9478
J. Am. Chem. Soc. 2001, 123, 9478-9479
Scheme 1
Asymmetric Catalysis with the Normally
Unresolvable, Conformationally Dynamic
2,2′-Bis(diphenylphosphino)-1,1′-biphenyl (Biphep)
Jennifer J. Becker, Peter S. White, and Michel R. Gagne´*
Department of Chemistry CB No. 3290
UniVersity of North Carolina
Chapel Hill, North Carolina 27599-3290
ReceiVed May 9, 2001
ReVised Manuscript ReceiVed July 19, 2001
Achieving high enantioselectivites in asymmetric catalysis
usually requires rigid, conformationally restricted chiral ligands
(e.g., BINAP, DuPhos).1 Recently a new approach has emerged
where conformationally flexible ligands are intentionally used to
magnify the effect of other chiral ligands.2 For example, Et2Zn
addition to aldehydes can be made enantioselective with titanium
catalysts containing flexible bis(sulfonamide) ligands in combina-
tion with a normally ineffective chiral alkoxide ligand.2a The chiral
alkoxide is proposed to magnify its chirality by inducing a
selective conformation in the sulfonamide ligand. Similarly, chiral
amine or amine N-oxide additives induce chiral conformations
in otherwise achiral (salen)manganese(II) complexes and lead to
enantioselective alkene epoxidation catalysis.2b-d 1,1′-Bis(diphe-
nylphosphino)biphenyl (biphep) interconverts between two atrop
forms, one of which is favored (3:1) when coordinated to the
chiral ruthenium fragment, Cl2Ru(S,S-dpeda), (S,S-dpeda ) (S,S)-
1,2-diphenylethylenediamine).2e-f This 3:1 mixture selectively
hydrogenates ketones in up to 92% ee.
Each of the above cases utilizes a flexible ligand to magnify
the stereochemical induction of a chiral catalyst. More rare is
the utilization of a flexible ligand that is locked into a metastable
state to provide the only source of asymmetry for catalysis.
Brintzinger’s biphenyl-bridged bisCp titanium catalyst functions
in this manner.3 In this unique case, thermolysis of a mixture of
diastereomers leads to a single complex where BINOL’s prefer-
ence for one atrop form of the ansa ligand provides an enantio-
and diastereopure titanocene precatalyst, which after BINOL
removal hydrogenates 1-phenylpyrroline in up to 98% ee. The
sole source of stereocontrol in this catalyst comes from the
biphenyl-bridged ligand. Expanding this strategy to chiral ligands
with broad utility in catalysis (e.g., diphosphines) promises to
expand the chiral ligand base beyond those normally considered
resolvable.
triflic acid (TfOH, 2 equiv) to generate5 P2Pt(OTf)2 Lewis acids,6
we reasoned that diastereopure (biphep)Pt(S-BINOL) complexes
could be a source of the enantiopure (biphep)Pt2+ Lewis acid
fragment, a potential catalyst whose chirality was solely engen-
dered in the coordinated biphep ligand (Scheme 1).
The availability of pure λ(S) and 95:5 δ(S) (biphep)Pt(S-
BINOL)4,7 provides the means to test this hypothesis. To also
generate the dicationic catalysts by traditional AgOTf treatments,
the BINOLate complexes were converted to the dichlorides by
treatment with HCl, a process that occurs with retention of
configuration. Fortunately, the 95:5 mixture of δ-PtCl2 and
λ-PtCl2 obtained from the mixture of diastereomers in δ(S)
recrystallized to enantiopure δ-PtCl2 (Scheme 1).8 The three
obtainable enantio- and diastereopure catalyst precursors were
utilized to test if biphep was viable for use in asymmetric catalysis.
To first confirm that the stereochemistry of (biphep)Pt2+ was
indeed stable in the absence of the BINOL’s bias, we carried out
quenching experiments on the dication (λ-Pt+2) derived from
triflic acid treatment of λ(S). The in situ generated ditriflate, was
quenched after variable periods of time with S,S-dpeda to form
diamine complexes (eq 1).9 Analysis of the diastereomeric ratio
of the diamine complexes by 31P NMR conveniently probes for
unintended atrop inversion during catalyst activation. The data
in Table 1 show that while slight erosion does occur, the ditriflate
is kinetically stable for at least 8 h (rt) without the BINOL.
