Electron-transfer induced rearrangement of thiocyanate to isothiocyanate

Article abstract of DOI:10.1016/S0040-4039(99)02063-8

Benzyl thiocyanates with an electron-withdrawing group (1a, 1b) and 9- fluorenyl thiocyanate (1e) rearrange to the corresponding isothiocyanate (2) under 9,10-diphenylanthracene (DPA) sensitization. The rearrangement would proceed via carbon-sulfur bond cleavage in an anion radical of 1 produced by photoinduced electron transfer, followed by back electron transfer to DPA·⁺ and recombination of the resulting radical or ion pair.

Full text of DOI:10.1016/S0040-4039(99)02063-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 365–369
Electron-transfer induced rearrangement of thiocyanate to
isothiocyanate
∗
Kan Wakamatsu, Jun Dairiki, Tetsuo Etoh, Hiroshi Yamamoto, Satoshi Yamamoto and
Yasumasa Shigetomi
Department of Chemistry, Faculty of Science, Okayama University of Science, Okayama 700-0005, Japan
Received 12 October 1999; revised 27 October 1999; accepted 29 October 1999
Abstract
Benzyl thiocyanates with an electron-withdrawing group (1a, 1b) and 9-fluorenyl thiocyanate (1e) rearrange to
the corresponding isothiocyanate (2) under 9,10-diphenylanthracene (DPA) sensitization. The rearrangement would
proceed via carbon–sulfur bond cleavage in an anion radical of 1 produced by photoinduced electron transfer,
·+
followed by back electron transfer to DPA and recombination of the resulting radical or ion pair. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: photochemistry; electron transfer; thiocyanates; rearrangements.
Organic thiocyanates and their isomeric compounds, i.e. isothiocyanates, are useful for syntheses
1
of compounds containing both sulfur and nitrogen. Although thermal or photochemical reactions
of thiocyanates have been well investigated,1 only a few studies of their ion radicals have been
reported. Electrochemical studies of 4-nitrobenzyl thiocyanate (1a) have revealed that the benzylic
carbon–sulfur bond of the anion radical of 1a could be efficiently cleaved at the rate constant of about
–6
4
−1
7
2
×10 s , affording 4-nitrobenzyl radical and thiocyanate anion. However, under these conditions the
0
only products were 4,4 -dinitrobibenzyl and 4-nitrotoluene arising from the 4-nitrobenzyl radical; the
rearrangement to the corresponding isothiocyanate (2a) did not occur. Photoinduced electron transfer
(PET) has also been useful for a generation of ion radicals and it is expected that neutral radicals
or ion pairs, resulting from back electron transfer, will play an important role. We report here the
thiocyanate–isothiocyanate rearrangement via an anion radical under PET conditions.
∗
Corresponding author. Fax: +81 86 254 2891; e-mail: waka@chem.ous.ac.jp (K. Wakamatsu)
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02063-8
tetl 15991

