Novel cofacial oxidative coupling reaction of phosphinine in the presence of Cu(I) and ClO4-

Article abstract of DOI:10.1039/b308892h

2,4,6-Triphenylphosphinine (TPP) underwent unprecedented cofacial oxidative coupling to form a novel C₂-symmetric cage compound, having extremely long C-C bonds.

Full text of DOI:10.1039/b308892h

Novel cofacial oxidative coupling reaction of phosphinine in the presence
2
of Cu(
I
) and ClO4 †
Takahiko Kojima,* Yoshitaka Ishioka and Yoshihisa Matsuda*
Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashi-Ku, Fukuoka
812-8581, Japan. E-mail: cosyscc@mbox.nc.kyushu-u.ac.jp (T. K.); Fax: 81 92 642 2570; Tel: 81 92 642
4179
Received (in Cambridge, UK) 29th July 2003, Accepted 12th December 2003
First published as an Advance Article on the web 16th January 2004
2,4,6-Triphenylphosphinine (TPP) underwent unprecedented
cofacial oxidative coupling to form a novel C2-symmetric cage
compound, having extremely long C–C bonds.
obliquely (3–5). The two phosphorus atoms are oxidized and
connected together with a m-oxo linkage. The molecule has a C2
axis, which penetrates the bridging oxygen (O2) and the middle of
the C2–C25 bond. Interestingly, all the eight coupled positions (P1,
P2, C1, C2, C5, C24, C25 and C28) are stereogenic. All the
stereogenic centres in one molecule are in the R configuration and
are in the S configuration in the other molecule, indicating the
coupling process proceeds with complete stereoselectivity. The
bond lengths of newly formed C–C bonds are 1.611(5) Å for C1–
C28, 1.650(5) Å for C5–C24 and 1.545(5) Å for C2–C25. The first
of these two bonds are extremely long for C–C single bonds and are
comparable to those of highly distorted prismane-like compounds
having C–C bonds longer than 1.60 Å.11
Phosphinine is a heteroaromatic compound containing a phospho-
rus atom, which was first synthesized by Märkl.1 Its chemistry has
been strongly extended by the synthesis of a variety of phosphinine
families,2 including the preparation of their organometallic com-
plexes3 by Le Floch and Mathey. A remarkable property of
phosphinines is that they have a low-lying p* orbital as the LUMO
which has a large coefficient at the phosphorus atom. This property
renders the phosphinine a strong p-acceptor to undergo p-back
bonding from a metal centre. Recently, Rh( )–phosphinine com-
I
plexes have been applied to catalytic hydroformylation of olefins.4
Thus, the chemistry of phosphinine is now developing in several
directions, allowing us to access a new frontier of organometallic
chemistry and catalysis. In order to consider the reactivity and
properties of phosphinine–metal complexes, it is necessary to gain
more insights into the reactions of phosphinines with metal ions.
The first phosphinine synthesized was 2,4,6-triphenylphosphi-
nine (TPP)1 and its transition metal complexes have been reported.
1
6
A variety of metal ions form h -,5 h -,6 and m2-TPP7 complexes,
including Cu(
I
).8 TPP has been known to undergo one-electron
oxidation to form a phosphinine cation radical, which is stable at
room temperature.9 Also, TPP can be reduced to form an anion
radical.10 Here, we present the first report of a novel oxidative
coupling of two phosphinines in the presence of Cu(
) and ClO42 to
I
form a stable phosphorus-containing cage compound of a dialkyl-
phosphoric anhydride with extremely long C–C single bonds.
The reaction of TPP with [Cu(CH3CN)4]ClO4 in CH2Cl2 at room
temperature under N2 gave the bis-TPP complex, [Cu(TPP)2]ClO4,
in good yield. Recrystallization from CH2Cl2–hexane under air at
room temperature ( > 25 °C) gave yellow block crystals of 1,
suitable for X-ray crystallography.‡ Compound 1 also can be
obtained in good yield by refluxing [Cu(TPP)2]ClO4 in CH2Cl2
under air. The reactions are summarized in Scheme 1. The crystal
structure of 1 was determined by X-ray crystallography and it
revealed that a pair of enantiomers exists in an asymmetric unit
(Fig. S1†). An ORTEP drawing of the R-isomer and its core
structure are depicted in Fig. 1 and selected bond lengths and angles
are listed in the figure caption.
Two TPP molecules are coupled together at the two 2- and
6-positions (2–2 and 6–6) and between the 3 and 5 positions
Fig. 1 (a) An ORTEP drawing of the R-isomer of 1 (R-1) with 50%
probability thermal ellipsoids. (b) The core structure of R-1. The blue-lined
scaffold represents the original TPP moiety and the newly formed C–C
bonds are shown as red lines. Selected bond lengths (Å) and angles (°): P1–
O1 1.464(3), P1–O2 1.630(3), P1–C1 1.895(4), P1–C5 1.841(4), P2–O2
1.621(3), P2–O3 1.465(3), P2–C24 1.884(4), P2–C28 1.824(4), C1–C28
1.611(5), C5–C24 1.650(5), C2–C25 1.545(5), C3–C4 1.328(6), C26–C27
1.339(5); O1–P1–O2 114.2(2), O2–P2–O3 113.6(2), P1–O1–P2,
103.7(1).
Scheme 1 Summary of reactions of TPP including the formation of 1.
† Electronic supplementary information (ESI) available: Synthetic route to
1, NMR data of 1, ORTEP drawing of the asymmetric unit. See http://
www.rsc.org/suppdata/cc/b3/b308892h/
366
C h e m . C o m m u n . , 2 0 0 4 , 3 6 6 – 3 6 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

