Mechanism of 2-iodophenol photolysis in aqueous solution

Article abstract of DOI:10.1039/b208423f

The photolysis of 2-iodophenol (2-IPhOH) was investigated by means of laser flash photolysis and product studies. Two major heterolytic dehalogenation pathways could be evidenced upon irradiation of anionic 2-IPhO^-: ring contraction leading to cyclopentadienic acids via a Wolff rearrangement (φ_a = 0.11 ± 0.02), and α-ketocarbene formation (φ_c = 0.03 ± 0.01) yielding products characteristic for triplet carbene reactivity. In contrast, the irradiation of neutral 2-IPhOH leads mainly to homolytic cleavage of the carbon-halogen bond (φ = 0.08 ± 0.01) with subsequent formation of biphenyls in deoxygenated solution. This specific reaction, which is not observed with other halogenated phenols, is explained by the low energy of the C-I bond. The relative efficiencies of the heterolytic pathways in the halogenophenol series are discussed in terms of the multiplicity of the excited states involved and of the internal heavy atom effect.

Full text of DOI:10.1039/b208423f

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Mechanism of 2-iodophenol photolysis in aqueous solution
a
Florent Bonnichon, Gottfried Grabner, Claire Richard* and Bernadette Lavedrine
b
a
a
´
a
´
´
Laboratoire de Photochimie Moleculaire et Macromoleculaire (CNRS UMR 6505),
´
´
`
Universite Blaise Pascal, Les Cezeaux, B. P. 45, 63177, Aubiere cedex, France.
E-mail: claire.richard@univ-bpclermont.fr; Fax: +33 (0)4 73 40 77 00
Tel: +33 (0)4 73 40 71 42
b
Institut fu¨r Theoretische Chemie und Molekulare Strukturbiologie, Universita¨t Wien,
Rennweg 95b, 1030, Wien, Austria
Received (in Montpellier, France) 29th August 2002, Accepted 5th November 2002
First published as an Advance Article on the web 9th January 2003
The photolysis of 2-iodophenol (2-IPhOH) was investigated by means of laser flash photolysis and product
studies. Two major heterolytic dehalogenation pathways could be evidenced upon irradiation of anionic 2-
IPhOꢀ: ring contraction leading to cyclopentadienic acids via a Wolff rearrangement (fa ¼ 0.11 ꢁ 0.02), and
a-ketocarbene formation (fc ¼ 0.03 ꢁ 0.01) yielding products characteristic for triplet carbene reactivity. In
contrast, the irradiation of neutral 2-IPhOH leads mainly to homolytic cleavage of the carbon-halogen bond
(f ¼ 0.08 ꢁ 0.01) with subsequent formation of biphenyls in deoxygenated solution. This specific reaction,
which is not observed with other halogenated phenols, is explained by the low energy of the C–I bond. The
relative efficiencies of the heterolytic pathways in the halogenophenol series are discussed in terms of the
multiplicity of the excited states involved and of the internal heavy atom effect.
Photocontraction (pathway a) was identified twenty years
Introduction
ago by Guyon and coworkers1,2 by means of photoproduct
analysis. It is observed both for the neutral and the anionic
forms of the halogenophenols (pKa ꢂ 8.52,5), but is more effi-
cient from the anion (fa ¼ 0.30 ꢁ 0.031 and 0.27 ꢁ 0.033 for
2-ClPhOꢀ and 2-BrPhOꢀ, respectively) than from the molecu-
lar form (fa ¼ 0.042 ꢁ 0.008 and 0.035 ꢁ 0.007 for 2-ClPhOH
and 2-BrPhOH, respectively4). Two transient intermediates
involved in this pathway were detected by laser flash photoly-
sis: cyclopenta-2,4-dienyl ketene (lmax ¼ 255 nm) and fulvene-
6,6-diol (lmax ¼ 295 nm in acidic or neutral medium and
The aqueous photochemistry of the ortho-halophenols, 2-
chlorophenol (2-ClPhOH) and 2-bromophenol (2-BrPhOH),
shows a complex and interesting behaviour. Previous investi-
gations1–6 have demonstrated that three characteristic reac-
tions occur subsequently to photoinduced heterolytic
dehalogenation: (a) ring contraction yielding cyclopentadienic
acids, (b) photohydrolysis into pyrocatechol, and (c) a-keto-
carbene formation giving rise to biphenyls in deoxygenated
solution (see Scheme 1).
