Effect of σ*P-O orbital on structure, stereomutation, and reactivity of C-apical O-equatorial spirophosphoranes

Article abstract of DOI:10.1080/10426500212284

Effect of σ*_P-O orbital of C-apical O-equatorial (O-cis) spirophosphorane was investigated both experimentally and theoretically. O-cis phosphoranes revealed to be much more electrophilic on the phosphorus atom than O-trans isomers by experimen

Full text of DOI:10.1080/10426500212284

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Effect of σ* P--O Orbital on Structure, Stereomutation,
and Reactivity of C -Apical O -Equatorial
Spirophosphoranes
Kin-ya Akiba a , Shiro Matsukawa b , Takahiro Adachi b , Yohsuke Yamamoto b , Suyong Re c &
Shigeru Nagase c
a Advanced Research Center for Science and Engineering , Waseda University , 3-4-1 Ohkubo,
Tokyo, 169-8555, Japan
b Department of Chemistry, Graduate School of Science , Hiroshima University , 1-3-1
Kagamiyama, Higashi-Hiroshima, 739-8526, Japan
c Department of Theoretical Studies , Institute for Molecular Science , Myodaiji, Okazaki,
444-8585, Japan
Published online: 27 Oct 2010.
To cite this article: Kin-ya Akiba , Shiro Matsukawa , Takahiro Adachi , Yohsuke Yamamoto , Suyong Re & Shigeru Nagase
(2002) Effect of σ* P--O Orbital on Structure, Stereomutation, and Reactivity of C -Apical O -Equatorial Spirophosphoranes,
Phosphorus, Sulfur, and Silicon and the Related Elements, 177:6-7, 1671-1675, DOI: 10.1080/10426500212284
To link to this article: http://dx.doi.org/10.1080/10426500212284
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:1671–1675
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DOI: 10.1080/10426500290093775
EFFECT OF
ORBITAL ON STRUCTURE,
O
P
STEREOMUTATION, AND REACTIVITY OF C-APICAL
O-EQUATORIAL SPIROPHOSPHORANES
Kin-ya Akiba,a Shiro Matsukawa,b Takahiro Adachi,b
Yohsuke Yamamoto,b Suyong Re,c and Shigeru Nagasec
Advanced Research Center for Science and Engineering, Waseda
University, 3-4-1 Ohkubo, Tokyo 169-8555, Japan;a Department
of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan;b
and Department of Theoretical Studies, Institute for Molecular
Science, Myodaiji, Okazaki 444-8585, Japanc
(Received July 29, 2001; accepted December 25, 2001)
Effect of
orbital of C-apical O-equatorial (O-cis) spirophos-
O
P
phorane was investigated both experimentally and theoretically. O-cis
phosphoranes revealed to be much more electrophilic on the phospho-
rus atom than O-trans isomers by experimental studies. Theoretically,
the energy of the
orbital of O-cis phosphorane was calculated
O
P
to be lower than that of the
orbital of O-trans phosphorane by
C
P
18.7 kcal/mol, and the result supports the enhanced electrophilicity of
O-cis spirophosphoranes compared with O-trans isomer.
Keywords: Hypervalent;
reversed apicophilicity
orbital; phosphorus; reactivity;
P
O
INTRODUCTION
We have reported on the synthesis, structure, and stereomutation
of C-apical O-equatorial (O-cis) spirophosphorane 1a and its stable
stereoisomer of O-apical C-equatorial (O-trans) spirophosphorane 2a.1
The phosphorane 1a was the first example exhibiting reversed api-
cophilicity, which could be converted to its stable pseudorotamer 2a.
O-cis phosphorane 1 has one of the two P O bonds on the equatorial
plane and therefore has a
O orbital on the same plane. On the other
P
Address correspondence to K.-y. Akiba, Advanced Research Center for Science
and Engineering, Waseda University, 3-4-1 Ohkubo, Tokyo 169-8555, Japan. E-mail:
akibaky@waseda.jp
1671

