Intramolecular photo-cyclization and consecutive rearrangement reactions of diazo-functionalized olefin-esters

Article abstract of DOI:10.1016/j.jphotochem.2012.12.005

A study on intramolecular photo-cyclization reactions with diazo-functionalized olefin-esters is presented. On irradiation of diphenyl diazo methane esters of cinnamic acids, not only the expected addition to the CC double bond is observed, yielding cyclopropanes, but also the addition to the carboxylic CO double bond is postulated as an intermediate. These formed intermediates undergo a rearrangement to cyclobutanones which further rearrange photo-assisted to oxetanes. Both were isolated and characterized. The reaction progress, the impact of the irradiation wavelength and solvent as well as the influence of the electron density of the olefin on the product distribution is described. Also, the detailed reaction mechanism for the photo-reaction cascade is discussed.

Full text of DOI:10.1016/j.jphotochem.2012.12.005

Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Intramolecular photo-cyclization and consecutive rearrangement reactions of
diazo-functionalized olefin-esters
Magnus R. Buchnera, Bernhard Wahlb, Klaus Ruhlandc,∗
a WACKER-Institut für Siliciumchemie, Lehrstuhl für Anorganische Chemie, Technische Universität München, Lichtenbergstr. 4, 85747 Garching bei München, Germany
b Lehrstuhl für Anorganische Chemie mit Schwerpunkt Neue Materialien, Technische Universität München, Lichtenbergstr. 4, 85747 Garching bei München, Germany
c Lehrstuhl für Chemische Physik und Materialwissenschaften, Universität Augsburg, Universitätsstr. 1, 86135 Augsburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A study on intramolecular photo-cyclization reactions with diazo-functionalized olefin-esters is pre-
sented. On irradiation of diphenyl diazo methane esters of cinnamic acids, not only the expected addition
Received 24 August 2012
Received in revised form 3 December 2012
Accepted 3 December 2012
Available online xxx
to the C C double bond is observed, yielding cyclopropanes, but also the addition to the carboxylic C
O
double bond is postulated as an intermediate. These formed intermediates undergo a rearrangement to
cyclobutanones which further rearrange photo-assisted to oxetanes.
Both were isolated and characterized. The reaction progress, the impact of the irradiation wavelength
and solvent as well as the influence of the electron density of the olefin on the product distribution is
described. Also, the detailed reaction mechanism for the photo-reaction cascade is discussed.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Photo-cyclization
Carbene
Ketene
Oxetane
1. Introduction
olefin electron density. Thus, we irradiated donor- and acceptor-
substituted diphenyl diazomethane olefin esters.
In many important natural products and biologically active
substances cyclopropane is a key building block [1]. Also, cyclo-
propanes are of great importance for organic synthesis in general
[2]. However, the formation of cyclopropanes is often performed
via cyclopropanation of olefins with less substituted diazo com-
pounds, rendering the method ineffective for total synthesis. This
problem has been overcome by the utilization of intramolecular
cyclopropanation reactions [3].
To broaden the reaction repertoire of intramolecular cyclo-
propanation we studied the photo-reactivity of a series of diphenyl
diazomethane olefin esters 1-R (Scheme 1). We anticipated the
addition of an in situ generated carbene to the olefinic double
bond with the formation of highly substituted cyclopropanes with
annellated six-membered lactones 2-R (left hand side; Scheme 1).
Interestingly, the reaction did not proceed cleanly to the desired
product but also yielded in cyclobutanones 3-R which subsequently
rearranged to oxetanes 4-R (right hand side; Scheme 1). To the best
of our knowledge, this kind of side reaction has not been observed
before. We therefore decided to study this finding in more detail
and to examine the product distribution in dependence on the
2. Materials and methods
2.1. General procedures
All manipulations and experiments were performed under
argon using standard Schlenck techniques and in a glove box filled
with argon unless otherwise stated. Pentane and dichloromethane
were dried and degassed using a two-column drying system
(MBraun) [4], diethyl ether and toluene were distilled from sodium.
All solvents were stored under an argon atmosphere. Deuterated
solvents were used as received from Deutero GmbH. Cinnamic acid,
4-toluenesulfonhydrazide, 4-toluenesulfonic acid monohydrate
and sodium hydride were purchased from Merck, 4-methoxy-
cinnamic acid, 4-methyl-cinnamic acid and 4-nitro-cinnamic acid
from Sigma–Aldrich and used without further purification. The acid
chlorides were synthesized according to literature procedures [5].
Silica with a particle size of 40–63 ␮m and technical grade solvents
were used for flash-chromatography. Column diameter and filling
height were chosen according to Still et al. [6] 1H NMR and 13C
NMR measurements were performed on a Bruker Avance III 400.
1H NMR (400 MHz) and 13C NMR (100 MHz) chemical shifts are
given relative to the solvent signal for CDCl3 (7.26 and 77.2) [7],
ESI-MS was conducted on a Finnigan LCQ in acetonitrile. Micro
analytical analysis was performed in the micro analytical lab of
the Technische Universität München. FT-IR was carried out on a
∗
Corresponding author. Tel.: +49 821 598 3361; fax: +49 821 598 3227.
E-mail address: klaus.ruhland@physik.uni-augsburg.de (K. Ruhland).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.12.005

184
M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
Scheme 1. Products of the photocyclization of diazo-compound 1-R. R Me (Me), p-C6H4OMe (PhOMe), p-C6H4Me (PhMe), Ph (Ph), p-C6H4NO2 (PhNO2).
Jasco FT/IR-460 Plus spectrometer. UV/vis spectra were recorded
on a Jasco V-550 spectrophotometer using quartz cuvettes. X-ray
single crystal parameters were obtained the following: The single
crystals were stored under perfluorinated oil, transferred into a Lin-
demann capillary, fixed and sealed. Preliminary examination and
data collection were carried out on an area detecting system (APEX
II, k-CCD) at the window of a rotating anode (Bruker AXS, FR591)
C17H14O3 (266.29 g/mol): calcd.: C 76.68, H 5.30 found: C 76.40,
H 5.10.
2-benzophenyl 4-methoxycinnamate (0-PhOMe): light yellow
solid, yield: 83%, route: Ester-I, extraction with an Et2O/CH2Cl2
mixture (1:1). Single crystals suitable for X-ray analysis were
obtained by slow evaporation of a saturated ethanol solution. 1H
3
NMR (CDCl3): ı = 3.81 (s, 3H, OCH3), 6.15 (d, JHH = 15.9 Hz, 1H,
3
˚
and graphite monochromated MoKa radiation (l = 0.71073 A). Raw
CHCHCO), 6.87 (d, JHH = 8.8 Hz, 2H, HAr), 7.23–7.63 (m, 10H, HAr
,
data were corrected for Lorentz, polarization, and, arising from
scaling procedure, for latent decay and absorption effects. The
structures were solved by a combination of direct methods and
difference Fourier syntheses. All non-hydrogen atoms were refined
with anisotropic displacement parameters, whereas all hydrogen
atoms were refined with isotropic displacement parameters. Full-
matrix least-square refinements were carried out by minimizing
CArCHCH), 7.80 (d, 3JHH = 7.1 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 55.5
(OCH3), 113.9 (CHCHCO), 114.4 (CarH), 123.3 (CArH), 125.6 (CAr),
126.8 (CArH), 128.4 (CArH), 129.9 (CArH), 130.1 (CArH), 130.4 (CArH),
132.0 (CAr), 132.2 (CArH), 133.1 (CArH), 137.7 (CAr), 146.5 (CArCHCH),
148.9 (CArO), 161.8 (CArOCH3), 165.1 (CHCOO), 195.1 (CarCOCAr).
