Photo-Wolff rearrangement of 2-Diazo-1,2-naphthoquinone: Stern-Volmer analysis of the stepwise reaction pathway

Article abstract of DOI:10.1111/php.12341

Abstract 2-Diazo-1,2-naphthoquinone (1) and its derivatives are the photoactive components in Novolak photoresists. A femtosecond infrared study has established that the photoreaction of 1 proceeds largely by a concerted Wolff rearrangement yielding the ketene 1H-inden-1-ylidene-methanone (3) within 300 fs after excitation, but earlier trapping studies gave evidence for a minor reaction path via a carbene intermediate 1-oxo-2(1H)-naphthalenylidene (2) with a lifetime of about 10 ps. Here, we provide a quantitative assessment of the stepwise pathway by Stern-Volmer analysis of the trapping of 2 by methanol to yield 2-methoxy-1-naphthol (4). We conclude that the lifetime of the carbene 2 is at least 200 ps. Moreover, [3 + 2]cycloaddition of 2 and acetonitrile yielding 2-methylnaphth[2,1-d]oxazole (5) was observed. A comparison of the yields of 5 formed upon photolysis and upon thermolysis of 1 in acetonitrile provides evidence that a substantial part of the hot nascent carbene 2 formed photolytically rearranges to the ketene 3 during its vibrational relaxation (hot ground-state reaction). A quantitative assessment of the stepwise versus concerted photodeazotization pathways of 2-diazo-1,2-naphthoquinone (1) forming ketene 3 is provided. Trapping of the carbene intermediate 2 by methanol yields 2-methoxy-1-naphthol (4) in up to 12% yield. [3+2]Cycloaddition of 2 and acetonitrile yielding 2-methylnaphth[2,1-d]oxazole (5) was also observed. The lifetime of the thermalized carbene 2 is at least 200 ps. A comparison of the yields of 5 formed upon photolysis and upon thermolysis of 1 in acetonitrile provides evidence that a substantial part of the hot nascent carbene 2 formed photolytically rearranges to the ketene 3 during its vibrational relaxation (hot ground-state reaction).

Full text of DOI:10.1111/php.12341

Photochemistry and Photobiology, 2015, 91: 678–683
Photo-Wolff Rearrangement of 2-Diazo-1,2-naphthoquinone: Stern–Volmer
Analysis of the Stepwise Reaction Pathway†
Manfred Ladinig‡, Markus Ramseier§ and Jakob Wirz*
Department of Chemistry, University of Basel, Basel, Switzerland
Received 27 May 2014, accepted 31 August 2014, DOI: 10.1111/php.12341
ABSTRACT
a short-lived a-ketocarbene intermediate 2 with a lifetime of
about 20 ps precedes the formation ketene 3 (stepwise reaction,
Fig. 1). However, a more recent study employing femtosecond
time-resolved vibrational spectroscopy (8,9) has established that
the major primary product, ketene 3, is formed within 300 fs.
The nascent ketene has an excess of vibrational energy; vibra-
tional cooling of 3 occurs with a time constant of about 10 ps in
methanol and 3 ps in water. Thus, partial formation of the ketene
3 via a short-lived carbene intermediate 2 with a lifetime on the
order of 10 ps was not ruled out (8). Finally, the involvement of
an oxirene intermediate was considered (10), but this was later
ruled out on the basis of isotopic labeling experiments (11).
