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sponding diphosphites
4
and
12.
However, in contrast to ligand
12c,
which offered
high stereoselectivities of around 90% ee, ligands
53-55
gave rise to only moderate
selectivities.
9.1.4
Mixed Phosphorus Donor and P-S Donor Ligands
Ligands containing two different phosphorus donor sites have only been sparingly
applied in hydroformylation reactions, even though they were among the first
ligands of the type to be tested for this transformation. Börner and coworkers
reported a family of structurally simple phosphine-phosphite furanoside ligands
with one diphenylphosphino group and phosphites derived from both enantiom-
ers of axially chiral BINOL and from simple catechol [20]. The authors tested
ligands
56
with a 2,3-
cis,
and ligands
57
with a 2,3-
trans,
relationship between the
two donor sites. While all ligands gave good conversion, the regioselectivity was
in all cases moderate to poor. For the ligands with 2,3-
cis
architecture,
56e
R
ax
and
56e
S
ax
, the stereodirecting influence of the axially chiral BINOL unit overrode that
exerted by the carbohydrate framework, but matched/mismatched effects between
BINOL and carbohydrate configuration were apparent. With the 2,3-
trans
architec-
ture, both ligands
57e
R
ax
and
57e
S
ax
produced predominantly the same enantiomer
irrespective of the BINOL configuration, but again strong matched/mismatched
effects were observed. Even in the matched cases, the stereoselectivities did not
exceed 45% ee, and tests with catechol phosphite ligand
56g
proved again that the
phosphite donor has to have bulky residues to achieve any stereoinduction
(Figure 9.6).
The group of Ruiz explored d-
xylo
configured furanosides
58
with a phosphine
donor at position 6 and various phosphite donors at position 3 [21]. These ligands,
which are again related to xylose diphosphites
4,
only offered moderate ees, when
the phosphite part contained bulky groups; the introduction of BINOL as an addi-
tional chiral motif had hardly no effect on the ee (Figure 9.6). One report on
carbohydrate-based phosphite-phosphoramidite ligands was published by Diéguez
and coworkers, who prepared ligands
59
and
60
with a d-xylo- and a d-ribofuran-
osidic scaffold [22]. Moderate stereoselectivities were obtained for
xylo
-ligands
59
and again the ee was significantly influenced by the substituents of the phos-
phoramidite and phosphite donors. Ligands
60
with
ribo
-configuration gave
modest ee but the same enantiomer of the product (Figure 9.6).
So far only one report on mixed P-S donor ligands based on carbohydrates has
appeared. The structures contained various alkylthio moieties at position 5 and
biphenyl-derived phosphites in position 3 of a xylose. While these ligands pro-
moted the reaction in good conversion they, unfortunately, offered no stereoselec-
tivity [23].
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