In the 1H NMR spectrum of 1 in CDCl3 (see Fig. S2†), a
characteristic AMX quartet was observed at 3.35 ppm (3JHH = 15
phosphinines and have established its structure unambiguously by
X-ray crystallography. Our results described here represent the
discovery of a new reactivity of phosphinines and pave the way for
other novel phosphorus-containing cage compounds to be synthe-
sized and discovered.
3
Hz and JPH = 11 Hz), which was assigned to the 3(5)-H at the
linked position.12
The formation of 1 was governed by Cu(
2
I
) and ClO4 and
moderate temperature. The requirement for perchlorate anion was
confirmed by observing no reaction in the case of [Cu(TPP)2]X (X
We thank Prof. T. Shinmyozu (IFOC, Kyushu University) for his
helpful discussion and the generous use of his diffractometer, and
Dr M. Suenaga (Dept. of Chem., Kyushu University) for his help
with NMR spectroscopy.
= BF4, PF6). The requirement for Cu(
I
) was also indicated because
) and in the presence of
). In addition, the reaction
TPP remained intact in the absence of Cu(
I
2
ClO4 (n-Bu4NClO4) without Cu(
I
temperature was revealed to be critical for the formation of 1. In
CH2Cl2, [Cu(TPP)2]ClO4 decomposed to form free TPP+· at lower
temperatures ( < 20 °C), which was detected by ESR to show a
doublet (AP = 23 G at g = 2.0020), consistent with observations
reported by Dimroth et al.9b We estimate the spin density at the
phosphorus atom to be 11.7% based on the hyperfine coupling
constant. In contrast, refluxing a CH2Cl2 solution of [Cu(TPP-
Notes and references
‡ Crystal data: 1, C92H68O6P4, M
monoclinic, space group P21/n, a = 23.4513(5), b = 13.4607(2), c =
24.5485(5) Å, b = 116.1286(4)°, V = 6957.3(2) Å3, Z = 4, R1 = 0.072 (I
> 2s(I)), Rw = 0.185 (all data), GOF = 1.02. CCDC 207184. See http://
www.rsc.org/suppdata/cc/b3/b308892h/ for crystallographic data in CIF or
other electronic format.
= 1393.44, T = 2160(1) °C,
)2]ClO4 gave 1 in moderate yield. This clearly indicates that the
2
oxidation of Cu(
I
)-coordinated TPP by ClO4 is the first step to
form TPP+· followed by the rate-limiting coupling reaction. The
1 (a) G. Märkl, Angew. Chem., Int. Ed. Engl., 1966, 5, 846; (b) G. Märkl,
F. Lieb and A. Merz, Angew. Chem., Int. Ed. Engl., 1967, 6, 458; (c) G.
Märkl, F. Lieb and A. Merz, Angew. Chem., Int. Ed. Engl., 1967, 6,
944.
2 (a) A. J. Ashe III, J. Am. Chem. Soc., 1971, 93, 3293; (b) J. M. Alcaraz,
A. Breque and F. Mathey, Tetrahedron Lett., 1982, 23, 1565; (c) P. Le
Floch, L. Ricard and F. Mathey, Bull. Chem. Soc. Fr., 1994, 131, 330;
(d) N. Avarvari, N. Mézailles, L. Ricard, P. Le Floch and F. Mathey,
Science, 1998, 280, 1587.
3 P. Le Floch and F. Mathey, Coord. Chem. Rev., 1998, 179–180, 771, and
references cited therein.
4 B. Breit, R. Winde, T. Mackewitz, R. Paciello and K. Harms, Chem.
Eur. J., 2001, 7, 3106.
5 H. Vahrenkamp and H. Nöth, Chem. Ber., 1973, 106, 2227.
6 (a) H. Vahrenkamp and H. Nöth, Chem. Ber., 1972, 105, 1148; (b) F.
Nief, C. Charrier, F. Mathey and M. Simalty, J. Organomet. Chem.,
1980, 187, 277.
7 M. T. Reetz, R. Bohres, R. Goddard, M. C. Holthausen and W. Thiel,
Chem. Eur. J., 1999, 5, 2101–2108.
8 M. Fraser, D. G. Holah, A. N. Hughes and B. C. Hui, J. Heterocycl.
Chem., 1972, 9, 1457.
9 (a) K. Dimroth and F. W. Steuber, Angew. Chem., Int. Ed. Engl., 1967,
6, 445; (b) K. Dimroth, G. W. Städe and F. W. Steuber, Angew. Chem.,
Int. Ed. Engl., 1967, 6, 711.
stoichiometry of coupling is controlled by the number of TPP
ligands coordinated to the Cu( ) centre, therefore, no other coupling
I
products are observed. This also suggests that the coupling reaction
takes place in an intramolecular manner, and not in an inter-
molecular manner.
It has been reported that TPP undergoes [4+2] cycloaddition with
hexafluoro-2-butyne to form a 1,4-addition product.13 Actually,
