Triplet sensitized photolysis of a vinyl azide: Direct detection of a triplet vinyl azide and nitrene

Article abstract of DOI:10.1021/jo501898p

Photolysis of vinylazide 1, which has a built-in acetophenone triplet sensitizer, in argon-saturated toluene results in azirine 2, whereas irradiation in oxygen-saturated toluene yields cyanide derivatives 3 and 4. Laser flash photolysis of azide 1 in argon-saturated acetonitrile shows formation of vinylnitrene 1c, which has a _max at ～300 nm and a lifetime of ～1 ms. Vinylnitrene 1c is formed with a rate constant of 4.25 × 10⁵ s^-1 from triplet 1,2-biradical 1b. Laser flash photolysis of 1 in oxygen-saturated acetonitrile results in 1c-O (_max = 430 nm, τ ≈ 420 μs acetonitrile). Density functional theory (DFT) calculations were used to aid in the characterization of the intermediates formed upon irradiation of azide 1 and to validate the proposed mechanism for its photoreactivity.

Full text of DOI:10.1021/jo501898p

Article
pubs.acs.org/joc
Triplet Sensitized Photolysis of a Vinyl Azide: Direct Detection of a
Triplet Vinyl Azide and Nitrene
Sridhar Rajam,† Abhijit V. Jadhav,† Qian Li,† Sujan K. Sarkar,† Pradeep N. D. Singh,† Ahleah Rohr,†
Tamara C. S. Pace,‡ Rui Li,‡ Jeanette A. Krause,† Cornelia Bohne,‡ Bruce S. Ault,†
and Anna D. Gudmundsdottir*,†
†Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, United States
‡Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W 3 V6, Canada
S
* Supporting Information
ABSTRACT: Photolysis of vinylazide 1, which has a built-in acetophenone
triplet sensitizer, in argon-saturated toluene results in azirine 2, whereas
irradiation in oxygen-saturated toluene yields cyanide derivatives 3 and 4.
Laser flash photolysis of azide 1 in argon-saturated acetonitrile shows
formation of vinylnitrene 1c, which has a λmax at ∼300 nm and a lifetime of
∼1 ms. Vinylnitrene 1c is formed with a rate constant of 4.25 × 105 s−1
from triplet 1,2-biradical 1b. Laser flash photolysis of 1 in oxygen-saturated
acetonitrile results in 1c-O (λmax = 430 nm, τ ≈ 420 μs acetonitrile).
Density functional theory (DFT) calculations were used to aid in the
characterization of the intermediates formed upon irradiation of azide 1 and
to validate the proposed mechanism for its photoreactivity.
biradical character (Scheme 1), to form an intermediate with a
new C−O bond.4
1. INTRODUCTION
Nitrenes are fascinating reactive intermediates that have been
used in various applications such as photoaffinity labeling, cross-
linking of polymers, and surface modifications, and they also have
potential as building blocks for high-spin assemblies and are
therefore of general interest.1,2 Because nitrenes are used in such
a broad variety of applications, it is important to determine how
they can be formed selectively and to identify the factors that
control their reactivity. Generally, the reactivity of nitrenes is
controlled by their spin configuration, as nitrenes with singlet
configuration are generally short-lived and highly reactive,
whereas their triplet counterparts are more stable and less
reactive. However, it is not only the electronic configuration that
controls the reactivity of nitrenes but also their substitution.
Recently, we reported the first direct detection of triplet
vinylnitrenes in solution by performing laser flash photolysis
studies of 2H-azirine and oxazole derivatives.3−5 Triplet
vinylnitrenes display unique reactivity among nitrenes, as they
are short-lived intermediates with lifetimes of a few micro-
seconds. Vinylnitrenes decay by intersystem crossing to yield
azirines, although vinylnitrenes that have a γ-carbonyl group
intersystem cross to form isoxazole derivatives in competition
with formation of azirines.3,4,6 The singlet−triplet energy gap in
vinylnitrenes has been calculated to be ∼15 kcal/mol.7 On the
basis of this, the lifetimes of triplet vinylnitrenes are limited by
their rate constant for intersystem crossing to their singlet
products, which for the triplet vinylnitrene is on the order of
105−106 s−1. Another unique property of triplet vinylnitrenes is
that they react efficiently with oxygen, due to their significant 1,3-
Scheme 1
In comparison, triplet nitrenes with electron-donating
substituents, such as alkyl- and phenylnitrenes, are long-lived
intermediates with lifetimes of several milliseconds in solution.8,9
Triplet alkyl- and arylnitrenes are highly unreactive because they
do not undergo intersystem crossing to react with themselves nor
the solvent but rather they decay by dimerizing to form azo
compounds (Scheme 2).8−11 Additionally, because they do not
have carbon atoms with unpaired electrons, triplet alkyl- and
arylnitrenes react with O2 to form the corresponding nitro-
compounds.
The most common precursor for nitrenes is the corresponding
azido compound that can be activated either thermally or
Received: August 17, 2014
Published: August 27, 2014
© 2014 American Chemical Society
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solutions yields 1,2-biradical 1b, which extrudes a N2 molecule to
form triplet vinylnitrene 1c. The latter then slowly intersystem
crosses to form azirine 2. In oxygen-saturated acetonitrile
solutions, laser flash photolysis shows that 1b is intercepted to
form peroxide radical 1c-O. In cryogenic argon matrices
irradiation of vinyl azide 1 yields benzoic and imine radicals.
