The diastereoselectivity of the Paternò-Büchi reaction between 2,3-dihydrofuran and aromatic carbonyl compounds in organized medium

Article abstract of DOI:10.1016/j.jphotochem.2016.05.001

The Paternò-Büchi reaction between benzaldehyde and 2,3-dihydrofuran can be performed within the cavity of NaY zeolite. An increased stereoselectivity has been observed. When the reaction was performed with substituted benzaldehyde derivatives, different

Full text of DOI:10.1016/j.jphotochem.2016.05.001

Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 69–75
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The diastereoselectivity of the Paternò-Büchi reaction between
2,3-dihydrofuran and aromatic carbonyl compounds in organized
medium
*
Maurizio D’Auria , Francesco Pellegrino, Licia Viggiani
Dipartimento di Scienze, Università della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy
A R T I C L E I N F O
A B S T R A C T
Article history:
The Paternò-Büchi reaction between benzaldehyde and 2,3-dihydrofuran can be performed within the
cavity of NaY zeolite. An increased stereoselectivity has been observed. When the reaction was
performed with substituted benzaldehyde derivatives, different stereoselectivity has been observed. The
stereoselectivity can be understood assuming an interaction in the biradical intermediate between the
HSOMO and the LSOMO.
Received 24 March 2016
Accepted 1 May 2016
Available online 4 May 2016
Keywords:
Dihydrofuran
Aromatic aldehydes
Paternò-Büchi reaction
DFT
ã 2016 Elsevier B.V. All rights reserved.
1. Introduction
The endo stereoselectivity observed when benzaldehyde was
used as substrate (Scheme 2) has been the object of several studies.
When Paternò and Chieffi in 1909 reported the photochemical
reaction of amylene with benzaldehyde they considered this
reaction as the first significant use of a photochemical reaction in
the organic synthesis [1,2]. The success of this reaction is well
documented by the large number of reviews on this argument
[3–21]. A lot of efforts has been performed in order to obtain
stereoselective reactions. These efforts are motivated by the need
to use this type of reactions in the synthesis of bioactive
compounds and to the need of perform photochemical reactions
on the basis of the principle of the atom economy in green
chemistry.
One of the approach able to give a stereoselective reaction is
connected with the use of zeolites when the photochemical
reaction occurs within their cavity [22,23]. This approach has been
successfully used in the case of intramolecular photorearrange-
ments while few examples are reported in the case of the Paternò-
Büchi reaction [24,25].
The reaction showed a de = 76% [29–37].
The explanation of this observed stereoselectivity can involve
steric interactions in the triplet state biradical [29–37] or the
possible interaction between the radical carbon atoms in the
biradical intermediate [38].
Recently, an application of this reaction to the synthesis of
(+)-goniofufurone, a compound isolated from Goniothalamus trees
[39,40] and showing interesting anticancer properties [41–43], has
been reported [44]. In this contest, the improvement of the
stereoselectivity of the reaction can be an interesting synthetic
target. Zeolites have been used in order to improve the stereo-
selectivity of a photochemical reaction [22,23]. However, in our
knowledge, the photochemical reaction of 2,3-dihydrofuran with
benzaldehyde has not been studied within a zeolite, and in this
article we want to report the results of a study of this reaction.
2. Materials and methods
In 1967 Ogata and coworkers reported that the irradiation of
2,3-dihydrofuran (1) in the presence of benzophenone in benzene
gave the corresponding adduct 2 (Scheme 1) [26–28]. The reaction
worked also in the presence of acetone and acetaldehyde.
Dichloromethane was used after distillation on CaH2. GC–MS
analyses were performed on Hewlett-Packard 6890 gaschromato-
graph equipped with a Hewlett-Packard 5973 mass spectrometer.
1H NMR were determined in CDCl3 using tetramethylsilane as an
internal standard. They were obtained by using a Varian INOVA
500 spectrometer at 500 MHz.
Enantiomeric excesses were determined on a HPLC apparatus
Jasco PU-1580 with a Varian 2550 UV–vis detector and a chiral
HPLC column (Chiralcel OD).
