2-diphenylphosphino-2'-diphenylphosphinyl-1,1'-binaphthalene (BINAPO), an axially chiral heterobidentate ligand for enantioselective catalysis

Article abstract of DOI:10.1016/S0957-4166(98)00009-3

Racemic 2-diphenylphosphino-2'-diphenylphosphinyl-1,1'-binaphthalene (BINAPO) 5 has been prepared in four steps from 1,1'-binaphthalene-2,2'-diol (BINOL) 1 and has been resolved with the aid of the C,N-cyclopalladated complex with N,N-dimethyl (S)-1-(1-naphthyl)ethylamine 6. P,O-chelate binding to palladium occurs in solution and this is confirmed in the solid state by X-ray analysis. (S)-BINAPO is an immediate precursor of (S)-BINAP and is itself an effective chiral inducer for the Pd-catalyzed asymmetric hydrosilylation of styrene (over 70% e.e.).

Full text of DOI:10.1016/S0957-4166(98)00009-3

Pergamon
Tetrahedron: Asymmetry 9 (1998) 391–395
2-Diphenylphosphino-20-diphenylphosphinyl-1,10-binaphthalene
(BINAPO), an axially chiral heterobidentate ligand for
enantioselective catalysis
Serafino Gladiali,a,∗ Sonia Pulacchini,a Davide Fabbri,b Mario Manassero c and
Mirella Sansoni c
aDipartimento di Chimica, Università di Sassari, Via Vienna 2, 07100 Sassari, Italy
bIstituto C.N.R. I.A.T.C.A.P.A., Via Vienna 2, 07100 Sassari, Italy
cDipartimento di Chimica Strutturale e Stereochimica Inorganica and Centro C.N.R., Università di Milano, Via Venezian 21,
20133 Milano, Italy
Received 6 November 1997; accepted 8 January 1998
Abstract
Racemic 2-diphenylphosphino-20-diphenylphosphinyl-1,10-binaphthalene (BINAPO) 5 has been prepared in
four steps from 1,10-binaphthalene-2,20-diol (BINOL) 1 and has been resolved with the aid of the C,N-
cyclopalladated complex with N,N-dimethyl (S)-1-(1-naphthyl)ethylamine 6. P,O-chelate binding to palladium
occurs in solution and this is confirmed in the solid state by X-ray analysis. (S)-BINAPO is an immediate precursor
of (S)-BINAP and is itself an effective chiral inducer for the Pd-catalyzed asymmetric hydrosilylation of styrene
(over 70% e.e.). © 1998 Elsevier Science Ltd. All rights reserved.
In the course of a recent investigation aimed at the preparation of unsymmetrical binaphthalene-
templated diphosphines,1 we have developed an efficient methodology for appending one phosphino
and one phosphinyl group onto the 2,20-carbon atoms of the binaphthalene backbone. This new pro-
cedure provides an easy entry into axially chiral phosphine–phosphine oxide derivatives, an almost
unprecedented class of products, and makes them available in substantial amounts. This prompted us
to study some aspects of their coordination chemistry and to explore their potential as chiral inducers
in enantioselective catalysis. Here we report the first results obtained with 2-diphenylphosphino-20 -
diphenylphosphinyl-1,10 -binaphthalene (5; BINAPO),2 the most accessible member of this novel family
of axially chiral, potentially heterobidentate ligands.
∗
Corresponding author. Fax: (+)39-79-229559; e-mail: gladiali@ssmain.uniss.it
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00009-3

