Pentacoordinate 1H-phosphirenes: Reactivity, bonding properties, and substituent effects on their structures and thermal stability

Article abstract of DOI:10.1021/jo060358z

The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted square pyramidal (SP) arrangement around the phospho

Full text of DOI:10.1021/jo060358z

Pentacoordinate 1H-Phosphirenes: Reactivity, Bonding Properties,
and Substituent Effects on Their Structures and Thermal Stability
Shohei Sase, Naokazu Kano, and Takayuki Kawashima*
Department of Chemistry, Faculty of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
takayuki@chem.s.u-tokyo.ac.jp
ReceiVed February 21, 2006
The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-
phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted
square pyramidal (SP) arrangement around the phosphorus. NMR studies confirm that they retain the SP
structure in solution and demonstrate that the endocyclic P-C bonds in the three-membered ring have a
very high degree of p character, which results from their being both basal bonds in the SP structure and
endocyclic bonds of the three-membered ring. Structural parameters of the three-membered ring of the
pentacoordinate phosphirenes obtained by experiment and theoretical calculations are very close to those
of a tetracoordinate phosphirenium cation. Thus, by analogy with tetracoordinate phosphirenium cations,
it can be considered that a σ*-π interaction between the σ* orbital of the apical bond and the π orbital
of the CdC bond in the three-membered ring is operative in pentacoordinate phosphirenes. The σ*-π
interaction is found to lower the reactivity of the CdC bond of the three-membered ring. The reactivities
of the pentacoordinate phosphirenes are also affected by the substituent on the carbon atom in the three-
membered ring.
1. Introduction
tional heteroatom in the ring, have been intensively studied.
Many examples of such three-membered ring compounds
bearing a tri- or tetracoordinate phosphorus atom have been
reported, and their structures and reactivities have been eluci-
dated.4,5 However, reports on three-membered ring compounds
bearing a pentacoordinate phosphorus atom are limited.6
Pentacoordinate phosphorus compounds are among the most
important hypervalent species, being intermediates of various
reactions such as the Wittig reaction,7 transphosphorylation in
biological systems,8 and so on. The typical structures of
pentacoordinate phosphorus compounds are trigonal bipyramidal
(TBP) and square pyramidal (SP).9,10 A phosphorane with TBP
Three-membered ring compounds have been studied for a
long time both experimentally and theoretically because of their
structural features and high strain energy.1 Much attention has
been paid to three-membered heterocycles,2 and the investigation
of such compounds showed that the electronic properties of the
molecule are influenced by the bond angle around the hetero-
atom.3 Among them, three-membered ring compounds involving
one phosphorus atom, as well as those containing another addi-
* Corresponding author. Fax: (+81) 3-5800-6899.
(1) (a) Liebman, J. F.; Greenberg, A. Chem. ReV. 1976, 76, 311. (b) de
Meijere, A. Angew. Chem., Int. Ed. Engl. 1979, 18, 809. (c) Liebman, J.
F.; Greenberg, A. Chem. ReV. 1989, 89, 1225.
(4) For reviews on phosphiranes and phosphirenes, see: (a) Mathey, F.
Chem. ReV. 1990, 90, 997. (b) Mathey, F.; Regitz, M. In ComprehensiVe
Heterocyclic Chemistry II; Katritzky, A. R., Rees, C. W., Scriven, E. F.,
Eds.; Pergamon: Amsterdam, 1996; Vol. 1A, pp 277-304. (c) Mathey,
F.; Regitz, M. In Phosphorus-Carbon Heterocyclic Chemistry: The Rise
of a New Domain; Mathey, F., Ed.; Pergamon: Amsterdam, 2001, pp 17-
55. (d) Heydt, H. Sci. Synth. 2002, 9, 85.
(2) For a review of three-membered heterocycles, see: ComprehensiVe
Heterocyclic Chemistry II; Katritzky, A. R., Rees, C. W., Scriven, E. F.,
Eds.; Pergamon: Oxford, 1996; Vol. 1A.
(3) Aue, D. H.; Webb, H. M.; Davidson, W. R.; Vidal, M.; Bowers, M.
T.; Goldwhite, H.; Vertal, L. E.; Douglas, J. E.; Kollman, P. A.; Kenyon,
G. L. J. Am. Chem. Soc. 1980, 102, 5151.
10.1021/jo060358z CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/22/2006
5448
J. Org. Chem. 2006, 71, 5448-5456

Pentacoordinate 1H-Phosphirenes
CHART 1. Three-Membered Ring Compounds Bearing a
Pentacoordinate Phosphorus Atom with TBP Structure
(Left) and SP Structure (Right)a
CHART 2. Pentacoordinate Chalcogenaphosphiranes 1a
and 1b and Pentacoordinate Phosphirenes 2a-e
a La ) apical ligand; Le ) equatorial ligand; Lb ) basal ligand; E )
main group element.
geometry has three equatorial bonds and two apical bonds
around the phosphorus atom, whereas a phosphorane with SP
geometry has one apical bond and four basal bonds. In both
geometries, the phosphoranes have electron-deficient multi-
center bonds that have a high degree of p character and are
highly polarized; the apical bonds in the TBP structure involve
three-center four-electron (3c-4e) bonding, and the basal bonds
in the SP structure involve five-center six-electron (5c-6e)
bonding.
If three-membered ring compounds bearing a pentacoordinate
phosphorus atom have a structure similar to the ideal TBP
structure, one of the endocyclic bonds will form the apical bond,
as theoretical calculations have predicted (Chart 1).11 If it has
a structure similar to the ideal SP structure, two ring bonds will
form the basal bonds. In both cases, at least one ring bond in
the three-membered ring will show electron-deficient multicenter
bonding that has a high degree of p character and is highly
polarized. Furthermore, considering that endocyclic bonds of a
three-membered ring generally have a high degree of p
character,12 the endocyclic bonds of the three-membered ring
bearing a pentacoordinate phosphorus atom should have a very
high degree of p character. Such a three-membered ring
compound is an attractive synthetic target in organophosphorus
chemistry, because it is expected to show interesting properties.
We have been interested in the chemistry of three-membered
ring compounds bearing a pentacoordinate phosphorus atom.6e,g
We have recently reported the syntheses of pentacoordinate
thiaphosphirane 1a and selenaphosphirane 1b, shown in Chart
2, by taking advantage of the Martin ligand and have elucidated
their highly distorted TBP structures in the crystalline state,
solvent-dependent change of polarization of the phosphorus-
chalcogen bonds in solution, and some reactivities. Before our
investigations,NMRobservationofapentacoordinatephosphirane6b
and syntheses of pentacoordinate azaphosphirenes6a and
thiaphosphiranes6d had been described, but the properties of the
three-membered rings were still unknown. In addition, Regitz
and co-workers reported in 1990 the synthesis of pentacoordinate
1H-phosphirenes 2a-e and the X-ray structure of 2a.6c To
understand the properties of three-membered ring compounds
bearing a pentacoordinate phosphorus atom, it is significant to
elucidate the reactivities of pentacoordinate phosphirenes.
