Intra- vs inter-molecular electron transfer processes in C[sbnd]N bond forming reactions. Photochemical, photophysical and theoretical study of 2′-halo-[1,1′-biphenyl]-2-amines

Article abstract of DOI:10.1016/j.tet.2016.08.051

N-Arylation reaction is obtained when 2′-halo-[1,1′-biphenyl]-2-amines are irradiated in basic medium. On the basis of photochemical, photophysical experiments and computational studies we propose that carbazoles are formed by intermolecular electron tran

Full text of DOI:10.1016/j.tet.2016.08.051

Accepted Manuscript
Intra- vs intermolecular electron transfer processes in C–N bond forming reactions.
Photochemical, photophysical and theoretical study of 2′-halo-[1,1′-biphenyl]-2-
amines
Walter D. Guerra, María E. Budén, Silvia M. Barolo, Roberto A. Rossi, Adriana B.
Pierini
PII:
S0040-4020(16)30827-4
10.1016/j.tet.2016.08.051
TET 28032
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 June 2016
Revised Date: 12 August 2016
Accepted Date: 16 August 2016
Please cite this article as: Guerra WD, Budén ME, Barolo SM, Rossi RA, Pierini AB, Intra- vs
intermolecular electron transfer processes in C–N bond forming reactions. Photochemical, photophysical
and theoretical study of 2′-halo-[1,1′-biphenyl]-2-amines, Tetrahedron (2016), doi: 10.1016/
j.tet.2016.08.051.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical Abstract
Intra vs Intermolecular Electron Transfer Processes
in C-N bond Forming Reactions. Photochemical,
Photophysical and Theoretical Study of 2'-Halo-[1,1'-
biphenyl]-2-amines
Leave this area blank for abstract info.
Walter D. Guerra a , María E. Budén a, , Silvia M. Baroloa, , Roberto A. Rossia and Adriana B. Pierini a
INFIQC, Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de
Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Intra- vs Intermolecular Electron Transfer Processes in C-N Bond Forming
Reactions. Photochemical, Photophysical and Theoretical Study of 2'-Halo-[1,1'-
biphenyl]-2-amines
Walter D. Guerra a , María E. Budén a, , Silvia M. Baroloa, , Roberto A. Rossia and Adriana B. Pierini a
aINFIQC, Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba,
Argentina.
ARTICLE INFO
ABSTRACT
Article history:
Received
N-Arylation reaction is obtained when 2'-halo-[1,1'-biphenyl]-2-amines are irradiated in a basic
medium. On the basis of photochemical, photophysical experiments and computational studies
Received in revised form
Accepted
we propose that carbazoles are formed by intermolecular electron transfer via the SRN
mechanism.
1
Available online
In general, biphenylamines with an EDG like Me or OMe behave in the same way as H giving
both, cyclized and reduced products. On the other hand, biphenylamines containing EWG like
CN, COOEt or CF3 gave only the corresponding carbazole. Herein, we report for the first time
the chain length for the propagation cycle of intramolecular SRN1 reactions and explain that
differences in the distribution of products suggest differences regarding the overall mechanism
involved.
Keywords:
Electron Transfer
Photochemical
Arylation Reaction
SRN1
2009 Elsevier Ltd. All rights reserved
.
Radical anions
1. Introduction
version of reductive ET mechanisms was used for ring closure
systems (Scheme 3).
Aromatic nucleophilic photosubstitution has proven to be a
versatile tool in organic synthesis. The photosubstitution
reactions between an aromatic substrate and a nucleophile could
be achieved by several mechanisms and its classification is based
on the key intermediates involved.1 When the first step involves
the excitation of the substrate in photosubstitution mechanisms,
the substrate in its excited state could achieve product, for
example, by: heterolysis and SN1Ar* via cation phenyl
intermediate,2 SN2Ar* (addition-elimination mechanisms)3
(Scheme 1) or via electron transfer process (ET),4 among others.
-
R
X
Nu
X
*
hν
S
2
A
r
*
Nu
X
R
R
-
R
-
We could classify the mechanisms involving ET processes in
photo-oxidations (via radical cation intermediates, Scheme 2) or
photo-reductions (via radical anion intermediates, Scheme 3).
The oxidative ET processes are more common with electron-
donating substituted aromatics in water or other ionic solvents
(unless a good nucleofugal group is present). The resulting
radical cation may react with a nucleophile and the resulting
radical ends with rearomatization (Scheme 2).5 A reductive ET
commonly requires the presence of donors such as enolate ions,
amines, arenes or alkenes. Mechanisms involving reductive ET
processes include SN(ET)Ar*, radical-radical collapse, SRN1 chain
R
Scheme 1. Polar Photosubstitution Mechanisms
X
X
*
X
Nu
Nu
X
ν
ET
Nu
h
R
R
R
R
R
Scheme 2. Oxidative Electron Transfer Mechanism
process6 or other photochemical processes. The intramolecular
———
María E. Budén. Tel.: +54-0351-5353867; eugebuden@yahoo.com.ar
Silvia M. Barolo. Tel.: +54-0351-5353867; sbarolo@fcq.unc.edu.ar

