Trans-cis isomerization of vinylketones through triplet 1,2-biradicals

Article abstract of DOI:10.1021/jp504174t

The irradiation of trans-vinylketones 1a-c yields the corresponding cis isomers 2a-c. Laser flash photolysis of 1a and 1b with 308 and 355 nm lasers results in their triplet ketones (T_1K of 1), which rearrange to form triplet 1,2-biradicals 3a and 3b, respectively, whereas irradiation with a 266 nm laser produces their cis-isomers through singlet reactivity. Time-resolved IR spectroscopy of 1a with 266 nm irradiation confirmed that 2a is formed within the laser pulse. In comparison, laser flash photolysis of 1c with a 308 nm laser showed only the formation of 2c through singlet reactivity. At cryogenic temperatures, the irradiation of 1 also resulted in 2. DFT calculations were used to aid in the characterization of the excited states and biradicals involved in the cis-trans isomerization and to support the mechanism for the cis-trans isomerization on the triplet surface.

Full text of DOI:10.1021/jp504174t

Article
pubs.acs.org/JPCA
Trans−Cis Isomerization of Vinylketones through Triplet 1,2-
Biradicals
R. A. A. Upul Ranaweera,† Tianeka Scott,† Qian Li,† Sridhar Rajam,† Alexander Duncan,† Rui Li,‡
Anthony Evans,§ Cornelia Bohne,‡ John P. Toscano,§ Bruce S. Ault,† and Anna D. Gudmundsdottir*,†
†Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States
‡Department of Chemistry, University of Victoria, Victoria, BC V8P 5C2, Canada
§Department of Chemistry, John Hopkins University, Baltimore, Maryland 21218, United States
S
* Supporting Information
ABSTRACT: The irradiation of trans-vinylketones 1a−c
yields the corresponding cis isomers 2a−c. Laser flash
photolysis of 1a and 1b with 308 and 355 nm lasers results
in their triplet ketones (T1K of 1), which rearrange to form
triplet 1,2-biradicals 3a and 3b, respectively, whereas irradiation
with a 266 nm laser produces their cis-isomers through singlet
reactivity. Time-resolved IR spectroscopy of 1a with 266 nm
irradiation confirmed that 2a is formed within the laser pulse.
In comparison, laser flash photolysis of 1c with a 308 nm laser
showed only the formation of 2c through singlet reactivity. At
cryogenic temperatures, the irradiation of 1 also resulted in 2.
DFT calculations were used to aid in the characterization of the
excited states and biradicals involved in the cis−trans isomerization and to support the mechanism for the cis−trans
isomerization on the triplet surface.
More recently, Garcia-Exposito et al. studied intermolecular
photosensitization of 2-pentenoate esters and found that they
formed long-lived triplet 1,2-biradicals (Scheme 1).19 The
authors explained that these biradicals are long-lived because
the decay from the triplet to the singlet surface is enhanced by
spin−orbit coupling20 which is negligible in the minimal energy
conformer of the 1,2-biradicals. The large energy gap between
the singlet and triplet surfaces restricts efficient intersystem
crossing, and although the energy gap is at minimum in the
minimal energy conformer of the 1,2-biradical, it was theorized
that the intersystem crossing from the triplet surface to the
singlet one takes place where the two 1,2-biradicals centers are
located ∼45° apart, where the spin−orbit coupling is more
significant. Caldwell and co-workers came to a similar
conclusion for the spin−orbit coupling in ethylene.21
In this paper, we report the photochemistry of simple alkenes
1 that have a built-in intramolecular sensitizer. The photo-
chemistry of 1a and 1b is wavelength dependent, as laser flash
photolysis of them with 355 and 308 nm lasers allowed us to
detect both triplet 1,2-biradicals and their triplet ketone
precursors. The biradicals have a lifetime of ∼3−6 μs in
acetonitrile and reacted with O2 with a rate constant of the
1. INTRODUCTION
The cis−trans isomerization of alkenes has potential
applications in light-triggered photoswitches, photoremovable
protecting groups, and image storage devices, and these
applications are of general interest.1,2 In the 1970s, Mulliken
suggested that the cis−trans isomerization of ethylene on the
triplet surface takes place via a twisted 1,2-biradical
intermediate, *P3, known as the phantom state.3,4 The twisted
geometry of the 1,2-biradical offsets the electronic repulsion
between the unpaired electrons on the adjacent carbon atoms.
Ab initio theory studies predict the energy gap between the
ground state and the *P3 of ethylene to be ∼60 kcal/mol. Only
recently has laser flash photolysis revealed the property of S1 of
ethylene to have a lifetime of less than 200 fs.5,6 Intersystem
crossing to *P3 of ethylene was not observed, but internal
conversion to S0 occurred instead.
Despite the importance of cis−trans isomerization, simple
1,2-biradicals have been studied sporadically because the direct
irradiation of simple alkenes does not result in intersystem
crossing to their triplet surface.7,8 In comparison, the cis−trans
isomerization of styrene on its singlet and triplet surface has
been studied extensively.9−18 For example, Caldwell et al. have
shown that triplet-sensitized photolysis of styrene derivatives
results in triplet 1,2-biradicals. The lifetimes of acyclic flexible
1,2-biradicals were found to be within ∼100 ns, whereas for
rigid or cyclic styrene derivatives, which have limited rotation,
the lifetimes increased to a few microseconds.9
Special Issue: Current Topics in Photochemistry
Received: April 29, 2014
Revised: June 29, 2014
Published: July 2, 2014
© 2014 American Chemical Society
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Scheme 1
Scheme 2. Product Studies of 1a−c
order of 109 M−1 s−1. The mechanism for the cis−trans
isomerization on the triplet surface was supported with density
functional theory calculations. In comparison, laser flash
photolysis of 1a and 1b with 266 nm laser results in cis−
trans isomerization on the singlet surface as no triplet transients
were detected.
It should be noted that cis 2a, 2b, and 2c were stable in
solution and did not revert back to their trans isomers.
Irradiations of 1a−c in oxygen-saturated chloroform-d yielded
the corresponding cis isomers 2a−c as the only products. The
conversion of E to Z isomer for 1a and 1b decreased in oxygen-
saturated chloroform-d in comparison to the case in argon-
saturated chloroform-d but remained the same for 1c. It should
be noted that irradiation conditions for each compound in
argon- and oxygen-chloroform-d were identical whereas the
irradiations times were different for different compounds.
Because the product ratios are affected by O2, we theorize
there are two pathways for 1 to yield 2, one, which is insensitive
to oxygen, takes place on the singlet surface of 1; the other
pathway must be on the triplet surface of 1 as it is affected by
oxygen.
2. RESULTS
2.1. Product Studies. Photolysis of 1a−c in argon-
saturated chloroform-d through a Pyrex filter (>300 nm) at
298 K yielded 2a−c as the only products (Scheme 2, Table 1).
Table 1. Percent Conversion of 1 into 2 upon Irradiation
a
through Pyrex (>300 nm) in CDCl3
b
compound
atmosphere
product ratio 1:2
Studies were performed with irradiation at different wave-
lengths, which led to different transient phenomena. The
assignments made are shown in Scheme 3 to guide the reader
through the results that support these assignments. We propose
that the aryl ketone and the vinyl ketone chromophores in 1a,
1b, and 1c are in cross-conjugation and can both absorb light,
but that only the singlet excited aryl ketone, formed when the
solution is irradiated at 308 nm, undergoes intersystem crossing
to its triplet excited state (Scheme 3, horizontal pathway). The
triplet ketone rearranges to form the triplet 1,2 biradical 3. In
b
1a
Ar
O2
Ar
O2
Ar
O2
67
74
55
68
95
94
33
26
45
29
5
b
1b
1c
4
a
2a, 2b, and 2c are stable at room temperature for more than 7 days.