1672
K.-y. Akiba et al.
FIGURE 1
hand, O-trans phosphorane 2 has the corresponding P C bond on the
equatorial plane; hence C, too (Figure 1). Since the energy level of
should be significantly lower than that of
P
C, there should be
P
to show the difference between 1 and 2 in
P
O
an apparent effect of
P
O
reactivity such as nucleophilicity and stability of -carbanions. Now we
describe the effects of
theoretical viewpoint.
O orbital of 1 from both an experimental and
P
RESULTS AND DISCUSSION
Synthesis of O-cis Spirophosphoranes by Oxidative
Cyclization of the Dianion
As reported in 1996, we succeeded to isolate O-cis spirophosphorane 1a
by thermal cyclization of P H(apical) monocyclic phosphorane.1 How-
ever this method is not suitable for synthesis of O-cis phosphoranes
because the thermal conditions accerelate stereomutation to the cor-
responding O-trans isomer. Indeed, 1a was obtained in 71% yield and
2a in 29% yield by this thermal method. To improve this problem, a
milder oxidative cyclization method was developed in our laboratory2
(Scheme 1). The new protocol provided 1a and 1b exclusively at low
temperatures in high yields, and aryl derivative 1c could also be syn-
thesized for the first time.
The possible reaction mechanism is shown in Scheme 2. The reaction
of P H (equatorial) phosphorane A with more than 2 equivalents of RLi
provides the dianion B, where one equivalent of RLi works as a base
and the other as a nucleophile. In the resulting dianion B, the lone pair
electrons on phosphorus occupy an equatorial position because of its
extreme electron-donicity. In this situation, oxidant I2 reacts with the

Effect of
Orbital on Spirophosphoranes
O
1673
P
SCHEME 1
SCHEME 2
dianion B to produce P I (equatorial) phosphorane C, and the
remaining alkoxide anion rapidly cyclizes with simultanious elimina-
tion of I to form O-cis spirophosphorane 1 exclusively.
Enhanced Electrophilicity of O-cis Phosphorane 1: Effect
of
Orbital in the Equatorial Plane
P
O
Reactions of O-cis 1a and O-trans 2a with nucleophiles were exam-
ined. Using TBAF (tetrabutylammonium fluoride; [n-Bu]4N+F ) as a
nucleophile, the reaction of 1a readily afforded a hexacoordinate phos-
phate bearing a P F bond 3 (1 JP = 706 Hz) while 2a did not re-
F
act at all. Configuration of the phosphate 3 could not be determined
because of rapid decomposition by trace amounts of H2O. However,
we have already characterized the corresponding hexacoordinate flu-
oroantimonate with two Martin ligands by X-ray analysis;3 therefore,
the fluorine atom in phosphate 3 is also likely to be located anti to
the oxygen due to the trans influence as the fluoroantimonate. In the
case of the reaction with MeLi, 1a reacted under mild conditions to af-
ford a monocyclic phosphorane 4 after aqueous work up but 2a did not
(Scheme 3).

1674
K.-y. Akiba et al.
SCHEME 3
Stabilization of Lone Pair Electrons Adjacent to the
Phosphorus of O-cis 1: Effect of
Equatorial Plane
Orbital in the
P
O
Deprotonation of O-cis 1d with KHMDS (potassium hexamethyldisi-
lazide; (Me3Si)2N K+) occurred in THF at 0 C during 30 min and gave
-deuterated product (40% yield) after quenching the mixture with D2O.
However, O-trans 2d could not be deprotonated under the same condi-
tions. Moreover, the benzyl anion of 1d was much more stable thermally
than the neutral molecule of 1d. In THF, O-cis 1d was totally converted
to O-trans 2d at 60 C during 5 h. Under the same conditions, the benzyl
anion of 1d remained almost unchanged, which shows the significant
stability of the O-cis benzyl anion compared to the corresponding neu-
tral molecule.
Theoretical Study on
Orbitals of O-cis and O-trans
Phosphoranes in the Equatorial Plane
In order to evaluate a difference in energy between the
O orbital of
P
O-cis phosphorane and that of O-trans isomer, theoretical calculations
for 1b and 2b were carried out (B3LYP/6-31G(d)). As shown in Figure 2,
the energy of
of 1b (LUMO+4) is lower than that of
of 2b
(X = O or C)
P
O
P
C
(LUMO+5) by 18.7 kcal/mol. Other LUMOs below
P
X
were distributed on the aromatic rings of the Martin ligands. There-
fore, the responsible orbitals toward nucleophilicity and stabilization
of -carbanion should be the
orbitals. As a result, the large
P
X

Effect of
Orbital on Spirophosphoranes
O
1675
P
FIGURE 2
difference in energy between
and
orbital provided theo-
C
P
O
P
retical evidence for the enhanced reactivity of O-cis spirophosphorane
1 compared with O-trans isomer 2, as shown in Scheme 3.
REFERENCES
[1] S. Kojima, K. Kajiyama, M. Nakamoto, and K.-y. Akiba, J. Am. Chem. Soc., 118, 12866
(1996).
[2] K. Kajiyama, M. Yoshimune, M. Nakamoto, S. Matsukawa, S. Kojima, and K.-y.
Akiba, Org. Lett., 3, 1873 (2001).
[3] S. Kojima, Y. Doi, M. Okuda, and K.-y. Akiba, Organometallics, 14, 1928 (1995).