2
Tetrahedron
ACCEPTED MANUSCRIPT
X
Nu
N
-X-
Z
*
X
Z
X
X
hν
Nu
Nu
ETintra
Nu
Nu
Z
Z
Z
-X-
D
D
ETinter
Nu
SRN
1
Z
ETinter
X
Z
-X-
Nu
Nu
Nu
Z
Z
Scheme 3. Aromatic Nucleophilic Photosubstitution Mechanisms by Reductive Electron Transfer
The reductive ET mechanisms have been proposed for the
X
synthesis of several heterocycles systems. For example, Park et.
al proposed an intramolecular SN(ET)Ar* mechanism (Scheme 3)
for the synthesis of 2-pyridinylbenzoxazole from N-(2-
halophenyl)pyridinecarboxamide7 and the synthesis of 2-(4-R-
phenyl)-1,3-benzoxazole and 9-R-phenanthridin-6(5H)-one from
2´-chloro-4-R-benzaniline.8 The same mechanism was proposed
for the synthesis of benzoxazole[3,2-b]isoquinolin-11-one from
tetrahydroisoquinoline-1,3-diones under basic media.9 On the
other hand, indolo benzoxazoles were prepared from N-(2-
halophenyl)-indolo-carboxamides,10 and the base-catalyzed
synthesis of substituted indazoles from (Z)-2-bromoacetophenone
N-tosylhydrazones with a catalytic amount of trans-N,N’-
dimethylcyclohexane-1,2-diamine11 via radical-radical collapse
mechanism (Scheme 3).
An SRN1 synthetic strategy (Scheme 3) to obtain heterocyclic
compounds has been recently applied to the synthesis of 1-
phenyl-1-oxazolinoindan derivatives and their related
compounds;12 tetracyclic isoquinoline derivatives;13 a series of
substituted 9H-carbazoles14,15 and carbolines,16 pyrroles, indoles,
and pyrazoles,17 pyrido[1,2-a]benzimidazoles,18 dibenzosultams
from N-aryl-2-halobenzenesulfonamides by intramolecular C-C
photoinduced arylation,19 or by visible-light-promoted
denitrogenative cyclization of 1,2,3,4-benzothiatriazine-1,1-
dioxides,20 in addition to other heterocyles.21 However, studies of
photochemical and photophysical properties still remain under
script in intramolecular SRN1 approach.
EWG
t-BuOK, hν
R
NH
R = CN, COOEt, CF3
NH2
R
EDG
ν
t-BuOK, h
R
+
NH
R
NH2
R= H, Me, OMe
Scheme 4. Intramolecular C-N bond forming reaction and
reduction of 2'-halo-[1,1'-biphenyl]-2-amines
Our proposal is to investigate the mechanism of this photo-
substitution from the study of experimental reaction conditions,
photophysical properties (UV-vis, steady state fluorescence and
time-resolved fluorescence), photochemical studies (the quantum
yields measured) and computational data (M06-2X DFT
functional and 6-311+G* basis set). Herein, we report for the first
time the chain length for the propagation cycle of intramolecular
SRN1 reactions.
2. Results and Discussion
The photostimulated reaction (45 min) of 2'-chloro-[1,1'-
biphenyl]-2-amine 1a in the presence of t-BuOK (2 equiv) as a
base in DMSO afforded 9H-carbazol 2a in 26% yield and [1,1'-
biphenyl]-2-amine 3a in 17% yield, with a ratio of cyclized
versus reduced products of 1.5:1 (Table 1, entry 1). The reaction
was completed after 180 minutes and the ratio between cyclic
and reduced product went up to 4.4:1 (57% yield of 2a and 13%
yield of 3a; entry 2). This reaction is completely suppressed in
The varied panorama illustrates that different pathways could
be possible and the actual path followed, as well as efficiency of
the overall reaction, will depend on a host of factors such lifetime
of the singlet or triplet excited state, their redox properties,
chemical reactivity and nature of the nucleophile/electron donor,
medium and so on. Developing a comprehensive demonstration
of the mechanisms involved is not simple and in many cases
computational data appear as a supporting and complementary
tool.22
dark conditions, excluding
a benzyne and other polar
mechanisms (entry 3). The addition of 25 mol% of m-
dinitrobenzene (m-DNB), a well-known electron acceptor, caused
46% of inhibition (14% yield of 2a, entry 4). The same behavior
was observed when 50 mol% of m-DNB was added (85% of
inhibition, 4% yield of 2a, entry 5). The reduced product
followed the same tendency. Here, the inhibition was
proportional to the amount of m-DNB used, showing that ET
processes are involved in the formation of products 2a and 3a
and a chain process could be involved for 2a.
We recently reported the intramolecular C-N bond forming
reactions of 2'-halo-[1,1'-biphenyl]-2-amines to synthesize
different 9H-carbazoles (Scheme 4).15 Even the SRN1 reaction
was proposed to be in play, a full mechanism description is here
reported. In general, biphenylamines with an electron donating
group (EDG) like Me or OMe behave in the same way as H
giving both, cyclized and reduced products. On the other hand,
biphenylamines containing electron-withdrawing groups (EWG)
like CN, COOEt or CF3 gave only the corresponding carbazole.
These differences in the distribution of products suggest
differences regarding the overall mechanism involved.
It is known that t-BuO- anion is a good electron donor.23 With
the aim of determining whether the reaction was initiated by ET
from t-BuO- anions to the substrate, the reaction was carried out

