Chemistry Reference
In-Depth Information
SH
CS
2
, HO
-
SH
S
S
-C
2
H
4
+
S
S
S
S
O
S
H
+
S R
Figure 2.6. Synthesis of the 1,3-dithiole-2-thiones.
S3 are non-selective and lead to mixtures while S4 and S5 can be completely or
partially selective. We illustrate these four strategies in a schematic way in Fig. 2.5
with the synthesis of EDT-TTF (Fourmigue
et al.
, 1993; Garreau
et al.
, 1995). The
underlying mechanisms involving intermediate species will not be discussed here.
In general such mechanisms are only partially understood. The methods described
here can also be employed for the preparation of molecules containing selenium,
such as TSF-derivatives, or molecules containing both sulfur and selenium. How-
ever, we shall only show examples involving sulfur.
S2 and S3 are based on the coupling of 1,3-dithiole-2-thiones (under the ac-
tion of phosphine or phosphites) and of dithiolium salts (in a basic medium), re-
spectively. S2 has been and continues to be largely used for the preparation of
asymmetric TTFs as well as more complex structures when they can be easily
separated from the formed mixtures. In general the yields are acceptable. The
principal advantage of this strategy is the relatively easy access to a very large
variety of 1,3-dithiole-2-thiones used as starting compounds in these reactions.
Indeed, the synthesis of the 1,3-dithiole-2-thiones is very important and can be
achieved in several ways (Schukat
et al.
, 1987). Figure 2.6 summarizes only a few of
them.
When using substituents in the starting molecules the success of the cross-
coupling strongly depends on such substituents. Many functions have thus been
introduced on TTF and TSF, in particular ester groups, which can then be easily
converted into carboxylic acid or amide or into the corresponding non-substituted
derivative by a simple decarboxymethylation. Other functional groups incorpo-
rated include nitrile and halogens such as chlorine, bromine and iodine. The reac-
tion yields obtained with S3 are lower than for S2 and hence this strategy is only
occasionally used. This is in part due to the fact that the reactivities of the two
dithiolium salts used can be quite different from each other.
