Wavelength Selective Generation of Aryl Radicals and Aryl Cations for Metal-Free Photoarylations

Article abstract of DOI:10.1021/acs.joc.6b01619

Photochemical reactions have become an important tool for organic chemists. Visible (solar) light can be conveniently adopted, however, only when using colored organic compounds or in photocatalyzed processes induced by visible light absorbing photocatalysts. Herein we demonstrate that a photolabile, colored moiety could be incorporated in a colorless organic compound with the aim of generating highly reactive intermediates upon exposure to visible (solar) light. Arylazo sulfones, colored thermally stable derivatives of aryl diazonium salts, were used as valuable substrates for the photoinduced metal-free synthesis of (hetero)biaryls with no need of a (photo)catalyst or of other additives to promote the reaction. Noteworthy, selective generation of aryl radicals and aryl cations can be attained at will by varying the irradiation conditions (visible light for the former and UVA light for the latter).

Full text of DOI:10.1021/acs.joc.6b01619

Article
pubs.acs.org/joc
Wavelength Selective Generation of Aryl Radicals and Aryl Cations
for Metal-Free Photoarylations
Stefano Crespi, Stefano Protti,* and Maurizio Fagnoni*
Department of Chemistry, PhotoGreen Lab, Viale Taramelli 12, 27100 Pavia, Italy
S
* Supporting Information
ABSTRACT: Photochemical reactions have become an
important tool for organic chemists. Visible (solar) light can
be conveniently adopted, however, only when using colored
organic compounds or in photocatalyzed processes induced by
visible light absorbing photocatalysts. Herein we demonstrate
that a photolabile, colored moiety could be incorporated in a
colorless organic compound with the aim of generating highly
reactive intermediates upon exposure to visible (solar) light.
Arylazo sulfones, colored thermally stable derivatives of aryl
diazonium salts, were used as valuable substrates for the
photoinduced metal-free synthesis of (hetero)biaryls with no
need of a (photo)catalyst or of other additives to promote the reaction. Noteworthy, selective generation of aryl radicals and aryl
cations can be attained at will by varying the irradiation conditions (visible light for the former and UVA light for the latter).
are required to carry out reactions under solar/visible
irradiation.
INTRODUCTION
■
The majority of organic compounds are colorless. Nonetheless,
the use of solar and visible light to promote key photochemical
steps in organic synthesis has rapidly grown in recent years.1
Two approaches have been followed, and in both cases the
presence of suitable moieties in the starting compounds has the
role of directing the reactivity. In the first case, an organic
molecule is activated by a chemical interaction with a visible
light absorbing catalyst via an electron1 or a hydrogen atom
transfer process.2 Transition metal photoredox catalysis has
been the most rapidly growing field in the past decade (see a
general scheme in Figure 1a).1 The incorporation in the
starting substrates of redox sensitive moieties (X) is required to
facilitate the monoelectronic oxidation/reduction step (path a).
These moieties, known as electroauxiliary groups,3 have the
further advantage to be easily eliminated at the end of the
process by fragmentation of the resulting radical ions (R−X•+
or R−X•−, path b) to give reactive radicals.
We describe herein the application of arylazo sulfones in
metal-free photochemical arylations (Figure 1c). These
substrates were easily prepared from colorless anilines and
have been sparsely described8 as suitable precursors of chemical
intermediates, including phenyl radicals and cations, upon
heating (>80 °C) or by treatment with a strong acid (e.g.,
CF3COOH) or a base (pyridine as solvent).9a−e Heating of
substituted arylazo sulfones in the presence of potassium iodide
or N,N-dimethylformamide gave iodoarenes and desulfonylated
arenes, respectively.9f More attention has been given to the
electrophilic character of the NN bond in the reaction with
nucleophiles (e.g., with selenolate ion9g or Grignard
reagents9h). Recently, aryl azosulfones have been employed in
desulfonylative [3 + 2] cycloadditions for the synthesis of
substituted pyrazoles,9i where, however, the azo moiety was
maintained in the final product.
