Chemistry Reference
In-Depth Information
N
N
S
S
[red]
R-CN
SNS
AsF
6
R
R
N
N
S
S
46
AsF
6
47
37
NC-X-CN
N
S
S
N
N
S
S
[red]
N
S
S
X
X
N
N
N
S
N
S
2 AsF
6
48
49
Scheme 9.13
S
R
N
S
50
The feature of 1,3,2,4-dithiadiazolyl radicals
37
which has garnered the most attention is their ther-
mal or photochemical rearrangement to the isomeric 1,2,3,5- radicals
35
. The thermal reaction occurs in
solution
120,143,145,146
and the solid state,
147
and can also be induced photochemically.
143,144
This transforma-
tion is most conveniently monitored in solution using EPR spectroscopy, although UV-Visible spectroscopy
has also been employed.
144
The substituent has a significant influence on the rate of this reaction: strongly
electron withdrawing R groups tend to rearrange within minutes while more electron rich substituents
can be isolated without rearrangement and persist for days or even weeks in solution in the dark. The
proposed mechanism for the thermal rearrangement (Scheme 9.14) is based on the observation that the
isomerization rate is concentration dependent, specifically second order in radical.
145
The process involves
association (
π
dimerization) of the 1,3,2,4-radicals (the enthalpy of dimerization has been measured to
4.5 kcal/mol
144
) followed by a skeletal rearrangement of the dimers
51
to
trans
-type
be
dimers
52
of the 1,2,3,5-isomer. The lone X-ray structure of a 1,3,2,4-dithadiazoly (di)radical provides supporting
evidence for this mechanism, as each radical associates in exactly the structure
51
proposed in the original
mechanism (Figure 9.11).
148
Interestingly, studies on the
photochemical
rearrangement suggest that this
process is unimolecular; a mechanism has been proposed involving initial fragmentation of the radical into
a nitrile and the SNS
•
radical.
143,144
∼−
π
N
S
N
S
R
N
N
N
N
R
S
S
S
S
R
R
S
S
R
N
S
N
N
R
N
S
S
N
S
N
37
51
52
35
Scheme 9.14
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