Scheme 2
2. RESULTS
photochemically. For example, singlet aryl- and alkoxynitrenes
are formed by direct photolysis of the corresponding azido
precursors that intersystem cross to form the ground-state triplet
nitrenes.2 The singlet−triplet energy gap in alkoxynitrene has
been reported in the 3−8 kcal/mol range, and alkoxynitrene
intersystem crosses with a rate constant of ∼108 s−1 from its
singlet to its triplet.12 In comparison, in phenylnitrene the
singlet−triplet energy gap is 14.8 kcal/mol,13 and it intersystem
crosses with a rate constant of ∼3 × 106 s−1 at cryogenic
temperatures (Scheme 3).14 Alkylnitrenes, however, cannot be
2.1. Product Studies. Irradiation of vinyl azide 1 in argon-
saturated toluene via Pyrex filter resulted in 2 as the only product
(Scheme 5). The photolysis of vinyl azide 1 was followed by 1H
Scheme 5
Scheme 3
NMR spectroscopy, which showed that vinyl azide 1 is converted
selectively to 2. Photolysis of vinyl azide 1 in oxygen-saturated
toluene did not yield azirine 2 but resulted in 3 and 4 (Scheme 5).
From the product studies we theorize that irradiation of vinyl
azide 1 forms the first triplet excited state of the ketone (T1K) of
1, which then undergoes energy transfer to yield biradical 1b.
Extrusion of a N2 molecule from 1b results in vinylnitrene 1c,
which intersystem crosses to form 2 (Scheme 6). In comparison,
we propose that in oxygen-saturated solutions 1b is formed and
trapped with oxygen to form 1b-O that expels a N2 molecule to
form 1c-O, which results in formation of 3 and 4 (Scheme 7). It
was not possible to isolate the peroxide precursor to 3 (Scheme
7) using silica column chromatography.
2.2. Molecular Modeling. To support the reaction
mechanisms for vinyl azide 1 proposed in Schemes 6 and 7, we
performed calculations using Gaussian03 at the B3LYP level of
theory with the 6-31+G(d) basis set.17−19 The optimized
structure of vinyl azide 1 is very similar to its X-ray crystal
structure (Figure 1). Time-dependent density functional theory
(TD-DFT) calculations of vinyl azide 1 place the first triplet
excited (T1K) and second triplet excited (T2K) states of the
ketone of 1 at 74 and 76 kcal/mol, respectively, above the S0 of 1.
The energies of T1K and T2K are similar to those reported for
valerophenone derivatives.20 Visualization of the molecular
orbitals revealed, as expected, that T1K of 1 has a (n,π*)
configuration of the acetophenone moiety, and it is ∼3 kcal/mol
more stable than the second excited state (T2K) of the ketone
with a (π,π*) configuration.
formed by direct photolysis of azidoalkanes because direct
photolysis yields imine products by either concerted reactions in
the singlet excited state or through short-lived singlet nitrenes
that form the imine products faster than they can intersystem
cross to the more stable triplet.8,15 The energy gap between
singlet and triplet alkylnitrenes has been reported to be on the
order of 31.2 kcal/mol,16 and thus intersystem crossing is
expected to be slow. Hence, triplet alkylnitrenes can only be
formed by intra- or intermolecular sensitization of azidoalkanes
(Scheme 4).8
In this article, we demonstrate that a vinylazide with a built-in
triplet sensitizer can serve as a precursor to triplet vinylnitrenes.
Photolysis of azide 1 in argon-saturated solutions results in
azirine 2, whereas in oxygen-saturated solutions 3 and 4 are
formed. Laser flash photolysis of azide 1 in argon-saturated
As the T1K of 1 is the lowest triplet excited state of the ketone
chromophore, it is possible to optimize its structure. The
optimized structure of T1K of 1 is located 65 kcal/mol above the
ground state (S0) of 1; this is considerably lower than the energy
obtained from TD-DFT calculations. However, DFT calcu-
lations are known to underestimate the triplet energy of ketones
with a (n,π*) configuration.21 The C−O bond in T1K of 1 is
prolonged to 1.29 Å, in comparison to the CO bond in the S0
of 1, which is 1.22 Å. The elongation of the C−O bond fits well
with what has been observed for triplet aryl ketones with a (n,π*)
configuration.22 The N−N−N angle is almost linear (173°), and
the calculated IR stretch for the azido group is 2247 cm−1, similar
Scheme 4
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Scheme 6
Scheme 7
Figure 1. (A) Molecular structure of vinyl azide 1 as determined by X-ray analysis. Only one of the two independent molecules is shown here (50%
probability ellipsoids). Selected bond distances (Å) and angles (deg): C1A−O7A = 1.217(1), C1B−O7B = 1.221(1), N1A−C10A = 1.423(2), N1B−
C10B = 1.422(2), C9A−C10A = 1.322(2), C9B−C10B = 1.322(2), N1A−N2A−N3A = 171.6(2), N1B−N2B−N3B = 171.7(2), O1A−C7A−C8A−
C9A = 6.8(2), O1B−C7B−C8B−C9B = 9.8(2), C7A−C8A−C9A−C11A = 73.6(2), C7B−C8B−C9B−C11B = 73.8(1), C8A−C9A−C10A−N1A =
−178.6(1), C11A−C9A−C10A−N1A = 1.6(2), C8B−C9B−C10B−B1B = −178.9(1), C11B−C9B−C10B−N1B = 0.7(2), C9A−C10A−N1A−N2A
= −168.9(2), C9B−C10B−N1B−N2B = −177.9(1). (B) Optimized structure of 1.
to what was calculated for the S0 of 1; thus, it is postulated that
the excited state is localized on the carbonyl group because the
azido moiety in T1K of 1 is similar to the S0 of 1. Spin density
calculations demonstrate that the spin density is localized on the
carbonyl oxygen atom and the phenyl ring, further supporting
the (n,π*) configuration for T1K of 1 (Figure 2).
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Figure 2. Optimized structure of T1K of 1, triplet biradical 1b, and triplet vinylnitrene 1c. The calculated bond distances are in Å. The calculated spin
densities are written next to the corresponding atoms.
Figure 3. Calculated rotational barrier around the CH2−CC−N bond in triplet vinylnitrene 1c.
Figure 4. Calculated stationary points on the triplet surface of 1. Numbers in parentheses are energies in kcal/mol. The energies of T2K of 1 and T1K of 1
(65) were obtained by TD-DFT calculations, and the other ones were obtained by optimization calculations.