From H NMR integration ( 1%).
b
1
Scheme 3. Proposed Mechanism for the Cis−Trans Isomerization of 1a−c
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Figure 1. Calculated orbitals for the lowest energy electronic transfer in 1a, 1b, and 1c.
contrast, the vinyl chromophore absorbs light at 266 nm to
form its singlet excited state, which then decays to form 1 and
2, directly on the singlet surface (Scheme 3, left vertical
pathway).
We also optimized the structure of T1K of 1 and 2, which
placed the energy of these triplet ketones considerably lower
than the TD-DFT calculations. We have previously shown that
DFT optimization of triplet ketones with (n,π*) configuration
underestimates their energy;25−27 however, it is more likely that
the energy difference is due to structural changes between the
optimized structure of the T1K of 1 and its S0. Analysis of the
calculated bond lengths in the T1K of 1 and 2 shows that the
CO and the CαCβ bonds are elongated in all of the
ketones in comparison to the calculated bond lengths in the
ground state (S0, Figure 2). This indicates that the triplet
ketones have a (n,π*) configuration. Spin density calculations
further support this notation as they place the unpaired
electrons on the carbonyl oxygen atom and in the π*-orbital on
the vinyl bond (Scheme 4). The calculations suggest that T1K of
1 and 2 are remarkably similar and are not affected by their p-
phenyl substituent, thus demonstrating that these triplet
ketones are best described as vinyl ketones rather than triplet
acetophenones.
2.2. Calculations. To better understand the isomerization
of 1a−c, we calculated stationary points on the triplet and
singlet surfaces of 1a−c using the Gaussian09 at the B3LYP
level of theory and with the 6-31+G(d) basis set.22−24
We optimized ketones 1a, 1b, and 1c and calculated the
electronic transitions due to their ground state absorptions.
These ketones have weak absorption bands at longer
wavelengths that for 1a and 1b are due mainly to an electronic
transition out of the lone pair on the oxygen atom into a π*-
orbital (Figure 1). In comparison, the major electronic
transition for 1c above 300 nm is an electronic transition out
of a vinylic π-orbital into π*-antibonding orbital. Thus, the
calculations support that irradiation at longer wavelengths
should mainly result in absorption of the ketone chromophores
in 1a and 1b but not for 1c.
Optimization of triplet biradicals 3a, 3b, and 3c show that
they are located just a few kcal/mol below the T1K of 1 and 2.
The calculated Cα−Cβ bonds in 3a, 3b, and 3c are ∼1.5 Å, as
they are single bonds, and the torsion angles between the CO−
Cα−Cβ−Cγ are calculated to be ∼80° to avoid any overlap
between the two radical centers. Spin density calculations place
the unpaired electrons on the Cβ and Cα atoms, which have
spin densities of −0.9 and −0.7, respectively (Scheme 5). The
radical on the Cα atom is in conjugation with the carbonyl
oxygen atom that has a spin density of −0.5, whereas the spin
density is localized on the Cβ-atom. The calculations suggest
that biradicals 3a, 3b, and 3c are all remarkably similar and not
affected by their p-substituent.
We calculated the transition state for the T1K of 1 and 2
rearranging to form biradical 3. These transitions states are
located only a few kcal/mol above the T1K of 1 and 2, and in
Figure 3, we have plotted stationary points on the triplet surface
of 1 and 2.
We calculated the rotational barrier for biradicals 3a−c
around their Cα−Cβ bond. As mentioned above, the biradicals
We used TD-DFT calculations to locate the T1K and T2K of 1
and 2, and the results are listed in Table 2. The calculations
support that T1K of 1a and 1b have similar energies, whereas
the T1K of 1c is slightly lower in energy. The T2K are just a few
kcal/mol above the T1K for 1 and 2.
Table 2. Energies of the T1K and T2K of 1a, 1b, and 1c in
kcal/mol
a
b
c
phosphorescence
optimization
TD-DFT
T1
T1
T1 T2
1a
1b
1c
2a
2b
2c
73
72
71
62
62
60
63
63
61
67
68
64
65
68
65
71
70
69
73
73
72
a
b
Obtained from phosphorescence. Obtained from optimization
c
calculations. Obtained from TD-DFT calculations.
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Figure 2. Optimized conformers of 1a, the T1K of 1a, 2a, and 3a. The bond lengths are given in Å.
The calculated rotational barrier for 3a to reach the T1K of 2a is
slightly higher (8 kcal/mol), due to the steric demands of the
methyl group. As expected, the calculated rotational barrier for
the T1K of 1a around CO−Cα−Cβ−CH3 yields the same
rotation graph as for 3a. From the graph, it can be seen that the
rotational barriers for the T1K of 1a and 2a forming 3a are
comparable to the transition state barrier for the T1K of 1a
forming 3a. We obtained similar results for 3b and 3c.
Scheme 4. Calculated Spin Densities for the T1K of 1a, 1b,
and 1c
Finally, we optimized the peroxide radicals 4, resulting from
the addition of O2 to biradicals 3a, 3b, and 3c (Scheme 6). The
calculations show that radicals 3a, 3b, and 3c are expected to
react similarly with O2. The addition of O2 to the radical
centered on the Cβ atom results in resonance stabilized radical
4. We cannot calculate the transition state barrier for O2 adding
to biradicals 3 due to spin restriction. In contrast, the calculated
transition state barrier for peroxide radicals 4 extruding an O2
molecule and forming 1 is only 3 kcal/mol above 4.
2.3. Phosphorescence. We obtained phosphorescence
spectra of 1a, 1b, and 1c at 77 K in frozen ethanol matrices
(Figure 5). The phosphorescence spectra of 1a, 1b, and 1c
show that they have emissions that do not have well-resolved
emission bands as typically observed for ketones with (n,π*)
configurations. Furthermore, the onset of the emission was
are the most stable as the two radical centers are almost
perpendicular to each other or when the CO−Cα−Cβ−CH3
torsion angle is 80°. The calculations suggest that further
rotation around the Cα−Cβ bond brings the two radical
centers closer together and destabilizes the biradicals. For
example, for 3a, the calculations imply that 6 kcal/mol (Figure
4) are required to reach the first rotational barrier where this
torsion angle is 160° (Δ 80°). Interestingly, further rotation
that makes the torsion angle closer to 180° yields the T1K of 1a.
Scheme 5. Calculated Spin Densities for Triplet Biradicals 3a, 3b, and 3c
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Figure 3. Calculated stationary points on the energy surface of 1a, 1b, and 1c. TD-DFT calculations for 1b place the triplet ketone with (π,π*)
configuration below the (n,π*), whereas optimization places the (n,π*) ketone lower in energy.
used to estimate the (0,0) bands and is listed in Table 2. The
energies from the emission spectra are considerably higher than
energies obtained both from the optimization and from TD-
DFT calculation of the T1K of 1. The calculated energies of the
T2K of 1 match better with the observed emission, and thus, we
speculate that the emission is due to the T2K of 1. In addition, it
should be noted that the relative intensitiy of phosphorescense
for 1c is considerably less than for 1a and 1b.
