Perfectly anti-apicophilic phosphoranes: Conversion of a 1-hydro-5-carbaphosphatrane into 1-alkyl- and 1-aryl-5-carbaphosphatranes

Article abstract of DOI:10.1080/10426500902748080

1-Alkyl- and 1-aryl-5-carbaphosphatranes were prepared by the reaction of 1-hydro-5-carbaphosphatrane with nucleophiles and subsequent intramolecular oxidative cyclization. X-ray crystallographic investigation of the t-butyl derivative revealed that it ha

Full text of DOI:10.1080/10426500902748080

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Phosphorus, Sulfur, and Silicon
and the Related Elements
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Perfectly “Anti-Apicophilic”
Phosphoranes: Conversion of a
1-Hydro-5-carbaphosphatrane
into 1-Alkyl- and 1-Aryl-5-
carbaphosphatranes
Junji Kobayashi a & Takayuki Kawashima a
a Department of Chemistry, Graduate School of
Science , The University of Tokyo , Tokyo, Japan
Published online: 26 Mar 2009.
To cite this article: Junji Kobayashi & Takayuki Kawashima (2009) Perfectly “Anti-
Apicophilic” Phosphoranes: Conversion of a 1-Hydro-5-carbaphosphatrane into 1-Alkyl-
and 1-Aryl-5-carbaphosphatranes, Phosphorus, Sulfur, and Silicon and the Related
Elements, 184:4, 1028-1035, DOI: 10.1080/10426500902748080
To link to this article: http://dx.doi.org/10.1080/10426500902748080
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Phosphorus, Sulfur, and Silicon, 184:1028–1035, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902748080
Perfectly “Anti-Apicophilic” Phosphoranes: Conversion
of a 1-Hydro-5-carbaphosphatrane into 1-Alkyl- and
1-Aryl-5-carbaphosphatranes
Junji Kobayashi and Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of
Tokyo, Tokyo, Japan
1-Alkyl- and 1-aryl-5-carbaphosphatranes were prepared by the reaction of 1-hydro-
5-carbaphosphatrane with nucleophiles and subsequent intramolecular oxidative
cyclization. X-ray crystallographic investigation of the t-butyl derivative revealed
1
that it has a perfectly “anti-apicophilic” arrangement. The apical JPC coupling
constants of the 5-carbaphosphatranes were extraordinarily large.
Keywords Apicophilicity; carbaphosphatrane; coupling constant; trigonal bipyramidal
structure
INTRODUCTION
Usually 10-P-5 phosphoranes assume trigonal bipyramidal structures
in the ground state in which there are two distinctive sites: the api-
cal and the equatorial positions.1 It is well known that site prefer-
ence of the substituents is dominated by their electronegativity and
π-donating ability, as well as steric hindrance in the pentacoordinate
species. In pentacoordinate phosphoranes, electronegative and small
groups prefer the apical positions, whereas electropositive and bulky
groups have a tendency to occupy the equatorial positions; this pref-
erence is called the apicophilicity.2 The apical bond is known to be a
three-centered four-electron bond involving a p orbital of the central
Received 5 January 2008; accepted 20 February 2008.
Dedicated to Professor Marian Mikołajczyk from the CBMiM PAN in Ło´dz´, Poland,
on the occasion of his 70th birthday.
This study was supported by the Grants-in-Aid for The Global COE Program for
Chemistry Innovation (T.K.) and for Scientific Research (T.K.) from the Ministry of Ed-
ucation, Culture, Sports, Science and Technology of Japan. We thank Tosoh Finechem
Corporation for the generous gifts of alkyllithiums.
Address correspondence to Takayuki Kawashima, Department of Chemistry, Gradu-
ate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan. E-mail: takayuki@chem.s.u-tokyo.ac.jp
1028

