o-benzoquinone photoreduction products in the presence of N,N-dimethylanilines

Article abstract of DOI:10.1007/s11172-006-0458-x

Photoreduction of o-benzoquinones in the presence of para-substituted N,N-dimethyl-anilines under irradiation at λ ≥ 500 nm affords pyrocatechol monoethers of the 2-(amino-methoxy)phenol type. In the subsequent dark reaction, these monoethers undergo quantitative decomposition by a heterolytic mechanism to give the corresponding pyrocatechols and nitrogen-containing compounds. The rate of this decomposition decreases with decreasing size of the substituent at the position adjacent to the ether bond that is formed upon photoreduction. The redox characteristics of such pyrocatechol monoethers can serve as the criterion of their stability. A weakening of the electron-withdrawing properties of quinones and the electron-donating properties of amines leads to an increase in stability of their reaction products.

Full text of DOI:10.1007/s11172-006-0458-x

Russian Chemical Bulletin, International Edition, Vol. 55, No. 9, pp. 1585—1592, September, 2006
1585
oꢀBenzoquinone photoreduction products
in the presence of N,Nꢀdimethylanilines
M. P. Shurygina,ꢀ Yu. A. Kurskii, S. A. Chesnokov, N. O. Druzhkov,
G. K. Fukin, G. A. Abakumov, and V. K. Cherkasov
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831 2) 62 7497. Eꢀmail: sch@iomc.ras.ru
Photoreduction of oꢀbenzoquinones in the presence of paraꢀsubstituted N,Nꢀdimethylꢀ
anilines under irradiation at λ ≥ 500 nm affords pyrocatechol monoethers of the 2ꢀ(aminoꢀ
methoxy)phenol type. In the subsequent dark reaction, these monoethers undergo quantitative
decomposition by a heterolytic mechanism to give the corresponding pyrocatechols and nitroꢀ
genꢀcontaining compounds. The rate of this decomposition decreases with decreasing size of
the substituent at the position adjacent to the ether bond that is formed upon photoreduction.
The redox characteristics of such pyrocatechol monoethers can serve as the criterion of their
stability. A weakening of the electronꢀwithdrawing properties of quinones and the electronꢀ
donating properties of amines leads to an increase in stability of their reaction products.
Key words: photoreduction, oꢀbenzoquinones, N,Nꢀdimethylanilines, pyrocatechol
monoethers.
Scheme 1
It is known that oꢀquinones are efficiently subjected to
photoreduction in the presence of various hydrogen doꢀ
nors (DH2).1,2 Studies of photoreduction of 9,10ꢀphenꢀ
anthrenequinone3,4 and 1,2ꢀnaphthoquinone5 demonꢀ
strated that the reaction pathway is largely determined by
the nature of the hydrogen donor. For example, photoreꢀ
duction of 9,10ꢀphenanthrenequinone in the presence of
alcohols,6,7 ethers (dialkyl5 or alkylaryl3), or aldehydes1
gives rise to compounds containing the 2ꢀalkoxyphenol
fragment (Scheme 1, product I). Photoreactions in the
presence of hydrocarbons (alkanes,8,9 alkenes,2,10 alkylꢀ
arenes,8 or alkynes2) produce the corresponding ketols
(products II). Prolonged sunlight irradiation of 9,10ꢀphenꢀ
anthrenequinone in a mixture with oꢀ and pꢀxylenes and
1,2,4ꢀtrimethylbenzene affords 2ꢀalkoxyphenols.1 These
reactions produce also pyrocatechol derivatives III idenꢀ
tified as quinhydrones.1,5 Photoreduction of camphoroꢀ
quinone (1,7,7ꢀtrimethylbicyclo[2.2.1]heptaneꢀ2,3ꢀdiꢀ
one) in the presence of xylenes and lower alcohols affords
2,3ꢀketols,11 whereas photoreduction in the presence of
tertiary amines gives the corresponding 2,3ꢀdiol as the
major product.12 Photoreduction of oꢀbenzoquinones has
received much less attention. Under UV or visible light
irradiation, these compounds undergo photoreduction in
the presence of such hydrogen donors as tertiary amines.13
The photoreaction of oꢀbenzoquinone with 4ꢀbromoꢀ
N,Nꢀdimethylaniline produces a mixture of compounds I
and III.14
In the present study, we investigated the products,
which were obtained by photoreduction of sterically hinꢀ
dered oꢀbenzoquinones in the presence of paraꢀsubstiꢀ
tuted N,Nꢀdimethylanilines, by NMR spectroscopy. We
also examined the influence of the electronic and steric
factors of substituents in the reagent molecules on the
kinetics of dark decomposition of 2ꢀalkoxyphenols I,
which are the primary products of the process.
Results and Discussion
Visible light irradiation of benzene solutions of oꢀbenꢀ
zoquinones (1) and paraꢀsubstituted N,Nꢀdimethylꢀ
anilines (2) is known15 to lead to decoloration of the
solutions due to photoreduction of oꢀbenzoquinones.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1528—1535, September, 2006.