Meanwhile, Ho and co-workers have recently reported that a
pentacoordinate dioxaphosphirane, which was generated by the
reaction of a phosphine with singlet oxygen at low temperature,
can undergo nonradical oxygen atom-transfer reactions with
olefins.6f
(5) For tri- and tetracoordinate heteraphosphiranes, see: (a) Niecke, E.;
Wilbredt, D. A. J. Chem. Soc., Chem. Commun. 1981, 72. (b) Caira, M.;
Neilson, R. H.; Watson, W. H.; Wisian-Neilson, P.; Xie, Z. M. J. Chem.
Soc., Chem. Commun. 1984, 698. (c) Niecke, E.; Boeske, J.; Krebs, B.;
Dartmann, M. Chem. Ber. 1985, 118, 3227. (d) Niecke, E.; Symalla, E.
Chimia 1985, 39, 320. (e) Xie, Z. M.; Wisian-Neilson, P.; Neilson, R. H.
Organometallics 1985, 4, 339. (f) Appel, R.; Casser, C. Chem. Ber. 1985,
118, 3419. (g) Bauer, S.; Marinetti, A.; Ricard, L.; Mathey, F. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1166. (h) Driess, M.; Pritzkow, H. Angew.
Chem., Int. Ed. Engl. 1992, 31, 751. (i) Ma¨rkl, G.; Ho¨lzl, W.; Kallmuenzer,
A.; Ziegler, M. L.; Nuber, B. Tetrahedron Lett. 1992, 33, 4421. (j) Streubel,
R.; Kusenberg, A.; Jeske, J.; Jones, P. G. Angew. Chem., Int. Ed. Engl.
1994, 33, 2427. (k) Toyota, K.; Takahashi, H.; Shimura, K.; Yoshifuji, M.
Bull. Chem. Soc. Jpn. 1996, 69, 141. (l) Ruf, S. G.; Dietz, J.; Regitz, M.
Tetrahedron 2000, 56, 6259. (m) Vlaar, M. J. M.; Ehlers, A. W.; de Kanter,
F. J. J.; Schakel, M.; Spek, A. L.; Lutz, M.; Sigal, N.; Apeloig, Y.;
Lammertsma, K. Angew. Chem., Int. Ed. 2000, 39, 4127.
(6) For three-membered ring compounds bearing a pentacoordinate
phosphorus atom, see: (a) Burger, K.; Fehn, J.; Thenn, W. Angew. Chem.,
Int. Ed. Engl. 1973, 12, 502. (b) Campbell, B. C.; Denney, D. B.; Denney,
D. Z.; Shih, L. S. J. Chem. Soc., Chem. Commun. 1978, 854. (c) Ehle, M.;
Wagner, O.; Bergstra¨sser, U.; Regitz, M. Tetrahedron Lett. 1990, 31, 3429.
(d) Abdou, W. M.; Yakout, E.-S. M. A. Tetrahedron 1993, 49, 6411. (e)
Sase, S.; Kano, N.; Kawashima, T. J. Am. Chem. Soc. 2002, 124, 9706. (f)
Ho, D. G.; Gao, R.; Celaje, J.; Chung, H.-Y.; Selke, M. Science 2003, 302,
259. (g) Sase, S.; Kano, N.; Kawashima, T. Chem. Lett. 2004, 33, 1434.
(7) (a) Maryanoff, B. E.; Reitz, A. B. Chem. ReV. 1989, 89, 863. (b)
Vedejs, E.; Peterson, M. J. Top. Stereochem. 1994, 21, 1.
We report here some reactivities of pentacoordinate 1H-
phosphirenes bearing a tetrachlorocatecholate ligand and the
substituent effects on the structure and thermal stability obtained
by experiments and theoretical calculations to elucidate the
bonding properties of the three-membered ring.
2. Results and Discussion
2.1. Synthesis of Pentacoordinate Phosphirenes. Regitz et
al. succeeded in synthesizing 2a-e, which have halogen or
pseudohalogen ligands, such as the azido group and the cyano
group, on the phosphorus atom.6c Considering that a P-C bond
is less reactive against substitution reactions than the P-X
bonds, pentacoordinate phosphirenes that have a P-C bond in
place of the P-X bond seem to be better for investigation of
the properties of a phosphirene moiety. In this context, we chose
a phenyl group as a ligand on the phosphorus atom.
Pentacoordinate phosphirenes bearing a phenyl group on the
phosphorus atom were prepared according to the method
reported by Regitz and co-workers.6c,13 Treatment of tricoor-
dinate phosphirenes 3a and 3b14 with 1 equiv of o-chloranil in
toluene at room temperature afforded pentacoordinate phos-
phirenes 4a and 4b in 58 and 52% yields, respectively, as pale
orange solids (Scheme 1). The 31P NMR chemical shifts of 4a
(8) Hengge, A. C. Acc. Chem. Res. 2002, 35, 105 and references therein.
(9) (a) Holmes, R. R.; Deiters, J. A. J. Am. Chem. Soc. 1977, 99, 3318.
(b) Holmes, R. R. Acc. Chem. Res. 1979, 12, 257.
(10) Hoffmann, R.; Howell, J. M.; Muetterties, E. L. J. Am. Chem. Soc.
1972, 94, 3047.
(11) (a) Chang, J.-W.; Taira, K.; Urano, S.; Gorenstein, D. G. Tetrahedron
1987, 43, 3863. (b) Ikeda, H.; Inagaki, S. J. Phys. Chem. A 2001, 105,
10711.
(13) Osman, F. H.; El-Samahy, F. A. Chem. ReV. 2002, 102, 629.
(14) Me´zailles, N.; Avarvari, N.; Bourissou, D.; Mathey, F.; Le Floch,
P. Organometallics 1998, 17, 2677.
(12) Weigert, F. J.; Roberts, J. D. J. Am. Chem. Soc. 1967, 89, 5962.
J. Org. Chem, Vol. 71, No. 15, 2006 5449

Sase et al.
TABLE 1. Selected Bond Lengths (Å) and Angles (deg) for the
X-ray Structures of Phosphirenes 3a, 4a, 2a, and 6
SCHEME 1. Synthesis of 4a and 4b
3aa
4a
2ab
6c
P1-C1
1.820(2)
1.821(3)
1.299(3)
1.842(2)
41.8(1)
1.740(2)
1.7343(19)
1.357(9)
1.796(2)
45.97(9)
89.53(7)
66.79(11)
67.24(12)
112.56(9)
115.20(9)
103.90(8)
100.53(8)
1.709(4)
1.715(4)
1.360(6)
1.784(4)
46.8(2)
90.3(1)
66.9
1.732(11)
1.730(12)
1.36(2)
1.799(10)
46.1(5)
P1-C2
C1dC2
P1-C3
C1-P1-C2
O1-P1-O2
P1-C1-C2
P1-C2-C1
C3-P1-C1
C3-P1-C2
C3-P1-O1
C3-P1-O2
69.1(1)
69.1(1)
104.4(1)
104.4(1)
66.9(7)
67.0(7)
66.3
112.3(2)
108.2(2)
97.3(2)
102.0(2)
a Reference 15. b Reference 6c. c Reference 16.
