Photoreduction of o-benzoquinones in the presence of p-bromo-N,N- dimethylaniline

Article abstract of DOI:10.1023/A:1023931428206

Photoreduction of o-benzoquinones in the presence of p-bromo-N,N- dimethylaniline under irradiation (λ, > 500 nm) affords the corresponding pyrocatechols and hydroxyphenyl ethers. The latter are unstable and, in turn, decompose in the dark reaction to pyrocatechols. The ratio between pyrocatechol and hydroxyphenyl ether formed upon the photoreaction is determined by the structure of o-quinone, namely, the presence and bulk of substituents in positions 3 and 6 of the ring. The yield of pyrocatechol is maximal (60-65%) if the substituents are the same (H and H, Bu^t and Bu^t) or insignificantly differ (Prⁱ and Bu^t), regardless of its bulk.

Full text of DOI:10.1023/A:1023931428206

718
Russian Chemical Bulletin, International Edition, Vol. 52, No. 3, pp. 718—724, March, 2003
Photoreduction of oꢀbenzoquinones in the presence
of pꢀbromoꢀN,Nꢀdimethylaniline*
S. A. Chesnokov,ꢀ V. K. Cherkasov, G. A. Abakumov, Yu. A. Kurskii,
M. P. Shurygina, O. N. Mamysheva, and A. S. Shavyrin
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831 2) 66 1497. Eꢀmail: cherkasov@imoc.sinn.ru
Photoreduction of oꢀbenzoquinones in the presence of pꢀbromoꢀN,Nꢀdimethylaniline unꢀ
der irradiation (λ > 500 nm) affords the corresponding pyrocatechols and hydroxyphenyl
ethers. The latter are unstable and, in turn, decompose in the dark reaction to pyrocatechols.
The ratio between pyrocatechol and hydroxyphenyl ether formed upon the photoreaction is
determined by the structure of oꢀquinone, namely, the presence and bulk of substituents in
positions 3 and 6 of the ring. The yield of pyrocatechol is maximal (60—65%) if the substituents
are the same (H and H, But and But) or insignificantly differ (Pri and But), regardless of
its bulk.
Key words: oꢀbenzoquinone, symmetry, pꢀbromoꢀN,Nꢀdimethylaniline, photoreduction,
radical pair.
оꢀQuinones (A) efficiently enter into the photoreducꢀ
affords ketols I,7 and the corresponding pyrocatechol8 III
tion reaction in the presence of appropriate Hꢀdonating
compounds (DH).1,2 The process includes the formation
is the main product in the presence of tertiary amines.
The photoreduction of оꢀbenzoquinones (Q) is studꢀ
ied to a lesser extent. It is known that these compounds
are reduced under irradiation with visible and UV light in
the presence of such Hꢀdonors as tertiary amines (AmH2)
to form intermediate radical products.9 It is established
that, unlike the quinones listed above, the direction of
photoreduction of Q is determined by the structure of
quinone itself.10 As shown for two оꢀbenzoquinones,
hydroxyphenyl ether II or pyrocatechol III are the final
reaction products, depending on the molecular strucꢀ
ture of quinone. Perhaps, for different quinones, the
3[QH•, AmH•] radical pairs either decompose with exit
of free radicals from the solvent cage (disproportionation
of the QH• semiquinone radicals should form pyrocatꢀ
echol), or undergo recombination to form hydroxyphenyl
ethers II.