3
without t-BuO- anions, but in the presence of NaH as a base.
Under this condition, no products were found (entry 6).
When 2'-chloro-5-carbonitrile-[1,1'-biphenyl]-2-amine (1c)
ACCEPTED MANUSCRIPT
was used as a substrate model with a EWG, only cyclic product
2b was obtained in 68% and 90% yield, after 15 and 45 minutes
of irradiation, respectively (entries 10 and 11). Unlike chlorinated
substrate 1a, photostimulated reaction of biphenyl amine 1c in
the presence of NaH as a base (instead t-BuO- anions) gave cyclic
product 2b in 71% yield together with 7% yield of 3b (entry 12).
Both reactions (with t-BuOK and NaH as bases) were inhibited in
dark conditions (entries 13 and 14). Moreover, the addition of
25% mol of m-DNB caused 51% of inhibition (33% yield of 2b,
entry 15). These results indicate that anion 1c- rather than t-BuO-
anion acts as a donor in the initial ET (homo-coupled redox
reaction) and this step is induced by light.
To elucidate the reaction mechanisms, the photo-substitution
reaction was carried out in the presence of oxygen atmosphere
(entry 7). In this case, product 2a was obtained in 38% yield
while reduction product 3a was completely inhibited. These
results imply that the single excited state of biphenylamine 1a
may be mainly involved in the photo-substitution product 2a and
triplet state may be involved in the reduced product 3a.7
Table 1. Photostimulated reactions of biphenylamines 1a-c
in DMSO. a
To support our initial evidence of the mechanism, we
performed different studies with steady-state and time-resolved
fluorescence. First, we studied photophysical properties of the
anions involved (anions 1a-, 1b-, 1c- and 3a-). UV-vis absorption
spectra of the anions were measured in DMSO under N2
atmosphere.25 It is important to notice that both neutral and anion
absorbed in the same region for pairs 1a/1a- (Figure 1A), 1b/1b-
and 3a/3a-, with almost identical λmax, but substantially changed
for 1c/1c- (Figure 1B).
R
R
+
R
X
DMSO
NH2
+ X-
NH2
NH
t-BuOK, hν
R=H, X=Cl (1a)
R=H, X=Br (1b)
R=CN, X=Cl (1c)
2a
2b
3a
3b
Since the shape of the UV spectra of neutral and anion is
almost identical, we could not determine the proportion of base
required to complete the formation of the corresponding anion.
To solve this problem, we performed a steady-state fluorescence
titration with increasing amounts of base (t-BuOK). The
preparation of anion 1a- is shown as an example in Figure 1C.
During titration, the band corresponding to the neutral species
disappeared (λem=340 - 380 nm) and a new band at a longer
wavelength was observed (λem=410 - 470 nm), attributed to anion
1a-. With this procedure, we determined that 10 equiv of t-BuOK
were necessary to completely form anion 1a-, 20 equiv for 1c-
and 50 equiv for 3a-.25
Furthermore, for all the anions, emission and excitation
steady-state fluorescence spectra were acquired25 and excited
singlet state energy (E0-0) was estimated (Table 2). Relative
fluorescence quantum yields (Фf) for all anions were also
calculated using anthracene as a reference26 (Table 2). The
relative quantum yield for dehalogenated anion 3a- was greater
than those for halogenated anions (1c- > 1a- > 1b-). For anion 1c-
Фf was bigger than that of the other halogenated anions which
could be related to more efficient fluorescence decay from
excited singlet state.
Biphenylamine
Entry
Yield
Yield
Reaction condition
b
b
1a-1c, yield (%)
of 2
of 3
1
2
3
4
5
6c
7
8
1a, 56
1a, --
45 min, hν
26
57
--
17
13
--
180 min, hν
45 min, dark
1a, 84
1a, 70
1a, 81
1a, 84
1a, 23
1b, 41
45 min, hν, 25% mol m-DNB 14
6
45 min, hν, 50% mol m-DNB
45 min, hν, NaH
45 min, hν, O2
4
4
--
--
38
25
--
45 min, hν
21
1a (24) and
1b (19)
9
45 min, hν
47
16
10
1c, 25
1c, 8
15 min, hν
68
90
71
--
--
--
7
11
45 min, hν
12 c
1c, 14
1c, 93
1c, 96
1c, 58
45 min, hν, NaH
45 min, dark
45 min, NaH, dark
13
--
--
--
14 c
--
Table 2. Photophysical properties for neutral (1a-c and 3a)
and anions (1a-c- and 3a-) in DMSOa
15
15 min, hν, 25% mol m-DNB 33
a
λmax excb
Anion
λmax emb
Anion
Фf
Photostimulated reactions were performed in inert atmosphere (nitrogen)
with [1a-c]= 0.1 M and [t-BuOK]= 0.2 M in DMSO. Irradiation was
conducted in a reactor equipped with two high-pressure lamps of model
Phillips HPI-T plus 400 W (air- and water-refrigerated), effective wavelength
irradiation longer than 350 nm.
λmax abs
λmax abs
c
Comp.
E0-0
Neutralb
Anionb
Aniond
1a
305
312
305
306
297, 334
348, 364
84.6
72.6
0.03
0.02
297, 320, 423, 449,
397, 417
1b
b Determined by GC using the internal standard method, error 5%.
477
c 2 equivalents of NaH instead t-BuOK was used as a base.
1c
284
313
302, 348 304, 355
313 276, 322
398, 440
408
77.9
78.8
0.37
0.96
3a
The photostimulated reaction proceeded with the precursor
(2'-bromo-[1,1'-biphenyl]-2-amine, 1b) under the same
conditions to yield cyclized and reduced products 2a-3a (25%
yield of 2a and 21% yield of 3a; entry 8) with a ratio close to
1.2:1. The competition experiment between bromo and chloro
derivatives, 1a and 1b, was calculated according to eq 124 (entry
9). We found that the bromo precursor is 1.3 times more reactive
than chloro precursor, in agreement with the reactivity order of
an ET reaction.
a
All UV-vis and fluorescence spectra were recorded in N2 atmosphere and
DMSO as a solvent.
b Wavelength (λ) is given in nm.
c
Values for Eo-o are given in kcal.mol-1 and were determined from the
intersecting wavelength of the normalized excitation and emission spectra of
the anions.
d
Φ
Fluorescence quantum yields (
) for anions were estimated using
f
anthracene in EtOH ( f = 0.27) as a reference. 26
Φ
(1)
kBr k1b ln(
[
1a
]
/
/
[
1a
]
−
−
[
1a t
]
]
)
)
0
0
global
=
=
kCl k1a ln(
[1b 0
]
[1b 0
]
[1b t

4
Tetrahedron
ACCEPTED MANUSCRIPT
A
B
6000
5000
4000
3000
2000
1000
0
12000
Cl
10000
NH2
Cl
NH2
8000
1a
NC
1c
1c
1c
1a
1a
6000
4000
2000
0
-
-
280
300
320
340
360
380
400
300
350
400
450
λ (nm)
λ (nm)
C
Cl
800
Cl
NH
NH2
Neutral
600
400
200
0
-5
3.7 x 10 M t-BuOK
1a
1a
-5
5.6 x 10 M t-BuOK
-5
7.4 x 10 M t-BuOK
-5
9.2 x 10 M t-BuOK
-4
1.5 x 10 M t-BuOK
-4
1.8 x 10 M t-BuOK
350
400
450
500
550
λ (nm)
Figure 1. A) UV-vis spectra for compound 1a (black solid line, [1a]= 9.3 x 10−5 M) and anion 1a- (blue solid line, [1a-]= 2 x 10−5
M). B) UV-vis spectra for compound 1c (black solid line, [1c]= 8.3 x 10−5 M) and anion 1c- (blue solid line, [1c-]= 4.1 x 10−6 M) in
DMSO under N2 atmosphere. C) Steady-state fluorescence titration with t-BuOK for biphenyl amine 1a ([1a]= 2 x 10−5 M) in DMSO
under N2 atmosphere. Fluorescence emission spectra were collected using an excitation wavelength of 305 nm.
2,0
A
-4
B
1.8 x 10 M t-BuOK
800
600
400
200
0
-4
3.6 x 10 M t-BuOK
1,8
1,6
1,4
1,2
1,0
-4
5.3 x 10 M t-BuOK
-4
7.1 x 10 M t-BuOK
-4
8.8 x 10 M t-BuOK
-3
1.1 x 10 M t-BuOK
-3
1.2 x 10 M t-BuOK
400
450
500
550
0
100
200
300
[t-BuOK] x 10-5
M
λ (nm)
Figure 2. A) Fluorescence emission spectra of anion 1a- with t-BuO- anion as a quencher. The concentration of 1a- was fixed at 2 x
10−5 M; [t-BuO- anion] from 18 to 122 x 10-5 M; T = 298 K, λexc = 305 nm. Both excitation and emission slits were 10 nm. B) Stern-
Volmer plots for 1a- ( ), 1b- ( ), 1c- ( ) and 3a- ( ).
The photoinduced reductive ET could be an intramolecular or
intermolecular process (bimolecular) where the substrate in
excited state receives an electron from a corresponding donor
(Scheme 3). If ETinter is taking place, the donor could be the
substrate (corresponding anion) or the base (t-BuO- anion). To
reveal which donor is involved we performed a steady-state and
time-resolved fluorescence quenching study.
where I0 and I are fluorescence intensities (or areas) in the
absence or presence of the quencher respectively, [t-BuO- anion]
is the concentration of quencher, and Ksv is the Stern–Volmer
quenching constant. As shown in Figure 2, increasing
concentrations of t-BuO- anion decreases the fluorescence
intensity (or total area) of anion 1a-, while shape and maxima of
the emission spectra remained unchanged. This indicates that no
exciplex formation is implicated in the quenching mechanisms.
Table 3 shows the static KSV quenching constant values.
All anions experimented steady-state fluorescence quenching
with t-BuO- anion and followed Stern–Volmer linear relationship
(eq 2),27

