Effect of a cobalt(II) complex on the radical reaction of vinyl type sulfides. A 'radico-catalysis'

Article abstract of DOI:10.1039/a808576e

Vinyl type sulfides show increased radicophilicity at the β-position via the coordination to a cobalt(II) complex, and hence the balance between Smiles rearrangement and an ortho-substitution, in vinyl type sulfides having an intramolecular alkyl radical, is lost to favor the latter reaction.

Full text of DOI:10.1039/a808576e

Effect of a cobalt(II) complex on the radical reaction of vinyl type sulfides. A
‘radico-catalysis’
Masaru Tada,* Tomohiro Uetake and Yoshinobu Hanaoka
Department of Chemistry, the Advanced Research Center for Science and Engineering, and Materials Research
Laboratory for Bioscience and Photonics, Waseda University, Shinjuku, Tokyo 169-8555, Japan.
E-mail: mtada@mn.waseda.ac.jp
Received (in Cambridge, UK) 4th November 1998, Accepted 27th November 1998
Vinyl type sulfides show increased radicophilicity at the b-
position via the coordination to a cobalt(ii) complex, and
hence the balance between Smiles rearrangement and an
ortho-substitution, in vinyl type sulfides having an intra-
molecular alkyl radical, is lost to favor the latter reaction.
study by us7 and characterized by back donation from cobalt(II
)
to sulfur. The bonding force of this back donation makes the
unstable two-centered three electron bond feasible.
Homolysis of triphenyltin cobaloxime produces a triphe-
nyltin radical and cobaloxime(II) [eqn. (6)] but the former
dissipates via reaction with a halide to leave an organo radical
[eqn. (7)]. Thus the organo radical and cobaloxime(II) coexist in
the reaction system and associate by the coordinative interaction
discussed above [eqn. (8)]. The interaction of the vinyl type
Acid and base catalysis generates a ‘paired electron hole’ [eqn.
(1)] and a ‘paired electron pool’ [eqn. (2)], respectively.
Likewise radical catalysis can be defined as the generation of
‘unpaired electron density’ at the reaction center [eqn. (3)].
Ph3Sn[Co]L
Ph3Sn•
R•
(R•) [COII]L
+
[CoII]L
+ Ph3SnBr
(6)
(7)
(8)
Rδ+
Rδ–
Rδ•
X
X
X
Acidδ–
Baseδ+
Radδ•
(1)
(2)
(3)
RBr
R•
+
•SnPh3
[COII]L
Acid catalysis:
Base catalysis:
RX
RX
+
+
+
Acid
+
Base
•(Rad)
Radico-catalysis: RX
sulfide and the cobalt(II) species may increase the radicophi-
licity of the vinyl moiety.
We have demonstrated that a cobalt(II) complex, cobalox-
ime(II),1† accelerates the radical substitutions on the sulfur of a
thioester group [eqn. (4)]2 and the radical 1,2-rearrangement of
a thioester group [eqn. (5)].2,3 Caddick et al.,4 Aldabbagh and
Thus 3-(benzothiophen-2-ylthio)-2,2-dimethylpropyl bro-
mide 1 was treated with triphenyltin cobaloxime under heating
and the products 3 and 5 were obtained as shown in Scheme 1.
The thermolysis gives the cobaloxime(II) radical and the
intermediate radical A which interact with each other by the
coordination discussed above to give radical B and thus product
3 (Scheme 2).
[CoII]L
MeSAr + RCO•
Me• + RCOSAr
(4)
(5)
[CoII]L
Ph
Ph COSEt
Me CH2•
•
C
CH2COSEt
Me
H2
C
H2
C
Me
Me
[CoII]L
Me
Me
H
•
•
Me
Me
Bowman,5 and Tada et al.6 showed the usefulness of the sulfur
function in the radical annelation on indole and benzimidazole
systems, in which the intramolecular alkyl radicals attack the
ipso-position to yield indoleno or benzimidazoleno carbocycles
by the extrusion of the sulfur function. We have suggested the
effect of cobaloxime(II) in the radical annelation of 2-phenyl-
thioindole via an intramolecular N-alkyl radical.6
S
S
S
S
S
S
•
[CoII]L
A
B
C
Ph3SnH
[CoII]L
5
6
3
2
Here we report another type of radico-catalysis by cobalox-
ime(II) in the radical annelation of benzothiophene and uracil
SnPh3
S•
•
S
H
Me
Me
•
derivatives. Coordination of a vinyl type sulfide to cobalt(II
)
4
Me
Me
generates an unpaired electron (spin) density on the sulfur,
which can delocalize to the b-position of the vinyl group as
shown in Fig 1. The coordination has been proven via an EPR
S
S
Me
S
S
Me
D
E
F
Scheme 2
Treatment of bromide 1 with Ph3SnH–AIBN gave the
products 4 and 6 after Smiles rearrangement by an addition–
elimination mechanism (A?D?E in Scheme 2)8 and the
reduction product 2. Thus the cobaloxime(II) in the reaction
system evidently accelerates the radical attack on the 3-position
of benzothiophene and ortho-substitution becomes competitive
with the radical Smiles rearrangement.8–10 On the other hand, in
•
•
R
R
S
S
[CoII]L
[CoII]L
Fig. 1 Resonance expression of spin delocalization in a sulfur–cobalt(II
)
complex.
Br
Me
Me
Me
Me
SH
Me
S)2
Me
Me
Me +
H3C
S
CH3
Me
Me
+
+
+
S
S
S
S
S
Me
Me
S
S
S
6
1
3
4
5
2
—
17%
—
—
70%
—
—
Ph3Sn[Co]L / 130 °C‡
Ph3SnH–AIBN / 80 °C§
11%
71%
14%
Scheme 1
Chem. Commun., 1999, 75–76
75