Little is known on the photoreactivity of arylazo sulfones and
arylazo sulfonates.8−10 At least in principle, a phenyl radical10 or
a phenyl cation may be generated upon irradiation in what
seems to be a solvent dependent process. As an example, the
photodecomposition of p-alkylphenylazo sulfonates used as
photolabile surfactants was investigated in micellar systems.11 A
heterolytic cleavage occurred in bulk aqueous phase to yield a
phenyl cation, whereas in micelles homolytic cleavage forming
the corresponding phenyl radical took place.11,12 Photolysis of
phenylazo-p-tolyl sulfones in aromatic solvents under visible
light irradiation was suggested to proceed via aryl radicals.10
In the second approach a photocleavable colored moiety
(responsible for the absorption of visible light) is introduced in
the starting substrate.4 However, this approach is limited to few
examples, such as Barton esters5 where stable carboxylic acids
are converted into colored photoactive thiohydroxamate esters
(Figure 1b). Photohomolysis of the labile N−O bond in these
esters gave (substituted) carbon-centered (aliphatic) radicals
(upon carbon dioxide loss from carbonyloxy radicals).4,5
In some instances uncatalyzed processes can be carried out
under solar light irradiation even for colorless compounds6 or
by the in situ formation of colored electron donor−acceptor
(EDA) complexes.7
Nonetheless, key motivations in devising photolabile visible
Received: July 6, 2016
light absorbing groups are that no (expensive) photocatalysts
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Table 1. Solar Light Induced Synthesis of 2-Arylfurans 2a−
a
h
Figure 1. Visible light generation of intermediates. (a) Photoredox
catalysis is mainly based on the presence of a redox sensitive moiety
(an electroauxiliary group, X) that makes organic molecules more
oxidizable and reducible, thus facilitating an electron transfer reaction
with a photoexcited photocatalyst. These X groups have the further
advantage to be lost in the reaction to give radicals. (b) A colored
moiety could be introduced in an organic compound by a functional
group interconversion (FGI) to allow the generation of reactive
intermediates by converting a strong bond to a weak photolabile bond.
(c) The introduction of an azosulfone group in colorless stable anilines
formed colored photolabile arylazo mesylates for the photogeneration
of either aryl radicals or aryl cations.
a
Reactions were performed on 5 mL solutions placed in glass vessels
in a Solarbox apparatus; 1 (0.1 M), furan (1 M). Letters in the
products refer to the same substituents as in compounds 1. Reaction
carried out by exposing the reaction vessel under natural sunlight (3
days, 8 h a day). 1 (0.05 M) and furan (2 M).
b
c
carried out under natural sunlight (3 days irradiation, 56% yield,
see further Figure S2).
We then investigated the arylation of other electron-rich
heteroaromatics such as thiophene, 2-methylthiophene, and N-
Boc-pyrrole (Table 2). Irradiation of azosulfones 1a,b in the
presence of thiophene led to the corresponding 2-arylth-
iophenes 4a,b in more than 70% yield. Sunlight was again
convenient to induce the synthesis of 4a. Azosulfones bearing
an electron-donating group on the aromatic ring (1e,f) gave
again the corresponding arylated products but accompanied by
a significant amount of 3e,f (ca. 30%). Arylation of 2-
methylthiophene with 1a gave a mixture of 2-aryl-5-
methylthiophene (5a) and 2-methyl-3-arylthiophene (5a′) in
72% overall yield (5a/5a′ 3:1 ratio). In a single case, N-Boc-
pyrrole was arylated (compound 6a, isolated as the exclusive
isomer, 59% yield), and the use of LED irradiation (λ = 450
nm) was found convenient in this case.
The challenging arylation of unactivated arenes was then
tested (Table 3). Arylation of benzene, though satisfactory in
some cases, is not a clean process (3 and acetanilide 9 as the
byproducts). However, when photolysis was carried out in neat
benzene, or upon sunlight irradiation, biaryls 7 were exclusively
formed. Biaryls 8a and 8b were obtained in 56% and 32%
yields, respectively by the reaction between electron-poor
azosulfones 1a,b and mesitylene. In the latter case, however, the
adoption of a 366 nm phosphor coated Hg lamp as the light
source improved the arylation yields up to 70% (Table 3).
Mechanistic Investigations on the Photoreactivity of
Azosulfones 1. Experiments were carried out to investigate
the mechanism of the reaction. No appreciable thermal
decomposition occurred when the solutions of 1a−h were
irradiated in the Solarbox protected from light. Azosulfones
1a,b,e,f were irradiated in the Solarbox in neat MeCN−H2O
9:1. Interestingly, along with 3, solvolysis products, namely,
acetanilide 9 and phenol 10, were obtained in a significant
As for the above, azosulfones can be viewed as the colored
and stable form of the corresponding highly reactive and rather
unstable aromatic diazonium salts. We reasoned that the use of
such azosulfones may widen the application of diazonium salts
(recently adopted for the photoredox catalytic generation of
aryl radicals13), overcoming, at the same time, their limitations
related to their electrophilicity and difficult handling.