* Corresponding author.
E-mail address: maurizio.dauria@unibas.it (M. D’Auria).
http://dx.doi.org/10.1016/j.jphotochem.2016.05.001
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

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M. D’Auria et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 69–75
zeolite with absorbed benzophenone and (À)-ephedrine, benzene
(80 ml) and 2,3-dihydrofuran (4.75 mmol) were added. The
mixture were irradiated for 12 h. The solvent was evaporated
and the reaction mixture was chromatographed on a semi-
preparative TLC eluting with 7:3 hexanes/ethyl acetate. Yield: 62%.
2.4. Reaction of benzaldehyde with 2,3-dihydrofuran
Scheme 1. The Paternò-Büchi reaction between 2,3-dihydrofuran and benzophe-
none.
The reaction was performed in a quartz test tube by using
benzaldehyde (0.108 mmol) and 2,3-dihydrofuran (1.31 mmol) in
benzene (6 ml). The irradiation was stopped after four hours. The
solvent was evaporated and the reaction mixture was chromato-
graphed on a semipreparative TLC eluting with 75:25 hexanes/
ethyl acetate. Yield: 84%. Endo isomer: 1H NMR (500 MHz, CDCl3)
d:
7.2–7.4 (m, 5 H), 6.37 (m, 1 H), 5.53 (dd, J1 = J2 = 5 Hz, 1 H), 5.06 (dd,
J1 = J2 = 5 Hz, 1 H), 3.98 (m, 2 H), 2.11 (dd, J1 = 17 Hz, J2 = 7 Hz, 1 H),
and 1.32 ppm (d, J = 7 Hz, 1 H); MS, m/z (%): 174 (4), 150 (3), 120 (9),
91 (36), 77 (14), 70 (100), 65 (7), 51 (12), 42 (49). Exo isomer: 1H
NMR (500 MHz, CDCl3) d: 7.5–7.3 (m, 5 H), 6.37 (m, 1 H), 5.52 (s,
Scheme 2. Paternò-Büchi reaction of 2,3-dihydrofuran and benzaldehyde.
1 H), 4.68 (s,1 H), 4.15 (m, 2 H), 2.07 (m,1 H), and 1.31 ppm (m,1 H);
MS, m/z (%): 174 (4), 150 (3), 120 (9), 91 (36), 77 (14), 70 (100), 65
(7), 51 (12), 42 (49).
The photochemical reactions were performed in an immersion
apparatus with a 125 W high pressure mercury arc (Helios-
Italquartz) surrounded by a Pyrex water jacket.
2.5. Reaction of benzaldehyde with 2,3-dihydrofuran in the presence of
zeolite
NaY zeolite (6 g) was maintained at 500 ꢀC for 3 h. After this
period, zeolite was suspended in 4:1 hexane/dichloromethane
(80 ml) and benzaldehyde (0.59 mmol) was added. The mixture
was stirred for 7 h. Then, the mixture was filtered under reduced
pressure and the residue was washed with hexane. To the zeolite
with absorbed benzaldehyde benzene (80 ml) and 2,3-dihydro-
furan (5.90 mmol) were added. The mixture were irradiated for
12 h. The solvent was evaporated and the reaction mixture was
chromatographed on silica gel eluting with 7:3 hexanes/ethyl
acetate. Yield: 74%.
2.1. Reaction of benzophenone with 2,3-dihydrofuran
The reaction was performed in a quartz test tube by using
benzophenone (19.8 mg, 0.108 mmol) and 2,3-dihydrofuran
(1.08 mmol) in benzene (6 ml). The irradiation was stopped after
7 h. The solvent was evaporated and the reaction mixture was
chromatographed on
75:25 hexanes/ethyl acetate. Yield: 82%. 1H NMR (500 MHz, CDCl3)
: 7.5–7.3 (m, 10 H), 5.46 (dd, J1 = J2 = 4.5 Hz, 1 H), 5.12 (d, J = 4.5 Hz,
a semipreparative TLC eluting with
d
1 H), 4.00 (t, J = 6 Hz, 2 H), and 1.37 ppm (d, J = 6 Hz, 2 H); MS, m/z
(%): 252 (4), 183 (100), 165 (23), 152 (12), 105 (55), 77 (37), 70 (74),
and 42 (35).