ESI-MS: m/z (%): 161.2 (99) [C10H9O2+], 381.2 (100) [C23H18O4Na+],
421.6 (79) [C23H17O4H2ONa2+], 739.0 (41) [(C23H18O4)2Na+], 754.8
(6) [(C23H18O4)2K+], 869.0 (7) [(C23H18O4)2(HCOOH)(HCOO)NaK+].
elemental analysis for C23H18O4 (358.39 g/mol): calcd.: C 77.08, H
5.06 found: C 76.92, H 5.08.
P(F0 − Fc2)2 with SHELXL-97 weighting scheme [8]. The final resid-
2
ual electron density maps showed no remarkable features. Neutral
atom scattering factors for all atoms and anomalous dispersion cor-
rections for non-hydrogen atoms were taken from International
Tables for Crystallography [9].
2-benzophenyl 4-methylcinnamate (0-PhMe): yellow crys-
tals, yield: 86%, route: Ester-I. 1H NMR (CDCl3): ı = 2.36
3
(s, 3H, CH3), 6.24 (d, JHH = 16.0 Hz, 1H, CHCHCO), 7.16 (d,
3JHH = 8.0 Hz, 2H, HAr), 7.27–7.62 (m, 10H, HAr, CArCHCH), 7.80
3
(d, JHH = 7.2 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 21.6 (CH3),
2.2. General procedures for the synthesis of 2-benzophenyl esters
115.4 (CHCHCO), 123.4 (CArH), 125.7 (CAr), 128.4 (CArH), 128.5
(CArH), 129.7 (CArH), 129.9 (CArH), 130.5 (CArH), 131.4 (CArH),
132.0 (CAr), 132.3 (CArH), 133.1 (CArH), 137.7 (CAr), 141.3
(CarCH3), 146.9 (CArCHCH), 148.9 (CArO), 165.0 (CHCOO), 195.1
(CarCOCAr). ESI-MS: m/z (%): 145.2 (85) [C10H9O+], 365.2 (98)
[C23H18O3Na+], 405.7 (100) [C23H17O3H2ONa2+], 475.8 (39)
[C23H18O3(HCOOH)(HCOO)NaK+], 717.0 (41) [(C23H18O3)2Na+],
837.1 (10) [(C23H18O3)2(HCOOH)(HCOO)NaK+]. elemental analysis
for C23H18O3 (342.39 g/mol): calcd.: C 80.68, H 5.30 found: C 79.56,
H 5.54.
Ester-I: 2.49 mmol alcohol was added to 1 equiv. (2.49 mmol,
59.7 mg) of NaH in 5 ml thf at 0 ◦C and warmed to ambient tem-
perature. After 15 min 1 equiv. (2.49 mmol) of the appropriate acid
chloride was added and stirring continued over night. The reaction
was quenched with 15 ml water and extracted three times with
Et2O. The combined organic layers were washed twice with sat-
urated NaHCO3 solution and once with brine, dried over Na2SO4,
filtered and the solvent removed under reduced pressure to give
the anticipated ester.
2-benzophenyl cinnamate (0-Ph): off-white solid, yield: >99%,
Ester-II: 16.5 mmol alcohol was dissolved in 5 ml pyridine,
26.4 mmol (1.6 equiv.) of acid chloride were added and the reac-
tion mixture was heated to 100 ◦C for 2 h. The reaction mixture
was poured into a mixture of 200 g of ice and 350 ml of 0.5 m
hydrochloric acid and extracted three times with Et2O. The com-
bined organic layers were washed twice with saturated NaHCO3
solution and once with brine, dried over Na2SO4, filtered and the
solvent removed under reduced pressure to give the product.
2-benzophenyl crotonate (0-Me) 1H NMR (CDCl3): ı = 1.79
3
route: Ester-II. 1H NMR (CDCl3): ı = 6.29 (d, JHH = 16.0 Hz,
1H, CHCHCO), 7.30–7.43 (m, 9H, HAr), 7.50–7.60 (m, 4H,
3
HAr
,
CArCHCH), 7.81 (d, JHH = 7.3 Hz, 2H, HAr). 13C NMR
(CDCl3): ı = 116.6 (CHCHCO), 123.4 (CArH), 125.8 (CArH),
128.4 (CArH), 128.5 (CArH), 129.0 (CArH), 130.0 (CArH), 130.5
(CArH), 130.8 (CArH), 132.0 (CAr), 132.3 (CArH), 133.2 (CArH),
134.2 (CAr), 137.8 (CAr), 146.9 (CArCHCH), 148.9 (CArO), 164.9
(CHCOO), 195.0 (CarCOCAr). ESI-MS: m/z (%): 351.2 (100)
[C22H16O3Na+], 391.8 (50) [C22H16O3(CH3CN)Na+], 475.9 (77)
[C22H16O3(CH3CN)3Na+], 565.1 (57) [C22H16O3(CH3CN)C14H9O2+],
3
3
(d, JHH = 6.9 Hz, 3H, CH3), 5.72 (d, JHH = 14.7 Hz, 1H, CHCHCO),
3
3
6.84 (dq, JHH = 6.9 Hz, JHH = 14.7 Hz, 1H, CH3CHCH), 7.21–7.26
(m, 1H, HAr), 7.28–7.35 (m, 1H, HAr), 7.38–7.46 (m, 2H, HAr),
7.48–7.59 (m, 3H, HAr), 7.73–7.81 (m, 2H, HAr). 13C NMR (CDCl3):
ı = 18.3 (CH3), 121.5 (CArH), 123.4 (CHCHCO), 125.7 (CArH), 128.5
(CArH), 130.0 (CArH), 130.5 (CArH), 132.1 (CAr), 132.3 (CArH), 133.1
(CArH), 137.8 (CAr), 147.4 (CH3CHCH), 148.9 (CArO), 164.3 (CHCOO),
195.1 (CArCOCAr). ESI-MS: m/z (%): 267.2 (17) [C17H14O3H+], 289.2
(100) [C17H14O3Na+], 306.5 (80) [C17H14O3H2ONa+], 419.1 (47)
[(C17H14O3)3HK2+], 503.1 (28) [C17H14O3(CH3CN)C13H9O2+], 543.5
(17) [C17H14O3(CH3CN)H2ONaC13H9O2+]. elemental analysis for
605.6
(85)
[C22H16O3(CH3CN)C14H9O2K+],
678.9
(19)
[(C22H16O3)2Na+]. elemental analysis for C22H16O3 (328.36 g/mol):
calcd.: C 80.47, H 4.91 found: C 80.20, H 4.86.
