Products and mechanisms of photochemical transformations of o-quinones

Article abstract of DOI:10.1134/S0018143910030148

The photochemical transformations of quinones by the action of light at λ > 500 nm, namely, the photodecarbonylation and photoreduction reactions were studied with the use of a series of o-benzoquinones and 9,10-phenanthrenequinone as examples. The two-stage mechanism of the decarbonylation reaction of o-benzoquinones was established. At the first stage, rearrangement of a photoexcited quinone molecule into a bicyclic compound that spontaneously decomposes in the dark reaction into cyclopentadienone and CO takes place. It has been found that the formation of the photoreduction products of both o-benzoquinones and 9,10-phenanthrenequinone in the presence of various H donors (N,N-dimethylanilines and polymethylbenzenes) follows the same mechanism. In the first step, a phenol ether is produced, which subsequently undergoes quantitative transformation into pyrocatechol or ketol via the heterolytic mechanism. The stability of phenol ethers is determined by the structure and redox properties of the reactants.

Full text of DOI:10.1134/S0018143910030148

ISSN 0018ꢀ1439, High Energy Chemistry, 2010, Vol. 44, No. 3, pp. 234–238. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © M.P. Shurygina, Yu.A. Kurskii, N.O. Druzhkov, S.A. Chesnokov, G.A. Abakumov, 2010, published in Khimiya Vysokikh Energii, 2010, Vol. 44, No. 3,
pp. 262–266.
MOLECULAR PHOTONICS
Products and Mechanisms of Photochemical Transformations
of oꢀQuinones
M. P. Shurygina, Yu. A. Kurskii, N. O. Druzhkov, S. A. Chesnokov, and G. A. Abakumov
Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences
ul. Tropinina 49, Nizhni Novgorod, 603950 Russia
eꢀmail: sch@iomc.ras.ru)
Received September 8, 2009
Abstract—The photochemical transformations of quinones by the action of light at
photodecarbonylation and photoreduction reactions were studied with the use of a series of
and 9,10ꢀphenanthrenequinone as examples. The twoꢀstage mechanism of the decarbonylation reaction of
ꢀbenzoquinones was established. At the first stage, rearrangement of a photoexcited quinone molecule into
a bicyclic compound that spontaneously decomposes in the dark reaction into cyclopentadienone and CO
takes place. It has been found that the formation of the photoreduction products of both ꢀbenzoquinones
λ
> 500 nm, namely, the
o
ꢀbenzoquinones
o
o
and 9,10ꢀphenanthrenequinone in the presence of various H donors (N,Nꢀdimethylanilines and polymethꢀ
ylbenzenes) follows the same mechanism. In the first step, a phenol ether is produced, which subsequently
undergoes quantitative transformation into pyrocatechol or ketol via the heterolytic mechanism. The stability
of phenol ethers is determined by the structure and redox properties of the reactants.
DOI: 10.1134/S0018143910030148
Under the action of light, quinones experience
numerous transformations involved in important proꢀ a series of sterically hindered
cesses, such as photosynthesis of green plants (1a 1f) (Scheme 1) with light of
and photodegradation of vat dyes [1]. Photoreactions ied [10]. The NMR study of the photoreaction of
of ꢀquinones form the basis for many systems used to ꢀbenzoquinones revealed the existence of an interꢀ
initiate radical photopolymerization [2, 3]. ꢀQuinoꢀ mediate compound (2) containing threeꢀ and fiveꢀ
In our works, the photolysis of benzene solutions of
ꢀbenzoquinones
> 520 nm was studꢀ
o
⎯
λ
о
o
o
nes exhibit absorption bands in the range of 400 to membered cycles. The intermediate quantitatively
650 nm and can act as photosensitizers [4] or can degrades already at room temperature to the single
undergo various transformations; namely, they are final product (main product for 1d), the correspondꢀ
capable of degradation with elimination of CO [5] or ing 3,4ꢀdisubstituted 2,5ꢀdiꢀtertꢀbutylcyclopentadiꢀ
entering into the photoreduction reaction with various enone (3) (Scheme 1). All cyclopentadienones 3a–3g
hydrogen donors [3, 5, 6]. The Razuvaev Institute of were isolated and were characterized by instrumental
Organometallic Chemistry possesses a unique collecꢀ techniques.
