Zirconocene-catalyzed alkylative dimerization of 2-methylene-1,3-dithiane via a single electron transfer process to provide symmetrical vic-bis(dithiane)s

Article abstract of DOI:10.1016/j.jorganchem.2006.12.029

A mixture of tertiary alkyl halide and 2-methylene-1,3-dithiane was treated with butylmagnesium bromide in the presence of a catalytic amount of zirconocene dichloride. The reaction resulted in alkylative dimerization to yield the corresponding vic-bis(dithiane). The reaction would proceed as follows. A single electron transfer from low-valent zirconocene to alkyl halide would generate the corresponding alkyl radical. The radical adds to 2-methylene-1,3-dithiane to afford the corresponding radical stabilized by the two sulfur atoms. A couple of the stable radicals finally undergo dimerization.

Full text of DOI:10.1016/j.jorganchem.2006.12.029

Journal of Organometallic Chemistry 692 (2007) 3110–3114
www.elsevier.com/locate/jorganchem
Communication
Zirconocene-catalyzed alkylative dimerization
of 2-methylene-1,3-dithiane via a single electron transfer
process to provide symmetrical vic-bis(dithiane)s
*
*
Suguru Yoshida, Hideki Yorimitsu , Koichiro Oshima
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Received 22 November 2006; received in revised form 22 December 2006; accepted 23 December 2006
Available online 9 January 2007
Abstract
A mixture of tertiary alkyl halide and 2-methylene-1,3-dithiane was treated with butylmagnesium bromide in the presence of a
catalytic amount of zirconocene dichloride. The reaction resulted in alkylative dimerization to yield the corresponding vic-bis(dithiane).
The reaction would proceed as follows. A single electron transfer from low-valent zirconocene to alkyl halide would generate the cor-
responding alkyl radical. The radical adds to 2-methylene-1,3-dithiane to afford the corresponding radical stabilized by the two sulfur
atoms. A couple of the stable radicals finally undergo dimerization.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Zirconium; Single electron transfer; Dithianes; Dimerization
1
. Introduction
oxidative addition involves the intermediacy of alkyl radi-
cals generated by single electron transfer from zirconocene
complexes to alkyl halides. However, the synthetic utility of
the zirconocene-mediated single electron transfer remains
largely unexplored.
We have been interested in single electron transfer from
low-valent zirconocene complexes to organic halides and
have applied the zirconocene-mediated electron transfer
to intramolecular radical cyclization reactions [4]. Here
we report zirconocene-mediated intermolecular radical
addition reactions of alkyl halides with 2-methylene-1,3-
dithiane. The radical addition proceeded smoothly to yield
vic-bis(dithiane)s through dimerization of the sulfur-stabi-
lized radicals generated by the radical addition.
Since the discovery of a convenient method for generat-
ing a low-valent zirconocene complex Cp Zr(1-butene), so-
2
called the Negishi reagent, organic synthesis by using the
zirconocene complex has been extensively developed [1].
The reagent provides the corresponding zirconacycles upon
treatment with unsaturated compounds such as alkene or
alkyne. On the other hand, little attention has been paid
to the reaction of Cp Zr(1-butene) with organic halides.
2
Although the reactions with alkenyl or aryl halides result
in the conventional oxidative addition reactions [2], the
reactions are usually complicated when alkyl halides are
subjected to the reactions. From a mechanistic point of
view, Schwartz investigated oxidation of Cp ZrL
2
2
(
L = PMePh or PMe Ph) with RX (RX = alkyl halide)
2. Results and discussion
2
2
and showed that the formal oxidative addition product
Cp ZrRX was formed [3]. It has been established that the
Treatment of tert-butyl bromide (1a, 0.50 mmol) and 2-
methylene-1,3-dithiane (2, 0.60 mmol) with butylmagne-
sium bromide (1.2 mmol) in the presence of a catalytic
amount of zirconocene dichloride (0.10 mmol) in THF
at 25 °C provided vic-bis(dithiane) 3a in 71% yield (Table
2
*
Corresponding authors.
E-mail addresses: yori@orgrxn.mbox.media.kyoto-u.ac.jp (H. Yori-
mitsu), oshima@orgrxn.mbox.media.kyoto-u.ac.jp (K. Oshima).
0
022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.12.029

S. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 3110–3114
3111
1
, entry 1) [5,6]. No products of the direct dimerization or
The following experiment justifies the intermediacy of
alkyl radicals (Scheme 2). Treatment of 1h under similar
reaction conditions afforded 3h having two cyclopentane
rings in high yield. Single electron transfer to 1h furnishes
6-methyl-5-heptenyl radical (4), which undergoes a well-
known 5-exo-trig radical cyclization [8] to yield radical 5.