The optimized structure of triplet 1,2-biradical 1b is located at
55 kcal/mol above the S0 of 1. The Cβ−Cγ bond length is 1.47 Å,
and thus it can be described as having single-bond character, as
opposed to the same bond in the S0 of 1 where it has double-
bond character (1.34 Å). The dihedral angle between the CH3−
Cβ−Cγ−H is 97° in 1b, in order to minimize the overlap
between the two adjacent radical centers. Furthermore, the C−N
bond was shortened to 1.37 Å in 1b, from 1.42 Å in the S0 of 1.
The stretching frequency of the azido group is 2201 cm−1 in 1b,
which is only slightly lower in frequency than that calculated for
the azido stretch in the S0 of 1 of 2251 cm−1. Spin density
calculations further support that 1b is a 1,2-biradical as the spin
densities of the Cβ and Cγ are −0.88 and −0.67, respectively.
The unpaired electron on the Cγ is conjugated with the azido
moiety as the terminal N atom has a spin density of −0.32.
Vinylnitrene 1c and N2 molecule are located 4 kcal/mol above
the S0 of 1. In 1c the CC and CN bond distances are 1.41
and 1.30 Å, respectively, similar to what we have reported
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previously for vinylnitrenes using DFT calculations.3 The
calculated spin density of vinylnitrene 1c demonstrates that the
unpaired electrons are located mainly on the N atom (−1.41)
and the Cβ atom (−0.66) (Figure 2). The calculated rotational
barrier around the CH2−CC−N bond in vinylnitrene 1c is
displayed in Figure 3. The calculated rotational barriers around
the CC bond of 16 and 14 kcal/mol are very significant.
The calculated stationary points on the triplet surface of 1 are
shown in Figure 4. The transition state for forming triplet
vinylnitrene 1c from extrusion of a N2 molecule from biradical 1b
is located ∼1 kcal/mol above 1b. We also calculated the
transition states for T1K of 1 and vinylnitrene 1c (Figure 4) to
undergo α-cleavage. The transition state for α-cleavage of
vinylnitrene 1c is located 22 kcal/mol above 1c and results in 1g
and 1h. In comparison, the transition state for T1K of 1
undergoing α-cleavage is located 9 kcal/mol above T1K of 1 at 65
kcal/mol and yields 1g and 1i. The transition state for 1i to form
1h with extrusion of a N2 molecule is 2 kcal/mol above 1i. These
calculations show that the energies necessary to form biradical 1b
from T1K of 1 and vinylnitrene 1c from biradical 1b are feasible at
ambient temperature. Similarly, α-cleavage for T1K of 1 is
expected to be feasible at ambient temperature, although α-
cleavage products were not observed in solution. On the other
hand, the transition state barrier for α-cleavage of vinylnitrene 1c
is significantly larger, and thus this reaction is not expected to
take place thermally.
1b-O has a spin density of −0.70 and −0.39 on the peroxide
oxygen atoms and −0.65 on the carbon atom adjacent to the
azido moiety, which is resonance-stabilized with the terminal N
atom in the azido group that has a spin density of −0.30. In
comparison, the spin density of 1c-O is localized on the N atom
and the peroxide oxygen atoms (Figure 5). Finally, we calculated
stationary points on the energy surface of 1b reacting with
oxygen. Biradical 1b can react with oxygen to form biradical 1b-
O, which is 32 kcal/mol more stable than 1b and O2 (Figure 6).
Biradical 1b-O can extrude a N2 molecule to form 1c-O, which is
40 kcal/mol more stable than 1b-O. It can also be theorized that
the expulsion of N2 and addition of O2 to 1b is concurrent, but
due to spin restrictions, we cannot obtain the transition states for
1b adding to O2 to form 1b-O or 1c-O. The transitions state for
1b-O to expel an N2 molecule to form 1c-O is 2 kcal above 1b-O,
and thus similar for 1b expelling N2 molecule.
2.3. Matrix Isolation. We deposited 1 into argon matrices at
14 K. Two azido bands were observed for 1, an intense band at
2114 cm−1 and a band with lower intensity at 2136 cm−1 (Figure
7a). Presumably, these two bands are due to different conformers
of 1 or entrapping of 1 in different matrix sites. After 2 h of
irradiation via Pyrex filter, the azido band at 2114 cm−1 was
depleted, whereas the azido band at 2136 cm−1 was not affected
(Figure 7). In addition, bands at 1704, 1691, 1347, 1336, 1298,
and 700 cm−1 were also depleted. The IR spectrum obtained after
irradiation showed formation of several new bands, the most
significant being at 1826, 1717, 1691, 929, 829, and 746 cm−1.
We assign the bands at 1826 and 746 cm−1 (Figure 7b) to
benzoyl radical 1g. The carbonyl group in benzoyl radical has
been reported to be at 1828 cm−1.23 The bands at 1717, 1691,
1677, 929, and 829 cm−1 were attributed to 1h (Figure 8). The
coupled CN and CC stretching of 1hA are calculated to be
at 1695 and 1684 cm−1 and at 1724 and 1670 cm−1 for 1hB,
whereas the bending of the CH2 in 1hA and 1hB are calculated to
be at 948 and 952 cm−1, respectively (Figure 7b).
We optimized 1c-S, the open-shell singlet of 1c, and
established that the singlet−triplet energy gap is 15 kcal/mol.
The singlet−triplet energy gap for 1c is similar to what Parasuk
and Cramer calculated for simple vinylnitrene CH2CH−N:.7
We optimized the structures obtained from 1b and 1c reacting
with O2 (Figure 5). Spin density calculations show that peroxide
Thus, photolysis of 1 in argon matrixes leads to α-cleavage to
form 1g and 1h. Because we did not observe any product
attributable to benzoyl radical formation in solution, we theorize
that vibrationally hot 1c cleaves to form 1g and 1h, rather than 1
undergoing α-cleavage. This is further supported by our
calculations, which demonstrate that in matrices vibrationally
hot 1c is formed with sufficient energy to overcome the transition
state barrier to form 1g and 1h. There are many examples of
azido and nitrene derivatives that have been proposed to react
from vibrationally hot states.24,25 However, we cannot rule out
that T1K of 1 undergoes α-cleavage directly. As irradiation of vinyl
azide 1 in matrices did not result in any new IR bands that were
Figure 5. Calculated structures of (a) 1b-O and (b) 1c-O. The numbers
stated are the calculated spin densities.