CHART 3. Tri, Tetra-, and Pentacoordinate Phosphirenes
FIGURE 1. Crystal structure of 4a.
and 4b, which are close to those of 2a-e, support the formation
of the desired products.
4a suggest that apical ligands on the phosphorus do not affect
the structures of the three-membered ring so much. The
O1-P1-O2, C3-P1-O1, and C3-P1-O2 angles of 2a and
4a are relatively close to those of the ideal SP structure,9,17
whereas the angles between the apical bond and the basal bonds
in the three-membered rings are somewhat wider than the value
of 105° for the ideal SP structure. This deviation seems to be
characteristic of a phosphorane involving a strained ring around
the phosphorus.18
As for tricoordinate 3a and pentacoordinate 4a, there are
several differences in the structural parameters. The exocyclic
P1-C3 bond length of 4a is shorter than that of 3a, which can
be explained by a high degree of s character of the apical bond
in the SP geometry. Also, shortening of the endocyclic P1-C1
and P1-C2 bonds, elongation of the C1dC2 bond, and
enlargement of the C1-P1-C2 angle were observed in 4a as
compared with 3a. In contrast, the structural parameters of the
three-membered ring of 4a have several similarities to those of
tetracoordinate 6. Although the ligands attached to the phos-
phorus are different between 4a and 6, these trends in the
Phosphirene 4a was relatively thermally stable, whereas 4c
bearing two trimethylsilyl groups on the three-membered ring
was thermally unstable. When 3c14 was treated with o-chloranil
in toluene at 0 °C, a signal at δP -92.7 was observed in the 31
P
NMR spectrum, indicating the formation of 4c. However, its
isolation failed, and phosphorane 5 bearing two tetrachlorocat-
echolate ligands on the phosphorus was obtained in 21% yield.
These results indicate that the stability of the pentacoordinate
phosphirenes is influenced by substituents on the endocyclic
carbon atoms and is reduced by attachment of electron-donating
groups to the carbon atoms.
2.2. X-ray Crystallographic Analysis of 4a. The crystal
structure of 4a was established by X-ray crystallographic
analysis. The selected bond lengths and angles of 4a together
with tricoordinate 3a,15 tetracoordinate 6 (Chart 3),16 and
pentacoordinate 2a6c are summarized in Table 1. Phosphirene
4a has a highly distorted SP arrangement at the phosphorus with
two oxygen atoms of the tetrachlorocatecholate ligand and two
carbon atoms in the three-membered ring at the basal posi-
tions and with the ipso-carbon atom of the phenyl group at the
apical position, as shown in Figure 1. The endocyclic P-C
and C1dC2 bond lengths in 4a are close to those in 2a.
The C1-P1-C2 bond angle of 4a is also similar to that in 2a.
These similarities of the structural parameters between 2a and
(17) For X-ray crystallographic analysis on phosphoranes with nearly
SP arrangement, see: (a) Sarma, R.; Ramirez, F.; Marecek, J. F. J. Org.
Chem. 1976, 41, 473. (b) Clark, T. E.; Day, R. O.; Holmes, R. R. Inorg.
Chem. 1979, 18, 1668. (c) Day, R. O.; Sau, A. C.; Holmes, R. R. J. Am.
Chem. Soc. 1979, 101, 3790.
(18) For X-ray crystallographic analysis on four-membered ring com-
pounds bearing a pentacoordinate phosphorus atom, see: (a) Howard, J.;
Russell, D. R.; Trippett, S. J. Chem. Soc., Chem. Commun. 1973, 856. (b)
Althoff, W.; Day, R. O.; Brown, R. K.; Holmes, R. R. Inorg. Chem. 1978,
17, 3265.
(15) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Chem. Soc.,
Chem. Commun. 1984, 45.
(16) Vural, J. M.; Weissman, S. A.; Baxter, S. G.; Cowley, A. H.; Nunn,
C. M. J. Chem. Soc., Chem. Commun. 1988, 462.
5450 J. Org. Chem., Vol. 71, No. 15, 2006

Pentacoordinate 1H-Phosphirenes
CHART 4. Model Compounds of Phosphirenes for
Theoretical Calculations
FIGURE 2. σ*-π interaction in (a) phosphirenium cations and (b)
pentacoordinate phosphirenes.
TABLE 2. Selected Bond Lengths (Å) and Angles (deg) for Model
Compounds of Phosphirenes Optimized at B3LYP/6-31G(d)
SCHEME 2. Electron Transfer in Pentacoordinate
Phosphirene Reported by Regitz and Co-Workers
7a
7b
7c
8
9
(X ) H) (X ) H) (X ) CN) (X ) H) (X ) H)
P1-C1
1.762
1.762
1.350
1.403
45.0
1.739
1.795
1.373
1.402
45.0
1.753
1.751
1.351
1.796
45.4
1.855
1.855
1.297
1.444
40.9
1.764
1.764
1.318
1.400
43.9
P1-C2
C1dC2
P1-X
C1-P1-C2
P1-C1-C2
P1-C2-C1
The calculated bond angles in the three-membered rings of
tricoordinate 8 and pentacoordinate 7a are close to the crystal-
lographically determined bond angles of 3a and 4a, respectively,
while the calculated bond lengths of the endocyclic P-C bonds
are slightly longer by about 0.03 Å than the experimental values.
In the theoretical calculations of 7a and 8, however, a shortening
of the endocyclic P-C bonds, elongation of the CdC bond,
and enlargement of the C-P-C angle were observed in 7a as
compared with 8 as shown in the real systems of 3a and 4a.
Furthermore, the theoretical calculations revealed a shortening
of the apical P-H bond length in 7a relative to the P-H bond
length in 8, supporting a higher degree of s character of the
apical P-H bond in 7a than that in 8. Calculation on tetra-
coordinate 9 showed that the structural parameters of the three-
membered ring of 9 are almost consistent with those of 7a,
although the CdC bond length in the three-membered ring of
9 is somewhat shorter than that of 7a.
The comparable structural parameters of the phosphirene rings
of the pentacoordinate phosphirenes and the tetracoordinate
phosphirenium cations indicate some common characteristics
in electronic structures of both the species. Theoretical study
on phosphirenium cations by Clark and co-workers showed that
the interaction between the σ* orbital of the phosphorus-ligand
bond and the π orbital of the carbon-carbon double bond in
the phosphirenium cations has a weak but significant effect on
the stability of the phosphirenium cations (Figure 2a).20 By
analogy with the phosphirenium cations, which exhibit delo-
calization of the double-bond character in the three-membered
ring, such a σ*-π interaction21 can be applicable to the
pentacoordinate phosphirenes. It can be thought that the σ*
orbital of the apical bond interacts with the π orbital of the
CdC bond, resulting in both the shortening of the endocyclic
P-C bond and the elongation of the CdC bond (Figure 2b).