2.4. Laser Flash Photolysis. We performed laser flash
photolysis to identify the excited state and intermediates
formed upon irradiation of 1a, 1b, and 1c. Laser flash
photolysis (excimer laser, 308 nm, 17 ns)28 of 1a in argon-
saturated acetonitrile produced a transient absorption with λmax
at ∼360 nm and at ∼460 nm (Figure 6). We assigned these
absorption bands to the overlapping absorption of the T1K of 1a
and 3a, on the basis of their calculated spectra. The calculated
TD-DFT absorption spectrum of the T1K of 1a in acetonitrile
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Figure 4. Calculated rotational barrier for 3a.
has major electronic transitions at 469 (f = 0.1423), 327 (f =
0.0570), 326 (f = 0.0812), 316 (f = 0.1618), and 309 nm (f =
0.2403) (Figure 7), which fits well with the observed spectra.
Furthermore, the calculated TD-DFT absorption spectrum of
3a in acetonitrile is also similar to the observed spectrum as it
has calculated electronic transitions at 469 (f = 0.0375), 437 (f
= 0.0127), and 337 nm (f = 0.0452).
Kinetic analysis of the transient absorption further supported
these assignments. The transient absorption at 420 nm in
argon-saturated acetonitrile was fitted to a sum of two
exponentials resulting in rate constants of 3.4 × 106 s−1 (τ =
308 ns) and 1.73 × 105 s−1 (τ = 5.7 μs, Figure 6). In O2-
saturated acetonitrile, the transient is formed faster than the
time resolution of the instrument, and it decays with a rate
constant of 1.16 × 107 s−1 (τ = 86 ns). As the decay in oxygen-
saturated acetonitrile is a monoexponential decay, we conclude
that oxygen shortens the lifetime of the T1K of 1a so we do not
observe it; its lifetime must be less than the time resolution of
the laser flash apparatus or 17 ns. Oxygen also reduces the
lifetime of 3a to 86 ns. As the concentration of O2 in
acetonitrile is 0.009 M,29 we can determine that 1,2-biradical 3a
reacts with O2 with a rate constant of 1.3 × 109 M−1 s−1.
Analysis of the kinetics at 360 nm revealed similar results as
obtained from the kinetics at 460 and 500 nm, but at 360 nm,
there is a significant residual absorption due to the formation of
2a, which makes it more complicated to obtain accurate
lifetimes.
Figure 5. Phosphorescence of 1a, 1b, and 1c in ethanol at 77 K with
300 nm excitation.
We used isoprene to quench the T1K of 1a (Figure 8). We
measured the yield of absorbance versus isoprene concentration
at 360 and 500 nm. This was required because it was
complicated to directly measure the rate constant for the decay
of 3a as a function of isoprene concentration considering that
the lifetime for ketone decay in the absence of quencher was
already close to the time resolution of the system. The yields of
the absorbance at 360 or 500 nm for the biradical decreased
with increased isoprene concentration whereas the lifetime of
the decay was not affected. Thus, isoprene quenches the T1K of
1a but not 3a. Using the Stern−Volmer equation,
Figure 6. (a) Transient spectra obtained by laser flash photolysis of 1a
in argon-saturated acetonitrile with 308 nm laser excitation. (b)
Kinetic trace obtained by laser flash photolysis of 1a with 308 nm laser
at 420 nm in argon-saturated acetonitrile.
A0
A
= 1 + kqτ0[Q]
quenching rate constant of the triplet state; τ0 is the lifetime of
the T1 of 1a without any isoprene. [Q] is the concentration of
the isoprene.
A0 is the absorption of 3a without any quencher, and A is the
absorption of 3a at various isoprene concentrations. kq is the
Scheme 6
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Figure 7. Calculated electronic transition for the T1K of 1a (top) and
3a (bottom) in acetonitrile using the TD-DFT level of theory with the
UB3LYP/6-31+G(d) basis set.
Figure 9. Laser flash photolysis of 1a in nitrogen-saturated acetonitrile
with (a) λ = 355 nm irradiation and (b) λ = 266 nm irradiation.
at 0.009 M concentration is 8.8 × 108 M−1 s−1,29 which is in
agreement, within experimental error, with the rate constant
determined from the 308 nm irradiation.
We also performed laser flash photolysis of 1a using 266 nm
irradiation (Nd:YAG laser, 266 nm, 10 ns)31,32 in nitrogen-
saturated acetonitrile that produced a broad absorption with a
λmax at ∼300 nm (Figure 9). This absorption remained constant
at shorter time scales but decayed with a rate constant of ca.
0.38 s−1 (τ = 2.6 s) at longer time scales. We assigned this
absorption to formation of 2a and the decay to the diffusion of
2a out of the detection pathway.31
Laser flash photolysis (Excimer laser, 308 nm, 17 ns) of 1b in
argon-saturated acetonitrile produced a broad transient
absorption with λmax at ∼360 nm that trails out to 600 nm
(Figure 10a). We assigned this absorption to the overlapping
Figure 8. Stern−Volmer plot for quenching the T1K of 1a with
isoprene at 360 nm.
We obtained a kq value of 8 × 106 M−1 s−1 using the lifetime
of the T1K of 1a as 308 ns at ambient temperature. The kq value
is considerably less than the rate constant for a diffusion
controlled process, which was expected because the triplet
energy of isoprene is 60 kcal/mol and thus similar to the triplet
energy of the T1K of 1a that we estimated to be 62 kcal/mol.30
Thus, the isoprene quenching demonstrated that the T1K of 1a
is the precursor to 3a and that the energy of the T1K of 1a is
similar to the energy of the triplet excited isoprene.
To better evaluate the wavelength dependency of the
photochemistry of 1a, laser flash photolysis with a 355 nm
Nd:YAG laser that has a 10 ns time resolution was
performed.31,32 The transient spectra obtained with 355 nm
irradiation show a negative absorption at approximately 350 nm
due to the bleaching of 1a, as a higher concentration of the
starting material is needed in this case compared to the
experiments conducted with 308 nm irradiation to ensure that
for the excitation at 355 nm the absorption was sufficiently high
for the detection of transients (Figure 9). In addition, the
transient spectra formed from 355 nm irradiation were
obtained using a flow cell and, thus, had less residual absorption
due to product formation than observed in the transient spectra
obtained with the 308 nm irradiation. Therefore, excitation at
355 nm resulted in spectra with a somewhat broader
appearance.
Figure 10. Transient spectra obtained by laser flash photolysis of 1b:
(a) argon-saturated acetonitrile with 308 nm irradiation and (b)
nitrogen-saturated acetonitrile with 355 nm irradiation.