Figure 6. Stationary points on the energy diagram for 1b reacting with oxygen. The numbers in parentheses are energies in kcal/mol.
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Figure 7. (a) Comparison of IR spectra of 1 before (red) and after (black) irradiation in argon matrices. (b) Expansion and (c) calculated spectra of
benzoyl and 1hA and 1hB radicals.
at 310 and 330 nm in argon-saturated solvents was formed with a
rate constant of 4.25 × 105 s−1 (τ = 2.4 μs) in acetonitrile and 3.0
× 105 s−1 (τ = 3.3 μs) in methanol (Figure 10) and had a lifetime
on the order of a millisecond (τ ≈ 1 ms). We assign this transient
absorption to have contributions from both 1b and 1c based on
TD-DFT calculations that place the major electronic transitions
for 1b at 354 (f = 0.011), 350 (f = 0.034), and 300 (f = 0.012) nm
and for the two minimal energy conformers of 1c, 1cA 332 (f =
0.009) and 318 (f = 0.005) nm and 1cB at 323 (f = 0.010) and
318 (f = 0.018) nm (Figure 11). Furthermore, since the
absorption of 1b and 1c overlap, 1c must have stronger
absorption than 1b, as the transient absorption is observed
growing in at shorter time scales, suggesting that 1b forms 1c.
The decay rate constant for 1b is the same as the rate constant for
forming 1c. Thus, 1b has a lifetime of 2.4 μs in acetonitrile and it
Figure 8. Conformers A and B of 1h.
depleted upon further irradiation, we can rule out vinylnitrene 1c
forming 1g and 1h as secondary photoproducts.
2.4. Laser Flash Photolysis. Laser flash photolysis studies of
1 in argon-saturated acetonitrile showed a broad transient
between 300 and 360 nm (Figure 9a). The transient monitored
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Figure 9. Laser flash photolysis of 1 in (a) argon-saturated acetonitrile
and (b) oxygen-saturated acetonitrile with 308 nm laser.
Figure 11. Calculated TD-DFT spectra of (a) 1c, (b) 1b, and (c) T1K of
1.
Figure 10. Kinetic trace recorded from laser flash photolysis of 1 in
argon-saturated (a) acetonitrile at 330 nm and (b) methanol at 310 nm.
Figure 12. Calculated TD-DFT spectra for (a) 1b-O and (b) 1c-O.
decays to form 1c, which in turn has a lifetime on the order of 1
ms.
methanol, and the decay rate constant of 1c-O is only 1.9 × 105
s−1 (τ = 5.2 μs, Figure 13b), which is a reflection of 1c-O decaying
by abstracting H atom from the solvent to form 3 and 4.
To further support the assignment of the transient absorption
to vinylnitrene 1c, we performed laser flash photolysis studies of
1 in oxygen-saturated acetonitrile that resulted in a different
transient spectrum with λmax at 420 nm (Figure 9b). Similar
transient absorption was obtained in air-saturated acetonitrile as
in oxygen-saturated solutions. We assign this transient to 1c-O
based on its calculated spectrum that has major electronic
transitions at 424 nm (f = 0.0055, Figure 12). Analysis of the
kinetics reveals that the absorption forms with a rate constant of
7.5 × 105 s−1 (τ = 1.33 μs) and 3.7 × 106 s−1 (τ = 270 ns) in air-
and oxygen-saturated acetonitrile, respectively. In oxygen-
saturated acetonitrile, the transient absorption at 430 nm
decayed with a rate constant of 2.4 × 104 s−1 (τ = 420 μs). As
the rate of forming 1c-O increased at higher O2 concentration,
we conclude that 1b reacts with O2 to form 1b-O with a slower
rate at the oxygen concentrations employed than 1b-O expels N2,
and thus we only observe the absorption of 1c-O and not 1b-O. It
can also be theorized that addition of O2 to 1b takes place at the
same time as N2 extrusion. In addition, the rate constant for
forming 1c-O is 4.9 × 106 s−1 (τ = 204 ns) in oxygen-saturated
3. DISCUSSION
The triplet sensitized photolysis of 1 led to the formation of 1,2-
biradical 1b, which expels N2 to form triplet vinylnitrene 1c. The
transition state barrier for the extrusion of the N2 molecule is
small enough that it takes place spontaneously at ambient
temperature with a rate constant of 3−5 × 105 s−1. We have
demonstrated that carbon-based radicals adjacent to the azido
group lead to the expulsion of N2 to form imine radicals, and the
rate constant for 6a extruding N2 is faster than the time
resolution of the laser flash photolysis apparatus, or 6 × 107 s−1.26
The extrusion of N2 from 1b is significantly slower than for 6a
(Scheme 8). Although, the calculated transition state barrier for
1b-O extruding N2 molecule to form 1c-O is higher than for 1b
expelling a N2 molecule to form 1c, the rate for forming 1c-O is
faster than for 1c, which supports the notion that addition of O2
and extrusion of N2 from 1b take place concurrently, rather than
stepwise.
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rotational barriers for vinylnitrenes 7 and 8, which are smaller
than that for 1c and they are therefore shorter lived than 1c.
Thus, the intersystem crossing in vinylnitrenes is similar to what
has been proposed for intersystem crossing in 1,2-biradicals.28
As mentioned earlier, triplet vinylnitrenes are unique as they
react more efficiently with O2 than triplet alkyl- and arylnitrenes
due to their 1,3-biradical character that places significant
unpaired electron density on their β-carbon atom. Biradical 1b
also reacts efficiently with O2 in acetonitrile and methanol, but
with a rate constant that is significantly lower than for a diffusion-
controlled process, or between 4 and 5 × 108 M−1 s−1, which is
somewhat less than the reported rate constant for vinylnitrenes 7
and 8 reacting with O2. Thus, biradical 1b and vinylnitrenes 7 and
8 are intercepted with O2 with rate constants that are less than
expected for a diffusional process, because they are electron-
deficient biradicals due to their azido and nitreno substitution.