2-benzophenyl 4-nitrocinnamate (0-PhNO2): white solid,
yield: 85%, route: Ester-I. 1H NMR (CDCl3): ı = 6.45 (d,
3JHH = 16.0 Hz, 1H, CHCHCO), 7.31 (d, JHH = 8.1 Hz, 1H, HAr),
3
3
3
7.38 (t, JHH = 7.5 Hz, 1H, HAr), 7.43 (t, JHH = 7.6 Hz, 2H, HAr),
3
7.48–7.66 (m, 6H, HAr, CArCHCH), 7.80 (d, JHH = 7.6 Hz, 2H, HAr),
8.22 (d, JHH = 16.0 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 120.9
3

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(CHCHCO), 123.2 (CArH), 124.6 (CArH), 126.0 (CArH), 128.5 (CArH),
129.0 (CArH), 130.0 (CArH), 130.7 (CArH), 131.7 (CAr), 132.5 (CArH),
133.2 (CArH), 137.6 (CAr), 140.1 (CAr), 143.6 (CArCHCH), 148.7
(CArO), 148.8 (CArN), 164.0 (CHCOO), 194.8 (CarCOCAr). ESI-MS:
m/z (%): 176.1 (64) [C9H6NO3+], 768.8 (45) [(C22H15NO5)2Na+].
elemental analysis for C22H15NO5 (373.36 g/mol): calcd.: C 70.77,
H 4.05, N 3.75 found: C 70.61, H 4.12, N 3.64.
2-(phenyl(2-tosylhydrazono)methyl)phenyl
cinnamate
(0TH-Ph): off-white solid, yield: 53%. 1H NMR (CDCl3): ı = 2.16
3
(s, 3H, CH3), 5.98 (d, JHH = 16.0 Hz, 1H, CHCHCO), 7.17–7.47
(m, 16H, HAr, CArCHCH), 7.54–7.61 (m, 1H, HAr), 7.78 (s, 1H,
3
NH), 7.89 (d, JHH = 8.3 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 21.6
(CH3), 115.7 (CHCHCO), 123.7 (CArH), 125.8 (CAr), 127.3 (CArH),
127.5 (CArH), 128.5 (CArH), 128.6 (CArH), 129.1 (CArH), 129.5
(CArH), 130.1 (CArH), 130.3 (CArH), 131.1 (CArH), 131.8 (CArH),
133.9 (CAr), 135.6 (CAr), 136.1 (CAr), 143.9 (CAr), 147.4 (CArCHCH),
148.3 (CArO), 151.1 (CarCNCAr), 165.5 (CHCOO). ESI-MS: m/z (%):
367.1 (16) [C20H18N2O3SH+], 497.1 (21) [C29H24N2O4SH+], 519.2
(100) [C29H24N2O4SNa+], 535.2 (7) [C29H24N2O4SK+], 764.2 (10)
[(C29H24N2O4S)3HK2+], 1014.9 (19) [(C29H24N2O4S)2Na+], 1031.0
(17) [(C29H24N2O4S)2K+]. elemental analysis for C29H24N2O4S
(496.58 g/mol): calcd.: C 70.14, H 4.87, N 5.64, S 6.46 found: C
69.94, H 4.89, N 5.68, S 6.46.
2.3. General procedure for the synthesis of hydrazones
4.19 mmol carbonyl compound was dissolved in toluene,
780 mg (4.19 mmol) of tosylhydrazine and one crystal of para-
toluene sulfonic acid monohydrate added and refluxed over night.
The solvent was removed under reduced pressure and the residue
stirred in pentane and filtered off to give the product.
2-(phenyl(2-tosylhydrazono)methyl)phenyl crotonate (0TH
-
2-(phenyl(2-tosylhydrazono)methyl)phenyl
4-
Me): white solid, yield: 67%, single crystals suitable for X-ray
nitrocinnamate (0TH-PhNO2): creamy solid, yield: 61%. 1H
3
analysis were obtained by slow evaporation of a saturated ethanol
NMR (CDCl3): ı = 2.27 (s, 3H, CH3), 6.20 (d, JHH = 16.1 Hz, 1H,
solution. 1H NMR (CDCl3): ı = 1.70 (d, JHH = 6.8 Hz, 3H, CHCH3),
CHCHCO), 7.21–7.49 (m, 11H, HAr, CArCHCH), 7.53 (d, 3JHH = 8.3 Hz,
3
3
3
2.42 (s, 3H, CArCH3), 5.40 (d, JHH = 14.3 Hz, 1H, CHCHCO), 6.52
2H, HAr), 7.60 (t, JHH = 7.8 Hz, 1H, HAr), 7.73 (s, 1H, NH), 7.91 (d,
3
3
3
(dq, JHH = 7.0 Hz, JHH = 15.4 Hz, 1H, CH3CHCH), 7.15–7.46 (m,
10H, HAr), 7.50–7.60 (m, 1H, HAr), 7.73 (s, 1H, NH), 7.90 (d,
3JHH = 8.1 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 18.3 (CHCH3), 21.8
(CArCH3), 120.7 (CarH), 123.7 (CHCHCO), 125.8 (CAr), 127.3 (CArH),
127.4 (CArH), 128.4 (CArH), 128.7 (CArH), 129.5 (CArH), 130.0
(CArH), 130.2 (CArH), 131.7 (CArH), 135.8 (CAr), 136.1 (CAr), 143.7
(CAr), 148.0 (CH3CHCH), 148.3 (CArO), 151.2 (CArCNCAr), 164.9
(CHCOO). ESI-MS: m/z (%): 367 (21) [C20H19N2O3S+], 435.1 (22)
[C24H22N2O4SH+], 457.2 (100) [C24H22N2O4SNa+]. elemental anal-
ysis for C24H22N2O4S (434.51 g/mol): calcd.: C 66.34, H 5.10, N 6.45,
S 7.38 found: C 65.83, H 5.10, N 6.44, S 7.34.
3JHH = 7.7 Hz, 2H, HAr), 8.24 (d, JHH = 8.2 Hz, 2H, HAr). 13C NMR
(CDCl3): ı = 21.7 (CH3), 120.2 (CHCHCO), 123.6 (CarH), 124.3 (CArH),
125.2 (CAr), 127.3 (CArH), 127.7 (CArH), 128.5 (CArH), 128.5 (CArH),
129.0 (CArH), 129.5 (CArH), 130.1 (CArH), 130.3 (CArH), 131.9 (CArH),
135.6 (CAr), 136.0 (CAr), 139.9 (CAr), 143.9 (CAr), 144.2 (CArCHCH),
148.0 (CArO), 148.9 (CArN), 150.6 (CArCNCAr), 164.3 (CHCOO).
ESI-MS: m/z (%): 542.2 (56) [C29H23N3O6SH+], 564.2 (100)
[C29H23N3O6SNa+], 1105.0 (47) [(C29H23N3O6S)2Na+], 1121.1
(41) [(C29H23N3O6S)2K+]. elemental analysis for C29H23N3O6S
(541.57 g/mol): calcd.: C 64.31, H 4.28, N 7.76, S 5.92 found: C
63.97, H 4.33, N 7.64, S 5.73.