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The preparation of 5 from racemic BINOL 1 has been accomplished according to a four step reaction
path involving sequential substitution of the homotopic triflate groups of 2 by diphenylphosphine oxide.3
Dissymmetrization of the substrate is achieved in the second step of the synthetic scheme where the first
phosphorated substituent is introduced in the form of a diphenylphosphinyl group by means of palladium
chemistry following a reported procedure.4 For the second triflate group to be substituted, removal of
the oxygen of the phosphinyl group of 3 is required (HSiCl3/Et3N; xylene; 120°; 12 h; 84% yield). The
diphenylphosphino triflate 4 is then converted into BINAPO 5 by further reaction with diphenylphosphine
oxide in the presence of Pd- or Ni-catalysts (DMF; 100°C; 10 h; 70% yield). Following this set of
reactions, racemic 5 is obtained in over 45% overall yield. In the 31P-NMR spectrum, compound 5 shows
one singlet at −14.70 (sharp) and one at 27.69 ppm (fairly broad) due to the phosphino and phosphinyl
groups, respectively.
Reaction of racemic 5 with the enantiopure chloride-bridged C,N-cyclopalladated complex 65 affords
an equimolar diastereomeric mixture of two mononuclear cationic complexes, which could be isolated
in crystalline form after exchanging the chloride counterion with hexafluorophosphate.6 In the 31P-NMR
spectrum of this mixture, the resonances of the phosphino and phosphinyl groups appear as singlets
at 44.44, 43.41, 42.28 and 34.68 ppm. While the two peaks at lower field are quite sharp, some line
broadening is apparent in the other two signals and this supports the attribution to the phosphinyl
substituents. In comparison with the free ligand, all the 31P-resonances are shifted downfield and this
provides convincing evidence for the chelate coordination of BINAPO to the Pd-atom. As expected, this
deshielding effect is larger for the P-centre of the phosphino group, which is directly bound to the metal
(∆δ≈60 ppm), and is less pronounced for that of the phosphinyl group, which is separated from the
metal by the oxygen atom (∆δ 7–15 ppm). The structure of the complex can be confidently formulated
as 7 since in these Pd-complexes the softer donor is expected to take up the position trans to the NMe2
group.7
One single crystallization of this mixture from CH2Cl2–petroleum ether was sufficient to attain the
selective separation of the less soluble diastereomer in pure form and in a crystalline shape suitable for
X-ray structure determination. An ORTEP view and the most representative parameters of the structure
are reported in Fig. 1.8

S. Gladiali et al. / Tetrahedron: Asymmetry 9 (1998) 391–395
393
Fig. 1. (a) ORTEP view of the cation (less soluble diastereoisomer) of compound 7 (X=PF6). For clarity the phenyl substituents
at the phosphorus centres not involved in π-stacking are represented by a single carbon atom (C27 and C39). (b) Perspective
view of the eight-membered chelate ring
The crystal contains discrete mononuclear units of 7 in which BINAPO (S configuration) acts
as a bidentate ligand providing an eight-membered chelate ring with an irregular, partly boat-like
conformation.9 As anticipated, the harder oxygen atom occupies the coordinative position trans to
the carbon. In the pentatomic metallacycle formed by N,N-dimethyl (S)-1-(1-naphthyl)ethylamine, the
benzylic methyl group is located in an axial position. This feature is common to all the recorded X-ray
structures of Pd-complexes derived from 6 and it is due to the necessity to avoid steric repulsion between
the C(56) methyl and the hydrogen atom bound to the C(52) of the naphthylamine.10 The Pd atom is in a
butterfly-distorted square planar coordination, with the atoms Pd, P(1), O and C(46) strictly planar and the
atoms Pd, N, O and C(46) also planar, whereas the dihedral angle between their best planes is 11.7(6)°.
In the pentatomic metallacycle, the atoms Pd, C(46), C(45) and C(55) are essentially planar, whereas the
nitrogen atom is displaced 0.75 Å out from this best plane. As usual, the naphthyl rings of the binaphthyl
framework are significantly bent. They show a dihedral angle of 82.1(1)° between their average best
planes which allows the opposing hydrogens at C(17) and C(27) to be located at a comfortable distance.
These figures suggest that the chiral diaryl backbone is basically free from torsional strain. A significant
π-stacking interaction involves one of the phenyl substituents on each P-atom and the phenyl ring which
is more remote from the metal of each naphthyl group of the binaphthalene template. The dihedral angles
between the best planes of the π-stacked aromatic rings are 13.1(5)° and 8.8(8)° for the interactions
involving the naphthyl rings C(1)–C(10) and C(11)–C(20), respectively. The distances of the atoms
C(33)–C(38) from the best plane of the C(1)–C(10) naphthyl ring are in the range 3.03(1)–3.63(1) Å

