Photolysis of Regioisomeric α,α-Diphenyl-Substituted Diazotetrahydrofuranones: Primary and Secondary Photochemical Processes

Article abstract of DOI:10.1134/S1070428010100179

The formation of C-H insertion products in the course of direct photolysis of α,α-diphenylsubstituted diazo ketones of the tetrahydrofuran series was rationalized by secondary photochemical processes which give rise to benzophenone acting as a sensitizer. Triplet excited states of diazo ketones generated by the action of benzophenone are capable of undergoing bimolecular transformations.

Full text of DOI:10.1134/S1070428010100179

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 10, pp. 1542–1545. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © L.L. Rodina, O.S. Galkina, M.B. Supurgibekov, Ya.M. Grigor’ev, V.A. Utsal, 2010, published in Zhurnal Organicheskoi Khimii,
2010, Vol. 46, No. 10, pp. 1536–1539.
Photolysis of Regioisomeric α,α-Diphenyl-Substituted
Diazotetrahydrofuranones: Primary and Secondary
Photochemical Processes
L. L. Rodina, O. S. Galkina, M. B. Supurgibekov, Ya. M. Grigor’ev, and V. A. Utsal’
St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia
e-mail: LRodina@VN6646.spb.edu
Received August 26, 2009
Abstract—The formation of C–H insertion products in the course of direct photolysis of α,α-diphenyl-
substituted diazo ketones of the tetrahydrofuran series was rationalized by secondary photochemical processes
which give rise to benzophenone acting as a sensitizer. Triplet excited states of diazo ketones generated by the
action of benzophenone are capable of undergoing bimolecular transformations.
DOI: 10.1134/S1070428010100179
Singlet excited states generated by direct irradiation
of diazo carbonyl compounds are capable of under-
going only very fast unimolecular transformations.
Among the latter, the most typical is Wolff rearrange-
ment which involves elimination of nitrogen molecule
and leads to the formation of ketenes and carboxylic
acid derivatives [1]. This reaction is especially effec-
tive with cyclic and heterocyclic diazo ketones, and the
yields of 1,2-nucleophilic rearrangement (ring contrac-
tion) products reach 80–90% and more [1].
be specially emphasized that the described reaction
was the first example of photochemical transformation
of diazo ketones without elimination of nitrogen.
Insofar as we failed to effect similar transforma-
tion of tetraalkyl-substituted analogs [3], we presumed
that the presence of phenyl substituents in the α-posi-
tions with respect to the diazo and/or carbonyl group
in the initial diazo ketone is crucial. Presumably,
α-phenyl groups are responsible for the formation of
long-lived triplet states from diazo ketones.
On the other hand, bimolecular processes were also
observed with some diazo ketones of the tetrahydro-
furan series. For example, we previously [2] described
bimolecular photochemical transformation of 4-diazo-
2,2,5,5-tetraphenyltetrahydrofuran-3-one which re-
acted with the solvent (tetrahydrofuran) to produce the
corresponding substituted hydrazone, i.e., the process
was accompanied by elimination of nitrogen. It should
Thus the goal of the present work was to study the
above previously unknown photochemical reaction [4]
using as substrates regioisomeric α,α-diphenyl-substi-
tuted diazo ketones Ia and Ib belonging to the tetra-
hydrofuran series [5]. Initially, analysis of the reaction
mixtures showed that the main path of photochemical
transformations of both diazo ketones Ia and Ib is the
Wolff rearrangement (to 73%) leading to the formation
Scheme 1.
O
HN
O
Nu
O
N2
O
N
O
R'
O
R
R'
hν, THF–NuH
+
+
+
R
R
R'
R'
R
R
R'
R'
R
R
R
R
R'
R'
R'
O
O
O
O
R
R'
Ia, Ib
II (49–73%)
IIIa, IIIb (3–22%)
IVa, IVb (2–10%)
IVa, Vb (4–22%)
R = Me, R′ = Ph (a); R = Ph, R′ = Me (b); Nu = OH, OMe, NMe2.
1542