1066ꢀ5285/06/5509ꢀ1585 © 2006 Springer Science+Business Media, Inc.

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Shurygina et al.
In the present study, we examined the reactions of steriꢀ
cally hindered oꢀbenzoquinones 1a—e with anilines 2a—c.
Photoreduction of these oꢀbenzoquinones was found to
give mixtures of pyrocatechols 3—6 and 2ꢀ(aminoꢀ
methoxy)phenols 7—14 (Scheme 2).
a
Scheme 2
7
6
5
4
3
2
δ
b
1.6
1.5
1.4
1.3
1.2
1.1
δ
c
Compound
R1
R2
Compound
R1
R2
1.6
1.5
1.4
1.3
1.2
1.1
δ
1a, 10
1b, 3, 7, 12
1c, 6, 11
But
But
But
MeO
H
F
1d, 5, 9
1e, 4, 8, 13, 14
Me
H
H
But
Fig. 1. 1H NMR spectra recorded during photoreduction of
3,6ꢀdi(tertꢀbutyl)ꢀ1,2ꢀbenzoquinone (1b) (3•10–2 mol L–1) with
4ꢀbromoꢀN,Nꢀdimethylaniline (2b) (1.5•10–1 mol L–1) in C6D6
before irradiation (a), within 2 min after irradiation (b), and
within 6 min after irradiation (c). The irradiation time was 1 min.
X = MeO (2a, 13), Br (2b, 7—11), CN (2c, 12, 14)
NCC are nitrogenꢀcontaining compounds.
1
The typical dynamics of the changes in the H NMR
spectra of the reaction mixture during irradiation and in
the course of the dark reaction is presented in Fig. 1. The
spectrum of the starting solution of 1b and 2b in C6D6
shows signals for the protons of the methyl groups in
compound 2b (δH = 2.31) and two equivalent tertꢀbutyl
groups of 1b (δH = 1.11) (see Fig. 1, a). Figure 1, b shows
the spectrum of the reaction mixture recorded at 2 min
after completion of irradiation (the irradiation time was
1 min). Irradiation of the solution leads to a decrease in
the intensity of the signals for the protons of the tertꢀbutyl
groups of 1b and the appearance of new signals for the
protons of the tertꢀbutyl groups of 3,6ꢀdi(tertꢀbutyl)pyroꢀ
catechol 3 (δH = 1.37) and the protons of two tertꢀbutyl
groups, the methyl group, and methylene group of
2ꢀ[Nꢀ(4ꢀbromophenyl)ꢀNꢀmethyl]aminomethoxyꢀ3,6ꢀ
di(tertꢀbutyl)phenol (7) (δH = 1.40, 1.54, 2.34, and 4.48,
respectively). The signal intensity ratio of the tertꢀbutyl
groups in compounds 3 and 7 is 3 : 1. In the course of the
dark process, the intensities of the signals of pyrocatechol 3
increase, whereas the intensities of the signals of 2ꢀ(amiꢀ
nomethoxy)phenol 7 decrease (see Fig. 1, c). Toward the
end of the fourth minute of the dark reaction, this ratio
reaches 15 : 1. Analogous changes in the 1H NMR spectra
are characteristic of photoreduction of all tested oꢀbenzoꢀ
quinones in the presence of tertiary amines.
Therefore, 2ꢀ(aminomethoxy)phenols 7—14 produced
in photoreactions are unstable compounds and decomꢀ
pose in the dark reaction to give pyrocatechol and nitroꢀ
genꢀcontaining compounds. Earlier,14 we have attempted
to identify nitrogenꢀcontaining photoreaction products
of compounds 1b and 2b. However, the 1H and 13C NMR
and DEPT spectra revealed only that the reaction proꢀ
duced a mixture containing presumably 4ꢀBrC6H4NHMe,
[4ꢀBrC6H4N(Me)]2CH2, and ArN(Me)CH2Brꢀtype comꢀ
pounds.
Figure 2 shows the kinetic curves of the dark decomꢀ
position reaction of 2ꢀ(aminomethoxy)phenol 9 (curve 1)
and accumulation of pyrocatechol 5 (curve 2) in the reacꢀ

Photoreduction of oꢀbenzoquinones
I (rel. units)
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 9, September, 2006
1587
Table 1. Effective rate constants of accumulation (kacc) and
decomposition (kdec) of 2ꢀ(aminomethoxy)phenols and the exꢀ
perimental (Рexp) and theoretical (Рcalc) normalized concentraꢀ
tions of 2ꢀ(aminomethoxy)phenols in the reaction mixture at
the reaction time τ = τ + τ , where τ is the irradiation time and
2.5
2
2.0
1.5
1
2
1
τ is the dark reaction time (C6D6, 298 К)
2
Comꢀ
pound
kacc•104 kdec•104 Рexp
Рcalc
τ
τ
2
1
1.0
s–1
%
s
1
0.5
7
8
9
54.7
40.7
46.3
13.6
46.4
14.5
22.9
29.0
33.0
0.26
0.22
0.14
17.0
0.60
3.0
25
53
62
14
33
23
20
14
3
5
6
1
3
2
2
1
19
52
56
61
31
17
23
16
7
4
4
5
6
2
3
2
60
90
120
120
120
120
120
120
120
180
180
700
120
120
120
60
300
600
t/min
Fig. 2. Kinetic curves of decomposition of 2ꢀ(aminomethꢀ
oxy)phenol 9 (1) and accumulation of pyrocatechol 5 (2) (the
reaction medium, without irradiation, vacuum, T = 298 К).
