Photocyclization of iminyl radicals: Theoretical study and photochemical aspects

Article abstract of DOI:10.1021/jo7025542

(Graph Presented) The irradiation of acyloximes was studied by theoretical methods. CASPT2/6-31G*//CASSCF/6-31G* calculations, using an active space of 14 electrons in 11 orbitals, indicate that S₂ should be the spectroscopic state, and its rel

Full text of DOI:10.1021/jo7025542

Photocyclization of Iminyl Radicals: Theoretical Study and
Photochemical Aspects
Rafael Alonso, Pedro J. Campos, Miguel A. Rodr´ıguez,* and Diego Sampedro*
Departamento de Qu´ımica, UniVersidad de La Rioja, Grupo de S´ıntesis Qu´ımica de La Rioja, Unidad
Asociada al C.S.I.C., Madre de Dios, 51, 26006 Logron˜o, Spain
miguelangel.rodriguez@unirioja.es
ReceiVed NoVember 30, 2007
The irradiation of acyloximes was studied by theoretical methods. CASPT2/6-31G*//CASSCF/6-31G*
calculations, using an active space of 14 electrons in 11 orbitals, indicate that S2 should be the spectroscopic
state, and its relaxation leads directly to N-O bond breakage due to coupling between the imine π* and
the σ* N-O orbitals. Subsequent calculations at the B3PW91/6-31+G* level suggest that the resulting
iminyl radicals are able to cyclize to the five- or six-membered ring, depending on the presence of a
phenyl group as a spacer, a process that has been verified experimentally. The photochemical aspects of
the more common five-membered ring formation, such as excited-state quenching, quantum yield, excited-
state sensitizers, laser flash photolysis experiments, Stern-Volmer plot, and luminescence measurements,
were investigated. These studies indicate that singlet and triplet excited states undergo the same reaction.
Emission lifetimes of ca. τ ) 10.6 µs for compound 11 are suggestive of triplet parentage, while no
fluorescence was detected, in agreement with the computed MEP energy profile.
SCHEME 1
Introduction
Iminyl radicals, generated by both thermal and photochemical
methods, have proven to be valuable intermediates for the
synthesis of five-membered nitrogen heterocyclic rings.1-5 We
recently described the synthesis of a range of isoquinoline
derivatives through light-induced iminyl radical formation
followed by addition to an unsaturated system (Scheme 1).6 This
capacity for linkage to generate either five- or six-membered
* Corresponding authors. Tel: +34-941299647. Fax: +34-941299621.
(1) For, a review see: Fallis, A. G.; Brinza, A. M. Tetrahedron 1997,
53, 17543.
(2) Gagosz, F.; Zard, S. Synlett 1999, 1978.
(3) Uchiyama, K.; Hayashi, Y.; Narasaka, K. Tetrahedron 1999, 55, 8915.
(4) Mikami, T.; Narasaka, K. Chem. Lett. 2000, 338.
(5) Kitamura, M.; Mori, Y.; Narasaka, K. Tetrahedron Lett. 2005, 46,
2373.
rings prompted us to carry out an in depth study. Herein, we
report a theoretical investigation and a photochemical study on
the photocyclization of iminyl radicals generated by this method.
Computational Details
All reaction paths were computed using fully unconstrained ab
initio quantum chemical computations in the framework of a
(6) Alonso, R.; Campos, P. J.; Garc´ıa, B.; Rodriguez, M. A. Org. Lett.
2006, 8, 3521.
10.1021/jo7025542 CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/22/2008
2234
J. Org. Chem. 2008, 73, 2234-2239

Photocyclization of Iminyl Radicals
CASPT2//CASSCF strategy.7 This requires the reaction coordinate
to be computed at the complete active space self-consistent field
(CASSCF) level of theory and the corresponding energy profile to
be re-evaluated at the multiconfigurational second-order Møller-
Plesset perturbation theory level (here we use the CASPT2 method
implemented in MOLCAS-6.4)8 to take into account the effect of
electron dynamic correlation. The calculation of the reaction path
was carried out using the 4-pentenal O-acyloxime 1 and the reaction
coordinate was obtained via minimum energy path (MEP) computa-
tions at the CASSCF level with the 6-31G* basis set and an active
space of 14 electrons in 11 orbitals (π and π* orbitals of the imine,
alkene, and carbonyl moieties, N and O lone pairs, and σ and σ*
orbitals for the N-O bond). The zeroth-order wave function used
in the single-point CASPT2 calculations needed for the re-evaluation
of the MEP energy profile (see above) is a three-root (S0, S1, S2)
state average CASSCF wave function. The same type of wave
function was used where necessary in order to avoid convergence
problems.