3
of triplet radical pairs (RP) [АH•, D•], whose transforꢀ
mation affords addition products at one (I) or both carboꢀ
nyl groups (II) or pyrocatechol (III).3,4
The study of photoreduction of 9,10ꢀphenanthreneꢀ
quinone (PQ)3,5 and 1,2ꢀnaphthoquinone4 shows that the
predominant formation of one or another product is deꢀ
termined by the nature of the donor DH. For example, in
the presence of alkenes1 or alkylarenes,6 PQ is reduced
under irradiation to the corresponding ketols I. Photoreꢀ
duction in the presence of ethers (dialkyl4 or alkyl aryl3)
and aldehydes2 proceeds via the mechanism of αꢀhydrogen
elimination to give hydroxyphenyl ether II. Pyrocatechol,
which was isolated as quinhydrone,2,4 was also identified
among the reaction products. The photoreduction of
camphorquinone (1,7,7ꢀtrimethylbicyclo[2.2.1]heptaneꢀ
2,3ꢀdione) in the presence of xylenes and lower alcohols
The progress in the synthesis of new оꢀbenzoquinones11
made it possible to perform more detailed studies of the
influence of the molecular geometry of quinones on the
direction of photoreduction. The present work is devoted
to the solution of this problem.
Experimental
NMR spectra were recorded on a Bruker DPXꢀ200 specꢀ
trometer. IR spectra were obtained on a Specord M 80 spectroꢀ
photometer.
* Dedicated to Academician I. P. Beletskaya on the occasion of
her anniversary.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 688—693, March, 2003.
1066ꢀ5285/03/5203ꢀ718 $25.00 © 2003 Plenum Publishing Corporation

Photoreduction of oꢀbenzoquinones
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 3, March, 2003
719
A KGMꢀ24ꢀ150 lamp with a focusing device was used as the
light source. Radiation with λ ≥ 500 nm was separated from the
luminous flux of the lamp using the light filter. Solutions of
оꢀquinone and pꢀbromoꢀN,Nꢀdimethylaniline in a ratio of
1 : 5 in C6D6 were deaerated, saturated with Ar, placed in
an NMR tube, and exposed at a distance of 15 cm from
the focusing device for 3—5 min, selecting irradiation conꢀ
ditions in such a way that the conversion of quinone would
be ∼50%.
2ꢀ(NꢀpꢀBromophenylꢀNꢀmethylamino)methoxyꢀ6ꢀtertꢀbutylꢀ
3ꢀisopropylphenol (3b). 1H NMR (C6D6), δ: 1.34 (d, 6 H, Pri,
J = 6.8 Hz); 1.42 (s, 9 H, But); 4.57 (s, 2 H, OCH2N).
2ꢀ(NꢀpꢀBromophenylꢀNꢀmethylamino)methoxyꢀ6ꢀtertꢀbutylꢀ
3ꢀmethylphenol (3c). 1H NMR (C6D6), δ: 1.47 (s, 9 H, But);
2.07 (s, 3 H, Me); 2.33 (s, 3 H, MeN); 4.57 (s, 2 H, OCH2N).
2ꢀ(NꢀpꢀBromophenylꢀNꢀmethylamino)methoxyꢀ4,5ꢀdiꢀ
methoxyphenol (3e). 1H NMR (C6D6), δ: 2.38 (s, 3 H, MeN);
3.30 (s, 3 H, MeO); 3.47 (s, 3 H, MeO); 4.64 (s, 2 H, OCH2N).
Solvents (benzene, hexane) were purified using standard proꢀ
cedures.12 оꢀBenzoquinones 1a—e and corresponding pyrocꢀ
atechols were synthesized using previously described proceꢀ
dures.11 pꢀBromoꢀN,Nꢀdimethylaniline (2) (Aldrich) was reꢀ
crystallized from aqueous MeOH.