10
11A,B
12
13
14
0.50
tion mixture (photoreduction products of 3ꢀmethylꢀ6ꢀ
tertꢀbutylꢀ1,2ꢀbenzoquinone in the presence of 4ꢀbromoꢀ
N,Nꢀdimethylaniline). Analogous dependences were obꢀ
tained in the study of the kinetics of decomposition of
other 2ꢀ(aminomethoxy)phenols. As is evident from these
data, compound 9 is transformed into pyrocatechol 5 in
an equimolar amount in the course of the dark reaction.
Decomposition of 2ꢀ(aminomethoxy)phenols starts at
the moment of their formation and occurs during irradiaꢀ
tion and setꢀup of the NMR spectrometer (2 min, see the
Experimental section). Correspondingly, based on the kiꢀ
netic curves of decomposition of 2ꢀ(aminomethoxy)pheꢀ
nols and accumulation of pyrocatechols, it is impossible
to determine whether photoreduction of quinones affords
exclusively 2ꢀ(aminomethoxy)phenols, whereas pyroꢀ
catechols appear in the mixture only as decomposition
products of the latter or, alternatively, 2ꢀ(aminomethꢀ
oxy)phenols and pyrocatechols are generated simultaꢀ
neously. Assuming that 2ꢀ(aminomethoxy)phenols are the
only primary photoreduction products of quinones, the
change in their concentration during irradiation and in
the course of the dark reaction should obey the system of
kinetic equations corresponding to the consecutive (1)
and monomolecular (2) reactions.
constants kacc and kdec determined under the same exꢀ
perimental conditions in Eqs (1) and (2), respectively, one
can calculate the theoretical concentration of 2ꢀ(aminoꢀ
methoxy)phenol at the reaction time τ = τ1 + τ2 and
compare this concentration with the experimental value.
An agreement between the experimental and theoretical
constants will imply that 2ꢀ(aminomethoxy)phenol is the
only primary photoreduction product of oꢀbenzoquinones
in the presence of N,Nꢀdimethylanilines. Table 1 gives
the effective rate constants of accumulation and decomꢀ
position of 2ꢀ(aminomethoxy)phenols and their experiꢀ
mental (Рexp) and calculated (Рcalc) normalized concenꢀ
trations (which are equal to the ratio of the current conꢀ
centration of 2ꢀ(aminomethoxy)phenol to the initial conꢀ
centration of quinone), which were determined at the
reaction time τ equal to the total time of irradiation (τ1)
and the dark reaction (τ2).
The rate constants of photoreduction of the quinones
under study and, correspondingly, the rate constants of
accumulation of 2ꢀ(aminomethoxy)phenols are similar to
each other (see Table 1). At the same time, the constants
kdec vary by two orders of magnitude depending on the
nature of quinone and amine. The calculated curves of the
changes in the normalized concentrations of 2ꢀ(aminoꢀ
methoxy)phenols 7 and 8 with time are shown in Fig. 3.
The curve Рcalc = f(τ) for compound 11 is similar to that
for 7 (see Fig. 3, a), and the curves for compounds 12—14
are similar to those for compound 8 (see Fig. 3, b).
A comparison of Рexp and Рcalc shows that Рexp are
lower than Рcalc only for 2ꢀ(aminomethoxy)phenol 10.
For the other compounds of this series, Рexp and Рcalc are
equal within experimental error. These results indicate
that 2ꢀ(aminomethoxy)phenols 7—9 and 11—14 are the
major photoreduction products of oꢀquinones. However,
since the experimental accuracy is low, we cannot state
with assurance that photoreduction of oꢀbenzoquinones
where [PE]τ is the current concentration of 2ꢀ(aminoꢀ
methoxy)phenol, [oꢀQ]0 is the initial concentration of
oꢀbenzoquinone, kacc is the rate constant of accumulaꢀ
tion of 2ꢀ(aminomethoxy)phenol, which is equal to the
rate constant of photoreduction of oꢀbenzoquinone, kdec
is the rate constant of decomposition of 2ꢀ(aminoꢀ
methoxy)phenol, τ1 is the irradiation time of the reaction
solution, and τ2 is the dark reaction time, including the
time of setꢀup of the NMR spectrometer. Substituting the

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Shurygina et al.