Two recent quantum dynamics calculations for the behavior
of the singlet excited state of 1 (12,13) produced similar results:
both the concerted Wolff rearrangement to ketene 3 and the step-
wise reaction forming carbene 2 were predicted to occur on a
time scale of hundreds of femtoseconds. Both studies predicted
that reaction via 2 predominates. While the (partial) occurrence
2-Diazo-1,2-naphthoquinone (1) and its derivatives are the
photoactive components in Novolak photoresists. A femtosec-
ond infrared study has established that the photoreaction of
1 proceeds largely by a concerted Wolff rearrangement yield-
ing the ketene 1H-inden-1-ylidene-methanone (3) within
300 fs after excitation, but earlier trapping studies gave evi-
dence for a minor reaction path via a carbene intermediate
1-oxo-2(1H)-naphthalenylidene (2) with a lifetime of about
10 ps. Here, we provide a quantitative assessment of the step-
wise pathway by Stern–Volmer analysis of the trapping of 2
by methanol to yield 2-methoxy-1-naphthol (4). We conclude
that the lifetime of the carbene 2 is at least 200 ps. Moreover,
[3 + 2]cycloaddition of 2 and acetonitrile yielding 2-meth-
ylnaphth[2,1-d]oxazole (5) was observed. A comparison of the
yields of 5 formed upon photolysis and upon thermolysis of 1
in acetonitrile provides evidence that a substantial part of the
hot nascent carbene 2 formed photolytically rearranges to
the ketene 3 during its vibrational relaxation (hot ground-
state reaction).
1
of a concerted rearrangement 1* ? 3 on a subpicosecond time
scale is thus well established, the amount proceeding in a step-
wise fashion and the lifetime of the putative carbene 2 intermedi-
ate remain still uncertain. Michael Kasha devoted much of his
recent work on the elucidation of concerted versus stepwise
photo-induced double proton reactions (14).
INTRODUCTION
An attempt to trap 2 with pyridine or 2-adamantanethione
failed (15). In a subsequent study of the 5-sulfonate salt of 1 in
aqueous alcohols, the formation of 2-alkoxy-1-naphthols and 1,2-
naphthalenediol was observed, in addition to the main indene-
carboxylic acid products formed from the ketene 3. These side
products were attributed to the addition of water or alcohols to
carbene 2 (4). Similar findings obtained upon preparative irradia-
tion of 1 in methanol had been reported previously (16). We
report a detailed Stern–Volmer analysis of the products formed
by trapping of 2 by methanol, 2-methoxy-1-naphthol (4) and by
acetonitrile, 2-methylnaphth[2,1-d]oxazole (5), (Fig. 2). The for-
mation of 5 upon photolysis or pyrolysis of 1 in acetonitrile has
been observed previously (17) and a similar reaction was
reported for diazoacenapthenone (18).
€
In 1944, Sus identified indenecarboxylic acid as the main prod-
uct formed by irradiation of 2-diazo-1,2-naphthoquinone (1) in
acidic aqueous solution (1). He also proposed a mechanism for
the ring contraction (Wolff rearrangement via ketocarbene 2) and
designed a lithographic printing plate based on the increased sol-
ubility of those areas of a Novolak film that were exposed to
light. Novolak mixtures containing 1 are still used in the fabrica-
tion of integrated circuits.
The mechanism of this reaction has been studied extensively.
The product of the photo-Wolff rearrangement, ketene 3, has
been generated in rigid matrices at low temperatures and was
identified by its UV (2–4) and IR (2,4,5) spectra. In aqueous
solution, ketene 3 hydrolyzes through a long-lived enol interme-
diate, which finally ketonizes to a mixture of indenecarboxylic
acids (6,7). Based on picosecond transient absorption measure-
ments with the 5-sulfonate derivative of 1, it was argued (4) that
MATERIALS AND METHODS
Instrumentation. GC–MS analysis was performed on a Hewlett–Packard
GC–MS (GC: 5890 series II, 25 m SE-52 capillary column; MS: 5970A
series). For NMR measurements, a 300 MHz instrument (Varian VX300)
was used. Irradiation experiments were carried out with a high-pressure
mercury arc lamp (Hanau St 41, 200 W) equipped with various band-
pass filters. Fluorescence spectra were measured at ambient temperature
*Corresponding author email: J.Wirz@unibas.ch (Jakob Wirz)
†This manuscript is part of the Special Issue dedicated to the memory of Michael
Kasha
‡Present address: Boverfeld 44, D-44227 Dortmund, Germany
§Present address: Ebenrainweg 13, CH-4450 Sissach, Switzerland
© 2014 The American Society of Photobiology
(ca. 22°C) on
a Spex Fluorolog (Model 111c) spectrophotometer
678

Photochemistry and Photobiology, 2015, 91 679
ketene intermediate 3 is efficiently trapped by the small amount
of methanol. In this case, a different side product was formed,
which was isolated by preparative TLC and identified as 2-meth-
ylnaphth[2,1-d]oxazole (5, Fig. 2).