Regitz and co-workers have reported that the contribution of
the canonical structures B and C, as shown in Scheme 2, could
explain the shortening of the endocyclic P-C bonds and the
elongation of the CdC bond in the three-membered ring.6c The
σ*-π interaction can be considered as an interpretation of
Regitz’s description from a standpoint of the molecular orbital
method.
67.5
67.5
69.6
65.2
67.2
67.4
69.5
69.5
68.0
68.0
structural parameters of the phosphirene rings indicate that the
structures of phosphirene rings depend on the coordination
number of the phosphorus atom.
2.3. Theoretical Calculations on Tri-, Tetra-, and Penta-
coordinate Phosphirenes. Theoretical calculations on the real
systems of 3a, 4a, and 4b were carried out with density
functional theory (B3LYP) using basis sets 6-31G(d) (see
Supporting Information).19 The calculated geometries of 3a and
4a roughly agreed with those of the X-ray structures. Although
their calculated bond lengths were overestimated by maximum
0.05 Å (for the P-O bonds) compared to the X-ray structures,
the computation reproduces reasonably the experimental results
such as the shortening of the endocyclic P-C bonds and
elongation of the CdC bond in the three-membered ring in 4a
compared with 3a.
To discuss further the structure of the phosphirene rings and
effects of the coordination number of the phosphorus atom on
the structure, we also performed theoretical calculations on
model compounds of tri-, tetra-, and pentacoordinate phos-
phirenes to shorten computation time. The model compounds
that we chose for the theoretical calculations are shown in Chart
4. In the model compounds 7a-c, the tetrachlorocatecholate
ligand in the real systems was replaced by a 1,2-ethenediolate
ligand (7a: X ) R ) H; 7b: X ) H, R ) SiH3; 7c: X ) CN,
R ) H). In the model compounds 8 and 9, hydrogen was used
as all the ligands of the phosphirenes. Geometry optimization
on these model compounds was carried out with density
functional theory (B3LYP) using basis sets 6-31G(d).19 Table
2 shows selected bond lengths and angles of the optimized
structures of 7a-c, 8, and 9.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11.4; Gaussian,
Inc.: Pittsburgh, PA, 1998.
To confirm the influence of the electronic properties of the
substituents on the three-membered ring, we carried out
(20) Go¨ller, A.; Heydt, H.; Clark, T. J. Org. Chem. 1996, 61, 5840.
(21) For an investigation on σ*-π interaction in silacyclopropenes
through X-ray crystallographic analysis, see: Tsutsui, S.; Sakamoto, K.;
Kabuto, C.; Kira, M. Organometallics 1998, 17, 3819.
J. Org. Chem, Vol. 71, No. 15, 2006 5451

Sase et al.
TABLE 3. Selected NMR Data of Phosphirenes 3a,b and 4a,b
in C6D6
SCHEME 3. Thermolysis of 4a and 4b
downfield shifts of 4a and 4b in 31P NMR spectroscopy relative
to 3a and 3b are attributable to the absence of a lone pair in the
pentacoordinate phosphirenes.
3a
Ph
3b
Me3Si
4a
Ph
4b
Me3Si
R
δP
C (C1)
1J(P,C1)a
C (C2)
1J(P,C2)a
C (C3)
1J(P,C3)a
-188.1
123.0
44.6
-204.6
138.8
53.6
123.3
51.9
-95.2
159.8
17.6
-91.3
185.3
9.2
170.3
not obsd
135.4
178.4
δ
In the 13C NMR spectra, the signals of the 13C nuclei of the
three-membered ring of 4a and 4b lie at lower fields relative to
the chemical shifts of 3a and 3b.24 In both cases, the coupling
δ
1
constants, J(P,C), of the endocyclic P-C bonds in 4a and 4b
δ
142.2
65.3
143.8
65.5
134.7
175.1
are smaller than those of 3a and 3b. It is worth noting that the
signal of C2 in 4b, to which a trimethylsilyl group is attached,
was observed as a singlet at δC 170.3, and no coupling between
the phosphorus and carbon nuclei was detected. Considering
that the coupling constant increases as the s character of each
bond increases,25 the endocyclic P-C bonds in 4a and 4b have
a high degree of p character, which reflects their being both
basal bonds in an SP structure and endocyclic bonds in three-
membered ring. The endocyclic P-C2 bond in 4b, in particular,
has a considerably high degree of p character. In contrast,
coupling constants of the exocyclic bonds between the phos-
phorus atom and the ipso-carbon atom of the phenyl group in
4a and 4b are larger than those of 3a and 3b, indicating a high
degree of s character of the exocyclic P-C bond in 4a and 4b.
Such a high degree of s character is a characteristic of apical
bonds in SP. These NMR studies show that 4a and 4b have an
SP arrangement around the phosphorus in the solution state,
which is consistent with the fact that the structures of penta-
coordinate phosphorus compounds in the solid and the solution
states do not differ substantially.26
2.5. Reactivities of Pentacoordinate Phosphirenes: Ther-
molyses of 4a and 4b. It has been reported that a pentacoor-
dinate phosphirane with a saturated three-membered ring
synthesized by Denney et al. is thermally unstable and detectable
only at -80 °C, decomposing to give the corresponding phos-
phonite and ethylene at room temperature.6b In contrast, 4a is
thermally stable; no decomposition of 4a was observed in the
NMR spectra after being heated at 60 °C for 40 h. Upon heating
at 80 °C for 36 days, 4a underwent a reductive elimination
reaction to give the corresponding phosphonite 10 and diphen-
ylacetylene, although some of 4a still remained in the solution
(Scheme 3).
a Coupling constants are shown in Hz.
theoretical calculations on 7b bearing a SiH3 group on the carbon
atom in the three-membered ring and 7c bearing a CN group
on the phosphorus atom. The structural parameters of the
phosphirene ring of 7c are very close to those of 7a. Substitution
of a CN group has little influence on the structural parameters
of the phosphirene ring, as observed in the X-ray crystal
structures of 2a and 4a, and these results suggest the small
influence of the apical ligand at the phosphorus on the structures
of the pentacoordinate phosphirenes. In contrast, attachment of
a SiH3 group to the carbon in the three-membered ring alters
the structure of the phosphirene ring to some extent. In 7b, the
distance between the phosphorus and the carbon with the silyl
group is elongated by about 0.03 Å compared with the basal
P-C bond length of 7a; unfortunately, the structural parameters
of 4b are not experimentally available. These results suggest
that the structures of the three-membered rings in the pentaco-
ordinate phosphirenes are influenced by the substituents on the
carbon atoms in the three-membered ring rather than by the
ligand on the phosphorus atom.
