Concerted wolff rearrangement in two simple acyclic diazocarbonyl compounds

Article abstract of DOI:10.1021/jp108690n

The photochemistry of two simple acyclic diazo carbonyl compounds, azibenzil and diazoacetone, were studied using the tools of ultrafast time-resolved spectroscopy. In the former case, UV-vis detection allows observation of an absorption band of singlet benzoylphenylcarbene, decaying with a 740 ± 150 ps time-constant in acetonitrile. IR detection shows that the ketene product of Wolff rearrangement (～2100 cm^-1) is formed by two parallel pathways: a stepwise mechanism with carbene intermediacy with a slow rise time-constant of 660 ± 100 ps, and directly in the diazo excited state as confirmed by the immediate formation of an IR band of a nascent hot ketene species. Photolysis (270 nm) of diazoacetone in chloroform leads mainly to the ketene species through a concerted process, consistent with the predominance of the syn conformation in the diazoacetone electronic ground state and a zero quantum yield of the internal conversion process.

Full text of DOI:10.1021/jp108690n

J. Phys. Chem. A 2010, 114, 13065–13068
13065
Concerted Wolff Rearrangement in Two Simple Acyclic Diazocarbonyl Compounds
,
†
‡
‡
,‡
Gotard Burdzinski,* Yunlong Zhang, Jin Wang, and Matthew S. Platz*
Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz UniVersity, 85 Umultowska,
Poznan 61-614, Poland, and Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue,
Columbus, Ohio, 43210, United States
ReceiVed: September 12, 2010; ReVised Manuscript ReceiVed: October 30, 2010
The photochemistry of two simple acyclic diazo carbonyl compounds, azibenzil and diazoacetone, were studied
using the tools of ultrafast time-resolved spectroscopy. In the former case, UV-vis detection allows observation
of an absorption band of singlet benzoylphenylcarbene, decaying with a 740 ( 150 ps time-constant in
-
1
acetonitrile. IR detection shows that the ketene product of Wolff rearrangement (∼2100 cm ) is formed by
two parallel pathways: a stepwise mechanism with carbene intermediacy with a slow rise time-constant of
6
60 ( 100 ps, and directly in the diazo excited state as confirmed by the immediate formation of an IR band
of a nascent hot ketene species. Photolysis (270 nm) of diazoacetone in chloroform leads mainly to the ketene
species through a concerted process, consistent with the predominance of the syn conformation in the
diazoacetone electronic ground state and a zero quantum yield of the internal conversion process.
1
. Introduction
Photolysis of diazo carbonyl compounds promotes the extru-
this rule for acyclic diazo carbonyl compounds.3,4 Small cyclic
diazo carbonyl compounds are structurally locked in a syn
conformation and largely decompose and rearrange by a
concerted mechanism, however, a stepwise pathway does still
slightly contribute to the overall WR process.5 Since the typical
lifetimes of the singlet excited states of the diazo precursors
are less than 0.4 ps, the concerted process can be confirmed by
sion of molecular nitrogen and the formation of carbene, and
in many cases the formation of a ketene. This reaction is known
as the Wolff rearrangement (WR) and it has been extensively
studied by stationary and time-resolved spectroscopic, chemical,
,6
1
and theoretical methods. The data demonstrate that ketene
6
a fast product rise (<0.4 ps). The stepwise process, which
formation can proceed directly in the excited diazo state in
concert with molecular nitrogen extrusion. Ketene can also be
produced in a stepwise process in which a carbene is first formed
and then undergoes a subsequent isomerization.
Over 40 years ago, Kaplan and Meloy proposed that the
concerted WR process is favored in the syn conformers of
R-diazo carbonyl compounds, while the heat or light activation
of anti conformers promotes the stepwise process, as demon-
strated by the intermediacy of chemically trappable carbenes.2
Time-resolved infrared experiments confirmed the validity of
involves a carbene intermediate, results in a slower product rise
(
>1 ps) which is dependent on the carbene structure. The shortest
lifetime of any carbene measured to date is 2.3 ps (a small ring
carbene dilactone in acetonitrile).5
In a singlet carbene, the carbonyl group prefers to be
orthogonal to the plane defined by the carbene carbon atom and
6
,7
the two atoms directly bonded to the carbene carbon.
Incorporating a carbonyl carbene within a small ring enforces
planarity (or near planarity), destabilizes the singlet carbene,
and accelerates WR, thus the rigid enforcement of planarity (or
SCHEME 1
near planarity) of the molecule can be a major factor in ketene
formation.8
,9
In acyclic diazo carbonyl compounds, the quantum yield of
*
To whom correspondence should be addressed. E-mail: Gotardb@
ketene formation depends upon substituents (R
). Other pathways in the diazo excited state that are competitive
with WR include internal conversion to reform the diazo
1 2
and R , Scheme
amu.edu.pl, platz.1@osu.edu.
1
†
Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz
University.
‡
6
10
Department of Chemistry, The Ohio State University.
precursor in the ground state, cyclization when possible, or
1
0.1021/jp108690n  2010 American Chemical Society
Published on Web 11/30/2010