2.6. Reaction of 4-chlorobenzaldehyde and 2,3-dihydrofuran
The reaction was performed in a quartz test tube by using 4-
chlorobenzaldehyde (15 mg, 0.108 mmol) and 2,3-dihydrofuran
(1.31 mmol) in benzene (6 ml). The irradiation was stopped after
12 h. The solvent was evaporated and the reaction mixture was
2.2. Reaction of benzophenone with 2,3-dihydrofuran in the presence
of zeolite
NaY zeolite (1.02 g) was maintained at 500 ꢀC for 3 h. After this
period, zeolite was suspended in 4:1 hexane/dichloromethane
(10 ml) and benzophenone (0.10 mmol) was added. The mixture
was stirred overnight. Then, the mixture was filtered under
reduced pressure and the residue was washed with hexane. To the
zeolite with absorbed benzophenone, benzene (5 ml) and 2,3-
dihydrofuran (1.00 mmol) were added. The mixture were irradiat-
ed for 48 h. The solvent was evaporated and the reaction mixture
was chromatographed on silica gel eluting with 7:3 hexanes/ethyl
acetate. Yield: 65%.
chromatographed on
75:25 hexanes/ethyl acetate. Yield: 45%. Endo isomer: 1H NMR
(500 MHz, CDCl3) : 7.3–7.2 (m, 4 H), 6.14 (m, 1 H), 5.49 (dd,
a semipreparative TLC eluting with
d
J1 = J2 = 4 Hz, 1 H), 5.01 (dd, J1 = J2 = 4 Hz, 1 H), 4.30 (m, 1 H), 3.64 (m,
1 H), 2.08 (dd, J1 = 14 Hz, J2 = 6 Hz, 1 H), and 1.63 ppm (m, 1 H). Exo
isomer: 1H NMR (500 MHz, CDCl3)
d: 7.3–7.2 (m, 4 H), 6.14 (m, 1 H),
5.42 (dd, J1 = J2 = 4 Hz,1 H), 4.53 (dd, J1 = J2 = 4 Hz), 4.30 (m,1 H), 3.64
(m, 1 H), 2.17 (dd, J1 = 14 Hz, J2 = 6 Hz, 1H), 1.63 (m, 1H).
2.7. Reaction of 4-chlorobenzaldehyde with 2,3-dihydrofuran in the
presence of zeolite
2.3. Reaction of benzophenone with 2,3-dihydrofuran in the presence
of zeolite and (À)-ephedrine
NaY zeolite (6 g) was maintained at 500 ꢀC for 3 h. After this
period, zeolite was suspended in 4:1 hexane/dichloromethane
(80 ml) and 4-chlorobenzaldehyde (84 mg, 0.59 mmol) was added.
The mixture was stirred overnight. Then, the mixture was filtered
under reduced pressure and the residue was washed with hexane.
To the zeolite with absorbed 4-chlorobenzaldehyde benzene
(80 ml) and 2,3-dihydrofuran (5.90 mmol) were added. The
mixture were irradiated for 12 h. The solvent was evaporated
and the reaction mixture was chromatographed on silica gel
eluting with 7:3 hexanes/ethyl acetate. Yield: 36%.
NaY zeolite (6 g) was maintained at 500 ꢀC for 3 h. After this
period, zeolite was suspended in 4:1 hexane/dichloromethane
(75 ml) and (À)-ephedrine (634 mg, 3.84 mmol) wad added. The
mixture was stirred overnight. Then, the mixture was filtered
under reduced pressure and the residue was washed with hexane.