3
66
We used substituted benzyl thiocyanates (1a–1d) and 9-fluorenyl thiocyanate (1e) as the target
thiocyanates, and 9,10-diphenylanthracene (DPA) as an electron-donative sensitizer. As listed in Table 1,
the rate constant for fluorescence quenching of DPA by 1 (k ) correlates with the free enthalpy changes
q
(
∆G ) for the radical–ion pair formation in acetonitrile. Since the quenching rate in benzene is of the
et
same order as in acetonitrile, we assume the formation of an exciplex (or a contact radical–ion pair) in
benzene as well as the formation of a solvent-separated radical–ion pair in acetonitrile. Unfortunately,
†
the direct evidence of exciplex formation in benzene solution could not be obtained because of the lack
of the exciplex emission, probably due to some chemical reactions (vide infra).
Table 1
a
The rate constants for DPA fluorescence quenching, the photoreaction results for 1 with DPA, and the
b
approximated free enthalpy changes for electron transfer
−
3
The results for photolyses of the solution containing both 1 (0.01 mol dm ) and DPA (0.001 mol
−3
dm ) under nitrogen atmosphere are shown in Table 1. It is found that only the stronger acceptors 1a,
b, and 1e afforded the isomeric mixture of 1 and 2 and they were more reactive in benzene than in
1
acetonitrile.
Under the same conditions we also investigated the reverse reaction, i.e. the rearrangement from the
isothiocyanate 2b, but found that 2b was almost completely recovered after irradiation for 2 hours both
‡
in benzene and in acetonitrile. This would be due to the difference in strength between the carbon–sulfur
·
−
·−
bond in 1b and the carbon–nitrogen bond in 2b .
Thermolysis and direct photolysis of benzyl thiocyanates (1a–1d) also afforded the corresponding
isothiocyanates (2a–2d), whereas in the case of 9-fluorenyl thiocyanate (1e) either thermolysis in DMF
or direct photolysis resulted in yielding a complex mixture, and the only isolated product in thermolysis
0
10
was 9,9 -bifluorenylidene (3%).
In order to gain insight into the reaction, we performed laser flash photolysis (LFP) experiments. When
−3
–3
a deaerated acetonitrile solution containing both 1e (0.01 mol dm ) and DPA (0.3 mmol dm ) was
irradiated by a 355-nm nanosecond laser pulse (Nd-YAG), a sharp absorption band at around 500 nm was
·+
11
12–15
observed concomitant with absorption due to DPA (Fig. 1c). With reference to previous reports
and because of the nature of rapid quenching by molecular oxygen, the sharp 500-nm absorption can be
†
In the present case, free enthalpy changes for the radical–ion pair formation in benzene may be comparable to those in
acetonitrile, because the decrease of the solvation energies will be cancelled by the increase of the Coulombic interaction
between the more contacted radical–ions. See Refs. 8 and 9.
‡
Prolonged irradiation (30 h) of the benzene solution containing DPA and 1b and/or 2b afforded a complex mixture due to
some secondary reactions, so that we could not find the stationary state.

3
67
·
−
·−
assigned to a fluorenyl radical (FL·). If 1e survived, we expected to observe the absorption of 1e at
§
the same region (550–750 nm) as the fluorene anion radical. The spectrum shape in this region at 1 µs
after laser pulse, however, is identical to the spectrum of DPA without 1e, which would be generated
via photoionization as already mentioned by Wayner et al., so that within at least 1 µs 1e should be
·+
11
·−
−
cleaved to FL· and SCN . The above observations indicate that the cleavage of the carbon–sulfur bond
definitely occurs even in acetonitrile solutions in which the recovery of 1 is relatively high.
Fig. 1. Transient decay traces (a, b) and absorption spectra (c, d) after 355-nm laser excitation in deaerated solutions containing
−3
−3
1
e and DPA ([1e]=0.01 mol dm , [DPA]=0.3 mmol dm ).
§
It is reported that the fluorene anion radical absorbs light at 650 and 707 nm in MTHF matrix. See Ref. 16.