Scheme 1
DOI: 10.1039/b208423f
New J. Chem., 2003, 27, 591–596
591
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283 nm at pH ¼ 12), the latter being produced by addition of
water on the ketene.5,6 This ring contraction corresponds to a
Wolff rearrangement;7 it is also observed in the photolysis of
a-diazoketones.8
Photohydrolysis (pathway b) is a minor pathway observed
in acidic medium (fb ¼ 0.012 ꢁ 0.001 and 0.009 ꢁ 0.001 for
2-ClPhOH and 2-BrPhOH, respectively4).
Laser flash photolysis
Transient absorption experiments were carried out using a fre-
quency-quadrupled Nd:YAG laser (Quanta-Ray DCR-1, 266
nm, pulse duration 10 ns or Quanta-Ray GCR-130-1, 266
nm, pulse duration 9 ns). The procedures used for transient
absorption spectroscopy measurements have been described
previously.17,18 Absorbances within the wavelength range
250–320 nm were corrected for bleaching.
The third process consisting in the formation of an a-keto-
carbene, 2-oxo-cyclohexa-3,5-dienylidene (pathway c) was
demonstrated recently.4 This species (lmax ¼ 370 and 388
nm) was firmly assigned on the basis of its characteristic triplet
carbene reactions, that is addition onto molecular oxygen to
yield the ortho-benzoquinone-O-oxide (lmax ¼ 475 nm) and
abstraction of an H atom from H-donors to give the phenoxyl
radical (lmax ¼ 400 nm). Addition of the a-ketocarbene on the
starting molecule yields substituted biphenyls and abstraction
of an H atom from H-donors like alcohols leads to phenol.
This reactive behaviour is fully analogous to that of the related
triplet carbenes, 4-oxocyclohexa-2,5-dienylidene9 and 4-imino-
cyclohexa-2,5-dienylidene,10 produced in the heterolytic
photodehalogenation of 4-halogenophenols and 4-halogeno-
anilines, respectively. The absorption spectra of the three car-
benes, featuring a characteristic two-peak structure, are also
similar.4,9,10 Photochemical production of the a-ketocarbene
was shown to be about ten times more efficient from 2-
BrPhOH (fc ¼ 0.04 ꢁ 0.01) than from 2-ClPhOH (fc ¼
0.003 ꢁ 0.001).4 This strong substituent effect is consistent with
a mechanism in which intersystem crossing at the molecular
level precedes carbene formation, since the intersystem cross-
ing rate is expected to be higher in the molecule containing
the heavier halogen atom.
In order to gain further insight into the primary steps of the
reaction, we undertook a photochemical study of 2-iodophenol
(2-IPhOH), which should exhibit even faster intersystem cross-
ing through the heavy atom effect. On the other hand, the
weakness of the carbon-iodine bond (64 kcal molꢀ111–13) could
favour a homolytic cleavage process, which is not observed in
the other 2-halogenophenols. Previous results obtained on
related systems are conflicting: if no homolytic C–I scission
was reported in the photolysis of aqueous 4-IPhOH,14 in con-
trast, the photolysis of 3-iodotyrosine in water was shown to
produce 3-tyrosyl free radicals and iodine atoms.15 In this
work, we report the photolysis of 2-IPhOH investigated by
means of laser flash photolysis and product studies. Both the
anionic and the molecular forms of the molecule (pKa ¼
8.4616) were studied.
Steady-state irradiations
Aqueous 2-IPhOH (4 ꢃ 10ꢀ4 mol lꢀ1) was irradiated at 282 nm
using a high pressure xenon lamp (1600 W) and a Schoeffel
monochromator. Potassium ferrioxalate was used as a chemi-
cal actinometer. Solutions were deoxygenated by argon bub-
bling for 20 min. prior to irradiation and were
suroxygenated by oxygen bubbling. The pH was adjusted
using HClO4 or NaOH.
Identification of photoproducts
Phenol, pyrocatechol and cyclopentadienic acids were identi-
fied by reference to authentic samples. Cyclopentadienic acids
dimerise in concentrated solutions1 and do not exist as pure
dry products. We prepared a solution of authentic cyclopenta-
dienic acids by irradiating 2-ClPhOꢀ (pH ¼ 12) at 280 nm.