1
3066 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Burdzinski et al.
alkene formation by a 1,2-H shift process when R
1
is an alkyl
state with a lifetime shorter than our instrumental response
group.11 The WR quantum yield significantly decreases when
the carbonyl unit is a part of an ester group as the rearrangement
function (<0.3 ps). This short lifetime value is in line with our
6
previous studies of other diazo compounds. Diazo excited state
6
now requires loss of ester resonance energy. For diazo esters,
decay is followed by the formation of the vibrationally excited
carbene species at shorter wavelength (370 nm). Subsequent
vibrational relaxation leads to the thermalized carbene band
observed after a delay time of 14 ps (Figure 1A). The UV
spectrum observed is consistent with TD-DFT calculations of
singlet benzoylphenylcarbene in the gas phase (Table S1 of the
Supporting Information, SI) which predict significant carbene
oscillator strength at 429 nm (f ) 0.0275) and 333 nm (f )
0.1298). Analysis of the transient absorption kinetic trace
recorded at 370 nm (Figure 1B and S1 of the SI) results in 3
characteristic time-constants: < 0.1 ps (diazo excited state
lifetime), 6 ( 1 ps (vibrational cooling) and 740 ( 150 ps
(carbene lifetime). The transient lifetime, as expected for a
singlet carbene, was found to be significantly shorter in a
reactive solvent such as methanol (140 ( 20 ps).
Kaplan and Meloy’s rule does not apply in force as it does with
ketones. For example, the syn conformers of acyclic or
macrocyclic diazomalonates do not undergo concerted WR upon
photodecomposition and only carbene-derived products are
formed.9
,12
Singlet carbenes are known to react rapidly with alcohol
solvents or may undergo intersystem crossing to the triplet state,
6
when the triplet carbene is the ground state. This test is used
to assign putative singlet carbene absorption bands because if
the carrier of the band is truly a carbene, then its lifetime will
be dramatically shortened in alcohol relative to acetonitrile,
halogenated, or hydrocarbon solvent.
In our previous ultrafast UV-vis and IR studies, we have
focused on diazo carbonyl compounds containing aromatic
chromophores because the diazo excited states and carbenes
absorb at longer wavelengths than the smaller alkyl analogues.4
However, the photochemistry of a simple acyclic diazo ketone
has not been previously subjected to ultrafast laser spectroscopic
studies. Thus, the extent to which simple diazo ketone photo-
chemistry resembles that of their aromatic analogues remains
According to Trozzolo and co-workers, photolysis of azibenzil
in solution produces ketene as the major photoproduct
,7,13
18
(85%-90%). The rise of ketene could not be observed in the
ultrafast UV-vis experiment, since its absorption band lies at
about 313 nm.19 This is outside our detection window, in line
with calculations (Table S2 of the SI) which predict ketene
absorption at 283 nm (f ) 0.1973). To observe the ketene rise
(λex ) 270 nm), we used infrared detection (Figure 2) at ∼2100
unclear. This motivated this study of diazoacetone (R
1
) H, R
2
)
CH ) and azibenzil (R , R ) Ph) for the sake of comparison.
3
1
2
-1
cm (calculations predict a strong ketene vibrational transition
-
1
20
2
. Experimental Section
Azibenzil and diazoacetone were synthesized according to
at 2088 cm , Table S2 of the SI). A positive ketene band
-
1
rises at 2113 cm with two characteristic time constants: 45
10 ps (attributed to vibrational cooling of the hot ketene
(
1
4,15
procedures described in the literature.
The mid-IR and
UV-vis femtosecond transient absorption systems have been
7
,16
previously described.
Samples were prepared in 50 mL of
solution with an absorption of ∼1.0 at the excitation wavelength
measured in a 1 mm optical flow cell. The entire set of
pump-probe delay positions (cycle) is repeated at least three
times, to ensure data reproducibility from cycle to cycle. To
avoid rotational diffusion effects, the angle between the
polarizations of the pump beam and the probe beam was set to
the magic angle (54.7°). Kinetic traces are analyzed by fitting
to a sum of exponential terms. Convolution with a Gaussian
response function is included in the global fitting procedure.
The instrument response was ∼300 fs (fwhm). All experiments
were performed at room temperature.
2
.1. Calculations. DFT and TD-DFT calculations were
performed using the Gaussian 03 suite of programs at The Ohio
Supercomputer Center. Geometries were optimized at the
B3LYP/6-31+G(d) level of theory with single-point energies
obtained at the B3LYP/6-311+G(d,p)//B3LYP/6-31+G(d) level
of theory. Vibrational frequency analyses at the B3LYP/6-
3
1+G(d) level were utilized to verify that the stationary points
obtained corresponded to energy minima. The calculated
frequencies were then scaled by a factor of 0.9614.17 The
electronic spectra were computed using time-dependent density
functional theory of Gaussian 03 at the B3LYP/6-311+G(d,p)
level, and 20 allowed electronic transitions were calculated.
3
. Results
.1. Azibenzil. The photochemistry of azibenzil is amenable
3
to study by both time-resolved UV-vis (over the spectral probe
range 350-600 nm) and IR spectroscopy and the results
obtained with this system will be detailed first. Ultrafast
photolysis (307 nm) of azibenzil in acetonitrile produces the
transient UV-vis spectra of Figure 1A. The initial broad band
peaking at about 400 nm can be assigned to the diazo excited
Figure 1. Transient UV-vis spectra recorded for azibenzil in
acetonitrile after excitation at 307 nm, (A). Kinetic traces selected at
370 nm (points) fitted with a 3-exponential function: < 0.1 ps, 6, and
740 ps (solid line), (B).