Pcalc (%)
were generated in equal amounts and decompose at an
equal rate (kdec = 17.0•10–4 s–1). In the other cases, the
formation of only one isomer was observed, and the conꢀ
stants kdec in Table 1 correspond to the process of decomꢀ
position of this isomer. Presumably, the substantial difꢀ
ference between Рexp and Рcalc for 2ꢀ(aminomethoxy)pheꢀ
nol 10 is attributed to the fact that the constant kacc correꢀ
sponds to the rate constant of accumulation of both isoꢀ
mers of compound 10, whereas kdec corresponds to only
a
20
1
2
10
0
1
one of the isomers. The H NMR spectra of the reaction
200
400
τ/s
products of 1a and 2b show signals in the regions characꢀ
teristic of methylene and methyl groups; however, these
signals cannot be unambiguously identified and assigned.
Based on the overall 1H NMR spectrum, it is imꢀ
possible to determine which regioisomer of 2ꢀ(aminoꢀ
methoxy)phenol is produced in the reaction. Only phoꢀ
toreduction of benzoquinone 1b in the presence of comꢀ
pound 2b afforded individual stable 2ꢀ[Nꢀ(4ꢀbromoꢀ
phenyl)ꢀNꢀmethyl]aminomethoxyꢀ4,6ꢀdi(tertꢀbutyl)pheꢀ
nol (8), which was characterized by elemental analysis,
NMR and IR spectroscopy, and Xꢀray diffraction study.
Xꢀray diffraction data demonstrated that compound 8 was
formed as regioisomer B (Fig. 4).
Pcalc (%)
2
40
20
1
b
0
1000
2000 τ/s
Fig. 3. Calculated curves of the changes in the normalized conꢀ
centrations of 2ꢀ(aminomethoxy)phenols 7 (a) and 8 (b) during
irradiation (1) and in the course of the dark process (2).
Molecule 8 consists of the 2ꢀ[Nꢀ(4ꢀbromophenyl)ꢀ
Nꢀmethyl]aminomethoxy and 4,6ꢀdi(tertꢀbutyl)phenol
fragments. The Br(1), N(1), C(15) and O(1), O(2),
C(11), C(7) atoms lie in the planes of the phenyl rings
C(17)—C(22) and C(1)—C(6), respectively. The average
deviation of these atoms from the planes of the phenyl
rings is 0.06 Å. The dihedral angle between the planes of
the Ph rings is 59.8(3)°. The C(16) atom deviates from the
plane of the Ph ring C(17)—C(22) by 0.210(9) Å. In
molecule 8, the intramolecular O(1)...H(2´) distance is
shortened (2.31(5) Å; O(1)...H(2´)—O(2), 116.8(9)°), and
this distance is substantially smaller than the sum of the
van der Waals radii of the O and H atoms (2.45 Å)16 but is
larger than the specific O...H distance characteristic of
hydrogen bonding (2.15 Å).16 It should be noted that
the IR spectrum of compound 8 in solution shows an
OH stretching band at 3490 cm–1 (see Ref. 14), which is
under study occurs only through the intermediate formaꢀ
tion of the corresponding 2ꢀ(aminomethoxy)phenols.
The rates of formation and decomposition of
2ꢀ(aminomethoxy)phenols are substantially different (see
Fig. 3 and Table 1). Depending on the nature of quinone
and amine, the constants kdec vary by two orders of magꢀ
nitude. The structure is one of the factors responsible for
stability of these compounds. Theoretically, photoreducꢀ
tion of oꢀbenzoquinones under study (except for 1b) can
afford two isomeric forms of 2ꢀ(aminomethoxy)phenol
(A and B). However, two isomers 11A,B were experimenꢀ
tally detected only upon photoreduction of 3,6ꢀdi(tertꢀ
butyl)ꢀ4ꢀfluoroꢀ1,2ꢀbenzoquinone (1c) in the presence
of 4ꢀbromoꢀN,Nꢀdimethylaniline (2b). These isomers
C(12)
C(13)
C(11)
C(9)
C(5)
C(4)
C(14)
C(6)
C(1)
C(7)
O(2)
C(3)
C(2)
H(2´)
C(10)
C(8)
O(1)
C(19) C(18)
C(15)
C(16)
Br(1)
N(1)
C(20)
C(17)
C(22)
C(21)
Fig. 4. Molecular structure of 2ꢀ[Nꢀ(4ꢀbromophenyl)ꢀNꢀmeꢀ
thyl]aminomethoxyꢀ4,6ꢀdi(tertꢀbutyl)phenol (8).

Photoreduction of oꢀbenzoquinones
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 9, September, 2006
1589
Table 2. Selected bond lengths (d) and bond angles (ω) in
2ꢀ[Nꢀ(4ꢀbromophenyl)ꢀNꢀmethyl]aminomethoxyꢀ4,6ꢀdi(tertꢀ
butyl)phenol (8)
Selected bond lengths and bond angles are typical of
pyrocatechols (Table 2). In the crystalline state, molꢀ
ecules 8 are linked to each other to form chains through
the normal van der Waals interactions H(2´)...O(1)
(2.47(6) Å) and Br(1)...H(13) (2.93(5) Å) between the
terminal molecules (Fig. 5).