The zeolite was suspended in 1:4 hexane/dichloromethane (60 ml)
and benzophenone (86.5 mg, 0.47 mmol) was added. The mixture
was stirred overnight. Then, the mixture was filtered under
reduced pressure and the residue was washed with hexane. To the

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71
2.8. Reaction of pyridine-3-carbaldehyde and 2,3-dihydrofuran
reaction was performed in the presence of (À)-ephedrine a very
low enantiomeric excess was observed (ee = 11%).
The reaction was performed in a quartz test tube by using
pyridine-3-carbaldehyde (0.106 mmol) and 2,3-dihydrofuran
(1.07 mmol) in benzene (6 ml). The irradiation was stopped after
12 h. The solvent was evaporated and the reaction mixture was
chromatographed on silica gel eluting with 8:2 hexanes/ethyl
When an aromatic aldehyde was used, we observed the
formation of three chiral carbon atoms. The reaction of benzalde-
hyde with 2,3-dihydrofuran gave a mixture of diastereoisomers at
C-7 (Scheme 2) with 88:12 endo-exo ratio (de = 76%). The
diastereoisomeric excess has been estimated on the basis of the
observed 1H NMR spectrum (Fig. 1). Our results confirmed the
results described in the literature [29–37].
The irradiation in the presence of NaY zeolite gave the
corresponding mixture of the oxetanes 3 and 4 in 95:5 ratio
(de = 90%). Also in this case, the corresponding pinacol, usually
obtained during the irradiation of benzaldehyde, was not observed.
The presence of (À)-ephedrine in the reaction cavity induced an
increased stereoselectivity: in fact, in this case, an endo-exo ratio
(3/4) of 98:2 was observed (de = 96%) (Fig. 1).
We performed the reaction with aromatic aldehydes different
from benzaldehyde, in order to understand if an electronic effect
can be responsible of the observed diastereoselectivity. At this
purpose, we performed the reaction in the presence of aromatic
aldehydes bearing both an electron withdrawing substituent (–Cl)
and an electron donating one (-OCH3). The reaction of
4-chlorobenzaldehyde (5a) with 2,3-dihydrofuran in benzene
solution gave the corresponding oxetanes 6a and 7a showing a
98:2 endo-exo ratio (de = 96%) (Scheme 3). When the same
aldehyde reacted with 2,3-dihydrofuran within NaY the observed
dastereoisomeric ratio decreased to 95:5 (de = 90%). The reaction of
4-methoxybenzaldehyde (5b) with 2,3-dihydrofuran in benzene
solution gave the corresponding oxetanes 6b and 7b with a
de = 70% (85:15 endo-exo ratio) (Scheme 3). The same reaction,
when performed within NaY, gave a de = 76% (88:12 endo-exo
ratio).
acetate. Yield: 80%. Endo isomer: 1H NMR (500 MHz, CDCl3)
d: 8.53
(m, 2 H), 7.60 (m, 1 H), 7.31 (m, 1 H), 6.13 (m, 1 H), 5.58 (dd,
J1 = J2 = 4 Hz, 1 H), 5.10 (dd, J1 = J2 = 4 Hz, 1 H), 4.40 (m, 1 H), 3.67 (m,
1 H), 2.15 (m, 1 H) and 1.71 ppm (m, 1 H). Exo isomer: 1H NMR
(500 MHz, CDCl3) d: 8.53 (m, 2 H), 7.60 (m, 1 H), 7.31 (m, 1 H), 6.13
(m, 1 H), 5.55 (s, 1 H), 4.66 (s, 1 H), 4.39 (m, 1 H), 3.67 (m, 1 H), 2.25
(m, 1 H), and 1.71 ppm (m, 1 H).
2.9. Reaction of pyridine-3-carbaldehyde with 2,3-dihydrofuran in the
presence of zeolite
NaY zeolite (6 g) was maintained at 500 ꢀC for 3 h. After this
period, zeolite was suspended in 4:1 hexane/dichloromethane
(80 ml) and pyridine-3-carbaldyde (0.59 mmol) was added. The
mixture was stirred overnight. Then, the mixture was filtered
under reduced pressure and the residue was washed with hexane.