The kinetic analysis obtained with 355 nm excitation showed
a decay that can be fitted to a sum of two exponentials with
lifetimes of 295 ns and 6 μs, which matches, within
experimental errors, the lifetimes of the T1K of 1 and 3a
obtained from 308 nm irradiation. In oxygen-saturated
acetonitrile, the faster component, T1K of 1a is fully quenched
and the absorption of 3a decays with a rate constant of 7.9 ×
106 s−1 (τ = 126 ns) or the rate constant for 3a reacting with O2
absorption of the T1K of 1b and the 3b on the basis of the TD-
DFT calculations. The calculated TD-DFT absorption
spectrum of the T1K of 1b in acetonitrile has major electronic
transitions at 498 (f = 0.034), 477 (f = 0.1351), 335 (f =
0.1856), and 327 nm (f = 0.3323), which fits well with the
observed spectrum (Figure 11). In comparison, the calculated
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constant of 1.4 × 107 s−1 (τ = 74 ns); thus, we conclude that
oxygen shortens the lifetime of the T1K of 1b so we do not
observe it, its lifetime must be less than the time resolution of
the laser flash apparatus or 17 ns. It also reduces the lifetime of
3b to 74 ns. As the concentration of O2 in acetonitrile is 0.009
M,29 the rate constant for 3b reacting with oxygen was
estimated to be 1.5 × 109 M−1 s−1.
Laser flash photolysis (excimer laser, 308 nm, 17 ns) of 1c in
argon-saturated acetonitrile produced a transient absorption
with a λmax at ∼360 nm (Figure 13). The intensity of the band
Figure 11. Calculated electronic transition of the T1K of 1b (top) and
3b (bottom) in acetonitrile using the TD-DFT level of theory with the
UB3LYP/6-31+G(d) basis set.
TD-DFT absorption spectrum of 3b in acetonitrile has major
electronic transitions at 564 (f = 0.0640), 347 (f = 0.0594), and
300 nm (f = 0.0615). The calculations demonstrate that 3b has
less intense electronic transitions that are buried under the
absorption for the T1K of 1b at shorter wavelengths, but at
longer wavelengths 3b has more significant absorption, which
fits nicely with the observed experimental spectra. Laser flash
photolysis (Nd:YAG laser, 355 nm, 10 ns) of 1b in argon-
saturated acetonitrile allowed us to measure the absorption out
to 700 nm, which showed a broad transient absorption with a
λmax at ∼360 nm that trailed out to 700 nm (Figure 10b).
Kinetic analyses of the transient absorption obtained with
308 and 355 nm irradiation were the same, within experimental
error, and further supported the assignment of the transient
absorption to the T1K of 1b and 3b. The transient absorption at
400 nm can be fitted as a sum of two exponentials, yielding rate
constants of 4.6 × 106 s−1 (τ = 215 ns) and 3.2 × 105 s−1 (τ =
3.1 μs) (Figure 12). We assigned the shorter decay to the T1K
of 1b and the longer one to 3b. The absorption at 330 nm grew
in with a rate constant of 2.5 × 105 s−1 (τ = 3.9 μs), which is
consistent with 2b being formed with the same rate constant as
observed for the decay of 3b. In oxygen-saturated acetonitrile, a
monoexponential decay was observed with a decay rate
Figure 13. (a) Laser flash photolysis of 1c in argon-saturated
acetonitrile. (b) Kinetics obtained by laser flash photolysis of 1c in
argon-saturated acetonitrile at 360 nm with 308 nm irradiation.
at 360 nm remains constant on shorter time scales, but on
millisecond time scales, we observe some decay of the signal.
The calculated spectra for the T1K of 1c (Figure 14) fit with the
observed spectra, but it is not reasonable to expect a triplet
ketone to have lifetime on the millisecond time scale. The
calculated absorption for 3c does not match with the observed
spectra, and therefore, we assign the absorption at 360 nm to
the formation of photoproduct 2c and the decay to diffusion of
the transient out of the detection volume.
Figure 12. Kinetic traces obtained at (a) 400 nm in nitrogen-saturated
acetonitrile and (b) 440 nm in oxygen-saturated acetonitrile with 355
nm irradiation of 1b.
Figure 14. Calculated UV of the T1K of 1c (top) and the TBR of 1c
(bottom) in acetonitrile using TD-DFT level of theory with the
UB3LYP/6-31+G(d) basis set.
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Kinetic analysis of the transient absorption further supports
this assignment. The transient absorption at 360 nm does not
decay on shorter time scales but does on millisecond time
scales with a rate constant of <62 s−1 (τ >16 ms, Figure 13b),
which must be due to 2c diffusing out of the monitoring path.
Thus, laser flash photolysis confirms that irradiation of 1a
and 1b results in formation of their triplet ketones, which decay
to form biradicals 3a and 3b, respectively. Biradicals 3a and 3b
have a lifetime of a few microseconds and react with molecular
oxygen with rate constants on the order of 0.88 and 1.5 × 109
M−1 s−1. In comparison, laser flash photolysis of 1c did not
yield the T1K of 1c or biradical 3c, either because the T1K of 1c
and biradical 1c are too short-lived or because 1c reacts only on
the singlet excited state surface to yield 2c.
2.5. Time-Resolved Infrared (TRIR) Spectroscopy. To
verify that the irradiation of 1a with 266 nm irradiation yields
2a in the singlet surface, we performed time-resolved infrared
(TRIR) spectroscopy of 1a in cyclohexane. TRIR difference
spectra from 1750 to 1500 cm−1 were monitored over the first
45 μs following 266 nm laser photolysis (Figure 15).
cm−1 concurrently with the decrease of the bands at 1688,
1668, 1654, 1612, 1562, 1447, 1299, 1219, 1222, 1025, 831,
757, and 720 cm−1 (Figure 16). We assigned all the new bands
Figure 16. IR spectra of 1a (a) before irradiation (red) and (b) after
irradiation (black) and (c) the difference spectrum (purple).
to 2a, on the basis of comparison to its reported IR and
calculated spectra. In Table 3 are listed all the IR bands that
were formed or depleted upon irradiation, along with their
assignments based on calculations.
Table 3. IR Bands Formed or Reduced upon Irradiation of
1a in Argon Matrices and Comparison of the Observed and
Calculated IR Bands of 1a and 2a
experimental IR bands cm−1
calculated IR bands cm−1
formed
reduced
Δ
2a
1a
Δ
1678
1661
1609
1558
1453
1688
1668
1612
1562
1447
1299
1219
1025
831
−10
−7
−3
−4
+6
1727 (190)
1670 (131)
1628 (11)
1503 (23)
1495 (46)
1293 (2)
1736 (156)
1678 (199)
1627 (13)
1506 (47)
1498 (9)
−11
−8
+1
−3
−3
1331 (82)
1243 (162)
1046 (24)
843 (11)
1229
1014
837
+10
−11
+6
1249 (233)
1031 (49)
853 (10)
+6
−15
+10
−24
+14
737
757
−20
+10
751 (90)
775 (30)
730
720
699 (17)
685 (9)
Figure 15. TRIR difference spectra averaged over the time scales
indicated following 266 nm laser photolysis of 1a in (a) argon-
saturated cyclohexane at room temperature. (b) Calculated CO and
CC vibrations of 1a and 2a using TD-DFT level of theory with
B3LYP/6-31+G(d) basis set (scaling factor = 0.961333).
Irradiation of 1b in argon matrices at 14 K led to the
formation of new absorption bands at 1676, 1605, 1538, 1268,
1242, 1233, 1171, 1088, 795, and 787 cm−1 concurrently with
the decrease of the bands at 1681, 1608, 1539, 1299, 1260,
1225, 1221, 1173, 812, and 801 cm−1. As before, we assigned
the new bands to 2b on the basis of its reported spectrum and
calculations (Figure 17, Table 4).
