Laser flash photolysis of nanocrystalline α-azido-: P -methoxy-acetophenone

Article abstract of DOI:10.1039/c7ob01731f

Irradiation of nanocrystals of azide 1 results in a solid-to-solid reaction that forms imine 2 in high chemical yield. In contrast, solution photolysis of azide 1 yields a mixture of products, with 7 as the major one. Laser flash photolysis (LFP) of a nanocrystalline suspension of azide 1 in water shows selective formation of benzoyl radical 4 (λ_max ～ 400 nm), which is short-lived (τ = 833 ns) as it intersystem crosses to form imine 2. In comparison, LFP of azide 1 in methanol reveals the formation of triplet alkylnitrene 10 (λ_max ～ 340 nm). The selectivity observed in the solid-state is related to stabilization of the triplet ketone with (n,π?) configuration by the crystal lattice, which results in α-cleavage being favored over triplet energy transfer to the azido chromophore. Both the solid-state and solution reaction mechanisms are further supported by density functional theory calculations. Thus, laser flash photolysis has been used to effectively elucidate the medium dependent reaction mechanisms of azide 1.

Full text of DOI:10.1039/c7ob01731f

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10.1039/C7OB01731F.
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DOI: 10.1039/C7OB01731F
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ARTICLE
Laser flash photolysis of nanocrystalline α-azido-p-methoxy-
acetophenone
Sujarna K. Sarkar,a DeVonna M. Gatlin,a Anushree Das,a Breyinn Loftin,a Jeanette A. Krause,a
Received 00th January 20xx,
Accepted 00th January 20xx
Manabu Abeb and Anna D. Gudmundsdotti
DOI: 10.1039/x0xx00000x
www.rsc.org/
Irradiation of nanocrystals of azide 1 results in a solid-to-solid reaction that forms imine 2 in high chemical yield. In contrast,
solution photolysis of azide 1 yields a mixture of products, with 7 as the major one. Laser flash photolysis (LFP) of a
nanocrystalline suspension of azide 1 in water shows selective formation of benzoyl radical 4 (λmax ~400 nm), which is short-
lived (τ = 896 ns) as it intersystem crosses to form imine 2. In comparison, LFP of azide 1 in methanol reveals formation of
triplet alkylnitrene 10 (λmax ~340 nm). The selectivity observed in the solid-state is related to stabilization of the triplet
ketone with (n,π*) configuration by the crystal lattice, which results in α-cleavage being favored over triplet energy transfer
to the azido chromophore. Both the solid-state and solution reaction mechanisms are further supported by density
functional theory calculations. Thus, laser flash photolysis have been used to effectively elucidate the medium dependent
reaction mechanisms of azide 1.
making it possible to elucidate solid-state reaction mechanisms
in detail.5-7
Introduction
Solid-state photochemistry holds promise for being an
important tool for sustainable organic synthesis.1 Because
photoreactions can be initiated by sunlight, a natural resource,
or energy-efficient LEDs, they are sustainable. In addition, as
solid-state reactions do not require any solvents, they are
environmentally benign.2 Owing to the rigidity of crystals,
molecular motions are limited; therefore, solid-state reactions
are generally more selective than their counterparts in
solution.3, 4 However, one of the challenges for developing solid-
state photoreactions for synthetical applications is the limited
general knowledge of solid-state reaction mechanisms.
Although the structure–reactivity correlations of starting
materials and photoproducts yield some insight into reaction
mechanisms in crystals, they not reveal the excited states and
reactive intermediates formed within crystal lattices. Not
knowing the detailed reaction mechanism makes it difficult to
optimize the reaction conditions. However, Garcia-Garibay and
co-workers have recently shown that laser flash photolysis (LFP)
of nanocrystals in water suspensions can be used effectively to
characterize excited states and reactive intermediates formed
in the solid-state and to determine solid-state kinetics, thus
W
e have previously reported that solid-state photolysis
of
α-azidoacetophenone derivatives is remarkably selective and
yields imine products via solid-to-solid reactions,8 whereas
solution photolysis gives a complex mixture of photoproducts
(Scheme 1).9, 10 In our earlier report, based on calculations, we
proposed that the solid-state reaction takes place through -
cleavage, but this mechanism was not investigated by transient
spectroscopy. Therefore, we have extended our work and in this
study, we compared solid-state and solution LFP of
methoxyacetophenone to identify the reactive
intermediates formed in the solid-state and to elucidate how
the crystal lattice renders the solid-state reaction selective.
