Unprecedented electrophilic behaviour of tetrathiafulvalenium salts

Article abstract of DOI:10.1039/a704690a

Primary tetrathiafulvalenium salts display a diverse and unprecedented reactivity towards nucleophiles.

Full text of DOI:10.1039/a704690a

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Unprecedented electrophilic behaviour of tetrathiafulvalenium salts
Owen Callaghan, Xavier Franck and John A. Murphy*
Department of Pure and Applied Chemistry, The University of Strathclyde, 295 Cathedral Street, Glasgow, UK G1 1XL
Primary tetrathiafulvalenium salts display a diverse and
unprecedented reactivity towards nucleophiles.
alternative reactions too slow to observe. We now report the
results of exposing primary tetrathiafulvalenium salts to various
nucleophiles.
Tetrathiafulvalene (TTF) and its derivatives have attracted
widespread interest in the preparation of novel materials.1
Recently, the synthetic value of TTF has become apparent as a
result of its catalysis of ‘radical-polar crossover2 chemistry’.
This leads to the formation of tetrathiafulvalenium salts 2
(Scheme 1) which have been isolated. In appropriate cases, e.g.
2a, these compounds undergo facile SN1 substitution at room
temperature by such nucleophiles as water, alcohols and MeCN.
Since this happens in situ, TTF is regenerated and so functions
as a catalyst. The catalyst is effective down to a concentration of
ca. 5 mol% but, below this level incomplete conversions are
seen. It is clear that the catalyst is slowly destroyed during such
reactions.
The salt 2b was subjected to attack by sodium azide in
acetone (Scheme 2). The principal products from this reaction
were tetrathiafulvalene (45%) and the vinyl azide 4 (46%).
Although the appearance of tetrathiafulvalene initially sug-
gested that an SN2 substitution had occurred, this is clearly not
the case. To explain the formation of these products, we propose
the sequence of steps depicted in Scheme 2. The thioketene 5
has not been detected, but could suffer attack by azide, which in
turn would lead to fragmentation to afford the stabilised carbene
7 together with either carbon monosulfide and azide ion or
thiocyanate ion plus nitrogen gas. The carbene 7 is an
established synthetic precursor of TTF in other contexts.3
One of the goals for the future of this research is to develop
catalysts with greater efficiency, i.e. with greater turnover
numbers. To achieve this, it is necessary to develop an
understanding of what processes other than substitution can
occur with TTF salts. In the case of salts such as 2a, these
processes can only compete poorly with the substitution
chemistry. This means that side-products are formed in very
small amounts in such cases, and hence it is difficult to
determine the nature of the side-reactions which ultimately limit
the catalytic life of TTF.
A logical way to determine the nature of side-reactions would
be to study TTF salts where SN1 substitution reactions are
inhibited. Alternative reactions observed under such conditions
then provide useful information on potential side-reactions in
normal radical-polar reactions. We have previously described1
that when the TTF moiety is attached to a primary carbon, as in
2b, no substitution was observed. It is totally reasonable that
substrates such as 2b should not undergo SN1 substitution, and
we assumed that the mildness of the nucleophiles rendered any
S
S
O
NaN3
S
2b
S
N3
+
–
S
S
O
O
–
N3
S
+ 0.5 TTF
S
S
+
4
N3
S
S
S
S
–
N3
S
S
–
–
+
N3
C
S
+
C
S
N3
S
5
7
6
O
O
O
TTF
–N
C S
0.5 TTF
S
+ N2 +
S
+
N2
•
•
N2
R
–
R
R
7
BF4
1a R = Me
b R = H
Scheme 2
–
BF4
+ •
S
To determine the generality of this process, two further
primary tetrathiafulvalenium salts 8 and 9 were prepared and
treated with sodium azide in acetone, producing the vinyl azides
10 (36%) and 11 (41%) respectively (Scheme 3). In both cases,
S
S
O
O
S
S
S
S
•
S
+
R
–
–
R
BF4
BF4
S
2a R = Me
b R = H
R1
X
R1
X
S
S
N3
+ TTF
S
S
NaN3
O
O
+
H2O
acetone
R2
8 X = NMs, R1 = R2 = H
9 X = O, R1 = CH2OH, R2 = OMe
Scheme 3
R2
10 X = NMs, R1 = R2 = H
11 X = O, R1 = CH2OH, R2 = OMe
OH
+
–
Me
Me
BF4
3
Scheme 1
Chem. Commun., 1997
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H
–
CO2Et
CO2Et
H
S
O
HO–
S
S
S
+
S
–
2b
S
Ms
N
+
S
O
17
HO–
8
CO2Et
CO2Et
14
S
S
–
Ms
N
+
S
S
OH
–
12
S
H
S
–
S
O
O
S
S
S
O
+
S
H
S