Results and Discussion
Colored organic solutions of mixtures of оꢀbenzoꢀ
quinones 1a—e with pꢀbromoꢀN,Nꢀdimethylaniline (2)
become colorless upon irradiation with visible light. A
change in the spectral characteristics of the solutions is
related to the transformation of оꢀbenzoquinone into pyꢀ
rocatechol and its derivatives, which is confirmed by the
1
3,6ꢀDiꢀtertꢀbutylpyrocatechol. H NMR (C6D6), δ: 1.36 (s,
18 H, But); 4.78 (s, 2 H, OH); 6.82 (s, 2 H, C(4)H, C(5)H).
6ꢀtertꢀButylꢀ3ꢀisopropylpyrocatechol. 1H NMR (C6D6),
δ: 1.04 (d, 6 H, Me2CH, J = 6.8 Hz); 1.49 (s, 9 H, But); 2.51
(sept, 1 H, CHMe2, J = 6.8 Hz); 3.90 and 5.48 (both s,
1 H each, OH); 6.64 and 6.78 (both d, 1 H each, C(4)H, C(6)H,
J = 8.3 Hz).
1
NMR data. For example, the H NMR spectrum of the
6ꢀtertꢀButylꢀ3ꢀmethylpyrocatechol. 1H NMR (C6D6), δ: 1.50
(s, 9 H, But); 1.61 (s, 3 H, Me); 2.10 (s, 2 H, OH).
initial solution of 1d and 2 (1 : 5) in C6D6 contains highꢀ
field signals. They are assigned to protons of the methyl
groups in compound 2 (δ = 2.31) and nonequivalent
tertꢀbutyl groups in compound 1d (δ = 1.13 and 0.74).
The irradiation (λ ≥ 500 nm) of the solution results in a
decrease in the intensities of signals for the protons of the
tertꢀbutyl groups in 1d and the appearance of new signals
attributed to protons of the tertꢀbutyl groups in 3,5ꢀdiꢀ
tertꢀbutylpyrocatechol (δ = 1.29 and 1.55) and protons of
two tertꢀbutyl groups, methyl group, and methylene group
of hydroxyphenyl ether 2ꢀ(NꢀpꢀbromophenylꢀNꢀmethylꢀ
amino)methoxyꢀ4,6ꢀdiꢀtertꢀbutylphenol (3d) (δ = 1.32,
1.57, 2.38, and 4.68, respectively) (Fig. 1). Compound 3d
3,5ꢀDiꢀtertꢀbutylpyrocatechol. 1H NMR (C6D6), δ: 1.27 and
1.55 (both s, 9 H each, But); 3.61 and 5.42 (both s, 1 H each,
OH); 6.23 (d, 1 H, C(6)H, J = 2.3 Hz); 7.07 (d, 1 H, C(4)H,
J = 2.3 Hz).
4,5ꢀDimethoxypyrocatechol. 1H NMR (C6D6), δ: 3.39 (s,
6 H, MeO); 4.57 (s, 2 H, OH).
2ꢀ(NꢀpꢀBromophenylꢀNꢀmethylamino)methoxyꢀ4,6ꢀdiꢀtertꢀ
butylphenol (3d). 3,5ꢀDiꢀtertꢀbutylꢀ1,2ꢀbenzoquinone (1d)
(0.44 g) and pꢀbromoꢀN,Nꢀdimethylaniline (2) (2 g) were disꢀ
solved in benzene (20 mL). A glass cylindrical tube 30 mm in
diameter filled with the solution was irradiated using the ZhSꢀ17
light filter for 3 h at a distance of 18 cm from the focusing
device, and the solvent was removed in vacuo. The residue was
washed with hot hexane, and compound 3d was isolated from
the hexane solution as white crystals with m.p. 122—124 °C.
Found (%): C, 62.8; H, 7.1; Br, 19.1. C22H30NO2Br. Calcuꢀ
lated (%): C, 63,41; H, 7.31; Br, 19.00. IR (KBr), ν/cm–1: 810
(H(3), H(5)); 855, 875 (H(2´), H(3´), H(5´), H(6´)); 1300
(CОaryl); 3490 (ОН). 1H NMR (C6D6), δ: 1.32 (s, 9 H, But);
1.57 (s, 9 H, But); 2.37 (s, 3 H, MeN); 4.67 (s, 2 H, OCH2N);
5.85 (s, 1 H, OH); 6.27—6.35 (m, 2 H, C(2´)H, C(6´)H); 6.69
(d, 1 H, C(6)H, J = 2.1 Hz); 7.15—7.26 (m, 3 H, C(3´)H,
C(5´)H, C(4)H)).
