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reaction of methyl-2-acetamido acrylate 6, which was studied by Pringle,4 the situation is some-
what dierent. In this series the ethyl derivative 3b delivers the highest enantioselectivity (94%
ee), but a clear trend with respect to the variation of the R-group is not discernable. The t-butyl
derivative 3d leads to an ee value of 93%, in agreement with Pringle's data (92%).4y This value is
slightly higher than that obtained by the use of 1 (90% ee) but considerably lower than the
enantioselectivity resulting from the use of the ferroceno-based ligand 2 (>99.5% ee).1 3 Both 1
and 2 are chelating bidentate ligands in contrast to the monodendate phosphonites 3a±d.
The results presented here and those discussed recently4 are relevant with respect to the long-
term discussion concerning the origin of enantioselectivity in Rh-catalyzed hydrogenation.6 10
The ®rst examples of asymmetric transition metal-catalyzed hydrogenation of prochiral ole®ns
were reported in 1968 independently by Horner11 and Knowles.12 They used chiral mono-
phosphines such as methyl-n-propylphenylphosphine which resulted in low enantioselectivities
(ee=3±15%). Other monodendate ligands also turned out to be rather ineective.6 10 In 1971
Kagan introduced the concept of chiral chelating diphosphines (DIOP) having stereogenic centers
in the backbone, which marked a breakthrough because high enantioselectivities were possible for
the ®rst time (70±80% ee).13 Shortly thereafter Knowles described the chelating diphosphine
DIPAMP, the P-atoms being the stereogenic centers.14 The highly ecient BINAP, ®rst described
by Takaya and Noyori, is also a bidentate ligand.15 The success of these and other chelating
ligands was explained by hindered rotational freedom around the metal±donor atom bond in the
intermediate metal chelates.6 10,13 16 On the basis of limited data Pringle has concluded that the
above dogma does not necessarily pertain.4 We agree to a certain extent. However, upon
scrutinizing all of the present data concerning the monophosphonites 3a±d, it becomes clear that
the eects are not as pronounced as perhaps suggested.4 Firstly, in no case concerning the use of 3
has an ee value been observed which exceeds 94%. Secondly, Pringle cites only the enantio-
selectivities resulting from ligand 1 as reported by us, yet the ee values obtained from the use of 2
are consistently much higher (>99% ee).1 3 Thus, the positive chelation eect on enantioselectivity
is in fact operating, provided the appropriate backbone is chosen.{
In summary, surprisingly high enantioselectivities are observed upon using BINOL-derived
monophosphonites in the Rh-catalyzed hydrogenation of two dierent types of prochiral ole®ns
(up to 94% ee).x However, data more extensive than that considered by Pringle4 show that
enantioselectivities are considerably higher when using the proper chelating diphosphonites.1 3
The potential advantage of chiral monophosphonites 3 in asymmetric hydrogenation has to do
with their ready synthesis and the possibility to vary the R-group. Thus, the modular synthesis of
chiral monophosphonites constructed from BINOL (or other optically active diols) and from a
wide variety of phosphorus compounds RPCl2 may well allow the optimization of a given
asymmetric hydrogenation reaction.
y
The ee value obtained from the use of the methyl derivative 3a (92%; Table 1, Entry 5) diers somewhat from the one
reported by Pringle4 (78% ee). The source of this discrepancy is unclear; all other relevant ee values are identical
within experimental error. Our ee values are the average of two runs; in no case did the respective two values dier
considerably.
{
3
The butano-bridged analog of 1 shows considerably lower ee values in Rh-catalyzed hydrogenation.1
x
Typical procedure for hydrogenation: All hydrogenations were carried out under the conditions previously reported
using chiral diphosphonites:1,2 1.3 bar H2; solvent CH2Cl2; rt; 20 h (most reactions are complete within 3±4 h). The
catalyst was separated from the reaction mixture by ®ltering through a short SiO2 column. Conversion and ee value
were determined by gas chromatography using a chiral stationary phase.