5
ACCEPTED MANUSCRIPT
1,8
1,6
1,4
1,2
1,0
A
B
1,3
1,2
1,1
1,0
-
-
-
1a - t-BuO
-
1c - t-BuO
*
-
-
-
(1a ) - 1a
-
-
-
Sterm-Volmer for 1a - t-BuO
*
-
(1c ) - 1c
-
Sterm-Volmer for 1c - t-BuO
*
-
-
-
Sterm-Volmer for (1a ) - 1a
*
-
Sterm-Volmer for (1c ) - 1c
5
10
15
20
25
30
35
100
300
400
500
[1a- ] x 10-7
[1c- ] x 10-8
M
[t-BuOK] x 10-5
M
M
Figure 3. A) Stern-Volmer plots for 1a- or 1c- - t-BuO- anion system. B) Stern-Volmer plots for 1a- and 1c- with the corresponding
anion as a quencher (self-quenching).
Table 3. Quenching values for anions 1a-, 1b-, 1c- and 3a-.
nature of the ET involved in the initiation step, we carried out
computational calculations. Our study was performed from the
excited state of anions 1a- and 1c- using M06-2X as functional, 28
6-311+G* basis set in polarized continuum model (PCM). 29 The
study in excited state was carried out employing TD-DFT with
the same functional and basis set.
Relation
[anion]/[t-
BuO-]
Static KSV
t-BuO-
–
Dynamic KSV
-
Dynamic KSV-
Comp.
t-BuO- (M-1)b
Anion (M-1)c
(102 M-1)a
1a-
1b-
1c-
9 – 61
(8.9 ± 0.3)
(8.5 ± 0.7)
(3.4 ± 0.1)
(1.73 ± 0.03)
(8.0 ± 0.8) x102
No quenching
40-112
2500 - 4500
3 – 210
--
No quenching
--
--
(1.2 ± 0.2) x106
Figure 4 shows the ground and singlet excited states of both
1a- and 1c-. It is important to notice that both anions in the
excited state (after optimization) exhibit distortions in both
dihedrals, the one that involved C-Cl experimenting a bending,
and the biphenyl moiety dihedral. This change in structure could
be related to the changes in photo-physical properties between
the ground and the excited state.
3a-
--
Values obtained from linear regression of I0/I as a function of [t-BuO-
anion].
a
b Values obtained from τ0/τ as a function of [t-BuO- anion].
c Values obtained from τ0/τ as a function of [1a-] or [1c-].
As observed in Figure 2B, anion 1c- seemed not to have
quenching process; however, at a higher concentration of t-BuO-
anion (more than 2500 equiv of t-BuO- anion) subject had a
quenching with a KSV of (3.4 ± 0.1) x 102 M-1 (Table 3).
In addition to study the initiation step, we employed the
Marcus-Hush theory30 to calculate the activation free energy
(∆GET*) involved in an outer-sphere ET (eq 3)
λ
∆Grel


2

Next, we evaluated the effect of t-BuO- anion added to the
singlet state lifetime (τf) of anions 1a- and 1c- using time-resolved
fluorescence measurements. For anion 1a- we observed that τf
changed by addition of t-BuO- anion (dynamic quenching),
indicating that the excited state of the biphenylamine anion 1a-
interacts with t-BuO- anion (ET process). However, for this
anion, fluorescence lifetime remained constant by increasing
anion concentration (Figure 3). In contrast, for anion 1c-, τf
remained unchanged at different concentrations of t-BuO- anion
(in the range that steady-state fluorescence quenching was
performed, Figure 3A). Increasing the anion concentration of 1c-,
we observed changes in τf, indicating the presence of self-
quenching in the single excited state (Figure 3B). All these
results have allowed us to conclude that we are in the presence of
a mechanism initiated by an ETinter (SRN1 mechanisms), dynamic
KSV showing a 104 magnitude order higher for 1c- than for 1a-.
∆GET
=
1+
(3)
4
λ


where ∆Grel represents the relative free energy difference
between the reactive and products, and λ is the reorganization
energy (interpreted as the vertical energy difference between the
minimum of the product curve and the point where the reactant
curve overlaps with this on the potential energy surface).
Scheme 5 summarizes the different activation free energy
∆GET*. In both cases, we can find that the process is more
endergonic from the ground state than from the excited state (S0).
Further, for anion 1a-, ET reaction from t-BuO- anion to excited
state of (1a-)* is favored than homo-coupled redox reaction
(11.52 vs 17.20 kcal.mol-1, respectively). By contrast, for anion
1c-, we found that ∆GET* is slightly favored from the excited
anion (1c-)* rather than from t-BuO- anion (11.80 vs 12.06
kcal.mol-1). These results are consistent with those found in
experimental quenching (Figure 3).
In order to support the photophysical and photochemical
phenomena observed in anions 1a- and 1c- and to understand the