RESULTS
■
We therefore deemed it worthwhile to investigate the
photochemistry of a set of arylazo mesylates (1a−h, Table
1). Compounds 1a−h were easily obtained as yellow/orange
crystalline solids from the corresponding anilines (see
Supporting Information for further details). The electronic
spectra of azosulfones 1 exhibit a low intensity band in the
visible (ε = 102 M−1 cm−1) and an intense band in the UV
region (ε = 104 M−1 cm−1),8 the latter considerably red-shifted
when an electron-donating group is present (see Table S1 and
Figure S1). These bands were safely attributed to the nπ* and
the ππ* transitions, respectively.8,10
Photochemical Arylation of (Hetero)aromatics via
Arylazo Mesylates. We carried out preliminary irradiation
experiments on 1a in the presence of furan. We found it
convenient to adopt a solar simulator (Solarbox) equipped with
a Xe lamp (500 W) as the light source. A MeCN/water 9:1
mixture was found to be the best solvent to obtain the
corresponding arylated furan and to minimize the undesired
formation of byproducts (mainly benzonitrile, Table S2). The
reaction was tested on arylazo mesylates 1a−h, and in each case
heterobiaryls 2a−h were obtained in satisfactory yields.
Noteworthy, in the case of 1a the reaction can be likewise
B
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
a
a
Table 2. Metal-Free Synthesis of Heterocycles 4−6
Table 3. Synthesis of Biaryls 7, 8
a
Conditions (see Table 2): 1 (0.05 M), benzene or mesitylene (2 M)
irradiated in MeCN−H2O 9:1. In italics: reaction carried out in neat
a
Reactions were performed on 5 mL solutions placed in glass vessels
in a Solarbox apparatus; 1 (0.05 M), heteroaromatics (2 M).
Compound 3 detected in <10% yield except where indicated.
b
benzene. Reaction carried out by exposing the reaction vessel under
c
d
natural sunlight (3 days, 8 h a day). MeCN used as the solvent. The
solution was irradiated in a 1 mm quartz cuvette for 5 h by using a 366
nm phosphor coated Hg lamp as the light source (GC yields).
b
Reaction carried out by exposing the reaction vessel under natural
c
sunlight (3 days, 8 h a day). Reaction carried out on 1 mL of solution
by using 450 nm LED as the light source.
DISCUSSION
■
The obtained data suggest that a wavelength dependent
generation of intermediates is involved in the photoreactivity
of 1 (Scheme 1). Thus, irradiation at 450 nm populates the
1nπ* state (path a), and homolysis of the S−N bond (path b)
occurs to afford the aryl radical (Ar•)/methanesulfonyl radical
amount, except for 1b, where acetophenone 3b was exclusively
formed (Table 4). The nature of the products obtained
resembles that found previously by our group in the irradiation
of benzenediazonium tetrafluoroborate salts.14 Ion chromato-
graph analyses of the photolyzed solutions revealed the
presence of methanesulfinic acid (CH3SO2H, 63% yield in
the case of 1a).
•
(CH3SO2 ) pair. In neat solvent, both radicals undergo
hydrogen abstraction from the solvent to give 316 and sulfinic
acid (CH3SO2H)17 (path c). However, when a radical trap
(furan in Scheme 1) is present at a sufficient concentration,
trapping of Ar• (path d) to form radical adduct 12• competes
efficiently with reduction.18 Hydrogen abstraction from 12• by
We were then interested to ascertain if a wavelength
dependent reactivity of the azosulfone exists. Accordingly, we
repeated some experiments on sulfone 1f by using a phosphor
coated Hg lamp (λ = 366 nm) and a LED (λ = 450 nm) in
order to reach selectively the ππ* and nπ* states, respectively.
Irradiation at these two wavelengths in neat MeCN−H2O
9:1 led to a markedly different distribution of products (at 366
nm the main product is acetanilide 9f), but the presence of
ascorbic acid (a reducing agent) led exclusively to anisole 3f in
both cases. Arylation of furan is not wavelength dependent,
which is different from the case of allyl phenyl sulfone, where
estragole 11f (48%) was isolated as the major product in the
irradiation of 1f at 450 nm but not at 366 nm.
Experiments carried out in the presence of triplet quencher
3,3,4,4-tetramethyl-1,2-diazetine dioxide (TMDD, ET ≤ 42.0
kcal mol−1, 0.025 M)15 demonstrated that while no effect was
observed at 450 nm, a dwarfing in the consumption of 1f (from
86% down to 13%) was apparent at 366 nm. Finally, a
comparison with the photoreactivity of 4-methoxyphenyldiazo-
nium tetrafluoroborate (4-MeOC6H4N2BF4) was carried out at
366 nm in the presence of furan, but, contrary to what was
observed for 1f under the same conditions, no arylation took
place.