Parameter
Bond
Value
Parameter
Value
d/Å
Bond
d/Å
1
The H NMR spectrum of individual compound 8 is
Br(1)—C(20) 1.902(5)
C(7)—C(10)
C(11)—C(12)
C(11)—C(13)
C(11)—C(14)
C(17)—C(18)
C(17)—C(22)
C(18)—C(19)
C(19)—C(20)
C(20)—C(21)
C(21)—C(22)
1.524(6)
1.539(6)
1.528(6)
1.534(6)
1.410(6)
1.388(6)
1.384(6)
1.371(7)
1.381(7)
1.389(7)
identical to the spectrum of this compound in the reacꢀ
tion mixture. This is evidence that only sterically less
hindered isomer B of 2ꢀ(aminomethoxy)phenols rather
than two regioisomers is generated upon photoreduction.
In turn, stability of 2ꢀ(aminomethoxy)phenols is deterꢀ
mined also by steric hindrance resulting from the ether
bond formation. For example, compound 7 prepared by
the photoreaction of 1e with 2b is much less stable than
isomer 8 (the constant kdec changes from 33.0•10–4 s–1
for 7 to 0.26•10–4 s–1 for 8). Consequently, a decrease in
the size of the substituents at the position adjacent to the
new ether bond leads to an increase in stability of
2ꢀ(aminomethoxy)phenols. This is also confirmed by staꢀ
bility of compound 9 containing the methyl group at posiꢀ
tion 3 of the quinoid ring (kdec = 0.22•10–4 s–1).
O(1)—C(1)
O(1)—C(15)
O(2)—C(6)
N(1)—C(15)
N(1)—C(16)
N(1)—C(17)
C(1)—C(2)
C(1)—C(6)
C(2)—C(3)
C(3)—C(4)
C(3)—C(7)
C(4)—C(5)
C(5)—C(6)
C(5)—C(11)
C(7)—C(8)
C(7)—C(9)
1.397(5)
1.457(5)
1.375(5)
1.411(6)
1.453(6)
1.400(6)
1.386(6)
1.393(6)
1.380(6)
1.403(6)
1.539(6)
1.396(6)
1.404(6)
1.536(6)
1.526(8)
1.536(8)
Angle
ω/deg
C(1)—O(1)—C(15) 112.5(3)
C(15)—N(1)—C(16) 119.2(4)
C(17)—N(1)—C(15) 120.8(4)
C(17)—N(1)—C(16) 119.0(4)
N(1)—C(15)—O(1) 114.4(4)
Analysis of the data in Table 1 also shows that the
electronꢀwithdrawing and electronꢀdonating properties of
quinones and amines, respectively, can serve as the
second criterion responsible for stability of 2ꢀ(aminoꢀ
methoxy)phenols. For example, stability of the products
indicative of the presence of an intramolecular hydrogen
bond and agrees well with the published data.17 Thereꢀ
fore, 2ꢀ(aminomethoxy)phenol 8 contains two planar fragꢀ
ments linked to each other by the O(1)—C(15) bond.
Br(1K)
Br(1J)
Br(1L)
H(13BH)
H(13BK)
H(13BL)
H(13BI)
Br(1I)
H(13BJ)
c
H(13BG)
Br(1H)
Br(1G)
0
b
H(13BD)
Br(1D)
Br(1E)
Br(1F)
H(13BF)
H(13BE)
H(13BC)
H(13BA)
Br(1A)
H(13BB)
Br(1B)
Br(1C)
a
Fig. 5. Fragment of the crystal packing of 2ꢀ[Nꢀ(4ꢀbromophenyl)ꢀNꢀmethyl]aminomethoxyꢀ4,6ꢀdi(tertꢀbutyl)phenol (8).

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Shurygina et al.
derivative 2c (E1/2 = 1.12 V)15 also leads to a substantial
decrease in stability of 2ꢀ(aminomethoxy)phenols preꢀ
pared by photoreactions of these compounds with
quinone 1b. The constant kdec increases by more than
two orders of magnitude from 0.6•10–4 s–1 for 12 to
33.0•10–4 s–1 for 7. An analogous effect was observed for
the photoreactions of 1e with 2a (E1/2 = 0.49 V),15
2b (E1/2 = 0.71 V), and 2c (E1/2 = 1.12 V). The conꢀ
stants kdec of the resulting 2ꢀ(aminomethoxy)phenols are
3.0•10–4 s–1 for 13, 0.26•10–4 s1 for 8, and 0.5•10–4 s–1
for 14. Therefore, it can be stated that an increase in the
electronꢀwithdrawing properties of quinone and the elecꢀ
tronꢀdonating properties of amine leads to a decrease in
stability of 2ꢀ(aminomethoxy)phenols.