The following radical addition and dimerization provided
3h.
disproportionation of the alkyl halide were observed,
which underscores the excellent reactivity of 2 as a radical
acceptor. Other tertiary alkyl bromides participated in the
reaction (entries 2–4). Unfortunately, the reactions of sec-
ondary alkyl bromides such as 1e resulted in much lower
yields (entry 5), which was due to lower conversions as
well as formations of rather complicated mixtures of prod-
ucts. Use of secondary alkyl iodides led to full conver-
sions, slightly improving the yields of 3 (entries 6 and
3. Summary
7
). The reactions of activated tertiary alkyl bromides such
as 2-bromo-N,N-diethyl-2-methylpropanamide afforded
complex mixtures in which no desired products were
observed.
The alkylative dimerization of 2-methylene-1,3-dithiane
proceeds with the aid of the BuMgBr/cat. Cp ZrCl com-
2
2
bination. Tertiary alkyl bromides are the best alkyl
sources. The transformation features single electron trans-
fer from electron-rich zirconocene to alkyl halides. The
products, vic-bis(dithiane)s, had not been readily accessi-
ble. The present easy preparation has made bis(dithiane)s
accessible and hence being potentially interesting interme-
diates [9].
We are tempted to assume the reaction mechanism as
follows (Scheme 1), based on our previous reports
II
[
4b,4c]. A low-valent zirconocene complex Cp Zr is ini-
2
tially formed. The zirconocene complex effected single elec-
III
tron transfer to an alkyl halide 1 to generate Cp Zr and
2
the radical anion of 1. The radical anion immediately liber-
ates a halide ion to yield an alkyl radical. The alkyl radical
adds to 2 to generate the corresponding sulfur-stabilized
4
. Experimental
radical. The radical is so stabilized and bulky that the rad-
III
ical is neither captured by Cp Zr nor abstracts hydrogen
2
4
.1. Instrumentation and chemicals
from a solvent molecule. Pair of the long-lived radicals
III
finally dimerize to yield 3 [7]. Cp Zr would be reduced
2
1
13
H NMR (500 MHz) and C NMR (126 MHz) spectra
by the action of an excess of BuMgBr to form the initial
II
were taken on Varian UNITY INOVA 500 spectrometers
low-valent Cp Zr , although the exact mechanism of the
2
and were recorded in CDCl . Chemical shifts (d) are in parts
3
reduction process is not clear.
1
per million relative to tetramethylsilane at 0.00 ppm for H
and relative to CDCl at 77.23 ppm for C unless otherwise
1
3
3
noted. IR spectra were determined on a SHIMADZU
FTIR-8200PC spectrometer. X-ray crystal structure analy-
sis was carried out with a Bruker SMART APEX CCD diffrac-
tometer with Mo Ka radiation. The structure was solved by
direct methods and refined by full-matrix least squares
Cp ZrIVCl2 2 BuMgBr Cp Zr
II
2
2
R X
1
BuMgBr
2
III
methods on F with the SHELXL-97. Mass spectra (FAB
Cp Zr
R X
2
unless otherwise noted) were determined on a JEOL Msta-
tion 700 spectrometer. TLC analyses were performed on
commercial glass plates bearing 0.25-mm layer of Merck Sil-
ica gel 60F254. Silica gel (Wakogel 200 mesh) was used for
column chromatography. Elemental analyses were carried
out at the Elemental Analysis Center of Kyoto University.
Unless otherwise noted, materials obtained from com-
mercial suppliers were used without further purification.
Zirconocene dichloride was purchased from TCI. Butylmag-
nesium bromide was prepared from magnesium metal and 1-
bromobutane in THF. THF was purchased from Kanto
Chemical Co., stored under nitrogen, and used as it is. All
reactions were carried out under argon atmosphere. Halide
1h was prepared according to the literature [10]. Dithiane 2
was prepared as described in the following section.
X
2
3
R
S
S
R
Scheme 1.