To the zeolite with absorbed pyridine-3-carbadehyde, benzene
(80 ml) and 2,3-dihydrofuran (5.94 mmol) were added. The
mixture were irradiated for 12 h. The solvent was evaporated
and the reaction mixture was chromatographed on silica gel
eluting with 7:3 hexanes/ethyl acetate. Yield: 76%.
2.10. Reaction of p-anisaldehyde and 2,3-dihydrofuran
The reaction was performed in a quartz test tube by using
p-anisaldehyde (0.107 mmol) and 2,3-dihydrofuran (1.07 mmol) in
benzene (6 ml). The irradiation was stopped after 12 h. The solvent
was evaporated and the reaction mixture was chromatographed on
silica gel eluting with 8:2 hexanes/ethyl acetate. Yield: 76%. Endo
Finally, we tested the possibility to use this methodology by
using an aromatic heterocyclic aldehyde, such as pyridine-3-
carbaldehyde (8). The reaction between pyridine-3-carbaldehyde
isomer: 1H NMR (500 MHz, CDCl3)
d: 7.24 (m, 2 H), 6.91 (m, 2 H),
6.12 (m, 1 H), 5.5 (dd, J1 = J2 = 4 Hz, 1 H), 5.03 (dd, J1 = J2 = 4 Hz, 1 H),
4.02 (m, 1 H), 3.85 (m, 1 H), 3.81(s, 3 H), 2.15 (m, 1 H), and 1.69 ppm
(m, 1 H). Exo isomer: 1H NMR (500 MHz, CDCl3)
d: 7.24 (m, 2 H),
6.91 (m, 2 H), 6.12 (m, 1 H), 5.49 (s, 1 H), 4.63 (s, 1 H), 4.02 (m, 1 H),
3.85 (m, 1 H), 3.81 (s, 3 H), 2.22 (m, 1 H), and 1.69 ppm (m, 1 H).
2.11. Reaction of p-anisaldehyde with 2,3-dihydrofuran in the
presence of zeolite
NaY zeolite (6 g) were maintained at 500 ꢀC for 3 h. After this
period, zeolite was suspended in 4:1 hexane/dichloromethane
(80 ml) and p-anisaldehyde (0.591 mmol) was added. The mixture
was stirred overnight. Then, the mixture was filtered under
reduced pressure and the residue was washed with hexane. To the
zeolite with absorbed p-anisaldehyde, benzene (80 ml) and 2,3-
dihydrofuran (5.94 mmol) were added. The mixture were irradiat-
ed for 12 h. The solvent was evaporated and the reaction mixture
was chromatographed on silica gel eluting with 7:3 hexanes/ethyl
acetate. Yield: 75%.
3. Results and discussion
The reaction of 1 with benzophenone gave the oxetane 2 in very
good yields (82%) (Scheme 1). The reaction can be performed also
in the presence of NaY zeolite. In this case, the same reaction
product can be obtained in lower yields. However, it is noteworthy
that benzopinacol, a compound usually observed when benzo-
phenone was irradiated, in this case was not observed. When the
Fig. 1. Section of the 1H NMR spectrum of the product of the reaction between
benzaldehyde and 2,3-dihydrofuran: a) reaction in benzene solution; b) reaction in
the presence of NaY zeolite.

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endo
exo
exo
ndo
e
Ph
Ph
H
H
O
O
O
O
H
H
H
CH
O
Ph
CH
3
O
3
O
O
Ph
H
12
13
14
15
Scheme 4. The possible conformations of the biradical intermediate in the reaction
between benzaldehyde and 2,3-dihydrofuran.
withdrawing properties of the nitrogen atom in the pyridine ring
should induce a reduced stability of the biradical intermediate and,
then, an increased stereoselectivity, but the observed de (lower
than that observed when benzaldehyde was used) does not
confirm the hypothesis.
Finally, a fundamental problem was found in the hypothesis
described in the Scheme 4. We tried to calculate the energy of the
conformers 13 and 14. The calculations were performed at DFT/
B3LYP/6-31G+(d,p) level of theory on Gaussian 09 [45]. Conformers
13,14, and 15 did not exist: only the conformer 12 represents a
stable structure. Hereupon, it is impossible to justify the formation
of exo isomer by using the above described theory.