α-azido-p-
(1)
O
O
O
O
H
N
Ar
hv
N3
Ar
Ar
Ar
Ar
hv
O
Solution
Crystals
N
=
Ar
p-Cl-Ph, p-Br-Ph, p-Ph-Ph
O
Ar
Scheme 1. Medium-dependent photochemistry of
α-
azidoacetophenone derivatives
a.Department of Chemistry, University of Cincinnati, Cincinnati OH 45221, USA
b.Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-
1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
†Electronic Supplementary Information (ESI) available: Description of calculations,
X-ray
diffraction,
phosphorescence,
and
LFP
experiments.
See
DOI: 10.1039/x0xx00000x
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Results and discussion
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DOI: 10.1039/C7OB01731F
O
O
O
hv
N3
Ar
N
Ar
Ar
N
H
NaCNBH3
Crystals
=
Ar
p-MeOPh
1
2
3
Fig. 2. Crystal of azide 1 a) before and b) after irradiation. The scale
is show in the lower right corner of b) and in the ESI
Scheme 2. Photolysis of azide 1 in crystals
Product studies. Irradiation of azide 1 in a KBr pellet with a mercury
arc lamp through a Pyrex filter resulted in formation of imine 2 as the
product (Scheme 2 and Fig. 1). After irradiation for 3 h, the azido
band at 2125 cm-1 was almost fully depleted along with the carbonyl
stretch at 1684 cm-1. Concurrently to the depletion of these bands,
new bands were formed at 1675, 1665, 1460, and 1173 cm-1, which
are assigned to imine 2 based on comparison with its calculated IR
spectrum. The calculations located the coupled stretches of the
ketone and imine chromophores in 2 at 1744 and 1709 cm-1, and
scaling by 0.961311 placed them at 1677 and 1643 cm-1, which fits
nicely with the observed broad bands at 1675 and 1665 cm-1. The
bands at 1460 and 1173 cm-1 were assigned to wagging of the CH2
moiety in 2, as calculations and scaling placed them at 1447 and 1179
cm-1, which is in excellent agreement with the experimentally
observed bands.
In an attempt to achieve higher conversion in the solid-state, we
ground crystals of azide 1 using a mortar and pestle to increase their
surface area. Once initial pictures of the nanocrystals were recorded,
they were irradiated for 4 h, and then images of the irradiated
sample were recorded (Fig. 3). The nanocrystals turned yellow upon
irradiation, but their appearance was similar before and after
irradiation. The IR spectrum of the irradiated crystals shows
complete depletion of the azido band at 2114 cm-1 (Fig. 1f),
confirming that azide 1 was fully consumed.
a)
b)
a)
d)
Fig. 3. Nanocrystals of azide 1 a) before and b) after irradiation. The
scales are shown in lower right corners.
b)
c)
e)
f)
The observation of full conversion only for nanocrystals supports
the notion that the solid-state reaction takes place on the crystal
surface. Furthermore, since azide 1 can also be fully depleted in KBr
pellets, it can be assumed that the environment is similar to
that in nanocrystals. Furthermore, the crystals release air bubbles
from their surface during irradiation. Thus, we theorize that imine 2
crystalizes into a new crystalline phase on the surface of the azide
crystals. However, as imine 2 is hygroscopic, it is complicated to
recrystallize it from organic solvents to verify whether it is formed in
a stable or meta-stable phase. In addition, owing to its hygroscopic
nature, we were unable to successfully characterize imine 2 after
dissolution in organic solvents. Thus, we relied on reduction of the
solid-state reaction mixture with sodium cyanoborohydride, as this
reaction transforms imine 2 to amine 3 but does not reduce azide 1
(Scheme 2). The reduction showed that imine 2 is the only solid-state
photoproduct and confirmed that the reaction can be carried out to
full conversion.
Fig. 1. IR spectra of azide 1 in a KBr pellet a) before irradiation and b)
after irradiation. c) Calculated IR spectrum of imine 2. IR spectra of
azide 1 in d) crystals, e) after irradiation of crystals, f) after irradiation
of nanocrystals.
Colorless crystals of azide 1 were grown from acetone solution
and placed on the stage of a Keyence VHX-1000E digital microscope.
