Completely stereoselective synthesis of a chiral tetra(tertiary phosphine). Crystal and molecular structure of [OC-6-22(/?,/?)]-(±)-dichloro{l,2-bis[(2-dimethyIphosphinophenyl) methylphosphino]benzene-P,P',P",P'"}cobalt(ni) hexafluorophosphate f

Article abstract of DOI:10.1039/b002662j

Reaction of (±)-(2-chlorophenyl)(2-dimethylphosphinophenyl)methylphosphine with sodium (2-dimethylphosphinophenyl)methylphosphide is completely stereoselective giving (.R,A)-l,2-bis[(2-dimethylphosphinophenyl)methylphosphino]benzene, (R,R)-1; as confirmed

Full text of DOI:10.1039/b002662j

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Completely stereoselective synthesis of a chiral tetra(tertiary
phosphine). Crystal and molecular structure of [OC-6-22-
(R*,R*)]-( )-dichloro{1,2-bis[(2-dimethylphosphinophenyl)-
methylphosphino]benzene-P,PЈ,PЉ,Pٞ}cobalt(III)
hexafluorophosphate†
Swarup Chatterjee,a Roy Doyle,a David C. R. Hockless,b Geoffrey Salem*a and
Anthony C. Willisb
a Chemistry Department, The Faculties, Australian National University, Canberra,
A. C. T. 0200, Australia. E-mail: Geoff.Salem@anu.edu.au
b Research School of Chemistry, Australian National University, Canberra, A. C. T. 0200,
Australia
Received 4th March 2000, Accepted 11th May 2000
Published on the Web 26th May 2000
Reaction of ( )-(2-chlorophenyl)(2-dimethylphosphino-
phenyl)methylphosphine with sodium (2-dimethylphos-
phinophenyl)methylphosphide is completely stereoselective
giving
(R*,R*)-1,2-bis[(2-dimethylphosphinophenyl)-
methylphosphino]benzene, (R*,R*)-1; as confirmed by a
crystal structure determination of cis-ꢀ-[CoCl2{(R*,R*)-1}]-
PF6.
Following the reported preparation of tris(2-diphenylphos-
phinophenyl)phosphine in 1963,1 a variety of tetra(tertiary
phosphines) have been synthesised and utilised as ligands in
coordination chemistry.2 Of particular relevance to the current
work are the linear tetradentate phosphines, of which 1,2-
bis{(diphenylphosphinoethyl)phenylphosphino}ethane, tetra-
phos,3 is arguably the best known example. Several examples
of ligands of this type have been reported in the literature in
which the four phosphorus centres are linked by one or more
methylene groups.2 Furthermore, they typically contain two
stereogenic donor atoms (the terminal phosphorus donor
atoms are non-stereogenic) and hence exist in racemic and meso
diastereomeric forms. The two diastereomers have been separ-
ated for certain of these linear tetradentate phosphines, includ-
ing tetraphos. In addition, the (R*,R*) form of tetraphos has
been successfully resolved by the method of metal complex-
ation.4 To date, however, there has been no report of a stereo-
selective synthesis of a linear tetra(tertiary phosphine). This is
an important consideration if ligands of this type are to be
investigated as potential chiral auxiliaries in asymmetric
synthesis.
In this communication, we report on the completely stereo-
selective synthesis of the dissymmetric tetra(tertiary phosphine)
(R*,R*)-1,2-bis[(2-dimethylphosphinophenyl)methylphos-
phino]benzene, (R*,R*)-1. An essential feature of the ligand is
the presence of three 1,2-phenylene linkages. Their presence is
not only responsible for the high stereoselectivity observed in
the synthesis of (R*,R*)-1 but also for the exclusive formation
of the cis-α diastereomer on complexation of the tetra(tertiary
phosphine) to cobalt().
Scheme 1 Only one of the enantiomers of (R*,R*)-1 and ( )-2 is
depicted. Reagents and conditions: (i) 2 BunLi, THF; 2 MeI, THF;
(ii) K, NH3(l); MeI, THF; (iii), Na, THF; 1,2-dichlorobenzene,
THF; (iv), sodium (2-dimethylphosphinophenyl)methylphosphide,
THF; (v), [Co(H2O)6]Cl2, MeOH, air; (vi), MeOH, NH4PF6 in water.
phide (Scheme 1). This approach was taken for two reasons:
firstly, the coupling of appropriately designed optically active
bidentate ligands could provide a general synthetic route to
optically active quadridentate ligands; and secondly, the reso-
lution of chiral bidentate ligands via the method of metal com-
plexation is particularly well established.5 We have previously
demonstrated the appropriateness of this approach in the stereo-
selective synthesis of a related chiral quadridentate ligand with
As2NP donor atoms, viz. (R*,S*)-( )-1-[(2-dimethylarsino-
phenyl)methylarsino]-2-[(2-aminophenyl)methylphosphino]-
benzene.6,7
Compound ( )-2 was prepared via stepwise alkylation of
1,2-phenylenebis(phosphine)8 (Scheme 1). Deprotonation of
the bis(primary phosphine) by reaction with two equivalents
of n-butyllithium in THF followed by the addition of a solution
of methyl iodide in the same solvent gave (R*,R*)- and (R*,S*)-
1,2-phenylenebis(methylphosphine) in high yield.8 Mono-
deprotonation of the bis(secondary phosphine) was achieved
using potassium in liquid ammonia and gave a ca. 4:1 mixture
of ( )-(2-dimethylphosphinophenyl)methylphosphine and 1,2-
phenylenebis(dimethylphosphine), 3, upon the addition of
methyl iodide. Subsequent reaction of the ca. 4:1 mixture
with sodium in THF followed by the addition of 1,2-dichloro-
The basic strategy behind the synthesis of (R*,R*)-1 involved
the coupling of two suitably designed bidentate ligands, ( )-(2-
chlorophenyl)(2-dimethylphosphinophenyl)methylphosphine,
( )-2, and sodium (2-dimethylphosphinophenyl)methylphos-
† Electronic supplementary information (ESI) available: experimental
and spectroscopic data for ( )-(2-dimethylphosphinophenyl)methyl-
phosphine, ( )-2, 3, cis-( )-[CoCl2{(R*,R*)-1}]PF6 and trans-[CoCl2-
(3)2]Cl. See http://www.rsc.org/suppdata/dt/b0/b002662j/
DOI: 10.1039/b002662j
J. Chem. Soc., Dalton Trans., 2000, 1829–1830
This journal is © The Royal Society of Chemistry 2000
1829

