Chemistry Reference
In-Depth Information
R
R
H
1. metathesis
2. base
3. R+
SOCl
2
NEt
3
N
N
H
2
N
N
NH
2
N
N
N
N
N
N
N
FeCp*
2
S
S
S
S
S
S
Se
Se
Se
Se
Se
Se
MeSe
SeMe
97c
105
106
104
SeO
2
AcOH
R
R
N
N
N
N
N
N
FeCp*
2
Se
Se
Se
Se
Se
Se
Se
Se
97d
107
Scheme 9.22
E
red
1
reduced to anions (
−
0
.
62
>
>
−
1
.
06 V) although the latter process is not always reversible; when
R
1
=
hydrogen, subsequent proton transfer events render the electron transfer processes irreversible. The
effects of the substituents (R
1
and R
2
) and chalcogen (E
1
and E
2
) on the redox potentials are qualitatively
predictable based on atom or group electronegativities.
R
1
N
R
2
108
The majority of bis(dithiazole)s
97
, including selenium variants, exist as unassociated radicals in the
solid state. There are a few exceptions whose associated structures are quite different from typical thiazyl
radical dimers. Bis(selenathiazole)
97b
(R
1
=
H) adopts a dimeric structure
109
with a Se-Se
bond length (2.46 A) close to that of a “normal” sigma bond.
185
The S-Se bond within the ring is essentially
ruptured as a consequence of the electronic structure reorganization suggested by the resonance structure
109
(and supported by the bond metrics of the structure). Pyrazine-based bis(dithiazole)
101
(R
1
=
ethyl, R
2
=
ethyl)
also adopts this dimeric structure, with the inter-radical S-S bond (2.17 A) again being very close to that of
a typical disulfide.
183
Interestingly, this molecule is bimoprhic; the second isomer is an unusual (for thiazyl
radicals) carbon-carbon
bonded dimer
110
with a relatively long (1.61 A) C-C bond. Computational
studies on dimerization processes on all four radical types (
98a-d
) suggest very low dimerization enthalpies
to form chalcogen based dimers,
184
and the C-C dimer
111
is predicted to be weakly endothermic and
as such was described as a kinetic product. Other derivatives of both of these radicals do not form these
unusual dimers, providing further evidence of the subtleties of the dimerization process in these delocalized
radicals.
182,186
σ
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