A triphenylphosphine mediated photo-rearrangement and methanol addition of aryl chalcones to 1-propanones

Article abstract of DOI:10.1039/c5pp00009b

Aryl chalcones rearrange and add methanol to give substituted propane-1-ones upon UV-A irradiation in the presence of PPh₃. We propose two possible mechanisms for this photo-rearrangement. The reaction involves either the formation of a phosphi

Full text of DOI:10.1039/c5pp00009b

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A triphenylphosphine mediated photo-
rearrangement and methanol addition of
Cite this: DOI: 10.1039/c5pp00009b
aryl chalcones to 1-propanones†
Qiu Sun,a Chang Jiang Yaoa,b and Burkhard König*a
Aryl chalcones rearrange and add methanol to give substituted propane-1-ones upon UV-A irradiation in
the presence of PPh3. We propose two possible mechanisms for this photo-rearrangement. The reaction
involves either the formation of a phosphine-carbonyl intermediate, nucleophilic addition of MeOH and
1,2-aryl migration or the formation of ylide and carbene intermediates. The intermediates trapped from
the reaction mixture support the first mechanistic hypothesis.
Received 7th January 2015,
Accepted 16th February 2015
DOI: 10.1039/c5pp00009b
www.rsc.org/pps
Introduction
The photochemistry of chalcones has always attracted the
interest of organic chemists. In recent years, the reactions of
aryl enones under visible light photoredox catalysis have been
studied.1–10 Typical reaction conditions use a ruthenium
complex as a visible light absorbing photoredox catalyst and a
tertiary amine as a sacrificial electron donor to initiate a
photoinduced electron transfer that reduces the enone to its
corresponding radical anion, which undergoes an inter- or
intra-molecular [2 + 2] cycloaddition4,7 or a reductive coup-
ling.9 The photochemistry of aryl ketones in the presence of
PPh3 was studied more than 40 years ago,11–14 but the investi-
gations were focused on the photogeneration of ylides15 and
Norrish type II reactions.14 Pandey et al. described in 1997 a
photocatalytic system for the reductive cyclization of enones,
where 9,10-dicyanoanthracene (DCA) was employed as photo-
redox catalyst and PPh3 as the sacrificial electron donor.16 In
addition to its role as an electron donor similar to tertiary
amines, PPh3 has some unique properties: it is sterically more
hindered; it has no hydrogen atom donor and it is an efficient
quencher of the carbonyl triplet state (Scheme 1). Therefore,
Scheme 1 Photolysis of aryl ketones in the presence of PPh3.
the photochemical behavior of α,β-unsaturated ketones in the
presence of PPh3 grabbed our interest.
We investigated the photoreaction of a variety of chalcone
derivatives in the presence of PPh3 using different solvents,
catalysts and light sources. The reaction of chalcone 1a with
10 mol% of DCA and PPh3 (1 equiv.) in MeOH after 20 h of
irradiation at 400 nm gave an unexpected rearrangement and
methanol addition product 2a (Scheme 2) instead of the
expected cyclization or Michael addition product. Further
studies showed that the reaction proceeds without the addition
of a photosensitizer but not in the dark, indicating a direct
photochemical process. Similar rearrangements have been per-
formed using hypervalent iodine17,18 or thallium reagents via
oxidative processes.19 However, since PPh3 is not an oxidative
reagent, we propose a different mechanism and developed
a convenient experimental procedure for this interesting
rearrangement.
aInstitute of Organic Chemistry, University of Regensburg, Universitätstr. 31, 93053
Regensburg, Germany. E-mail: Burkhard.Koenig@ur.de
bBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
People’s Republic of China
†Electronic supplementary information (ESI) available: Characterization details
with NMR spectra and UV-vis spectra for supplementary information. Details of
the crystal structure analysis of compound 2n. CCDC 1041821. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c5pp00009b
Scheme 2 Photorearrangement and methanol addition of 2-thienyl
chalcone 1a.