Assuming a yield of photocontraction from 2-ClPhOꢀ equal
to 0.3,3 we could use this solution to quantify the formation
of cyclopentadienic acids from 2-IPhOH. In preliminary
experiments, we found that irradiating NaI in the presence
of 2-IPhOH efficiently yielded the same biphenyls as the irra-
diation of 2-IPhOH alone. So, we chose to accumulate biphe-
nyls using the following procedure: a 100 ml deoxygenated
solution containing NaI (6.8 ꢃ 10ꢀ2 mol lꢀ1) and 2-IPhOH
(5.8 ꢃ 10ꢀ3 mol lꢀ1) was irradiated at 254 nm in a quartz reac-
tor using a germicide lamp. Three biphenyls were separated
by preparative HPLC and analyzed by 1H-NMR and mass
spectrometry.
2-Iodo-6-(20-hydroxyphenyl)phenol. lmax/nm (H2O) 250 and
285; 1H NMR (CD3OD, 400 MHz): d 8.28 (1H, d, J 2.0),
8.01 (1H, d, J 2.1), 7.91 (1H, dd, J 8.7 and 2.1), 7.71 (1H, d,
J 2.5), 7.19 (1H, d, J 8.7); MS (EI): m/z 312, and fragments
at 185 (40%), 157 (45), 128 (80)
2-Iodo-4-(20-hydroxyphenyl)phenol. lmax/nm (H2O) 252 and
295; 1H NMR (CD3OD, 400 MHz): d 8.22 (1H, d, J 2.3),
7.86 (1H, dd, J 8.7 and 2.3), 7.41 (1H, d, J 2.5), 7.31 (1H,
dd, J 8.7 and 2.2), 7.05 (1H, d, J 8.7); MS (EI): m/z 312,
and fragments at 185 (50%), 157 (45), 128 (60).
Experimental
4-(20-Hydroxyphenyl)phenol or isomers. lmax/nm (H2O) 247
and 285; MS (EI): m/z 186, and fragments at 167 (20%), 157
(40), 128 (35).
Material and methods
2-IPhOH was purchased from Aldrich and purified by subli-
mation at room temperature before use. Phenol, pyrocatechol,
2-ClPhOH and 2-propanol were of the highest purity grade
available and were used as received. Water was purified with
a Milli-Q (Millipore) device. H NMR spectra were recorded
on a Bruker AC400 spectrometer. Mass spectrometry analyses
Results and discussion
1
Photolysis of 2-IPhOH
´
were performed by the Service d’Analyse of the Universite
d’Orleans, France. UV-visible spectra were recorded on a Cary
Laser flash photolysis experiments. Fig. 1 shows the transient
spectra measured in a deoxygenated solution of 2-IPhOꢀ
(2.0 ꢃ 10ꢀ4 mol lꢀ1) at pH 12. Based on analogy to previous
work,5,6 the pulse end transient exhibiting an absorption max-
imum around 255 nm was assigned to the ketene and the sec-
ondary species with maximum at 283 nm to the dianionic
fulvene-6,6-diol, the former being converted into the latter
with k ¼ 1.3 ꢃ 106 sꢀ1. A further species absorbing in the near
UV and decaying by a first-order kinetics with k ¼ 5.0 ꢃ 105
sꢀ1 was detected at pulse end. Its two-band structure
(lmax ¼ 388 and 375 nm) was made clear by subtracting the
´
3 (Varian) spectrophotometer. Analytical HPLC was carried
out using a Waters apparatus equipped with a photodiode
array detector and a conventional reverse phase 5 mm column.
Pyrocatechol was titrated on a Merck apparatus equipped with
a Hitachi F-1050 fluorescence detector; the mobile phase was a
mixture of water with H3PO4 (0.1%) and MeOH (40–60, v/v).
Preparative HPLC was performed on a Gilson apparatus with
UV detection using a semi-preparative 3 mm Microsorb
column.
592
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Fig. 3 Transient absorption spectra from aqueous 2-IPhOꢀ at pH
12. Absorbances normalised to 2 mJ per pulse, A(266) ¼ 0.77. (S) in
oxygenated medium, (X) in deoxygenated medium containing 2-propa-
nol (0.17 mol lꢀ1).
Fig. 1 Transient absorption spectra from aqueous 2-IPhOꢀ at pH
12. Absorbances normalised to 2 mJ per pulse and corrected for
bleaching, A(266) ¼ 0.28. (S) extrapolated to pulse end, (X) measured
4 ms after the pulse end.