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(average 3.32 Å); those of the atoms C(21)–C(26) from the best plane of C(11)–C(20) naphthyl are in
the range 3.07(1)–3.47(1) Å (average 3.28 Å).
From this complex, (S)-BINAPO 5 could be recovered in quantitative yield by treatment with a slight
excess of ethylenediamine in the biphasic system CHCl3–H2O.11 Its enantiomeric purity was determined
by deoxygenation with trichlorosilane which afforded enantiopure (S)-BINAP in excellent yield. The
same diphosphine can be obtained in a single step from complex 7 by reaction with LiAlH4 (ether; 35°C;
3 h; 70% yield). This provides an alternative synthesis of enantiopure BINAP from racemic BINOL
which adds to the other ones already available.12
(S)-BINAPO has been tested as a chiral inducer in the Pd-catalyzed asymmetric hydrosilyla-
tion of styrene. The reaction has been performed on the neat substrate at room temperature using
[Pd(allyl)Cl]2–(S)-BINAPO (1:2) as an in situ catalyst at a substrate to metal ratio 1000:1.13 A quan-
titative conversion with complete regioselectivity for the branched isomer is attained in 70 h affording,
after oxidative work-up, (S)-1-phenylethanol in 18% e.e. The e.e. increases up to 72% when the reaction
is run using benzene as a solvent. This result is comparable with the one obtained on the same substrate
with MOP14 as the chiral inducer and encourages future applications of this new ligand to asymmetric
catalysis.
Acknowledgements
Financial support from CNR, Rome ‘Progetti Finalizzati’ and M.U.R.S.T., Rome is gratefully ac-
knowledged.
References
1. S. Gladiali, A. Dore, S. Pulacchini and D. Fabbri, work in preparation.
2. (R)-BINAPO has been isolated as a co-product in the preparation of Pd(0)[(R)-BINAP]2 from palladium(II) diacetate and
(R)-BINAP. F. Ozawa, A. Kubo and T. Hayashi, Chem. Lett. 1992, 2177.
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quantitative yield, but proved unsuitable for the resolution.
6. Selected data for (S)-5: mp 230–233°C; [α]20 −92.2 (c=1.02, CHCl3); IR (KBr): 1208 cm−1; 1H-NMR (CDCl3) δ: 6.63
D
(d, J=8.1 Hz, Ar, 1H), 6.72 (dt, J=1.2, 6.6 Hz, Ar, 1H), 6.81–7.43 (series of m, Ar, 23H), 7.60 (m, Ar, 3H), 7.79 (d, J=8.1
Hz, Ar, 1H), 7.81 (d, J=8.4 Hz, Ar, 1H), 7.82 (d, J=7.8 Hz, Ar, 1H), 7.93 (dd, J=2.4, 8.4 Hz, Ar, 1H); 31P-NMR (CDCl3)
δ: −14.70, 27.69. Anal. calcd. for C44H32OP2: C, 82.74; H, 5.05. Found: C, 82.68; H, 5.16. Selected data for 7 (X=PF6):
(S,Sa)-7 (less soluble complex): mp 210–215°C (dichloromethane–petroleum ether); [α]20D −311.6 (c=0.584, CHCl3); 1H-
NMR (CDCl3) δ: 1.85 (d, J=6.6 Hz, 3H), 2.20 (d, J=6.2 Hz, 3H), 2.41 (d, J=3.2 Hz, 3H), 4.28 (m, 1H), 5.80 (d, J=8.4 Hz,
1H), 6.12 (dd, 1H), 6.53 (t, J=7.8 Hz, 1H), 6.72 (d, J=8.4 Hz, 1H), 6.85–8.12 (series of m, 34H); 31P-NMR (CDCl3) δ:
43.41, 42.28. Anal. calcd. for C58H48F6NOP3Pd·0.5CH2Cl2: C, 60.2; H, 4.32; N, 1.2. Found: C, 60.44; H, 4.48; N, 1.25.
1
(S,Ra)-7 (more soluble complex): mp 215–220°C; H-NMR (CDCl3) δ: 1.80 (d, J=6.6 Hz, 3H), 2.60 (d, J=6.4 Hz, 3H),
2.81 (d, J=3.0 Hz, 3H), 4.28 (m, 1H), 6.15–8.22 (series of m, 38H); 31P-NMR (CDCl3) δ: 44.44, 34.68.
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395
8. Crystal data for compound 7: C58H48F6NOP3Pd·0.5CH2Cl2, M=1130.8, monoclinic, space group C2 (no. 5), a=21.161(3),
b=12.111(2), c=21.437(3) Å, β=104.28(1)°, U=5324(2) Å3, Z=4, Dc=1.411 g cm−3, µ=5.4 cm−1, F(000)=2308. Reflections
measured=6667, unique=5685 with I≥3σ(I), refined parameters=666. For the correct enantiomorph final R=0.040 and
RW=0.051; for the incorrect one R=0.044 and RW=0.056. Full structural details have been deposited with the Cambridge
Crystallographic Centre.
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