Photolysis of 1 in matrices led to α-cleavage but not
intersystem crossing to ketene imine or azirines as has been
observed for most vinylnitrenes in cryogenic matrices.6,29−34
Thus, it seems reasonable to propose that vinylnitrene 1c is
formed from vibrationally hot biradical 1b, and it cleaves to form
1g and 1h. However, we cannot rule out the possibility that at
cryogenic temperatures the α-cleavage takes place from T1K of 1
that has (n,π*) configuration and the thermal population of T2K
of 1 (π,π*) cannot compete with α-cleavage from T1K of 1 (n,π*)
as in solutions. Nevertheless, vinylazide 1 displays unique
reactivity in matrices.
Figure 13. Kinetic trace recorded from laser flash photolysis of 1 in
oxygen-saturated methanol at 400 nm over (a) 1 μs and (b) 50 μs time
window.
Scheme 8
4. CONCLUSIONS
We have shown that irradiation of vinyl azide 1, which has a built-
in triplet sensitizer, yields 1,2-biradical 1b through intramolecular
sensitization. Biradical 1b reacts by expelling a N2 molecule to
form vinylnitrene 1c. Thus, triplet sensitization can be used to
efficiently form triplet vinylnitrene 1c from vinyl azides 1.
Therefore, we conclude that triplet sensitization can be used to
overcome the limitation of using vinyl azides as precursor for
vinylnitrenes, as intersystem crossing from the singlet excited
states of vinyl azides and singlet vinylnitrenes to their triplet
configuration is inefficient. Intramolecular sensitization of vinyl
azides can therefore be used in the pursuit of forming stable
triplet vinylnitrenes.
Interestingly, vinylnitrene 1c is significantly longer lived than
previously reported vinylnitrenes 7 and 8 that have lifetimes of a
few μs at ambient temperature (Scheme 9). Because vinyl-
Scheme 9
5. EXPERIMENTAL SECTION
5.1. Calculations. All geometries were optimized at the B3LYP level
of theory and with 6-31G+(d) basis set as implemented in the
Gaussian03 programs.17−19 All transition states were confirmed to have
one imaginary vibrational frequency. Intrinsic reaction coordinate
calculations were used to verify that the located transition states
corresponded to the attributed reactant and product.35,36 The
absorption spectra were calculated using time-dependent density
functional theory (TD-DFT).37−41
5.2. Synthesis. Synthesis of 3-Methyl-1-phenyl-but-3-en-1-ol.
The method of Imai and Nishida was followed to make this
compound.42 To a magnetically stirred solution of benzaldehyde (5.3
g, 50 mmol) and 3-chloro-2-methylpropene (5.4 g, 60 mmol) in
dimethylformamide (DMF) (100 mL) was added successively SnCl2·
2H20 (17 g, 75 mmol) and NaI (11 g, 75 mmol); a mild exothermic
reaction was noted at the early stage. After stirring for 20 h at room
temperature, 30% aqueous NH4F (100 mL) and diethyl ether (200 mL)
were added. The mixture was stirred, and the two layers separated. The
aqueous phase was re-extracted with diethyl ether. The ether extracts
were combined, washed with saturated aqueous NaCl (100 mL), and
dried over MgSO4. The removal of solvent yielded 3- methyl-1-phenyl-
but-3-en-1-ol (7.6 g, 47 mmol, 95% yield). IR (neat): 3410, 1640, 910
cm−l; 1H NMR (CDCl3, 250 MHz): 1.77 (s, 3H), 2.31 (s, 1H), 2.40 (d,
nitrenes 1c, 7, and 8 all decay by intersystem crossing to form the
corresponding azirines, 1c must intersystem cross slower than 7
and 8. Wenthold showed that the singlet−triplet energy gap in
vinylnitrene can be reduced with electron delocalization;27
however, the calculated energy gap between the triplet and
singlet configuration of vinylnitrenes 1c, 7, and 8 (Scheme 9) are
similar and thus cannot be used to explain the difference in their
lifetimes. We theorize, however, that intersystem crossing to the
singlet surface of 1c is facilitated by rotation around the CC
bond in vinylnitrenes, which makes accessible higher-energy
conformers with better spin-orbit coupling than the minimal
energy conformer 1c. In these high-energy conformers, the spins
on the vinyl C and N atoms are in closer proximity and thus spin-
orbit coupling is enhanced along with intersystem crossing.
Further support for this notion comes from the calculated
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2H, 6.8 Hz), 4.76 (t, 1H, 6.8 Hz), 4.82 (s, lH), 4.89 (s, lH), 7.21−7.40
(m, 5H) ppm. MS (m/e): 162 (M+, <1%).
Photolysis in Oxygen-Saturated Solutions. A solution of E-4-azido-
3-methylphenyl-but-3-en-1-one (20 mg, 0.0995 mmol) in toluene (3
mL, dried over Na) was bubbled with oxygen for 30 min and photolyzed
for 30 min through a Pyrex filter. The reaction was monitored with high-
performance liquid chromatography (HPLC), and irradiation was
stopped when 15% of the precursor had reacted. Analysis of the HPLC
trace showed formation of two products in the ratio of 2:1.
