108
K. Liu et al. / Catalysis Communications 20 (2012) 107–110
Scheme 1. Catalytic dimerization of 1a.
analyses were performed using silica gel plates, and column chroma-
tography was conducted using silica gel (mesh 200–300), both of
which were obtained from Qingdao Ocean Chemicals.
pure nitrogen atmosphere (Table 1, entry 4). In contrast, the dimer
4a was obtained in 76% yield under an oxygen atmosphere (Table 1,
entry 5). This result demonstrated that the dimerization requires a
synergistic relationship between the styrene-based compounds,
alkyl phenylphosphinate and Mn(II)/Co(II)/O2 redox system. In addi-
tion, by changing the ratio of 1a and 2a from 1:3 to 1:0.5, the dimer-
ization also proceeded; however, the yield of 4a decreased to 30%
(Table 1, entry 6). The radical mechanism for alkene hydrophosphor-
ylation using Mn(II)/Co(II)/O2 is well known, so a free radical inhibi-
tor, p-hydroquinone, was selected to further confirm the mechanism
(Table 1, entry 7). It demonstrated that the dimerization was inhib-
ited completely by addition of p-hydroquinone.
2.2. Synthesis of 3-ethyl-2-methyl-3-(2″,4″,5″-trimethoxy)
phenyl-1-(2′,4′,5′- trimethoxy)phenyl-1-propene (4a)
A typical dimerization reaction was carried out as follows. Butyl
phenylphosphinate (3.0 mmol), Mn(OAc)2 (0.05 mmol) and
Co(OAc)2 (0.05 mmol) were added to
a solution of 2,4,5-tri-
methoxy-1-propenylbenzene (1.0 mmol) in ethyl acetate (15 mL),
and the mixture was heated at 90 °C for 5 h. After cooling, the mixture
was extracted with ethyl acetate, and the organic layer was washed
with saturated NaCl until the pH was neutral. The resulting oil was
purified by column chromatography and eluted with 1:6 ethyl ace-
tate/petroleum ether to afford the title product as a white solid
(0.158 g, 76%).
Having optimized the catalytic conditions, the corresponding di-
mers were obtained conveniently in the presence of several alkyl
phenylphosphinates (Me, Et, and Bu). This suggested that alkyl
groups of alkyl phenylphosphinate had little effect on the dimerization
of styrenes. We further investigated the influence of styrene-based
compounds with different substituents on the scope and limitations of
dimerization. As shown in Table 2, styrene-based compounds bearing
methoxy substituents on a phenyl ring gave good results (Table 2, en-
tries 1–7). The yield of the dimers increased upon increasing the num-
ber of methoxy groups, while there was no obvious difference to the
reaction when using monomethoxy and dimethoxy substituents. No
dimer was obtained from styrene bearing an electron-withdrawing
substituent such as NO2 (Table 2, entries 11). These results indicate
that the electronic effect of the substituents played an important role
in the present dimerization reaction. However, it was worth noting
the dimerization could not proceed for styrenes with other electron-
donating substituents such as dimethylamino and methyl substituents
(Table 2, entries 8 and 9). A likely explanation is that methoxy groups
have higher donating efficiency compared with those of dimethylamino
and methyl groups, which facilitate to form the strong p–π conjugation.
These results demonstrated that sufficient donating efficiency in sub-
strate preferred the dimerization of styrenes. When the substrate was
a terminal alkene, the 1H NMR spectrum exhibited a doublet of doublets
from the two vinylic protons with a coupling constant of 16 Hz, typical
for the coupling between vicinal vinylic protons at trans-position
(Table 2, entries 2–4 and 6–7). Previously reported procedures for pre-
paring alkene dimers catalyzed by transition metals are generally limit-
ed to terminal alkenes [3–6], while the present reaction could be
applied to dimerization of non-terminal alkenes (Table 2, entries 1
and 5). In addition, this electronic effect did not account for the sub-
strate with a phenolic hydroxyl group, for which no reaction took
place (Table 2, entry 12). Presumably the reason was due to quinone
formation of oxidized phenol under the oxygen conditions.
3. Results and discussion
Due to the hypothesis that alkyl phenylphosphinate compound
acts as a catalyst rather than a substrate we first examined influence
of substrate structure on dimerization reaction. It was found the di-
merization reaction failed when substrate was replaced by a linear al-
kene (amylene) instead of a styrene-based compound, indicating that
the alkene structure was a significant factor in this reaction, and is
discussed later. We then aimed to test the effects of individual cata-
lytic factor on the mentioned reaction. As shown in Table 1, the corre-
sponding results were not obtained in the absence of PhP(O)HOBu
(Table 1, entry 1). Similarly, no products were detected in the absence
of Mn(OAc)2 or Co(OAc)2 (Table 1, entries 2 and 3). When the same
catalyst system was used, the dimerization did not proceed under a
Table 1
a
Dimerization of 1a by 2a using Mn(II)/Co(II)/O2
.
Entry
2a
(mmol)
Mn(OAc)2
(mmol%)
Co(OAc)2
(mmol%)
Inhibitorb
(mmol%)
Isolated yieldc
(%)
1
2
–
5
–
5
5
5
5
5
5
5
–
5
5
5
5
–
0
0
0
3
3
3
3
0.5
3
–
3
–
4d
5
–
0
–
76
30
0
6
7
–
10
a
1a (1 mmol) was catalyzed by 2a, Mn(OAc)2 and Co(OAc)2 under O2 at 90 °C for 5 h.
p-Hydroquinone.
Based on the amount of 1a used.
O2 was replaced by N2.
While a detailed reaction mechanism is not clear, the results can
be well explained by a radical mechanism as depicted in Scheme 2.
For the initiating step, the reaction is known to proceed by the
b
c
d
Liu, Kaijian