Scheme 3
The influence of the redox properties of quinones and
amines on stability of their photoreaction products sugꢀ
gests that decomposition of these products occurs by a
heterolytic mechanism shown in Scheme 3. The introꢀ
duction of the fluorine atom at position 4 of the quinone
ring (R = F) stabilizes the anion in the intermediate ion
pair due to which compound 11 decomposes much
faster than 10. The replacement of electronꢀwithdrawꢀ
ing substituents X (Br or CN) in the para position of
N,Nꢀdimethylaniline with the electronꢀdonating subꢀ
stituent (MeO) increases stability of the intermediate
iminium cation due to which decomposition of comꢀ
pound 13 occurs more rapidly compared to compounds
8 and 14.
NCC are nitrogenꢀcontaining compounds.
prepared by the reaction of quinone 1a (E1/2 = –0.48 V)18
with amine 2b is substantially higher than that of comꢀ
pounds prepared by the corresponding reaction with more
electronꢀwithdrawing quinone 1c (E1/2 = –0.34 V)18 (the
constant kdec increases from 0.14•10–4 s–1 for 10 to
17.0•10–4 s–1 for 11). At the same time, an increase in the
electronꢀdonating properties of amine in bromo derivaꢀ
tive 2b (E1/2 = 0.71 V)15 compared to those in cyano
The heterolytic mechanism of decomposition of
2ꢀ(aminomethoxy)phenols is confirmed by the fact that
the decomposition rate of these compounds is sensitive to
Scheme 4
NCC are nitrogenꢀcontaining compounds.

Photoreduction of oꢀbenzoquinones
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 9, September, 2006
1591
action mixture.* The spectra show only one isomeric form, exꢀ
cept for the products prepared by photoreduction of 3,6ꢀdi(tertꢀ
butyl)ꢀ4ꢀfluoroꢀ1,2ꢀbenzoquinone in the presence of 4ꢀbromoꢀ
N,Nꢀdimethylaniline. The 1H NMR spectroscopic data for 2ꢀ
(aminomethoxy)phenols are given below and were reported in
the earlier publication.14
2ꢀ[Nꢀ(4ꢀBromophenyl)ꢀNꢀmethylamino]methoxyꢀ3,6ꢀdi(tertꢀ
butyl)ꢀ4ꢀmethoxyphenol (10). 1H NMR (C6D6), δ: 1.66 and
1.81 (both s, 9 H each, But); 3.00 (s, 3 H, MeO); 4.58 (s,
2 H, OCH2N).
the solvent polarity. The influence of the solvent polarity
was examined using compound 8 as an example. Meaꢀ
surements demonstrated that the rate constant of decomꢀ
position in deuterochloroform (ε = 4.39)19 is an order of
magnitude higher than that in deuterobenzene (ε = 2.12)19
(0.41•10–7 and 3.13•10–7 s–1, respectively).* Based on
the above facts, photoreduction of oꢀbenzoquinones in
the presence of N,Nꢀdisubstituted anilines can be deꢀ
scribed by Scheme 4.
2ꢀ[Nꢀ(4ꢀBromophenyl)ꢀNꢀmethylamino]methoxyꢀ3,6ꢀdi(tertꢀ
butyl)ꢀ4ꢀfluorophenols (two isomers 11A,B). 1H NMR (C6D6),
δ: 1.26, 1.36, and 1.54 (all s, 9 H each, But); 1.69 (d, 9 H, But,
J = 3 Hz); 4.42 and 4.48 (both s, 2 H each, OCH2N).
3,6ꢀDi(tertꢀbutyl)ꢀ2ꢀ[Nꢀ(4ꢀcyanophenyl)ꢀNꢀmethylamiꢀ
no]methoxyphenol (12). 1H NMR (C6D6), δ: 1.37 and 1.47
(both s, 9 H each, But); 2.31 (s, 3 H, MeN); 4.49 (s,
2 H, OCH2N).
4,6ꢀDi(tertꢀbutyl)ꢀ2ꢀ[Nꢀ(4ꢀmethoxyphenyl)ꢀNꢀmethylamiꢀ
no]methoxyphenol (13). 1H NMR (C6D6), δ: 1.33 and 1.59
(both s, 9 H each, But); 2.61 (s, 3 H, MeN); 3.36 (s,
2 H, OCH2N).
The proton abstraction from the methyl group of amine
by the photoexcited oꢀquinone molecule gives rise to radiꢀ
cal products, viz., the oxyphenoxy and aminomethyl radiꢀ
cals. Recombination of the oxyphenoxy and aminomethyl
radicals affords aminoformal, viz., 2ꢀ(aminomethoxy)pheꢀ
nol, which is the major primary photoreduction product
of oꢀbenzoquinone. Disproportionation of the hydroxyꢀ
phenoxy radicals giving rise to pyrocatechol and oꢀquinone
cannot be ruled out as well. In the subsequent dark reacꢀ
tion, 2ꢀ(aminomethoxy)phenols (vacuum, room temperaꢀ
ture) undergo quantitative decomposition to form pyroꢀ
catechols.