Product yield of 4. Quantitative analysis of the photoproduct 2-
methoxy-1-naphthol (4) is not straightforward, because it is a
rather reactive compound. Kral et al. (24) reported that 2-alkoxy
substituted 1-naphthols readily undergo oxidative dimerization to
form diones; the dione formed from 4 has a violet color
(kmax = 542 nm). Irradiated solutions of 1 in methanol turned
violet when they were concentrated for GC–MS analysis.
1H-NMR, HPLC and GC–MS, which require relatively high con-
centrations, were therefore not suitable for a quantitative analysis
of product 4. We found that 4 is strongly fluorescent, whereas
the major product 6 is not. A solution of 1 (4 9 10ꢀ5 M) in
methanol was irradiated at 405 nm in a quartz cell until no fur-
ther changes were observed in the absorption spectrum. As only
compound 1 absorbs at the irradiation wavelength of 405 nm,
prolonged irradiation is unproblematic. The fluorescence excita-
tion and emission spectra of an exhaustively photolyzed solution
of 1 and of an authentic sample of 4 (5 9 10ꢀ6 M) in methanol
were in good agreement (Fig. 3). Slight differences in the
wavelength region below 300 nm of the excitation spectrum may
be attributed to the inner filter effect of absorption by 6 in the
photoproduct mixture.
Figure 1. Concerted and stepwise pathways for the photo-Wolff rear-
rangement of 1.
equipped with an EMI R928 phototube; excitation spectra were corrected
by reference to a Rhodamine 6 G quantum counter. Emission spectra are
uncorrected.
Synthesis of 2-methylnaphth[2,1-d]oxazole (5). 2-Diazo-1,2-naphtho-
quinone (1) was available from a previous study (7). 2-Methylnaphth
[2,1-d]oxazole (5) was obtained by refluxing 0.9 g of 1 in acetonitrile at
80°C for 115 h. The solvent was evaporated and the residue chromato-
graphed with methylene chloride over neutral aluminum oxide. The main
fraction was distilled in vacuo (150°C) and crystallized from hexane,
m.p. 38–39°C, lit. (19) 36–37°C. 1H-NMR and UV–Vis spectra are in
agreement with published data (20,21). 1H-NMR (300 MHz, CD3OD): d
2.76 (s, 3H), 7.57 (t, 1H), 7.67 (t, 1H), 7.72 (d, 1H), 7.85 (d, 1H), 8.03
(d, 1H), and 8.21 ppm (d, 1H). MS: m/e 183 (M+).
Product studies by GC–MS. Solutions of
1
(5 9 10–5 M, not
Thus, the fluorescence emission induced by excitation at
wavelengths >300 nm provides a sensitive technique for the
exclusive detection of 4 in the photoproduct mixtures without
any work-up. The fluorescence spectrum of 4 depends on the
solvent, kmax(methanol) = 390 nm, kmax(hexane) = 367 nm. The
relative yield of 4, r(4) = /4//dec, was determined by calibration
with solutions containing known concentrations of authentic 4 in
the same solvent mixtures. The quantum yields, /4 and /dec, are
those for the formation of 4 and the decomposition of 1. Opti-
cally dilute solutions were used (absorbances at the excitation
wavelength <0.02) to ensure that the calibration plots showed
strictly linear dependence on the concentration of 4.
