Chemistry of carbaphosphatranes

Article abstract of DOI:10.1080/10426500701734398

A 5-carbaphosphatrane and a 6-carbaphosphatrane were synthesized by utilizing triarylmethyl-type tetradentate ligands. Their structures were determined by X-ray crystallographic analysis to reveal that they have nealy ideal trigonal bipyramidal structures with a perfectly anti-apicophilic arrangement. 1JPH and 1JPC values of carbaphosphatranes were shown to be extraordinarily large as an apical coupling constant. The reactivities of carbaphosphatranes based on the tautomerization with their corresponding phosphonites were demonstrated in some reactions. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon
and the Related Elements
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Chemistry of
Carbaphosphatranes
Takayuki Kawashima a
a Department of Chemistry, Graduate School of
Science , The University of Tokyo , 7-3-1 Hongo,
Bunkyo-ku, Tokyo, 113-0033, Japan
Published online: 20 Jun 2008.
To cite this article: Takayuki Kawashima (2008) Chemistry of Carbaphosphatranes,
Phosphorus, Sulfur, and Silicon and the Related Elements, 183:2-3, 306-312, DOI:
10.1080/10426500701734398
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Phosphorus, Sulfur, and Silicon, 183:306–312, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701734398
Chemistry of Carbaphosphatranes
Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
A 5-carbaphosphatrane and a 6-carbaphosphatrane were synthesized by utilizing
triarylmethyl-type tetradentate ligands. Their structures were determined by X-ray
crystallographic analysis to reveal that they have nealy ideal trigonal bipyramidal
structures with a perfectly anti-apicophilic arrangement. 1 JPH and 1 JPC values of
carbaphosphatranes were shown to be extraordinarily large as an apical coupling
constant. The reactivities of carbaphosphatranes based on the tautomerization with
their corresponding phosphonites were demonstrated in some reactions.
Keywords Anti-apicophilic; hypervalent; phosphorane; tautomerism; trigonal bipyra-
midal; X-Ray crystallography
INTRODUCTION
While a variety of main group atranes have been reported so far,1 there
has been no example of an atrane bearing a carbon atom at 5-position
instead of a group 15 element such as nitrogen or phosphorus. Recently,
we have reported the synthesis and structure of 5-carbaphosphatrane
1, the first example of 5-carbon analogues of phosphatranes. Beside
of its thermodynamically disfavored anti-apicophilic arrangement, 5-
carbaphosphatrane 1 showed remarkably thermal stability.2 In the
course of our study, we also synthesized ring-expanded analog, 6-
carbaphosphatrane 2 as the mixture of its trivalent tautomer.3 Herein
we report the syntheses, structures and reactivities of carbaphospha-
tranes.
This work was partly supported by Grants-in-Aid for The 21st Century COE Program
for Frontiers in Fundamental Chemistry (T.K.) and for Scientific Research No. 15015001
(T.K.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
We also thank Shin-etsu Chemical Co., Ltd. and Tosoh Finechem Corp. for the generous
gifts of silicon reagents and alkyllithiums, respectively.
Address correspondence to Takayuki Kawashima, Department of Chemistry, Gradu-
ate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 133-0033
Japan. E-mail: takayuki@chem.s.u-tokyo.ac.jp
306