•
CH3SO2 then affords the heterobiaryl 2 (path e). The
intervention of an aryl radical is further supported by the
reaction of 1f with allyl phenyl sulfones (a typical selective trap
for aryl radicals19), which gives estragole 11f as the main
product. On the other hand, irradiation at 366 nm populates
1
the ππ* state (path f). In this case, intersystem crossing to
3ππ* (path g) is followed by heterolytic cleavage of the S−N
bond to generate an excited aryl diazonium salt in the same
multiplicity (3ArN2 , path h). The intermediacy of a triplet state
+
is confirmed here by the efficient quenching observed in the
presence of TMDD (path g′), not observed upon irradiation at
450 nm (Table 4).
3
+
Reduction of ArN2 by ascorbic acid to the corresponding
radical,20 however, leads efficiently to 3 (path c′). Heterolysis of
14,21
+
3
the Ar−N bond in ArN2
generates a triplet aryl cation
(3Ar+, path i) that is then reduced to 3 by the solvent (path
j).14,22 The presence of solvolysis products 9, 10 (path l) is
diagnostic of the formation of a singlet aryl cation 1Ar+ (by ISC
from 3Ar+, path k)23 that depends on the nature of the aromatic
C
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
a
Table 4. Investigation on the Wavelength Dependent Behavior of Azosulfones 1
b
b
b
compd
light source
additive
1 (% cons.)
3 (%)
9/10 (%)
arylated (%)
c
1a
1b
1e
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
Solarbox
100
100
100
100
70
3a, 57
3b, 67
3e, 22
3f, 34
3f, 56
3f, 7
9a, 15; 10a, 6
c
c
c
Solarbox
Solarbox
Solarbox
9e, 20; 10e, 8
9f, 28, 10f, 14
d
d
d
d
d
LED (450 nm)
e
Hg (366 nm)
87
9f, 57; 10f, 12
f
LED (450 nm)
ascorbic acid
100
100
100
100
100
100
64
3f, 48
3f, 50
3f, 21
3f, 16
3f, 23
3f, 8
e
f
Hg (366 nm)
ascorbic acid
g
LED (450 nm)
furan
2f, 68
e
g
Hg (366 nm)
furan
9f, 5
2f, 58
h
h
LED (450 nm)
allyl phenyl sulfone
allyl phenyl sulfone
11f, 48
11f, 11
e
Hg (366 nm)
9f, 46
i
LED (450 nm)
TMDD
3f, 56
3f, <5
3f, <5
e
i
Hg (366 nm)
TMDD
13
e
g
4-MeOC6H4N2BF4
Hg (366 nm)
furan
100
9f, 25
2f, <5
a
b
c
Conditions: 1 or diazonium salt (0.05 M) in MeCN−H2O 9:1. Yields based on the consumption of 1. 1 mL of solution irradiated for 15 h in a
d
e
vial in the Solarbox. The solution was irradiated in a 1 mm quartz cuvette for 5 h. The solution was irradiated in a 1 mm quartz cuvette for 1.5 h.
f
g
h
i
Ascorbic acid 0.025 M. Furan 2 M. Allyl phenyl sulfone 0.2 M. 4,4-Tetramethyl-1,2-diazetine dioxide (TMDD) 0.025 M.
substituents. A singlet cation reactivity is almost exclusively
observed when irradiating the 4-methoxyphenyldiazonium
tetrafluoborate salt evidencing a role of the azosulfone group
phenylhydrazine,26b and pyridone based macrocycles.26c The in
situ generated aryl radical then reacts via SRN1 mechanism with
an unactivated arene (in most cases, benzene) that acts as
reagent and solvent. A milder approach makes use of ascorbic
acid to reduce the starting diazonium salts to give aryl
radicals.18
As for metal-free photochemical approaches, aryl radicals can
be accessed by Eosin Y photocatalyzed reduction of aryl13a and
(hetero)aryl27a diazonium salts or by two photon perylene
bisimide photocatalyzed reduction of aryl halides in the
presence of triethylamine as the electron donor.27b The
uncatalyzed formation of aryl radicals can be promoted by
UV irradiation of the in situ generated diazo anhydrides.27c
Finally, the photoinduced metal-free borylation of aryl halides
and ammonium salts was suggested to proceed via either aryl
radicals or triplet aryl cations.27d
As a common feature of these processes, the addition of the
aryl radical onto a heteroaromatic is particularly successful
when the radical bears an electron-withdrawing substituent on
the aromatic ring.13a,20,27b,c The arylation of simple arenes is
likewise feasible but only in the presence of a large excess of the
arene.13g The same holds even in the present case, as is
apparent in Tables 1−3.
in the population of 3ArN2 (Table 4). However, in the
+
presence of π-bond nucleophiles, such as (hetero)aromatics,
efficient trapping of 3Ar+ occurs and heterobiaryls (2 in Scheme
1) are obtained via Wheland intermediate 12+ (paths m, m′). In
contrast, the presence of a radical trap, namely, allyl phenyl
sulfone, gives allylated 11f only as a minor product. When using
a xenon lamp (such as that present in the Solarbox apparatus),
both Ar• and 3Ar+ are generated in solution, and this
successfully leads to (hetero)aromatics. To improve the yields,
however, a more selective generation of one of these
intermediates was adopted (see Tables 2, 3).