2-(phenyl(2-tosylhydrazono)methyl)phenyl
4-
methoxycinnamate (0TH-PhOMe): creamy solid, yield: 86%.
2.4. General procedure for the synthesis of diazo compounds
1H NMR (CDCl3): ı = 2.21 (s, 3H, CH3), 3.85 (s, 3H, OCH3), 5.87
3
3
(d, JHH = 15.9 Hz, 1H, CHCHCO), 6.89 (d, JHH = 8.8 Hz, 2H, HAr),
7.19–7.4 (m, 9H, HAr), 7.40–7.44 (m, 3H, HAr, CArCHCH), 7.53–7.59
1.42 mmol tosyl hydrazone was added to 34.1 mg (1.42 mmol)
NaH in 10 ml thf at 0 ◦C, warmed to ambient temperature and
stirred over night. The solvent was removed in vacuo, the residue
taken up in 15 ml toluene and heated to 75 ◦C for 45 min. Again the
solvent was removed in vacuo afterwards, the residue extracted
with pentane and filtrated until the filtrate was colorless. The diazo
compounds were isolated by removal of the solvent in vacuo.
2-(diazo(phenyl)methyl)phenyl crotonate (1-Me): red oil,
yield: 43%. 1H NMR (CDCl3): ı = 1.87 (d, 3JHH = 6.9 Hz, 3H, CH3),
(m, 2H, HAr), 7.83 (s, 1H, NH), 7.90 (d, JHH = 8.3 Hz, 2H, HAr). 13C
3
NMR (CDCl3): ı = 21.6 (CH3), 55.6 (OCH3), 113.1 (CHCHCO), 114.5
(CArH), 123.7 (CArH), 125.8 (CAr), 126.7 (CAr), 127.3 (CArH), 128.0
(CArH), 128.0 (CArH), 128.4 (CArH), 128.5 (CArH), 129.4 (CArH),
130.0 (CArH), 130.2 (CArH), 131.7 (CArH), 135.7 (CAr), 136.2 (CAr),
143.7 (CAr), 147.1 (CArCHCH), 148.4 (CArO), 151.1 (CArCNCAr), 162.1
(CArOCH3), 165.8 (CHCOO). ESI-MS: m/z (%): 161.2 (14) [C10H9O2+],
367.2 (9) [C20H18N2O3SH+], 527.0 (2) [C30H26N2O5SH+], 549.3
(100) [C30H26N2O5SNa+], 565.1 (2) [C30H26N2O5SK+], 1075.0 (15)
[(C30H26N2O5S)2Na+], 1091.1 (7) [(C30H26N2O5S)2K+]. elemental
analysis for C30H26N2O5S (526.60 g/mol): calcd.: C 68.42, H 4.98, N
5.32, S 6.09 found: C 67.27, H 5.06, N 5.18, S 6.24.
3
3
5.89 (dd, JHH = 1.7 Hz, JHH = 15.5 Hz, 1H, CHCHCO), 7.00 (dq,
3JHH = 6.9 Hz, 3JHH = 15.5 Hz, 1H, CH3CHCH), 7.07–7.13 (m, 2H, HAr),
7.20–7.36 (m, 6H, HAr), 7.42–7.46 (m, 1H, HAr). 13C NMR (CDCl3):
ı = 18.4 (CH3), 121.5 (CArH), 122.3 (CArCNCAr), 123.8 (CArH), 123.9
(CHCHCO), 124.9 (CArH), 126.5 (CArH), 128.5 (CArH), 129.1 (CArH),
130.0 (CArH), 130.3 (CAr), 130.5 (CAr), 147.5 (CH3CHCH), 148.5
(CArO), 164.3 (CHCOO). FT-IR (KBr plates, cm−1): 3089 (w), 3060
(m), 3030 (w), 2925 (m), 2851 (m), 2046 (s, N2), 1970 (w), 1737 (s,
CO), 1654 (s), 1597 (m), 1500 (w), 1494 (s), 1440 (s), 1381 (w), 1299
(s), 1254 (m), 1194 (m), 1149 (m), 1090 (m), 1030 (w), 976 (s), 910
(w), 828 (w), 816 (m), 814 (w), 753 (m), 693 (m), 647 (m), 579 (m),
523 (w), 477 (m). UV/vis (pentane, nm): 208 (s, Ar), 286 (s, N2).
2-(diazo(phenyl)methyl)phenyl 4-methoxycinnamate (1-
PhOMe): red oil, yield: 49%. 1H NMR (CDCl3): ı = 3.85 (s, 3H, OCH3),
2-(phenyl(2-tosylhydrazono)methyl)phenyl
4-
methylcinnamate (0TH-PhMe): creamy solid, yield: 67%. 1H
NMR (CDCl3): ı = 2.41 (s, 3H, CH3), 2.46 (s, 3H, CH3), 6.43 (d,
3JHH = 16.0 Hz, 1H, CHCHCO), 6.63–6.76 (m, 1H, HAr), 7.02 (d,
3JHH = 8.1 Hz, 1H, HAr), 7.09–7.16 (m, 1H, HAr), 7.19–7.34 (m, 6H,
3
3
HAr, NH), 7.38 (d, JHH = 8.1 Hz, 2H, HAr), 7.48 (d, JHH = 8.0 Hz, 3H,
3
HAr), 7.57–7.60 (m, 2H, HAr), 7.80 (d, JHH = 7.2 Hz, 1H, CArCHCH),
3
7.84 (d, JHH = 8.3 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 21.5 (CH3),
21.6 (CH3), 115.9 (CHCHCO), 117.6 (CArH), 118.7 (CAr), 118.8
(CArH), 127.1 (CArH), 127.8 (CArH), 127.9 (CArH), 128.4 (CArH), 129.3
(CArH), 129.7 (CArH), 129.7 (CArH), 130.0 (CArH), 130.1 (CArH), 130.6
(CArH), 130.7 (CArH), 131.3 (CAr), 131.8 (CArH), 134.7 (CArH), 141.4
(CarCNCAr), 147.5 (CArCHCH), 158.8 (CArO), 172.2 (CHCOO). ESI-MS:
m/z (%): 145.2 (9) [C10H9O+], 367.3 (24) [C20H18N2O3SH+], 511.1
(2) [C30H26N2O4SH+], 533.3 (100) [C30H26N2O4SNa+], 549.2 (3)
[C30H26N2O4SK+], 1043.0 (12) [(C30H26N2O4S)2Na+], 1059.1 (4)
[(C30H26N2O4S)2K+].
