Stereospecific construction of a trans-1,4-diphosphacyclohexane skeleton

Article abstract of DOI:10.1021/ol800261j

(Chemical Equation Presented) trans-1,4-Diphosphacyclohexanes were successfully synthesized by the stereospecific intramolecular coupling reaction of the optically active bisphosphine. This is a new route for the construction of the trans-1,4-diphosphacyclohexane skeleton. A cis isomer was also prepared along with the trans isomer from a mixture of rac- and meso-bisphosphines. The coordinated boranes were easily removed to afford the corresponding 1,4-diphosphacyclohexanes.

Full text of DOI:10.1021/ol800261j

ORGANIC
LETTERS
2008
Vol. 10, No. 7
1489-1492
Stereospecific Construction of a
trans-1,4-Diphosphacyclohexane
Skeleton
Yasuhiro Morisaki,* Hiroaki Imoto, Yuko Ouchi, Yuuya Nagata, and
Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
chujo@chujo.synchem.kyoto-u.ac.jp; ymo@chujo.synchem.kyoto-u.ac.jp
Received February 5, 2008
ABSTRACT
trans-1,4-Diphosphacyclohexanes were successfully synthesized by the stereospecific intramolecular coupling reaction of the optically active
bisphosphine. This is a new route for the construction of the trans-1,4-diphosphacyclohexane skeleton. A cis isomer was also prepared along
with the trans isomer from a mixture of rac- and meso-bisphosphines. The coordinated boranes were easily removed to afford the corresponding
1,4-diphosphacyclohexanes.
There has been a long-standing interest in the synthesis,
structure, reactivity, and coordination properties of heteroa-
tom-containing ring systems. Among them, diphosphacy-
cloalkanes play a considerably important role in organome-
tallic chemistry. cis-Diphosphacycloalkanes are useful bidentate
ligands for transition metals,1,2 while trans-diphosphacy-
cloalkanes have potential application as building blocks for
the construction of metal-organic crystal architectures. In
particular, porous coordination polymers, which comprise
transition metals and organic ligands through a self-assembly
process, are currently receiving considerable attention in view
of their possible functions such as sorption, separation, guest
alignment, and molecular storage.3 Therefore, the develop-
ment of new synthetic routes for cis- and trans-diphosphacy-
cloalkanes is of considerable importance for organometallic
and coordination chemistry.
Diphosphacycloalkanes reported to date are generally
obtained as a mixture of stereoisomers, and they are separated
by chromatography.4 The stereoselective synthesis of cis-
diphosphacycloalkane derivatives was attained by Alder and
co-workers, in which bicyclic P-P bridged bisphosphines
were ingeniously used for precursors (Scheme 1).5 Synthetic
strategy for cis-1b,4e,5c,d or trans-diphosphacycloalkanes5b,d is
(1) (a) Mercier, F.; Mathey, F. Tetrahedron Lett. 1985, 26, 1717. (b)
Cucciolito, M. E.; Felice, V. D.; Orabona, I.; Panunzi, A.; Ruffo, F. Inorg.
Chim. Acta 2003, 343, 209. (c) Arbuckle, B. W.; Musker, W. K. Polyhedron
1991, 10, 415.
(2) (a) Hockless, D. C. R.; Kang, Y. B.; McDonald, M. A.; Pabel, M.;
Willis, A. C.; Wild, S. B. Organometallics 1996, 15, 1301. (b) Cucciolito,
M. E.; De Felice, V.; Orabona, Ida; Panunzi, A.; Ruffo, F. Inorg. Chim.
Acta 2003, 343, 209. (c) Mason, L. J.; Perrault, E. M.; Miller, S. M.; Helm,
M. L. Inorg. Chem. Commun. 2006, 9, 946.
Scheme 1. Stereoselective Route to cis-Diphosphacycloalkane
Derivatives Reported by Alder and Co-workers5
(3) (a) Moulton, B.; Zaworotko, M. J. Chem. ReV. 2001, 101, 1629. (b)
Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.
Kim, J. Nature 2003, 423, 705. (c) Janiak, C. Dalton Trans. 2003, 2781.
(d) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43,
2334. (e) Bradshaw, D.; Claridge, J. B.; Cussen, E. J.; Prior, T. J.;
Rosseinsky, M. J. Acc. Chem. Res. 2005, 38, 273. (f) Fe´rey, G.; Mellot-
Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res. 2005, 38, 217. (g)
Uemura, T.; Horike, S.; Kitagawa, S. Chem. Asian. J. 2006, 1-2, 36.
10.1021/ol800261j CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/13/2008