Upon irradiation of 1c, new bands were formed at 2240,
1681, 1624, 1370, 1368, 1223, 1026, 1015, 909, 789, 787, 768,
716, and 552 cm−1 concurrently as the intensity of the
following bands decreased 2237, 1688, 1671, 1634, 1380, 1378,
1301, 1220, 1217, 1041, 1020, 918, 811, 809, 760, and 542
cm−1. As before, we assign the new bands to 2c on the basis of
its IR spectrum and calculations (Figure 18, Table 5).
The matrix isolation experiments show that 1a, 1b, and 1c
undergo efficient cis−trans isomerization at cryogenic temper-
atures. However, we cannot identify whether these isomer-
izations take place through the singlet or triplet surfaces of 1a−
c.
Immediately after the laser pulse, two negative bands were
observed at 1685 and 1635 cm−1, which are due to the depleted
CO and CC stretches of 1a. Concurrent to the depletion
of the bands at 1685 and 1635 cm−1, two new bands were
formed at 1670 and 1610 cm−1, which are assigned to the C
O and CC stretches of 2a. These assignments are based on
the observed and calculated IR spectrum of 2a (Figure 15b).
Analysis of the kinetics showed that on microsecond time scales
the bands due to depletion of 1a and formation of 2a did not
change over time. Furthermore, these bands were not affected
by molecular oxygen. Thus, the 266 nm irradiation of 1a results
in cis−trans isomerization on its singlet excited state surface.
2.6. Matrix Isolation. We studied the photochemistry of
1a, 1b, and 1c in argon matrices at 14 K to confirm that they
undergo cis−trans isomerization at cryogenic temperatures.
Upon irradiation of 1a, significant new bands were formed at
1678, 1661, 1609, 1558, 1453, 1229, 1014, 837, 737, and 730
3. DISCUSSION
The photochemistry of 1a and 1b is wavelength dependent,
where irradiation at longer wavelengths allows for selective
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Table 5. IR Bands Formed or Reduced upon Irradiation of
1c in Argon Matrices and Comparison of the Observed and
Calculated IR Bands of 1c and 2c
experimental IR bands cm−1
calculated IR bands cm−1
formed
reduced
2237
Δ
2c
1c
Δ
2240
1681
1624
1370
1368
+3
2336 (36)
1730 (194)
1670 (165)
1423 (18)
2336 (36)
1737 (165)
1678 (235)
1434 (2)
0
1688
1634
1380
1378
1301
1220/1217
1041
1020
918
−7
−7
−8
−11
−10
−10
−10
1294 (2)
1245 (244)
1047 (20)
1030 (47)
925 (57)
808 (60)
1331 (75)
1243 (171)
1062 (29)
1035 (13)
934 (81)
−37
+2
1223
1026
1015
909
+3/+6
−15
−5
−15
−5
−9
Figure 17. Matrix IR spectra of 1b before irradiation (red) and after
irradiation (black) and the difference spectrum (purple).
−9
789
811
−22
−22
+8
831 (38)
−23
Table 4. IR Bands Formed or Reduced upon Irradiation of
1b in Argon Matrices and Comparison of the Observed and
Calculated IR Bands of 1b and 2b
787
809
768
760
784 (16)
731 (13)
564 (15)
774 (14)
748 (2)
+10
−17
+5
716
experimental IR bands cm−1
calculated IR bands cm−1
552
542
+10
559 (10)
formed
reduced
1681
Δ
2b
1b
Δ
possible. Furthermore, as the calculated properties of the T1K of
1c and 2c are similar to those of the T1K of 1a and 1b and
biradicals 3a and 3b, it is unlikely that the T1K of 1c and 2c are
too short-lived to be observed. Further support that 1c does
not undergo triplet reactivity comes from the phosphorescence
studies that show that the emission relative intensity for 1c is
significantly lower than for 1a and 1b.
Biradicals 3a and 3b reacted with molecular O2 with rate
constants of (0.88−1.3) × 109 and 1.5 × 109 M−1 s−1,
respectively, which is somewhat lower than the quenching of
their triplet ketone precursors with O2. Spin restriction
prevents us from calculating the transition state for O2 adding
to biradicals 3a and 3b (Scheme 7) to form 4. However, it can
1676
1605
1538
−5
−3
−1
1723 (182)
1649 (262)
1504 (25)
1733 (158)
1653 (263)
1507 (53)
1327 (32)
1296 (271)
1248 (250)
−10
−4
−3
--
1608
1539
1299
1268
1233
1242
1171
1088
787
1260
+8
1302 (205)
1259 (351)
1293 (16)
1201 (255)
1043 (57)
810 (59)
+6
1225/1221
+8/+12
+11
1173
−2
1210 (143)
1050 (36)
840 (32)
794 (1)
−9
−7
−30
+7
812
801
−25
−6
795
801 (22)
Scheme 7
Figure 18. Matrix IR of 1c before irradiation (red) and after irradiation
(black) and the difference spectrum (purple).
irradiation into the ketone chromophore, as supported by the
TD-DFT calculations.
The singlet excited ketones intersystem cross to their triplet
excited states, which rearrange to yield triplet 1,2-biradicals 3a
and 3b. In contrast, irradiation at shorter wavelengths leads to
excitation into the vinyl excited state that results in cis−trans
isomerization on the singlet surface (Scheme 3). It was not
possible to selectively irradiate into the ketone chromophore of
1c, and thus, its excited state only undergoes isomerization on
its singlet surface. The TD-DFT calculations support that
selective excitation into the ketone chromophore 1c is not
be theorized that peroxy radical 4 extrudes molecular O2 to
form 1a and 1b, as the calculated transition state barrier for
extruding an O2 molecule is only a few kcal/mol above the
peroxide biradical. It is also possible that molecular O2
quenches biradicals 3a and 3b by energy transfer similarly to
how O2 quenches the T1K of 1a and 1b. Thus, adding O2 to
triplet 1,2-biradicals is not expected to yield any new products
but rather to regenerate 1a and 1b or 2a and 2b.
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Phosphorescence emission of 1 results in an emission that
has considerably higher energy than the one obtained from
calculations, and thus, it seems plausible that at lower
temperatures we are observing emission from the T2K of 1
rather than the T1K of 1. It should be highlighted that the
quenching studies with isoprene, however, support that the
precursor to 3a is the T1K of 1 that has triplet energy similar to
that for isoprene.
The lifetimes of 3a and 3b are similar to the lifetimes that
Garcia-Exposito et al. reported for triplet biradicals from 2-
pentenoate esters that have lifetimes between 1 and 16 μs.19 As
mentioned earlier, the authors theorized that the lifetime of
their biradicals was limited due to intersystem crossing, which
depends on the energy gap between the singlet and the triplet
surface and is enhanced by spin−orbit coupling. Furthermore,
the authors concluded that the largest spin−orbit coupling was
obtained when the torsional angle between the two radicals is
approximately 45°, and therefore, intersystem crossing should
take place as the biradicals rotate into this conformer. The
calculated energy between the ground state of 1 and triplet
biradicals 3 is similar to the 2-pentenoate system, and thus,
these biradicals must decay by a similar mechanism.
oxygen, contained the solution used. The flow rate was
increased until the interference from product formation
disappeared; i.e., the shape of the decay and the final offset
were the same when the flow rate was increased further.