3
3
6.32 (d, JHH = 16.0 Hz, 1H, CHCHCO), 6.91(d, JHH = 8.7 Hz, 2H,
HAr), 7.06–7.14 (m, 3H, HAr), 7.24–7.39 (m, 6H, HAr), 7.41–7.49 (m,
2H, HAr), 7.63 (d, JHH = 16.0 Hz, 1H, CArCHCH). 13C NMR (CDCl3):
3
ı = 55.6 (OCH3), 114.1 (CHCHCO), 114.5 (CArH), 122.2 (CAr), 123.8
(CArH), 123.9 (CArH), 124.9 (CArH), 126.5 (CArH), 127.0 (CAr), 128.4
(CAr), 128.5 (CArH), 129.1 (CArH), 129.9 (CArH), 130.2 (CArH), 146.6
(CArCHCH), 148.6 (CArO), 161.9 (CArOCH3), 165.2 (CHCOO). FT-IR
(KBr plates, cm−1): 3066 (m), 3036 (m), 2954 (m), 2931 (m), 2842

186
M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
´
(m), 2046 (s, N2), 1726 (s, CO), 1636 (m), 1602 (s), 1568 (w), 1512
(m), 1486 (m), 1442 (m), 1419 (w), 1307 (m), 1255 (m), 1195 (m),
1173 (w), 1128 (s), 1030 (m), 982 (m), 919 (w), 852 (w), 828 (m),
777 (m), 694 (m), 665 (w), 636 (w), 568 (w), 553 (m), 509 (m).
UV/vis (pentane, nm): 208 (s, Ar), 227 (s, Ar), 293 (s, N2).
˚
˚
(300 nm: RPR-3000 A; 419 nm: RPR-4190 A) for the stated time.
After removal of the solvent under reduced pressure the crude
product mixture was dissolved in CDCl3 and the NMR measurement
was performed.
Concentrations:
2-(diazo(phenyl)methyl)phenyl 4-methylcinnamate (1-
PhMe): red oil, yield: 25%. 1H NMR (CDCl3): ı = 2.38 (s, 3H, CH3),
1-p-anisyl: 0.0067 mol/l;
1-p-tolyl: 0.0071 mol/l;
1-phenyl: 0.0073 mol/l;
1-p-nitrophenyl: 0.0065 mol/l;
1-Me: 0.0090 mol/l.
3
6.40 (d, JHH = 16.0 Hz, 1H, CHCHCO), 7.07–7.13 (m, 3H, HAr),
3
7.20 (d, JHH = 7.8 Hz, 2H, HAr), 7.27–7.42 (m, 7H, HAr), 7.47 (d,
3
3JHH = 7.6 Hz, 2H, HAr), 7.64 (d, JHH = 16.0 Hz, 1H, CArCHCH). 13C
NMR (CDCl3): ı = 21.7 (CH3), 155.6 (CHCHCO), 122.3 (CAr), 123.8
(CArH), 123.9 (CArH), 124.9 (CArH), 126.5 (CArH), 128.5 (CArH), 129.1
(CArH), 129.8 (CArH), 130.0 (CArH), 130.5 (CAr), 131.6 (CAr), 141.4
(CAr), 147.0 (CArCHCH), 148.6 (CArO), 165.0 (CHCOO). FT-IR (KBr
plates, cm−1): 3029 (w), 2925 (s), 2850 (m), 2043 (s, N2), 1730 (s,
CO), 1687 (w), 1628 (m), 1599 (m), 1496 (m), 1449 (m), 1411 (w),
1374 (w), 1314 (w), 1269 (w), 1240 (s), 1195 (s), 1180 (s), 1142
(w), 1098 (w), 1031 (w), 987 (m), 946 (m), 919 (m), 866 (w), 815
(m), 753 (m), 697 (m), 628 (w), 499 (m), 473 (w). UV/vis (pentane,
nm): 208 (m, Ar), 222 (m, Ar), 286 (s, N2).
1-(4-Methoxyphenyl)-7b-phenyl-1,1a-
dihydrocyclopropa[c]chromen-2(7bH)-one (2-PhOMe): white
solid, Rf = 0.35. 1H NMR (CDCl3): ı = 2.80 (d, JHH = 4.8 Hz, 1H,
3
3
CHCHCO), 3.04 (d, JHH = 4.8 Hz, 1H, CArCHCH), 3.73 (s, 3H, OCH3),
6.82–7.22 (m, 9H, HAr), 7.29–7.55 (m, 6H, HAr). 13C NMR (CDCl3):
ı = 31.9 (CHCOO), 38.3 (CArCHCH), 41.8 (CArC), 55.4 (OCH3), 113.8
(CArH), 114.6 (CAr), 117.8 (CArH), 124.4 (CArH), 126.2 (CAr), 127.6
(CArH), 128.2 (CArH), 128.7 (CArH), 128.9 (CArH), 130.0 (CAr),
132.1 (CArH), 135.0 (CArH), 149.5 (CArO), 158.8 (CArO), 166.1
(CHCOO). ESI-MS: m/z (%): 313.3 (100) [C21H17O2H+], 343.3 (27)
[C23H18O3H+]. FT-IR (KBr plates, cm−1): 2953 (m), 2924 (m), 2834
(w), 1749 (s, CO), 1606 (m), 1515 (s), 1455 (m), 1249 (s), 1032 (m),
829 (w), 757 (m), 702 (m).
2-(diazo(phenyl)methyl)phenyl cinnamate (1-Ph): red oil,
3
yield: 34%. 1H NMR (CDCl3): ı = 6.44 (d, JHH = 16.0 Hz, 1H,
CHCHCO), 7.04–7.17 (m, 2H, HAr), 7.25–7.42 (m, 5H, HAr), 7.44–7.52
(m, 2H, HAr), 7.66 (d, 3JHH = 16.0 Hz, 1H, CArCHCH). 13C NMR (CDCl3):
ı = 116.8 (CHCHCO), 122.3 (CArCNCAr), 123.8 (CArH), 123.9 (CArH),
125.0 (CArH), 126.7 (CArH), 128.5 (CArH), 128.6 (CArH), 128.9 (CAr),
129.1 (CArH), 129.2 (CArH), 130.0 (CArH), 130.5 (CAr), 130.9 (CArH),
134.3 (CAr), 147.0 (CArCHCH), 148.5 (CArO), 164.9 (CHCOO). FT-IR
(KBr plates, cm−1): (m), 3029 (m), (w), 2925 (m), 2856 (w), 2045
(vs, N2), 1957 (w), 1732 (vs, CO), 1666 (w), 1634 (s), 1597 (m), 1577
(w), 1496 (s), 1449 (s), 1308 (s), 1235 (s), 1193 (s), 1133 (s), 1100
(w), 1029 (w), 979 (s), 949 (w), 945 (w), 916 (w), 860 (m), 751 (s),
693 (s), 619 (m), 577 (w), 561 (w), 473 (m). UV/vis (pentane, nm):
208 (s, Ar), 282 (s, N2).