degassed) in methanol or acetonitrile/methanol mixtures were irradiated
with light of various wavelengths (254, 405, and 436 nm) filtered from a
medium-pressure mercury arc (Hanau St.41). The reaction progress was
followed by optical spectroscopy. For qualitative GC–MS analysis,
approximately 50 mL of the irradiated solutions was concentrated to an
end volume of ca. 200 lL. Preparative TLC was performed on silica gel
(Merck, silica gel 60 F254; a 1:1 mixture of hexane and ethyl acetate was
used as eluent).
Actinometry. Quantum yields were determined by UV-spectrophoto-
metric monitoring of the reaction progress using azobenzene actinometry
(22,23).
RESULTS
In neat methanol, the yield of 4 was 15 ꢁ 1% after exhaus-
tive irradiation (6 min) of 1 at 405 nm (mercury arc). It
decreased substantially when lower wavelengths of irradiation
were used: 13 ꢁ 1% were obtained by 351 nm excitation (XeF
excimer laser), and about 8% by irradiation at either 254 nm
(mercury arc) or at 248 nm (KrF excimer laser). The photoprod-
uct 4 is not entirely stable to 248 or 254 nm irradiation; irradia-
tion of a solution of 4 for the same period reduced its
fluorescence intensity by 15%. However, this small reduction
cannot account for the much lower yield of 4 obtained by 248 or
254 nm irradiation of 1, indicating that the yield of carbene 2 is
reduced to about ½ upon irradiation at short wavelengths.
Trapping reactions of 2
Methanol. Solutions of 1 (5 9 10–5 M) in methanol were irradi-
ated with light of various wavelengths (254, 405 or 436 nm).
The product mixture was analyzed by GC–MS. The major prod-
uct, methyl indene-3-carboxylate (6), and a trace (1–2%) of 2-
methoxy-1-naphthol (4), were identified using authentic samples
for comparison. 1-Naphthol was detected only upon triplet sensi-
tization of 1 with excess benzophenone.
Acetonitrile. On irradiation of 1 in acetonitrile containing 1% of
methanol, ester 6 was again the main product, indicating that the
Figure 2. Trapping reactions of the carbene intermediate 2.

680 Manfred Ladinig et al.
/
kMeOH½MeOHꢂ
/
2
4
rð4Þ ¼
¼
ꢃ
/
dec
ð1Þ
/
kMeOH½MeOHꢂ þ kWR
dec
ꢀ
ꢁ
1
rð4Þ
kWR
kMeOH½MeOHꢂ
/
dec
¼
1 þ
ꢃ
/
2
ð1aÞ
A “dual reciprocal plot” of the data, (1/r(4) versus 1/[MeOH]),
was linear, in agreement with Eq. (1a). The fit parameters kMeOH
/
–1
kWR = 3.2 ꢁ 0.2 M and /2//–N2 = 0.12 ꢁ 0.01 were, never-
theless, best determined by nonlinear least-squares fitting of
Eq. (1) to the experimental data r(4) to avoid distortion by the
increasing experimental errors in 1/r(4) with decreasing methanol
concentrations [MeOH]; the standard errors in r(4) are constant. It
is seen from Fig. 4 that the yields r(4) are well reproduced by the
fitted function. At high methanol concentration nearly all of car-
bene 2 is trapped by methanol. The asymptotic yield of 4 obtained
from Eq. (1) for [MeOH]/M ? ∞ and thus r(4) ? /4//dec is
12 ꢁ 1%. Thus, the Wolff rearrangement appears to proceed
mainly (88%) along a “concerted” pathway.
Figure 3. Fluorescence emission and excitation spectra of a 4 9 10ꢀ5
M
solution of 1 in methanol after exhaustive irradiation at 405 nm (dotted
line), and of a 5 9 10ꢀ6 M solution of 4 in methanol (solid line). Excita-
tion wavelength for emission: 330 nm. Emission wavelength chosen for
the excitation spectrum: 390 nm.