relatively undeveloped;6 in particular, the stereoselective
synthesis of trans-diphosphacycloalkanes remains challeng-
ing.
phosphine from oxidation. The crude product was purified
by column chromatography on SiO2 to obtain trans-1,4-di-
tert-butyl-1,4-diphosphacyclohexane-diborane trans-2a and
trans-1,4-diphenyl-1,4-diphosphacyclohexane-diborane trans-
2b stereospecifically in 73% and 56% isolated yields,
respectively.14 On the other hand, by using 1.2 equiv of sec-
BuLi/(-)-sparteine in the lithiation step, optically active
oligomers, i.e., tetraphosphine and hexaphosphine possessing
four and six chiral phosphorus atoms, respectively, were also
formed through the intermolecular oxidative coupling
reaction.9b,c
The structures of trans-2 were confirmed by 1H, 13C, and
31P NMR, mass analysis, elemental analysis, and X-ray
crystallography. The NMR spectra of trans-2 exhibited a
chair conformation in solution. It is expected that bulky tert-
butyl and phenyl groups are equatorial. X-ray crystal-
lographic analysis revealed their structures in the solid state,
and Figure 1 shows the structure of trans-2b.15,16 Bisphos-
On the other hand, we have investigated the synthesis of
optically active polymers,7 dendrimers,8 and oligomers9
containing the P-chiral bisphosphine unit in their main chain,
in which (S,S)-1,2-bis(boranato(tert-butyl)methylphosphino)-
ethane (S,S)-1a10,11 has been employed as the key monomer.
During our studies on the incorporation of the (S,S)-1a
skeleton into the polymer main chain, we discovered the
stereospecific intramolecular coupling reaction of (S,S)-1a
to form the trans-1,4-di-tert-butyl-1,4-diphosphacyclohexane
skeleton efficiently. In this paper, we report the new synthetic
route to trans-1,4-diphosphacyclohexane, as well as the cis
stereoisomer by means of the stereospecific intramolecular
oxidative coupling reaction of the optically active bisphos-
phine compounds.
Optically active bisphosphine (S,S)-1a,10 reported by
Imamoto and co-workers, and (S,S)-1b,12 reported by Evans
and co-workers, were prepared as precursors. The treatment
of (S,S)-1 with a slightly excess of sec-BuLi with (-)-
sparteine (2.2 equiv based on (S,S)-1) generated the dilithi-
ated intermediate, as shown in Scheme 2. CuCl2 was added
Scheme 2. Stereospecific Synthesis of
trans-1,4-Diphosphacyclohexane-diborane trans-2a,b
to the reaction mixture, and then it was treated with aqueous
NH3. Although this scheme does not involve an asymmetric
reaction, we employed (-)-sparteine to activate the alkyl-
lithium reagent.13 Borane coordinated to phosphorus atom
enables the lithiation of the methyl group and also protects
Figure 1. (A) ORTEP drawing of trans-2b. Thermal ellipsoids
are drawn at the 50% probability level. (B) Top view of crystal
packing structure of trans-2b. The shortest intermolecular distance
of phenyl rings is included.
(4) (a) Issleib, K.; Wichmann, H. Chem. Ber. 1968, 101, 2197. (b)
Hackney, M. L. J.; Norman, A. D. J. Chem. Soc., Chem. Commun. 1986,
850. (c) Hackney, M. L. J.; Schubert, D. M.; Brandt, P. F.; Haltiwanger, R.
C.; Norman, A. D. Inorg. Chem. 1997, 36, 1867. (d) Schubert, D. M.;
Norman, A. D. Inorg. Chem. 1984, 23, 4131. (e) Brooks, P. J.; Gallagher,
M. J.; Sarroff, A.; Bowyer, M. Phosphorus Sulfur Silicon Relat. Elem. 1989,
44, 235. (f) Toto, S. D.; Arbuckle, B. W.; Bharadwaj, P. K.; Doi, J. T.;
Musker, W. K. Phosphorus Sulfur Silicon Relat. Elem. 1991, 56, 27. (g)
Musker, W. K. Coord. Chem. ReV. 1992, 117, 133. (h) Schubert, D. M.;
Brandt, P. F.; Norman, A. D. Inorg. Chem. 1996, 35, 6204.
(5) (a) Alder, R. W.; Read, D. Coord. Chem. ReV. 1998, 176, 113. (b)
Alder, R. W.; Ganter, C.; Harris, C. J.; Orpen, A. G. J. Chem. Soc., Chem.
Commun. 1992, 1170. (c) Alder, R. W.; Ellis, D. D.; Hogg, J. K.; Mart´ın,
A.; Orpen, A. G.; Taylor, P. N. Chem. Commun. 1996, 537. (d) Alder, R.
W.; Ganter, C.; Gil, M.; Gleiter, R.; Harris, C. J.; Harris, S. E.; Lange, H.;
Orpen, A. G.; Taylor, P. N. J. Chem. Soc., Perkin Trans. 1 1998, 1643.
phine trans-2b adopts the chair conformation with equatorial
phenyl groups and axial boranes as shown in Figure 1A. The
(6) Recently, Gates reported the synthesis of diphosphiranium (P2C, i.e.,
diphosphacyclopropane skeleton) and diphosphetanium (P2C2, i.e., 1,3-
diphosphacyclobutane skeleton) cyclic cations from the corresponding
phospha-alkenes: Bates, J. I.; Gates, D. P. J. Am. Chem. Soc. 2006, 128,
15998.
(7) (a) Morisaki, Y.; Ouchi, Y.; Tsurui, K.; Chujo, Y. J. Polym. Sci.
Part A: Polym. Chem. 2007, 45, 866. (b) Morisaki, Y.; Ouchi, Y.; Tsurui,
K.; Chujo, Y. Polym. Bull. 2007, 58, 665. (c) Ouchi, Y.; Morisaki, Y.;
Ogoshi, T.; Chujo, Y. Chem. Asian J. 2007, 2, 397.
(8) Ouchi, Y.; Morisaki, Y.; Chujo, Y. Polym. Bull. 2007, 59, 339.
1490
Org. Lett., Vol. 10, No. 7, 2008