5.3. Time-Resolved IR Methods. TRIR experiments were
conducted (with 16 cm−1 spectral resolution) following the
method of Hamaguchi and co-workers,46,47 as has been
described previously.48 Briefly, the broadband output of a
MoSi2 IR source (JASCO) is crossed with excitation pulses
from a Continuum Minilite II Nd:YAG laser (266 nm, 5 ns, 2
mJ) operating at 15 Hz. Changes in IR intensity are monitored
using an AC-coupled mercury/cadmium/tellurium (MCT)
photovoltaic IR detector (Kolmar Technologies, KMPV11-J1/
AC), amplified, digitized with a Tektronix TDS520A
oscilloscope, and collected for data processing. The experiment
is conducted in dispersive mode with a JASCO TRIR 1000
spectrometer. Sample preparations were performed as
described the literature.48
5.4. Matrix Isolation. Matrix isolation studies were
performed using conventional equipment.49 Matrix isolation
experiments were conducted using standard high vacuum
techniques. Argon gas flowed over the solid sample, entraining
the vapor above the solid, and was deposited onto a CsI cold
window cooled to 14 K by a CTi closed cycle refrigerator.
Infrared spectra were recorded at 1 cm−1 resolution by a
PerkinElmer Spectrum One FTIR during and after matrix
deposition. The matrix was then irradiated by the Pyrex-filtered
light of a medium pressure mercury arc lamp after which
additional spectra were recorded.
4. CONCLUSION
Thus, we have shown that intramolecular sensitization can be
used to selectively form triplet 1,2-biradicals from simple
vinylalkenes. The biradicals have lifetimes of a few micro-
seconds in acetonitrile and are efficiently quenched with
molecular O2. As cis−trans isomerization has potential use in
several applications, it is important to evaluate how spin
delocalization and rotational barriers affect the lifetimes of the
1,2-biradicals and, thus, the cis−trans isomerization.
5.5. Phosphorescence. Ethanol solutions of 1a (1 mM),
1b (0.02 M), and 1c (0.02 M)) were prepared, and their
phosphorescence spectra were obtained at 77 K (Horiba
Instruments Inc., Edison, NJ, USA; Horiba Jobin-Yvon
fluorolog with 5 nm as the emission monochromator
bandwidth). The solutions were irradiated at 300 and 260
nm and the emission spectra recorded between 300 and 800
nm.
5. EXPERIMENTAL SECTION
5.1. Calculations. All geometries were optimized at B3LYP
level of theory and with 6-31G+(d) basis set as implemented in
the Gaussian09 programs.22−24 All transition states were
confirmed to have one imaginary vibrational frequency.
Intrinsic reaction coordinates calculations were used to verify
that the located transition states corresponded to the attributed
reactant and product.34,35 The absorption spectra were
calculated using time-dependent density functional theory
(TD-DFT).36−40 The effect of solvation was calculated using
the self-consistent reaction field (SCRF) method with the
integral equation formalism polarization continuum model
(IEFPCM) with acetonitrile, methanol, and dichloromethane as
solvents.41−45
5.2. Laser Flash Photolysis. Laser flash photolysis was
performed with an excimer laser (308 nm, 17 ns)28 or Nd:YAG
laser (266 or 355 nm, 10 ns).31,32 A stock solution of 1a−c in
acetonitrile was prepared with spectroscopic grade acetonitrile,
such that the solutions had absorption between 0.3 and 0.6 at
the excitation wavelength. Typically, ∼1 mL of the stock
solution was placed in a 10 mm × 10 mm wide, 48 mm long
quartz cuvette and was purged with argon for 5 min or oxygen
for 5 min. The rate constants were obtained by fitting an
average of three to five kinetic traces.
5.6. Preparations of Starting Materials. 5.6.1. Synthesis
of 1a. 5.6.1.1. 1-Phenylbut-3-en-1-ol. 1-Phenylbut-3-en-1-ol
was synthesized following a modified procedures of El
Bouakher et al.50 and Bates and Sridhar.51 Benzaldehyde
(10.6 g, 0.10 mol) was dissolved in DMF (25 mL) and to the
resulting solution was added allyl bromide (14.4 g, 0.12 mol,
1.2 equiv) and SnCl2 (40.0 g, 0.20 mol, 2 equiv). NaI (20.0 g,
0.13 mol, 1.3 equiv) was added one spatula at a time slowly
over a period of 10 min. Additional DMF (75 mL) was added
to the mixture, and it was stirred overnight (20 h). NH4F (30.0
g, 1.10 mol, 11 equiv) was dissolved in H2O (100 mL), and the
solution was added to the reaction mixture to quench the
reaction. Diethyl ether (100 mL) was added, and the mixture
stirred for further 30 min. The ether layer was extracted and
washed twice with water (100 mL) and dried over anhydrous
MgSO4. The ether layer was evaporated under reduced
pressure to give crude 1-phenylbut-3-en-1-ol (14.3 g, 0.10
1
mol, 96.8% yield). H NMR, 13C NMR, and IR spectra match
the ones reported in the literature.52 1H NMR (CDCl3, 400
MHz): δ 2.4 (t, J = 6.8 Hz, 2H, −CH2−), 2.7 (br s, 1H, −OH),
4.6 (t, J = 6.8 Hz, 1H, −CH), 5.08−5.13 (m, 2H, CH2), 5.7−
5.8 (m, 1H, CH), 7.2−7.3 (m, 5H, Ph-H) ppm. IR (neat):
3371 (br, OH), 3076, 3030, 3007, 2979, 2933, 2906, 1950,
1875, 1811, 1641, 1603, 1494, 1454, 1433, 1416, 1358, 1316,
1198, 1115, 1077, 1048, 1000, 916, 871, 830, 758, 700, 643,
609, 538 cm−1.
For the experiments using the Nd:YAG laser for excitation, a
home-built flow cell was employed where the irradiation
occurred in a cell with a 7 mm × 7 mm cross section. A syringe
pump was used to pump the solution through the cell and a
reservoir, which was continuously purged with nitrogen or
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1H NMR, 13C NMR, and IR spectra match those reported in
the literature.57 1H NMR (CDCl3, 400 MHz): δ 3.7 (d, J = 6.8
Hz, 2H, −CH2−), 3.9 (s, 3H, −OCHH3), 5.1−5.2 (m, 2H, 
CH2), 6.0−6.1 (m, 1H, CH), 6.9 (d, J = 8.8 Hz, 2H, Ph-H),
8.0 (d, J = 8.8 Hz, 2H, Ph-H) ppm. IR (neat): 2841, 1676 (s,
CO), 1601, 1576, 1510, 1420, 1331, 1261, 1215, 1170, 1030,
912, 834, 733, 608 cm−1.
5.6.2.3. Synthesis of (1-(4-Methoxyphenyl)-but-2-en-1-
one) 1b. Following the procedure described above 1-(4-
methoxyphenyl)-but-3-en-1-one (4.3 g, 0.02 mol) resulted in
1b (0.8 g, 4.60 mmol, 19% yield). The spectroscopic data of 1b
match those reported in the literature.55,58 1H NMR (CDCl3,
400 MHz): δ 2.0 (d, J = 6.0 Hz, 3H, −CH3), 3.9 (s, 3H,
−OCHH3), 6.9−7.0 (m, 3H, CH, Ph-H), 7.0−7.3 (m, 1H,
CH), 7.9 (d, J = 8.4 Hz, 2H, Ph-H) ppm. IR (neat): 2840,
1664 (s, CO), 1618, 1598, 1507, 1440,1335, 1299, 1259,
1224, 1170, 1024, 963, 917, 809 cm−1. GC/MS (EI): m/z: 176
(M+), 161, 135 (100%), 107, 92, 77, 69.
