Chemistry Reference
In-Depth Information
The products that were isolated possessed only slightly lower enantiomeric excess than those
obtained with the corresponding small molecule catalyst.
In a similar manner, Sellner et al. [
20
] prepared a variety of polystyrene beads with embedded
a
-tertiaryl-l,3-dioxolane-4,5-dlmethanol groups. They were subsequently used to form an
immobilized catalyst with recyclable chiral
,
a
,
a
,
a
titanium Lewis acid for addition of Bu
2
Zn to
benzaldehyde.
O
O
OH HO
They also reported preparations of various cross-linkers containing tertiary aryl groups with
polymerizable carbon-carbon double bonds [
20
].
Itsuno et al. [
21
] synthesized a cross-linked polymer support with a chiral 1,2-diamine for
enantioselective ruthenium transfer hydrogenation catalysis of aromatic ketones.
A variation on the process was to functionalize polystyrene by incorporating groups like hydroxy,
acetoxy, or nitrile onto the backbone of polystyrene, through copolymerization of monomers [
22
].
Following are two examples:
+
CH
2
CN
+
or
O
S
O
O
Similar to the work by Frechet who grafted functional groups to heterogeneous polystyrene (see
above), Hodges used living free-radical polymerization to prepare what he referred to as
Rasta resin
[
23
]. The Merrifield resin was first functionalized with reduced TEMPO to form a resin core. With the
help of Rhodamine dye, it was confirmed that the grafts and associated functional groups were located
