948
Nonpolar solvents
Polar solvents
O
O
O
H O
O
O
OH
O
O
O
O
O
H
O
O
hydrogen bond
H
O
H
O
H
O
OH
OH
O
HO
O
H
δ+
δ−
HO
HO
H
O
H
O
O
O
OH/π interaction
(HQ1)2
(QQ1)(HH1)
energetically favorable
energetically favorable
Figure 3. Hydrogen-bonded dimers existing in a QQ1-HH1-HQ1 system in nonpolar solvents (left) and monomeric species in polar solvents (right).
HQ1, QQ1, and HH1, were detected, thus strongly indicating that
HQ1 is expected to be an equilibrium product. HQ1 could be
trapped as MQ1 by adding dimethyl sulfate and potassium
carbonate to the mixed solution of QQ1 and HH1; its molecular
structure was determined by comparing with that of the sample
shown in Figure 2. The exchange rate was considerably slow in
all solvents, and it took from a few hours to more than twenty
hours to achieve equilibrium (see Supporting Information15). This
contrasts the mixed solution of duroquinone and durohydroqui-
none, which reaches equilibrium imediately.1a
thus, QQ1 did not form a charge-transfer (CT) complex with
hydroquinone.14 Nevertheless, a new absorption band was
observed in UV-vis spectra of the mixed solution of QQ1 and
HH1, which is attributable to HQ1 on the basis of K1. As shown
in Figure 5, the longest wavelength absorption peak (-max) of
HQ1 is red-shifted compared to QQ1 and HH1. It is seen that
-
for HQ1 is significantly red-shifted in the less polar solvent
max
(chloroform) compared to that in acetone. This negative solva-
tochromism indicates that in HQ1, the ground state is more
charge separated than the excited state because of electron transfer
from H to Q; this is expected to allow rapid Q/H exchange in
HQ1. Similar negative solvatochromism is observed in MQ1,
however the shift in -max was smaller (19 nm whereas 25 nm in
HQ1) because of the weak electron-donating behavior of the
2,5-dimethoxyphenyl moiety.
The Q/H exchange was also observed in the mixed solution
of QQ2 and HH2 and yielded 2-(2,5-dihydroxyphenyl)-p-
quinone (HQ2). Compared to K1, the values of K2 were low
being about 0.36-3.1 (benzene, 0.36; chloroform, 0.75, dichloro-
methane, 0.85; acetone, 3.1; acetonitrile, 3.1). The positions of the
tert-butyl groups affect the equilibrium constants, and as steric
repulsions between the tert-butyl groups are negligible in both
intra- and intermolecular processes, it is inferred that solvent
interactions play a critical role. It has been reported that in the
intramolecular cyclization of QQ1 and QQ2,8 the substitution of
bulky tert-butyl groups affects the solvation of the adjacent
substituents.
In conclusion, the Q/H exchange was observed in both the
mixtures of QQ1 and HH1 and QQ2 and HH2 to yield HQ1 and
HQ2, respectively, and HQ1 was generated more favorably than
HQ2 in all solvents. The Q/H exchange was slow between QQ1
and HH1 and considerably fast in the hydrogen-bonded dimer
(HQ1)2 presumably due to the intramolecular electron transfer.
Our future research will focus on the substituent effect on the ratio
[QH]2:[QQ][HH] as well as the exchange rate of intra- and
intermolecular processes.
In all solvents used, HQ1 is present as a major component
and K1 (eq 1, n = 1) is about 15-37 (benzene, 23; chloroform,
21, dichloromethane, 15; acetone, 37; acetonitrile, 31). This is
because in nonpolar solvents with small dielectric constants (¾),
there are strong hydrogen bonds between virtually all the
molecules.12 The formation of (HQ1)2 appears to be the most
favorable, because all hydroxy groups form hydrogen bonds
(Figure 3). In contrast, in the hydrogen-bonded complex
(QQ1)(HH1), just two hydroxy groups of HH1 form hydrogen
bonds with the carbonyl groups in QQ1 and the other two
participate in OH/³ interactions as found in 2,2¤-biphenyldiol,13
thus the sum of the interaction energies is less than that of (HQ1)2.
The formation of (HQ1)2 is also advantageous in nonpolar
solvents because the molecules are aligned head-to-tail for the
molecular dipole moments to cancel out. In contrast, (QQ1)(HH1)
is polar. On the other hand, in polar solvents, the molecules are
monomeric because of the small hydrogen-bonding energies,12
therefore in the polar environment, HQ1 should be favorable
compared to QQ1 and HH1 due to its polar character (Figure 3).
Existence of (HQ1)2 is confirmed by phase-sensitive
2D NOESY NMR spectra (303 K, mixing time = 0.5 s). As shown
in Figure 4a, the cross peaks arising from the chemical exchange
of the two tert-butyl groups of HQ1 (Figure 4c) were observed
in CDCl3. In contrast, no cross peaks were observed in the
(CD3)2CO solution, as shown in Figure 4b. This result agrees
with the hypothesis that HQ1 exists as a monomer.
The Q/H exchange between HQ1s is so rapid as to be
observable in NMR measurement, which is in marked contrast to
the slow Q/H exchange between QQ1 and HH1. The slow
exchange in the mixed solution of QQ1 and HH1 is sterically
hindered by the tert-butyl groups and the highly twisted ³-planes.
These prohibit the electron transfer in the initial step of the Q/H
exchange. In fact, we previously reported that the torsional angle
of two quinone planes was 38° in the X-ray structure of QQ1;6
We gratefully acknowledge the financial support provided
by the Research Foundation for Materials Science, Saneyoshi
Scholarship Foundation, and the Fund for Future Technology,
University of Toyama.
This paper is in celebration of the 2010 Nobel Prize awarded
to Professors Richard F. Heck, Akira Suzuki, and Ei-ichi Negishi.
Chem. Lett. 2011, 40, 947-949
© 2011 The Chemical Society of Japan
Hayashi, Naoto