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Table 2 Scope of aryl chalcones in the photorearrangement/addition
reaction
Results and discussion
Initially, the required amount of PPh3 for the photoreaction
was investigated (Table 1). The desired product was obtained
in 84% yield with 0.5 equiv. of PPh3. Decreasing the amount of
PPh3 to 0.25 equiv. does not reduce the product yield, but with
catalytic amounts of less than 10 mol% PPh3, the yield of the
reaction dropped to 28%. During work up, 30%–40% of the
PPh3 could be recycled by column chromatography. Triphenyl-
phosphine oxide was isolated as a by-product. The results indi-
cate that PPh3 acts as a catalyst but decomposes during the
reaction yielding PPh3vO. Using PPh3vO, PHPh2 and DIPEA
(N,N-diisopropylethylamine) instead of PPh3 did not yield
the desired product 2a, but the formation of small amounts of
[2 + 2] cycloaddition product was observed (see ESI†). Control
experiments without PPh3, without light or under reflux con-
ditions gave no product, revealing that PPh3 and light were
essential (entries 8, 9 and 10). MeOH was replaced with EtOH, 12
i-PrOH or CF3CH2OH but no product formation was detected
in these solvents by GC-MS analysis of the reaction mixture.
Moreover, the reaction was also carried out with photosensiti-
zers, e.g., Ru(bpy)3Cl2·6H2O and Eosin Y, at 450 nm and
530 nm, respectively. The formation of the [2 + 2] cycloaddition
product and a reductive coupling product were observed when
Ru(bpy)3Cl2·6H2O was used in the reaction, while no reaction
occurred when it was irradiated with Eosin Y at 530 nm.
Aryl-
Yielda,b
Product [%]
Entry chalcone R1
R2
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
2-Thienyl
2-Thienyl
2-Thienyl
2-Thienyl
2-Thienyl
2-Thienyl
2-Thienyl
2-Thienyl
Ph
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
84
42
55
37
32
79
Trace
—
78
4-F-C6H4
4-Br-C6H4
4-Cl-C6H4
3-Br-C6H4
4-Me-C6H4
4-MeO-C6H4
4-CN-C6H4
Ph
4-Me-C6H4
4-Br-C6H4
Ph
Ph
Ph
9
10
11
Ph
2j
2k
2l
2m
2n
60
—
Trace
—
48
4-MeO-C6H4
4-MeO-C6H4
2-NO2-C6H4
2-Naphthyl
13
14
a Isolated yield. b The reactions were carried out in 1.0 mL of CH3OH
under a N2 atmosphere.
Next, we investigated the substrate scope of the aryl chal-
cones in the photoreaction, and the results are summarized in
Table 2. Phenyl and naphthyl chalcones rearrange using the
abovementioned reaction conditions. The X-ray structure ana-
lysis of compound 2n confirmed the structure of the molecule
(Fig. 1). Chalcones bearing moderate electron-withdrawing,
neutral and electron-donating substituents reacted smoothly
to afford the corresponding products in moderate to good
Fig. 1 Structure of compound 2n in the solid state obtained from the
photorearrangement/addition reaction of aryl chalcone 1n.
Table 1 Investigation of different reaction conditions for the photo-
rearrangement/addition reaction of 1a
yield. Strong electron-withdrawing and electron-donating
substituents such as –OCH3, –NO2 and –CN on either of the
aromatic rings inhibit the rearrangement reaction; the pro-
ducts of the [2 + 2] cycloaddition were observed in these cases
(see ESI†).
Yielda,b
[%]
Entry
Conditions
Several reactions were performed to investigate the mechan-
ism of the photorearrangement/addition reaction. Initially, the
reaction was performed in deuterated methanol giving pro-
ducts 1o and 1o′ in a ratio of approximately 5 : 1 (Scheme 3).
The more acidic α-hydrogen atom was fully deuterated, while
the less reactive β-hydrogen atoms were not completely
exchanged by deuterium.
1
2
3
4
5
6
7
8
PPh3 (1.0 equiv.)
PPh3 (0.5 equiv.)
56
84
73
28
0
0
0
PPh3 (0.25 equiv.)
PPh3 (0.10 equiv.)
PPh3vO (1.0 equiv.)
PHPh2 (1.0 equiv.)
DIPEA (1.0 equiv.)
No PPh3
0
The presence of the persistent radical 2,2,6,6-tetramethyl-
piperidine-1-oxyl (TEMPO) did not affect the reaction and no
radical trapping products were identified, indicating the
absence of a radical mechanism. On the basis of the related
9
10
PPh3 (1.0 equiv.), no light
PPh3 (1.0 equiv.), no light, reflux
0
0
a Isolated yield. b The reactions were carried out in 1.0 mL of CH3OH
under a N2 atmosphere.