The products of photoreduction of 3,6ꢀdiꢀtertꢀbutylꢀ (1a),
3ꢀtertꢀbutylꢀ6ꢀisopropylꢀ (1b), 3ꢀtertꢀbutylꢀ6ꢀmethylꢀ (1c),
and 4,5ꢀdimethoxybenzoquinoneꢀ1,2 (1e) in the presence of
pꢀbromoꢀN,Nꢀdimethylaniline (2) were qualitatively and quanꢀ
titatively analyzed and the reaction course was monitored by the
NMR method.
We failed to isolate compounds 3a—c,e because of their
instability. The NMR data for these compounds in the reaction
mixture are presented below.*
2ꢀ(NꢀpꢀBromophenylꢀNꢀmethylamino)methoxyꢀ3,6ꢀdiꢀtertꢀ
butylphenol (3a). 1H NMR (C6D6), δ: 1.39 (s, 9 H, But); 1.53 (s,
9 H, But); 2.33 (s, 3 H, MeN); 4.47 (s, 2 H, OCH2N).
Compound
1
R
a
But
H
b
Pri
H
c
Me
H
Me
H
d
H
e
H
OMe
H
MeO
MeO
H
R1
R2
R3
R4
R5
R6
But
H
3
But
H
Pri
H
But
H
But
* Signals for aromatic protons are not presented because they
overlap with signals of the starting compounds.
H
But
H
But
H
But

720
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 3, March, 2003
Chesnokov et al.
X1
X2
a
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
δ
X1
X2
Z2
Z1
b
Y2
Y1
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
δ
X1
X2
Z2
Y2
Z1
c
Y1
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
δ
Fig. 1. NMR spectra of a solution of 3,5ꢀdiꢀtertꢀbutylꢀ1,2ꢀbenzoquinone (1d) and pꢀbromoꢀN,Nꢀdimethylaniline (2) (1 : 5) in C6D6
before irradiation (a), immediately after irradiation (b), and 60 min after irradiation (c). The signals for protons of the tertꢀbutyl
groups of quinone (X1 and X2), pyrocatechol (Y1 and Y2), and hydroxyphenyl ether (Z1 and Z2) are denoted.
is unstable and decomposes in air to pyrocatechol (the
product formed from the nitrogenꢀcontaining moiety of
3d was not identified). In a solution, the rate of transforꢀ
mation of 3d into pyrocatechol is much higher. The ratio
of content of ether 3d and pyrocatechol in the reaction
mixture immediately after irradiation and after 60ꢀmin
storage in the dark changes from 3.5 : 1 to 1 : 1.
pounds of this type. Therefore, an attempt to synthesize
hydroxyphenyl ethers 3a—c and 3e in preparative amounts
was unsuccessful, and their formation was detected only
in situ by the NMR method. The low stability of hydroxyꢀ
phenyl ethers explains the fact that no formation of comꢀ
pound 3a has been found previously10 when studying the
products of photoreduction of quinone 1a.
