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X. Peng et al. / Tetrahedron Letters 49 (2008) 4862–4864
the reactions for both vinyl acetate and styrene (entries 1–3). How-
ever, the sterically more demanding N-arylacyl substituents (R =
4-t-Bu) in ligand 3d are obviously unfavorable to the control of
enantioselectivity. In fact, virturally racemic hydroformylation
product was obtained in the reaction of styrene (entry 4). Ligands
3e and 3f exhibited a catalytic performance similar to that of 3b
(entries 5 and 6 vs 2). Remarkably, changing the substituent R0 of
phenoxy moieties from H to methyl group can also lead to the dra-
matic changes of regio- and enantioselectivities, as ligands 3g and
3h exhibited decreased ees but improved b/l ratios relative to those
of 3b and 3c (entries 2 vs 7 and 3 vs 8, respectively). Ligand 1 was
also tested in the reactions under the above-optimized conditions,
with relatively poor enantioselectivity and regioselectivity being
observed in the asymmetric hydroformylation of both substrates
(entry 9).
Acknowledgments
Financial supports from the National Natural Science Founda-
tion of China (Nos. 20532050, 20423001), the Chinese Academy
of Sciences, the Major Basic Research Development Program of
China (Grant No. 2006CB806106), the Science and Technology
Commission of Shanghai Municipality, and the Merck Research
Laboratories are gratefully acknowledged.
Supplementary data
Supplementary data associated with this article can be found, in
References and notes
These observations suggested that the stereocontrol capabilities
of ligand 3 are highly sensitive to the subtle changes in the substit-
uents on the ligand backbone, and the N-arylacyl substituents
remote from the catalytic center can also exert a control of the
enantioselectivity to some extent.7 These facts along with the mod-
ular nature of this type of ligands suggest that their asymmetric
induction capabilities could in principle be fine tuned by appropri-
ate modifications of the R and R0 substituents on the skeleton of the
ligands, which represents a valuable feature for ligand optimiza-
tion. Within the ligand series 3a–h and 1, 3c afforded the best
enantiocontrol (83% ee) and moderate regioselectivity (b/l 15.1)
for asymmetric hydroformylation of vinyl acetate, while 3h gave
the optimal enantioselectivity (55% ee) and good regioselectivity
(b/l 16.9) for asymmetric hydroformylation of styrene.
In summary, we have developed a new class of ferrocene-based
modular chiral diphosphonite ligands, which exhibited moderate
to good enantioselectivities and regioselectivities in the Rh(I)-cata-
lyzed asymmetric hydroformylations of vinyl acetate (up to 83% ee
and 15.1 b/l ratio) and styrene (up to 55% ee and 16.9 b/l ratio),
respectively. This type of ligands has salient features such as cheap
starting material and facile preparation, structural tunability, as
well as remote stereocontrol capability of backbone substituents.
Further work on modifying the ligand structure for improvement
of the regio- and enantioselectivity9 as well as broadening the sub-
strate scope of the asymmetric hydroformylation is undergoing in
this lab.
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