tion of sterically hindered oꢀbenzoquinones, which
allowed us to perform a systematic study of the prodꢀ
ucts and mechanisms of photochemical transformaꢀ
tions of oꢀquinones, namely, the photodecarbonylaꢀ
tion and photoreduction reactions.
tꢀBu
tꢀBu
tꢀBu
X
X
X
X
O
O
X
X
O
λ > 520 nm
O
−
CO
tꢀBu
tꢀBu
tꢀBu
O
PHOTODECARBONYLATION REACTION
1
2
3
OF oꢀBENZOQUINONES
X = H(a); F (b); Cl (c); OCH3 (d);
The studies devoted to the photolysis of quinones
have been basically limited to investigation into the
kinetics or products produced by the action of UV
radiation only [7–12]. It is known that 3,6ꢀ [7, 8] and
O
N
S
S
(e);
(f);
(g)
C O
O
N
3,5ꢀdiꢀtertꢀbutyl derivatives [9] of
decomposed by UV irradiation (
cyclopentadienones, ejecting CO. This reaction can
o
λ
ꢀbenzoquinone are
< 405 nm) into
Scheme 1.
Thus, the photodecarbonylation reaction of oꢀbenꢀ
be considered a method for the synthesis of cyclopenꢀ zoquinones is a twoꢀstep process. In the first step, the
tadienones, which are used as ligands for organomeꢀ rearrangement of the cyclohexadienone ring of a phoꢀ
tallic compounds.
toexcited oꢀbenzoquinone molecule into the correꢀ
234

PRODUCTS AND MECHANISMS OF PHOTOCHEMICAL TRANSFORMATIONS
235
sponding bicyclic compound takes place. The second ferred product for 9,10ꢀphenanthrenequinone
stage comprises the elimination of CO from the cycloꢀ
propane moiety and the formation of the final product
cyclopentadienone with a yield close to quantitative.
(4) [12–14] is determined by the nature of the hydroꢀ
gen donor. For example, 9,10ꢀphenanthrenequinone
in the presence of alcohols [15], ethers [12], and aldeꢀ
hydes [11] is reduced to corresponding phenol ethers
(Scheme 2, product 5) As a result of the photoreaction
of 4 in the presence of hydrocarbons [15–18], correꢀ
sponding ketols are produced (Scheme 2, product 6).
PHOTOREDUCTION REACTION
OF QUINONES
The photoreduction reaction of carbonyl comꢀ
pounds consists in the abstraction of a hydrogen atom
from an Hꢀdonor molecule by a photoexcited accepꢀ
tor molecule to give radicals, which yield various prodꢀ
ucts via their subsequent transformations [11]. It has
Earlier, we have found [19] that some oꢀbenzoquinoꢀ
nes in the presence of tertiary amine are effectively
photoreduced with UV and visible light, yielding a
mixture of products: phenol ether 5 and pyrocatechol
been found that the formation of one or another preꢀ (Scheme 2, compound 7).
O
O
ODH
OH
OH
OH
O
hν
R
+ DH2
;
DH
R
;
R
R
OH
5
7
6
Scheme 2.
This reaction was studied in detail [20, 21] with the that the only product, corresponding phenol ether
use of a number of ꢀbenzoquinones (1a, 1h–1k) (5a–5h), is formed, which subsequently degrades in
(Scheme 3) in the presence of ꢀsubstituted N,Nꢀ the dark to yield pyrocatechol (7a–7d) and a set of
o
p
dimethylanilines (8a–8c).Owing to the detailed NMR nitrogen compounds (Scheme 3) [21].
