Chemistry Reference
In-Depth Information
that may vary with temperature. If this variable
behaviour were understood better, it would be more
predictable and new clean methods could result.
Investigations into organic synthesis in high-
temperature water, carried out with the MBR and
CMR, have gone some way towards such outcomes
[9,64,102].
myrcene, a-terpinene, limonene, g-terpinene, the
ocimenes, a-terpinolene and alloocimenes (see
Scheme 17.10).
Nerol and linalool both underwent considerably
more elimination than did geraniol, to give the same
array of hydrocarbons. The major products and their
relative proportions were consistent with those
for carbocationic rearrangement of derivatives of
linalool, nerol and geraniol under acidic conditions
[107,108]. At first glance, an aqueous environment
may seem inappropriate for dehydration reactions.
However, dehydration often proceeds readily under
such conditions, by E1 mechanisms and
via
carbo-
cation intermediates [109,110].
Established methods for the conversion of a- and
b-ionones to ionene have involved catalysis by
hydriodic acid along with small amounts of phos-
phorus, or distillative heating in the presence of
0.5% iodine [111]. The latter procedure is
known to co-produce about 10% 1,1,6-trimethyl-
1,2-dihydronaphthalene [112]. The cyclisation also
occurred by heating b-ionone in water at 250°C in
the MBR (see Scheme 17.11). Work-up did not
require the usual exhaustive washing procedures
[106]. Similarly, carvacrol was prepared almost
quantitatively, by isomerisation of carvone in water
at 250°C for 10 min (Scheme 17.11) [102]. A litera-
ture method utilised acidic conditions, took a longer
time and proceeded in lower conversion [113]. The
microwave reactions showed that elevated tem-
peratures under neutral pH conditions can offer
advantages over acidic (or basic) reagents at lower
temperatures.
7.1 Reactions in high-temperature water
Biomimetic reactions
As suggested by the lower dielectric constant,
high-temperature water behaves somewhat like an
organic solvent, dissolving organic compounds that
are much less soluble at ambient temperature. This
property was exploited for the hydrolysis of naturally
occurring monoterpene alcohols without the need
for prior derivatisation with typical water-
solubilising groups such as phosphates or glycosidic
units [106]. Because of the enhanced dissociation of
high-temperature water, biomimetic reactions that
normally would be acid-catalysed proceeded on the
underivatised compounds in the absence of added
acidulant. Cooling of the mixtures rendered the
products insoluble, readily isolatable and the
aqueous phase did not require neutralisation before
work-up. This approach should be beneficial in
the flavour and fragrance industry, where pro-
ducts derived by clean processing can command a
premium.
For example, geraniol, nerol and linalool are prac-
tically insoluble in water at ambient temperature.
Although acid labile, they do not react readily in
water at moderate temperature and neutral pH. In
unacidified water at 220°C in the MBR they reacted
within minutes [106]. Geraniol rearranged to a-
terpineol and linalool predominantly and gave
lesser amounts of the monoterpene hydrocarbons,
Indoles
In research into indole preparation and transforma-
tions, 2,3-dimethylindole was obtained in 67% yield
from phenylhydrazine and butan-2-one by a one-pot
Scheme 17.10
Reaction of geraniol in
water at 220°C [106].