6
Tetrahedron
1
1 ACCEPTED MANUSCRIPT
(1-)*opt
2
2
5
3
4
= (1c- )*
(1a-)*=
5
4
3
Dihedral1,2,3,4=-19º
Dihedral1,2,3,4= -25º
Dihedral2,3,4,5= -43º
Dihedral2,3,4,5= -43
h
ν
∆
E= 67 kcal/mol
∆
E= 75 kcal/mol
1
1
2
2
5
3
4
1a- =
3
= 1c-
Dihedral1,2,3,4= -2º
4
5
1-
Dihedral1,2,3,4= -2º
Dihedral2,3,4,5= -75º
Dihedral2,3,4,5= -74º
Figure 4. Representation of anions 1a- and 1c- in ground and singlet excited states.
From ground state
*
G
∆
ET
R
(kcal.mol-1)
R
Comp.
DMSO
hν
NH
+ t-BuO
1a-
80.82
78.02
+ t-BuO
NH
Cl
1c-
Cl
From excited state
*
∆GET
R
R
*
(kcal.mol-1)
Comp.
Donor
t-BuO-
11.52
17.20
12.06
11.80
NH
NH
DMSO
hν
1a-
1a-
1c-
1c-
+ Donor
+ Donor
Cl
1a-
Cl
(
)*
t-BuO-
1c-
(
)*
Scheme 5. Calculations of activation free energy (∆GET*) for ET reactions from ground and excited state by Marcus-Hush theory
In addition, we evaluated the feasibility of different steps
presence of reduced product (3a) in the cyclization reaction of
1a-. The only difference found was that Eafragmentation for 1c- is
twice bigger than that for 1a-. Moreover, 1a- cyclizes with lower
activation energy than 1c- (6.0 kcal.mol-1 vs 8.3 kcal.mol-1). We
also analyzed the hydrogen-abstraction reaction by inclusion of a
discrete molecule of the solvent (DMSO), and found that this
step is higher in energy to compete with C-N coupling.32
(fragmentation, C-N coupling and ETinter for propagation step31
and hydrogen abstraction) of the proposed mechanism by DFT
calculations, and anions 1a- and 1c- were taken as representatives.
The main results for activation energies for all the steps
mentioned are summarized in Figure 5. The evaluation of these
activation energies leads us to conclude that there is not major
difference in EaC-N, EaET or Eaabstraction that may account for the

7
ACCEPTED MANUSCRIPT
*
Cl
DMSO
Cl
NH
NH
t
-BuOK
ν
R
h
R
D
R= H, 11.52 kcal.mol-1 (D-= t-BuO-)
R= CN, 11.80 kcal.mol-1 (D-= [1c-]*)
D
FRAGMENTATION
R= H, 6.30 kcal.mol-1
R= CN, 3.50 kcal.mol-1
Cl
NH
NH
R
R
ETinter
HYDROGEN
ABSTRACTION
R= H, 13.4 kcal.mol-1
R= CN, 13.1 kcal.mol-1
H
NH
NH
H3C
R= H,
10.1 kcal.mol-1
R
S O
R
H3C
H+
R= CN,
9.8 kcal.mol-1
Cl
NH
NH
R
R
NH2
C-N COUPLING
R
R= H, 6.0 kcal.mol-1
R= CN, 8.3 kcal.mol-1
Figure 5. Activation energies for different steps in the mechanisms proposed
The SRN1 reaction is a chain mechanism, hence the overall
Φ
reactivity, as well as the quantum yield of the products ( global),
NH2
depends on the efficiency of the initiation, propagation and
NH2
(6)
DMSO
Φ
termination steps. The magnitude of the chain length (
)
propagation
t-BuOK, hν
could be calculated by the ratio of the overall quantum yield and
the quantum yield of the initiation step ( initiation) (eq 4).33
Φ
Cl
1d
3a
Φ global
Chain Lenght = Φ propagation
=
Our measurements of quantum yields were performed at
(4)
λ=300 nm.34 For 1a,
was calculated from both
Φinitiation
Φ
initiation
Φ
To calculate the quantum yield of the initiation step (
)
approximations reaching a similar chain length around 2 (Table
5, entries 1-3). This was achieved since the addition of 1,4-
cyclohexadiene was a good way to suppress the product cyclized.
initiation
we proposed two different approximations. The first one includes
the suppression of the substitution product (cyclized product) by
the addition of a good hydrogen donor like 1,4-cyclohexadiene.
In this condition the main product formed is reduced product (eq
5). The second approximation involves the use of biphenylamine
1d. This precursor could only afford the reduced product 3a (eq
6).
For 1c chain length was calculated; it was more than twice than
Φ
for 1a (
= 4.8) (Table 5, entry 6). In this case, the
propagation
propagation step could not be calculated suppressing the cyclized
product (eq 5) due to the fact that the cyclization process was
more efficient for 1c than for 1a. The difference in
Φ propagation
could be related to the fact that the turnover of the reaction for 1c
was larger than for 1a. These results could account for the
difference in the distribution product found.
R
R
R
NH2
NH2
DMSO
NH +
Cl
(5)
t-BuOK, hν
1a or 1c
2a-b
3a-b