1,7b-Diphenyl-1,1a-dihydrocyclopropa[c]chromen-2(7bH)-
one (2-Ph): white solid, Rf = 0.60. Single crystals suitable for
X-ray analysis were obtained by slow evaporation of a saturated
CHCl3 solution. 1H NMR (CDCl3): ı = 2.85 (d, JHH = 5.4 Hz, 1H,
3
3
CHCHCO), 3.14 (d, JHH = 5.4 Hz, 1H, CArCHCH), 6.70–6.80 (m, 2H,
HAr), 2.80–2.91 (m, 2H, HAr), 7.07–7.19 (m, 6H, HAr), 7.20–7.30
(m, 4H, HAr). 13C NMR (CDCl3): ı = 31.8 (CHCOO), 38.7 (CArCHCH),
42.1 (CArC), 117.8 (CArH), 124.4 (CArH), 125.6 (CAr), 127.2 (CArH),
127.7 (CArH), 127.9 (CArH), 128.2 (CArH), 128.3 (CArH), 128.3 (CArH),
128.7 (CArH), 132.0 (CArH), 134.4 (CArH), 134.9 (CArH), 149.5 (CArH),
166.0 (CHCOO). ESI-MS: m/z (%): 283.3 (100) [C21H14OH+], 313.2
(38) [C22H16O2H+], 378.3 (28) [C24H19NO2Na+]. FT-IR (KBr plates,
cm−1): 3061 (m), 2923 (m), 1757 (vs, CO), 1604 (w), 1486 (m),
1453 (m), 1211 (s), 1112 (w), 957 (w), 750 (s), 699 (s), 571 (w).
1-(4-Nitrophenyl)-7b-phenyl-1,1a-
2-(diazo(phenyl)methyl)phenyl
4-nitrocinnamate
(1-
PhNO2): orange solid, yield: 39%, extraction with Et2O instead
3
of pentane. 1H NMR (CDCl3): ı = 6.54 (d, JHH = 16.0 Hz, 1H,
CHCHCO), 7.05–7.13 (m, 3H, HAr, CArCHCH), 7.27–7.40 (m, 6H,
HAr), 7.48 (d, 3JHH = 7.5 Hz, 1H, HAr), 7.60–7.69 (m, 2H, HAr), 8.25 (d,
3JHH = 8.7 Hz, 2H, HAr). 13C NMR (CDCl3): ı = 121.0 (CHCOO), 122.1
(CAr), 123.7 (CArH), 123.9 (CArH), 124.3 (CArH), 125.1 (CArH), 126.9
(CArH), 128.5 (CArCNCAr), 128.6 (CArH), 129.0 (CArH), 129.1 (CArH),
130.1 (CArH), 130.3 (CAr), 140.2 (CAr), 143.7 (CArCHCH), 148.2
(CArO), 148.8 (CArN), 163.8 (CHCOO). FT-IR (KBr plates, cm−1):
3091 (m), 2942 (m), 2852 (m), 2046 (s, N2), 1734 (s, CO), 1643 (w),
1598 (m), 1520 (s), 1486 (m), 1449 (w), 1404 (w), 1345 (s), 1237
(w), 1199 (m), 1140 (m), 976 (w), 844 (w), 755 (m), 699 (m), 662
(w), 579 (w). UV/vis (pentane, nm): 208 (s, Ar), 291 (s, N2).
dihydrocyclopropa[c]chromen-2(7bH)-one (2-PhNO2): yellow
solid, Rf = 0.24, eluent (9:2 pentane/Et2O). 1H NMR (CDCl3): ı = 2.92
(d, 3JHH = 5.2 Hz, 1H, CHCHCO), 3.21 (d, 3JHH = 5.2 Hz, 1H, CArCHCH),
3
6.87 (d, JHH = 8.5 Hz, 2H, HAr), 6.97–7.21 (m, 5H, HAr), 7.27–7.54
(m, 4H, HAr), 7.99 (d, JHH = 8.5 Hz, 2H, HAr). 13C NMR (CDCl3):
3
ı = 29.9 (CHCOO), 38.0 (CArCHCH), 43.1 (CArC), 118.0 (CArH), 123.4
(CArH), 124.7 (CArH), 127.7 (CArH), 128.5 (CArH), 128.8 (CArH), 128.9
(CArH), 129.1 (CArH), 129.2 (CAr), 131.8 (CArH), 133.8 (CAr), 142.5
(CAr), 147.0 (CArN), 149.4 (CArO), 165.0 (CHCOO). ESI-MS: m/z (%):
282.3 (100) [C21H13OH+], 328.2 (29) [C21H13NO3H+], 358.2 (10)
[C22H15NO4H+]. FT-IR (KBr plates, cm−1): 2925 (s), 1760 (s, CO),
1602 (m), 1520 (s), 1455 (w), 1345 (s), 1213 (m), 1110 (w), 853
(w), 760 (s), 703 (s).
2.5. Irradiation
General irradiation conditions:
Irradiations were performed in a Rayonet RPR-100 reactor
(Southern New England Ultra Violet Company, Connecticut, USA)
˚
˚
using RPR-4190 A and RPR-3000 A fluorescent lamps [10].
General preparative irradiation procedure:
2-(4-Methoxyphenyl)-7b-phenyl-2,2a-
dihydrobenzo[d]cyclobuta[b]furan-1(7bH)-one
(3-PhOMe):
50 mg diazo compound was dissolved in 20 ml dry, degased
toluene and distributed evenly into four Duran irradiation tubes
under argon and the reaction mixture was irradiated at the given
white solid, Rf = 0.65. 1H NMR (CDCl3): ı = 3.78 (s, 3H, OCH3), 4.76
3
3
(d, JHH = 3.7 Hz, 1H, CHCHO), 5.43 (d, JHH = 3.7 Hz, 1H, CHCHO),
3
3
6.87 (d, JHH = 8.4 Hz, 2H, HAr), 6.96 (t, JHH = 7.4 Hz, 1H, HAr), 7.04
(d, 3JHH = 8.2 Hz, 1H, HAr), 7.15–7.23 (m, 3H, HAr), 7.26–7.31 (m, 2H,
HAr), 7.32–7.43 (m, 4H, HAr). 13C NMR (CDCl3): ı = 55.5 (OCH3), 69.7
(CHCHO), 80.4 (CHCCO), 85.5 (CHCHO), 111.8 (CArH), 114.6 (CArH),
122.4 (CArH), 126.0 (CArH), 126.1 (CAr), 126.3 (CArH), 127.6 (CAr),
128.0 (CArH), 128.6 (CArH), 129.1 (CArH), 130.4 (CArH), 137.3 (CAr),
159.2 (CArO), 159.6 (CArO), 203.0 (CCOCH). ESI-MS: m/z (%): 313.3
(100) [C21H17O2H+]. FT-IR (KBr plates, cm−1): 2957 (w), 2927 (m),
˚
˚
wavelength (300 nm: RPR-3000 A; 419 nm: RPR-4190 A) for 2 h.
After removal of the solvent under reduced pressure the products
were separated by flash chromatography (SiO2; 9:1 pentane/Et2O).
General analytical irradiation procedure:
10 mg diazo compound was dissolved in 4 ml of the desired
dry, degased solvent in a Duran irradiation tube under argon
and the reaction mixture was irradiated at the given wavelength

M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
187
O2S
O
O
O
NH
R
O
O
R
O
N2
R
O
N
1. NaH, thf,
tosyl hydrazine
0°C->RT, 16 h
toluene, reflux
16 h
2. toluene, 75°C
45 min
0TH-R
0-R
1-R
R
Yield [%]
R
Yield [%]
Me
67
86
67
53
61
Me
43
49
25
34
39
PhOMe
PhMe
Ph
PhOMe
PhMe
Ph
PhNO2
PhNO2
Scheme 2.