Concerted Wolff Rearrangement
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13067
Figure 2. Transient IR spectra produced by ultrafast photolysis of
azibenzil in acetonitrile (λexc ) 270 nm) (A). Kinetic trace at 2113
Figure 3. Transient IR spectra produced by ultrafast photolysis of
diazoacetone in chloroform (λ ) 270 nm) (A). Kinetic trace selected
exc
-
1
-1
cm (points) fitted by a 2-exponential function with 45 and 660 ps
components (solid line) (B).
at 2124 cm (points) approximated by a fit with 1-exponential function:
40 ( 10 ps (solid line) (B).
formed in the concerted WR process) and 660 ( 100 ps (in
frequency upshift of the initial positive IR band over a 100 ps
time window (Figure 3). The relaxed ketene band, at about
agreement with the previously mentioned carbene lifetime of
5
7
40 ps).
1
00 ps, strongly overlaps the νNdN bleaching band to a greater
Inspection of the transient absorption spectra in Figure 2A,
at early delays, shows a negative band at 2090 cm- caused by
1
extent than is the case with azibenzil. The ketene infrared signal
is constant up to 2 ns post excitation pulse (Figure 3B).
Experiments performed in a carbene-trapping solvent, such as
methanol, leads to very similar transient infrared spectra.
Interestingly, the ratio of initial bleach amplitude to the relaxed
ketene signal is the same in chloroform and methanol. Thus,
little to no methanol trappable carbene is formed from diaz-
oacetone with λex ) 270 nm, implying that all of the ketene is
formed by the concerted WR process under these conditions.
immediate bleaching of the diazo vibration and a broad positive
band between 2030-2060 cm-1 assigned tentatively to vibra-
tionally excited ketene. The latter band might be slightly affected
by overlap with the spectrum of a hot diazo species produced
by internal conversion. The partial recovery of the ground state
bleach takes place over a 100 ps time window indicating a
reaction quantum yield, Φ
agreement with a previously reported value (0.5) in benzene
R
) 0.5, of diazo decomposition, in
1
9
solution upon irradiation with λ ) 313 nm.
.2. Diazoacetone. Excitation (270 nm) of diazoacetone, in
4
. Discussion
3
chloroform, generates the IR transient absorption spectra shown
The transient spectra and kinetics obtained upon ultrafast
in Figure 3. At initial delays, a positive band is present at about
2
photolysis of azibenzil in time-resolved IR and UV-vis studies
020 cm- and a negative band is seen at 2110 cm . The initial
1
-1
are reminiscent of those with p-biphenyl methyldiazo ketone,
4
positive band can be clearly assigned to the hot ketene species.
An alternative attribution of this band to vibrationally hot
diazoacetone in its electronic ground state (formed by internal
conversion) can be ruled out, as the bleaching of the IR signature
of the precursor is constant over a 2 ns time window, as deduced
from νCdO recorded at about 1650 cm- (Figure S2 of the SI).
Initially, the nascent ketene species undergoes vibrational
relaxation as confirmed by band-narrowing and the spectral
BpCN
2
COCH
3
. However, in studies of BpCN
2
COCH
3
the
carbene band was better resolved spectroscopically than in this
work. The maximum absorbance of the carbene was at 380 nm
and a fast reaction pathway from hot carbene species to ketene
could be ruled out based on the constant band area over 0.8-30
1
4,21
ps, indicating a constant carbene concentration.
In the case of azibenzil, calculations indicate that the dominant
conformer is syn (62% in polar solvent estimated from Boltz-