Methanol in dioxane. Similar results were obtained with
solutions of methanol in dioxane (not shown). These solvents are
miscible in all proportions. Again, 6 was the only product identi-
fied by GC–MS and 4 was the only product detected by fluores-
cence spectroscopy. The parameters obtained by nonlinear
ꢀ1
least-squares fitting of Eq. (1) are kMeOH/kWR = 1.2 ꢁ 0.2 M
and /2//–N2
=
0.15 ꢁ 0.01 and the data are accurately
reproduced by the fitted function.
Methanol in acetonitrile. In the solvent mixtures of methanol
with hexane and dioxane used above, only methanol was suffi-
ciently reactive to intercept the carbene 2 prior to its rearrange-
ment to ketene 3. Other solvent addition products were not
detected even at very low methanol concentrations. A different
situation arises in mixtures of methanol and acetonitrile, because
carbene 2 is also trapped by acetonitrile (Fig. 2). Hence, the rela-
tive yield of 4 is expected to depend on the concentrations of
both methanol and acetonitrile, Eq. (2).
Figure 4. Relative yields of 4, r(4), obtained by 405 nm irradiation of 1
in hexane as a function of added methanol. The solid line represents the
best fit of Eq. (1) to the experimental data. A linear regression to the
same data according to Eq. (1a) is shown in the inset.
Stern–Volmer analyses; methanol in hexane. Methyl indenecarb-
oxylate (6) was the only product detected by GC–MS after irra-
diation of 1 (5 9 10ꢀ5 M) in hexane containing 1% of methanol.
Neither 1-naphthol nor a hexane insertion product was found.
Fluorescence revealed the presence of a trace of 4; the yield of 4
increased with increasing methanol concentration, but rapidly
approached a plateau of about 12% at the highest achievable
methanol concentration of 2 M. Saturation of trapping would not
be expected, if the Wolff rearrangement proceeded exclusively
via the trappable carbene intermediate 2. On the contrary, related
work has shown excess reactivity of singlet carbenes toward
methanol at high methanol concentrations in an inert solvent
(25). The relative yields of 4 determined after exhaustive photol-
ysis of 1 at 405 nm, r(4) = /4//dec, as a function of methanol
concentration up to 1 M is shown in Fig. 4.
/
kMeOH½MeOHꢂ
/
2
4
rð4Þ ¼
¼
ꢃ
/
/
kMeOH½MeOHꢂ þ kMeCN½MeCNꢂ þ kWR
dec
dec
ð2Þ
The molar concentrations of pure methanol and acetonitrile
are [MeOH] = 24.6 M and [MeCN] = 18.9 M at 25°C. Excess
molar volumes in mixtures of these solvents are slightly nega-
tive, but never below ꢀ0.165 cm3 molꢀ1 (26), i.e., the reduction
in volume upon mixing is well below 1% in all mixtures. Ignor-
ing these small changes of the partial molar volumes, the
concentration of acetonitrile may be expressed as a linear
function of the methanol concentration in acetonitrile/methanol
mixtures.
The observed saturation of the trapping yield implies that the
reaction proceeds predominantly by a “concerted” formation of
ketene 3, which bypasses the trappable carbene 2. The relative
yield r(4) as a function of methanol concentration is given by
Eq. (1), as derived in the Appendix by analysis of the kinetic
scheme shown in Fig. 5.
18:9
24:6
½MeCNꢂ ffi 18:9m ꢀ
½MeOHꢂ
ð3Þ
Combining Eqs. (2) and (3), the relative yield of 4 can be
expressed as a function of methanol concentration only, Eq. (4).

Photochemistry and Photobiology, 2015, 91 681
Figure 5. Kinetic scheme for the trapping reaction of 2 by methanol.