Radical ground-state calculations were carried out using the
Gaussian 03 program package.9 Becke’s three-parameter hybrid
exchange potential (B3)10 was used with the Perdew-Wang
(PW91)11 gradient-corrected correlation functional, B3PW91. This
method has been shown to describe bond cleavage more accurately
than pure DFT methods.12 The standard split-valence 6-31+G* basis
set was employed. Geometry was fully optimized without any
symmetry constraint for all model compounds. Optimized structures
were characterized as minima or saddle points by frequency
calculations, which also allowed ZPE and thermal corrections to
be obtained.
FIGURE 1. Calculated transitions (blue) for model 1 and experimental
UV spectrum (black) for compound 2.
electrons in 11 orbitals. We included π and π* orbitals for the
CdC, CdN and CdO double bonds, lone pairs for the nitrogen
and oxygen atoms and the σ and σ* orbitals for the N-O bond.
The light absorption of model compound 1 was initially
evaluated and the results are shown in Figure 1. The calculated
n-π* S0 f S1 transition has an oscillator strength of 0.014
with a λmax of 233 nm (123 kcal/mol, 5.32 eV), while the π-π*
S0 f S2 transition has an oscillator strength of 0.2 with a λmax
of 212 nm (135 kcal/mol, 5.86 eV). These values are consistent
with the experimental spectrum of 5-hexen-2-one O-acetyloxime
2 (Figure 1), in which the bands appear at 234 nm with f )
0.08 and at 204 nm with f ) 0.23. These data show that S2 is
the spectroscopic state while S1 is an excited dark state. Excited-
state relaxation was studied through an MEP calculation, as
shown in Figure 2. Every step was obtained by minimization
of the PES on a hypersphere centered on the initial geometry
with a predefined radius at the CASSCF level (Figure 2a).
Subsequent CASPT2 calculations were performed for the states
of interest (S0, S1, and S2) at each point of the MEP (Figure 2 b).
As can be seen, relaxation on the S2 PES leads directly to
N-O bond cleavage. This is due to the coupling between the
imine π* and the σ* N-O orbitals, which increases the
occupancy of the σ* orbital. Indeed, the first part of the MEP
coordinate is controlled by N-O bond elongation, which leads
to stabilization of the S2 state and, as a consequence, to a
decrease in the S2-S1 energy gap. The main geometry modi-
fication is the increase in the N-O bond length, while the rest
of the molecule remains almost unchanged. Population of S1
will be possible at this point given that the energy gap between
S1 and S2 is small. Subsequent elongation of the N-O bond
leads to further stabilization of S1 (and S2) and this in turn leads
to a situation where two radicals are formed and the three PES
become almost degenerate. The main geometric change in this
part of the MEP corresponds with the two C-O distances which
progressively become similar, as expected for the acetyl radical
formation. The main features of the MEP are the same for both
CASSCF and CASPT2 calculations. It should be noted,
however, that the S2/S1 crossing takes place early after the
Results and Discussion
Theoretical Calculations. In order to gain further insights
into the reaction mechanism, we decided to explore this process
by means of theoretical calculations. We tried to obtain a
comprehensive view of the reaction path from light absorption
to product formation. Thus, the level of theory used depended
on the main process involved. The first part of the reaction
mechanism involves a photochemically driven N-O cleavage
and the CASPT2//CASSCF strategy was therefore used as this
has proven to give reliable results for this kind of reaction.7,13
In particular, we used model compound 1 to explore the
photochemical reaction path using an active space of 14
(7) Olivucci, M. Computational Photochemistry; Elsevier: Amsterdam
2005.
(8) Karlstrom, G.; Lindh, R.; Malmqvist, P.-A.; Roos, B. O.; Ryde, U.;
Veryazov, V.; Widmark, P.-O.; Cossi, M.; Schimmelpfennig, B.; Neogrady,
P.; Seijo, L. Comp. Mater. Sci. 2003, 28, 222.
(9) Gaussian 03, Revision C.02: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J.