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Scheme 5 Synthesis of the 2-substituted terminal enone 8 using the
PPh3 mediated photorearrangement/addition reaction of compound 1i
and the subsequent elimination of methanol. Reaction conditions: (a)
0.5 equiv. PPh3, CH3OH, 20 °C, 20 h, 78%. (b) 1% NaOCH3, PhCH3,
160–170 °C, 1.5 h, 72%.
Scheme 3 The photorearrangement/addition reaction of 1a in CD3OD
leads to deuterium incorporation. Reaction conditions: (a) 0.5 equiv.
PPh3, CD3OD, 20 °C, 20 h.
reports,14,15,20 we propose two possible mechanisms shown in
Scheme 4. Photochemical excitation of the α,β-unsaturated
ketone 1 and ISC gives its triplet state 1,3 which is captured by
PPh3 to form 3 through the intervention of an exciplex. The
electronic structure of compound 3 is described by four reson-
ance structures. In pathway 1, compound 3 undergoes nucleo-
philic attack by MeOH to give the intermediate 4. A 1,2-aryl
To demonstrate its synthetic application, we used the
photoreaction product 2i in the synthesis of the 2-substituted
enone 8 (Scheme 5). Functionalized terminal enones are
useful compounds in organic synthesis. They are highly reac-
tive and can undergo conjugate addition reactions with
nucleophiles yielding a variety of bioactive products. The reac-
tion of chalcone 1i with PPh3 under the standard photoreac-
tion conditions provided the corresponding product 2i, which
was then further converted into 1,2-diphenylprop-2-en-1-one 8
upon heating to 160–170 °C with 1% NaOCH3 in toluene.22
shift and 1,2-hydrogen shift provide product
2 of the
rearrangement addition reaction sequence. In pathway 2, a
1,2-aryl migration occurs, giving the phosphonium ylide 5. It
is known that ylides can be photochemically cleaved to car-
benes.20 Carbene 6 is formed upon irradiation and quickly
trapped by MeOH to give product 2. Our attempts to identify
the intermediate presence of a phosphonium ylide by reaction
with benzaldehyde or carbene with styrene, cyclohexene and
other alcohols were without success. We also did not observe
any products arising from the carbene intermediate as
described in previous reports.14,15 Intermediate 3 tends to
react via O–P bond cleavage to give the products.21 The
extended π system of the α,β-unsaturated ketone may stabilize
the carbonyl group. Therefore, the mechanistic hypothesis of
pathway 1 may be more likely, but we cannot rule out a reac-
tion along pathway 2 from our experimental results.
Experimental
General
1H and 13C NMR spectra were obtained at 298 K using a
Bruker AVANCE 300 spectrometer (operating at 300.13 MHz for
1H and 75.47 MHz for 13C) and a Bruker AVANCE 400 spectro-
meter (operating at 400.13 MHz for 1H and 100.62 MHz for
13C), respectively. The spectra were obtained using chloroform-d
(99.8%, Deutero GmbH) and referenced against the non-
deuterated (1H)/deuterated (13C) solvents. The shift values (δ H
and δ C) are reported in ppm with J values in Hz. The melting
points were measured using a Stanford Research Systems Opti-
Melt MPA 100. The high resolution mass spectra were obtained
using a Finnigan MAT SSQ 710 A spectrometer at 70 eV
(HREIMS, positive and negative mode) or an Agilent 6540 UHD
(HRESIMS, positive and negative mode). Automated flash
chromatography was performed on a Biotage® IsoleraTM
Spektra One device. Silica gel 60 M (40–63 μm, Merck) was
used for flash column chromatography. The starting materials
and reagents were purchased from commercial suppliers and
used without further purification. The solvents were p.a. grade
for the reaction mixtures and industrial grade for flash
column chromatography. Analytical TLC was performed on
silica gel coated alumina plates (MN TLC sheets ALUGRAM®
Xtra SIL G/UV254). UV–Vis analyses were performed with
Varian Cary 50 UV/Vis spectrophotometer and Agilent 8453
UV-Vis spectrometer. For UV measurements, 10 mm Hellma
fluorescence quartz cuvettes (117.100F-QS) with a screw cap
and PTFE-coated silicon septum were used. Irradiation Source:
Philips LUXEON® Rebel (purple, max = 400
10 nm,
1000 mA, 1.2 W). Chalcone 1i was purchased from Sigma-
Scheme 4 The suggested mechanistic hypothesis for the photo-
rearrangement/addition reaction of aryl chalcones in the presence of PPh3.