1
3068 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Burdzinski et al.
TABLE 1: B3LYP/6-311+G(d,p)//B3LYP/6-31+G(d)
diazo excited state nitrogen extrusion proceeds. From the
ultrafast rise of ketene species (<0.4 ps time constant), we can
deduce that the lifetime of the diazo excited state populated
must be shorter than 0.4 ps.
-1
Calculations of Gibbs Free Energy Differences (kcal ·mol )
for Syn and Anti Conformers of Diazoacetone and Azibenzil
in the Gas Phase, Cyclohexane, Chloroform, and Acetonitrile
Diazoacetone produces ketene with a large quantum yield
upon UV excitation because the diazo carbonyl compound exists
predominantly in the syn conformation favoring the concerted
process (Kaplan and Meloy’s rule) and its simple structure does
1 0
not facilitate the internal conversion process (S f S ), and
therefore, IC is not a competitive deactivation channel of the
diazo excited state.
∆G298K syn % anti % ∆G298K syn % anti %
gas phasea
cyclohexane
chloroform
acetonitrile
1.14
1.05
1.10
1.21
87.3
85.5
86.5
88.5
12.7
14.5
13.5
11.5
-0.21
0.00
0.17
41.2
50.0
57.1
61.6
58.8
50.0
42.9
38.4
b
b
Acknowledgment. Ultrafast studies were performed at The
Ohio State University Center for Chemical and Biophysical
Dynamics. Support of this work by the National Science
Foundation and by the Ohio Supercomputer Center are gratefully
acknowledged. G.B. thanks the Foundation for Polish Science
b
0.28
a
The zero-point energy (ZPE) for the gas-phase calculations is
corrected by a factor of 0.9806.17 b The solution-phase calculations
used PCM models, and the geometries are from the gas-phase
B3LYP/6-31+(d) calculations.
(
FNP) for a “Homing” Grant in the year 2009.
mann distribution at 293 K, Table 1). The short (less than 100
fs) lifetime of the diazo excited state indicates that the syn and
anti forms produced with 270 nm light do not interconvert, thus
the photochemistry proceeds from the Franck-Condon state.
Our experimental observation that both concerted and stepwise
paths are operative is in agreement with the presence of two
diazo excited state conformers and with Kaplan and Meloy’s
rule, assuming that syn favors concerted WR and the anti
conformer is disposed toward carbene formation. Calculations
of diazoacetone predict that the electronic ground state exists
mainly in the syn form (87%) in gas phase and in solution (Table
Supporting Information Available: Spectra and kinetics
of ultrafast studies, DFT, and TD-DFT calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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(
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(
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1
) in agreement with the 90% value determined by NMR in
22
acetone. Indeed time-resolved IR spectroscopy indicates that
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(
(
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(
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2
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(
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-1
carbene carbonyl infrared band at about 1563 cm , Table S7
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(
(
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2
3
carbene species reported earlier.
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(
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(
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as azibenzil and BpCN
Excitation of diazoacetone and azibenzil with 270 nm
radiation pumps the diazo compound to the S (n g 4) state as
indicated by time-dependent density functional theory (TD DFT)
2
COCH
3
(ΦIC ≈ 0.5).
Ernsting, N. P. Chem. Phys. Lett. 2000, 323, 312.
(
(
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2
4,25
calculations (Tables S3-S6 of the SI).
diazo S (n g 4) state may undergo an ultrafast IC process to
populate the S state. Thus, we do not know from which singlet
The initially excited
(
25) Stratmann, R.; Scuseria, G.; Frish, M. J. Chem. Phys. 1998, 8218.
n
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