Once initial pictures of the crystals were recorded, the crystals were
irradiated for 12 h. Afterwards, images of the irradiated crystals were
also obtained. Comparison of the crystal images revealed a distinct
color change to yellow upon exposure to UV light (Fig. 2). The
crystals, however, do not show any structural deformation. The IR
spectrum of the irradiated crystals showed that the azido band at
2114 cm-1 did not decrease significantly during irradiation. Thus, the
photoreaction must mainly take place on the surface of the crystals.
In comparison, irradiation of 1 in argon-saturated toluene for 20
h resulted in the formation of products 7, 8, and 9 in a ratio of 5:1:1
(Scheme 3). The major photoproduct, 7, was isolated and then
1
characterized using X-ray crystallography, IR spectroscopy, and H-
NMR spectroscopy, whereas ketone 8 and aldehyde 9 were
characterized by comparison to authentic samples. The solution
photochemistry of 1 is similar to that of similar α-azido derivatives.
a)
b)
2 | J. Name., 2012, 00, 1-3
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O
O
O
O
to alkylnitrene 10 based on comparison to its calculaVteiedw AsrptieclcetOrunlmine.
DOI: 10.1039/C7OB01731F
TD-DFT calculations of alkylnitrene 10 placed the major electronic
H
ν
h
N3
N
Ar
Ar
Ar
Ar
Ar
H
O
Ar
Pyrex
Toluene
transitions above 300 nm at 324 nm (f = 0.0300) and 349 nm (f =
0.0107) (Fig. 4d), which fits well with the experimental spectrum. In
argon-saturated methanol, on a shorter timescale, the kinetic trace
at 340 nm could be fitted as a mono-exponential growth with a rate
constant of 1.50 × 107 s-1 (τ = 66 ns), whereas ton a longer timescale
the kinetic trace could be fitted as a mono-exponential decay with a
rate constant of ~765 s-1 (τ ~ 6 ms) (Fig. 4f). This kinetic analysis
further supports the assignment of the transient absorption to
alkylnitrene 10, as triplet alkylnitrenes are long-lived intermediates
in solution.12
1
7
8
9
=
Ar
p-MeOPh
Products
72
14
14
(%)
Scheme 3. Photolysis of azide 1 in toluene
LFP. We used LFP to identify the intermediates formed in the solid-
state and in solution upon exposing azide 1 to UV light. LFP of 1 in a
nanocrystalline suspension produced a transient spectrum with λmax
of ~400 nm (Fig. 4a). We assigned this absorption to the p-methoxy
benzoyl radical (4) based on the similarity between its calculated
spectrum and the observed one. Time-dependent density functional
theory (TD-DFT) calculations predicted the most intense electronic
transition above 300 nm for radical 4 to be at 382 nm (f = 0.0005)
(Fig. 4c), which fits nicely with the observed spectra. Radical 4
decayed with a rate constant of 1.2 × 106 s-1 (τ = 896 ns) (Fig. 4e).
We proposed previously that α-azidoacetophenone in solution
undergoes both α-cleavage to form benzoyl and azidomethyl
radicals, and energy transfer to the azido chromophore, which falls
apart to yield triplet alkylnitrene. Major product 7 is formed when
the benzoyl radical intercepts triplet alkylnitrene 10, as shown in
Scheme 5. Although the products formed in solution indicate that
azide 1 undergoes α-cleavage to form benzoyl radical 4, which is
intercepted by the triplet alkylnitrene to form major product 7, we
have previously demonstrated that triplet alkylnitrenes are
sufficiently long-lived to cleave photochemically and form benzoyl
radicals in solution.13 We theorize that photoproduct 8 results form
β-cleavage of azide 1 to from p-methoxyacetophenone and azido
radicals, as we have previously reported light-induced C-N bond
cleavages of 2-azido-1,3-diphenyl-propan-1-one derivatives.14
a)
c)
e)
b)
d)
*
3
*
3
O
O
O
.
.
α
O
ν
-cleavage
h
f)
N3
N3
N3
+
N3
Ar
Ar
T2
Ar
Ar
π
,
1
π π
*)
Toulene
*)
(n,
(
and
T1
Energy
Transfer
.
.
.
.
O
O
O
*
-N2
ν
h
N
N3
N
Ar
Ar
Ar
1
T
A of
Fig. 4. LFP of azide 1 a) in the solid-state and b) in methanol.