8
Tetrahedron
was calculated including PCM contribution under the state-
Table 5. Quantum yields and chain length calculated for
ACCEPTED MANUSCRIPT
anions 1a- and 1c-.a
specific approach.
c
Φ
products
General Considerations. Column chromatography was
Entry
Biphenylamine – 1 b
Chain length
carried out on silica gel. Melting points were determined using a
standard melting point instrument and were uncorrected. Gas
chromatographic analyses were performed with a flame-
ionization detector, on 30 m capillary column of a 0.32 mm x
0.25 µm film thickness, with a 5% phenylpolysiloxane phase.
GC-MS analyses were performed employing a 25 m x 0.2 mm x
2
3
1
1a
1a
1d
1c
1c
1d
2a, 5.4
2a, 0.1
---
3a, 6.1
3a, 9.4
3a, 5.4
3b, 1.8
3b, 3.5
3b, 1.9
1.8
2.1
n.d.
4.8
2d
3e
4
1
2b, 7.3
2b, 12.2
---
0.33 µm with a 5% phenylpolysiloxane phase column. H NMR
spectra and 13C NMR spectra were recorded on a 400.16 MHz in
CDCl3, Dimethyl sulfoxide-d6 (CD3SOCD3) or acetone-d6
(CD3COCD3) as solvent with TMS as internal standard. Coupling
constants are given in Hz and chemical shifts are reported in δ
values in ppm. Data are reported as follows: chemical shift,
multiplicity (s = singlet, s br = broad singlet, d = doublet, t =
triplet, dd = double doublet, dt = double triplet, ddd = double
double doublet, m = multiplet), coupling constants (Hz), and
integration. Copies of 1H NMR and 13C NMR spectra are
provided.
5d
6f
a
Performed under N2 atmosphere, using [1a] or [1c] = 0.1 M and [t-
BuOK]= 0.2 M in DMSO. Irradiation was conducted in a photoreactor
(Rayonet) equipped with 10 lamps of Hg (air refrigerated), effective
wavelength irradiation at 300 nm. The products were quantified by GC using
the internal standard method.
bFor 1a the reaction time was 45 minutes; for 1c the reaction time was 12
minutes.
General Methods and Materials
c Determined at λmax=300 nm using Fe(III) as actinometer.
d In the presence of 0.5M of 1,4-cyclohexadiene.
e Irradiated for 45 minutes for comparison with 1a.
f Irradiated for 12 minutes for comparison with 1c.
Methods. Irradiation was performed in a photochemical
reactor equipped with two HPI-T 400 W lamps metallic iodide
(air and water refrigerated). Irradiation for quantum yield was
carried out using 10 lamps of 300 nm each in a photochemical
reactor (Rayonet) at room temperature (air refrigerated). The
purification of the products was done by column chromatography
on silica gel. Gas chromatographic analyses were obtained with a
flame-ionization detector, on 30 m capillary column of a 0.32
mm x 0.25 µm film thickness, with a 5% phenylpolysiloxane
phase. Quantification by GC was performed by the internal
standard method. Mass spectra were performed employing a 25
m x 0.2 mm x 0.33 µm with a 5% phenylpolysiloxane phase
column. 1H NMR (400.16 MHz) and 13C NMR (100 MHz)
spectra were obtained in CDCl3 or DMSO-d6 as solvents.
Coupling constants (J) are given in Hz units and chemical shifts
are reported in δ values in ppm. Melting points were recovered
with an Electrothermal 9100 instrument and were uncorrected.
3. Conclusions
Here, a complete mechanistic picture of the photoinduced ET
cyclization reaction is discussed. A comparative and detailed
study at the initiation level has allowed us to conclude that ET
process involves bimolecular interaction (SRN1), excluding ETintra
mechanisms like SN(ET)Ar* and radical-radical collapse.
The difference of photophysical properties and photochemical
reactivity is in agreement with the nature of the initiation step.
For anion 1a- is proposed that ET from t-BuO- anion to (1a-)*
could be involved meanwhile for 1c- homo-coupled redox
reaction take place. The computational data using Markus-Hush
from the excited state show a similar tendency than that of the
photophysical study.
Materials. t-BuOK, NaH, m-DNB, TEMPO, 1,4-
cyclohexadiene, anthracene were commercially available and
used as received from the supplier. DMSO was stored under
molecular sieves (4 Å). All solvents were analytical grade. Silica
gel (0.063−0.200 mm) was used in column chromatography.
Our study involves the first calculation of a chain length in an
intramolecular SRN1 process finding that 1c has twice chain
length than 1a. Both initiation and propagation could be involved
in favour to the formation of only one product for 1c (cyclized
product 3c), under SRN1 mechanism.
Spectroscopic measurements. All measurements were
carried out under an inert atmosphere of nitrogen, in quartz
cuvettes, at room temperature. Solutions of t-BuOK were
prepared at time of use. UV-vis spectra were recorded on UV-vis
spectrophotometer. Fluorescence spectra were performed in a
fluorescence spectrometer. Fluorescence lifetimes were measured
by the single-photon counting technique irradiating with a laser
at 340 nm. Quantum yield of fluorescence was determined from
the integrated emission spectra, using anthracene as the reference
following the methods reported.
Total quantum measurements.35 The ferrioxalate
actinometer was prepared by mixing equal volumes of 0.02 M
ammonium ferric sulfate dodecahydrate with 0.06 potassium
oxalate-1-hydrate both in 0.1N H2SO4 in the dark. After
illumination the product Fe2+ was analyzed by adding 0.5 mL of
4. Experimental Section
Computational Procedure. All calculations were performed
with the Gaussian09 program. The conformers obtained were
refined with complete geometry optimization within the M06-2X
DFT functional and 6-311+G* basis set. The geometries thus
found were used as starting points for the evaluation of the
reaction profiles by using the distinguished reaction coordinate
scan. The effect of DMSO as a solvent was evaluated through
Tomasi’s Polarized Continuum Model (PCM) as implemented in
Gaussian09. The inclusion of the solvent in the calculations is a
requisite for evaluating valence radical anions. The
characterization of stationary points was done by Hessian matrix
calculations. The energy informed for TSs, anions and radical
anions includes zero-point corrections. The vertical excited
singlet stated (1S) of anion 3a was calculated with TD-DTF, the
M06-2X functional and the 6-311+G* basis set. The energy of 1S
0.1% (w/v) 1,10-phenanthroline monohydrate in 1.8
M
anhydrous sodium acetate to 3 mL of the actinometer. The
absorbance of [Fe(1,10-phen)3]2+ was measured after 30 min in
the dark ε510= (5.9 ± 0.3) x103 M-1 cm-1 for the calculation of
concentration.

9
Representative procedure for synthesis of 2'-halo-[1,1'-
biphenyl]-2-amines
crude was extracted in acid media (pH = 1, H2SO4) with ethyl
ACCEPTED MANUSCRIPT
acetate (3 × 30 mL); the combined organic layers were discarded.
The resulting aqueous phase was extracted in basic media (pH =
10, NaOH) with ethyl acetate (3 × 30 mL); washed with H2O (20
mL) and dried over anhydrous MgSO4; and the solvent was
evaporated under vacuum. The product was purified by
preparative TLC eluting with pentane/ethyl acetate (98:2 %) as a
colorless liquid. Spectroscopy data agreed with the literature.
The procedure for the synthesis of biphenylamines 1a-c, d
followed previous reports.15
2'-Bromo-[1,1'-biphenyl]-2-amine (1b).36 The product was
purified by column chromatography on silica gel eluting with
petroleum ether/EtOAc (100:0 → 95:5 %). Brown solid was
obtained in 70% yield (0.17 mg, 0.7 mmol). 1H NMR (400 MHz,
CDCl3): δ 7.69 (dd, J = 8.0, 0.8, 1H), 7.38 (td, J = 7.4, 1.2, 1H),
7.32 (dd, J = 7.6, 2.0, 1H), 7.18-7.25 (2H, m), 7.02 (dd, J = 7.4,
1.2, 1H), 6.82 (td, J = 7.4, 1.2, 1H), 6.77 (dd, J = 8.0, 0.8, 1H),
3.52 (br.s, 2H); 13C NMR (100 MHz, CDCl3): δ 143.5, 140.0,
133.1, 131.8, 130.2, 129.2, 129.1, 127.8, 127.1, 124.2, 118.2,
115.5. GC-MS (EI) m/z 249 (M++2, 12), 247 (M+, 8), 169 (14),
168 (100), 167 (79), 140 (12), 139 (16), 84 (25), 83 (10).
Acknowledgments. We thank Dr. Prof. Guillermo Montich,
Universidad Nacional de Córdoba for time-resolved fluorescence
facilities. This work was supported in part by Agencia Córdoba
Ciencia, Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET), Secretaría de Ciencia y Tecnología,
Universidad Nacional de Córdoba (SECyT) and Agencia
Nacional de Promoción Científica y Técnica (ANPCyT). W.D.G.
gratefully acknowledges the receipt of a fellowship from
CONICET.
4'-Chloro-[1,1'-biphenyl]-2-amine (1d).37 A solution of 1-
chloro-4-iodobenzene
(86
mg,
0.5
mmol),
(2-
Supporting Information Available. Copies of UV-vis
spectra, steady-state fluorescence spectra, excitation and
emission spectra, 1H NMR and 13C NMR spectra for the
substrates and products and theoretical section (xyz of stationary
points) are available in Supporting Information. This material is
available free of charge via the Internet.
aminophenyl)boronic acid hydrochloride (173.0 mg, 1.0 mmol),
Pd(PPh3)2Cl2 (70.2 mg, 0.1 mmol), PPh3 (52.5 mg, 0.2 mmol)
and K2CO3 (552 mg, 4 mmol) in toluene:ethyl alcohol (10:4 mL)
was stirred at room temperature for 5 min. H2O (2 mL) was
added, and the resulting mixture was slightly degassed, sealed
and stirred at 120 ºC for 2 h. After being cooled to room
temperature, the mixture was extracted with Et2O or EtOAc. The
extracts were combined, dried over Na2SO4, and filtered. After
removal of volatile components from the filtrate, the resulting
crude product was purified by column chromatography on silica
gel eluting with pentane/EtOAc (100:0 → 80:20%)). Light
References and Notes
1. Fagnoni, M.; Albini, A. in Photonucleophilic Substitution Reactions in
Organic Photochemistry and Photophysic, Ramamurthy, V.; Schanze, K.
(eds), CRC Press. Taylor & Francis, Boca Raton, 2006, 131-170.
1
yellow oil was obtained in 75% yield (153 mg, 0.75 mmol). H
2. (a) Havinga, E.; Cornelisse, J. Chem. Rev. 1975, 75, 353-388. (b) Yang,
N. C.; Huang, A.; Yang, D. H. J. Am. Chem Soc. 1989, 111, 8060-8061.
NMR (400.16 MHz, CDCl3): δ 7.40-7.35 (m, 4H), 7.14 (t, J =
7.6, 1H), 7.06 (d, J = 7.6, 1H), 6.80 (t, J = 7.6, 1H), 6.73 (d, J =
8.0, 1H), 3.68 (br.s, 2H); 13C NMR (100.62 MHz, CDCl3): δ
143.3, 137.8, 132.9, 130.3, 130.2, 128.8, 128.7, 126.1, 118.6,
115.6.
3. Cornelisse, J. In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. H., Song, P.-S., Eds; CRC Press: New York,
1995, p 250.
4. Ceroni, P.; Balzani, V. Photoinduced Energy and Electron Transfer
Processses, in The Exploration of Supramolecular System and
Nanostructures by Photochemical Techniques, Ed. Ceroni, P. Springer
Science+Business Media B., 2012, Chapter 2.
Representative Procedure for Photostimulated Reactions.
Preparation of Carbazole Derivatives in DMSO. The
following procedure is representative of all these reactions. They
were carried out in a flame-dried Schlenk tube equipped with a
nitrogen inlet and magnetic stirrer at rt. To 5 mL of dry and
degassed DMSO under nitrogen was added 0.4 mmol (2.0 equiv.,
0.449 g) of t-BuOK and after 5 min, 0.2 mmol (1 equiv., 0.041 g)
of biphenylamine. The reaction mixture was irradiated for the
corresponding time. If the biphenylamine was oil, it was added
dissolved in dry ethyl ether. The reaction was quenched with an
excess of ammonium nitrate and water (60 mL). The mixture was
extracted three times with ethyl acetate (30 mL each), the organic
extract was washed twice with water, dried over Na2SO4 and
quantified by GC using the internal standard method.
5. Fukuzumi, S.; Ohkubo, K. in Photoinduced Reactions of Radicals Ions
via Charge Separation, in Encyclopedia of Radicals in Chemistry, Biology &
Materials, Chatgilialoglu, C., Studer, A., Eds., John Wiley & Sons Ltd,
Chichester, UK 2012, pp 365-393.
6. For reviews see: (a) Budén, M. E., Martín, S. E., Rossi, R. A. Recent
Advances in the Photoinduced Radical Nucleophilic Substitution Reactions,
in CRC Handbook of Organic Photochemistry and Photobiology, 3th ed.,
Eds. Griesbeck, A. G., Oelgemöller, M., and Ghetti, F. CRC Press Inc. Boca
Raton, 2012, Chapter 15, p.p. 347-368. (b) Bardagí, J. I., Vaillard, V. A.,
Rossi, R. A. in The SRN1 Reaction in Encyclopedia of Radicals in Chemistry,
Biology & Materials, Chatgilialoglu, C., Studer, A., Eds., John Wiley & Sons
Ltd, Chichester, UK 2012, pp 333-364. (c) Rossi, R. A.; Pierini, A. B.;
Peñéñory, A. B. Chem. Rev. 2003, 103, 71-167. (d) Rossi, R. A., Guastavino,
J. F., Budén, M. E. The SRN1 Reaction in “Arene Chemistry: Reaction
Mechanisms and Methods for Aromatic Compounds”. Part 2. Nucleophilic
Aromatic Substitution, Editor J. Mortier, John Wiley & Sons Ltd, Chichester,
UK, 2016, Chapter 10, pp 243-268.
The procedure for the isolation of carbazoles 2a and 2b and
reduced product 3a followed previous reports.15 Spectroscopy data
agreed with the literature.
6-Amino-[1,1'-biphenyl]-3-carbonitrile (3b).39 The product
was purified by column chromatography on silica gel eluting
with pentane/EtOAc (100:0 → 75:25%). Light yellow crystals
1
were obtained in 88% yield (171 mg, 0.881 mmol). H NMR
(400.16 MHz, CD3SOCD3): δ 7.49-7.37 (m, 6H), 7.33 (d, J = 2,
1H), 6.82 (1H, d, J = 8.4), 5.81 (2H, br.s); 13C NMR (100.62
MHz, CD3SOCD3): δ 150.1, 137.9, 134.4, 132.8, 129.4, 129.1,
128.0, 126.0, 120.8, 115.4, 97.3; GC-MS (EI) m/z 195 (13), 194
(M+, 98), 193 (100), 192 (37), 166 (11), 83 (12).
7. Park, Y-T; Jung, C-H; Kim, K-W. J. Org. Chem. 1999, 64, 8546-8856.
8. Mayouf, A. M.; Park, Y-T. J. of Photochem. Photobiol., A 2002, 150,
115-123.
9. Senthilvelan, A.; Ramakrishman, V. T. Tetrahedron Lett. 2002, 43,
38
2-Amino-2`-deuteriobiphenyl.32,
The reaction was carried
8379-8381.
out with the same procedure using 2.5 mL of DMSO-d6. The