DFT calculations for TPP indicated that HOMO mainly populates
at the phosphorus and the 4-position.14 However, in the present
case, no 1,4-addition product was observed and the phosphorus
atom was oxygenated instead. Our observation of radical formation
from the Cu(
can be oxidized more easily than the free form due to p-back
donation from the Cu( ) centre, which renders the TPP more
I)-precursor complex suggests that coordinated TPP
I
electron-rich. This argument is supported by the large negative shift
of oxidation potentials of TPP (0.88 V vs. Ag/AgNO3) compared to
that of [Cu(TPP)2]ClO4 (0.66 V),15 and a large upfield coordination
shift from 183.0 ppm for TPP to 144.4 ppm for [Cu(TPP)2]ClO4
(Dd = 238.6 ppm).16 In addition to the p-back donation, the role
of Cu( ) is assumed to converge the reactants at a close distance to
I
facilitate the two TPP molecules to react with each other. The
oxidation reduces the HOMO–LUMO gap between TPP and TPP+·
to couple together. Recently, Mathey and coworkers have reported
radical coupling reactions to give P–P coupled products of a
macrocyclic phosphinine via the formation of an anion radical.17
However, in our case, such P–P coupling was not observed and
10 K. Dimroth, G. W. Städe, F. W. Steuber, W. Sauer and L. Duttka,
Angew. Chem., Int. Ed. Engl., 1967, 6, 85.
11 (a) C. Lim, M. Yasutake and T. Shinmyozu, Tetrahedron Lett., 1999,
40, 6781; (b) K. Matohara, C. Lim, M. Yasutake, R. Nogita, T. Koga, Y.
Sakamoto and T. Shinmyozu, Tetrahedron Lett., 2000, 41, 6803; (c) C.
Lim, M. Yasutake and T. Shinmyozu, Angew. Chem., Int. Ed., 2000, 39,
578; (d) R. Nogita and T. Shinmyozu, Hikarikagaku, 2002, 33, 106.
12 NMR spectra and assignments are given as Figs. S2–S6 in the ESI†.
13 G. Märkl and F. Lieb, Angew. Chem., Int. Ed. Engl., 1968, 7, 733.
14 G. Frison, A. Sevin, N. Avarvari, F. Mathey and P. Le Floch, J. Org.
Chem., 1999, 64, 5524.
such reactions should be prevented by the coordination to Cu( )
I
through the phosphorus atoms and also by the steric hindrance of
two 2- and 6-phenyl groups. We note the report of an intra-
molecular and ionic 5s + 5s [6+4] cycloaddition to form a similar
scaffold by the reaction of phosphinine derivatives with diazoalk-
anes without oxidation of phosphorus centres.18 In sharp contrast to
1, the compounds referred to as diphosphachiropteradienes19
undergo decomposition to give the corresponding phosphinines and
more complicated coupling products.18 The two phosphorus atoms
in 1 are irreversibly oxidized to form the phosphoric anhydride
linkage, which can contribute to the stabilization of 1. The
15 Cyclic voltammograms were recorded in CH2Cl2 (0.1 M [n-Bu4N]-
ClO4) under N2 at room temperature. The oxidation waves were
irreversible at a scan rate of 50–500 mV s21
.
16 31P{1H} NMR data (d/ppm in CDCl3): 145.7 (s) ([Cu(TPP)2]PF6) and
144.4 (s) ([Cu(TPP)2]ClO4).
17 L. Cataldo, S. Choua, T. Berclaz, M. Geoffroy, N. Mézailles, L. Ricard,
F. Mathey and P. Le Floch, J. Am. Chem. Soc., 2001, 123, 6654.
18 G. Märkl, H. J. Beckh, K. K. Mayer, M. L. Ziegler and T. Zahn, Angew.
Chem., Int. Ed. Engl., 1987, 26, 236.
19 The compound reported in ref. 18, in sharp contrast to 1, exhibits normal
C–C bond lengths of 2–2 and 6–6 bondings (1.567 Å), but a longer C–C
bond length (1.627 Å) for the 3–5 linkage. These data were obtained
from a Chem 3D model based on the CSD data (CSD-52330).
participation of Cu(
) ion and ClO42, and the stability of 1,
I
however, make the reaction described here different from those
reported by Märkl and coworkers. Further investigation on the
reaction mechanism is underway.
In conclusion, we have succeeded in synthesizing and isolating
an unprecedented Cu( )-assisted oxidative coupling product of
I
C h e m . C o m m u n . , 2 0 0 4 , 3 6 6 – 3 6 7
367