PHOTOLYSIS OF REGIOISOMERIC α,α-DIPHENYL-SUBSTITUTED DIAZOTETRAHYDRO...
1543
of oxetane-3-carboxylic acid II (Nu = OH) or its
derivatives (Nu = OMe, NMe2) (Scheme 1). The con-
tributions of concurrent photochemical processes lead-
ing to C–H insertion products IIIa and IIIb, products
of 1,2-migration of methyl or phenyl group (com-
pounds IVa, IVb), and hydrogen abstraction products
Va and Vb were considerably smaller, and the product
ratio was almost independent on the initial regioisomer
structure.
tion to 3-methylbut-2-enoic acid (VIIIa) or its amide
VIIIb] upon short-wave UV irradiation of oxetanecar-
boxylic acid II or its amide (Scheme 3).
Scheme 3.
Me
hν, THF–NuH
hν'
CONu
Ia
II
+
VI
–N2
Me
VIIIa, VIIIb
Nu = OH (a), NMe2 (b).
However, detailed examination of the reaction mix-
tures by gas chromatography–mass spectrometry, IR
spectroscopy, and H NMR revealed that prolonged
1
Our results led us to conclude that irradiation with
short-wave UV light of initially formed ketene VII, as
well as of oxetanecarboxylic acid II and its derivatives
(Nu = OH, NMe2), induces photochemical decomposi-
tion of the oxetane heteroring with elimination of ben-
zophenone. This cycloreversion of oxetane derivatives
may be regarded as the reverse Patterno–Büchi
reaction [6].
UV irradiation (1–2 h) resulted in the formation of
compounds II–IV and small amounts of seven or eight
other compounds. Among the latter we identified
benzophenone (VI), which is likely to be formed as
a result of secondary photochemical reactions. Our
subsequent experiments confirmed this assumption and
made it possible to identify some secondary photolysis
products.
Furthermore, we have found that photolysis of di-
azo ketone Ia in the presence of 1.5 equiv of benzo-
phenone leads to C–H insertion product IIIa in a good
yield (56%) together with a small amount of ketone Va
(7%). These data are direct experimental proofs for
electronic excitation energy transfer by benzophenone
and formation of long-lived triplet states from initial
diazotetrahydrofuranones I. Such triplet states are
capable of undergoing bimolecular transformations
with participation of solvent molecules (tetrahydro-
furan in our case). As a result, α-oxo hydrazones III
are obtained which may formally be regarded as inser-
tion products of excited diazo ketone molecules into
C–H bond of tetrahydrofuran.
When diazo ketone Ia was subjected to photolysis
in THF in the absence of nucleophile, the IR spectrum
of the reaction mixture recorded after irradiation for
20 min contained absorption bands assignable to
ketene VII (2127 and 790 cm–1). Judging by the
intensity ratio of the absorption bands at 2091 and
2127 cm–1, the ratio Ia:VII was 2:1 in 20 min (see
figure, curve 1) and 4.5:1 in 40 min (curve 2).
After irradiation for 60 min, ketene VII completely
disappeared from the reaction mixture, and benzo-
phenone (VI) was identified among photochemical
decomposition products of ketene VII (Scheme 2).
Benzophenone (VI) can readily be detected and quan-
1
Thus the results of the present study showed that
the formation of C–H insertion products III in the
tified by the H NMR spectrum which contained
a downfield doublet at δ 7.79–7.82 ppm from the
ortho-protons; as a rule, their signal was not over-
lapped by signals of the other products. Benzophenone
VI was also detected in the reaction mixture [in addi-
Scheme 2.
O
1
2
·
O
N2
Me
Me
Ph
Ph
hν
Me
Me
Ph
Ph
–N2
O
Ia
VII
hν
O
2200
2160
2120
2080 ν, cm–1
+
Other products
Ph
Ph
IR spectrum of intermediate ketene VII generated by photol-
ysis of diazo ketone Ia in THF in the absence of nucleophile.
VI (2–10%)
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 10 2010