Synthesis of 3-Methyl-1-phenyl-but-3-en-1-one.43 To a stirred
slurry of 3-methyl-1-phenyl-but-3-en-1-ol (7.6 g, 47 mmol) in acetone
(100 mL) was added Jones reagent (10 g of chromium trioxide, 10 mL of
sulfuric acid, and 30 mL of water) until the color changed to an orangish-
yellow solution. The solution was filtered and the acetone was removed
in vacuo to give a red−brown slurry that was dissolved in diethyl ether
(250 mL), and then the ether layer was washed with saturated sodium
bicarbonate solution and dried (MgSO4). Removal of the diethyl ether
in vacuo at room temperature produced 3-methyl-1-phenyl-but-3-en-1-
one (6.08 g, 38 mmol, 80% yield). IR (neat): 3076, 2971, 2913, 1687,
1448, 1207, 690 cm−l. 1H NMR(CDCl3, 250 MHz): 7.99 (d, 2H, 8 Hz),
7.56 (t, 1H, 7 Hz), 7.48 (d 2H, 8 Hz) 4.98 (s, 1H), 4.85 (s, 1H), 3.68 (s,
2H), 1.82 (s, 3H) ppm. MS (m/e): 160 (M+, <1%).
A solution of E-4-azido-3-methylphenyl-but-3-en-1-one (124 mg,
0.614 mmol) in dry distilled toluene (24 mL) was photolyzed for 30 min
(Pyrex filter). Removal of solvent in vacuum, followed by isolation using
HPLC, yielded 2-hydroxy-2-methyl-4-oxo-4-phenylbutyronitrile 3 (4
mg, 0.021 mmol, 3% yield) and (E)-2-methyl-4-oxo-4-phenyl-but-2-
enenitrile 4 (2 mg, 0.011 mmol, 1.6% yield) and recovered the starting
material. The spectroscopic characterizations of 3 and 4 fit with those in
the literature.45,46
Synthesis of 4-Azido-3-iodo-3-methyl-1-phenylbutanone. The
azido−iodo compound was prepared following the procedure of Molina
et al.44 To a stirred slurry of sodium azide (6.0 g, 0.087 mol) in
acetonitrile (100 mL) at −10 °C was added slowly iodine monochloride
(6.56 g, 0.04 mol) over a period of 20 min. The reaction mixture was
stirred for an additional 5−10 min, and 3-methyl-1-phenyl-but-3-en-1-
one (6.08 mg, 0.038 mol) was added, allowed to warm to room
temperature, and stirred for 8−20 h. The red−brown slurry was poured
into water (250 mL), and the mixture was extracted with diethyl ether
(250 mL) in three portions. These were combined and washed with 5%
sodium thiosulfate (150 mL), leaving a colorless ethereal solution. The
solution was washed with water (900 mL) in four portions and dried
over magnesium sulfate. Removal of the diethyl ether in vacuo at room
temperature produced the iodo−azide adduct (7.42 g, 0.022 mol, 60%
yield), slightly orange in color. IR (neat): 2104, 1688, 1596, 1580, 1448,
1221, 1002 cm−1.
4-Azido-3-methyl-1-phenyl-but-3-en-1-one (7.4 g, 0.02 mol) was
reacted with sodium azide (1.5 g, 0.022 mol) in dimethylformamide
(100 mL, dried over molecular sieves, type 4 Å) at room temperature.
The solution was then poured into a mixture of water−diethyl ether, and
the diethyl ether layer was washed several times with water and dried
over MgSO4. Removal of the diethyl ether yielded a mixture of cis- and
trans-azido-3-methyl-1-phenyl-but-3-en-1-one (2.7 g, 13.3 mmol, 60%
yield). The crude product was purified by column chromatography
using an ethyl acetate and hexane system in the increasing order of
polarity to yield E-form (540 mg, 2.6 mmol, 20% yield) and Z-form (324
mg, 1.6 mmol, 12% yield) of 4-azido-3-methyl phenyl-but-3-en-1-one.
E-4-Azido-3-methyl-1-phenyl-but-3-en-1-one. IR (neat): 2105,
1675, 1600, 1574, 1510, 1259, 1170 cm−1.1H NMR (CDCl3, 250
MHz): 7.96 (d, 2H, 8 Hz), 7.57 (t, 1H, 7 Hz), 7.46 (t, 2H, 8 Hz), 6.10 (s,
1H), 3.62 (s, 2H), 1.69 (s, 3H) ppm. 13C NMR (CDCl3, 250 MHz): 18,
40, 121, 123.16, 128, 128, 133, 136, 196 ppm. HRMS: Calculated for
C11H12N3O = 202.0980. Found =202.0980. HRMS = high-resolution
mass spectrometry.
Compound 3.45 IR (CDCl3): 3455, 3063, 2992, 2919, 1681, 1597,
1
1580 cm−1. H NMR (CDCl3): 1.752 (s, 3H), 3.25 (d, 1H, 18 Hz),
3.723 (d, 1H, 18 Hz), 5.169 (s, 1H), 0.5.61 (t, 2H, 8 Hz), 7.69 (t, 1H, 7
Hz), 7.99 (d, 2H, 8 Hz) ppm. 13C NMR (CDCl3): 198, 135, 134, 129,
128, 121, 65, 47, 27 ppm. HRMS: Calculated mass (M+23) = 212.0687.
Experimental (M+23) = 212.0684.
Compound 4.46 IR (CDCl3): 3061, 2920, 2850, 2219, 1669, 1609,
1596, 1579 cm−1. 1H NMR (CDCl3): 2.304 (s, 3H), 7.421 (s, 1H), 7.55
(t, 1H, 8 Hz), 7.67 (t, 1H, 7 Hz), 8.0 (d, 2H, 8 Hz) ppm.
5.4. Laser Flash Photolysis. Laser flash photolysis experiments
were done with a YAG laser (355 and 266 nm, 15 ns)47 or an excimer
laser (308 nm, 17 ns).26 Stock solutions of 1 in methanol or acetonitrile
were prepared with spectroscopic-grade solvents such that the solutions
had an absorption between 0.2 and 0.8 at the excitation wavelengths.
Solutions were purged with nitrogen or oxygen for at least 15 min.