5.6.1.2. Synthesis of 1-Phenylbut-3-en-1-one. The syn-
thesis of 1-phenylbut-3-en-1-one was carried out by following a
procedure reported by Hathaway and Paquette with some
minor modifications.53 CrO3 (4.0 g, 0.04 mol) was dissolved in
H2SO4 (4 mL, 12 N) acid to form a slurry, and water (12 mL)
was added to make the Jones reagent (H2CrO4). The crude 1-
phenylbut-3-en-1-ol (14.3 g, 0.10 mol) was dissolved in cold
acetone (100 mL) and then 0.8−1.0 mL of H2CrO4 was added
to the 1-phenylbut-3-en-1-ol while stirring at 0 °C until the
green color of the reaction mixture changed to orange. The
solution was filtered, the solvent was removed under reduced
pressure, and the residue was dissolved in diethyl ether (100
mL) and washed with water (100 mL), saturated NaHCO3
(100 mL), and brine (100 mL). The ether layer was dried over
anhydrous MgSO4, and the solvent was removed under reduced
pressure to give 1-phenylbut-3-en-1-one (11.1 g, 0.08 mol,
78.5% yield). 1H NMR, 13C NMR, and IR spectra match those
reported.54 1H NMR (CDCl3, 400 MHz): δ 3.7 (d, J = 6.8 Hz,
2H, −CH2−), 5.16−5.21 (m, 2H CH2), 6.0−6.1 (m, 1H, 
CH), 7.4 (t, J = 7.6 Hz, 2H, Ph-H), 7.5 (t, J = 7.6, Hz, 1H, Ph-
H), 7.9 (d, J = 7.2 Hz, 2H, Ph-H) ppm. IR (neat): 3081, 1683
(s, CO), 1645, 1598, 1581, 1449, 1425, 1395, 1334, 1276,
1210, 1180, 1004, 922, 755, 690, 617, 593 cm−1.
5.6.3. Preparation of 1c. 5.6.3.1. Synthesis of 4-(1-
Hydroxybut-3-enyl)benzonitrile. Following the method de-
scribed above 4-formylbenzonitrile (1.3 g, 0.01 mol) allyl
bromide (1.5 g, 0.01 mol, 1.2 equiv) yield 4-(1-hydroxybut-3-
1
enyl)benzonitrile in 77% yield (1.3 g, 7.70 mmol). H NMR,
5.6.1.3. Synthesis of 1-Phenylbut-2-en-1-one (1a). 1-
Phenylbut-3-en-1-one (11.1 g, 0.08 mol) from the reaction
above was dissolved in methanol (100 mL), and triethylamine
(3−4 drops) was added. The reaction mixture was stirred
overnight (20 h), the solvent was removed under reduced
pressure, and the residue was extracted with diethyl ether (100
mL) and washed with water (100 mL). The ether layer was
dried over anhydrous MgSO4, the solvent removed under
reduced pressure, and the residue was purified on a silica
column eluted with ethyl acetate/hexane (1:4) to yield 1a (1-
13C NMR, and IR spectra match the reported literature.52 1H
NMR (CDCl3, 400 MHz): δ 2.3 (s, 1H, −OH), 2.4−2.5 (m,
1H, −CH2), 2.5−2.6 (m, 1H, −CH2), 4.79−4.82 (m, 1H,
−CH), 5.15−5.20 (m, 2H, CH2), 5.7−5.8 (m, 1H, 
CH),7.5 (d, J = 8.4 Hz, 2H, Ph-H), 7.6 (d, J = 8.4 Hz, 2H, Ph-
H) ppm. 13C NMR (CDCl3, 100 MHz): δ 43.8, 72.4, 111.2,
118.8, 119.5, 126.5, 132.2, 133. 4, 149.1 ppm. IR (neat): 3435
(br, OH), 3077, 3006, 2980, 2931, 2906, 2229 (s, CN),
1928, 1641, 1609, 1504, 1412, 1303, 1199, 1173, 1105, 1056,
1018, 999, 920, 873, 840, 760, 649, 630, 566 cm−1.
1
phenylbut-2-en-1-one) (4.5 g, 0.03 mol, 31% yield). The H
NMR, 13C NMR, and IR spectra of 1a match those reported.55
1H NMR (CDCl3, 400 MHz): δ 2.0 (d, J = 6.8 Hz, 3H, −CH3),
6.9 (d, J = 15.2 Hz, 1H, =CH), 7.0−7.1 (m, 1H, CH), 7.4 (t,
J = 7.4 Hz, 2H, Ph-H), 7.5 (t, J = 7.4 Hz, 1H, Ph-H), 7.9 (d, J =
8 Hz, 2H, Ph-H) ppm. 13C NMR (CDCl3, 100 MHz): δ 18.6,
127.6, 128.5, 132.6, 137.9, 145.0, 190. Eight ppm. IR (neat):
3058, 2912, 1702, 1673 (s, CO), 1598, 1579, 1448, 1332,
1297, 1220, 1179, 1038, 1024, 965, 918, 830, 759, 693, 664
cm−1. GC/MS (EI): m/z: 146 (M+), 131, 117, 105 (100%), 77,
69.
5.6.3.2. Synthesis of 4-But-3-enoylbenzonitrile. Jones
oxidation of 4-(1-hydroxybut-3-enyl)benzonitrile (1.3 g, 7.70
mmol) as described above yielded 4-but-3-enoylbenzonitrile
(1.2 g, 7.00 mmol, 90% yield). 1H NMR (CDCl3, 400 MHz): δ
3.8 (d, J = 6.8 Hz, 2H, −CH2−), 5.2−5.3 (m, 2H, CH2),
6.0−6.1 (m, 1H, CH), 7.8 (d, J = 8.4 Hz, 2H, Ph-H), 8.0 (d,
J = 8.4 Hz, 2H, Ph-H) ppm. 13C NMR (CDCl3, 100 MHz): δ
43.6, 116.5, 117. 9, 119.6, 128.7, 130.0, 132.6, 139.4 ppm. IR
(neat): 3093, 3047, 2228 (s, CN), 1689 (s, CO), 1645,
1606, 1566, 1425, 1406, 1394, 1335, 1291, 1210, 1177, 1108,
1009, 995, 936, 856, 834, 822, 769, 602, 574, 534 cm−1. HRMS:
m/z calculated for C11H10NO+ [M + H]+, 172.0762; found,
172.0762
5.6.2. Preparation of 1b. 5.6.2.1. Synthesis of 1-(4-
Methoxyphenyl)-but-3-en-1-ol. Using the procedure described
above, anisaldehyde (13.5 g, 0.10 mol) and allyl bromide (14.4
g, 0.12 mol, 1.2 equiv) resulted in 1-(4-methoxyphenyl)but-3-
5.6.3.3. Synthesis of (E)-4-But-2-enoylbenzonitrile, 1c. 4-
But-3-enoylbenzonitrile (1.2 g, 7.0 mmol) was converted to 1c
1
en-1-ol (15.6 g, 0.09 mol) in 88% yield. H NMR, 13C NMR,
and IR spectra obtained of 1a match those reported.56 1H
NMR (CDCl3, 400 MHz): δ 2.0 (d, J = 2.8 Hz, 1H, −OH), 2.5
(t, J = 6.8 Hz, 2H, −CH2), 3.8 (s, 3H, −OCHH3), 4.7 (dt, J =
6.4 Hz, J = 2.4 Hz, 1H, −CH), 5.11−5.12 (m, 2H, CH2),
5.7−5.8 (m, 1H, CH), 6.9 (d, J = 8.8 Hz, 2H, Ph-H), 7.3 (d,
J = 8.8 Hz, 2H, Ph-H) ppm. 13C NMR (CDCl3, 100 MHz): δ
43.8, 55.3, 73.0, 113.8, 118.2, 127.1, 134.6, 136.1, 159.0 ppm.