Chemistry of Carbaphosphatranes
307
RESULTS AND DISCUSSIONS
Lithiation of triarylmethane 3 followed by the reaction with phosphorus
trichloride at 50◦C gave cyclic phosphinate 4 via intramolecular cycliza-
tion and hydrolysis. 1-Hydro-5-carbaphosphatrane 1 was synthesized
by the reaction of cyclic phosphinate 4 with tribromoborane in CHCl3.
Treatment of 4 with iodotrimethylsilane at room temperature in CDCl3
also afforded 1 (Scheme 1).
SCHEME 1
On the other hand, lithiation of methyl ether 5 with 2 equiv. of
lithium naphthalenide (LiNaph) and subsequent treatment with phos-
phorus trichloride under refluxing condition gave a bicyclic phospho-
nite as a result of two intramolecular cyclization reactions with loss
of chloromethane, which was oxidized to bicyclic phosphonate 6 by
aqueous H2O2. Phosphonium salt 7 was obtained by the reaction of
6 with 5 equiv. of methyl triflate. Treatment of 7 with 1 equiv. of
lithium tri(t-butoxy)aluminum hydride gave a mixture of 1-hydro-6-
carbaphosphatrane 2 and its tautomer, phosphonite 8 (Scheme 2).
The structure of 1 was established by X-ray crystallographic analy-
sis (Figure 1a). The apical bond lengths of 1 are 1.921(2) and 1.38(2)
˚
A for the P C and P H bonds, respectively, and the sum of the an-
gles between equatorial bonds is 359.7◦. The figure clearly shows that
1 has a nearly ideal trigonal bipyramidal structure, where the apical
positions are occupied by hydrogen and carbon atoms while three oxy-
gen atoms are located at the equatorial positions. It is well known that
phosphoranes usually bear electronegative atoms at the apical posi-
tions and electropositive atoms at the equatorial positions according to

308
T. Kawashima
SCHEME 2
the apicophilicity of the elements. The structure of 1 in the perfectly
“anti-apicophilic” arrangement is particularly noteworthy in this con-
nection.
Single crystals of 2 and 8 were obtained by the slow evaporation of
saturated solutions of a mixture of 2 and 8 in ether or benzene, for
2 and 8, respectively. To the best of our knowledge, this is the first
example of isolation and structural determination of both tri- and pen-
tavalent tautomers. It is interesting to point out that the ratios of 2/8
are 0.4/1 and 0.64/1 in ether and in benzene, respectively, and 8 exists
mainly in both solvents. The reason why solvent-dependent crystalliza-
tion occurred in this system remains unclear. The structures of 2 and
8 were definitively determined by X-ray crystallographic analysis (Fig-
ures 1b and 1c). 2 has a slightly distorted trigonal bipyramidal geometry
with an anti-apicophilic arrangement similar to that of 1 around the
FIGURE 1 ORTEP drawings of a) 1-hydro-5-caraphosphatrane 1, b) 1- hydro-
6-carbaphosphatrane 2, c) trivalent tautomer 8.

Chemistry of Carbaphosphatranes
309
TABLE I 1 J and 1
J
PC
Coupling Constants of Various
PH
Pentacoordinated Phosphorus Compounds
1
2
9
10
11
12
13
14
15
1 J
852
125
882
121
266
853
791
733
PH
1 J
116 (P-Bun)
125
122
PC
phosphorus. Unlike the C3v structure of 1, 2 has a propeller-like C3
structure due to three six-membered rings. While the P H and P O
bond lengths of 2 are typical lengths, as was the case for 1, the P C
bond length is slightly longer than typical values for the apical P C
bond of a phosphorane.
31P NMR spectra of 2 and 8 showed the signals at δP –58 and 123,
respectively. The signal of 2 was observed at a higher field region than
that of 1 (δP 2.7), attributable to the strong electron-donating ability of
1
1
benzyloxy groups to the phosphorus center. The J and J values
PH
PC
of 1, 2, and the related compounds are summarized in Table I.
These values of 1 and 2 are extraordinarily large for the coupling
constants of apical bonds in usual phosphoranes. It is well-known that
the coupling constants are dominated by the s character of each bond,
increasing as the s character increases. It is widely recognized that the
coupling constant of an apical bond of a phosphorane is smaller than
that of an equatorial bond since the apical bond is a three-center four-
electron bond formed from the p orbital of a phosphorus atom. Indeed,
the 1 J value of phosphorane 9 bearing an apical hydrogen is much
PH
smaller than that of phosphorane 12 bearing an equatorial hydrogen.4
1
Similarly the apical J value of a usual phosphorane is also smaller
PC
than that for an equatorial P C bond. However, the 1 J values of 1
PH
and 2, where the hydrogen atom is located at the apical position, are
much larger than that of the equatorial P H bond of phosphorane 12