(k ¼ 7.5 ꢃ 106 sꢀ1) was in excellent agreement with the known
rate constant of reaction between Iꢄ and Iꢀ (k ¼ 1.1 ꢃ 1010
absorbances measured 8 ms after pulse end from those mea-
sured at pulse end (Fig. 2). This transient is similar to that pro-
duced upon irradiation of 2-BrPhOH and is therefore assigned
to the a-ketocarbene.4 To firmly confirm this attribution, we
photolysed 2-IPhOꢀ first in the presence of oxygen
(1.3 ꢃ 10ꢀ3 mol lꢀ1) and then in that of 2-propanol (0.17 mol
molꢀ1 l sꢀ1 20–22. This result proves that C–I homolysis occurs
)
from the molecular form of 2-IPhOH. The anionic form was
alꢄsꢀo irradiated in the presence of NaI; the absorption of the
I2 radical was not detected in this case. Whether heterolytic
a-ketocarbene formation also occurs from neutral 2-IPhOH
was difficult to decide due to the overlapping absorption spec-
l
ꢀ1). As expected, the 388 nm transient decay rate was signifi-
ꢄ
ꢀ
cantly enhanced in both cases and secondary species were pro-
duced exhibiting spectral characteristics corresponding to
those of ortho-benzoquinone-O-oxide4 and the phenoxyl radi-
cal,19 respectively (Fig. 3). The transient spectroscopy of 2-
IPhOꢀ is thus very similar to that of 2-BrPhOH.4
tra of the carbene and of the I2 radical. A careful examina-
tion of the end-of-pulse absorbances at 388 nm revealed,
however, that a-ketocarbene formation was at least five times
less in neutral than in basic medium.
All transients shown in Figs. 1–4 were formed in monopho-
tonic processes as indicated by the linear dependences of their
absorbances on the laser pulse energy, P (Fig. 5). Using chemi-
cal actinometry as described in ref. 18, we computed the pro-
duct e ꢃ f of the quantum yield and extinction coefficient of
each transient from the slopes of the regression lines. In basic
medium, we found 2400 ꢁ 400 molꢀ1 l cmꢀ1 at 260 nm corre-
sponding mainly to the ketene, 1750 ꢁ 300 molꢀ1 l cmꢀ1 for
the dianionic fulvene-6,6-diol at 283 nm and 62 ꢁ 15 molꢀ1
l cmꢀ1 for the a-ketocarbene at 388 nm. In acidic medium,
we obtained 120 ꢁ 25 molꢀ1 l cmꢀ1 for the fulvene-6,6-diol
The irradiation of neutral 2-IPhOH also produced ketene
and fulvene-6,6-diol, but in very low yields. In addition, a
further transient showing a broad absorption band extending
from 300 up to 500 nm with a maximum at 385 nm was
ꢄ
ꢀ
detected (Fig. 4), reminiscent of the well-known I2 radical
anion.20–22 In order to confirm this assignment, we photolysed
2-IPhOH (5.0 ꢃ 10ꢀ4 mol lꢀ1
) in the presence of NaI
(6.2 ꢃ 10ꢀ4 mol lꢀ1) after ensuring that direct photolysis of
ꢄ
ꢀ
NaI alone at this concentration did not produce I2 radical
anions in measurable amounts. As expected, the addition of
NaI significantly enhanced the rate of formation of the
385 nm transient as well as the absorbance at 385 nm (Fig. 4
and insert). The observed formation rate constant
ꢄ
ꢀ
at 295 nm and 780 ꢁ 120 molꢀ1 l cmꢀ1 for the I2 radical at
385 nm. Using e ¼ 2100 molꢀ1 l cmꢀ1 for the a-ketocarbene
Fig. 4 Transient absorption spectra from aqueous neutral 2-IPhOH.