Perfectly “Anti-Apicophilic” Phosphoranes
1029
FIGURE 1 1-Hydro-5-carbaphosphatrane 1.
element. It has unique characteristics, being a weaker and more polar
bond. Coupling constants to the central element are smaller when com-
pared to those involving usual σ bonds. Recently, it was reported that a
specially designed tetraoxyphosphorane, in which a carbon substituent
occupies the apical position due to steric hindrance, could be isolated
as the only detectable isomer.3 Moreover, a spirophosphorane having
an apical carbon and equatorial oxygen five-membered ring structure
was synthesized as a kinetically favored isomer.4 Here we report the
synthesis of the perfectly “anti-apicophilic” phosphorane, in which all
equatorial positions are occupied with electronegative oxygen atoms
and apical positions with electropositive carbon atoms, by the reaction
of 1-hydro-5-carbaphosphatrane with various nucleophiles and subse-
quent oxidation.
RESULTS AND DISCUSSION
Recently we have reported the synthesis of 1-hydro-5-
carbaphospahtrane 1 (Figure 1), a five-carbon analog of phosphatranes
bearing the tetradentate ligand based on the triarylmethyl framework
with three oxygen atoms at 2,2ꢀ,2ꢀꢀ-positions.5,6
1-Hydro-5-carbaphosphatrane 1 showed similar characteristics to
known phosphatranes in the spectroscopic and structural aspects in
spite of the difference of the bond properties, whereas the reactivity of
1 is based on tautomerization with the trivalent isomer and is quite
different from those of known phosphatranes. 1 is also regarded as an
example of an anti-apicophilic phosphorane, in which all equatorial po-
sitions are occupied with three oxygen atoms. It has been reported that
the reactivity towards nucleophiles such as fluoride anion and alkyl-
lithiums is enhanced in the O-equatorial spirophosphoranes compared
to the O-apical ones.7
Treatment of 1 with alkyllithium reagents in THF followed by hy-
drolysis gave the bicyclic hydrophosphoranes 2a and 2b (Scheme 1). In

1030
J. Kobayashi and T. Kawashima
SCHEME 1 Reactions of 1 with nucleophiles and the synthesis of 1-alkyl- and
1-aryl-5-carbaphosphatranes 4. aYields are estimated by 1H NMR.
the 1H NMR spectra of 2a and 2b, three aromatic rings were observed
with a ratio of 2:1, strongly suggesting that the carbaphosphatrane
framework is not maintained and one five-membered ring is cleaved
by nucleophilic substitution. When the reaction was monitored by 31P
NMR spectroscopy, a new signal was observed at δP −67 with one bond
P H coupling (1JPH = 567 Hz). This signal can be assigned to pheno-
late 3 formed by nucleophilic substitution at the central phosphorus
atom, and the structure of 3 can be considered to be similar to that of
1
2a, since JPH value of 3 is close to that of 2a (645 Hz). The reactions
of an H-equatorial spirophosphorane with nucleophiles are reported
to proceed via deprotonation at the phosphorus atom and subsequent
nucleophilic attack.8 In the case of 1, however, the presence of a P-H
proton was still observable during the reaction of 1 with n-BuLi. This
different reactivity is derived from the structural difference. Because
the hydrogen atom in 1 is fixed at the apical position, its acidity is lower
than that of the H-equatorial spirophosphoranes, and thus nucleophilic
substitution is preferred over deprotonation in the case of 1. Variable
temperature 31P NMR experiments showed that a new signal appeared
at δP −89 at −80◦C in the presence of an excess amount of 12-crown-4
ether. These observations must be attributable to pseudorotation be-
tween the H-apical isomer 3a and the H-equatorial isomer 3b, but the
exact reason why two signals were observed in VT-NMR is unclear.
Phosphoranes 2a and 2b were slowly oxidized to 1-alkyl-5-
carbaphosphatranes 4a and 4b in air, respectively, and the reaction

Perfectly “Anti-Apicophilic” Phosphoranes
1031
SCHEME 2 Plausible structures of the intermediates.
was accelerated by treatment with I2. These reactions proceeded quan-
titatively, judging from 1H and 31P NMR spectra, and isolated by HPLC
in the yields described in Scheme 1. The structure of 4b was confirmed
by the result of single-crystal X-ray diffraction studies. An ORTEP
drawing of the molecular structure of 4b is shown in Figure 2.9
FIGURE 2 ORTEP drawing of the molecular structure of 1-t-butyl-5-
carbaphosphatrane 4b in the crystal; ellipsoids are drawn at 50% probability
level.

