Chemistry Reference
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1.9 equiv. K
2
CO
3
1 ml EtOH
MBM
18 s
-1
, 1 - 2 h
R
2
Br
O
+
+
R
1
P
R
1
R
2
Polystyrene
R
1
= C
6
H
5
, 4-CH
3
OC
6
H
4
, 4-NO
2
C
6
H
4
, 2-furyl, 2-naphthyl, 4-BrC
6
H
4
, CHCHC
6
H
5
, CH
2
CH
2
C
6
H
5
R
2
= CO
2
C
2
H
5
, CH
2
C
6
H
5
Scheme 2.27 Wittig reaction with functional resin.
O
2 equiv. K
2
CO
3
+
+
Br
PPh
3
Br
Br
MBM
60 s
-1
, 7 h
Scheme 2.28 One-pot Wittig reaction.
yield of 17% was achieved a rate enhancement could be observed for re-
actions with EtOH. Thereby, yields of 99% were gained within 2 h. Adding
small amounts of solvents has furthermore the advantage that the (E)/(Z)
ratio of the product was tunable by varying the solvent. Using the resins
offered several advantages like simple work up. By suspending the mixture
in EtOAc and subsequent filtration, the pure product could be obtained.
Balema et al. reported the Wittig reaction in a mixer ball mill under he-
lium atmosphere.
50
(Benzyl)triphenylphosphonium chloride was milled with
aromatic aldehydes, ketones and K
2
CO
3
. Samples were milled for 7 h in a
glove box, affording the Wittig product in 70-93% yield. To avoid heating
while milling, forced air cooling of the milling beakers of the Spex 8000 mill
was employed. Beside application of preformed phosphorus yields the
Wittig reaction can also be performed as one-pot reaction by using PPh
3
,an
organic halogenide, and an organic carbonyl compound (Scheme 2.28).
In contrast to reactions in solution, the Wittig reaction of (benzylidene)- or
(2-naphthylene)triphenylphosphoranes and aromatic carbonyl compound
preferably yields the more thermodynamically stable (E)-stilbenes instead of
(Z)-stilbenes or mixtures with nearly equal content of (Z)- and (E)-isomers.
Shearouse et al. also published details of a Horner-Wadsworth-Emmons
reaction for the synthesis of a,b-unsaturated esters (Scheme 2.29).
51
The
milling vessels were equipped with triethyl phosphonoacetate, 1.5 equiv.
aldehyde and 2 equiv. K
2
CO
3
and the reactants milled at 18 Hz for 6 h. Yields
of 50-96% could be achieved with (E) : (Z)-ratio of 99 : 1 in almost all cases.
As aldehyde was added in excess, after milling 4 equiv. Oxone
s
were added
to the reaction mixture, which was then further milled for 12 h to remove the
unreacted aldehyde. This strategy may offer an interesting starting-point for
work up of reaction mixtures in ball mills. The aldehyde was oxidized to the
carboxylic acid, which can be simply removed by acid-base extraction.
Boc-protected unsaturated amino esters were synthesized by Baron and
co-workers.
52
A Horner-Wadsworth-Emmons reaction between aminophos-
phoryl acetate (1.1 equiv.), aldehyde (1 equiv.) and a base (2 equiv.) gave the
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