 85% H3PO4 as standard references;
J/Hz. Selected spectroscopic data for 2a (Х8:1 mixture of 2a,b):
3
3
13C NMR: δ 2.9 [d, JPC 1.9, Si(CH3)3], 3.3 [d, JPC 2.8, Si(CH3)3], 33.3
14 G. M. Sheldrick, SHELXL-97, program for crystal structure
refinement, Universität Göttingen, 1997.
(d, 1JPC 16.7, CH), 108.1 (d, 2JPC 6.3 POC), 126.5 (s, Ph), 128.6 (s, Ph),
2496
J. Chem. Soc., Dalton Trans., 2000, 2495–2496