Importance of the Method. The development of metal-
free arylation procedures for the synthesis of (hetero)biaryls is a
pioneering field in organic chemistry.13a,24 Common strategies
involve the generation of reactive intermediates such as aryl
cations or radicals. Triplet aryl cations are obtained via UV
irradiation of aryl halides (mainly chlorides) and esters in protic
solvents,22 but the process is limited to electron-rich substrates.
Diazonium salts are likewise used as 3Ar+ precursors, but in the
case of electron-rich aromatics, the use of a triplet photo-
sensitizer (e.g., benzophenone) is mandatory.14
In the present work we showed that aryl azosulfones are
versatile substrates for the uncatalyzed metal-free arylation of
heterocycles and unactivated arenes with no need for additives
(e.g., bases) at ambient temperature. The introduction of an
azosulfone group allows for the wavelength selective formation
of aryl radicals and aryl cations albeit both species are generated
upon solar light irradiation. Furthermore, aryl azosulfones are
not simply a stable colored form of diazonium salts. The
presence of the azosulfonyl group is able to change the
photoreactivity of the corresponding salts as shown in the case
On the other hand, aryl radicals can be formed under metal-
free conditions by following either thermal or photochemical
approaches. The generation of aryl radicals is reported via
radical mediated halide atom abstraction from aryl iodides
taking place at room temperature in the presence of
(TMS)3SiH as chain carrier.25 The most recent proposals,
however, involve the monoelectronic reduction of aryl halides
(mainly iodides and bromides) followed by halide ion loss. The
process occurs at >80 °C in the presence of a strong base
(usually tBuOK) and organic catalysts such as quinolines,26a
D
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
CH2Cl2 was added sodium methanesulfinate (1 equiv except where
indicated) in one portion. The temperature was allowed to rise to
room temperature and the solution stirred overnight. The resulting
mixture was then filtered and the obtained solution evaporated. The
raw solid was purified by dissolution in cold CH2Cl2 and precipitation
by adding n-hexane.
Scheme 1. Wavelength Selective Generation of Aryl Radicals
(Ar•) and Aryl Cations (3Ar+)
4-Cyanophenylazo mesylate (1a): from 840 mg (3.87 mmol) of 4-
cyanobenzenediazonium tetrafluoroborate29 and 395 mg (3.87 mmol)
of sodium methanesulfinate in CH2Cl2 (13 mL). Compound 1a was
obtained in 52% yield (421 mg, yellow crystalline solid, mp 114.5−
1
115.6 °C dec). H NMR (300 MHz, CDCl3) δ: 8.08−7.90 (AA′BB′,
4H), 3.28 (s, 3H). 13C NMR (75 MHz, CDCl3) δ: 150.6, 133.6 (CH),
124.6 (CH), 118.0, 117.2, 34.9 (CH3). IR (KBr, ν cm−1): 2922, 2232,
1344, 1334, 1169, 1145, 957. Anal. Calcd for C8H7N3O2S: C, 45.92; H,
3.37; N, 20.08. Found: C, 46.1; H, 3.1; N, 19.8.
4-Acetylphenylazo mesylate (1b): from 1 g (4.27 mmol) of 4-
acetylbenzenediazonium tetrafluoroborate30 and 436 mg (4.27 mmol)
of sodium methanesulfinate in CH2Cl2 (14 mL). Compound 1b was
obtained in 87% yield (840 mg, orange crystalline solid, mp 120.5−
1
120.9 °C dec). H NMR (300 MHz, CDCl3) δ: 8.18−8.02 (AA′BB′,
4H), 3.27 (s, 3H), 2.70 (s, 3H). 13C NMR (75 MHz, CDCl3) δ: 196.6,
151.1, 141.4, 129.5 (CH), 124.4 (CH), 34.8 (CH3), 26.8 (CH3). IR
(KBr, ν cm−1): 1688. Anal. Calcd for C9H10N2O3S: C, 47.78; H, 4.45;
N, 12.38. Found: C, 47.9; H, 4.4; N, 12.5.