Scheme 3. Paternò-Büchi reaction between 2,3-dihydrofuran and some aromatic
aldehydes.
and 2,3-dihydrofuran in benzene solution gave the corresponding
oxetanes 9 and 10 in 82:18 endo-exo ratio (de = 64%) (Scheme 3).
The photochemical reaction between 1 and 8 within NaY gave the
corresponding oxetanes in 96:4 endo-exo ratio (de = 92%).
The endo-exo selectivity can be explained also by considering
the coupling between the HSOMO and the LSOMO in the biradical
intermediate [38]. The HSOMO was mainly localized on the
aromatic ring and it is extended to the radical carbon. The LSOMO
was mainly localized on the dihydrofuran ring. The coupling
between the radical carbons in these two orbitals was possible (the
atomic orbitals involved can superimpose themselves) only if the
endo isomer was formed (Fig. 3).
This hypothesis shows an important limitation. It cannot justify
the formation of the exo isomer. However, we can suppose that the
exo isomer was obtained on the basis of a different mechanism,
involving an electron transfer process. In fact, electron transfer
processes have been reported in Paternò-Büchi reactions on
electron-rich olefins [46]. In this case, the zwitterionic intermedi-
ate 16 is expected in the reaction of benzaldehyde with
2,3-dihydrofuran (Fig. 4). For the discussion of this hypothesis
see below.
The coupling between the radical carbon atoms is more efficient
the closer are the energies of the orbitals involved. Fig. 5 reports
the energies of LSOMOs and HSOMOs of the biradical intermedi-
ates involved in the reaction of benzaldehyde, p-chlorobenzalde-
hyde, and p-methoxybenzaldehyde with 2,3-dihydrofuran. The
calculations have been performed at DFT/B3LYP/6-31G+(d,p) level
of theory on Gaussian 09 [45].
It is evident that the presence of the chlorine substituent
decreases both the HSOMO and the LSOMO: however, the energy
difference between HSOMO and LSOMO was 0.02395 a.u.
(15.0 kcal molÀ1) in the chlorine substituted biradical intermediate
while, in the case of the hydrogen substituted biradical interme-
diate, we found an energy difference of 0.02614 a.u. (16.4 kcal
These results need
a discussion. The oxetanes 3 and 4
(Scheme 2) are supposed to be formed through the attack of
triplet excited state of the aromatic carbonyl compound on the
double bond of 2,3-dihydrofuran to give the corresponding
biradical intermediate 11 (Fig. 2). For the pronounced endo
selectivity in the reaction between aromatic aldehydes and 2,3-
dihydrofuran, a first hypothesis considers the two biradical
conformers 12 and 13 to be responsible, with the alkyloxy
substitutent localized in a pseudoequatorial position and 12 being
more populated because of fewer steric interactions (Scheme 4)
[29–37].
It is not simple to discuss the observed electronic effects on the
basis of the Scheme 4. We can suppose that the presence of an
electron withdrawing group in the biradical intermediate, such as
in the biradical obtained by using the aldehyde 5a, could reduce
the stability of the biradical inducing its fast ring closure reaction.
The decreased stability could prohibit the rotation along the OÀÀC
bond, inhibiting the possible equilibration of the conformers, and,
then, the formation of the exo isomer from the less stable
conformer 13. On the contrary, the same hypothesis could justify
the observed behavior of the aldehyde 5b. In fact, in this case, the
effect of the electron donating group could stabilize the biradical
intermediate, allowing it to rotate. The observed lower diaster-
eoselectivity could be in agreement with this observation.
However, this hypothesis was not confirmed by the use in the
reaction of the aldehyde 8. In fact, also in this case, the electron
Ph
O
Ph
H
Ph
O
O
H
Ph
O
O
O
H
H
O
11
O
Fig. 2. The biradical intermediate in the reaction of benzaldehyde with 2,3-
Fig. 3. Ring closure reaction in the formation of the endo isomer of the adduct
dihydrofuran.
between 2,3-dihydrofuran and benzaldehyde.