Solutions of 1 are relatively photolabile and could be excited only with a
small number of laser shots. For this reason, transient spectra were
collected using a flow system, where a freshly prepared solution was
pumped through a custom-designed Suprasil-cell (7 × 7 mm) at a rate of
1.5−2.0 mL/min. As a result, a fresh solution was irradiated by each laser
shot. Decays were collected either by using the flow system or by
subjecting the sample in static cells to a small number of laser shots and
checking for the integrity of the sample with UV−vis spectroscopy. The
kinetic traces were fitted so the chi square value (the ratio of the curve
fitting error to the standard deviation) is typically <0.02% of standard
deviation of the measured data.
5.5. Matrix Isolation. Matrix isolation studies were performed using
conventional equipment.48
5.6. X-ray Crystallography. Single crystals were obtained as
colorless plates from Et2O. For X-ray examination and data collection, a
suitable crystal, approximate dimensions 0.12 × 0.11 × 0.01 mm, was
mounted in a loop with paratone-N and transferred immediately to the
goniostat bathed in a cold stream. Intensity data were collected at 200 K
on a D8 goniostat equipped with a Bruker Platinum 200 CCD detector
at Beamline 11.3.1 at the Advanced Light Source (Lawrence Berkeley
National Laboratory) using synchrotron radiation tuned to λ = 0.77490
Å. For data collection, frames were measured for a duration of 2 s at 0.3°
intervals of ω. The data frames were collected using the APEX2 suite of
programs. The data were corrected for absorption and beam corrections
based on the multiscan technique as implemented in SADABS. The
structure was solved by a combination of direct methods in SHELXTL
and the difference Fourier technique and refined by full-matrix least-
squares on F2. Non-hydrogen atoms were refined with anisotropic
displacement parameters. The H atoms were calculated and treated with
a riding model. The H atom isotropic displacement parameters were
defined as a*Ueq(C) of the adjacent atom (a = 1.5 for CH3 and 1.2 for all
others). The refinement converged with crystallographic agreement
factors of R1 = 4.51%, wR2 = 11.49% for 2368 reflections with I > 2σ(I)
(R1 = 6.00%, wR2 = 12.45% for all data) and 274 variable parameters.
Z-4-Azido-3-methyl-1-phenyl-but-3-en-1-one. IR (neat): 2100,
1
1682, 1605, 1479, 1449, 1010 cm−1. H NMR (CDCl3, 250 MHz):
8.0 (d, 2H, 8 Hz), 7.57 (t, 1H, 7 Hz), 7.46 (t, 2H, 8 Hz), 6.17 (s, 1H),
3.72 (s, 2H), 1.71 (s, 3H) ppm. 13C NMR (CDCl3, 250 MHz): 15, 45,
123, 124, 128, 128, 133, 136, 197 ppm.
5.3. Photolysis of E-4-Azido-3-methylphenyl-but-3-en-1-one.
Photolysis in Argon-Saturated Solution. A solution of E-4-azido-3-
methylphenyl-but-3-en-1-one (20 mg, 0.0995 mmol) in toluene (3 mL,
dried over Na) was bubbled with argon for 20 min and photolyzed for 50
min (λ > 312 nm). The reaction was monitored with thin-layer
chromatography (TLC). Evaporation of toluene yielded 2-(2-methyl-
2H-azirin-2-yl)-1-phenyl ethanone (16.4 mg, 95 mmol, 82% yield) and
the starting material 1 (1.8 mg, 9 mmol, 8% yield). The photoproduct
was characterized by NMR, HRMS, and IR spectroscopy. IR (neat):
3060, 2975, 2952, 2922, 2107, 1683, 1640, 1597, 1579, 1448 cm−1. 1H
NMR (CDCl3, 400 MHz): 10.2 (s, 1H), 7.88 (d, 2H, 7 Hz), 7.54 (t, 1H,
7 Hz), 7.46 (d, 2H, 7 Hz), 3.42 (d, 1H, 16 Hz), 3.07 (d, 1H, 16 Hz), 1.35
(s, 3H) ppm. 13C NMR (CDCl3, 400 MHz): 197, 173, 136, 133, 128,
124, 47, 28, 24 ppm. HRMS (M+H); Calculated for C11H12N1O =
174.0919. Found = 174.0914.
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Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;
Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D.
K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M. G.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian, Inc.: Wallingford, CT, 2003.
ASSOCIATED CONTENT
* Supporting Information
NMR and IR spectra of 1−3. Cartesian coordinates, energies,
and vibrational frequencies are included. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: anna.gudmundsdottir@gmail.com.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(18) Becke, A. D. Chem. Phys. 1993, 98, 5648.
We thank the National Science Foundation and the Ohio
Supercomputer Center for supporting this work. S.R. thanks the
U. C. Chemistry Department for a Twitchell fellowship, and Q.
L. thanks the Zimmer program for their support. C. B., R. L., and
T. C. S. P. thank the Natural Sciences and Engineering Council of
Canada (NSERC) for support in the form of a discovery grant
(C. B.) and a CGS-D fellowship (T. C. S. P.). Crystallographic
data were collected through the SCrALS (Service Crystallog-
raphy at Advanced Light Source) program at Beamline 11.3.1 at
the Advanced Light Source (ALS), Lawrence Berkeley National
Laboratory. The ALS is supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the U.S. Department
of Energy under Contract No. DE-AC02-05CH11231.
(19) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of
Photochemistry; Marcel Dekker: New York, 1993.
(21) Muthukrishnan, S.; Mandel, S. M.; Hackett, J. C.; Singh, P. N. D.;
Hadad, C. M.; Krause, J. A.; Gudmundsdottir, A. D. J. Org. Chem. 2007,
72, 2757.
(22) Srivastava, S.; Yourd, E.; Toscano, J. P. J. Am. Chem. Soc. 1998,
120, 6173.
(23) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk, J.; Ingold, K.
U. J. Am. Chem. Soc. 1991, 113, 1869.
(24) Wentrup, C. Aust. J. Chem. 2013, 66, 852.
(25) Gritsan, N. P.; Platz, M. S. In Organic Azides: Syntheses and
Applications; Brase, S., Banert, K., Eds.; John Wiley & Sons, Ltd:
Chippenham, Wiltshire, U.K., 2010.