/
kMeOH½MeOHꢂ
/
2
4
À
Á
rð4Þ ¼
ffi
ꢃ
ð4Þ
18:9
/
/
dec
kMeOH ꢀ 24:6
k
½MeOHꢂ þ kMeCN ꢃ 18:9m þ kWR
MeCN
dec
The fluorescence spectra of exhaustively irradiated
(kirr = 405 nm) solutions of 1 (4 9 10ꢀ5 M) in acetonitrile/meth-
anol mixtures are shown in Fig. 7. The change of the fluorescent
products from predominantly 5 (kmax = 350 nm) in acetonitrile
to predominantly 4 (kmax = 390 nm) in methanol is clearly seen.
Relative yields of 4 in mixtures of methanol and acetonitrile
are shown in Fig. 6. The rate constant kWR was ill defined by
least-squares fitting of Eq. (4). That is reasonable, as we know
that essentially all of the interceptable carbene 2 is trapped in
these solvent mixtures. For example, using the ratio kMeOH
/
kWR = 3.2 Mꢀ1 obtained from the analysis of Eq. (1) (methanol in
hexane), the ratio kMeOH[MeOH]/kWR for neat methanol equals
about 80. Nonlinear least-squares fitting of Eq. (4) to the
experimental data was thus done by neglecting the term kWR rela-
tive to the other terms in the denominator, which gave an excel-
lent fit with the two parameters kMeOH/kMeCN = 1.7 ꢁ 0.1 and
/2//dec = 0.15 ꢁ 0.01. The limiting yield of 15 ꢁ 1% is the
same as that determined for the dioxane/methanol solvent mixture
above, but is reached only in neat methanol, because the compet-
ing reaction of 2 with acetonitrile is nearly as fast as that with
methanol.
Photochemical versus thermal deazotization of 1 in
acetonitrile
The relative yield of 5, r(5) = /5//dec, was determined by mea-
surement of the fluorescence intensities that were calibrated using
solutions of known concentrations of 5. Thermal decomposition of
1 by refluxing an acetonitrile solution at 80°C gave r(5) = 23%. A
solution heated to 190°C in an autoclave gave r(5) = 18%.
Exhaustive irradiation of a solution of 1 at 405 nm gave r
(5) = 7.6% in acetonitrile and r(5) = 5.3% in acetonitrile-d3.
Figure 6. Relative yields of 4, r(4), obtained by irradiation of 1 in ace-
tonitrile/methanol mixtures. The solid line represents the best fit of
Eq. (4) to the experimental data. A linear regression to the same data
(down to 0.025 M methanol) using the inverse of Eq. (4) is shown in the
inset. At still lower methanol concentrations the scatter becomes exces-
sive.
Figure 7. Uncorrected fluorescence spectra of the photoproducts of 1 in
methanol (vol-% shown)–acetonitrile mixtures. Excitation: 260 nm.

682 Manfred Ladinig et al.
Acknowledgement—This work was supported by the Swiss National
Science Foundation.
Quantum yield of the photoreaction of 1
Quantum yields of decomposition of 1, /dec, were measured by sev-
eral groups; values reported range from 0.14 to 0.79 (4,27,28). The
most recent value, measured by photolysis of 1 in methanol at
405 nm, is 0.54 (4). It has been reported that the quantum yield of
photoconversion of quinonediazides (aromatic 1,2-diazoketones)
depends on the wavelength of irradiation (27,28). We have
APPENDIX
The quantum yields of decomposition of 1 and of formation of the
interceptable carbene 2 may be expressed (36) in terms of the rate
constants kconcerted, kstepwise and knr for the concerted, stepwise and
nonradiative decay of the excited singlet state of 1, 11*,
measured
/dec(1) at various wavelengths of irradiation:
/
dec(254 nm) = 0.38 ꢁ 0.04, /dec(313 nm) = 0.52 ꢁ 0.04, /dec
(Fig. 5):
and
/
= (kconcerted + kstepwise)/(kconcerted + kstepwise + knr)
dec
(365 nm) = 0.62 ꢁ 0.04,
/
dec(405 nm) = 0.38 ꢁ 0.03. The
/2 = (kstepwise)/(kconcerted + kstepwise + knr).
