Chemistry Reference
In-Depth Information
Scheme 14.1
8
Regardless of the ability to recycle halide salts, use
of the Heck reaction has been limited because the
necessary bromides and iodides are expensive.
Recently, aryl diazonium salts, available from less
expensive nitroaromatics, have been demonstrated
as being useful in the Heck reaction and form the
basis of a new small-scale process for the production
of Novartis' herbicide Prosulfuron. The production
is currently 100 t year
-1
(Scheme 14.18) [68]. The
intermediate trifluoromethylethyl-substituted arene
is formed in one pot, minimising waste in its
synthesis.
Metachlor, an important herbicide marketed as
Dual, has been sold as a racemic mixture. However,
a more recent approach based on asymmetric hydro-
genation produces a single enantiomer of the ma-
terial. The difference in herbicidal activity is striking.
Identical performance was observed at only 65% of
the previous application rate, which has a significant
impact on the environmental issue of run-off from
agricultural operations. The process proceeds via
asymmetric hydrogenation of an intermediate imine
over a chiral Ir ferrocenyl complex in 80% ee. The
catalyst system is extremely active and exhibits a
turnover number of 1 ¥ 10
6
with a frequency of 2 ¥
10
5
h
-1
. About 1 ¥ 10
4
t year
-1
of optically pure Dual
are produced using this route (Scheme 14.19) [69].
Longer term, continued use of homogeneous
catalysis will rely on the ability to design highly
active transition metal species that exhibit both high
turnover number and turnover frequency. For
example, recent work has revealed new Pd catalysts
for the Heck reaction that exhibit turnover numbers
as high as 196 000 000 and a turnover frequency of
12 000 000. An air-stable Pd complex reported by
Milstein exhibits turnover numbers as high as 10
6
and a frequency of 8 ¥ 10
4
h
-1
. These reactions are
fairly substrate specific but demonstrate that
extremely high activity can be realised [70]. Further
research areas important to catalysis, and hence to
implementation of green process technology, have
been reported [9,27,71].
5 Renewables as Chemical Feedstocks
and Biocatalysis
Catalysis has proven to be a key contributor to green
chemical processes by virtue of its role in syntheti-
cally efficient manufacture of chemical products.
However, green process technology must not only
address the methodology used for chemical produc-
tion but also the starting materials employed. A truly
green chemical industry eventually will evolve from
non-sustainable feedstocks to renewable building
blocks. The unfamiliarity of renewables to the indus-
try has hindered their adoption. This situation has
started to change in recent years because of the sym-
biosis between biocatalysis and renewable feed-
stocks, particularly carbohydrates. Biocatalysis is
expanding rapidly, with an increasing number of
organisms able to convert sugars to a wide array of
products. As a result, renewables as chemical feed-
stocks are also receiving increased attention.
It is important to realise that the use of renewable
feedstocks does not
require
biotechnology, or vice
versa. Lichtenthaler has written extensively about
the use of conventional chemical technology to
convert carbohydrates to new products [72]. Exam-
ples of industrial use of biotechnology for the con-
version of non-renewable starting materials also
have appeared. As industrial biotechnology expands,
the use of renewables will grow, as will the comfort