Calculated spectra of c) benzoyl radical 4 and d) nitrene 10. Kinetic
traces at e) 360 nm in the solid-state and d) 340 nm in methanol.
=
Ar
O
p-MeOPh
O
H
N
Ar
*
Ar
Ar
O
O
O
O
O
-
N2
N3
O
N3
N3
hv
ISC
Ar
Ar
7
8
Ar
Ar
N
6
Ar
Ar
N
1
1
4
5
4
T
1 of
Scheme 5. Proposed solution photoreaction mechanism for azide 1
=
2
p-MeOPh
LFP of 1 clearly demonstrates that the reactivity of azide 1 in the
solid-state follows a different reaction pathway than in solution. The
crystal lattice facilitates formation of benzoyl radical 4, whereas in
solution, formation of triplet alkylnitrene 10 is favored over α-
cleavage.
Scheme 4. Proposed solid-state photoreaction mechanism for azide
1
Thus, LFP confirms that azide 1 in crystals undergoes α-cleavage
to form benzoyl radical 4, as shown in Scheme 4. Because benzoyl
radical 4 is short-lived, it can be concluded that the azido methyl
radical 5 rearranges effectively to yield imine radical 6, which
combines with benzoyl radical 4 to form product 2.
In contrast, LFP of azide 1 in methanol resulted in a different
transient spectrum with λmax of ~340 nm (Fig. 4b), which was assigned
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optimized, and the azido group was found to be in syn-alignment
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DOI: 10.1039/C7OB01731F
with the carbonyl group. In addition, we optimized an extended
conformer of azide 1, conformer C, that is more similar to that
adopted by azide 1 in crystals. Conformer 1C was located 2.5
kcal/mol above the minimal energy conformer, 1A.
TD-DFT calculations located the first and second triplet excited
states (T1 and T2) of 1A and 1C at 72 and 78 kcal/mol above their S0,
respectively. Furthermore, we optimized two T1 structures for 1: one
with a (n,π*) configuration, T1 of 1A, and the other with a (π,π*)
configuration, T1 of 1C. The C=O bond in T1 of 1A was elongated to
1.31 Å in comparison with the C=O bond in S0 of 1 (1.22 Å). Extension
of the C=O group is typical observed for triplet arylketones with
(n.π*) configurations. In contrast, the calculated C=O bond in T1 of 1C
was only 1.26 Å, and the phenyl C–C bonds did not have equivalent
bond lengths, as expected for triplet arylketones with (π, π*)
configurations. The calculated spin densities of T1 of 1A and T1 of 1C
further confirm that their assignment to (n,π*) and (π,π*)
configurations, respectively (Fig. 6). The energies of the optimized
structures of T1 and T2 of 1 are similar; T1 of 1A is located 67 kcal/mol
above S0 of 1A and T1 of 1C is located 70 kcal/mol above S0 of 1C.
Furthermore, as DFT calculations underestimate somewhat the
energies of triplet ketones with (n,π*) configurations,13 we can
conclude that the energies of the (n,π) and (π,π*) configurations are
similar and the environment can influence their relative order.
Because α-cleavage generally takes place from triplet ketones with
(n,π*) configurations, we theorize that energy transfer occurs from
the triplet ketone with (π,π*) configuration.8, 13
a) Azide 1
b) Product 7
Fig. 5. X-ray structure of a) azide 1 and its extended packing motif
and b) product 7.
X-ray structure. We obtained the X-ray structure of azide 1 to gain
more insight into how the crystal packing affects its solid-state
reactivity (Fig. 5). Azide 1 crystallizes in an extended, planar
conformer; the carbonyl and azide groups are co-planar with the
phenyl ring. Azide 1 adopts a π-stacked motif with an interplanar
stacking distance of 3.34 Å. The carbonyl carbon, C7, and azide α-N,
N1, are 2.41Å apart. This proximity of the benzoyl and imine radicals,
4 and 6, allows the recombination to form imine 2 to be a feasible
pathway. However, the α-N…α-N interatomic distance is only 3.84Å,
thus the crystal packing would make it feasible for two nitrene
molecules, if formed in the solid-state, to dimerize.15, 16
.
0.33
0.39
0.84
0.42
a) α-Cleavage
0.48
0.39
T1 of 1C
(π,π*) 72 kcal/mol
T1 of 1A
(n,π*) 67 kcal/mol
S0 of 1A (0 kcal/mol)
S0 of 1C (2.5 kcal/mol)
b) Energy Transfer
Fig. 6. Optimized structures of azide 1 and TK of 1. The red numbers
are the calculated spin densities. Energies in kcal/mol are shown in
parentheses in relevance to S0 of 1A.