2.4. NMR Studies of Phosphirenes. The selected NMR data
of tri- and pentacoordinate phosphirenes are summarized in
Table 3. The 31P NMR chemical shifts of 4a and 4b are in the
range of those of phosphirenes 2a-e (δP -126.6 to -69.8).6c
Compared with the 31P NMR chemical shift (δP -5.9) of
phosphorane 5,17b which has two five-membered rings involving
the phosphorus atom, the 31P resonances of 4a and 4b appear
at a much higher field, which is explained by the fact that the
nuclei of a three-membered ring resonate upfield relative to the
less strained compound.22 The 31P NMR chemical shifts of 4a
and 4b lie at a relatively low field compared with those of 3a
and 3b. Large upfield shifts of 3a and 3b in 31P NMR
spectroscopy are a typical feature of tricoordinate phosphirenes.4a,b
Considering that the upfield shifts in 31P NMR spectroscopy
are correlated with the degree of s character of the phosphorus
lone pair,4a,23 such upfield shifts of the tricoordinate phos-
phirenes are due to both high s character of the phosphorus
lone pairs and the three-membered ring effect. Hence, the
Phosphirene 4b is thermally less stable, and it decomposed
at room temperature to give 10 and phenyl(trimethylsilyl)-
acetylene. These results indicate that attachment of a trimeth-
ylsilyl group to the endocyclic carbon lowers the thermal
stability of the pentacoordinate phosphirenes. Considering that
a phosphorane with an SP structure is stabilized when an
(24) One of the reviewers suggested that the NMR data of Table 3 fit
the description of Me3Si as an accepting group, because C1 is more
deshielded when R is Me3Si than when R is Ph. We think that the Me3Si
groups behave as an electron-accepting group, but their electron-donating
nature is not negligible in the R-carbon, considering that C2 is more shielded
than C1 in 3b and 4b.
(22) (a) Burke, J. J.; Lauterbur, P. C. J. Am. Chem. Soc. 1964, 86, 1870.
(b) Seyferth, D.; Lambert, R. L. J.; Annarelli, D. C. J. Organomet. Chem.
1976, 122, 311. (c) Crimaldi, K.; Lichter, R. L. J. Org. Chem. 1980, 45,
1277.
(25) Jameson, C. J. In Multinuclear NMR; Mason, J., Ed.; Plenum: New
York, 1987; Chapter 4.
(23) Chesnut, D. B.; Quin, L. D.; Wild, S. B. Heteroatom Chem. 1997,
8, 451.
(26) Dennis, L. W.; Bartuska, V. J.; Maciel, G. E. J. Am. Chem. Soc.
1982, 104, 230.
5452 J. Org. Chem., Vol. 71, No. 15, 2006

Pentacoordinate 1H-Phosphirenes
SCHEME 4. Reaction of 4a and 4b with Proton Sources
SCHEME 5. Plausible Mechanism of Hydrolysis of 4
FIGURE 3. Calculated reaction profile of decomposition of 7a (top)
and 7b (bottom). Bond lengths are shown in Å. Energies calculated at
the B3LYP/6-311+G(2d,p)// B3LYP/6-31G(d) level, zero-point energy
corrected, are in kcal/mol.
hydrolysis of 4b was considerably faster than that for 4a,
showing that attachment of the silyl group to the carbon in the
phosphirene ring also affects the reactivity of pentacoordinate
phosphirenes toward proton sources.
electron-withdrawing ligand occupies the basal position,10 4b
is destabilized by a trimethylsilyl group, which is more electron-
donating than a phenyl group.
A possible mechanism for the reactions of 4a and 4b with
reagents affording protons is shown in Scheme 5. The first step
is thought to be protonation to the oxygen of 4, followed by
P-O bond cleavage to afford phosphorane intermediates 12 or
12′ with distorted TBP structures. In the case of unsymmetrical
4b (R ) Me3Si), 12b should be more stable than 12b′ because
the electron-donating silyl substituent on the carbon atom at
the equatorial position enhances the stability of the phosphorane
with the TBP arrangement.28 Considering that apical bonds in
a phosphorane with the TBP arrangement are weaker and more
reactive than equatorial bonds, further hydrolysis of 12b should
occur with cleavage of the apical P-C bond, resulting in
formation of 11b, which is consistent with the experimental
results. In the case of 4a (R ) Ph), an electron-withdrawing
phenyl group at the equatorial position should destabilize 12a,
thereby the initial protonation of 4a should be slower relative
to that of 4b. Thus, the difference of the reactivity between 4a
and 4b can be explained by comparison of the stability of the
phosphorane intermediates.
These results are supported by theoretical calculations on 7a
and 7b. They were used to examine activation energies for the
reductive elimination reaction of the pentacoordinate phos-
phirenes. Both the optimized geometries and calculated energies
of 7a and 7b are summarized in Figure 3. In both cases, the
decomposition reactions are exothermic, releasing 5.35 and
10.07 kcal/mol for 7a and 7b at the B3LYP/6-311+G(2d,p)//
B3LYP/6-31G(d) level, respectively. The calculated activation
barrier for reductive elimination in 7b (14.54 kcal/mol) is lower
than that in 7a (21.09 kcal/mol), supporting the experiment
results. The lower activation energy for 7b than that for 7a is
attributable to destabilization of the starting molecule that results
from attachment of an electron-donating silyl group to the carbon
at the basal position.27
Reactions of 4a and 4b with Proton Sources. Reactions of
the pentacoordinate phosphirenes with some reagents that
provide protons were examined. Phosphirene 4a reacted neither
with excess amount of water in CDCl3 nor with hydrogen
chloride in Et2O/THF at room temperature. Cleavage of the
endocyclic P-C bond, however, took place when much stronger
acid was used; treatment of 4a with trifluoromethanesulfonic
acid followed by aqueous workup gave vinylphosphinate 11a
in 89% yield (Scheme 4). The (E)-configuration of the olefin
moiety of 11a was determined by differential NOE experiments.
Although 4a was not hydrolyzed by water, silyl-substituted 4b
was hydrolyzed immediately in wet C6D6 to give 11b quanti-
tatively. The stereochemistry of the alkene moiety of 11b was
determined to be the (E)-isomer by X-ray crystallographic
analysis, which revealed that the P-C(Ph) bond was cleaved
regioselectively in this reaction. The reaction rate for the
Reaction of 4a with Bromine. When 4a reacted with
bromine at room temperature, cleavage of the P-C bond in the
three-membered ring took place and vinylphosphinate 14 was
obtained in 97% yield (Scheme 6). The regiochemistry of 14
was determined by X-ray crystallographic analysis, and the
olefin moiety was found to have the (Z)-configuration.29
Although a weak signal of an intermediate at δP -5 together
(27) NBO analyses on 4a and 4b at the B3LYP/6-31G(d) level show
the electron-donating nature of the SiMe3 group (4a: -0.324 and -0.325
for C1 and C2, respectively, 4b: -0.312 for C1 and -0.792 for C2).
(28) For a methyl group as an electron-donating substituent, see:
Kawashima, T.; Okazaki, R.; Okazaki, R. Angew. Chem., Int. Ed. Engl.