2830 (w), 1779 (s, CO), 1607 (s), 1513 (s), 1461 (m), 1251 (s), 1179
(w), 1068 (m), 1031 (m), 831 (w), 753 (m), 699 (w).
3. Results and discussion
3.1. Synthesis of diphenyl diazomethane olefin esters (1-R)
2,7b-Diphenyl-2,2a-dihydrobenzo[d]cyclobuta[b]furan-
1(7bH)-one (3-Ph): white solid, Rf = 0.86. 1H NMR (CDCl3): ı = 4.69
3
3
(d, JHH = 3.3 Hz, 1H, CHCHO), 5.35 (d, JHH = 3.3 Hz, 1H, CHCHO),
We chose the synthesis route shown in Scheme 2 to synthesize
our starting material 1-R.
3
3
6.83 (t, JHH = 7.5 Hz, 1H, HAr), 6.90 (d, JHH = 8.2 Hz, 1H, HAr),
7.06–7.29 (m, 12H, HAr). 13C NMR (CDCl3): ı = 70.2 (CHCHO), 80.5
(CCO), 85.1 (CHCHO), 111.8 (CArH), 122.4 (CArH), 126.0 (CArH), 126.3
(CArH), 127.5 (CArH), 127.6 (CAr), 127.8 (CArH), 128.0 (CArH), 129.1
(CArH), 129.2 (CArH), 130.4 (CArH), 134.0 (CAr), 137.2 (CAr), 159.6
(CArO), 202.5 (CCOCH). ESI-MS: m/z (%): 283.4 (100) [C21H14OH+].
FT-IR (KBr plates, cm−1): 3064 (w), 2929 (w), 1780 (s, CO), 1604
(m), 1495 (w), 1472 (w), 1461 (w), 1223 (m), 1063 (w), 752 (s),
697 (s).
The synthesis of benzophenyl-2-esters 0-R from the alcohol and
the adequate acid chloride, following standard literature proce-
dures [11], gave the anticipated esters in good to excellent yields.
The ester 0-R was then reacted with tosyl hydrazine. In con-
trast to the common procedures [13,14] no alcohols could be
used as solvent in this reaction because of ester cleavage. A pro-
cedure by Jones et al. [15] using CH2Cl2 as solvent resulted in
no conversion. Therefore, a synthesis by Guldi et al. [16] was
modified and the ester and tosyl hydrazine were refluxed in
toluene over night in the presence of catalytic amounts of toluene
sulfonic acid to result in the desired tosyl hydrazones in good
yields.
The synthesis of the diazo compounds was achieved, via the
Bamford–Stevens reaction [17], by deprotonation of the hydrazone
with sodium hydride over night and subsequent thermal rear-
rangement by a modified procedure of Tomioka et al. [18]. Due
to the low volatility of the diazo esters these could not be sep-
arated by distillation and therefore heating had to be performed
in toluene, not neat, and extraction with pentane or diethyl ether
depending on the solubility of the ester had to be performed. The
hydrazide salt is isolable and can be stored under an argon atmo-
sphere, but direct conversion of the deprotonation residue after
removal of the solvent in vacuo proved successful without loss of
yields.
2,8-Diphenyl-2,2a-dihydrooxeto[3,2-b]chromene
(4-Ph):
white solid, irradiation time: 4 h. 1H NMR (CDCl3): ı = 5.73 (s, 1H,
CHO), 5.85 (s, 1H, CHCAr), 6.71–6.84 (m, 2H, HAr), 6.89–7.03 (m,
3H, HAr), 7.21–7.34 (m, 6H, HAr), 7.37–7.47 (m, 3H, HAr). 13C NMR
(CDCl3): ı = 77.1 (CHO), 109.6 (CHCAr), 116.7 (CHCO), 121.2 (CArH),
123.4 (CArH), 126.0 (CArH), 127.4 (CArH), 128.1 (CArH), 128.5 (CArH),
128.8 (CArH), 128.9 (CArH), 129.4 (CAr), 129.7 (CArH), 131.1 (CArH),
136.9 (CAr), 138.2 (CAr), 140.7 (CArCCAr), 153.9 (CArO). ESI-MS: m/z
(%): 283.3 (100) [C21H14OH+]. FT-IR (KBr plates, cm−1): 3080 (m),
3050 (m), 2931 (m), 1736 (m), 1602 (m), 1479 (s), 1449 (s), 1263
(m), 1196 (s), 1061 (m), 750 (s), 698 (s).
(Z)-6-((E)-2-oxo-1-phenylpent-3-enylidene)cyclohexa-2,4-
dienone (6): white solid, Rf = 0.2. 1H NMR (CDCl3): ı = 1.87 (d,
3
3JHH = 7.2 Hz, 3H, CH3), 6.53 (d, JHH = 15.2 Hz, 1H, CHCHCO),
3
3
6.77 (dq, JHH = 7.2 Hz, JHH = 15.2 Hz, 1H, CH3CHCH), 7.27–7.61
(m, 7H, HAr), 7.75 (d, JHH = 7.8 Hz, 2H, HAr). 13C NMR (CDCl3):
3
ı = 18.7 (CH3), 128.5 (CArH), 128.9 (CArH), 129.0 (CArH), 129.9
(CArH), 130.0 (CArH), 130.1 (CArH), 131.0 (CArH), 133.1 (CHCHCO),
137.4 (CAr), 139.5 (CAr), 140.6 (CAr), 146.7 (CH3CHCH), 192.9 (CO),
197.4 (CO). ESI-MS: m/z (%): 209.2 (50) [C14H9O2+], 233.2 (53)
[C17H13O+], 251.1 (100) [C17H14O2H+]. FT-IR (KBr plates, cm−1):
3070 (w), 3040 (w), 2980 (w), 2936 (w), 1667 (s, CO), 1621 (m),
1448 (m), 1284 (s), 1147 (w), 969 (w), 931 (m), 760 (m), 704 (m),
637 (w).
The received diazo esters are stable in air. They can be handled
in air at ambient temperature and stored at −35 ◦C in air without
any decomposition even after months. However at ambient tem-
perature they decompose slowly in substance and within days in
solution. In the FT-IR spectra two prominent bands are observed,
the one around 1730 cm−1 being assigned to the C O stretching
frequency of the carboxyl C O double bond and the one at around
2045 cm−1 to the typical N N stretching vibration [19].

188
M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
Fig. 1. 1H NMR spectrum excerpt of a sample of 1-Ph irradiated at 300 nm for 1 h (products 7-Ph and 7ꢀ-Ph are explained below in the text).
Fig. 2. ORTEP [12] representation of cyclopropane 2-Ph. Thermal ellipsoids are given at 50% probability level. Hydrogen atoms are omitted for clarity.