Absorbances normalised to 2.2 mJ per pulse, A(266) ¼ 1.1. (G) mea-
sured 1 ms after the pulse end in solution containing only 2-IPhOH, (K)
measured 4 ms after the pulse end upon addition of NaI (6.2 ꢃ 10ꢀ4 mol
Fig. 2 Transient absorption spectra from deoxygenated aqueous
2-IPhOꢀ at pH 12. Absorbances normalised to 2 mJ per pulse,
A(266) ¼ 0.77. Difference between the absorbances measured at pulse
end and those measured 8 ms after pulse end.
l
ꢀ1). Insert: time course of absorbances at 385 nm
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photoproducts are due to photocontraction. The lack of
detectable photoproducts originating from the carbene path-
way in basic medium has already been noted in the case of
the 4-halogenophenolates.9,24 The biphenyls expected from
addition of the carbene on the substrate were found to be
stable in highly basic medium and should be detected if
formed. It must therefore be assumed that the intermediary
cyclohexadiene-type moieties resulting from the addition reac-
tion do not yield biphenyls under these pH conditions.
In acidic or neutral medium, ring contraction and a-ketocar-
bene formation are minor pathways, with efficiencies lower
than 0.005, and photohydrolysis is also negligible. The main
reaction route in this case is the formation of Iꢄ atoms through
ꢄ
homolytic cleavage of the carbon-iodine bond (fI ¼ 0.08).
The concomitantly generated hydroxyphenyl radicals (HOPhꢄ)
could not be detected by laser flash photolysis. Nevertheless,
their formation is evidenced by the detection of 2-iodo-6-(20-
hydroxyphenyl)phenol and 2-iodo-4-(20-hydroxyphenyl)phe-
nol among the photoproducts, which result from the addition
of HOPhꢄ on the nucleophilic C2 and C4 carbons of 2-IPhOH.
This addition is inhibited both by 2-propanol ꢄand oxygen in
accordance with the easy scavenging of HOPh .25 In the first
case, phenol is formed by reduction of HOPhꢄ, while, in the
second one, pyrocatechol is the end product of HOPhꢄ oxida-
tion. In the chloro and bromo analogues, formation of pyroca-
techol was observed whatever the oxygen concentration and it
was proposed to result from hydrolysis via a heterolytic path-
way. In the case of 2-iodophenol, pyrocatechol was observed
in oxygenated medium only. Its mechanism of formation is
therefore different; one possibility would be addition of oxygen
onto a HOPhꢄ radical, followed by a reduction step. The mass
balance in neutral oxygenated medium being poor, pyro-
catechol is only one of the possible end products of HOPhꢄ
oxidation.
Fig. 5 Dependence of absorbance on laser pulse energy (P). (X)
ketene at 260 nm in aqueous 2-IPhOꢀ, A(266) ¼ 0.41; (S) a-ketocar-
ꢄ
ꢀ
bene at 388 nm in aqueous 2-IPhOꢀ, A(266) ¼ 0.77; (G) I2 radical
at 385 nm in aqueous 2-IPhOH containing NaI (6.2 ꢃ 10ꢀ4 mol lꢀ1),
A(266) ¼ 1.1.
at 388 nm,4 we compute fc ¼ 0.03 ꢁ 0.01 in basic medium and
ꢄ
using e ¼ 10 000 molꢀ1 l cmꢀ1 for I2 at 385 nm,23 we get
ꢀ
ꢄ
fI ¼ 0.08 ꢁ 0.01 in neutral solution. The e ꢃ f value measured
at 260 nm in basic medium cannot be used to evaluate the
quantum yield of photocontraction accurately because the
a-ketocarbene also contributes to absorption in the far-UV
wavelength range.9
Steady-state irradiations. The quantum yield of 2-IPhOH
photodegradation was measured in various conditions of pH
and deoxygenation (Table 1). The formation of the main
photoproducts (cyclopentadienic acids, pyrocatechol, phenol
and biphenyls) was also studied. In basic medium, the quan-
tum yield of 2-IPhOH photolysis is equal to 0.17 and cyclopen-
tadienic acids are the only products observed; they account for
about 70% of converted 2-IPhOꢀ. In neutral or acidic solution,
the quantum yield of 2-IPhOH photolysis is three times smaller
than in basic medium and cyclopentadienic acids are produced
in low yield in agreement with the results obtained by laser
flash photolysis. In deoxygenated solution, biphenyls (mainly
2-iodo-4-(20-hydroxyphenyl)phenol) are produced along with
phenol; pyrocatechol is detected in traces. In oxygen-saturated
medium, the mass balance is very poor. The formations of
biphenyls and phenol are drastically reduced while the produc-
tion of pyrocatechol is enhanced. In deoxygenated solution
containing 2-propanol, the formation of biphenyls is reduced
too and that of phenol is favoured.