1997, 36, 2500.
J. Org. Chem, Vol. 71, No. 15, 2006 5453

Sase et al.
SCHEME 6. Plausible Mechanism of Reaction of 4a with
Bromine
SCHEME 7. Reaction of 4b with
2,3-Dimethyl-1,3-butadiene
with a strong signal due to 14 was observed in the 31P NMR
spectra, the intermediate could not be identified.
that 16 was produced by a [4 + 1]-cycloaddition reaction of 10
with the diene.32 Formation of 16 by the reaction of 10 with
the diene was independently confirmed by a control experiment.
These results indicate that the alkene moieties of 4a and 4b are
inert toward Diels-Alder reactions, and decomposition over-
comes the cycloaddition of 4b. The σ*-π interaction seems to
lower the reactivity of the CdC bond in the three-membered
rings.
A plausible mechanism for this reaction is illustrated in
Scheme 6. Bromine electrophilically attacks the exocyclic P-C
bond, resulting in formation of phosphonium bromide 13. Then,
water, which exists in the solution as impurity, attacks the
phosphorus of 13 to afford the hydroxyphosphorane intermedi-
ate, which is immediately converted to 14. Because there was
a possibility of electrophilic attack of bromine on the carbon-
carbon double bond in the three-membered ring, the molecular
orbitals of model compound 15 were examined. Selected MOs
of 15 are shown in Figure 4. The MO pictures show that the
endocyclic P-C σ bonds are involved in HOMO-2, while a
π-type orbital of the CdC bond is involved in HOMO-3, which
lies at lower energy. This is a strong indication that the
electrophilic attack of bromine on the P-C bond is more
favorable than that on the CdC double bond.
3. Conclusion
Several pentacoordinate P-phenyl-substituted 1H-phos-
phirenes were successfully synthesized. X-ray crystallographic
analysis of one of them revealed its distorted SP arrangement
around the phosphorus. NMR studies on the pentacoordinate
phosphirenes revealed that they persist in an SP arrangement
around the phosphorus in the solution state and that the
endocyclic P-C bonds in the three-membered ring have a very
high degree of p character. Such a high degree of p character
of the endocyclic P-C bonds resulted from their being both
basal bonds in the SP structure and endocyclic bonds of the
three-membered ring.
Crystallographically determined structural parameters of the
three-membered ring of the pentacoordinate phosphirenes are
very close to those of a tetracoordinate phosphirenium cation,
but they are considerably different from those of the tricoor-
dinate phosphirenes. Theoretical calculations on the phos-
phirenes reproduced these experimental results. Such a similarity
of the structural parameters of the three-membered ring between
tetra- and pentacoordinate phosphirenes is explained in terms
of a σ*-π interaction, where the σ* orbital of the apical bond
interacts with the π orbital of the CdC bond of the three-
membered ring.
The σ*-π interaction seems to lower the reactivity of the
CdC bond in the three-membered ring; Diels-Alder reactions
of the pentacoordinate phosphirenes with 1,3-butadienes did not
proceed. In contrast, the endocyclic P-C bond in the three-
membered ring was more reactive than the endocyclic CdC
bond, and hence P-C bond cleavage took place even in the
reaction with bromine. The reactivity of pentacoordinate phos-
phirenes is also affected by the substituent attached to the carbon
atom in the three-membered ring. Their thermal stability and
reactivity toward water are dramatically changed when an
electron-donating trimethylsilyl group is attached to the basal
carbon atom in the three-membered ring.
FIGURE 4. Kohn-Sham orbitals of 15. Energies (in hartree) are
shown in parentheses.
Reactions of 4a and 4b with 1,3-Dienes. Because a trivalent
phosphirene tungsten complex has been reported to undergo a
[4 + 2]-cycloaddition reaction with 2,3-dimethyl-1,3-buta-
diene,30 reactions of 4a and 4b with 1,3-butadienes were
performed to examine the reactivity of the CdC double bond
in the three-membered ring of the pentacoordinate phosphirenes.
On one hand, treatment of a toluene solution of 4a with 2,3-
dimethyl-1,3-butadiene at 60 °C and Danishefsky’s diene31 at
100 °C resulted in no reaction. On the other hand, reaction of
4b with 2,3-dimethyl-1,3-butadiene gave phosphorane 16 in 79%
yield (Scheme 7). In the course of the reaction of 4b with the
diene, a signal attributable to 10 that was formed from the ther-
molysis of 4b was detected by 31P NMR spectroscopy, indicating
(29) Because a single crystal of 14 was of low quality, the details of the
structural parameters could not be discussed, but the regiochemistry of the
olefin moiety of 14 was determined to be the (Z)-configuration by X-ray
crystallographic analysis. The crystallographic data for 14 (CCDC-285444)
can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(30) Marinetti, A.; Mathey, F. J. Am. Chem. Soc. 1985, 107, 4700.
(31) (a) Danishefsky, S.; Kitahara, T. J. Am. Chem. Soc. 1974, 96, 7807.
(b) Danishefsky, S. J.; Larson, E.; Askin, D.; Kato, N. J. Am. Chem. Soc.
1985, 107, 1246.
(32) Hasserodt, U.; Hunger, K.; Korte, F. Tetrahedron 1963, 19, 1563.