All substances where isolated as red solids or oils showing
3.2. Irradiation experiments with 1-Ph
absorption bands in the aromatic region of the UV/vis range as well
as at around 290 nm. The latter absorption band is caused by the
diazo group (Table 1).
On irradiation of 1-Ph at 300 nm, which was chosen due to its
proximity to the absorption frequency of the diazo group, for 1 h,
several different products were obtained, whereof three are shown
in the detail of the 1H NMR spectrum in Fig. 1 (3-Ph and 7-Ph with
two diastereomers).
Table 1
Selected UV/vis bands of diazo esters.
Compound 2-Ph could be isolated and single crystals of it
suitable for X-ray diffraction analysis could be gained by slow evap-
oration of a concentrated chloroform solution. Fig. 2 shows the
depiction of a molecule in the solid state.
On this basis 2-Ph could be unequivocally proven to be the
expected irradiation product, resulting from the elimination of N2
N2 (nm)
1-Me
286
293
286
282
291
1-PhOMe
1-PhMe
1-Ph
1-PhNO2

M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
189
Scheme 3.
on irradiation followed by the attack of the generated carbene on
the C C double bond (Scheme 3).
syn-configuration concerning the cyclobutyl hydrogens, while the
minor diastereomer is anti-substituted. The structure assignment
is further supported by COSY, HMQC and HMBC spectroscopy. The
The chemical shifts of 2.9 ppm and 3.1 ppm in the 1H NMR spec-
tra (3JHH = 5 Hz) are typical for ester substituted cyclopropanes as is
the shift of 31 ppm, 38 ppm and 42 ppm in 13C NMR spectroscopy
[20]. The carbonyl stretching frequency at ca. 1760 cm−1 is in the
typical range of six membered lactones [21]. All cyclopropanes 2-
R were isolated as colorless solids and show similar spectroscopic
properties to 2-Ph.
C
O stretching vibration of 1780 cm−1 is in the range expected for
four-membered cyclic ketones [21].
We propose the reaction cascade in Scheme 4 to explain the
formation of 3syn-Ph and 3anti-Ph.
The generated carbene reacts with the C O double bond rather
than with the C C double bond. The formed epoxide is radically
opened on further irradiation releasing ring strain and yielding in
two mesomerically stabilized radicals and a C O double bond. The
phenoxy radical attacks the C C double bond which again leads
to a mesomerically stabilized benzylic radical from which a ketene
and a C C double bond is reformed. This compound undergoes a
[2+2] cycloaddition to generate 3syn-Ph and 3anti-Ph. Support for
the correctness of this sequence is found by the fact that if 1-Me is
irradiated, instead of 3syn-Me and 3anti-Me the only product found
is compound 6 in Scheme 5.
A second product (3syn-Ph and 3anti-Ph) present in larger
amount in the irradiation mixture (Fig. 1) could be isolated by
column chromatography. These signals are assigned to the two pos-
sible syn/anti-diastereomers of the cyclobutanone 3-Ph shown in
Scheme 1 on the basis of our NMR and IR spectroscopic as well
as ESI-MS results. These cyclobutanones show extreme low field
shifts for the two hydrogen atoms tethered to the four-membered
ring as expected for phenyl substituted annellated keto ethers. The
3
major diastereomer shows JHH coupling constants of 3.3 Hz. The
3JHH coupling constant for the minor diastereomer is 7.8 Hz. On this
Compound 6 could be isolated by column chromatography
basis we conclude that the major diastereoisomer is substituted in a
and is assigned on the basis of the 1H and 13C NMR and IR
Scheme 4.

190
M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
Scheme 5.
Fig. 3. 1H NMR of the reaction progress on irradiation of 1-Ph.

M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
191
Scheme 6.
Fig. 4. Time conversion plot for the irradiation of diazo compound 1-Ph at 300 nm in toluene.
spectroscopic results as well as ESI-MS. In case of a methyl-
substituted C C-double bond a radical attack of the double bond
does not generate a mesomerically stabilized benzylic radical and
an internal electronic rearrangement is more favorable.
ketenes were not observed, though. The spectroscopic properties
of 3-phenyl benzofurane 5 match the ones given in the literature
[25].
Compounds 7-Ph and 7ꢀ-Ph (Fig. 1) were present in too small
amounts to be isolated and characterized unequivocally.
3.3. Kinetic evaluation of the irradiation reaction cascade
On further irradiation at 300 nm the signals accounted for the
cyclobutanone 3-Ph is depleted and two new doublets are detected
at about 5.7 and 5.9 ppm (Fig. 3). The amount of 2-Ph slightly
increases while that of 7ꢀ-Ph decreases accordingly. 7-Ph is not
effected by longer irradiation times. The newly formed product is
the oxetane 4-Ph which is generated besides 3-phenyl benzofurane
5 (Scheme 6). The structure of 4-Ph was assigned on the basis of its
1H and 13C NMR, COSY, IR- and ESI-MS spectra.
The time conversion plot in Fig. 4 shows that the diazo com-
pound is almost completely converted after 1 h and the formation
of oxetane 4-Ph and benzofurane 5 begins after a reasonable
amount of cyclobutanone 3-Ph has been generated on irradia-
tion at 300 nm. 3-Ph is, thus, the kinetic product, while 4-Ph
and 5 are the thermodynamically more stable final compounds
(Table 2).
The formation of 4-Ph and 5 is explained by a [2+2]-photo-cyclo-
reversion of 3-Ph (Scheme 6). This is in analogy to the opening
of cyclobutanones, generating a ketene and olefins described by
Staudinger [22] thermally, by Koda et al. [23] laser induced and was
achieved by Majima et al. [24] via photo-sensitizers. The postulated
3.4. Solvent, wavelength and C C electron density dependence
on the reaction cascade
Irradiation was also conducted in different solvents and at dif-
ferent wavelengths. Since the cyclobutanone ring opening was
Table 2
Composition of the reaction mixture in dependence of time when 1-Ph is irradiated at 300 nm in toluene.
Time (min)
1-Ph (%)
2-Ph (%)
3-Ph (%)
4-Ph (%)
5 (%)
7-Ph (%)
7ꢀ-Ph (%)
0
15
30
100
55.6
28
5
0
0
0
0
0
0
0
0
0
0
6.0
8.4
15.4
15.3
16.6
11.6
0
11.3
16.7
24.1
26.4
27.3
28.2
20.0
34.7
35.0
22.2
12.1
4.9
7.1
8.3
10.9
7.6
6.2
2.7
3.9
9.7
17.7
22.4
22.9
60
0
120
180
240
10.8
15.4
29.7
0

192
M.R. Buchner et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 183–193
Table 3
Appendix A. Supplementary data
Solvent dependent product distribution after 1 h irradiation in toluene of 1-Ph.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.jphotochem.2012.12.005.
ꢀ (nm)
2-Ph
3-Ph
4-Ph
7-Ph
Pentane
Pentane
PhMe
PhMe
Et2O
300
419
300
419
300
419
1
1
1
1
1
1
1.56
4.42
1.45
6.45
0.43
4.42
0.3
–
0.4
–
0.63
–
0.78
–
0.92
–
0.46
–
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