O
O
SnPh3
S
O
H
O
Br
S
O
O
H
H
Me
Me
Me
Me
Me
Me
Me
Me
H3C
S
MeN
O
MeN
O
MeN
O
MeN
O
MeN
O
MeN
O
(
)
n
DMF
+
+
+
+
(
)
n
(
)
n
(
)
n
(
)
n
Me
Me
N
CH3
N
S
N
N
N
S
Me
Me
Me
Me
Me
8
10
7
9
11
12
n = 1 Ph3Sn[Co]L‡
—
73%
—
—
—
n = 1 Ph3SnH–AIBN§
n = 2 Ph3Sn[Co]L‡
n = 2 Ph3SnH–AIBN§
—
—
38%
—
8%
—
45%
—
28%
82%
47%
—
—
—
—
Scheme 3
addition to product 2, only the products of Smiles rearrange-
ment were obtained without cobaloxime(II). The formation of
the product 6 is accounted for by radical substitution on sulfur
through the radical intermediate F. A reasonable mechanism for
formation of these products is shown in Scheme 2.
Next we tested the effect of cobaloxime(II) on the radical
from n-(1,3-dimethyluracil-5-ylthio)-2,2-dimethylalkyl bro-
mide 7 and a similar acceleration effect was observed for the
ortho-substitution, as shown in Scheme 3. Bromide 7 (n = 1)
All the experimental results shown here suggest an accelera-
tion effect of cobalt(II) species on the radical attack of an alkyl
radical on a vinyl type sulfide. Thus the coordination of a vinyl
sulfide to a paramagnetic cobalt(II) complex generates a spin
density at the b-position and makes the b-position more
radicophilic. We propose a term ‘radico-catalysis’ for this effect
even though the reaction is stoichiometric and not ‘catalytic’ in
the correct sense.
Experimental details and structural assignments of the
products will be reported in a full paper.
gave the substitution product 9 in the presence of cobaloxime(II
)
whereas the reaction with Ph3SnH gave the addition product 10
and the products 11 and 12 formed via Smiles rearrangement
(Scheme 4).
The present work was supported by Waseda University and
the Ministry of Education, Culture, and Sports of Japan through
an Annual Project Program and a Grant-in-Aid for Scientific
Research, respectively
Both Smiles rearrangement via a six-membered intermediate
(J) (n
= 2) and ortho-addition via a seven-membered
intermediate (I) (n = 2) are slow and the only product from the
reaction of Ph3SnH and bromide 7 (n = 2) is the direct
reduction product. The reaction with triphenyltin cobaloxime,
however, gave the ortho-substitution product 9 (n = 2) as a
major product, however the intramolecular radical addition is
still slow and hydrogen abstraction form the solvent to give
product 8 (n = 2) is the dominant process. A reasonable
mechanism for formation of 8–12 is illustrated in Scheme 4.
The intermediate radical I (n = 1) which gives 10 (n = 1) is
formed even without cobaloxime(II), although with lower
efficiency, while radical I (n = 2) which gives 9 (n = 2) is
formed only with the assistance by cobaloxime(II). The
intermediates I (n = 1,2) give products 9 (n = 1,2) with
hydrogen elimination by cobaloxime(II) and the product 10 (n =
1) via hydrogen abstraction from the tin hydride. Products 11
and 12 derive from the intermadiate L (n = 1) via hydrogen
elimination [route (a)] and the fragmentation process [route
(b)], respectively, after Smiles rearrangement (Scheme 4).
Notes and references
† Cobaloxime(II) is bis(dimethylglyoximato)(4-tert-butylpyridine)cobalt(II
and denoted here by [CoII]L.
‡ Reaction conditions: 1 or 7 (0.1 mmol), Ph3Sn[Co]L (0.3 mmol), DMF
(5.0 ml), 130 °C, 24 h.
§ Reaction conditions: 1 or 7 (0.1 mmol), Ph3SnH (0.2 mmol), AIBN (0.1
mmol), benzene (40 ml), 80 °C, 4 h.
)
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9
[CoII]L
O
H2•
H
Me
Me
•
Me
Me
Me
Me
H2C
C
([CoII]L])
MeN
O
(
)
n
(
)
n
(
)
n
S
N
S
S
•
Me
[CoII]L
G
H
I
Ph3SnH
7 M. Tada and R. Shino, J. Inorg. Biochem., 1991, 44, 89.
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8
11
12
10
(a)
(b)
(a)
H
•
Me
Me
S--SnPh3
S•
•
Ph3SnH
(
)
n
(
)
n
(
)
n
Me
Me
S
Me
Me
(b)
J
K
L
Scheme 4
Communication 8/08576E
76
Chem. Commun., 1999, 75–76