IR (neat): 3404 (br, OH), 3075, 3002, 2934, 2836, 1640, 1612,
1586, 1514, 1464, 1442, 1302, 1248, 1175, 1106, 1036, 917,
872, 832, 768, 629, 586, 548 cm−1.
1
(0.9 g, 7.00 mmol, 77% yield) as described above. The H
NMR, 13C NMR, and IR spectra of 1c match those reported.59
1H NMR (CDCl3, 400 MHz): δ 2.0 (dd, J = 6.8 Hz, J = 1.2 Hz,
3H, −CH3), 6.9 (dd, J = 15.4 Hz, J = 1.2 Hz, 1H, CH), 7.1−
7.2 (m, 1H, CH), 7.8 (d, J = 8.4 Hz, 2H, Ph-H), 8.0 (d, J =
8.4 Hz, 2H, Ph-H) ppm. 13C NMR (CDCl3, 100 MHz): δ 18.8,
115.8, 118.0, 127.0, 128.9, 132.4, 141.2, 147.3, 189.4 ppm. IR
(neat): 3095, 3048, 2972, 2914, 2850, 2231 (s, CN), 1672 (s,
CO), 1654, 1624 (s, CC), 1560, 1442, 1405, 1375, 1332,
1293, 1220, 1177, 1107, 1073, 1040, 1017, 965 920, 864, 839,
809, 759, 663, 642, 567, 544 cm−1. GC/MS (EI): m/z:
171(M+), 156, 142, 130, 115, 102, 75, 69 (100%), 63, 51.
HRMS: m/z calculated for C11H9NONa+ [M + Na]+,
5.6.2.2. Synthesis of 1-(4-Methoxyphenyl)-but-3-en-1-one.
Oxidization of 1-(4-methoxyphenyl)-but-3-en-1-ol (15.6 g, 0.09
mol) with Jones reagent as described above gave 1-(4-
methoxyphenyl)-but-3-en-1-one (4.3 g, 0.02 mol, 28% yield).
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Article
194.05764; found, 194.05767; HRMS: m/z calculated for
C11H10NO+ [M + H]+, 172.07571; found, 172.07569
purged with oxygen and photolyzed through a Pyrex filter for 1
h at 298 K. 1H NMR spectrum of the reaction mixture after 1 h
showed the formation of 2c (4%) and remaining starting
material 1c (96%). After 4 h of irradiation, the product ratio
was decreased to 2c (11%) and 1c (89%).
5.7. Photolysis of 1a−c. 5.7.1. Product Studies of 1a in
Argon-Saturated CDCl3. A solution of 1a (∼20 mg, 137 μmol)
in CDCl3 (2 mL) was purged with argon and photolyzed via a
1
Pyrex filter for 30 min at 298 K. H NMR spectrum of the
ASSOCIATED CONTENT
* Supporting Information
reaction mixture showed the formation of 2a (33%), with 67%
■
1
S
of remaining starting material. 2a: H NMR (CDCl3, 400
MHz): δ 2.146 (dd, J = 7.2 Hz, J = 1.6 Hz, 3H, −CH3), 6.399−
6.481 (m, 1H, CH), 6.827 (dd, J = 11.2 Hz, J = 1.6 Hz, 1H,
CH), 7.444−7.569 (m, 3H, Ph-H), 7.912−8.064 (m, 2H, Ph-
H) ppm. GC/MS (EI): m/z 145 ((M-H)+, 100%), 130, 116,
104, 77, 69, 50.
Cartesian coordinates and energies of 1−4, IR, NMR, and MS
spectra of 1 and 2 and the synthetic precursors to 1, matrix
isolation studies of 1b and 1c. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*A. D. Gudmundsdottir: e-mail, Anna.Gudmundsdottir@uc.
edu; phone, 513-556-3380.
5.7.2. Product Studies of 1a in Oxygen-Saturated CDCl3. A
solution of 1a (∼20 mg, 137 μmol) in CDCl3 (2 mL) was
purged with oxygen and photolyzed through a Pyrex filter for
■
1
30 min at 298 K. H NMR analysis of the reaction mixture
showed the formation of 2a (26%), with 74% remaining
starting material. The product was characterized by GC/MS
chromatography.
Notes
The authors declare no competing financial interest.
5.7.3. Product Studies of 1b in Argon-Saturated CDCl3. A
solution of 1b (∼20 mg, 114 μmol) in CDCl3 (2 mL) was
purged with argon and photolyzed via a Pyrex filter for 1.5 h at
ACKNOWLEDGMENTS
■
This work was supported by NSF (CHE-1057481) and the
Ohio Supercomputer Center. R.A.A.U.R. acknowledges a Harry
B. Mark Fellowship from the Department of Chemistry,
University of Cincinnati. R.L. and C.B. thank the Natural
Sciences Research Council of Canada (NSERC) for financial
support.
1
298 K. H NMR analysis of the reaction mixture showed the
formation of 2b (45%), with 55% remaining starting. The
product was characterized by GC/MS chromatography and 1H
1
NMR spectroscopy of the reaction mixture. 2b: H NMR
(CDCl3, 400 MHz): δ 2.1 (dd, J = 7.2 Hz, J= 1.6 Hz, 3H,
−CH3), 3.9 (s, 3H, −OCHH3), 6.35−6.40 (m, 1H, CH), 6.8
(dd, J = 11.6 Hz, J= 1.6 Hz, 1H, CH), 6.9−7.0 (m, 2H, Ph-
H), 8.0 (d, J = 8.2 Hz, 2H, Ph-H) ppm. GC/MS (EI): m/z 176
(M+), 161, 145, 135 (100%), 127, 115, 107, 92, 77, 69, 64.
5.7.4. Product Studies of 1b in Oxygen-Saturated CDCl3. A
solution of 1b (∼20 mg, 114 μmol) in CDCl3 (2 mL) was
purged with oxygen and photolyzed through a Pyrex filter for
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1
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1
mixture. 2c: H NMR (CDCl3, 400 MHz): δ 2.2 (dd, J = 7.2
Hz, J = 1.6 Hz, 3H, −CH3), 6.5−6.6 (m, 1H, CH), 6.9 (dd, J
= 11.6 Hz, J = 1.6 Hz, 1H, CH), 7.8 (d, J = 8.4 Hz, 2H, Ph-
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172.07571; found, 172.07569
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solution of 1c (∼20 mg, 117 μmol) in CDCl3 (2 mL) was
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