M. D’Auria et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 69–75
73
Ph
a)
O
H
O
b)
16
Scheme 5. a: radical reaction; b: electron transfer reaction.
Fig. 4. Zwitterionic intermediate in the reaction between benzaldehyde and 2,3-
dihydrofuran.
Fig. 5. HSOMO and LSOMO of the biradical intermediates involved in the reaction of benzaldehyde, 4-chlorobenzaldehyde, and 4-methoxybenzaldehyde with 2,3-
dihydrofuran.
molÀ1). The lower energy difference can induce a faster reaction
between the radical carbon atoms. In the case of the methoxy
substituted biradical intermediate, the presence of the substituent
increased the energy of both HSOMO and LSOMO: in this case, the
energy difference increased to 0.03401 a.u. (21.3 kcal molÀ1) in
agreement with the observed lower stereoselectivity.
Assuming that an electron transfer process was responsible of
the formation of the exo isomer, we performed some calculations
on the possible zwitterionic intermediate 16. The calculations were
performed at the same level of theory reported above. We have
found that 16 is less stable than the biradical intermediate 11 for
23 kcal molÀ1, in agreement with the observed diastereoisomeric
excess. Furthermore, the analysis of the HOMO and LUMO orbitals
in 16 showed that they allowed the formation of the exo isomer, in
agreement with the experimental results (Fig. 6).
reaction in the biradical intermediate (k-1) or to an increased rate of
the cyclization reaction (k2). In the second case, a reduced rate of
the electron transfer reaction can depend on the efficiency of the
back electron transfer reaction or on the reduced possibility to
have this type of reaction.
In the zeolites the cavity does not allow the conformational
equilibrium [25]. However, in our opinion, the conformational
equilibrium is not so relevant in this case, where we have not
several possible conformations of the biradical intermediate.
However, the reduced mobility of the reagents within the cavity of
the zeolite can reduce the rate of retrocleavage reaction of the
biradical intermediate (k-1).
It is known that zeolites accelerate the intersystem crossing
from the excited singlet state [47,48]. However, we used aromatic
carbonyl compounds with intersystem crossing of ca. 1. Therefore,
we think that this described effect of zeolites cannot be useful in
this case.
In the supercage of the zeolite the carbonyl compound is
complexed with the cation [23,49]. The formation of this complex
can modify the energy of the HSOMO and LSOMO in the biradical
intermediates changing the rate of the ring closure reaction (k2 in
Scheme 5). We optimized at the same level of theory described
above the structure of the complex obtained between the biradical
intermediate and one sodium cation. The results are reported in
Table 1. The formation of the complex between the biradical
intermediate and sodium cation in the framework of the zeolites
reduces the energy gap between HSOMO and LSOMO allowing to
have a more efficient coupling between these orbitals. These
results can explain the increased stereoselectivity observed by
using benzaldehyde and p-methoxybenzaldehyde. However, the
When the reaction is performed within the zeolite, assuming
the hypothesis described above, an enhanced diastereoselectivity
has to be due to an increased rate of the radical reaction (Scheme 5)
or to a decreased rate of the electron transfer reaction (Scheme 5).
In the first case, the result should be due to a reduced rate of back
Fig. 6. Ring closure reaction in the formation of the exo isomer of the adduct
between 2,3-dihydrofuran and benzaldehyde through a zwitterionic intermediate.

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M. D’Auria et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 69–75
Table 1
LSOMO-HSOMO energy difference in the biradical intermediate.
Aldehyde
LSOMO-HSOMO [a.u]
Biradical intermediate
Biradical intermediate-Na+ complex
Benzaldehyde
p-Chlorobenzaldehyde
p-Methoxybenzaldehyde
0.02614
0.02395
0.03401
0.01620
0.00748
0.02606
decreased stereoselectivity observed when p-chlorobenzaldehyde
was used cannot be explained.
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