4,6ꢀDi(tertꢀbutyl)ꢀ2ꢀ[Nꢀ(4ꢀcyanophenyl)ꢀNꢀmethylamiꢀ
no]methoxyphenol (14). 1H NMR (C6D6), δ: 1.32 and 1.56
(both s, 9 H each, But); 2.34 (s, 3 H, MeN); 4.64 (s, 2 H,
OCH2N); 4.92 (s, 1 H, OH).
Experimental
2ꢀ[Nꢀ(4ꢀBromophenyl)ꢀNꢀmethylamino]methoxyꢀ4,6ꢀdi(tertꢀ
butyl)phenol (8), which was prepared by photoreduction of
3,5ꢀdi(tertꢀbutyl)ꢀ1,2ꢀbenzoquinone in the presence of 4ꢀbromoꢀ
N,Nꢀdimethylaniline, was isolated in individual state14 and studꢀ
ied by Xꢀray diffraction. Single crystals of 8 were grown from an
nꢀhexane solution in an evacuated tube by slow cooling. At
100(2) К, crystals of 8 are triclinic: C22H30BrNO2, M = 420.38,
a = 10.026(1),–b = 10.131(2), c = 11.317(2) Å, V = 1053.5(3) Å3,
The electronic absorption spectra were recorded on a SFꢀ14
spectrometer. The NMR spectra were measured on a Bruker
DPXꢀ200 spectrometer. Photochemical reactions were carried
out with the use of an illuminator equipped with a focusing
system and a KGMꢀ24ꢀ150 lamp. Radiation at λ > 500 nm was
separated from the light flux of the lamp using a ZhSꢀ16 light
filter. All studies were performed in NMR tubes. A deaerated
benzene solution (2 mL) containing oꢀbenzoquinone (3•10–2
mol L–1) and amine (1.5•10–1 mol L–1) was placed in a tube,
which was sealed and then exposed at a distance of 7 cm from
the lens of the illuminator. The solvents (benzene and hexane)
were purified according to a standard procedure.19 4ꢀMethoxyꢀ
N,Nꢀdimethylaniline was synthesized according to a known proꢀ
cedure.20 4ꢀBromoꢀN,Nꢀdimethylaniline (Aldrich) and 4ꢀcyanoꢀ
N,Nꢀdimethylaniline (Aldrich) were twice sublimed. The synꢀ
thesis of oꢀbenzoquinone and pyrocatechol derivatives has been
described earlier.21 3,6ꢀDi(tertꢀbutyl)ꢀ4ꢀfluoropyrocatechol was
prepared by reduction of 3,6ꢀdi(tertꢀbutyl)ꢀ4ꢀfluoroꢀ1,2ꢀbenzoꢀ
quinone with hydrazine hydrate. 1H NMR (C6D6), δ: 1.23 (s,
9 H, But); 1.54 (d, 9 H, But, J = 2.5 Hz); 3.95 and 4.12 (both s,
1 H each, OH); 6.23 (s, 1 H, C(5)H).
space group P1, Z = 2, µ = 1.966 mm–1, dcalc= 1.325 g cm–3
,
5783 reflections were measured, of which 3680 indepenꢀ
dent reflections (Rint = 0.0261) were used in calculations;
R1 (I > 2σ(I )) = 0.0572, wR1 (based on complete data)
was 0.1630, GOOF (F 2) = 1.055. The structure was solved by
direct methods and refined against F 2 using the SHELXTL proꢀ
gram package.18 All nonhydrogen atoms were refined anisotroꢀ
pically. The hydrogen atoms were located in difference electron
density maps, except for the hydrogen atoms at C(10), which
were placed in geometrically calculated positions and refined
using a riding model. Selected bond angles and bond lengths are
given in Table 1.
The effective rate constants of photoreduction (kacc) of
oꢀbenzoquinones in the presence of paraꢀsubstituted N,Nꢀdiꢀ
methylanilines were determined spectrophotometrically based
on a decrease in the intensity of the Sꢀπ—π* absorption band of
quinone. The solution was placed in an NMR tube and irradiꢀ
ated under standard conditions. Under experimental conditions,
photoreduction initially obeys the firstꢀorder kinetic equation
until the conversion of quinone reached ~30%. The effective
rate constants of quinones were calculated from the slope of the
linear region of the ln(C0/Cτ)—τ plot, where C0 and Cτ are the
concentrations of oꢀbenzoquinone before irradiation and at the
The compositions of the products prepared by photoreducꢀ
tion of oꢀbenzoquinones in the presence of paraꢀsubstituted
N,Nꢀdimethylanilines were monitored based on the changes in
the 1H NMR spectra. All 2ꢀ(aminomethoxy)phenols are unꢀ
stable and were characterized by NMR spectroscopy in the reꢀ
* The fact that kdec in C6D6 determined in this experiment difꢀ
fers from kdec in Table 1 (kdec = 0.26•10–4 s–1) is, evidently,
attributed to the difference in the experimental conditions. The
solvent effect on kdec was estimated with the use of solutions of
individual compound 8, whereas the data in Table 1 correspond
to decomposition of 2ꢀ(aminomethoxy)phenols in the reaction
mixture.