310
T. Kawashima
(1 J = 733 Hz).4 The 1 JPH values of 1 and 2 are more than three times
PH
as large as that of the neutral phosphorane 9 (1 J = 266 Hz) bearing
PH
an apical P-H bond, whereas it is similar to those of phosphatranes 105
6
(1 J = 853 Hz) and 11 (1 J = 791 Hz). Although there is a differ-
PH
PH
ence that 1 and 2 are neutral compounds and 10 and 11 are ionic, these
results indicate that the spectroscopic properties of 1 and 2 are more
similar to those of usual phosphatranes than to those of known neutral
1
phosphoranes. The J values of 1 and 2 are also larger than that of
PC
7
phosphorane 13 (1 J (P–CH2) = 116 Hz) bearing an equatorial P C
PC
bond. It was reported that, in some phosphoranes such as 14 and 15,
where all the equatorial positions are occupied by oxygen and nitrogen
atoms, the apical P C coupling constants are as large as 117–128 Hz.8
These reports associated with the results given in Table II suggest that
anomalously large apical coupling constants are one of the characteris-
tics of pentacoordinate phosphorus compounds where all the equatorial
positions are occupied by electronegative atoms.
Usually, alkoxy- or aryloxy-substituted hydrophosphoranes showed
tautomerization with the trivalent tautomer. The tautomerization of 1-
hydro-5-carbaphosphatrane 1 was not observed by NMR even at the
higher temperature (Scheme 3), while 1-hydro-6-carbaphosphatrane
exists as the mixture of the pentavalent and trivalent tautomers.
SCHEME 3
Variable temperature NMR in various solvents was measured in or-
der to investigate the dynamic equilibrium between 2 and 8. The Keq
value for eq. 1 was determined over a temperature range of 307–342 K
and thermodynamic parameters were estimated from a van’t Hoff plot.
They were also obtained by theoretical calculations on the model com-
pounds 16 and 17 (without t-Bu groups) at the B3LYP/6-31G(d) level
(Table II).
While 2 is enthalpicly favorable, 8 is more stable due to entropy ef-
fects. Taking into consideration that an analogous trivalent species was
not observed in the case of 1, ring expansion from five-membered ring

Chemistry of Carbaphosphatranes
311
TABLE II Thermodynamic Parameters for eq (1) at 298K
K
eq
ꢀ
ꢁ
2(16)
8(17)
(1)
◦
ꢀS
solvent
Keq
ꢀG◦
ꢀH◦
Exp.
benzene
CHCl3
THF
1.3
2.4
4.5
12
−0.2
−0.5
−0.8
−1.5
3.4
2.8
2.5
0.9
50
46
46
34
Calcd.
to six-membered ring destabilizes the “anti-apicophilic” phosphorane
form. In addition, the ratio of 2 to 8 depends on the polarities of sol-
vents. The tautomer 8, which has a larger dipole moment than 2 as
predicted by theoretical calculations, is preferred to 2 in a more polar
SCHEME 4

312
T. Kawashima
solvent. Hydrogen bonding between 8 and THF may contribute to its
stability in THF.
Although tautomerization between 1-hydro-5-carbaphosphatrane 1
and the trivalent tautomer was not observed by NMR spectroscopy, ox-
idation and sulfurization of 1 to give cyclic phosphonate 18 and thiox-
ophosphonate 19 suggest the existence of tautomerization (Scheme 4).
On the other hand, 1-hydro-6-carbaphosphatrane 2 was oxidized or sul-
furized more easily than in the case of 1. These results consist with
the direct observation of tautomerization between the pentavalent and
trivalent species in the case of 1-hydro-6-carbaphosphatrane.
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