Hence,
quoted standard errors do not include possible systematic errors of
our measurements. The differences between the values obtained
at different excitation wavelengths may, therefore, not be
significant.
/2//dec = kstepwise/(kconcerted + kstepwise).
Similarly, the quantum yield for the formation of 4 by trap-
ping of carbene is given by /4 = /2 9 kMeOH[MeOH]/
2
(kMeOH[MeOH] + kWR), where kMeOH and kWR are the rate con-
stants for the trapping of 2 by methanol and its reaction by
Wolff rearrangement. Hence, the ratio r(4) = /4//dec is given by
Eq. (2).
DISCUSSION
The work of Wolpert et al. (8) has established that the photo-
Wolff rearrangement of 2-diazo-1,2-naphthoquinone (1) to the
ketene 3 is largely complete within 300 fs. Nevertheless, the
side product 2-methoxy-1-naphthol (4) is formed in methanol,
which indicates that a trappable carbene intermediate 2 is
formed as well (stepwise pathway, Fig. 1). The trapping of car-
bene 2 by methanol and acetonitrile (Fig. 2) indicates that it
reacts in its singlet state, which is presumably its ground state.
The ratios of the rate constants of trapping by methanol and of
Wolff rearrangement of 2, kMeOH/kWR, were determined as
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ꢀ1
3.2 ꢁ 0.2, 1.2 ꢁ 0.2 and 1.7 ꢁ 0.1 M in mixtures of metha-
nol with hexane, dioxane and acetonitrile, respectively. Bimo-
lecular rate constants for OH insertion by singlet carbenes are
generally close to the diffusion-controlled limit (29–33), i.e.,
around kMeOH ꢄ 5 9 109
M
s
ꢀ1. Assuming an upper limit of
ꢀ1
kMeOH < 1 9 1010
M
s
ꢀ1, these values indicate that the rate
ꢀ1
constant kWR for Wolff rearrangement of the trappable carbene
2 is less than 5 9 109 ꢀ1. Thus, the lifetime of 2 must be at
s
least 200 ps, more likely around 500 ps. This range lies well
above the value proposed by Vleggaar et al. (4), s2 = 20 ps.
Because the relative yield of 2 is less than 15% and the life-
time of 2 exceeds 200 ps, the stepwise reaction may well have
escaped detection in the experiments of Wolpert et al. (8),
whose measurements extended only up to 50 ps after pulsed
excitation.
The carbonyl carbene 2 is born with a huge excess energy
upon nitrogen elimination from the singlet excited state of 1.
It may well be that Wolff rearrangement of hot 2 competes
with its vibrational cooling, as was proposed for related carb-
enes by Platz and co-workers (34,35). Evidence for this
hypothesis is provided by the relative yields of oxazole 5 in
acetonitrile under different reaction conditions: thermal deazoti-
zation gives nearly three times as much 5 as the photochemi-
11. Blocher, A. and K. P. Zeller (1993) Photolyse von Naphth[2,3-d]
[1,2,3]oxadiazol – ein Beitrag zum Oxiren-Problem. Chem. Ber. 127,
551–555.
cal reaction. Moreover, the yield of
5 obtained upon
irradiation of 1 in acetonitrile-d3 is about 30% lower than in
acetonitrile-h3. The solvent with C–H bonds is a better accep-
tor for the excess vibrational energy of nascent 2 than that
having C–D bonds only. This may explain the apparent dis-
crepancy with quantum dynamical calculations (12,13), which
predicted that the stepwise reaction via 2 predominates over
the concerted Wolff rearrangement, while we find that the rel-
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