phenyl rings are found to be perpendicular to the diphos-
phacyclohexane skeleton, as shown in the side and top view
in Figure 1A. In the crystal packing structure of trans-2b
shown in Figure 1B, the shortest intermolecular distance of
phenyl rings is 5.001 Å; therefore, there is no effective π-π
stacking between neighboring phenyl groups. On the other
hand, trans-3b (vide infra), which does not have boranes,
was synthesized by another method and characterized by
X-ray crystallography.4e Interestingly, the ORTEP drawing
shows that trans-3b adopts chair-conformation possessing
the axial phenyl groups and equatorial lone pairs.4e
2. The separation of the isomers by column chromatography
on SiO2 was possible to afford the corresponding cis-2a and
trans-2a in 18% and 20% yields and cis-2b and trans-2b in
16% and 17% yields (Scheme 3).
1
The structures of cis-2 were also confirmed by H, 13C,
and 31P NMR, mass analysis, and elemental analysis. The
13C and 31P NMR spectra of cis-2a are shown in Figure 2 as
representative spectra.18 Compound cis-2a had two ethylene
carbons separated clearly at 12.88 and 12.91 ppm, which
In order to construct the corresponding cis stereoisomer,
optically inactive bisphosphine meso-1, i.e., (S,R)-1 and/or
(R,S)-1, should be prepared as a precursor.17 Bisphosphine
meso-1 was synthesized by the coupling reaction of dim-
ethylphosphine-borane without (-)-sparteine, and rac-bis-
phosphine rac-1 ((S,S)-1 and (R,R)-1) was also obtained
simultaneously (Scheme 3). Although the isolation of the
Scheme 3. Synthesis of
cis-1,4-Diphosphacyclohexane-diborane cis-2a,b and trans-2a,b
Figure 2. 13C NMR (150.9 MHz, CD2Cl2) and 31P NMR (242.9
MHz, CD2Cl2) spectra of cis-2a.18
were coupled with the phosphorus atoms with JCP ) 31.1
and 30.3 Hz, respectively. The methyl carbons of tert-butyl
groups appeared at 25.40 and 25.41 ppm. The two quaternary
carbons of the tert-butyl groups seemed to be overlapped,
and they appeared at 28.9 ppm (JCP ) 30.5 Hz). In the 31P
NMR spectrum, two types of doublet peaks with JPP ) 51.3
pure meso-1 was not achieved, the intramolecular coupling
reaction of a mixture of rac-1 and meso-1 was carried out
(Scheme 3). The reaction proceeded smoothly to yield cis-
1,4-diphosphacyclohexane-diborane cis-2, as well as trans-
(9) (a) Morisaki, Y.; Ouchi, Y.; Fukui, T.; Naka, K.; Chujo, Y.
Tetrahedron Lett. 2005, 46, 7011. (b) Morisaki, Y.; Ouchi, Y.; Naka, K.;
Chujo, Y. Tetrahedron Lett. 2007, 48, 1451. (c) Morisaki, Y.; Ouchi, Y.;
Naka, K.; Chujo, Y. Chem. Asian J. 2007, 2, 1166.
1
Hz were observed at +24.2 and +24.7 ppm. The H NMR
spectrum also exhibited two kinds of tert-butyl groups, as
shown in Figure S8. These spectra suggest that cis-2a adopts
a chair conformation with both equatorial and axial tert-butyl
groups in solution. Incidentally, it is reported that cis-2b acts
(10) Imamoto, T.; Watanabe, J.; Wada, Y.; Masuda, H.; Yamada, H.;
Tsuruta, H.; Matsukawa, S.; Yamaguchi, K. J. Am. Chem. Soc. 1998, 120,
1635. It is commercially available from Tokyo Kasei Kogyo (TCI).
(11) For recent reviews, see: (a) Cre´py, K. V. L.; Imamoto, T. Top.
Curr. Chem. 2003, 229, 1. (b) Cre´py, K. V. L.; Imamoto, T. AdV. Synth.
Catal. 2003, 345(1-2), 79.
(15) The crystal data for trans-2b are as follows. C16H24B2P2; fw )
299.91, triclinic, P1h (No. 2), a ) 6.761(3) Å, b ) 7.616(4) Å, c ) 9.075-
(12) Muci, A. R.; Campos, K. R.; Evans, D. A. J. Am. Chem. Soc. 1995,
117, 9075.
(6) Å, b ) 109.96(3)°, V ) 420.3(4) Å3, Z ) 1, Dcalcd ) 1.185 g cm-3
The refinement converged to R ) 0.044, Rw ) 0.137, GOF ) 1.092.
.
(13) The reaction without amine or with other amines such as N,N,N′,N′-
tetramethylethylenediamine (TMEDA) as a ligand proceede smoothly to
obtain the corresponding 1,4-diphosphacyclohexane; however, the best result
was obtained when (-)-sparteine was employed in the lithiation step.
(14) The oxidative intramolecular coupling reaction of enantiomer (R,R)-1
also provides the trans isomer (trans-2) stereospecifically. Synthesis of (R,R)-
1a was reported by Imamoto and Cre´py: Imamoto, T.; Cre´py K. V. L.
Tetrahedron Lett. 2002, 43, 7735.
(16) ORTEP drawing of trans-2a is shown in Figure S14 in the
Supporting Information, although a desired single crystal could not be
obtained.
(17) The stereoselective or stereospecific synthesis of meso-1 has not
yet been achieved and challenged.
(18) Original spectra are shown in Figures S9 and S10 in the Supporting
Information.
Org. Lett., Vol. 10, No. 7, 2008
1491