4-Chlorophenylazo mesylate (1c): from 1.2 g (5.30 mmol) of 4-
chlorobenzenediazonium tetrafluoroborate31 and 541 mg (5.30 mmol)
of sodium methanesulfinate in CH2Cl2 (17.5 mL). Compound 1c was
obtained in 50% yield (577 mg, yellow solid, mp 120.1−120.4 °C dec).
1H NMR (300 MHz, CDCl3) δ: 7.92−7.55 (AA′BB′, 4H), 3.22 (s,
3H). 13C NMR (75 MHz, CDCl3) δ: 147.2, 141.7, 130.0 (CH), 125.7
(CH), 34.8 (CH3). IR (KBr, ν cm−1): 3052, 2950, 1489, 1345, 1265,
954. Anal. Calcd for C7H7ClN2O2S: C, 38.45; H, 3.23; N, 12.81.
Found: C, 38.4; H, 3.1; N, 12.7.
Phenylazo mesylate (1d): from 1.2 g (6.25 mmol) of
benzenediazonium tetrafluoroborate29 and 638 mg (6.25 mmol) of
sodium methanesulfinate in CH2Cl2 (21 mL). Compound 1d was
obtained in 55% yield (633 mg, orange solid, mp 72.7−73.1 °C dec).32
1H NMR (300 MHz, CDCl3) δ: 8.00−7.95 (m, 2H), 7.70−7.65 (m,
1H), 7.60−7.55 (m, 2H), 3.24 (s, 3H). 13C NMR (75 MHz, CDCl3)
δ: 148.9, 135.1 (CH), 129.6 (CH), 124.5 (CH), 34.6 (CH3). IR (KBr,
ν cm−1): 3057, 1345, 1266, 1146, 905. Anal. Calcd for C7H8N2O2S: C,
45.64; H, 4.38; N, 15.21. Found: C, 45.5; H, 4.2; N, 15.1.
4-tert-Butylphenylazo mesylate (1e): from 958 mg (3.99 mmol) of
4-tert-butylbenzenediazonium tetrafluoroborate29 and 407 mg (3.99
mmol) of sodium methanesulfinate in CH2Cl2 (13 mL). Compound
1e was obtained in 40% yield (383 mg, orange needles, mp 70.3−71.0
of 1f (Table 4). This opens the way to the use of such versatile
azosulfones in a wide range of metal-free synthetic protocols.
1
°C dec). H NMR (300 MHz, CDCl3) δ: 7.92−7.59 (AA′BB′, 4H),
3.22 (s, 3H), 1.39 (s, 9H).13C NMR (75 MHz, CDCl3) δ: 159.7,
146.9, 126.6 (CH), 124.4 (CH), 35.4, 34.6 (CH3), 30.9 (CH3). IR
(KBr, ν cm−1): 3055, 2969, 1344, 1265, 1150, 955. Anal. Calcd for
C11H16N2O2S: C, 54.98; H, 6.71; N, 11.66. Found: C, 55.2; H, 6.5; N,
11.4.
EXPERIMENTAL SECTION
■
Aromatics used in the experiments (furan, thiophene, 2-methylthio-
phene, N-Boc-pyrrole, benzene, and mesitylene) were commercially
available and used as received, except for furan, which was freshly
distilled before use. Compounds 3, 9, 10, and 11f have been
characterized by comparison with authentic samples, and their yields
calculated by means of GC calibration curves. The reaction course was
followed by means of TLC and HPLC analyses (C18 column; eluant:
MeOH/water mixture). 1H and 13C NMR spectra were recorded on a
300 MHz spectrometer. The attributions were made on the basis of 1H
and 13C NMR, as well as DEPT-135 experiments; chemical shifts are
reported in ppm downfield from TMS. Ion chromatography analyses
were performed by means of an instrument equipped with a
conductometric detector and an electrochemical suppressor by using
the following conditions: eluant, NaHCO3 0.8 mM + Na2CO3 4.5
mM; flux, 1 mL min−1; current imposed at detector, 50 mA.
4-Methoxyphenylazo mesylate (1f): from 890 mg (4.01 mmol) of
4-methoxybenzenediazonium tetrafluoroborate33 and 409 mg (4.01
mmol) of sodium methanesulfinate in CH2Cl2 (14 mL). Compound
1f9i was obtained in 44% yield (378 mg, yellow needles, mp 81.5−82.6
1
°C dec). H NMR (300 MHz, CDCl3) δ: 7.97−7.06 (AA′BB′, 4H),
3.96 (s, 3H), 3.21 (s, 3H). 13C NMR (75 MHz, CDCl3) δ: 165.6,
143.1, 127.3 (CH), 114.8 (CH), 55.8 (CH3), 34.8 (CH3). IR (KBr, ν
cm−1): 3038, 2934, 1603, 1147, 1028. Anal. Calcd for C8H10N2O3S: C,
44.85; H, 4.70; N, 13.08. Found: C, 44.7; H, 4.9; N, 13.3.