Aldrich.
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General procedure for the preparation of α,β-unsaturated 131.3, 130.9, 130.5, 128.3, 127.0, 123.0, 74.2, 59.3, 54.7. HRMS
ketones 1a–1h and 1j–1n. Synthetic procedures and chemical (ESI) calcd for C14H13BrO2S [M + H]+ calcd for 324.9892, found
characterizations are available in the ESI.†
324.9891.
General procedure for the photorearrangement/addition
3-Methoxy-1-(thiophen-2-yl)-2-(p-tolyl)propan-1-one
(2f).
reaction. In a 5 mL snap vial equipped with magnetic stirring Colorless oil. 1H NMR (300 MHz, CDCl3) δ 7.75 (dd, J = 3.8,
bar, the PPh3 (0.5 equiv., 0.125 mmol) and aryl chalcone 0.9 Hz, 1H), 7.54 (dd, J = 4.9, 0.9 Hz, 1H), 7.28 (s, 1H), 7.12
derivative (1.0 equiv., 0.25 mmol) were added in 1 mL of (d, J = 8.0 Hz, 2H), 7.03 (dd, J = 4.9, 3.9 Hz, 1H), 4.71 (dd, J =
CH3OH and the resulting reaction mixture was degassed via 8.9, 5.3 Hz, 1H), 4.22–4.13 (m, 1H), 3.62 (dd, J = 9.1, 5.3 Hz,
three “pump–freeze–thaw” cycles using a syringe needle. The 1H), 3.35 (s, 3H), 2.29 (s, 3H). 13C NMR (75 MHz, CDCl3)
vial was irradiated through the vial’s plane bottom side using δ 191.3, 144.1, 137.4, 134.0, 133.3, 132.7, 129.7, 128.2, 74.4,
400 nm purple LEDs with a cooling device, maintaining the 59.2, 54.8, 21.1. HRMS (ESI) calcd for C15H16O2S [M + H]+
reaction temperature at around 20 °C. After 20 h of irradiation, 261.0944, found 261.0946.
the solvent was removed and the residue purified by flash
column chromatography using petroleum ether–ethyl acetate 1H NMR (300 MHz, CDCl3) δ 8.02–7.95 (m, 2H), 7.50–7.45
(99 : 1 to 99 : 5) as the eluent. (m, 1H), 7.43–7.37 (m, 2H), 7.32 (dt, J = 8.7, 1.7 Hz, 4H),
3-Methoxy-1,2-diphenylpropan-1-one (2i). Colorless oil.
3-Methoxy-2-phenyl-1-(thiophen-2-yl)propan-1-one (2a). Col- 7.25–7.21 (m, 1H), 4.90 (dd, J = 8.7, 5.3 Hz, 1H), 4.25–4.14
orless oil. 1H NMR (400 MHz, CDCl3) δ 7.75 (dd, J = 3.8, (m, 1H), 3.65 (dd, J = 9.1, 5.3 Hz, 1H), 3.36 (s, 3H). 13C NMR
1.1 Hz, 1H), 7.61–7.57 (m, 1H), 7.39–7.34 (m, 2H), 7.32 (ddd, (75 MHz, CDCl3) δ 198.3, 136.7, 136.3, 133.1, 129.0, 128.8,
J = 7.6, 4.5, 1.2 Hz, 2H), 7.06 (dd, J = 4.9, 3.9 Hz, 1H), 4.71 (dd, 128.6, 128.4, 127.6, 74.7, 59.2, 53.8. HRMS (ESI) calcd for
J = 8.9, 5.3 Hz, 1H), 4.18 (t, J = 9.0 Hz, 1H), 3.64 (dd, J = 9.1, C16H16O2 [M + H]+ calcd for 241.1223, found 241.1228.
5.3 Hz, 1H), 3.36 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 191.1,
144.1, 136.3, 134.0, 132.7, 129.0, 128.3, 128.1, 127.7, 74.4, 59.2, oil. H NMR (300 MHz, CDCl3) δ 8.04–7.94 (m, 2H), 7.49–7.45
55.3. HRMS (ESI) calcd for C14H14O2S [M + H]+ 247.0787, (m, 1H), 7.41–7.29 (m, 2H), 7.23 (d, J = 8.1 Hz, 2H), 7.12 (d, J =
3-Methoxy-1-phenyl-2-(p-tolyl)propan-1-one (2j). Colorless
1
found 247.0788.