The results of irradiation of solutions of other оꢀbenzoꢀ
quinones in the presence of 2 are analogous: the NMR
spectra exhibit a superposition of signals of the correꢀ
sponding pyrocatechols and hydroxyphenyl ethers. Then
the intensity of signals for protons of ethers decreases,
and that for protons of pyrocatechols increases. The kiꢀ
netics of disappearance of hydroxyphenyl ethers suggests
that the rate of transformation of 3d is by approximately
an order of magnitude lower than that of other comꢀ
The transformation products of aniline 2 were studied
in its reaction with 1а. The reaction was carried out at the
1 : 1 ratio of the reactants in benzene in an argon atmoꢀ
sphere under irradiation with λ > 500 nm. The solution
was irradiated until discoloration, after which benzene
was removed, and the residue was recrystallized from hexꢀ
ane, dissolved in CDCl3, and analyzed by NMR. The 1Н,
13C, and DEPT NMR spectra showed that the mixture
contained compounds bearing four different methyl groups

Photoreduction of oꢀbenzoquinones
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 3, March, 2003
721
(δH = 2.81, 2.85, 3.01, and 3.04; δC = 30.7, 36.4, 38.6,
and 39.0), three methylene groups (δH = 4.71, 4.78, and
4.95; δC = 57.6, 70.1, and 81.0), and aromatic fragments
(δH = 6.4, 6.7, and 7.3; δC in regions of 110, 115, 131.5,
and 148, groups of four signals each). The presence
of crossꢀpeaks of δH signals 2.85/4.71, 3.01/4.78, and
3.04/4.95 in the COSY spectra implies that the correꢀ
sponding pairs of methyl and methylene groups exist in
the same molecule. All groups listed above have crossꢀ
peaks with protons in the aromatic region. Based on the
chemical shifts, integral intensities, and correlation exꢀ
periments, one can suppose that a mixture of four prodꢀ
ucts is formed, of which three products contain the
—(Me)NCH2— fragment and one product contains the
—NH—Me fragment. The chemical shift of the methylꢀ
ene groups indicates that they are situated near an elecꢀ
tron acceptor (for example, nitrogen or oxygen
atoms). Presumably, these are the following comꢀ
pounds: pꢀBrC6H4NHMe, whose formation is deduced
from the presence of the methyl group (δH = 2.81,
δC = 30.1) bound to none of the methylene groups;
pꢀBrC6H4N(Me)CH2N(Me)(pꢀBrC6H4), whose formaꢀ
tion is evidenced by two methyl groups (δH = 2.85,
δC = 36.5) per methylene group (δH = 4.71, δC = 70.1)
and signals for the aromatic carbon atoms δC 110.0 (CBr),
115.2 (CH), 131.8 (CH), and 147.9 (CN), their chemical
shifts being in good accordance with those expected for
such a structure. Similarly, based on the chemical shifts
the second molecule is reduced due to hydrogen
elimination from AmH•. The formation of pyroꢀ
catechol from оꢀbenzoquinones is described by the
Scheme 1.
Analysis of the NMR spectra of the photoreduction
products of quinones 1a—e shows that the ratios of
hydroxyphenyl ethers to pyrocatechols are determined by
the nature of оꢀbenzoquinone. Below we present the yields
(P) of pyrocatechols upon the photoreduction of quinoꢀ
nes 1a—e after 3ꢀmin irradiation, which corresponds to
∼50% conversion.
Starting оꢀbenzoquinone
P (%)*
1a 1b 1c 1d 1e
60 65 25 20 60
* P is the yield of the corresponding pyrocatechol.
A comparison of the yields of pyrocatechols upon phoꢀ
toreduction of quinones 1a and 1d shows that the Р value
changes threefold, although the only difference between
these quinones is the position of one But group in the
quinonoid ring. The yield of pyrocatechol upon the phoꢀ
toreduction of quinone 1с almost coincides with that for
quinone 1d. The main photoreduction products of quinoꢀ
nes 1b and 1c, as well as of 1a, are the corresponding
pyrocatechols.