I
Cp ZrCl (0.30 eq)
BuMgBr (3.0 eq)
2
2
S
S
1
h
o
+
THF, 25 C, 12 h
S
S
2
(1.5 eq)
3
h
67%
4.2. Preparation of 2-methylene-1,3-dithiane (2)
4
5
Butyllithium (1.60 M in hexane, 15.9 mL, 25.4 mmol) was
Scheme 2.
added to a solution of 2-trimethylsilyl-1,3-dithiane (4.19 g,

3112
S. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 3110–3114
2
2.1 mmol) in THF (20 mL) at ꢀ40 °C. After stirring for 2 h
rated aqueous NH Cl and extracted with ethyl acetate. The
4
at the same temperature, paraformaldehyde (0.90 g,
combined organic layers were dried over anhydrous Na SO
2
4
3
0.0 mmol) was added to the resulting solution. The reaction
and concentrated in vacuo. Purification by chromatography
0
was allowed to warm to room temperature gradually for 12 h.
Water (20 mL) was added to the reaction mixture, and the
mixture was extracted with ethyl acetate (10 mL) three times.
The combined organic extract was washed with brine, dried
over sodium sulfate, and concentrated under reduced pres-
sure. Distillation (51 °C/0.1 Torr) in the presence of hydro-
quinone (a few milligram) gave 2-methylene-1,3-dithiane (2,
on a silica gel column provided 2,2 -bis(2,2-dimethylpropyl)-
0
2,2 -bi-1,3-dithianyl (3a, 66.0 mg, 71%).
4.4. Characterization data
4.4.1. 2-Bromo-4-(4-methoxyphenyl)-2-methylbutane (1d)
ꢀ
1 1
IR (neat) 2931, 1613, 1514, 1456, 1248, 1038 cm ; H
1
.68 g, 12.7 mmol, 58%). The product showed the identical
NMR (CDCl ) d 1.83 (s, 6H), 2.05–2.08 (m, 2H), 2.79–
3
NMR spectrum in the literature [11].
2.82 (m, 2H), 3.81 (s, 3H), 6.85 (dd, J = 6.5, 2.0 Hz, 2H),
1
3
7
.14 (dd, J = 6.5, 2.0 Hz, 2H); C NMR (CDCl ) d 32.2,
3
4.3. Typical procedure for zirconocene-catalyzed alkylative
dimerization (Table 1)
34.6, 50.0, 55.6, 67.9, 114.2, 129.6, 133.9, 158.2. HRMS
+
(FAB ) (m/z) Observed: 256.0460 (D = ꢀ1.2 ppm). Calcd
+
for C H OBr [M ]: 256.0463.
1
2
17
Zirconocene dichloride (29.0 mg, 0.10 mmol) was placed
in a flask under an atmosphere of argon. A THF (1.0 mL)
solution of 2-methylene-1,3-dithiane (2, 80.0 mg, 0.61 mmol)
and tert-butyl bromide (67.6 mg, 0.49 mmol) was added. A
THF solution of butylmagnesium bromide (0.86 M,
4.4.2. 1-Iodo-6-methyl-5-heptene (1h)
ꢀ1 1
IR (neat) 2929, 2855, 1436, 1377, 1224 cm ; H NMR
(CDCl ) d 1.44 (tt, J = 7.5, 7.5 Hz, 2H), 1.61 (s, 3H),
3
1.70 (d, J = 1.0 Hz, 3H), 1.83 (tt, J = 7.5, 7.5 Hz, 2H),
2.01 (dt, J = 7.5, 7.5 Hz, 2H), 3.20 (t, J = 7.5 Hz, 2H),
1
.40 mL, 1.2 mmol) was added, and the mixture was stirred
1
3
at 25 °C for 1 h. The reaction mixture was poured into satu-
5.10 (dt, J = 7.5, 1.0 Hz, 1H); C NMR (CDCl ) d 7.4,
3
Table 1
Zirconocene-mediated alkylative dimerization of 2-methylene-1,3-dithiane (2)
Cp ZrCl (0.20 eq)
2
2
S
S
S
BuMgBr (2.4 eq)
R
R
X
+
o
R
THF, 25 C, 1 h
S
S
S
1
2
(1.2 eq)
3
Entry
1
R–X
1
3
Yield (%)
71
1a
3a
Br
2
1b
3b
69
C H
Br
8
17
3
4
5
6
1c
1d
1e
1f
3c
3d
3e
3f
50
52
Br
p-MeOC H
Br
6
4
a
21
Br
b
44
I
b
7
1g
3g
42
I
a
The reaction was performed for 4 h.
b
1.5 equiv of 2 was used.

S. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 3110–3114
3113
1
7.9, 26.0, 27.1, 30.9, 33.4, 124.1, 132.3. Anal. Calc. for
38.2, 45.3, 68.3. Anal. Calc. for C H S : C, 59.64; H,
20 34 4
C H I: C, 40.35; H, 6.35. Found: C, 40.12; H, 6.30%.