5454 J. Org. Chem., Vol. 71, No. 15, 2006

Pentacoordinate 1H-Phosphirenes
) 7.3 Hz, 2H; ortho of PhA), 7.15 (d, 3J(H,H) ) 7.7 Hz, 2H; meta
4. Experimental Section
3
of PhB), 7.23 (d, J(H,H) ) 7.1 Hz, 1H; para of PhB), 7.30 (t,
4,5,6,7-Tetrachloro-1′,2′,3′-triphenylspiro[1,3,2-benzodioxa-
phosphole-2,1′λ5-[1H]-phosphirene] (4a). A toluene solution (1.5
mL) of 3a (68 mg, 0.24 mmol) and o-chloranil (57 mg, 0.23 mmol)
was stirred for 2 h at room temperature, and the solvent was
evaporated. In a glovebox under an argon atmosphere, the resulting
orange solid was washed with hexane (2 mL) to give 4a as pale
orange solid (57 mg, 58%). Pale orange solid; mp 143.5 °C (dec);
1H NMR (500 MHz, C6D6) δ 6.94-7.01 (m, 3H; para and meta of
P-Ph), 7.10-7.14 (m, 2H; para of C-Ph), 7.20 (t, 3J(H,H) ) 7.9
Hz, 4H; meta of C-Ph), 7.88-7.91 (m, 2H; ortho of P-Ph), 7.98
3J(H,H) ) 7.3 Hz, 2H; meta of PhA), 7.32-7.35 (m, 1H; para of
PhA), 7.44-7.48 (m, 2H; meta of Ph-P), 7.59-7.62 (m, 1H; para
3
of Ph-P), 7.73 (d, J(P,H) ) 24.1 Hz, 1H; CdC(Ph)H), 7.80-
7.85 (m, 2H; ortho of Ph-P), 11.0 (br, 1H; OH); 13C NMR (126
MHz, CDCl3, 25 °C) δ 122.7 (d, J ) 35.2 Hz), 123.9 (s), 125.1 (d,
1
J ) 5.2 Hz), 126.8 (d, J(P,C) ) 134.7 Hz; ipso of Ph-P), 128.4
3
(s; meta of PhB), 128.7 (d, J(P,C) ) 13.4 Hz; meta of Ph-P),
1
128.7 (s; para of PhA), 129.0 (s), 129.2 (d, J(P,C) ) 75.6 Hz;
1
CdC(Ph)(H)), 129.96 (s), 130.03 (s), 130.7 (d, J(P,C) ) 129.9
3
Hz; ipso of PhB), 130.7 (s), 132.7 (d, J(P,C) ) 10.3 Hz; ortho of
3
(d, J(H,H) ) 7.9 Hz, 4H; ortho of C-Ph); 13C{1H} NMR (126
Ph-P), 133.0 (d; J ) 9.3 Hz), 133.7 (d; J ) 20.8 Hz), 133.9 (s),
MHz, C6D6, 25 °C) δ 115.0 (d, 2J(P,C) ) 12.5 Hz), 124.5 (s), 129.0
(d, 3J(P,C) ) 10.5 Hz; ortho of C-Ph), 129.2 (s; meta of C-Ph),
129.5 (s; meta of P-Ph), 130.6 (s; para of C-Ph), 130.8 (d, 2J(P,C)
2
136.2 (d; J ) 10.5 Hz), 145.3 (s), 146.0 (d, J(P,C) ) 13.5 Hz;
CdC(Ph)(H)); 31P{1H} NMR (109 MHz, CDCl3, 25 °C) δ 41.8;
HRMS-FAB (m/z): [M + H]+ calcd for C26H18O335Cl4P, 548.9747;
found, 548.9739.
2
) 12.5 Hz; ipso of C-Ph), 131.1 (d, J(P,C) ) 7.3 Hz; ortho of
P-Ph), 131.8 (d, 4J(P,C) ) 3.5 Hz; para of P-Ph), 134.7 (d, 1J(P,C)
Reaction of 4b with Water. Pentacoordinate phosphirene 4b
was dissolved to wet C6D6 (0.5 mL). After the solution was allowed
to stand at room temperature for 5 min, only a signal due to 11b
was observed in the 31P NMR spectrum, and signals due to 11b
1
) 175.1 Hz; ipso of P-Ph), 143.5 (s), 159.8 (d, J(P,C) ) 17.6
Hz, P-C(Ph)); 31P{1H} NMR (202 MHz, C6D6, 25 °C) δ -95.2.
LRMS (FAB): m/z 530 (M+); Anal. Calcd for C26H15Cl4O2P: C,
58.68; H, 2.84. Found: C, 58.62; H, 3.06.
1
were observed in the H NMR spectrum.
4,5,6,7-Tetrachloro-1′,2′-diphenyl-3′-trimethylsilylspiro[1,3,2-
benzodioxaphosphole-2,1′λ5-[1H]-phosphirene] (4b). Similarly,
reaction of 3b (207 mg, 0.73 mmol) with o-chloranil (170 mg, 0.73
mmol) at room temperature for 30 min gave 4b as pale orange
solid (196 mg, 52%). Pale orange solid; 1H NMR (500 MHz, C6D6,
25 °C) δ 0.35 (s, 9H; (CH3)3Si), 6.98-7.10 (m, 3H; para of P-Ph
and meta of P-Ph), 7.11-7.15 (m, 1H; para of Ph), 7.21-7.24
(m, 2H; meta of Ph), 7.81-7.89 (m, 4H; ortho of Ph-P and ortho
of Ph); 13C{1H} NMR (126 MHz, C6D6, 25 °C) δ -0.93 (d, 3J(P,C)
) 2.6 Hz; (CH3)3Si), 114.7 (d, 2J(P,C) ) 11.7 Hz), 115.0 (d, 2J(P,C)
) 14.0 Hz), 124.0 (s), 124.6 (s), 129.0 (d, 3J(P,C) ) 10.7 Hz; meta
of Ph-P), 129.1 (d, 3J(P,C) ) 2.1 Hz; ortho of Ph), 129.2 (s; meta
2,3,4,5-Tetrachloro-6-hydroxyphenylphenyl [(E)-2-Phenyl-1-
trimethylsilylethenyl]phosphinate (11b): Colorless solid; mp
176.3 °C; 1H NMR (500 MHz, CDCl3, 25 °C) δ 0.11 (s, 9H;
(CH3)3Si), 7.28-7.30 (m, 2H; ortho of Ph), 7.37-7.40 (m, 3H;
meta and para of Ph), 7.45-7.50 (m, 2H; meta of Ph-P),
7.56-7.59 (m, 1H; para of Ph-P), 7.88-7.93 (m, 2H; ortho of
3
Ph-P), 8.23 (d, J(P,H) ) 36.4 Hz; P-C(TMS)dCH(Ph)), 11.3
(br, 1H; OH); 13C{1H} NMR (126 MHz, CDCl3, 25 °C) δ 1.11 (d,
4J(P,C) ) 6.7 Hz; (CH3)3Si), 122.7 (d, J ) 5.5 Hz), 124.9 (d, J )
1
5.4 Hz), 126.9 (d, J(P,C) ) 123.5 Hz; ipso of Ph-P), 128.0 (s;
3
ortho of Ph), 128.3 (s; meta of Ph), 128.8 (d, J(P,C) ) 13.4 Hz;
2
meta of Ph-P), 129.3 (s; para of Ph), 129.5 (s), 132.7 (d, J(P,C)
2
of Ph), 130.8 (d, J(P,C) ) 13.0 Hz; ortho of Ph-P), 131.2 (s;
) 11.0 Hz; ortho of Ph-P), 133.3 (d, 1J(P,C) ) 87.7 Hz;
P-C(TMS)dCH(Ph)), 133.8 (d, 4J(P,C) ) 2.6 Hz; para of
Ph-P), 136.2 (d, J ) 9.8 Hz), 137.7 (d, 3J(P,C) ) 28.9 Hz;
4
para of Ph), 131.7 (d, J(P,C) ) 3.7 Hz; para of Ph-P), 132.2 (d,
2J(P,C) ) 9.1 Hz; ipso of Ph), 135.4 (d, 1J(P,C) ) 178.4 Hz; ipso
of Ph-P), 142.6 (s), 144.4 (s), 170.3 (s; C(Ph)dC(TMS)), 185.3
(d, 1J(P,C) ) 9.2 Hz; C(Ph)dC(TMS)); 31P{1H} NMR (202 MHz,
C6D6, 25 °C) δ -91.3; HRMS-FAB (m/z): [M + H]+ calcd for
C23H20O235Cl4SiP, 526.9724; found, 526.9718.