as a bidentate ligand for transition metals such as nickel,2b
palladium,2b,c and platinum2b,c by adopting the boat confor-
mation.
The removal of the coordinated boranes is demonstrated
in the following deboranation reaction (Scheme 4). The
boranes of trans-2a were removed by reaction with excess
clohexane skeleton by the stereospecific intramolecular
oxidative coupling reaction of the optically active bisphos-
phines (S,S)-1. In addition, we demonstrated that cis-1,4-
diphosphacyclohexanes were readily obtained by the oxida-
tive coupling reaction of the mixture of rac- and meso-
bisphosphines (rac-1 and meso-1). Further investigations to
employ various optically active bisphosphines as a precursor
and to reveal the coordination behaviors of the trans- and
cis-1,4-diphosphacyclohexanes after deboranation are cur-
rently underway.
Scheme 4. Removal of Boranes
Acknowledgment. This work was supported by Grant-
in-Aid for Science Research on Priority Areas (No. 19027028,
Synergy of Elements) from Ministry of Education, Culture,
Sports, Science and Technology, Japan. The authors are
grateful to Dr. Hideo Koizumi in NTT Advanced Technology
Corporation for solving the crystal structure of trans-2a. The
authors are also grateful to Mr. Haruo Fujita and Mrs. Keiko
Kuwata in Kyoto University for NMR (600 MHz) and
HRMS analysis, respectively. Y.O. appreciates Research
Fellowships from the Japan Society for the Promotion of
Science for Young Scientists.
trifluoromethanesulfonic acid, and subsequently with potas-
sium hydroxide for alkylphosphine-boranes to obtain trans-
3a in 91% yield.19 Boranes of trans-2b could be easily
removed under relatively mild conditions by treatment with
1,4-diazabicyclo[2.2.2]octane (DABCO) to afford the cor-
responding trans-3b in 94% yield.
Supporting Information Available: Experimental pro-
cedures, compound characterization data, NMR spectra,
X-ray crystallographic data, and an X-ray crystallographic
file (CIF) for trans-2b. This material is available free of
charge via the Internet at http://pubs.acs.org.
In conclusion, we have developed the first and practical
method for the construction of the trans-1,4-diphosphacy-
(19) (a) McKinstry, L.; Livinghouse, T. Tetrahedron Lett. 1994, 35, 9319.
(b) McKinstry, L.; Livinghouse, T. Tetrahedron 1994, 50, 6145.
OL800261J
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Org. Lett., Vol. 10, No. 7, 2008