As in the case of other carbonylꢀcontaining comꢀ
pounds,13 the process of photoreduction of оꢀbenzoꢀ
quinones is the interaction of the oꢀbenzoquinone molꢀ
ecule excited to the lowest nπ*ꢀtriplet state with the amine
molecule9 to form the triplet exciplex ("collision comꢀ
plex" 14) 3(Q*, AmH2), which is transformed into the tripꢀ
let radical pair 3(QH•, AmH•) after the transfer of hydroꢀ
gen atom. It is known that the main products of benzopheꢀ
none photoreduction are the corresponding pinacones,13
which are the recombination products of ketyl radicals
formed, in turn, upon the decomposition of the triplet
RP. At the same time, PQ photoreduction in the presence
of arylꢀcontaining hydrogen donors mainly affords the
products of geminal recombination of the radicals, viz.,
ketols or hydroxyphenyl ethers.3,15 This indicates that, in
the case of PQ, the triplet RP is transformed into the
singlet RP with a high probability followed by radical
recombination. oꢀBenzoquinones studied in this work afꢀ
ford products of both disproportionation of two semiꢀ
quinone radicals (pyrocatechol) and recombination of the
QH• and AmH• radicals (hydroxyphenyl ether), and the
ratio between the products is determined by the quinone
structure. We believe that the triplet RP formed upon
the photoreduction of оꢀbenzoquinones: both decomꢀ
poses to radicals with formation of pyrocatechol and
amine dehydrogenation products and transforms into
the singlet RP followed by radical recombination to
hydroxyphenyl ether. The predominant direction of tripꢀ
let RP transformation is determined by the quinone
structure.
1
of the methylene groups of other products in the Н and
13С NMR spectra, the formation of compounds containꢀ
ing the ArN(Me)CH2Br fragments and amine oxidation
products including the ArN(Me)CH2O fragments can be
assumed.
A half of quinone 1a is photoreduced when 1a reacts
with 2 in a molar ratio of 2 : 1. This means that pyrocatꢀ
echol (as well as hydroxyphenyl ether) is formed in the
reaction of quinone and amine in a ratio of 1 : 1. In other
words, in the formation of pyrocatechol one amine molꢀ
ecule formally donates two hydrogen atoms for the reducꢀ
tion of one molecule of quinone 1a. Benzophenone
and fluorenone are similarly photoreduced in the presꢀ
ence of amines.13 These processes are described by the
scheme in which the first molecule of a carbonylꢀconꢀ
taining compound is reduced owing to the dehydrogenaꢀ
tion of the amine molecule to the AmH• radical, and
Scheme 1
Q + AmH2
Q + AmH•
2 QH•
QH• + AmH•
QH• + Am
Q + QH2
___________________________________
Q + AmH2 QH2 + Am

722
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 3, March, 2003
Chesnokov et al.
To explain the observed effects, let us consider changes
distances between the radical centers. Triplet RPs of the
first type (RP1) (with a higher <r> value) decompose to
component radicals, one of the transformation products
being pyrocatechol. Triplet RPs of the second type (RP2)
(with lower <r>) undergo the T→S transition followed by
the rapid recombination of the radicals with the singlet
RP to form hydroxyphenyl ether (Scheme 2). According
to the assumed mechanism, the quantitative ratio beꢀ
tween both types of an RP is determined by the symmetry
of the initial оꢀbenzoquinone. The lower the difference in
bulks of the substituents in positions 3 and 6 the greater
the fraction of the RPs of the first type and, correspondꢀ
ingly, the higher the content of pyrocatechol in the reacꢀ
tion products.
that can occur in the (QH•, AmH•) radical pair when
varying substituents in the QH• radicals. It follows from
the equality of the P values for pyrocatechols obtained
upon reduction of quinones 1a and 1e that shielding of
the radical center in QH• does not determine the direcꢀ
tion of transformation of the triplet RP. It is known that
hydrogen is transferred to the quinone molecule and the
RP forms when quinones 1а and 1d are photoreduced in
the presence of hydrogen donors (sterically hindered
phenols,16 pyrocatechol 1а,17 and diphenylamine18) in
glassy solutions. The RP structure and the distance beꢀ
tween the radical centers are also determined by the naꢀ
ture of оꢀbenzoquinone. For example, irradiation of 1a in
the presence of 2,4,6ꢀtriꢀtertꢀbutylphenol16 and diphenyꢀ
lamine18 results in the formation of the RP with the disꢀ
tance between unpaired electrons <r> ∼ 6.3 Å. When 1d is
irradiated in the presence of diphenylamine,18 two types
of an RP are formed: with <r> ∼5.3 and 6.3 Å; those with
the lower <r> value are mainly formed. Although the RP
states in a solution and solid matrix are not identical in
the general case, it can be assumed that the photoreducꢀ
tion of 1a—e in the presence of 2 in a solution also affords
two types of a triplet RP with different geometries and
3
The influence of the symmetry of the оꢀbenzoquinone
molecule on the RP geometry can be formulated as folꢀ
lows. In the semiquinone radical formed upon the transꢀ
fer of a hydrogen atom, the H atom migrates from one
oxygen atom to another with a frequency of ∼109 s–1 19
,
i.e., the semiquinone molecule exists in two isomeric forms
(Scheme 3).