study of the evolution of the process, it has been found
tꢀBu
H3C
CH3
tꢀBu
N
tꢀBu
OH
O
O
O
OH
hν
C6D6
X
+
+
nitrogenꢀcontaining
compounds
''R
R''
''R
R''
''R
R''
OH
R' H2C
N
R'
1a, h−k
X
R'
7a−d
CH3
5a−h
8a−c
R' = tꢀBu, R'' = H, X = Br (a);
R' = tꢀBu, R'' = H (a);
R' = H, R'' = tꢀBu (b);
R' = Me, R'' = H (c);
R' = tꢀBu, R'' = F (d)
X = MeO (a);
Br (b); CN (c);
R' = tꢀBu, R'' = H (a);
R' = tꢀBu, R'' = MeO (h);
R' = tꢀBu, R'' = F (i);
R' = Me, R'' = H (j);
R' = H, R'' = tꢀBu (k).
R' = Me, R'' = H, X = Br (b);
R' = H, R'' = tꢀBu, X = Br (c);
R' = tꢀBu, R'' = MeO, X = Br (d);
R' = tꢀBu, R'' = F, X = Br (e);
R' = tꢀBu, R'' = H, X = CN (f);
R' = H, R'' = tꢀBu, X = MeO (g);
R' = H, R'' = tꢀBu, X = CN (h)
Scheme 3.
We have revealed the factors that affect the stability of the quinone ring considerably reduces the rate of
of phenol ethers. One of them is the structure of degradation of phenol ethers. The degradation rate
benzoquinone. The replacement of the tertꢀbutyl with constant (kd) decreases by two orders of magnitude
the methyl group or hydrogen atom in the 3ꢀposition from 3.30
10–3 (5a) to 2.2 10–5 s–1 (5b) or 2.6
oꢀ
×
×
×
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SHURYGINA et al.
5
10⎯ s–1 (5c). Another factor affecting the stability of ety of phenol ether reduces its stability. This fact can be
phenol ether is the donor–acceptor properties of its explained in terms of the assumption that the degradaꢀ
substituents. The enhancement of the electronꢀwithꢀ tion of phenol ether follows the heterolytic mechaꢀ
drawing properties of the substituent in the quinine nism (Scheme 4).
moiety and the donating properties in the amine moiꢀ
H2C
Oᮎ
CH3
⊕
N
X
OH
X
OH
R
+
nitrogenꢀcontaining
compounds
R
R
O CH2
N
OH
OH
CH3
Scheme 4.
An additional support for the assumption on the threnequinone during irradiation with visible light. As
heterolytic degradation is the response of the degradaꢀ
tion rate to a change in the solvent polarity. When deuꢀ
terated benzene is replaced with deuterated chloroꢀ
form, the degradation rate increases by an order of
hydrogen donors, a number of anilines (8d–8f) were
taken. An analysis of proton NMR spectra has shown
[23] that irradiation of 9,10ꢀphenanthrenequinone in
the presence of anilines 8c and 8d results in the formation
of the corresponding phenol ether alone (Scheme 5), as
magnitude from 3.1
photoreduction of ꢀbenzoquinones in the presence of
× ×
10–7 to 4.1 10–8 s–1. Thus, the
o
in the case of oꢀbenzoquinones. An increase in the
N,Nꢀdimethylanilines leads to the formation of the
only product phenol ether in the first step, which
experiences degradation via the heterolytic mechaꢀ
nism yielding pyrocatechol.
The photoreduction products of 9,10ꢀphenanꢀ
threnequinone have been studied to the greatest
electronꢀdonating power of amines from Е1/2 = 1.12 V
(8c) to 0.84 V (8d) destabilizes the phenol ether in this
case. The values of the phenol ether degradation rate
constant increases from 3.3
× ×
10–4 (5i) to 5.3 10⎯4 s–1
(5j). During they dark reaction, phenol ether is quanꢀ
titatively rearranged into ketol (6a, 6b) following the
extent. It is known [22] that pꢀsubstituted N,Nꢀdimeꢀ
thylanilines effectively photoreduce 9,10ꢀphenanꢀ firstꢀorder rate law.