3
68
The decay of DPA + was over 20 times slower in the presence of 1e (k
·
DPA·+
=1.6×10 dm mol
9
3
−1
d
¶
−
1
·+
10
3
−1 −1
·+
s , Fig. 1a) than that of DPA without 1e (3.9×10 dm mol s ). In this case, DPA would
·+
be quenched by hole transfer to FL· and the fairly long lifetime of DPA might be due to the lower
k,**
oxidizability of FL· than the electron-attached acetonitrile.
We tried to determine the second-order rate constant of FL· decay. The decay rate constant of FL· at
·+
5
00 nm can be calculated by fitting the residual trace obtained by subtraction of the DPA component
DPA·+
at 500 nm, which can also be calculated by using the k
value and the OD ratio (Fig. 1b). Then the
d
FL·
FL·
6
–1
derived rate constant (k = kd /dε500 =3.01×10 s (d=1 cm)) and the reported molar extinction
obs
FL·
3
−1
–1 15
coefficient (ε5
) in methanol (3600 dm mol cm ) allow us to determine the second-order rate
00
FL·
10
3
−1 −1
FL·
constant, kd =1.1×10 dm mol s , assuming ε
is solvent-independent. FL· would react not
500
·+
−
only with DPA but also with SCN . Since the rate constant of the former reaction has already been
9
3
−1 −1
DPA·+
·+
determined as 1.6×10 dm mol
s
(k
) from the decay profile of DPA (Fig. 1a), we can obtain
d
9
3
–1 –1
the rate constant of the latter as 9.4×10 dm mol s . This undoubtedly shows the reaction between
−
·+
FL· and SCN can compete with the hole transfer from DPA to FL·.
The most reasonable pathway for the reaction between the benzyl-type radical, including FL·, and
−
−
−
SCN would be S 1-like nucleophilic attack of SCN to the radical. Since SCN has an ambident
RN
character, both the sulfur side and the nitrogen side of the anion can react as a nucleophilic center. As is
well-documented, the affinity between the two chemical groups can be interpreted by the hard–soft-acid-
1
9,20
−
base (HSAB) theory.
According to the HSAB theory, the softer sulfur atom of SCN can react with
a soft electrophile, whereas the harder nitrogen atom prefers the harder center. Actually, the variation
−
of the thiocyanate/isothiocyanate ratio in the nucleophilic reaction of SCN toward several carbocations
has been well explained by using the HSAB theory, and in the case of 1-phenylethyl cation, which can
3
be regarded as a harder carbocation, the ratio has been reported to be only 0.01. The benzyl-type radical
in the current case should be much softer than the corresponding benzyl-type cation. Therefore, the
−
extent of the nucleophilic attack of the sulfur side of SCN to the radical, leading to regeneration of the
thiocyanate anion radical, would be higher than that in the case of the attack to the cation.
·
+
In a similar experiment in benzene the absorption of DPA was not observed and only weak absorption
21
(
4
ca 0.005 at 1 µs) due to FL· was detected accompanied by the absorption of DPA triplet at around
50 nm (Fig. 1d). The weaker absorption of FL· would reflect a fast back electron transfer within the
−
·+
solvent-cage [FL· SCN DPA ].
The mechanism of the rearrangement from 1 to 2 under DPA sensitization would be explained by
Scheme 1. Both in acetonitrile and in benzene the stronger electron accepter (1a, 1b, 1e) effectively
·
−
·+
·−
quenches the DPA excited singlet, affording the 1 –DPA pair. The resulting 1 would be dissociated
−
to the benzyl-type radical and SCN within 1 µs. In acetonitrile, as revealed by LFP experiments, the
nucleophilic attack of SCN to the radical center can compete with the hole transfer from DPA to
the radical, and the lower yield of 2 can be explained by assuming the sulfur side of SCN does the
−
·+
−
nucleophilic attack more preferentially. On the other hand, in benzene, since the radical and the ions are
−
·+
kept in the same solvent cage, the rate for the electron transfer from the radical or SCN to DPA may
−
increase as well as that for the nucleophilic attack of SCN to the radical. However, the contribution of
¶
·+
The decay of DPA without 1e seems to be slightly faster than the diffusion-controlled process, and this will probably be
·+
due to the contribution of the decay of DPA pairing with the electron-attached acetonitrile molecule.
k
·+
The lifetime of DPA is affected by the presence of oxygen (longer than under a nitrogen atmosphere) and it is reasonably
·
+
explained that the counter of DPA (oxidation potential of DPA: +1.19 V versus SCE) is changed from FL· (oxidation potential:
17
+
0.76 V ) to the less reductive FL–O2·.
*
*
+
FL generated by the hole transfer could not be observed, which absorption should appear at around 515 nm as a broader
18
+
−
band than that of FL· This would be due to the rapid reaction of FL with SCN .

3
69
the former process would be more significant than that in acetonitrile, so that the recombination of the
benzyl-type radical (cation) and SCN radical (anion) resulting from the electron transfer within the cage
would lead to higher yield of 2 analogously to the usual thermal or photochemical rearrangement.
Scheme 1.
Acknowledgements
We are grateful to Professor Tsutomu Miyashi, Tohoku University, for the use of his LFP equipment.
We also thank Professor Michinori Oki, Okayama University of Science, and Professor Yasutake
Takahashi, Mie University, for their helpful suggestions in preparing this manuscript.
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