1032
J. Kobayashi and T. Kawashima
The apical P C bond length for P-t-Bu is 1.877(2) Å, which is slightly
longer than the sum of the covalent radii of carbon and phosphorus
(1.83 Å), while that for P CAr3 (1.959(2) Å) is much longer, because
of the steric bulkiness and rigidity. The equatorial P O bond lengths
fall within the normal range of P O bond lengths in pentacoordinate
phosphoranes. The axial C P C bond angle is 179.27(7)◦, and equa-
torial O P O bond angles are 118.95(6)–121.16(6)◦, the sum of equa-
torial bond angles being 360◦. These structural parameters indicate
that 4b has a nearly ideal trigonal bipyramidal structure; nevertheless
4b displays a perfect anti-apicophilic arrangement. Usually, bulky and
electropositive substituents prefer the equatorial positions. However,
in the case of 4b, a t-butyl group is located at the apical position in
spite of its steric hindrance and electropositive character. The forma-
tion of 4a and 4b in good yields can be attributed to the conformation
of their precursors 2a and 2b. Because the t-butyl group is the most
equatophilic substituent in 2b, the t-butyl group is located in equatorial
position, and one of the apical positions is occupied with the hydrogen
atom. The hydrogen atom at the apical position has hydridic character
and is easily eliminated with phenolic proton as hydrogen molecule.
Similar reactions have been reported by Akiba et al.,8 and the same
type of reaction was also observed using aromatic nucleophiles. When
1 was treated with aryllithium reagents, PhLi and MesLi (Mes = 2,4,6-
trimethylphenyl), phosphoranes 2c and 2d were obtained quantita-
tively. Thermolyses of 2c and 2d in a sealed tube readily gave 1-aryl-5-
carbaphosphatranes 4c and 4d, respectively. The yields were estimated
by 1H NMR of crude materials, and 4c was obtained in 74% yield with
the recovery of 1, while 4d was obtained quantitatively. Unfortunately
isolation of 4c and 4d failed due to their instability against air. The
difference of yields of these reactions can be rationalized by the struc-
tural difference of the reaction intermediate, as described above. The
apicophilicity of the mesityl group is smaller than that of the phenyl
group due to its steric hindrance. In 2d, H-apical isomer is much more
favorable than H-equatorial isomer, and the hydrogen atom is thought
to be located in the apical position. However, the apicophilicity of the
phenyl group is reported to be almost the same as that of the hydrogen
atom. The H-apical isomer and the H-equatorial isomer are in the equi-
librium in the case of 2c. When elimination of the hydrogen molecule
proceeds from the H-apical isomer, 1-phenyl-5-carbaphosphatrane 4c
is obtained. If the reaction proceeds from the H-equatorial isomer with
elimination of a benzene molecule, 1-hydro-5-carbaphosphatrane 1 is
recovered. Therefore, when the equatophilicity of the introduced sub-
stituent is large, anti-apicophilic phosphoranes are readily formed via
a dehydrogenation process in our system.

Perfectly “Anti-Apicophilic” Phosphoranes
1033
1
TABLE I JPC Values in 5-Carbaphosphatranes
1JPC[Hz]
R
R-P
P
CAr3
1
H
(853)[a,b]
207
208
124[b]
135
137
137
4a
4b
4c
4d
4e
n-Bu
t-Bu
Ph
Mes
Me
[c]
248
124
215[b]
133[b]
[a] 1JPH value. [b] ref. 5. [c] not determined.
NMR spectroscopy reveals unique characteristics of 4a–d. The 31P
NMR signals of 4a–d were observed at δP 22, 22, 8, and 29, respec-
tively. Usually, chemical shifts of pentacoordinate phosphorus atom
are observed at δP –100 to 0.1 The downfield shifts observed for 4a–
d indicate that the central phosphorus atom is deshielded because
the electronegative oxygen atoms occupied all equatorial positions. We
have already reported that the apical coupling constants of 1-hydro-5-
carbaphosphatrane 1 are extremely large for apical coupling constants
in a pentacoordinate phosphorane. It has widely been recognized that
the magnitude of the coupling constant is dominated by the s character
of the corresponding bond.10 Since the apical bonds of a phosphorane
involve a p orbital of the central phosphorus atom, the apical coupling
constants are usually smaller than the equatorial coupling constants.
However, the apical 1JPC values in 4a–d are much larger than the equa-
torial P C coupling constants of usual C-equatorial phosphoranes, as
shown in Table I, and similar to those of 1-hydro derivative 1 and its
ring-expanded analogs.6 One possible explanation for the extraordi-
narily large apical coupling constants is that the contribution of the
positive charge to the Fermi term11 is expected to be larger in the
apical coupling of 4a–d since the central phosphorus atom of 4a–d is
positively charged as indicated by the 31P NMR shifts.
In summary, we have presented the synthesis of perfectly anti-
apicophilic phosphoranes by nucleophilic addition to 1-hydro-5-
carbaphosphatrane and subsequent oxidation. In our system, when
the apicophilicity of nucleophiles decreases, perfectly anti-apicophilic
phosphoranes are formed more easily due to the rigid framework, which
is designed to stabilize anti-apicophilic trigonal bipyramidal struc-
ture. The t-butyl derivative was found to have a nearly ideal trigonal