RODINA et al.
1544
direct photolysis of α,α-diphenyl-substituted 4-diazo-
tetrahydrofuran-2-ones Ia and Ib is not related to
effect of phenyl groups in the α-position of the initial
diazo ketone. Alternatively, their formation is rational-
ized by secondary photochemical processes leading to
the appearance in the reaction mixture of benzo-
phenone as sensitizer. The latter promotes sensitized
photolysis of diazo ketones Ia and Ib in parallel to the
main photochemical reaction, eventually yielding C–H
insertion products III and ketones V.
using petroleum ether (bp 40–65°C) and diethyl ether
(or tert-butyl methyl ether) as eluents (gradient
elution) or identified in the reaction mixture by IR
spectroscopy and GC–MS.
2,2-Dimethyl-4,4-diphenyloxetane-3-carboxylic
acid (II). Colorless crystals, mp 184–186°C (petro-
leum ether), Rf 0.15 (petroleum ether–diethyl ether,
1
5:1). H NMR spectrum, δ, ppm: 1.44 s (3H, CH3),
1.62 s (3H, CH3), 4.23 s (1H, CH), 7.15–7.50 m (10H,
H
arom). 13C NMR spectrum, δC, ppm: 25.9 (CH3), 32.5
(CH3), 58.2 (CH), 80.8 (CMe2), 83.9 (Ph2C); 125.2,
126.5, 127.4, 127.5, 128.1, 128.7 (CHarom); 142.9,
148.2 (Carom); 174.6 (C=O) [7] .
EXPERIMENTAL
The H and 13C NMR spectra were recorded on
1
2,2-Dimethyl-5,5-diphenyltetrahydrofuran-3,4-
dione 4-(tetrahydrofuran-2-ylhydrazone) (IIIa).
Yellow crystals, mp 111–112°C (petroleum ether),
a Bruker DPX-300 spectrometer at 300 and 75.5 MHz,
respectively, from solutions in CDCl3 using tetrameth-
ylsilane as internal reference. Gas chromatographic–
mass spectrometric analysis was performed on
a GCMS-QP2010 Plus instrument. The formation of
ketene VII was monitored by IR spectroscopy using
Perkin–Elmer BXII and Specord IR-75 instruments
(quantitative measurements were performed by the
intensities of characteristic absorption bands at 2127
and 790 cm–1).
1
Rf 0.16 (petroleum ether–Et2O, 10:1). H NMR spec-
trum, δ, ppm: 1.26 s (3H, CH3), 1.33 s (3H, CH3),
1.90–2.22 m (4H, CH2CH2), 3.83–3.88 m (2H, OCH3),
5.37–5.43 m (1H, CHN, J = 3.7 Hz), 7.21–7.32 m (6H,
H
arom), 7.38–7.49 m (4H, Harom), 11.60–11.62 d (1H,
NH, J = 5.6 Hz). 13C NMR spectrum, δC, ppm: 25.0
(CH2), 25.24 and 25.27 (CH3), 30.17 (CH2), 67.9
(CH2O), 80.7 (CMe2), 82.9 (CPh2), 90.7 (CHN);
126.8, 126.9, 127.1, 127.1, 127.8, 127.9 (CHarom);
135.2 (C=N); 145.5, 145.7 (Carom); 200.2 (C=O) [5].
The reactions were carried out in anhydrous sol-
vents which were purified according to standard proce-
dures. The reaction mixtures were separated by column
or flash chromatography on silica gel (Silicagel L, 40–
100 μm; Chemapol) of activity grade I with gradient
elution. Silica gel 60 F254 plates were used for analyt-
ical TLC; mixtures of petroleum ether with diethyl
ether at different ratios, as well as tert-butyl methyl
ether, ethyl acetate, or their mixtures, were used as
eluents.
2,3-Dihydro-2,2-dimethyl-4,5-diphenylfuran-3-
one (IVa). Colorless crystals, mp 81–82°C (from pe-
troleum ether), Rf 0.23 (petroleum ether–diethyl ether,
1
10:1). H NMR spectrum, δ, ppm: 1.56 s (6H, CH3),
7.29–7.65 m (10H, Harom). 13C NMR spectrum, δC,
ppm: 23.4 (CH3), 87.0 (CMe2), 113.4 (C4); 127.6,
128.03, 128.07, 128.4, 128.6, 129.6, 130.1, 131.8
(Carom); 178.5 (C5), 205.4 (C=O) [7].
General procedure for photochemical reactions.
The reactions were carried out in a quartz cell under
irradiation with a Hanau S81 medium-pressure mer-
cury lamp (λ > 210 nm, 100–130 W). A solution of
0.43 g (1.5 mmol) of diazo ketone Ia or Ib and
15 mmol of the corresponding nucleophile in 10 ml of
THF was irradiated with UV light at 20–25°C over
a period of 1.5–2 h. Samples were withdrawn from the
reaction mixture at definite time intervals for kinetic
measurements, the solvent and nucleophile were re-
moved under reduced pressure (first at 10–15 mm and
then at 1–2 mm, 25–35°C), internal standard (methyl-
ene bromide) was added to the residue, and the mixture
was dissolved in CDCl3 and analyzed by 1H NMR. The
products were isolated by recrystallization (compound
II) and/or by column chromatography (III–VI, VIII)
2,2-Dimethyl-5,5-diphenyltetrahydrofuran-3-one
(Va). Colorless crystals, mp 61–62°C (from petroleum
ether), Rf 0.56 (petroleum ether–ethyl acetate, 5:1).
1H NMR spectrum, δ, ppm: 1.21 s (6H, CH3), 3.32 s
(2H, CH2), 7.20–7.28 m (2H, Harom), 7.29–7.32 m (4H,
Harom), 7.39–7.42 m (4H, Harom). 13C NMR spectrum,
δC, ppm: 25.3 (CH3), 48.1 (CH2), 81.4 (C2), 82.7 (C5);
126.0, 127.4, 128.3 (6Carom); 146.1 (2Carom) [7].
Benzophenone (VI). mp 48–49°C (from petroleum
ether) [8], Rf 0.67 (petroleum ether–MeOBu-t, 6:1).
1H NMR spectrum, δ, ppm: 7.46–7.51 t (4H, Harom),
7.57–7.62 t (2H, Harom), 7.79–7.82 d (4H, Harom).
13C NMR spectrum, δC, ppm: 128.2, 129.9, 132.3
(10CHarom); 137.6 (2Carom); 196.5 (C=O). Mass spec-
trum, m/z (Irel, %): 182 (48) [M]+, 105 (98), 77 (52).
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PHOTOLYSIS OF REGIOISOMERIC α,α-DIPHENYL-SUBSTITUTED DIAZOTETRAHYDRO...
1545
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1
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