2
ipso of Ph), 145.8 (d, J ) 2.9 Hz), 161.8 (d, J(P,C) ) 1.3 Hz;
P-CdCH(Ph)), a signal due to a quaternary carbon could not be
found; 31P{1H} NMR (109 MHz, CDCl3, 25 °C) δ 53.2; LRMS
(FAB) m/z ) 547 ([M + H]+); Anal. Calcd for C23H21Cl4O3PSi:
C, 50.57; H, 3.87. Found: C, 50.31; H, 4.05.
Thermolysis of 4b. Pentacoordinate phosphirene 4b (11 mg, 21
µmol) and a drop of 2,4,6-tri(t-butyl)benzene as an internal standard
were dissolved in C6D6 (0.5 mL), and the solution was degassed
and sealed in an NMR tube. After the solution was allowed to stand
at room temperature for 5 d, a signal at δP -91.3 due to phosphirene
4b disappeared, and the signal at δP 196.1 due to phosphonite 10
Reaction of 4a with Bromine. To a dichloromethane solution
(1 mL) of 4a (18 mg, 33 µmol) was added a dichloromethane
solution of bromine (0.39 M, 0.13 mL, 51 µmol) at room
temperature. After the reaction mixture was stirred for 30 min at
room temperature, the solvent was evaporated and the resulting
brown solid was exposed to air. The crude mixture was puri-
fied by GPLC (CHCl3) to give 14 (20 mg, 97%) as a colorless
solid.
1
was observed in the 31P NMR spectrum. The H NMR spectrum
showed a signal at δH 0.29 due to the trimethylsilyl group of phenyl-
(trimethylsilyl)acetylene. The yield of trimethylsilyl(phenyl)acety-
lene was estimated as 99% from the integral of the 1H NMR
spectrum.
Reaction of 4a with Trifluoromethanesulfonic Acid. To a
CDCl3 solution (0.6 mL) of 4a (20 mg, 38 mol) was added tri-
fluoromethanesulfonic acid (5 µL, 0.06 mmol). After the reaction
mixture was allowed to stand at room temperature for 10 min, the
reaction mixture was opened in air. After being washed with water,
the resulting organic layer was dried over MgSO4. The solvent was
removed under reduced pressure to give 11a (19 mg, 89%) as
colorless solid.
2,3,4,5-Tetrachloro-6-hydroxyphenyl [(Z)-2-Bromo-1,2-di-
phenylethenyl]phenylphosphinate (14): 1H NMR (500 MHz,
CDCl3, 25 °C) δ 7.05-7.23 (m, 10H), 7.47-7.53 (m, 2H: meta
of Ph-P), 7.62 (t, 3J(H,H) ) 7.5 Hz, 1H; para of Ph-P), 8.00 (dd,
3
3J(P,H) ) 13.6 Hz, J(H,H) ) 7.7 Hz, 2H; ortho of Ph-P), 10.8
(s; OH); 13C{1H} NMR (126 MHz, CDCl3, 25 °C) δ 122.5 (s),
1
122.9 (s), 124.9 (d, J ) 5.2 Hz), 127.4 (d, J(P,C) ) 134.7 Hz;
ipso of P-Ph), 127.9 (s), 128.2 (s; para of Ph-C), 128.3 (s), 128.8
(d, 3J(P,C) ) 14.5 Hz; meta of P-Ph), 129.0 (s), 129.3 (s), 129.5
(s), 130.4 (br), 132.0 (d, 2J(P,C) ) 11.5 Hz; ortho of P-Ph), 133.9
4
1
(d, J(P,C) ) 2.6 Hz; para of Ph), 135.1 (d, J(P,C) ) 141.0 Hz;
PC(Ph)dC), 135.9 (d, J ) 9.3 Hz), 136.1 (d, J ) 5.2 Hz), 139.4
(d, 2J(P,C) ) 13.5 Hz; ipso of P-C-Ph), 141.5 (d, 3J(P,C) ) 7.3
Hz; ipso of P-CdC(Ph)), 145.3 (s); 31P{1H} NMR (109 MHz,
CDCl3, 25 °C) δ 38.7; HRMS-FAB (m/z); [M + H]+ calcd for
C26H17O335Cl479BrP, 626.8853; found, 626.8824.
2,3,4,5-Tetrachloro-6-hydroxyphenyl [(E)-1,2-Diphenyleth-
Reaction of 4b with 2,3-Dimethyl-1,3-butadiene. To a C6D6
solution (0.5 mL) of 4b (20 mg, 38 µmol) in an NMR tube was
added 2,3-dimethyl-1,3-butadiene (30 µL, 0.26 mmol), and the
1
enyl]phenylphosphinate (11a). H NMR (500 MHz, CDCl3, 25
°C) δ 7.04 (d, 3J(H,H) ) 7.7 Hz, 2H; ortho of PhB), 7.10 (d, 3J(H,H)
J. Org. Chem, Vol. 71, No. 15, 2006 5455

Sase et al.
solution was degassed and sealed. After the reaction mixture was
allowed to stand at room temperature for 6 d, a signal at δP 14.0
due to 16 was observed. The sealed tube was opened in a glovebox
under an argon atmosphere, and the solvent was evaporated to give
red solid (22 mg) containing 16. Yield of 16 was estimated to be
electronic energies of the B3LYP/ 6-31G(d) geometries at the
B3LYP/6-311+G(2d,p) level were corrected by nonscaled zero-
point energies.
Acknowledgment. We thank Central Glass Co., Ltd., Tosoh
Finechem Corp., and Shin-etsu Chemical Co., Ltd. for the gifts
of organofluorine compounds, alkyllithiums, and organosilicon
compounds, respectively. This work was partially supported by
Grants-in-Aid for The 21st Century COE Program for Frontiers
in Fundamental Chemistry (T.K.) and for Scientific Research
Nos. 14044020 (T.K.), 15105001 (T.K.), and 16750028 (N.K.)
from Ministry of Education, Culture, Sports, Science and
Technology, Japan.
1
79% from the integral of the H NMR spectrum.
Theoretical Calculation. Theoretical calculations reported here
were performed with the Gaussian 98 (revision A.11.4).19 Full
geometry optimizations of the stable structures and the transition
states were performed with the density functional theory (B3LYP)
using basis sets 6-31G(d), where no symmetry constraints were
imposed in any case. The stationary points were identified exactly
by the curvature of the potential energy surface at these points
corresponding to the eigenvalues of the Hessian. All reported stable
structures exhibit exclusively positive Hessian eigenvalues, while
all transition states have exactly one negative Hessian eigenvalue.
The stationary points shown in Figure 3 are connected with each
other on the potential surface as confirmed by intrinsic reaction
coordinate (IRC) analysis.33 For decomposition of 7a and 7b, the
Supporting Information Available: General experimental
methods, ORTEP drawings of 11b and 14, details of theoretical
calculation (Cartesian coordinates and computed total energies),
and X-ray crystallographic data for 4a, 11b, and 14 (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(33) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154.
(b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
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