Synthesis, isolation, and skeletal inversion of 5,10-dihydrophosphanthrenes

Article abstract of DOI:10.1080/10426507.2010.515957

Two stereoisomers, the trans form 1a and the cis form 1b, of 2,3,7,8-tetramethyl- 5,10-diphenyl-5,10-dihydrophosphanthrene dioxide (1) were formed by spontaneous oxidation in air of the corresponding 5,10- dihydrophosphanthrenes 2 and were separated by recrystallization. Both 1a and 1b were characterized by X-ray crystallographic analysis, which revealed configurations at the phosphorus and conformations of the central tricyclic system. The 31P NMR of the reaction mixture showed at first two signals at-15.3 and-15.1 ppm, which shifted to 9.11 and 10.3 ppm after purification of the reaction mixture under aerobic conditions by oxidation of phosphorus atoms. The reaction of 1b with HSiCl3 in toluene-d8 at 100 °C gave no change, whereas 1a reacted with HSiCl3 to give reduced compounds 2a and 2b under the same conditions. The trans-cis isomerization of 2 occurred via vertex inversion at phosphorus atom. VT-NMR, X-ray analysis, and density functional theory (DFT) calculation showed that the conformational exchange of the cis form 1b proceeded via skeletal inversion of the tricyclic ring. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon and the
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Synthesis, Isolation, and
Skeletal Inversion of 5,10-
Dihydrophosphanthrenes
Yosuke Uchiyama a & Yasuhiro Mazaki a
a Department of Chemistry, School of Science , Kitasato University ,
Sagamihara , Kanagawa , Japan
Published online: 25 Apr 2011.
To cite this article: Yosuke Uchiyama & Yasuhiro Mazaki (2011) Synthesis, Isolation, and Skeletal
Inversion of 5,10-Dihydrophosphanthrenes, Phosphorus, Sulfur, and Silicon and the Related Elements,
186:4, 822-825, DOI: 10.1080/10426507.2010.515957
To link to this article: http://dx.doi.org/10.1080/10426507.2010.515957
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Phosphorus, Sulfur, and Silicon, 186:822–825, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.515957
SYNTHESIS, ISOLATION, AND SKELETAL INVERSION
OF 5,10-DIHYDROPHOSPHANTHRENES
Yosuke Uchiyama and Yasuhiro Mazaki
Department of Chemistry, School of Science, Kitasato University, Sagamihara,
Kanagawa, Japan
Abstract Two stereoisomers, the trans form 1a and the cis form 1b, of 2,3,7,8-tetramethyl-
5,10-diphenyl-5,10-dihydrophosphanthrene dioxide (1) were formed by spontaneous oxidation
in air of the corresponding 5,10-dihydrophosphanthrenes 2 and were separated by recrystal-
lization. Both 1a and 1b were characterized by X-ray crystallographic analysis, which revealed
configurations at the phosphorus and conformations of the central tricyclic system. The 31P
NMR of the reaction mixture showed at first two signals at −15.3 and −15.1 ppm, which shifted
to 9.11 and 10.3 ppm after purification of the reaction mixture under aerobic conditions by
oxidation of phosphorus atoms. The reaction of 1b with HSiCl3 in toluene-d8 at 100 ◦C gave
no change, whereas 1a reacted with HSiCl3 to give reduced compounds 2a and 2b under the
same conditions. The trans–cis isomerization of 2 occurred via vertex inversion at phosphorus
atom. VT-NMR, X-ray analysis, and density functional theory (DFT) calculation showed that
the conformational exchange of the cis form 1b proceeded via skeletal inversion of the tricyclic
ring.
Keywords Isomerization; skeletal inversion; stereochemistry; VT-NMR; X-ray crystallo-
graphic analysis
INTRODUCTION
Conformational and configurational exchanges are important and fundamental phe-
nomena in a molecular control system.1 5,10-Disubstituted-5,10-dihydroheteroanthrenes
with group 15 elements have been expected to be one of the interconversion sys-
tems allowing both of the configurational and conformational exchanges. 5,10-
Dihydrophosphanthrene and 5,10-dihydroarsanthrene were reported, but the structure and
reactivity to electrophiles of only configurational isomers, a trans form and a cis form, are
known. As for the conformational isomers of these compounds, an exo cis and an endo cis
form, the isolation and property studies remain to be investigated.2 We will report here the
synthesis and isolation of two stereoisomers, the trans form 1a and the cis form 1b, of
Received 24 July 2010; accepted 12 August 2010.
This study was partially supported by a Grant-in-Aid for Encouragement of Young Scientists (B) to Y. U.
(No. 16750041 and No. 18750037) from the Ministry of Education, Culture, Sports, Science and Technology of
Japan and a Kitasato University Research Grant for Young Researchers.
Address correspondence to Yosuke Uchiyama, Department of Chemistry, School of Science, Kitasato
University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan. E-mail: yosuke@kitasato-
u.ac.jp
822