(26) Muthukrishnan, S.; Sankaranarayanan, J.; Klima, R. F.; Pace, T. C.
S.; Bohne, C.; Gudmundsdottir, A. D. Org. Lett. 2009, 11, 2345.
(27) Wenthold, P. G. J. Org. Chem. 2012, 77, 208.
(28) Caldwell, R. A.; Carlacci, L.; Doubleday, C. E., Jr.; Furlani, T. R.;
King, H. F.; McIver, J. W., Jr. J. Am. Chem. Soc. 1988, 110, 6901.
(29) Inui, H.; Murata, S. Chem. Commun. 2001, 1036.
REFERENCES
■
(1) Nitrenes and Nitrenium Ions; Falvey, D. E., Gudmundsdottir, A. D.,
Eds. Wiley Ser. React. Intermed. Chem. Biol., Vol. 6; John Wiley & Sons,
Inc.: New York, 2013.
(2) Platz, M. S. In Reactive Intermediate Chemistry; Moss, R. A., Platz,
M. S., Jones, M., Jr., Eds.; John Wiley & Sons, Inc.: New York, 2004.
(3) Rajam, S.; Murthy, R. S.; Jadhav, A. V.; Li, Q.; Keller, C.; Carra, C.;
Pace, T. C. S.; Bohne, C.; Ault, B. S.; Gudmundsdottir, A. D. J. Org.
Chem. 2011, 76, 9934.
(4) Gamage, D. W.; Li, Q.; Ranaweera, R. A. A. U.; Sarkar, S. K.;
Weragoda, G. K.; Carr, P. L.; Gudmundsdottir, A. D. J. Org. Chem. 2013,
78, 11349.
(5) Zhang, X.; Sarkar, S. K.; Weragoda, G. K.; Rajam, S.; Ault, B. S.;
Gudmundsdottir, A. D. J. Org. Chem. 2014, 79, 653.
(30) Murata, S.; Tomioka, H. Chem. Lett. 1992, 57.
(31) Nunes, C. M.; Reva, I.; Fausto, R. J. Org. Chem. 2013, 78, 10657.
́ ́
(32) Begue, D.; Dargelos, A.; Berstermann, H. M.; Netsch, K. P.;
Bednarek, P.; Wentrup, C. J. Org. Chem. 2014, 79, 1247.
(33) Kvaskoff, D.; Vosswinkel, M.; Wentrup, C. J. Am. Chem. Soc. 2011,
133, 5413.
(34) Wentrup, C. Wiley Ser. React. Intermed. Chem. Biol. 2013, 6, 273.
(35) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154.
(36) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
(37) Density Functional Methods in Chemistry; Labanowksi, J. K.,
Andzelm, J. W., Eds.; Springer-Verlag: New York, 1991.
(38) Parr, R. G.; Weitao, Y. Density Functional Theory in Atoms and
Molecules; Oxford University Press: Oxford, U.K., 1989.
(39) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454.
(40) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998,
109, 8218.
(41) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. J.
Phys. Chem. 1992, 96, 135.
(42) Imai, T.; Nishida, S. Synthesis 1993, 1993, 395.
(43) Rieke, R. D.; Klein, W. R.; Wu, T. C. J. Org. Chem. 1993, 58, 2492.
(44) Molina, P.; Pastor, A.; Vilaplana, M. J.; Foces-Foces, C.
Tetrahedron 1995, 51, 1265.
(6) Nunes, C. M.; Reva, I.; Pinho e Melo, T. M. V. D.; Fausto, R.;
Solomek, T.; Bally, T. J. Am. Chem. Soc. 2011, 133, 18911.
̌
(7) Parasuk, V.; Cramer, C. J. Chem. Phys. Lett. 1996, 260, 7.
(8) Singh, P. N. D.; Mandel, S. M.; Sankaranarayanan, J.;
Muthukrishnan, S.; Chang, M.; Robinson, R. M.; Lahti, P. M.; Ault, B.
S.; Gudmundsdottir, A. D. J. Am. Chem. Soc. 2007, 129, 16263.
(9) Li, Y. Z.; Kirby, J. P.; George, M. W.; Poliakoff, M.; Schuster, G. B. J.
Am. Chem. Soc. 1988, 110, 8092.
(10) Sankaranarayanan, J.; Bort, L. N.; Mandel, S. M.; Chen, P.; Krause,
J. A.; Brooks, E. E.; Tsang, P.; Gudmundsdottir, A. D. Org. Lett. 2008, 10,
937.
(11) Leyva, E.; Platz, M. S.; Persy, G.; Wirz, J. J. Am. Chem. Soc. 1986,
108, 3783.
(45) Soldermann, C. P.; Vallinayagam, R.; Tzouros, M.; Neier, R. J.
Org. Chem. 2008, 73, 764.
(46) Trabulsi, H.; Rousseau, G. Synth. Commun. 2011, 41, 2123.
(47) Liao, Y.; Bohne, C. J. Phys. Chem. 1996, 100, 734.
(48) Ault, B. S. J. Am. Chem. Soc. 1978, 100, 2426.
(12) Buron, C.; Platz, M. S. Org. Lett. 2003, 5, 3383.
(13) Wijeratne, N. R.; Da Fonte, M.; Ronemus, A.; Wyss, P. J.;
Tahmassebi, D.; Wenthold, P. G. J. Phys. Chem. A 2009, 113, 9467.
(14) Gritsan, N. P.; Tigelaar, D.; Platz, M. S. J. Phys. Chem. A 1999, 103,
4465.
(15) Singh, P. N. D.; Mandel, S. M.; Robinson, R. M.; Zhu, Z.; Franz,
́
R.; Ault, B. S.; Gudmundsdottir, A. D. J. Org. Chem. 2003, 68, 7951.
(16) Travers, M. J.; Cowles, D. C.; Clifford, E. P.; Ellison, G. B.;
Engelking, P. C. J. Chem. Phys. 1999, 111, 5349.
(17) Gaussian 03 . Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.;
9334
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