A.; 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.; Bakken,
V.; 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.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,
J. A. Gaussian, Inc., Wallingford CT, 2004
(10) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(11) Perdew, J. P.; Burke, K.; Wang, Y. Phys. ReV. B 1996, 54, 16533.
(12) Jensen, F. Introduction to Computational Chemistry; Wiley: New
York, 1999; p 283.
(13) Kutateladze, A. G. Computational Methods in Photochemistry; CRC
Press: Boca Raton, 2005.
J. Org. Chem, Vol. 73, No. 6, 2008 2235

Alonso et al.
FIGURE 3. S0 reaction path for the cyclization for radical model 3.
FIGURE 2. Evolution of the ground and two lowest singlet excited
states for model 1 from the FC geometry along the S2 MEP. Selected
geometries are shown together with the N-O bond distance: (a)
CASSCF, (b) CASPT2.
inclusion of electron correlation (Figure 2 b). These data clearly
show that after irradiation the system readily undergoes a
homolytic N-O bond cleavage to generate two radicals in the
ground state: acetyl and iminyl radicals that can further react
further. This easy bond cleavage is the reason behind the use
of these kinds of systems as photoinitiators for UV-curable
coatings.14
FIGURE 4. Radical models for theoretical calculations.
Once the iminyl radical reaches S0, the remainder of the
reaction path is controlled by ground state chemistry. In order
to ensure this, we carried out a CASPT2 calculation of the
iminyl radical to find out that the excited states (both S1 and
S2) lie more than 70 kcal/mol above the ground state. Thus, the
progress of the reaction, the regioselectivity and the effect of
substituents were studied by means of DFT calculations. The
choice of B3PW91 was made on the basis of previous results
where this functional proved to give satisfactory results in radical
chemistry.15-19 After the photochemically induced homolysis
of the N-O bond, two radicals are generated, but we will focus
on the fate of the iminyl radical responsible for the formation
of the final product. Our first task was to explore the cyclization
of the iminyl radical. We calculated the structures of the iminyl
radical resulting from the photocleavage and the six-membered
alkyl radical formed after cyclization. These two minima are
connected through a transition structure (TS) as shown in Figure
3. In order to fully prove the relevance of the transition structure
we also computed the IRC (intrinsic reaction coordinate)
connecting these three critical points to confirm that the TS
really relates these two minima. As shown in Figure 3, the ring
closure must surpass a barrier of ca. 14 kcal/mol through a
gradual uphill path at the beginning (which corresponds to a
conformational adjustment and no relevant geometry modifica-
tion) and a progressively steeper incline (corresponding to the
radical addition to the alkene). At this point, the key variation
is the forming N-C bond. The iminyl radical approaches the
alkene moiety reaching the TS point at a distance of 2.095 Å.
At the same time, the CdC distance reflects the radical addition
with a change in distance from 1.335 Å in 3 to 1.370 Å in the
TS while the CdN distance lengthens from 1.251 to 1.262 Å.
The next step was to extend the theoretical exploration of
the cyclization to several models with different substitution
patterns. The compounds considered are shown in Figure 4, and
the free energy values for the three critical points for each case
are shown in Table 1. Model compounds 4 and 5 are useful to
explore the effect of the substituent on the iminic carbon, while
compound 6 illustrates the effect of the aromatic ring as a spacer.
Models 7 and 8 allow a complete evaluation of the ring closure
step as well as a comparison with experimental results.
(14) Belfield, K. D.; Crivello, J. V. Photoinitiated Polymerization;
American Chemical Society: Washington, DC 2003.
(15) Lorance, E. D.; Hendrickson, K.; Gould, I. R. J. Org. Chem. 2005,
70, 2014.
(16) Pinter, B.; DeProft, F.; VanSpeybroeck, V.; Hemelsoet, K.; Waro-
quier, M.; Chamorro, E.; Veszpremi, T.; Geerlings, P. J. Org. Chem. 2007,
72, 348.
(17) Pace, A.; Buscemi, S.; Vivona, N.; Silvestri, A.; Barone, G. J. Org.
Chem. 2006, 71, 2740.
(18) Matsubara, H.; Falzon, C. T.; Ryu, I.; Schiesser, C. H. Org. Biomol.
Chem. 2006, 4, 1920.
Computed geometries for the iminyl radicals 3-8 share
similar parameters. The main difference lies in the relative
arrangement of the alkene and imine moieties. While in models
3-5 both groups are parallel, in 6-8 they tend to be coplanar
(19) Arnone, M.; Engels, B. J. Phys. Chem. A 2006, 110, 12330.