5,10-DIHYDROPHOSPHANTHRENES
823
Figure 1 Molecular structure of the trans form 1a and the endo cis form 1bꢁ drawn with 50% probability.
5,10-dihydrophosphanthrene dioxide 1 and will describe the equilibrium of the trans–cis
and the endo cis–exo cis isomerizations.
RESULTS AND DISCUSSION
Bis(2-bromo-4,5-dimethylphenyl)phenylphosphine (3) was obtained by the reaction
of 4,5-dibromo-o-xylene with n-BuLi followed by the addition of PhPCl2. The reaction
of diarylphenylphosphine 3 with 4 equiv. of t-BuLi followed by the addition of PhPCl2
afforded the dioxide 1 in 58% yield (Scheme 1).3 The crude material reproducibly contained
1
two stereoisomers 1a and 1b in the ratio of 1:1, which was estimated by the H NMR
spectroscopy. In the 31P NMR of the reaction mixture, the signals were at first observed at
−15.3 and −15.1 ppm. After workup under aerobic conditions, the signals were shifted to
9.11 and 10.3 ppm. The results show that compound 2 formed at first was spontaneously
oxidized in air to afford the dioxide 1. Recrystallization of the stereoisomeric mixture of
1a and 1b from CH3CN gave the cis form 1b (δP 10.3). Then the solvent of the filtrate
was evaporated and the residue recrystallized from benzene to afford the trans form 1a (δP
9.11). The structures were determined by X-ray crystallographic analysis (Figure 1).
Ph
Ph
P
Ph
P
1) t-BuLi
O
O
P
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
2) PhPCl2
[O]
THF, –78 °C
Br Br
P
Ph
P
3
Ph
2 (δP –15.3, –15.1)
1 (58%, δP 9.11, 10.3)
1:1 mixture
Scheme 1
The trans form 1a has a planar structure because of the steric repulsion between
the substituents at 5,10-positions and the central tricyclic ring. In the cis form 1b, the
central six-membered ring adopts a boat conformation with the two phenyl groups in the
pseudoaxial positions, referred to as the endo cis form 1bꢁ, which is expected to be in
equilibrium with the exo cis form 1bꢁꢁ in solution.

824
Y. UCHIYAMA AND Y. MAZAKI
The reaction of 1a with HSiCl3 at 100◦C for 2 h gave products that have two 31P
signals at −15.7 and −16.6 ppm in toluene-d8. The results indicate that 1a was reduced to
give 5,10-dihydrophosphanthrene 2a, and the trans–cis isomerization occurred via vertex
inversion at the phosphorus to provide a mixture of 2a and 2b (Scheme 2). On the other
hand, the reduction of the cis form 1b did not proceed at all under the same condition as
1a, because of large steric repulsion between phenyl groups at 5,10-positions.
Ph
P
Ph
P
Ph
Ph
O
O
Me
Me
P
P
Me
Me
Me
Me
P
P
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
HSiCl3
[O]
HSiCl3
[O]
P
P
O
O
Ph
Ph
Ph
2a
Ph
2b
1a
1b
Scheme 2
1
In the H and 31P NMR spectra, it was difficult to detect the equilibrium between
1bꢁ and 1bꢁꢁ even at −105 ◦C, though the signals of the methyl groups at 2,3,7,8-positions
and the phosphorus signals were observed as somewhat broad signals. The conformational
exchange proceeded quickly in a CD2Cl2 solution due to the low energy barrier suggested
by density functional theory (DFT) calculations [B3LYP/6–31(d)] (Scheme 3).
Scheme 3
The transition state of the endo cis–exo cis isomerization of the dioxide without
methyl groups was obtained by the synchronous transit-guided quasi-newton (STQN)
method. The conformational isomerization proceeds via skeletal inversion, and the transi-
tion state is less stable than the ground states by 3.1 and 0.10 kcal/mol, in agreement with
the experimental results by the VT-NMR as well as the conformational behavior of the
9,10-dihydroanthracene derivatives.1,4
CONCLUSION
We report the synthesis, isolation, and skeletal inversion of 5,10-dihydrophosphan
threne dioxides 1. The structures of the trans and the cis forms were revealed by X-
ray crystallographic analysis. The trans–cis and the endo cis–exo cis isomerizations were
expected to proceed via vertex inversion at the phosphorus and skeletal inversion of the
tricyclic system based on experimental and calculated results, respectively.5,6 The skeletal
inversion would be new interconversion mode in a molecular control system and 5,10-
dihydrophosphanthrene skeletons have the possibilities of multifunctional abilities.

5,10-DIHYDROPHOSPHANTHRENES
825
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6. The inversion energies of PH3 were reported to be 35.0 (6a) and 34.7 (6b) kcal/mol, respectively.
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