Chemistry Reference
In-Depth Information
The oxidation of aromatic and benzylic thiols to
disulfides is described with the polyvinylpyridine-
bromine complex in acetonitrile [182].
A wide range of compounds thus can be prepared
provided that the required reagent is available.
Fortunately, several of these reagents are now com-
mercially available: polystyrene, chloromethylated
polystyrene, polyvinylpyridine, polyvinylpyrroli-
done and copolymers, polybenzimidazole and some
polyesters can be obtained from manufacturers
and some retailers; processed polymers containing
functional groups are found in the catalogues
of manufacturers and dealers: Advances Chem-
tech, Aldrich, Agilent, DuPont Pharma, Fluka,
Novabiochem, Polymer Laboratories and Poly-
sciences. Some significant reagents are reported in
Table 8.1.
3.21 Wittig and Wittig-related reactions
The syntheses of functionalised alkenes through
Wittig and Horner-Wadsworth-Emmons (HWE)
reactions were performed more conveniently when
the elimination of the spent phosphine oxide or
phosphate anion became easier. Two approaches
are described: either the aldehyde is linked to the
polymer and the organosphorus reagent is in the
solution; or the aldehyde is in the solution and is
reacted with a phosphonate-exchanged resin or a
phosphonium salt obtained by quaternisation of a
phosphane-linked polymer with the appropriate
alkyl halide (Fig. 8.64a).
A few examples show that for a given supported
phosphonium the reaction conditions must be
adapted to the base (to form the ylide) and to the
aldehyde [183-185]. The general scheme is applied
successfully in the synthesis of ethyl retinoate [186]
(Fig. 8.62a). Interestingly, functionalised phospho-
nates, when acidic enough, can be immobilised
under their anionic form to react with aldehydes and
ketones in solution; the phosphate anion remains on
the resin, affording a clean reaction [187].
When a labile phosphonate is covalently bound
to a support it can react with a variety of aldehydes
under mild basic conditions; an excess of aldehyde
gives high conversions to alkenes [188] (Fig. 8.64b).
In a reciprocal approach, phosphorane ylides
and phosphonate anions in solution react smoothly
with resin-bound aldehydes and ketones to provide
alkenes in good yields and purities, as assessed
after the cleavage of the final product; excellent
stereoselectivities are obtained with aldehydes
[189].
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