2236 J. Org. Chem., Vol. 73, No. 6, 2008

Photocyclization of Iminyl Radicals
TABLE 1. Free Energies (∆G, kcal/mol) for the Cyclization Step
TABLE 2. Free Energies for the Cyclization Step to Five- or
Six-Membered Rings (All Values Are Relative to the Corresponding
Iminyl Radical)
(All Values Are Relative to the Corresponding Iminyl Radical)a
compd
TS
alkyl radical
compd
TS-A
TS-B
product A
product B
3
4
5
6
7
8
16.0
15.5
15.3
5.6
8.1
8.1
-7.6
-9.1
-10.3
-27.7
-26.7
-25.4
3
6
12.6
6.4
16.0
5.6
-5.2
-10.6
-7.6
-27.7
SCHEME 3
SCHEME 4
a Thermochemical analysis for T ) 298 K.
SCHEME 2
TABLE 3. Irradiation of 11 in the Presence of Different Triplet
Quenchers and Sensitizers
entry
ketone
ETa (kcal/mol)
12/11 ratiob
1
2
3
4
5
benzophenone
2-acetylnaphthalene
1-acetylnaphthalene
none
69
59
56
3.93
1.88
0.14
0
9-fluorenone
50
0
a See ref 21. b Irradiation was carried out for 2 h
due to the influence of the phenyl group. In the case of the
transition state geometries, the relevant parameter is the bond
distance of the forming N-C bond. Once again, the difference
lies in the presence of the phenyl group as a spacer. While 3-5
have N-C bond distances of 2.095, 2.103, and 2.108 Å,
respectively, in models 6-8, where the phenyl ring contributes
to the radical stabilization, this distance increases to 2.169, 2.179,
and 2.175 Å, respectively.
Analysis of the data collected in Table 1 enables some
conclusions to be drawn. The transition structures involved in
the cyclization step have quite similar geometries, and the energy
values depend on the chemical structure. When both the imine
and alkene moieties are placed on the aromatic ring (models
6-8), the energy for the transition state is much lower. This
can be related to a stabilizing effect of the phenyl group, which
contributes by lowering the energy barrier. However, substitution
on the iminic carbon has a negligible effect in that both
geometries and energies for the two groups (3-5 and 6-8) are
very similar. In this case, substitution only becomes important
when the substituent is a hydrogen atom, allowing the formation
of nitrile and thus lowering the reaction yields through a side
reaction.6
account the calculations on 3, we believed it to be of interest to
test the results in an experimental way. Thus, we prepared
acyloxime 11, which could cyclize at both the five- or six-
positions. As shown in Scheme 4, the photoreaction of 11, in
the presence of 1,4-cyclohexadiene to prevent further polym-
erization, led to the formation of pyrroline 12 in 68% yield,
while the six-membered ring isomer was not detected in the
crude reaction mixture, a finding consistent with the theoretic
prediction. Thus, formation of A or B (Scheme 2) depends
exclusively on the radical precursor. Only one type of product
could be experimentally found, although in the case of 6 the
calculated energy difference for both processes seems not very
high.
Photochemical Aspects. Since the understanding of a
photochemical reaction requires knowledge of the processes
occurring at the molecular level from the absorptive act to the
formation of products, we report in this section results of the
standard studies such as excited-state quenching, quantum yield,
excited-state sensitizers, laser flash photolysis experiments, the
Stern-Volmer plot, and luminescence measurements.
Taking into account that the formation of pyrroline derivatives
from iminyl radicals is a more general procedure, we decided
to study the cyclization of acyloxime 11. The nature of the
excited-state involved in this process was first investigated by
carrying out the irradiation in the presence and in the absence
of molecular oxygen, which is an effective triplet state quencher
for a large number of photoreactions. Since we confirmed that
O2 slows the reaction down, we assessed the effect of different
triplet state quenchers and sensitizers on the irradiation of this
acyloxime. Compound 11 does not absorb above 330 nm and,
therefore, its irradiation was carried out through a solution filter
At this point, we considered the driving force to give five-
or six-membered rings, that is to say, the feasibility of the ring
closure depending on the position of the radical attack. To clarify
this point, we also calculated the cyclization step for 3 and 6 to
yield five-membered rings (Scheme 2). The results of these
calculations are shown in Table 2. As can be seen, while 6 shows
a small preference for the formation of the six-membered ring,
model 3 has a preference of more than 3 kcal/mol for the
formation of the five-membered ring. In agreement with the
calculations on 6, irradiation of compound 9 yields isoquinoline
10 in 38% yield (Scheme 3).6 On the other hand, taking into
J. Org. Chem, Vol. 73, No. 6, 2008 2237

Alonso et al.
