STERIC EFFECTS OF SUBSTITUENTS OF QUINONES IN CATALYTIC REACTIONS
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(C O) and the hydroxyl of NHPI, and thus the formation of free
—
radical PINO was blocked.
In order to further confirm the steric effects of the substituents
of quinone, the catalytic activities of NQ, MNQ, AQ, and EAQ
were tested in the ethylbenzene oxidation reactions. As shown in
Fig. 3, the catalytic activities varied in the following order:
NQ > MNQ > AQ > EAQ. NQ with the lowest steric effects
induced the highest conversion of ethylbenzene. With increasing
the number of substituents on the quinone ring, the reaction
activity decreased. EAQ with the highest sterically hindered
effects exhibited the lowest catalytic activity because of the large
steric effects which hindered the abstraction of hydrogen atom
from NHPI by quinone.
To give a straight insight into the steric effects of the
substituents, the oxidation of ascorbate catalyzed by quinones
was studied. Ascorbate oxidation catalyzed by quinones is a
convenient method to evaluate the reactivity of quinone.[17]
Moreover, this reaction can be used as a criterion to estimate the
host toxicity of quinones to organism, which has been proposed
as a tool to get rid of cancer cells.[18] Figure 4 shows the results of
the oxidation of ascorbate by O2 catalyzed by various quinones.
The conversions of ascorbate were much higher in the presence
of different quinones compared with that in the absence of
quinones, which implied that quinones could effectively catalyze
the oxidation of ascorbate. Among methyl benzoquinones, the
catalytic activity in the oxidation reaction of ascorbate varied in
the order of BQ < MBQ < DQ <TMQ < m-DMQ. As expected,
m-DMQ displayed the highest activity with a 96% conversion of
ascorbate within 10 h, which was much higher than that of BQ
or MBQ. Among AQ, NQ, MNQ and EAQ, NQ presented the
highest activity with a 97% ascorbate conversion. Under
same conditions, the conversions were 54, 36, and 15% by using
MNQ, AQ, or EAQ as the catalyst, respectively. In addition, the
oxidation of ascorbate was studied in the solution of water–
acetone, the catalytic activity also varied in the order of
NQ > MNQ > AQ > EAQ. The steric effects of quinones in the
oxidation of ascorbate were similar with those in the oxidation of
ethylbenzene. The effect of the concentration of NQ on the
oxidation of ascorbate was also investigated (Fig. 5). The
conversion of ascorbate increased quickly at a low concentration
Figure 2. The conversion of ethylbenzene catalyzed by BQ derivatives/
NHPI. Reaction conditions: 16 mmol of ethylbenzene, 0.4 mmol of qui-
none, 1.6 mmol of NHPI, 10 ml of CH3CN, 0.3 MPa of O2, 80 8C, 6 h
substituent methoxyl group was introduced, which further
proved our explanation for the decrease of the catalytic activity
of MBQ/NHPI. In addition, the use of BBQ bearing a tert-butyl
substituent and NHPI as the catalyst resulted in a higher
conversion than the use of MBQ/NHPI as the catalyst, which
implied that the bulky substituent in the mono-substituent
quinone benefits its catalytic activity.
Among methyl benzoquinones, p-DMQ with two substituents
showed a higher catalytic activity in the ethylbenzene oxidation.
Moreover, m-DMQ/NHPI and o-DMQ/NHPI exhibited just slightly
lower activity than p-DMQ/NHPI, which indicated that the
influence of the position of the substituents in the benzoquinone
ring on its catalytic activity was negligible. Further, DBQ bearing
two bulky substituents combined with NHPI presented a similar
conversion of ethylbenzene compared with m-DMQ/NHPI, which
showed that the catalytic activity of di-alkyl-quinone was no
more sensitive when the steric hindrance of the two substituents
was further increased. As mentioned above, the greater the
number of the alkyl groups in quinone ring, the lower the redox
potential of quinone and the lower catalytic activity. On the other
hand, the sterically hindered substituents limited the addition of
range and reached its maximum at
a
NQ amount of
—
free radicals to the C C double bonds of the quinone ring, and
—
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consequently restrained the formation of the low reactive
alkoxyphenoxyl radical that led to chain termination;[10] so the
catalytic activity was promoted. For DMQ, the positive steric
effects were dominant relative to the negative electronic effects
of methyl group. Therefore, the benzoquinones with two alkyl
substituents displayed a higher activity than MBQ. The catalytic
activity of TMQ/NHPI was slightly lower than that of p-DMQ/NHPI,
suggesting that the steric effects of the substituents were still
dominant for TMQ.
When the number of methyl substituents in the quinone ring
was further increased, the conversion of ethylbenzene decreased.
From Fig. 2, it can also be seen that the catalytic activity of DQ was
comparable to that of MBQ and was lower than that of BQ. DQ
bearing four methyl groups had a lower probability of quenching
free radicals in view of their high sterically hindered effects.
However, too many sterically hindered alkyl groups probably
hindered the interaction between the active centers of quinone
NQ
MNQ
AQ
EAQ
Figure 3. The conversion of ethylbenzene catalyzed by NQ, AQ, or their
derivatives with NHPI as co-catalyst; under the same reaction conditions
described in Fig. 2
J. Phys. Org. Chem. 2011, 24 693–697
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc