Chemistry Reference
In-Depth Information
tests show it to be three to four times as effective [59]. The 1S,aRS diastere-
omer (Outlook
®
, BASF) is marketed for corn and bean crops as a means of
controlling grasses and broad-leaf weeds.
Another example of an herbicide where chirality is important is meto-
lachlor (Dual
®
, Syngenta), also in class 15 and used for corn and soybeans.
Metolachlor has two chiral elements. The bond between the nitrogen and the
phenyl group is restricted from free rotation and therefore there is a chiral axis.
Chiral axes are not particularly common and it is coincidental that they appear
in both this and the previous example. Metolachlor also has an asymmetric
carbon at the methine carbon attached to nitrogen.
Chiral carbon
O
H
3
C
O
H
3
C
N
CH
2
Cl
Metolachlor
Metolachlor was originally introduced to the market as a mixture of all four
stereoisomers. Later it was found that the stereochemistry at the chiral carbon
controlled the activity. About 95% of the herbicidal activity comes from the
two diastereomers where the methine carbon has S configuration [60]. This
recognition led to a lengthy and major industrial research project to prepare
metolachlor with S configuration at the methine carbon. The project began
in 1981. Fifteen years later, after the evaluation of numerous routes, metal
catalysts, chiral ligands, and process conditions catalysts, S-metolachlor was
in full-scale production [61]. The key step is the selective hydrogenation of the
imine. At greater than 10,000 tons per year, it represents possibly the largest
scale enantioselective process. S-Metolachlor is marketed as Dual Gold
®
;
Dual Magnum
®
is used as the name in the U.S.
O
O
O
H
3
C
H
3
C
H
3
C
O
O
H
3
C
N
H
3
C
NH
H
3
C
N
CH
2
Cl
Enantioselective
hydrogenation
Cl
CH
2
Cl
Imine
(S)-metolachlor
S-metolachlor is also the active ingredient in Dual II Magnum
®
(Syn-
genta). This product also contains a safener, benoxacor. Safener is a name for
compounds that protect crops from herbicide injury without replacing herbi-
cide activity in targeted weeds. Various chemicals have been developed to act
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