FIGURE 5. (a) Triplet-state decay plots of a solution of xanthone in acetonitrile with different amounts of 11. (b) τ inverses (kD) versus the
amount of O-acetyloxime added.
that removed λ radiation under 330 nm.20 The data for the
reaction of acyloxime 11 (1 × 10-2 M), to give the 1-pyrroline
12, in the presence of different ketones in deoxygenated
acetonitrile are summarized in Table 3. Ketones can act as triplet
state sensitizers or quenchers depending on their relative energies
compared with that of 11. We used ketones in the range 79-
50 kcal/mol.21 The reaction gave 12 with benzophenone,
2-acetylnaphthalene or 1-acetylnaphthalene but not with 9-fluo-
renone. These results indicate that the reaction takes place under
triplet state conditions. The triplet state energy of 11 was
estimated to be between 56 and 50 kcal/mol.
The triplet state was also studied by laser flash photolysis.
The triplet state decay of a 4.03 × 10-4 M solution of xanthone
in acetonitrile was measured. Subsequently, 1, 2, 3, 4, 5, 6, and
8 equiv of 11 were added as a quencher, and the decay plots
were recorded.
All graphs were fitted with exponential functions in order to
find the xanthone life time, τ, for each experiment. The τ
inverses (disappearance constants, kD) were then plotted versus
the amount of O-acetyloxime added. The straight line obtained
shows compound 11 to be a triplet quencher (Figure 5). The
quenching rate, kD ) 7.8 × 109, is obtained from the linear fit
slope. This value is typical of diffusional quenching.
We proceeded to investigate the quenching of the photore-
activity of 11. We chose the common triplet-state quencher 2,5-
dimethyl-2,4-hexadiene, since it possesses an appropriate triplet
energy (42 kcal/mol)22 and does not show any absorption at
the irradiation wavelengths (solution filter, λ > 330 nm). The
plot of quantum yield of disappearance of 11 (φ0/φ, where the
superscript zero denotes the absence of quencher) versus
concentration of 2,5-dimethyl-2,4-hexadiene (see Figure 6 a)
is typical for a situation in which two excited states (singlet
and triplet) undergo the same reaction and only one of them is
selectively quenched.23 Extrapolation of the plateau to the φ0/φ
FIGURE 6. (a) Experimental plot of quantum yield rate of disappear-
ance of 11 versus concentration of 2,5-dimethyl-2,4-hexadiene for the
irradiation of 11. (b) Stern-Volmer plot for the reaction of O-acyloxime
11 after subtracting singlet contribution.
coordinate yields 0.24. Assuming that this unquenchable reaction
is due to the reaction of 11 from the singlet state, the contribution
(20) Concentrations: NaBr‚2H2O, 650 g/L; Pb(NO3)2, 3 g/L. See, for
of the triplet state reaction to the slope may be determined by
example: CRC Handbook of Organic Photochemistry; Scaiano, J. C., Eds.;
subtracting 0.24 from the experimental values of φ0/φ. Thus, a
CRC Press: Boca Raton, 1989; Vol. I, p 48.
(21) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; New York, 1993; p 54.
(22) See ref 21, p 60.
(23) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991; p 253.
normal Stern-Volmer plot results with an intercept of 0.86 (
0.01 and a slope of kqτ ) -2.3 ( 0.1 M-1, where kq is the
quenching rate constant, for the triplet reaction alone (Fig-
ure 6b).
2238 J. Org. Chem., Vol. 73, No. 6, 2008

Photocyclization of Iminyl Radicals
TABLE 4. Emission Data for a Solution of 11 in Methylene
Chloride
that iminyl radicals could evolve through cyclization to the six-
or five-membered ring, depending on the presence or absence
of a phenyl group as a spacer. This fact has been experimentally
verified by irradiation of 2-vinylbenzaldehyde O-acetyloxime
and (Z)-1-phenyl-4-hepten-1-one O-acetyloxime, respectively.