8.51. Found: C, 59.37; H, 8.25%.
8
15
0
0
0
0
4.4.8. 2,2 -Di(2-methylpropyl)-2,2 -bi-1,3-dithianyl (3f)
4
(
.4.3. 2,2 -Bis(2,2-dimethylpropyl)-2,2 -bi-1,3-dithianyl
3a)
ꢀ1 1
Mp 126–127 °C; IR (nujol) 2926, 2857, 1456 cm ; H
NMR (CDCl ) d 1.07 (d, J = 8.0 Hz, 12H), 1.92 (doublet
of septet, J = 8.0, 4.5 Hz, 2H), 1.99 (d, J = 4.5 Hz, 4H),
2.03–2.10 (m, 2H), 2.29–2.36 (m, 2H), 2.81 (ddd,
J = 14.5, 7.0, 5.0 Hz, 4H), 3.18 (ddd, J = 14.5, 9.0,
.5 Hz, 4H); C NMR (CDCl ) d 23.7, 25.7, 26.5, 29.2,
7.8, 68.9. Anal. Calc. for C H S : C, 54.80; H, 8.62.
Found: C, 54.77; H, 8.70%.
ꢀ
1
3
Mp 119–120 °C; IR (nujol) 2925, 2855, 1464, 1456 cm
H NMR (CDCl ) d 1.21 (s, 18H), 1.80–2.30 (m, 8H), 2.73
ddd, J = 13.5, 4.5, 4.5 Hz, 4H), 3.20–3.80 (br s, 4H);
NMR (CDCl ) d 23.1, 30.4, 31.9, 33.3, 52.3, 67.9. Anal.
Calc. for C H S : C, 57.08; H, 9.05. Found: C, 57.24;
H, 9.11%. Crystal data. CCDC 627902, C H S ,
M = 378.69, orthorhombic, a = 12.0851(13), b = 11.1164
12), c = 15.2028(17) A, V = 2042.4(4) A , T = 293(2) K,
space group Pbca, Z = 4, l(Mo Ka) = 0.461 mm
0,778 reflections measured, 2214 unique (Rint = 0.0273)
which were used in all calculations. The final wR(F ) was
.136 (all data). The largest residual electron density hole
was ꢀ0.181 e A˚
;
1
3
1
3
(
C
3
13
4
3
1
8
34 4
4
1
8
30 4
1
8
34 4
3
˚
˚
(
0
0
4
.4.9. 2,2 -Di(cyclohexylmethyl)-2,2 -bi-1,3-dithianyl (3g)
Mp 114–115 °C; IR (nujol) 2917, 2848, 1654, 1559,
ꢀ
1
,
1
ꢀ1
1
1
1
1
457 cm ; H NMR (CDCl ) d 1.00–1.06 (m, 4H), 1.13–
2
3
.18 (m, 2H), 1.26–1.34 (m, 4H), 1.55–1.68 (m, 6H),
.88–2.00 (m, 12H), 2.01–2.10 (m, 2H), 2.79 (ddd,
J = 14.0, 6.5, 4.5 Hz, 4H), 3.17 (ddd, J = 14.0, 9.0,
.0 Hz, 4H); C NMR (CDCl ) d 23.7, 26.6, 26.9, 29.3,
5.7, 36.1, 46.4, 69.0. Anal. Calc. for C H S : C, 61.34;
1
ꢀ
3
.
1
3
5
3
0
0
4
.4.4. 2,2 -Bis(2,2-dimethyldecyl)-2,2 -bi-1,3-dithianyl (3b)
1
3
2
2
38 4
ꢀ1
IR (neat) 2925, 2854, 1468, 1456 cm
;
H NMR
H, 8.89. Found: C, 61.30; H, 8.91%.
(
1
2
CDCl ) d 0.87 (t, J = 7.0 Hz, 6H), 1.18 (s, 12H), 1.19–
3
0
0
.33 (m, 24H), 1.43–1.48 (m, 4H), 1.70–2.50 (m, 8H),
4
.4.10. 2,2 -Di(2-cyclopentyl-2-methylpropyl)-2,2 -bi-1,3-
1
3
.62–2.78 (m, 4H), 3.00–3.90 (br s, 4H); C NMR (CDCl3)
dithianyl (3h)
Mp 131–132 °C; IR (nujol) 2924, 2855, 1684, 1653,
d 14.4, 23.0, 23.3, 24.5, 29.4, 29.7, 30.0, 30.7, 30.9, 32.2,
3
H, 10.87. Found: C, 66.57; H, 10.67%.