H3C
CH3
N
O
O
OH
λ > 520 nm
+
OH
O CH2
N
X
CH3
ᮎ
⊕
CH2
X
O
N
X
X = CN (8c), C(O)H (8d),
H (8e), CH3 (8f)
X = CN (5i), C(O)H (5j)
CH3
OH
ᮎ
OH
CH2
N
X
⊕
CH2
N
X
O
O
CH3
CH3
X = CN (6a), C(O)H (6b)
H (6c), CH3 (6d)
Scheme 5.
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PRODUCTS AND MECHANISMS OF PHOTOCHEMICAL TRANSFORMATIONS
237
The further enhancement of the electronꢀdonating
In the presence of polymethylbenzenes, the proꢀ
ability of amine on passing from 8e and 8f (Е1/2 = 0.71 cess occurs via another scheme. We studied the photoꢀ
and 0.65 V, respectively) apparently lowers the stability reaction of 9,10ꢀphenanthrenequinone in the presꢀ
of the corresponding phenol ethers to such an extent ence of a number of polymethylbenzenes (9a–9f). It
that they become undetectable. The 1H NMR spectra has been found [23] that the primary products of the
display only the signals of the corresponding ketols photoreduction reaction of 9,10ꢀphenanthreneꢀ
(6c, 6d). By analogy with the aboveꢀdiscussed mechaꢀ quinone with polymethylbenzenes 9a, 9b, 9d, and 9e
nism of degradation of phenol ethers produced from are the corresponding phenol ethers (5k–5p)
o
ꢀbenzoquinones (Scheme 4), it may be assumed that (Scheme 6). The phenol ethers produced in this reacꢀ
the rearrangement of phenol ethers due to 9,10ꢀ tion are stable in the dark and enter into a secondary
phenanthrenequinone and amine follows the heteroꢀ photoreaction with 4 upon subsequent irradiation,
lytic mechanism as well.
yielding corresponding ketols.
OH
CH2
O
9a, b, d, e
(CH3)n
O
O
5 k−p
λ > 520 nm
+
λ > 500 nm
4
(CH3)n
9
9c, f
O
4
OH
(CH3)n
CH2
6 e−j
n = 6 (a), 4 (b), 3 (c), 2 (o–d, n–e, m–f)
Scheme 6.
During the photoreaction of 4 in the presence of 9c of the final product of degradation of phenol ether.
or 9f, the formation of only the corresponding ketols is Phenol ether is rearranged into ketol in the case of
1
manifested in the H NMR spectra. All ketols 6a–6j phenanthrenequinone or pyrocatechol in the case of
were isolated in the individual form and were characꢀ
oꢀbenzoquinones. Moreover, along with the photoreꢀ
terized by instrumental techniques. Thus, we assume duction products, the products of the competing
that the photoreduction of 9,10ꢀphenanthreneꢀ quinone decarbonylation process are formed,
quinone in the presence of either polymethylbenzenes although the contribution of this process is insignifiꢀ
or tertiary amines is a twoꢀstep process, as in the case cant since the reaction rates differ by more than an
of the photoreaction of
The results obtained in this work suggest that the
formation of the photoreduction products of both
benzoquinones and 9,10ꢀphenanthrenequinone folꢀ
lows the same scheme at the first step. Namely, the
only product (phenol ether) is produced, which subseꢀ
o
ꢀbenzoquinones.
order of magnitude.
oꢀ
ACKNOLEDGMENT
This work was supported by the Russian Foundation for
Basic Reserch (grants nos. 08ꢀ03ꢀ97055ꢀr_povolj’ye_a, 09ꢀ
03ꢀ12286ꢀofi_m, 09ꢀ03ꢀ00668ꢀa, 10ꢀ03ꢀ00788ꢀa) and
Russian President grant supporing Scientific Schools
(NShꢀ7065.2010.3).
quently transforms into pyrocatechol (in the case of oꢀ
benzoquinone) or ketol (in phenanthrenequinone
reactions) depending on the nature of both reactants.
The variety and controversy of published data on the
products of phototransformations of quinones can be
explained by several factors. The first is the difference
in stability between phenol ethers; consequently, difꢀ
ferent product ratios will be observed depending on
the time of isolation of the products. The second is the
nature of the acceptor, which determines the structure
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