The photochemical aspects, such as excited-state quenching,
quantum yield, excited-state sensitizers, laser flash photolysis
experiments, and Stern-Volmer plot, of the more common five-
membered ring formation were investigated. These studies reveal
that singlet and triplet excited states undergo the same reaction.
Emission lifetimes of ca. τ ) 10.6 µs for compound 11 are
suggestive of triplet parentage, while no fluorescence was
detected, in agreement with the computed MEP energy profile.
T (K)
λ
exc (nm)
λ
em (nm)
τ (s)
298
77
309
300
380
350
10.6
10.8
Next, we studied the emission properties of compound 11.
Emission data for a solution 0.16 M in CH2Cl2 of this compound
are shown in Table 4. O-Acyloxime 11 shows an emissive state
at both room temperature and 77 K. These long emission
lifetimes (τ ) 10.6-10.8 µs) are suggestive of triplet parentage.
These lifetime values are in agreement with those found in
literature for O-acyloximes triplet states.24 From the triplet
lifetime at 298 K and the Stern-Volmer plot slope, kqτ, a
quenching rate constant, kq, value of ca. 2.2 × 105 M-1 s-1 is
calculated.
Consistent with the calculated MEP energy profile, where
relaxation on the S2 PES leads directly to N-O bond cleavage
(see Figure 2), no fluorescence was detected in those experi-
ments.
Finally, we also determined the quantum yield of the
efficiency of appearance of 12 from the photoreaction of 11.
trans-Azobenzene was used as an actinometer,25 and the
irradiation was performed at 313 nm using a monochromator.
The quantum yield for the formation of 12 for a 0.107 M
solution of 11 in deoxygenated acetonitrile was found to be ΦR
) 0.05 ( 0.01.
Experimental Section
General Procedure for Irradiation. The acyloxime 11 (0.5
mmol) and 10 equiv of 1,4-ciclohexadiene (5 mmol) were dissolved
in 50 mL of deoxygenated acetonitrile and irradiated at room
temperature under an Ar atmosphere through Pyrex glass with a
400 W medium pressure-mercury lamp until the oxime was
consumed (TLC, hexane/AcOEt, 4:1). The solvent was then
removed with a rotary evaporator, and the products were separated
by chromatography (silica gel, hexane/AcOEt).
5-Phenyl-2-propyl-3,4-dihydro-2H-pyrrole (12): colorless oil;
1
yield 64 mg, 68%; H NMR (300 MHz, CDCl3) δ 7.85 (dd, J )
3.0, 3.0 Hz, 2H), 7.44-7.36 (m, 3H), 4.28-4.12 (m, 1H), 3.01-
2.95 (m, 1H), 2.93-2.84 (m, 1H), 2.27-2.12 (m, 1H), 1.89-1.76
(m, 1H), 1.67-1.55 (m, 1H), 1.55-1.40 (m, 3H), 1.02-0.94 (t, J
) 6.0 Hz, 3H); 13C NMR (300 MHz, CDCl3) δ 171.83, 134.94,
130.32, 128.48, 127.75, 73.21, 39.08, 35.06, 28.69, 20.05, 14.41;
ES (+) m/z 188 (M + 1). Anal. Calcd for C13H17N: C, 83.37; H,
9.15; N, 7.48. Found: C, 83.42; H, 9.13; N, 7.45.
Conclusions
We have investigated the photochemically induced iminyl
radical cyclization reactions of acyloximes. Theoretical calcula-
tions (CASPT2/6-31G*//CASSCF/6-31G* level, using an active
space of 14 electrons in 11 orbitals) indicate that S2 should be
the spectroscopic state and its relaxation leads directly to N-O
bond breaking, due to the coupling between the imine π* and
the σ* N-O orbitals, with the formation of iminyl radicals. Our
calculations, carried out at the B3PW91/6-31+G* level, suggest
Acknowledgment. We thank the Spanish MEC (CTQ2007-
64197) and the Comunidad Auto´noma de La Rioja (ACPI2005/
06) for financial support. D.S. is financed by the Ramo´n y Cajal
program from the MEC. R.A. thanks the Comunidad Auto´noma
de La Rioja for his fellowship.
Supporting Information Available: Experimental procedures,
characterization data for new compounds, and Cartesian coordinates
for the calculated geometries. This material is available free of
charge via the Internet at http://pubs.acs.org.
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