3-Cyanophenylazo mesylate (1g): from 960 mg (4.42 mmol) of 3-
cyanobenzenediazonium tetrafluoroborate34 and 496 mg (4.86 mmol,
1.1 equiv) of sodium methanesulfinate in CH2Cl2 (15 mL).
Compound 1g was obtained in 34% yield (314 mg, orange solid,
General Procedure for the Synthesis of Arylazo Sulfones
1a−h. Diazonium salts were synthesized by following a known
procedure28 and purified by dissolving in acetone and precipitation by
adding cold diethyl ether before use. For the synthesis of 1a−h we
adapted a procedure previously described.9h To a cooled (0 °C)
suspension of the appropriate diazonium salt (1 equiv, 0.3 M) in
1
mp 121.9−122.1 °C dec). H NMR (300 MHz, CDCl3) δ: 8.27 (s,
1H), 8.21 (dd, J = 7.9, 1.0 Hz, 1H), 7.97 (dd, J = 7.9, 1.0 Hz, 1H), 7.77
(t, J = 7.9 Hz, 1H), 3.28 (s, 3H). 13C NMR (75 MHz, CDCl3) δ:
148.9, 137.5 (CH), 130.8 (CH), 128.2 (CH), 127.6 (CH), 116.9,
114.3, 34.9 (CH3). IR (KBr, ν cm−1) 3039, 2229, 1337, 1197, 965.
E
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Anal. Calcd for C8H7N3O2S: C, 45.92; H, 3.37; N, 20.08. Found: C,
45.8; H, 3.4; N, 20.3.
3-(Furan-2-yl)benzonitrile (2g): from 104 mg (0.5 mmol, 0.1M) of
1g and 365 μL (5 mmol, 1 M) of furan in MeCN−H2O 9:1 (5 mL).
Purification by column chromatography (eluant: cyclohexane/ethyl
acetate 95:5) afforded 57.5 mg of 2g (oil, 68% yield) The
spectroscopic data of 2g were in accordance with the literature.42
Anal. Calcd for C11H7NO: C, 78.09; H, 4.17; N, 8.28. Found: C, 77.8;
H, 4.3; N, 8.1.
2-(Furan-2-yl)benzonitrile (2h): from 104 mg of 1h, and 365 μL (5
mmol, 1 M) of furan in MeCN−H2O 9:1 (5 mL). Purification by
column chromatography (eluant: cyclohexane/acetate 95:5) gave 60
mg of 2i (oil, 71% yield). The spectroscopic data of 2h were in
accordance with the literature.43 Anal. Calcd for C11H7NO: C, 78.09;
H, 4.17; N, 8.28. Found: C, 78.4; H, 4.5; N, 8.0.
2-Cyanophenylazo mesylate (1h): from 1.56 g (7.19 mmol) of 2-
cyanobenzenediazonium tetrafluoroborate35 and 808 mg (7.91 mmol,
1.1 equiv) of sodium methanesulfinate in CH2Cl2 (24 mL).
Compound 1h9g was obtained in 60% yield, (902 mg, orange solid,
mp 117.7−118.4 °C dec). 1H NMR (300 MHz, CDCl3) δ: 8.02−7.92
(m, 2H), 7.87−7.79 (m, 2H), 3.31 (s, 3H). 13C NMR (75 MHz,
CDCl3) δ: 149.1, 135.0 (CH), 134.0 (CH), 133.8 (CH), 117.2 (CH),
115.9, 115.1, 34.5 (CH3). IR (KBr, ν cm−1) 3050, 2234, 1341, 1158,
963. Anal. Calcd for C8H7N3O2S: C, 45.92; H, 3.37; N, 20.08. Found:
C, 45.7; H, 3.6; N, 20.2.
General Procedure for Solar Light Metal-Free Arylations via
Aryl Azosulfones. A solution (5 mL) of the aryl azosulfone (1, 0.05−
0.1 M) and the (hetero)aromatic (1−2 M) in MeCN−H2O 9:1
mixture was poured into a glass Pyrex vessel, purged for 10 min with
nitrogen, capped, and exposed to solar simulated light (in a Solarbox)
or natural sunlight on a window ledge. In some cases, it was found
convenient to use LEDs (450 nm, 1 W) or phosphor coated Hg lamps
(366 nm 20 nm, 15 W) as light sources. After the completion of the
reaction (as detected by HPLC analysis), the solvent was removed in
vacuo from the photolyzed solution and the end products were
isolated by column chromatography (stationary phase, silica gel
chromatography; eluant, cyclohexane/ethyl acetate mixture).