For symmetrical semiquinones (derivatives of 1a
and 1e), these isomers are degenerate in energy, and the
probability that the H atom is situated near each O atom
Scheme 2

Photoreduction of oꢀbenzoquinones
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 3, March, 2003
723
Scheme 3
radicals mainly recombine in the T→S transitions of
the RPs.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos. 01ꢀ03ꢀ33040
and 00ꢀ15ꢀ97336ꢀl). Spectral studies were performed at
the Analytical Center of the Institute of Organometallic
Chemistry of the Russian Academy of Sciences and were
financially supported by the Russian Foundation for Baꢀ
sic Research (Project No. 00ꢀ03ꢀ42004).
is the same. Therefore, the AmH• radical located near
such a radical in the RP has not any predominant orientaꢀ
tion with respect to one of the carbonyl groups of the
semiquinone molecule. This should increase the distance
between the radical centers in the RP, weaken the interꢀ
action between the radicals, and increase the probability
of RP decomposition. In nonsymmetrical semiquinones,
the degeneration in energy of the isomers is eliminated.
One of the isomers is more stable than another, and the
probabilities that the hydrogen atom is situated near one
or another oxygen atom are different.20 Therefore, the
AmH• radical in the RP will take a certain orientation
with respect to the semiquinone radical QH•. The apꢀ
proach of the radical centers in the RP increases the probꢀ
ability of radical recombination for transitions from the
triplet to singlet state of the RP. The higher reactivity of
semiquinones of nonsymmetrical quinones is confirmed
by the fact that the rate constant for disproportionation of
semiquinone of nonsymmetrical quinone 1d is threeꢀ
fold higher than the similar constant for symmetrical
quinone 1а.21
Thus, оꢀbenzoquinones are photoreduced in the presꢀ
ence of pꢀbromoꢀN,Nꢀdimethylaniline to the correspondꢀ
ing pyrocatechols and hydroxyphenyl ethers. The quantiꢀ
tative ratio between them is determined by the difference
in bulks of the substituents in positions 3 and 6 of the
quinonoid ring: the lower the ratio the higher the content
of pyrocatechol. According to the proposed mechanism
of the reaction, the phototransfer of a hydrogen atom
results in the formation of triplet RPs, which can transꢀ
form by two variants: decomposition to radicals (pyrocatꢀ
echol is the final product of оꢀquinone photoreduction)
or recombination of radicals after the tripletꢀsinglet tranꢀ
sitions of the RP to form hydroxyphenyl ether. The quanꢀ
titative ratio between these directions of the reaction is
determined, most likely, by the difference in energies of
two isomeric forms of the semiquinone radical. This is
manifested as a difference in bulks of the substituents in
positions 3 and 6 of the ring. If, regardless of the bulk, the
substituents are the same or the difference between them
is small (for example, tertꢀbutyl and isopropyl groups),
then the RP mainly decomposes to radicals composing
this RP. If the difference in volumes of the substituents is
great (for example, the tertꢀbutyl and methyl groups
or the tertꢀbutyl group and hydrogen atom), then the
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Received June 4, 2002;
in revised form October 8, 2002