1034
J. Kobayashi and T. Kawashima
bipyramidal structure in spite of its thermodynamically disfavored ar-
rangement around the central phosphorus atom.
EXPERIMENTAL
4b: To a solution of 1 (35.4 mg, 72.5 µmol) in THF (5 mL), a solution of t-
BuLi in pentane (1.77 M, 0.22 mmol) at −78◦C was added. After stirring
for 1 h at −78◦C, the mixture was treated with H2O and extracted
with CHCl3. The extracts were dried over anhydrous MgSO4. After
removal of the solvent, 2b (37.1 mg) was obtained as a colorless oil. To
a solution of 2b in CDCl3 (0.5 mL), iodine (10.3 mg, 40.6 µmol) was
added in the presence of Et3N (0.05 mL). The mixture was stirred for
30 min at ambient temperature and treated with a Na2SO3 solution
and extracted with CHCl3. The extracts were dried over anhydrous
MgSO4. After removal of the solvent, the crude reaction products were
separated by HPLC to give 4b (23.5 mg, 60%). 4b: colorless crystals; mp
187–190◦C.1H NMR (500 MHz, CDCl3, TMS, 27◦C): δ = 1.32 (s, 27H),
1.43 (d, JPH = 19.2 Hz, 9H), 6.91 (d, J = 8.4 Hz, 3H), 7.12 (dd, J = 8.4,
1
2.1 Hz, 3H), 7.87 (br s, 3H). 13C{ H} NMR (125 MHz, CDCl3, 27◦C):
δ = 29.2 (d, JPC = 4.4 Hz), 31.6 (s), 34.4 (s), 40.3 (d, JPC = 136.9 Hz),
42.2 (d, JPC = 207.5 Hz), 111.9 (d, JPC = 8.8 Hz), 122.0 (d, JPC = 22.0
Hz), 124.2 (d, JPC = 11.6 Hz), 132.6 (d, JPC = 11.2 Hz), 145.6 (s), 149.8
(d, JPC = 2.9 Hz). 31P NMR (109 MHz, CDCl3, 27◦C): δ = 22. HRMS (EI
70 eV): m/z 502.2637, Calcd for C32H39O3P 502.2606.
4a, 4c, and 4d were prepared by a similar procedure.
4a: colorless solid. 1H NMR (500 MHz, CDCl3, TMS, 27◦C): δ = 1.00
(t, J = 7.4 Hz, 3H), 1.32 (s, 27H), 1.48–1.51 (m, 2H), 1.83–1.85 (m, 2H),
2.00–2.06 (m, 2H), 6.90 (d, J = 8.5 Hz, 3H), 7.12 (dd, J = 8.5, 2.1 Hz,
1
3H), 7.87 (br s, 3H). 13C{ H} NMR (125 MHz, CDCl3, 27◦C): δ = 14.0
(s), 24.5 (d, JPC = 24.8 Hz), 28.2 (d, JPC = 7.9 Hz), 31.5 (s), 34.4 (s), 35.9
(d, JPC = 207.3 Hz), 40.2 (d, JPC = 134.5 Hz), 111.9 (d, JPC = 9.5 Hz),
122.0 (d, JPC = 22.3 Hz), 124.0 (s), 132.5 (d, JPC = 12.0 Hz), 145.7 (s),
149.6 (d, JPC = 3.7 Hz). 31P NMR (109 MHz, CDCl3, 27◦C): δ 22.
1
4d: colorless solid. H NMR (500 MHz, toluene-d8, TMS, 27◦C): δ =
1.28 (s, 27H), 2.16 (s, 3H), 2.65 (d, JPH = 1.4 Hz, 6H), 6.78 (d, J = 8.4
Hz, 3H), 6.82 (d, JPH = 6.4 Hz, 2H), 6.90 (dd, J = 8.4, 2.1 Hz, 3H), 8.19
1
(br s, 3H). 13C{ H} NMR (125 MHz, toluene-d8, 27◦C): δ = 20.7 (s), 25.8
(d, JPC = 4.4 Hz), 31.5 (s), 34.4 (s), 46.0 (d, JPC = 123.8 Hz), 112.8 (d,
JPC = 9.5 Hz), 121.8 (d, JPC = 22.0 Hz), 124.5 (s), 130.1 (d, JPC = 19.9
Hz), 132.6 (d, JPC = 12.5 Hz), 137.1 (s), 137.9 (d, JPC = 11.0 Hz), 141.9
(d, JPC = 248.0 Hz), 145.5 (s), 151.0 (d, JPC = 4.8 Hz). 31P NMR (109
MHz, toluene-d8, 27◦C): δ = 29.

Perfectly “Anti-Apicophilic” Phosphoranes
1035
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[9] Crystal data for 4b: monoclinic, P21/n, colorless, a = 11.3312(9) Å, b = 17.9837(10)
Å. c = 15.6717(17) Å, β = 88.6517(8)◦, V = 3192.6(5) Å3, 123 K, Z = 4, R1 = 0.0394,
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