Scheme 1
The reaction of racemic BIPHEP-Rh complex6 in non-
polar solvents such as dichloromethane under hydrogen (1
atm) gave a dimeric BIPHEP-Rh complex (Scheme 1). The
dimer complexes7 consisted of homo- and hetero-chiral ones
in a 40:60 ratio, but the hetero-chiral complex isomerized
slowly to the homo-chiral one at room temperature over 48
h.8
Figure 1. Solvent effect on chiral control of BIPHEP-Rh/DABN
complex. The diastereomeric ratios were determined by 31P and
1H NMR at room temperature.
The complexation of the BIPHEP-Rh complex and an
equimolar amount of (R,R)-DPEN (1,2-diphenylethylene-
diamine) as a chiral controller was examined to give a
mixture of diastereomers in a 50:50 ratio (eq 1). Unfortu-
nately, no change was observed in the diastereomeric ratio
at room temperature regardless of solvents.
Next, an equimolar amount of (R)-DABN (2,2′-diamino-
1,1′-binaphthyl) was added under a similar condition, to give
a mixture of diastereomers in a R,R/S,R 46:54 ratio (eq 2).
However, the isomerization proceeded slowly in dichloro-
methane at room temperature, leading eventually to the single
R,R diastereomer after 17 days (Figure 1). As a solvent effect,
the complexation in polar solvents such as methanol gave a
different diastereomer ratio R,R/S,R 56:44, and the isomer-
ization was found to be faster to afford the single R,R
diastereomer within 72 h. The single R,R diastereomer could
also be obtained at 80 °C in dichloroethane within 5 h.
However, isomerization was not observed at 5 °C in either
solvent.
It is thus clarified that the tropos or atropos nature of
BIPHEP-Rh complexes at room temperature critically
depend on the amines complexed. The aliphatic DPEN
complex is atropos, whereas the aromatic DABN complex
is tropos. Furthermore, the amine-free BIPHEP-Rh com-
plexes are tropos, as proven after amine complexation.
However, at 5 °C or below, even amine-free BIPHEP-Rh
complexes are atropos and hence can be used as chiral
catalysts.
The enantiopure Rh-complexes obtained via heat pretreat-
ment (80 °C in dichloroethane for 5 h) could be used as
asymmetric catalysts for ene-type cyclization9 of 1,6-enyne
substrate 1 by addition of TfOH to protonate diamines off
the Rh metal center (Table 1). BIPHEP-Rh gave the desired
cyclic product 2 in good yield but in moderate enantio-
selectivity at room temperature (entry 1). Without TfOH,
the reaction was very slow (12.5 h) (entry 2). Using DM-
BIPHEP (DM ) 3,5-dimethy-phenyl) instead of BIPHEP,
(6) Crystal data for BIPHEP-Rh complex in X-ray analysis: formula
C46H44F6P2RhSbCl2, monoclinic, space group P21/n (No. 14), a ) 14.065-
(4) Å, b ) 13.737(4) Å, c ) 21.407(56) Å, â ) 89.5260(1)°, V ) 4097.9-
(21) Å3, Z ) 4, and D ) 1.732 g cm-3. X-ray diffraction data were collected
on a Rigaku Saturn 70 CCD system with graphite-monochromated Mo KR
(λ ) 0.71070 Å) at -100 °C and the structure was solved by direct methods
(SIR92). The final cycle of full-matrix least-squares refinement on F2 was
based on 12097 observed reflections and 542 variable parameters and
converged to R ) 0.076 and Rw ) 0.179. Goodness of fit ) 1.084, shift/
error ) 0.006. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-
241306. Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. Fax: (+44)1223-
(7) For the BINAP-Rh complex: Takaya, H.; Miyashita, A.; Souchi,
T.; Noyori, R. Tetrahedron 1984, 40, 1245-1253. For the X-ray analysis
of DPPE-Rh complex: Halpern, J.; Rily, D. P.; Chan, A. S. C.; Pluth, J. J.
J. Am. Chem. Soc. 1977, 8055-8057.
(8) 31P NMR (162 MHz, CD2Cl2): heterochiral δ 39.8 (dd, J ) 193.3,
49.6 Hz), 43.3 (dd, J ) 209.5, 49.6 Hz); homochiral δ 40.8 (dd, J ) 192.3,
48.8 Hz), 44.6 (dd, J ) 209.5, 48.8 Hz). For the homochiral dimer (R)-
BINAP-Rh complex: 31P NMR (162 MHz, CD2Cl2) δ 42.7 (dd, J ) 195.5,
43.6 Hz), 46.4 (dd, J ) 211.4, 43.6 Hz).
3700
Org. Lett., Vol. 6, No. 21, 2004