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atoms of the quadridentate ligand have the same relative
configurations.
Resolution of ( )-2 via the metal complexation method
is currently in progress. The optically active bidentate (R)-2
[or (S)-2] should again react with sodium (2-dimethylphos-
phino phenyl)methylphosphide in a completely stereoselective
manner to give (R,R)-1 [or (S,S)-1]. A stereoselective route to
chiral tetra(tertiary phosphines) which form cis-α complexes
exclusively on coordination to a transition metal ion has
important implications in asymmetric synthesis.
Acknowledgements
The authors gratefully acknowledge the financial support of
this project by the Australian Research Council.
Notes and references
‡ Crystal data for cis-α-[CoCl2{(R*,R*)-1}]PF6: C24H30F6Cl2CoP5,
M = 717.20, monoclinic, space group P21/a (no. 14), a = 12.630(4),
b = 19.034(4), c = 12.745(4) Å, β = 95.75(2)Њ, U = 3048(1) Å3, T = 23 ЊC,
Dc = 1.563 g cmϪ3 for Z = 4, F(000) = 1456, µ(Mo-Kα) = 10.47 cmϪ1. Of
5846 measured intensities, 5582 were considered observed [I > 3σ(I)].
After correction for Lorentz and polarisation effects, the structure was
solved by direct methods and expanded using Fourier techniques. Sub-
sequent refinement (full-matrix least squares) afforded R and Rw values
of 0.045 and 0.046, respectively. CCDC reference number 186/1979. See
http://www.rsc.org/suppdata/dt/b0/b002662j/ for crystallographic files
in .cif format.
Fig. 1 Molecular structure of the cation cis-α-[CoCl2{(R*,R*)-1}]ϩ.
Selected bond distances and angles are as follows: Co–P(1) 2.243(2),
Co–P(2) 2.176(2), Co–P(3) 2.173(2), Co–P(4) 2.248(2), Co–Cl(1)
2.314(2), Co–Cl(2) 2.270(2) Å; P(1)–Co–P(2) 85.73(6), P(1)–Co–Cl(2)
84.46(7), P(2)–Co–Cl(2) 168.18(7), P(2)–Co–P(4) 104.00(6), Cl(1)–Co–
P(3) 168.88(6), P(3)–Co–P(4) 86.95(6), P(1)–Co–Cl(1) 86.88(6), P(1)–
Co–P(3) 102.34(7), P(2)–Co–P(3) 88.12(6), Cl(1)–Co–Cl(2) 99.65(7),
Cl(1)–Co–P(4) 85.03(6) and Cl(2)–Co–P(4) 86.74(7)Њ.
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2 F. A. Cotton and B. Hong, Prog. Inorg. Chem., 1992, 40, 179, and
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benzene gave ( )-2 and unreacted 3, which were separated by
fractional distillation.
The product, (R*,R*)-1, from the reaction of ( )-2 with
sodium (2-dimethylphosphinophenyl)methylphosphide (con-
taining 3) in THF was separated from 3 by complexation to
cobalt(). Two complexes were isolated containing these ter-
tiary phosphines, viz. cis-α-[CoCl2{(R*,R*)-1}]Cl and trans-
[CoCl2(3)2]Cl. The former was converted to the corresponding
hexafluorophosphate salt by metathesis with an aqueous
solution of NH4PF6 and its structure has been confirmed by an
X-ray crystallographic analysis (Fig. 1).‡ The complex is a
racemic compound with the ∆-(S,S) and Λ-(R,R) forms of the
cation being present in the unit cell (only the former is shown
in Fig. 1). It is clear from the structural data that the cis-α
diastereomer is formed exclusively and that both stereogenic
6 R. J. Doyle, G. Salem and A. C. Willis, J. Chem. Soc., Chem.
Commun., 1994, 1587.
7 R. J. Doyle, G. Salem and A. C. Willis, J. Chem. Soc., Dalton Trans.,
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8 E. P. Kyba, M. C. Kerby and S. P. Rines, Organometallics, 1983, 2,
1877.
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