10
Tetrahedron
10. Vaillard, V. A.; Rossi, R. A.; Argüello, J. E. Org. Biomol. Chem.
27. Dewey, T. J. Biophysical and Biochemical Aspects of Fluorescence
ACCEPTED MANUSCRIPT
2012, 10, 9255-9261.
Spectroscopy, Plenum, New York, 1991, 1-41.
11. Thomé, I.; Besson, C.; Kleine, T.; Bolm, C. Angew. Chem., Int. Ed.
2013, 52, 7509-7515.
28. (a) Frei, R.; Wodrich, M. D.; Hari, D. P.; Borin, P-A.; Chauvier, C.;
Waser, J. J. Am. Chem. Soc. 2014, 136, 16563-16573. (b) Zhao, Y.; Truhlar,
D. G. Theor. Chem. Acc. 2008, 120, 215-241. (c) Zhao, Y.; Truhlar, D. G.
Acc. Chem. Res. 2008, 41, 157-167.
12. Marshall, L. J.; Roydhouse, M. D.; Slawin, A. M. Z.; Walton, J. C. J.
Org. Chem. 2007, 72, 898-911.
29. (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117-
129. (b) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239-245. (c) Cossi,
M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996, 255, 327-335.
13. Roydhouse, M. D.; Walton, J. C. Eur. J. Org. Chem. 2007, 1059-
1063.
14. Budén, M. E.; Vaillard, V. A.; Martín, S. E.; Rossi, R. A. J. Org.
Chem. 2009, 74, 4490-4498.
30. Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.
31. Using Marcus-Hush theory. See eq 3 and reference 30.
15. Guerra, W. D.; Rossi, R. A.; Pierini, A. B.; Barolo, S. M. J. Org.
Chem. 2015, 80, 928-941.
32. We carried out the reaction in the presence of DMSO-d6 and we
observed that deuterated reduced product, confirming that hydrogen-
abstraction reaction is between DMSO and phenyl radical intermediate.
16. Laha, J. K.; Barolo, S. M.; Rossi, R. A.; Cuny, G. D. J. Org. Chem.
2011, 76, 6421-6425.
33. Argüello, J. E.; Peñéñory, A. B.; Rossi, R. A. J. Org. Chem. 2000, 65,
17. Vaillard, V. A.; Budén, M. E.; Martín, S. E.; Rossi, R. A. Tetrahedron
Lett. 2009, 50, 3829-3832.
7175-7182.
34. This shorter wavelength was found to be more reductive due to the
fact that a higher proportion of reduced product was found for 1a. Even
precursor 1c afforded traces of corresponding reduced product, previously not
found. This more reductive wavelength probably is leading to conversion of
the excited anion to triplet state.
18. Barolo, S. M.; Wang, Y.; Rossi, R. A.; Cuny, G. D. Tetrahedron 2013,
69, 5487-5494.
19. Guerra, W. D.; Rossi, R. A.; Pierini, A. B.; Barolo, S. M. J. Org.
Chem. 2016, 81, 4965−4973.
35. Goldstein, S.; Rabani, J. J. Photochem. Photobiol. A Chem. 2008,
193, 50-55.
20. Han,Y.-Y.; Wang, H.; Yu, S. Org. Chem. Front. 2016, 3, 953-956.
21. Guastavino, J. F.; Rossi, R. A. J. Org. Chem. 2012, 77, 460-472.
36. Pan, X., Wilcox, C. S. J. Org. Chem. 2010, 75, 6445-6451.
37. Bu, M.-J; Lu, G.-P; Cai, C. Org. Chem. Front. 2013, 1-3.
22. Peisino, L. E.; Camargo Solorzano, G. P.; Budén, M. E.; Pierini, A. B.
RSC Advance, 2015, 5, 36374-36384.
38. Kim, B. S.; Lee, S. Y.; Youn, S. W. Chem. Asian. J. 2011, 6, 1952-
23. (a) Schmidt, L. C.; Argüello, J. E.; Peñéñory, A. B. J. Org. Chem.
2007, 72, 2936-2944. (b) Budén, M. E.; Guastavino, J. F.; Rossi, R. A. Org.
Lett. 2013, 15, 1174-1177.
1957.
39. Zuo, Z.; Liu, J.; Nan, J.; Fan, L; Sun, W.; Wang, Y.; Luan, X Angew.
Chem. Int. Ed. 2015, 54, 15385-15389.
24. Peñéñory, A. B.; Rossi, R. A. Gazz. Chim. Ital. 1995, 125, 605-609.
25. See supporting information (SI).
26. Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.