Fig. 7. Calculated stationary points on the triplet surface of azide 1 to
form a) imine 2 and b) nitrene 10. Energies in kcal/mol are shown in
parentheses in relevance to S0 of 1A.
Calculations. We performed density functional theory
(DFT/B3LYP/6-31+G(d)) calculations to support the proposed
reaction mechanisms for 1 in the solid-state and in solution.17-19 The
minimal energy conformer, A, of the ground state (S0) of 1 was
The calculated stationary points on the triplet surface of azide 1
to form imine 2 are depictured in Fig. 7. T1 of 1A undergoes α-
cleavage to form benzoyl radical 4 and azido methyl radical 5, and
the calculated transition state barrier is located 78 kcal/mol above S0
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of 1. Similarly, the calculated transition state barrier for extruding a Photolysis of 1 in KBr pellet. Crystals of 1 were ground and 4 mg was
View Article Online
DOI: 10.1039/C7OB01731F
N2 molecule from radical 5 to form imine 6 is only 1 kcal/mol. Thus, added to 400 mg of KBr. The mixture was thoroughly ground and
pressed into a pellet, which was then photolyzed with a mercury arc
lamp through a Pyrex filter. The progress of the solid-state reaction
was monitored by IR spectroscopy, and the reaction was considered
complete when the azido band at 2125 cm-1 was fully depleted.
the calculations support that α-cleavage is an efficient pathway for
T1 of 1 to decay and form imine 2.
a)
b)
Preparative photolysis of 1 in crystals.22 Dried crystals of 1 (20.0 mg,
0.1 mmol) were irradiated (mercury arc lamp/Pyrex filter) for 3 h and
then dissolved in methanol (2 mL). Sodium cyanoborohydride
(NaCNBH3, 12.6 mg, 0.2 mmol) was added to the solution and the
resulting mixture, with a pH of ~5, was stirred for 45 min at room
temperature. The solution was then poured into 3 mL of water and 6
N NaOH was added until the pH of the solution became ~10. The
resulting solution was saturated with sodium chloride and extracted
with diethyl ether (15 mL). The organic layer was dried over MgSO4
and the solvent was evaporated under reduced pressure to obtain
colorless solid of pure 3 in quantitative yield (14.6 mg, 0.09 mmol,
90% yield). The product was characterized using 1H-NMR
spectroscopy and its spectrum compared with reported literature
values.23, 24 1H-NMR (400 MHz, CDCl3): δ 7.74–7.72 (m, 2H), 6.9–6.91
(m, 2H), 6.08 (broad, s, 1H), 3.85 (s, 3H), 3.01-3.00 (d, J = 4 Hz, 3H)
ppm.
Fig. 8. Phosphorescence spectra of azide 1 at 77 K in a) mTHF and b)
in crystals
In addition, we calculated the triplet excited state of the azido
chromophore (TA) of 1 and triplet alkylnitrene 10. The transition state
barrier for TA of 1 to extrude a N2 molecule to form nitrene 10 is only
~0.1 kcal/mol. The stationary points on the triplet surface of azide 1
to form nitrene 10 shown in Fig. 7b indicate that formation of nitrene
10 via energy transfer is also feasible for azide 1.
Phosphorescence. To evaluate the energy of T1 of 1, we examined
the phosphorescence of azide 1 in methyl tetrahydrofuran (mTHF)
and crystals at 77 K (Fig. 8). In mTHF, the phosphorescence spectrum
exhibited a vibrational structure similar to that typically observed for
triplet ketones with (n,π*) configurations. The (0,0) band at 413 nm
corresponds to an energy of 69 kcal/mol, which fits well with the
calculated energy of T1 of 1A. Thus, the α-azido group must lower the
energy of the (n,π*) triplet ketone in azide 1, similar to the behavior
observed for α-chloro- and α-bromoacetophenones.20 In
comparison, the phosphorescence spectrum of the crystals had a
(0,0) band at 450 nm, which corresponds to a triplet energy of 64
kcal/mol. Thus, greater stabilization of T1 of 1A is realized in the
crystal lattice than in mTHF, and thus the crystal lattice renders α-
cleavage more feasible than energy transfer.
