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of PQ and DUR in C6D6 results only in the appearance in the
1H NMR spectrum of signals attributed to phenolether (1d).
Irradiation of the reaction mixture leads to the appearance
and accumulation of signals attributed to ketol (2f). This, the
phototransformation of phenolether 1d to ketol occurs faster
than 1c. So, after total decolouration of reaction mixtures
PQeHMB and PQeDUR, the ratios 1c:2e and 1d:2f become
1:1 and 1:3, respectively, and does not change further. Upon
further diminution of the electron-donor properties of polyme-
thylbenzene from E1/2¼1.46 V (HMB) and E1/2¼1.59 V
(DUR) to E1/2¼1.85 V (mesitylene) the formation of corre-
sponding phenolether was not observed. The 1H NMR spectrum
shows signals of the single product of the reaction - ketol (2g).
Ketol 2g was isolated and described.12 The difference in stability
of phenolethers can’t be explained in the context of the earlier
proposed mechanism for the conversion of phenolethers formed
from N,N-dimethylanilines. The diminishing of electron-donor
ability of polymethylbenzenes should cause increasing of stabil-
ity of phenolether but not vice versa.
O
O
OH
O
D
OH
hν
DH
+
O D
Scheme 6.
In the first stage a photoexcited molecule of PQ interacts
with the H-donor resulting in the formation of a phenolether.
In the second stage, under light and in the presence of the sec-
ond molecule 9,10-phenanthrenequinone formed phenolether
transforms into ketol. The weaker the electron-donor properties
of the polymethylbenzene, the greater the probability of photo-
transformation of the phenolether into ketol. Apparently, mesi-
tylene and m-xylene during initial photoreaction with PQ also
form corresponding phenolethers. But due to the fast second
stage, the formed phenolethers are rapidly converted into ketols.
Based on the aforementioned data, the following scheme of
transformation of phenolether where PQ is ‘phototransfer’ of
hydrogen can be suggested.
Photoexcited PQ molecule abstracts a proton from the hy-
droxyl or methylene group of the phenolether forming a radical
pair consisting of protonated semiquinone radical PQꢂH and
radical A (Scheme 7). Radical A then transforms into radical
B (Scheme 7). Interaction of B with radical PQꢂH gives ketol
and a molecule of initial PQ. The inner contradiction of sug-
gested mechanism is the fact that relatively stable radical A
spontaneously transforms into the certainly less stable radical
B. A possible explanation is that transformation of the phenol-
ether proceeds simultaneously with energy-favourable reac-
tion of dehydrogenation of PQꢂH with the formation of
ketol; the whole process occurs in the cell of solvent. Conse-
quently, the rate of reaction is determined by the probability of
conversion of radical A and stability of radical B, respectively.
Decreasing of the number of methyl groups in the benzyl frag-
ment (in A and B) increases its acceptor property. The increas-
ing of the electron-acceptor properties of the benzyl fragment,
in turn enhances the stability of radical B and accelerates the
photoreaction of phenolether with PQ. Analyzing the differ-
ence in behaviour of m- and p-xylenes, it is necessary to men-
tion the following: according to values of Taft’s constants,26
fragment CH2C6H4CH3-meta appears to be a stronger acceptor
than fragment CH2C6H4CH3-para (s*¼0.20 and 0.17, respec-
tively). Apparently, it is because of the less probability of a
secondary reaction of phenolether which was formed from
p-XYL, in comparison with phenolether formed from m-xylene,
and, consequently, the possibility to observe phenolether
which was formed upon photoreduction of PQ with p-XYL
in reaction mixture.
It should be mentioned that not only do the electron-donor
properties of polymethylbenzenes in examined row (HMB,
DUR, MES) decrease, but also the steric protection of ether
bond of phenolether decreases. It is possible that the absence
of a substituent in ortho-position of the methylene group of
the phenolether somehow facilitates its rearrangement to ketol.
Then, steric factors will be determinative in the photoreduc-
tion of PQ in the presence of xylenes with close values of
E
1
1/2: in the case of o-xylene the H NMR spectrum should
show phenolether, while in the cases of m- and p-xylenes
only the signals of corresponding ketols should be observed.
In reality, the irradiation of PQ during 1 min in the presence
of o-xylene leads to the appearance of superposition signals
1
in H NMR spectrum attributed to phenolether (1e) and ketol
(2h) in the ratio 1:2. But irradiation of PQ and m-xylene (dur-
ing 1 min) results only in ketol (2i). However, the presence of
p-xylene (the irradiation during 1 min) produces not only ketol
(2j) but also phenolether (1f) (ratio of 2j:1f is 2:1). The
obtained results are summarized in Scheme 5.
It was established here that the photoreduction of 9,10-phenan-
threnequinone by HMB, DUR, o- and p-XYL affords the corre-
sponding phenolethers as initial products. Unfortunately, the
attempts toisolatetheformedphenolethers werefailed. Theresult-
ing phenolethers are involved in a secondary photoreaction with
PQ togivethecorrespondingketols. Inthecourseofphotoreaction
of PQ with MES and m-XYL the existence of phenolethers is not
registered. The 1H NMR spectra contain only signals of the corre-
sponding ketols. We suggest that photoreduction of 9,10-phenan-
threnequinone in the presence of both polymethylbenzenes and
tertiary amines occurs as a two-stage reaction (Scheme 6).
Thus, on the basis of experimental and literature data it can
be concluded that the mechanism of formation of products of
photoreduction of 9,10-phenanthrenequinone and o-benzequi-
none is the same. The first stage of photoreaction results in the
formation of a 1,4-addition productdphenolether. And only in
a secondary reaction (photo- or dark reaction) is phenolether
converted into the 1,2- addition product (ketol in the case of
PQ). The range of products of photoreduction of o-quinones
PQ
+
HMB, DUR, o-XYL, p-XYL, MEZ, m-XYL
hν
hν
hν, PQ
Phenolether
Ketol
Scheme 5.