Chemistry Reference
In-Depth Information
whereas picric acid and
t
-BuOH performed poorly.
Interestingly, Sivorski and co-workers have shown
that phenol in low concentration (up to 0.001 M)
was an effective co-catalyst but at higher concentra-
tions its activity dropped sharply. This phenomenon
was attributed to the formation of the less-active
QY-HY adducts when the concentration of the addi-
tive exceeded the concentration of the quaternary
catalyst [73]. We have discovered recently that the
catalytic role of alcohols in the extraction of basicity
into organic phase by lipophilic onium salts could be
exercised for the conversion of sodium hydroxide
into other valuable hydroxides (such as potassium,
caesium or tetralkylammonium) via a two-stage
extraction process [74].
A related process was the isomerisation of alkenes
and alkynes under basic conditions. Shioiri [75] has
demonstrated a novel synthesis of diarylallenes via
isomerisation of arylbenzyl acetylenes (Scheme
10.8). When a chiral phase-transfer catalyst was used
in this system, up to 35% enantiomeric excess (
ee
)
could be obtained.
Vinylidene carbenes were prepared by Mizuno via
double HBr elimination of 1,1-dibromocyclopropane
under basic PTC conditions. The latter readily reacted
(Scheme 10.9) with electron-rich olefins to generate
vinylidene cyclopropanes [76].
Starting with 1,1-dibromo-2,3-diarylcyclopropane
in the absence of an alkene acceptor, the same group
was able to form alkynyl carbenes, which dimerised
easily to (
E
)- and (
Z
)-3-hexene-1,5-diynes (Scheme
10.10) [77].
Another PTC/base-catalysed isomerisation has
been the conversion of the carotenoid leutin to its
isomer zeaxanthin (a useful golden-yellow foodstuff
pigment) [78].
Weak uncharged bases such as dimethylamine also
could be extracted and activated by phase-transfer
techniques. This was shown by Alcaide [79], who
used benzyltributylammonium bromide to catalyse
H
Ar
10% TBAB
KOH, PhMe, rt
•
Ar
X
15-60 min
X
77-84% yield
Scheme 10.
8
R
1
R
2
Ar
Ar
Br
Br
R
1
R
2
NaOH, TBAH
+
R
3
benzene
R
3
R
4
R
4
Ar= C
6
H
5
,
p
-CH
3
C
6
H
4
,
p
-ClC
6
H
4
,
p
-CH
3
OC
6
H
4
R
1
-R
4
= H, CH
3
, C
6
H
5
, CH
3
CH
2
O
yield 42-96%
Scheme 10.9
Ar
Br
Br
Ar
Br
50% NaOH
TBAH 25
Ar
-
HBr
Ar
,
°
C, 2 h
Ar
Ar
Ar
Ar
Ar
Ar
+
Ar
Ar
Ar
Ar
E
Z
yield 68-83%, E/Z = ~1
Ar= C
6
H
5
-,
p
-CH
3
C
6
H
4
-,
p
-ClC
6
H
4
-
Scheme 10.10
