Chemistry Reference
In-Depth Information
Li
S
S
S
S
LDA
S
S
S
S
(a)
E
E
S
S
S
S
CO
2
Et
CO
2
Et
S
S
S
S
LDA
(b)
RI
S
S
S
S
R
S
S
S
S
LDA
(c)
RI
S
S
S
S
R
Figure 2.9. Electrophilic substitution of lithiated TTF (strategy S8). E stands for
electrophilic substituent. (a) Generic reaction, (b) monosubstituted TTF with the
electron withdrawing substituent CO
2
Et and (c) monosubstituted TTF with the
electron donor substituent CH
3
.E
=
Br, I, CO
2
H, CO
2
Et, CH
2
OH, CH
3
, CONR
2
,
CHNR
2
, CHO, etc.
give the corresponding functionalized or alkylated TTF by nucleophilic substitu-
tion (Fig. 2.9(a)). The main disadvantage of this strategy is the disproportionation of
tetrathiafulvalenyllithium into multilithiated species and often results in a complex
mixture of products along with the recovery of TTF.
In the case of further lithiation of monosubstituted TTF, the orientation of the
lithiation depends on the nature of the substituent already present on the TTF het-
erocycle. An electron withdrawing substituent such as CO
2
Et increases the acidity
of the adjacent hydrogen atom and leads to a 4,5-disubstituted TTF (Fig. 2.9(b))
whereas an electron donor substituent (CH
3
), which decreases this acidity, directs
the lithiation to the unsubstituted 1,3-dithiole ring (Fig. 2.9(c)). One major advan-
tage of this method is that it allows the preparation of a large variety of donors by
direct functionalization of the TTF or TSF skeleton.
Let us end this section with the synthesis of three relevant molecules: TCNQ,
α
-nitronyl nitroxides and CuPc. The synthesis route that efficiently leads to the
preparation of the strong acceptor TCNQ is depicted in Fig. 2.10. It is based on
the ready condensation of malonitrile with 1,4-cyclohexadione. Further treatment
with C
5
H
5
N and Br
2
gives TCNQ in excellent yields (Acker & Hertler, 1962). The
electronic band structure of neutral TCNQ will be discussed in Section 6.1.
α
-nitronyl nitroxides are a class of stable radicals with
p
-NPNN as the most
prominent representative. The growth and physical properties of thin
p
-NPNN