7.9 Hz, 2H), 4.87 (dd, J = 8.7, 5.3 Hz, 1H), 4.18 (t, J = 8.9 Hz,
2-(4-Fluorophenyl)-3-methoxy-1-(thiophen-2-yl)propan-1-one 1H), 3.63 (dt, J = 11.1, 5.5 Hz, 1H), 3.36 (s, 3H), 2.29 (m, 3H).
1
(2b). Colorless oil. H NMR (300 MHz, CDCl3) δ 7.74 (dd, J = 13C NMR (75 MHz, CDCl3) δ 198.4, 137.3, 136.7, 133.3, 133.0,
3.8, 1.1 Hz, 1H), 7.63–7.59 (m, 1H), 7.37–7.30 (m, 2H), 7.07 (dt, 129.8, 128.8, 128.5, 128.2, 74.7, 59.2, 53.4, 21.1. HRMS (ESI)
J = 7.3, 3.6 Hz, 1H), 7.04–6.96 (m, 2H), 4.73–4.66 (m, 1H), 4.13 calcd for C17H18O2 [M + H]+ 255.1380, found 255.1377.
(t, J = 8.9 Hz, 1H), 3.62 (dd, J = 9.1, 5.5 Hz, 1H), 3.35 (s, 3H).
3-Methoxy-1-(naphthalen-2-yl)-2-phenylpropan-1-one
(2n).
1
13C NMR (75 MHz, CDCl3) δ 191.1, 143.8, 134.3, 132.7, 129.9, Colorless solid. H NMR (300 MHz, CDCl3) δ 8.51 (s, 1H), 8.04
129.8, 128.2, 116.1, 115.8, 74.3, 59.2, 54.3. HRMS (ESI) calcd (dd, J = 8.7, 1.8 Hz, 1H), 7.92 (d, J = 7.9 Hz, 1H), 7.82 (dd, J =
for C14H13FO2S [M + H]+ 265.0693, found 265.0696.
12.2, 6.8 Hz, 2H), 7.54 (ddd, J = 9.2, 5.1, 1.4 Hz, 2H), 7.41–7.38
2-(4-Bromophenyl)-3-methoxy-1-(thiophen-2-yl)propan-1-one (m, 2H), 7.34–7.28 (m, 2H), 7.25–7.21 (m, 1H), 5.06 (dd, J = 8.7,
1
(2c). Colorless oil. H NMR (300 MHz, CDCl3) δ 7.72 (dd, J = 5.3 Hz, 1H), 4.25 (t, J = 8.9 Hz, 1H), 3.71 (dd, J = 9.1, 5.3 Hz,
3.8, 1.1 Hz, 1H), 7.60 (dd, J = 4.9, 1.1 Hz, 1H), 7.46–7.40 1H), 3.38 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 198.3, 136.5,
(m, 2H), 7.27–7.22 (m, 2H), 7.06 (dd, J = 4.9, 3.9 Hz, 1H), 4.67 135.5, 134.1, 132.4, 130.6, 129.7, 129.1, 128.5, 128.4, 127.7,
(dd, J = 8.5, 5.7 Hz, 1H), 4.20–4.03 (m, 1H), 3.62 (dd, J = 9.1, 127.6, 126.7, 124.4, 74.8, 59.2, 53.8. HRMS (ESI) calcd for
5.7 Hz, 1H), 3.34 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 190.7, C20H18O2 [M + H]+ 291.1380, found 291.1381.
143.7, 135.4, 134.5, 132.8, 132.1, 130.0, 128.3, 121.8, 74.1, 59.3,
54.5. HRMS (ESI) calcd for C14H13BrO2S [M + H]+ 324.9892, methoxide (1.0 mg, 0.019 mmol) to 3-methoxy-1,2-diphenyl-
found 324.9894. propan-1-one (0.10 g, 0.83 mmol) in 2 mL of toluene.