13
+
S
15
Scheme 4
+
16
–
O
–
H
O2C
S
TTF was also produced (49 and 21%, respectively). Compound
11 was converted into its crystalline 3,5-dinitrobenzoate ester.
However, attempts to produce a suitable crystal for X-ray
structure determination were not successful.
O
S
H2O
H
S
O
S
S
+
S
19
+
The fragmentations seen above represent a previously unseen
reaction of TTF salts. However, a completely different mode of
reaction was seen when 2b was treated with the anion of diethyl
malonate (Scheme 4). In this case, the product 12 (38%)
resulted from reaction at the central carbon–carbon double bond
of the salt. The structure 12 rather than 13 is suggested for the
product, since the 1H NMR spectrum exhibits a one-proton
singlet at d 3.1; this is in the expected region for a malonate
proton but is considerably upfield of the expected shift for the
proton in 13. In this reaction, no attack on the peripheral double
bonds of the TTF salt was observed. The reason for the
complete change in selectivity for azide and malonate as
nucleophiles is unknown.
Finally, the salt 2b was treated with potassium hydroxide as
base under different conditions (KOH in methanol, KOH in
DMSO, KOH in acetone). In these cases, an alternative but very
clean fragmentation to 14 (79% NMR yield, 66% isolated yield)
was seen. Possible mechanisms for this reaction are proposed in
Scheme 5. The base-sensitive dithiocarbonate 17 has not been
detected, even when only 1 equiv. of base was used.
–CO2
18
H
–
O
S
S
+
14
Scheme 5
cations are different from the alkylated TTF cations discussed
here, it is clear that quenching of the conducting properties of
these compounds could arise by attack by nucleophiles on the
TTF nucleus.
We thank the EPSRC and Merck Ltd. for funding and the
EPSRC National Mass Spectrometry Service Centre, Swansea,
for mass spectra. We thank Dr Richard Hartley, University of
Glasgow, for helpful discussions.
Footnote and References
The ylide proton in 14 appeared as a singlet at d 3.1, and this
correlated with a methine carbon at dC 84. The chemical shift of
both the carbon and the proton indicate that this structure is best
viewed as an ylide i.e. featuring 2C–S+ rather than a CNS
* E-mail: john.murphy@strath.ac.uk
1 M. R. Bryce, J. Mat. Chem., 1995, 5, 1481; S. Horiuchi, H. Yamochi,
G. Saito, K. Sakaguchi and M. Kusunoki, J. Am. Chem. Soc., 1996, 118,
8604; A. Charlton, A. E. Underhill, G. Williams, M. Kalaji, P. J. Murphy,
K. M. A. Malik and M. B. Hursthouse, J. Org. Chem., 1997, 62, 3098.
2 C. Lampard, J. A. Murphy and N. Lewis, J. Chem. Soc., Chem. Commun.,
1993, 295; C. Lampard, J. A. Murphy, F. Rasheed, N. Lewis,
M. B. Hursthouse and D. E. Hibbs, Tetrahedron Lett., 1994, 35, 8675;
M. J. Begley, J. A. Murphy and S. J. Roome, Tetrahedron Lett., 1994, 35,
8679; R. J. Fletcher, C. Lampard, J. A. Murphy and N. Lewis, J. Chem.
Soc., Perkin Trans 1, 1995, 1349; N. Lewis, J. A. Murphy, F. Rasheed
and S. J. Roome, Chem. Commun., 1996, 737; R. J. Fletcher, D. E. Hibbs,
M. B. Hursthouse, C. Lampard, J. A. Murphy and S. J. Roome, Chem.
Commun., 1996, 739; M. Kizil, C. Lampard and J. A. Murphy,
Tetrahedron Lett., 1996, 37, 2511.
p-bond. However, one interesting feature is that the 1H and 13
C
NMR spectra of 14 show no indications of the presence of
diastereoisomers. This contrasts with 2b, where signals due to
diastereoisomers are clearly seen. This may indicate that
inversion at the sulfonium sulfur in 14 occurs rapidly on the
NMR timescale.4
In conclusion, primary tetrathiafulvalenium salts exhibit a
rich and complex chemistry. The design of improved radical-
polar crossover catalysts should take into account the observed
nucleophilic attack on both the periphery and the internal alkene
of these TTF salts.
These studies may also prove useful in materials chemistry.1
A vast number of salts with intriguing electrical conductivity
properties are known. These salts result from electron transfer
3 See for example: A. J. Moore and M. R. Bryce, Synthesis, 1997, 407.
4 For rapid inversion at sulfur in a sulfur ylide, see: H. Nozaki, M. Takaku
and K. Kondo, Tetrahedron, 1966, 22, 2145.
from TTF to appropriate acceptor molecules, and hence the salts
+
·
formally involve the TTF radical-cation, TTF . Although these
Received in Cambridge, UK, 3rd July 1997; 7/04690A
1924
Chem. Commun., 1997