ꢀ1
1
6.1, 45.2, 50.3, 68.3. Anal. Calc. for C H S : C, 66.83;
32 62 4
1558 cm
1
1
;
H NMR (CDCl ) d 1.25–1.32 (m, 16H),
3
.41–1.50 (m, 8H), 1.61–1.68 (m, 4H), 1.70–2.40 (m,
0H), 2.73 (ddd, J = 13.5, 4.5, 4.5 Hz, 4H), 3.00–3.80 (br
s, 4H); C NMR (CDCl ) d 23.3, 25.8, 26.2, 27.4, 30.8,
3
37.8, 50.0, 53.6, 68.4. Anal. Calc. for C26
H, 9.52. Found: C, 63.96; H, 9.26%.
1
3
0
0
4.4.5. 2,2 -Di[(1-methylcyclohexyl)methyl]-2,2 -bi-1,3-
dithianyl (3c)
H S : C, 64.14;
46 4
ꢀ
1
Mp 107–108 °C; IR (nujol) 2925, 2855, 1456, 1448 cm
;
1
H NMR (CDCl ) d 1.34 (s, 6H), 1.38–1.68 (m, 20H), 1.80–
3
2
.50 (m, 8H), 2.73 (ddd, J = 13.5, 4.5, 4.5 Hz, 4H), 3.20–
Acknowledgements
1
3
3
.80 (br s, 4H); C NMR (CDCl ) d 22.4, 23.3, 24.8,
3
+
2
6.6, 30.8, 36.2, 40.5, 52.8, 68.5. HRMS (FAB ) (m/z)
This work is supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. We thank Prof. Masaki
Shimizu (Kyoto University) for allowing us to use an
X-ray diffractometer.
Observed: 459.2242 (D = ꢀ1.3 ppm). Calcd for C H S
2
4
43 4
+
[
MH ]: 459.2247.
0
4
2
.4.6. 2,2 -Bis[4-(4-methoxyphenyl)-2,2-dimethylbutyl]-
0
,2 -bi-1,3-dithianyl (3d)
IR (neat) 2919, 2859, 1506, 1464, 1456, 1247, 1176,
References
ꢀ
1 1
039 cm ; H NMR (CDCl ) d 1.28 (s, 12H), 1.60–2.50
3
1
[
[
1] (a) E. Negishi, T. Takahashi, Acc. Chem. Res. 27 (1994) 124;
(
m, 12H), 2.55–2.65 (m, 4H), 2.65–2.70 (m, 4H), 3.00–
(
(
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7
3
1
8
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3
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3
0.3, 30.8, 31.8, 36.3, 47.1, 49.5, 55.5, 66.4, 113.9, 129.5,
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4
50
2 4
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(
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0
0
4
.4.7. 2,2 -Di(cyclopentylmethyl)-2,2 -bi-1,3-dithianyl (3e)
Mp 118–119 °C; IR (nujol) 2924, 2854, 1456, 1416, 1377,
ꢀ
1
1
[3] (a) G.M. Williams, K.I. Gell, J. Schwartz, J. Am. Chem. Soc. 102
1
1
(
284 cm ; H NMR (CDCl ) d 1.18–1.25 (m, 4H), 1.47–
3
(
(
(
1980) 3660;
.64 (m, 8H), 1.90–1.97 (m, 2H), 1.98–2.05 (m, 6H), 2.24
d, J = 5.5 Hz, 4H), 2.37–2.46 (m, 2H), 2.84 (ddd,
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3
5
.0 Hz, 4H); C NMR (CDCl ) d 23.4, 25.0, 28.8, 35.3,
3

3114
S. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 3110–3114
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[7] The dimerization is practically irreversible.
(
(
2004) 1727;
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5] In the absence of zirconocene dichloride and 2, no reaction between
[
[
1
a and butylmagnesium bromide took place. The reaction of 1a with
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[9] A number of attempts to convert 3 to the corresponding 1,2-diketones
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[11] T. Okuyama, Tetrahedron Lett. 23 (1982) 2665.
butylmagnesium bromide in the presence of zirconocene dichloride
and in the absence of 2 afforded a mixture of 2-methyl-1-decene
(24%), 2-methyl-2-decene (10%), 2-methyldecane (34%), and
9,9,10,10-tetramethyloctadecane (8%).
6] Use of 0.05 mmol of zirconocene dichloride resulted in a slightly
lower yield (65%). Much lower catalyst loadings such as 0.025 mmol
led to poor reproducibility.