A comparison of the photoreactivities of the 2-halogenophenols
A comparison of the respective efficiencies of the three photo-
induced dehalogenation pathways in the 2-halogenophenol
series (2-ClPhOH, 2BrPhOH and 2-IPhOH) may serve to clar-
ify the mechanism of their photoreactions (Table 2). The gen-
eral mechanism deduced from this comparison is summarised
in Scheme 3 (written for the anionic halogenophenols, but
valid for neutral 2-ClPhOH and 2BrPhOH as well).
The cleavage of the C–X bond is always heterolytic except
with 2-IPhOH for which the homolytic pathway is dominant
(ꢂ90%). This specific behaviour, characteristic of the molecu-
lar form, can be explained by the low C–I bond energy (64 kcal
molꢀ1 compared to 79–80 kcal molꢀ1 for C–Br11–13).
The processes occurring after heterolysis are ring contrac-
tion, photohydrolysis and a-ketocarbene formation (Scheme
1). As can be seen in Table 2, changing the halogen atom
has a significant effect on the efficiency of these processes.
First, the photocontraction efficiency decreases in the order
Cl > Br > I, the difference being small between Cl and Br
but significant between Br and I. Second, it appears that a-
ketocarbene formation is more efficient from the iodo and
bromo derivatives than for the chloro derivative. The ratio
Interpretation. The reaction pathways of 2-iodophenol
photolysis emerging from the results presented above are sum-
marised in Scheme 2.
Ring contraction and a-ketocarbene formation are the
two dominant photoreaction pathways of 2-IPhOꢀ, with effi-
ciencies around 0.11 and 0.03, respectively. All observable
Table 1 Quantum yields of 2-iodophenol consumption (f) and product formation
a
a
Conditions
f
fcyclopentadienic acids
fpyrocatechol
fphenol
Detection of biphenyls
1.5
Argon
Argon
0.051 ꢁ 0.006
0.065 ꢁ 0.007
0.047 ꢁ 0.006
0.061 ꢁ 0.006
0.17 ꢁ 0.02
0.003 ꢁ 0.001
0.004 ꢁ 0.001
0.003 ꢁ 0.001
0.004 ꢁ 0.001
0.11 ꢁ 0.02
< 10ꢀ5
< 10ꢀ5
0.0070
< 10ꢀ5
< 10ꢀ5
0.010
0.0070
Yes
Yes
No
No
No
Neutral
Neutral
Neutral
12.5
Oxygen-saturated
Argon, 2-PrOH (0.17 mol Lꢀ1
Argon
< 10ꢀ4
0.038
)
< 10ꢀ4
a
Estimated error ꢁ15%
594
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Scheme 2
Table 2 Reaction pathways of 2-ClPhOH, 2-BrPhOH and 2-IPhOH
Heterolytic cleavage
ꢄ
X
Form
Reference
Homolytic cleavage fX
Photocontraction fa
Photohydrolysis fb
0.012 ꢁ 0.001
a-Ketocarbene formation fc
Cl
Br
I
Neutral
Anionic
Neutral
Anionic
Neutral
Anionic
4
0.042 ꢁ 0.008
0.30 ꢁ 0.03
0.003 ꢁ 0.001
nd
0
1
4
0.035 ꢁ 0.007
0.27 ꢁ 0.03
0.009 ꢁ 0.001
< 10ꢀ5
0.04 ꢁ 0.01
0.03 ꢁ 0.01
< 0.005
0
3
This work
This work
0.004 ꢁ 0.002
0.11 ꢁ 0.02
0.08 ꢁ 0.01
0.03 ꢁ 0.01
nd: not determined
of the carbene formation yield to the photocontraction yield
for the anionic forms increases from bromo to iodo
(0.11 against 0.27). Based on the action of the internal heavy
atom effect, these two observations are consistent with path-
way a arising from the singlet excited state and pathway c from
the triplet excited state. The first conclusion is consistent with
the general view of the photo-Wolff rearrangement as proceed-
ing on the excited singlet surface.7,26 The second conclusion
is in line with the characteristic triplet reactivity of the a-keto-
carbene. The quantum yield of 2-IPhOꢀ photolysis is not as
high as expected; this may be explained by fast non-radiative
deactivation processes. Lastly, the efficiency of photodrolysis,
while very low, also decreases in the order Cl > Br > I, sug-
gesting that this reaction may take place from the singlet
excited state, too.
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Scheme 3
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