Chemistry Reference
In-Depth Information
F
2
HCO
OCHF
2
HO
OH
Acetone, NaOH, TM
A
C
4.3 bar, 46
+ CHClF
2
NN
NN
°
C, -NaCl
SR
SR
R= C
1
-C
4
alkyl, phenyl, benzyl
59% yield
Scheme 10.1
8
Methyl methacrylate was transesterified with
various alcohols and diols in the presence of K
2
CO
3
and TBAB in hexane [117]. Peresters were prepared
from cumyl or
t
-butyl hydroperoxides via reaction
with acid halides using a tetra-
n
-butylammonium
hydrogen sulfate (TBAHS) catalyst [118].
Cl
Cl
KOH (aq), Toluen
e
PEG-200, KI
NO
2
NO
2
Cl
OH
95% yield
Phase-transfer catalysis in aldol and related
condensation reactions
Scheme 10.19
Because carbonyl compounds are stronger elec-
trophiles than typical PTC substrates such as alkyl
halides, there was no apparent call for catalysis in
condensation processes, e.g. Aldol, Knoevenagel,
Darzen, Perkin, Prins or Wittig reactions. This view
changed in recent years and PTC techniques cur-
rently are utilised routinely in this widespread family
of reactions.
Bentley
et al
. [119] have studied the condensa-
tion of benzophenones with acetonitrile initiated by
solid potassium hydroxide. Although this reaction
proceeds also in the absence of catalyst, a 30-fold rate
increase was monitored when Aliquat 336 was
added to the reaction mixture. Other phase-transfer
agents such as TBAI, TBAB, TEBA or TBAHS exhib-
ited inferior performance. Intriguingly, the rate of
this solid/liquid reaction was almost independent
of the amount and appearance (crushed or pellets)
of the KOH used and of the stirring rate. Reaction
rate was affected strongly by the presence of water,
temperature and
para
-substituents on the ben-
zophenone. The authors concluded that the rate-
determining step was the organic-phase reaction
between the acetonitrile anion and the ketone. It is
imperative to note in this context that acetonitrile
could not be alkylated with alkyl halides or sul-
fonates under PTC conditions.
Methylene-bridged bis(cyclopentadienyl) com-
pounds were prepared by condensation of formalde-
hyde with, for example, indene using NaOH and
TEBA [120]. The PTC Darzen reaction of chloroace-
was developed further into an industrial processes
by Sawant [109], who used benzoate esters for the
hydrolysis of benzyl chloride, and by Prasad, who
hydrolysed 2-chloroacetamides to 2-hydroxyac-
etamides via formate intermediates [110]. The same
procedure was used by Oda, who treated halo-
terminated polypropylene to hydroxy-terminated
polymer using formate ester intermediates [111].
4,4¢-Bis(hydroxymethyl)biphenyl was prepared in a
similar way by the hydrolysis of the corresponding
dichloride [112]. Aryl halides usually do not require
the engagement of ester intermediates, so 1,4-
dichloronitrobenzene was hydrolysed regioselec-
tively (Scheme 10.19) to 4-chloro-2-nitrophenol
[113].
Hydrolysis under basic conditions in PTC was used
also for the purification of epoxy resins by reducing
the remaining chlorine content. After treatment at
60°C with KOH
(aq)
/TEBA the chlorine content of the
polymer was reduced to 700 ppm [114].
Phase-transfer catalysis in transesterification
Sugar esters were prepared from a fatty acid alkyl
ester and a polyol such as mono- or disaccharide in
the presence of a base and a phase-transfer catalyst
(ammonium salt, crown ether or PEG) [115]. The
PTC/base transesterification of protected carbohy-
drates with methyl benzoate and dodecanoate was
enhanced by microwave irradiation [116].