Photolysis of 1 in solution. A toluene (5 mL) solution of 1 (200 mg,
1.047 mmol) was degassed with argon and then photolyzed for 20 h
(mercury arc lamp/Pyrex filter). The solvent was then removed and
the residue was eluted with 1:9 ethyl acetate:hexane on a silica
column. Major product 7 (198 mg, 0.662 mmol, 63%, colorless
crystals) was isolated along with two minor products: 8 (25 mg, 0.169
mmol, 16%, colorless liquid) and 9 (19 mg, 0.140 mmol, 13% white
solid-state). The total isolated yield was 92%. Major photoproduct 7
was characterized using X-ray crystallography, 1H-NMR
spectroscopy, and IR spectroscopy. The minor photoproducts were
characterized by comparison with authentic samples. The spectra of
photoproduct 7 fit with those reported:24 1H-NMR (400 MHz, CDCl3):
δ 8.04–8.02 (d, J = 8 Hz, 2H), 7.88–7.86 (d, J = 8 Hz, 2H), 7.01–6.96
(m, 4H), 4.91–4.90 (d, J = 4 Hz, 2H), 3.91 (s, 3H), 3.88 (s, 3H) ppm; IR
(neat): 3341, 2962, 2923, 2847, 1694, 1637, 1602, 1542, 1503, 1460,
1356, 1307, 1258, 1230, 1173 cm-1.
Experimental
Preparation of 1. 2-Bromo-4′-methoxyacetophenone (1.000 g, 4.367
mmol) was dissolved in 40 mL of acetone in a 100 mL round-
bottomed flask and then sodium azide (0.341 g, 5.240 mmol) was
added to the solution in portions. The reaction mixture was stirred
for 24 h to completion, and then acetone was removed under
reduced pressure and the residue was dissolved in diethyl ether. The
solution in ether was washed several times with water and dried over
MgSO4. The solvent was then removed under reduced pressure to
yield pure white solid of 1 (0.676 g, 3.539 mmol, 81% yield). The
product was characterized using 1H-NMR and IR spectroscopy and its
spectra matched those reported in the literature.21 1H-NMR (400
MHz, CDCl3): δ 7.90–7.88 (d, J = 8 Hz, 2H), 6.97–6.96 (d, J = 4 Hz, 2H),
4.51 (s, 2H), 3.89 (s, 3H) ppm; IR (neat): 3028, 2972, 2899, 2123,
1682, 1601, 1517 cm-1.
Single Crystal X-ray Diffraction. Diffraction data was collected for 1
and 7 at 150K on a Bruker APEX-II CCD detector at Beamline 11.3.1
at the Advanced Light Source using synchrotron radiation tuned to
λ=0.7749Å. The structures were solved by a combination of direct
methods and the difference Fourier technique and refined by full-
matrix least squares on F2. CCDC-1561493 and 1561494.
Laser Flash Photolysis. LFP experiments were performed with an
excimer laser (308 nm, 17 ns pulse width).25 A nanocrystalline
suspension of azide 1 was prepared by following the reprecipitation
technique described by Simoncelli et al.6 The absorbance of the
nanocrystalline suspensions and solutions was between 0.3 and 0.5
at 308 nm. For each kinetic trace, approximately 2 mL of the
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suspension was placed in a quartz cuvette with a 10 mm × 10 mm 8. S. M. Mandel, P. N. D. Singh, S. Muthukrishnan, M. Chang, J. A.
View Article Online
Krause and A. D. Gudmundsdóttir, Org. Lett., 2006, 8, 4207-
4210.
DOI: 10.1039/C7OB01731F
cross section, and the slurry purged with argon for ~5 min. T
9. S. M. Mandel, J. A. Krause Bauer and A. D. Gudmundsdottir, Org.
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The details of the mechanism by which azide 1 forms imine 2
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form benzoyl radical 4, which can combine with imine radical 6 to
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Acknowledgements
We thank the National Science Foundation (CHE-1464694) and the
Ohio Supercomputer Center (OSC) for their generous support of this
work. BL is grateful for a NSF-REU (CHE-1156449) fellowship.
Crystallographic data were collected through the SCrALS (Service
Crystallography at the 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 U.S. Department of
Energy, Office of Energy Sciences, under contract DE-AC02-
05CH11231. This work was supported by JSPS KAKENHI Grant
Number JP 17H03022.
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