C O M M U N I C A T I O N S
Scheme 2
combining phosphorus chirality and other chiralities have been
prepared. The preliminary results from some asymmetric hydro-
genation reactions show that the introduction of P-chirality into
well-known ferrocene-based phosphine ligands enhances the enan-
tioselective discrimination produced by the corresponding Rh
catalyst when a matching among the planar chirality, carbon
chirality, and the chirality at phosphorus exists.
Acknowledgment. We thank Dr. Jamie Bickley of Liverpool
University for the crystal structure analysis, and Dr. David Justice
for the synthesis of dehydrophenylalanine.
Supporting Information Available: Experimental procedures and
confirmation of the absolute configuration of (RC,SFc,SP)-2a, (RC,SFc,RP)-
2c, (RC,SFc,SP)-2a, and (RC,SFc,SP)-6b. This material is available free
Table 2. Asymmetric Hydrogenation of R-Dehydroamino Acid
Derivatives
References
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entry
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R
solvent
H
2 (psi)
time (h)
conv. (%)
ee (%)
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1
2
3
5
6
7
8
10
11
BoPhoz
4b
4c
BoPhoz
4b
4c
BoPhoz Ph THF
4b
4c
H
H
H
H
H
H
THF
THF
THF
MeOH
MeOH
MeOH
50
50
50
50
50
50
100
100
100
2.5
2.5
2.5
2.5
2.5
2.5
2
99
100
100
91.4
100
84.5
100
100
100
94.5
98.3
92.3
81.1
>99
90.3
95.7
97.0
92.4
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Ph THF
Ph THF
2
2
nately, most of transformations can be carried out at room
temperature, with no evidence of any epimerization.
The preliminary results of asymmetric hydrogenation of two
R-dehydroamino acid derivatives catalyzed by Rh complexes
prepared from the new ligands show that the introduction of
P-chirality in ferrocene-based phosphine ligands enhances the
enantioselective discrimination produced by the catalyst when a
cooperative effect operates among the planar chirality, carbon
chirality, and the chirality at phosphorus (Table 2). The activities
and enantioselectivities of the matched ligand (RC,SFc,SP)-4b are
higher than that of BoPhoz (entry 1 vs entry 2 and entry 8 vs entry
10), while the mismatched ligand (RC,SFc,RP)-4c is less active and
enantioselective than BoPhoz (entry 1 vs entry 3 and entry 8 vs
entry 11). Interestingly, the matched ligand (RC,SFc,SP)-4b seems
to be particularly suitable for hydrogenation conducted in methanol
(entry 5 vs entry 6).
In summary, a very simple, highly stereoselective synthesis of
ferrocene-based P-chiral phosphine ligands has been developed. This
new methodology offers modular construction of ligands and hence
generates many possibilities for modifying the structure of these
ferrocene-based phosphines. Indeed, on the basis of this new
method, several new families of ferrocene-based phosphine ligands
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