Synthesis of the 2-substituted terminal enone 8. Sodium
2-(4-Chlorophenyl)-3-methoxy-1-(thiophen-2-yl)propan-1-one The mixture was heated to 160–170 °C for 1.5 h. The solvent
(2d). Colorless oil. 1H NMR (300 MHz, CDCl3) δ 7.77–7.69 was evaporated and the residue purified by flash
(m, 1H), 7.65–7.54 (m, 1H), 7.29 (d, J = 2.1 Hz, 3H), 7.12–6.98 column chromatography using petroleum ether–ethyl acetate
1
(m, 1H), 4.68 (dd, J = 9.1, 5.7 Hz, 1H), 4.19–4.03 (m, 1H), 3.62 (50 : 1) as the eluent. White solid. H NMR (300 MHz, CDCl3)
(dd, J = 9.1, 5.7 Hz, 1H), 3.34 (s, 3H). 13C NMR (75 MHz, δ 7.95–7.86 (m, 2H), 7.60–7.32 (m, 8H), 6.08 (s, 1H), 5.65 (s,
CDCl3) δ 190.8, 143.7, 134.8, 134.4, 133.7, 132.8, 129.6, 129.2, 1H). The spectra was in accordance with that reported in the
128.3, 74.2, 59.3, 54.5. HRMS (ESI) calcd for C14H13ClO2S literature.23
[M + H]+ calcd for 281.0398, found 281.0398.
2-(3-Bromophenyl)-3-methoxy-1-(thiophen-2-yl)propan-1-one
1
(2e). Colorless oil. H NMR (300 MHz, CDCl3) δ 7.74 (dd, J =
Conclusions
3.8, 1.0 Hz, 1H), 7.62 (dd, J = 4.9, 1.0 Hz, 1H), 7.53 (dd, J = 6.4,
4.7 Hz, 1H), 7.41–7.34 (m, 1H), 7.34–7.28 (m, 1H), 7.23–7.16 In conclusion, we have reported the photorearrangement and
(m, 1H), 7.11–7.04 (m, 1H), 4.67 (dd, J = 8.6, 5.6 Hz, 1H), methanol addition reaction of aryl chalcones mediated by
4.17–4.08 (m, 1H), 3.63 (dd, J = 9.1, 5.6 Hz, 1H), 3.34 (s, 3H). PPh3 under UV-A irradiation. The reaction proceeds smoothly
13C NMR (75 MHz, CDCl3) δ 190.5, 143.8, 138.5, 134.6, 132.9, at room temperature without the presence of a sensitizer using
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400 nm emitting LEDs. The rearrangement product can be
further converted into 2-substituted terminal enones, which
Employing
Tactic, Org. Lett., 2014, 17, 3–5.
a
Tandem Photoisomerization/Cyclization
are interesting molecular structures with potential biological 10 R. Brimioulle and T. Bach, Enantioselective Lewis acid cata-
activity. We propose two possible mechanistic hypotheses for
this rearrangement/addition reaction, involving either the for-
lysis of intramolecular enone [2 + 2] photocycloaddition
reactions, Science, 2013, 342, 840–843.
mation of a phosphine-carbonyl intermediate, nucleophilic 11 J. C. Scaiano, Trapping by Di-Tert-Butyl Selenoketone of
addition of MeOH and 1,2-aryl migration or reaction via ylide
and carbene intermediates.
Biradicals Produced in Photochemistry of Phenyl Alkyl
Ketones - Kinetic Study, J. Am. Chem. Soc., 1977, 99, 1494–
1498.
12 R. D. Small and J. C. Scaiano, Photochemistry of Phenyl
Alkyl Ketones - Lifetime of Intermediate Biradicals, J. Phys.
Chem., 1977, 81, 2126–2131.
Acknowledgements
13 J. C. Scaiano, Photochemistry of Phenyl Alkyl Ketones in
Presence of Organophosphorus (V) Compounds, J. Org.
Chem., 1978, 43, 568–570.
14 Y. L. Chow and B. Marciniak, Photochemistry of Phenyl
Alkyl Ketones in the Presence of Triphenylphosphine,
J. Org. Chem., 1983, 48, 2910–2914.
This work was supported by the German Science Foundation
(DFG, GRK 1626, Chemical Photocatalysis). CJY thanks the
Sino-German Center for the Lindau fellowship.
15 M. A. Fox, Photo-Chemical Generation of Aryl Phos-
phonium Ylides, J. Am. Chem. Soc., 1979, 101, 5339–5343.
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