To assess the viability of the putative homochiral catalyst
formulations, the benchmark Lewis acid-catalyzed Diels-Alder
reaction in eq 2 was chosen. The catalyst λ-Pt2+ was generated
in two ways, by the addition of 1.7 equiv of HOTf to λ(S) and
(4) Tudor, M. D.; Becker, J. J.; White, P. S.; Gagne´, M. R. Organometallics
2000, 19, 4376-4484.
(5) Brunkan, N. M.; Gagne´, M. R. Unpublished results.
(6) For several recent references describing P2M+2 (M ) Pd, Pt) Lewis
acid catalysis, see: (a) Ghosh, A. K.; Matsuda, H. Org. Lett. 1999, 1, 2157-
2159. (b) Pignat, K.; Vallotto, J.; Pinna, F.; Strukul, G. Organometallics 2000,
19, 5160-5167. (c) Hao, J.; Hatano, M.; Mikami, K. Org. Lett. 2000, 2, 4059-
4062. (d) Koh, J. H.; Larsen, A. O.; Gagne´, M. R. Org. Lett. 2001, 3, 1233-
1236. (e) Oi, S.; Terada, E.; Ohuchi, K.; Kato, T.; Tachibana, Y.; Inoue, T.
J. Org. Chem. 1999, 64, 8660-8662. (f) Ferraris, D.; Young, B.; Dudding,
T.; Lectka, T. J. Am. Chem. Soc. 1998, 120, 4548-4549. (g) Hattori, T.;
Suzuki, Y.; Uesugi, O.; Oi, S.; Miyana, S. Chem. Commun. 2000, 73-74. (h)
Hori, K.; Kodana, H.; Ohta, T.; Furukawa, I. J. Org. Chem. 1999, 64, 5017-
5023. (i) Hori, K.; Ohta, T.; Furukawa, I. Tetrahedron 1998, 54, 12737-
12744.
We recently demonstrated that, even though the enantiomers
of biphep rapidly interconvert at room temperature, coordination
to a substitution-inert metal such as platinum(II) significantly
slows atropinversion.4 Since P2PtBINOL complexes react with
(1) Seyden-Penne, J. Chiral Auxiliaries and Ligands in Asymmetric
Catlalysis; Wiley: New York, 1995.
(2) (a) Balsells, J.; Walsh, P. J. J. Am. Chem. Soc. 2000, 122, 1802-1803.
(b) Miurra, K.; Katsuki, T. Synlett 1999, 783-785. (c) Hashihayata, T.; Ito,
Y.; Katsuki, T. Tetrahedron 1997, 53, 9541-9552. (d) Hashihayata, T.; Ito,
Y.; Katsuki, T. Synlett 1996, 1079-1081. (e) Mikami, K.; Korenaga, T.;
Terada, M.; Ohkuna, T.; Pham, T.; Noyori, R. Angew. Chem., Int. Ed. 1999,
38, 495-497. (f) Korenaga, T.; Aikawa, K.; Terada, M.; Kawauchi, S.;
Mikami, K. AdV. Synth. Catal. 2001, 343, 284-288.
(7) To distinguish biphep and BINOL stereochemistry we arbitrarily label
the biphep stereochemistry in terms of its skew conformation (λ/δ), and BINOL
by its axial designation (R/S).
(8) Assayed by halide displacement with S,S-dpeda and diastereopurity
analysis by 31P NMR, see ref 9.
(9) δ-PtN2 and λ-PtN2 were independently synthesized from (biphep)PtCl2
and completely characterized. (δ-PtN2: δ -0.92, JP-Pt ) 3325 Hz, λ-PtN2:
δ 0.62, JP-Pt ) 3366 Hz). δ-PtN2 was characterized by X-ray crystallography,
see Supporting Information.
(3) Ringwald, M.; Stu¨rmer, R.; Brintzinger, H. H. J. Am. Chem. Soc. 1999,
121, 1524-1527.
10.1021/ja016167p CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/31/2001