* The signals for the protons of the aromatic system of 2ꢀ(amiꢀ
nomethoxy)phenols are not reported because they overlap with
the signals of the starting reagents.

1592
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 9, September, 2006
Shurygina et al.
time τ, respectively, where τ is the total time of irradiation of the
solution. The numerical values of the effective rate constants
were averaged based on three measurements with the converꢀ
gence of the results within 10%.
9. H. Shindo, K. Maruyama, T. Otsuki, and T. Maruyama,
Bull. Chem. Soc. Jpn, 1971, 44, 2789.
10. K. Maruyama, T. Rwai, and I. Naruta, Bull. Chem. Soc. Jpn,
1978, 51, 2052.
The effective rate constants of decomposition of 2ꢀ(aminoꢀ
methoxy)phenols (kdec) were determined from the changes in
the intensity of the characteristic signals in the 1H NMR spectra.
A solution of oꢀbenzoquinone (3•10–2 mol L–1) and paraꢀsubꢀ
stituted N,Nꢀdimethylaniline (1.5•10–12 mol L–1) in C6D6 was
placed in an NMR tube and deaerated. The tube was exposed
with a constant geometry. The irradiation time was varied taking
into account the efficiency of photoreduction of the reaction
pair. The changes in the intensity of the characteristic signals of
the components of the mixture were recorded at intervals. Unꢀ
der experimental conditions, decomposition of 2ꢀ(aminoꢀ
methoxy)phenols obeys the firstꢀorder kinetic equation. The
constants kdec were calculated from the slope of the linear region
of the ln(I0/Iτ)—τ plot (I0 and Iτ are the intensities of the typical
signals of 2ꢀ(aminomethoxy)phenols before irradiation and at
the time τ, respectively, where τ is the dark reaction time). The
numerical values of kdec were averaged based on three measureꢀ
ments with the convergence of the results within 15—20%.
11. M. Monroe and S. A. Weiner, J. Am. Chem. Soc., 1969,
91, 450.
12. E. Andrzejewska, L. Linden, and J. F. Rabek, Macromol.
Chem. Phys., 1998, 199, 441.
13. G. A. Abakumov, S. A. Chesnokov, V. K. Cherkasov, and
G. A. Razuvaev, Izv. Akad. Nauk SSSR, Ser. Khim., 1985,
773 [Bull. Acad. Sci. USSR, Div. Chem. Sci., 1985, 34, 700
(Engl. Transl.)].
14. S. A. Chesnokov, V. K. Cherkasov, G. A. Abakumov, Yu. A.
Kurskii, M. P. Shurygina, O. N. Mamysheva, and A. S.
Shavyrin, Izv. Akad. Nauk, Ser. Khim., 2003, 688 [Russ.
Chem. Bull., Int. Ed., 2003, 52, 718].
15. C. A. Chesnokov, V. K. Cherkasov, Yu. V. Chechet, V. I.
Nevodchikov, G. A. Abakumov, and O. N. Mamysheva, Izv.
Akad. Nauk, Ser. Khim., 2000, 1515 [Russ. Chem. Bull., Int.
Ed., 2000, 49, 1506].
16. Yu. V. Zefirov and P. M. Zorkii, Usp. Khim., 1995, 64, 446
[Russ. Chem. Rev., 1995, 64 (Engl. Transl.)].
17. K. Nakanishi, Infrared Absorption Spectroscopy, HoldenꢀDay
Inc., San Francisko and Nankodo Company Limited, Tokyo,
1962, p. 211.
18. G. M. Sheldrick, SHELXTL V. 6.12, Structure Determination
Software Suite, Bruker AXS, Madison (Wisconsin,
USA), 2000.
19. J. Gordon and R. A. Ford, The Chemist´s Companion,
A Wiley Intercience Publication, J. Wiley and Sons, New
York—London—Sydney—Toronto, 1972.
This study was financially supported by the Council
on Grants of the President of the Russian Federation
(Program for State Support of Leading Scientific Schools
of the Russian Federation, Grant NShꢀ4947.2006.3) and
the Russian Foundation for Basic Research (Project Nos
05ꢀ03ꢀ32706, 06ꢀ03ꢀ33061ꢀa, and 06ꢀ03ꢀ08186ꢀofi).
20. WeygandꢀHilgetag, Organischꢀchemische Experimentierkunst,
Johann Ambrosios Barth, Leipzig, 1964, p. 464.
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Received March 23, 2006;
in revised form July 7, 2006