4-(Furan-2-yl)benzonitrile (2a): from 105 mg (0.50 mmol) of 1a
and 365 μL (5 mmol, 1 M) of furan in MeCN−H2O 9:1 (5 mL).
Purification by column chromatography (eluant: cyclohexane/ethyl
acetate from 95:5 to 9:1) afforded 59 mg of 2a (colorless solid, 70%
yield, mp 52.5−53.7 °C, lit.36 54−56 °C). A 56% yield of 2a was
obtained when exposing a 0.05 M solution of 1a in the presence of
furan (2 M) to natural sunlight for 3 days (8 h a day). The
spectroscopic data of 2a were in accordance with the literature.13a
Anal. Calcd for C11H7NO: C, 78.09; H, 4.17; N, 8.28. Found: C, 78.1;
H, 4.2; N, 8.0.
2-(4-(Acetyl)phenyl)furan (2b): from 113 mg of 1b and 365 μL
(0.50 mmol, 1 M) of furan in MeCN−H2O 9:1 (5 mL). Purification
by column chromatography (eluant: cyclohexane/ethyl acetate from
99:1 to 9:1) afforded 58 mg of 2b (colorless solid, 62% yield, mp
98.8−100.7 °C, lit.37 96−98 °C). Compound 2b was obtained in 87%
yield when irradiating a 0.05 M solution of 1a in the presence of furan
(2 M) in MeCN−H2O 9:1 (5 mL). The spectroscopic data of 2b were
in accordance with the literature.27a Anal. Calcd for C12H10O2: C,
77.40; H, 5.41. Found: C, 77.1; H, 5.2.
2-(4-Chlorophenyl)furan (2c): from 109 mg (0.50 mmol) of 1c and
365 μL (5 mmol, 1 M) of furan in MeCN−H2O 9:1 (5 mL).
Purification by column chromatography (eluant: cyclohexane/ethyl
acetate from 99:1 to 9:1) afforded 84 mg of 2c (94% yield colorless
solid, mp 64.8−66.1 °C, lit.38a 65−66 °C). The spectroscopic data of
2c were in accordance with the literature.38b Anal. Calcd for
C10H7ClO: C, 67.24; H, 3.95. Found: C, 67.1; H, 4.1.
2-Phenylfuran (2d): from 92 mg (0.5 mmol, 0.1 M) of 1d and 365
μL (5 mmol, 1 M) of furan in MeCN−H2O 9:1 (5 mL). Purification
by column chromatography (eluant: cyclohexane/ethyl acetate 99:1)
afforded 50.5 mg of 2d (oil, 70% yield). The spectroscopic data of 2d
were in accordance with the literature.39 Anal. Calcd for C10H8O: C,
83.31; H, 5.59. Found: C, 83.1; H, 5.3.
4-(Thiophen-2-yl)benzonitrile (4a): from 52 mg (0.25 mmol, 0.05
M) of 1a and 800 μL (10 mmol, 2 M) of thiophene in MeCN−H2O
9:1 (5 mL). Purification by column chromatography (eluant:
cyclohexane/acetate from 99:1 to 9:1) afforded 34 mg of 4a (pale
yellow solid, 74% yield, mp 88.9−89.3 °C, lit.44a 88 °C). Compound
4a was obtained in 68% yield when exposing the reaction vessel to
natural sunlight for 3 days (8 h a day). The spectroscopic data of 4a
were in accordance with the literature.44b Anal. Calcd for C11H7NS: C,
71.32; H, 3.81; N, 7.56. Found: C, 71.4; H, 3.7; N, 7.3.
2-(4-(Acetyl)phenyl)thiophene (4b): from 57 mg of 1b (0.25 mmol,
0.05 M) and 800 μL (10 mmol, 2 M) of thiophene in MeCN−H2O
9:1 (5 mL). Purification by column chromatography (eluant:
cyclohexane/acetate from 99:1 to 9:1) gave 38 mg of 4b (colorless
solid, 75% yield, mp 120.5−120.8 °C, lit.45a 120−122 °C). The
spectroscopic data of 4b were in accordance with the literature.45b
Anal. Calcd for C12H10OS: C, 71.25; H, 4.98. Found: C, 71.4; H, 5.1.
2-(4-(tert-Butyl)phenyl)thiophene (4e): from 60 mg of 1e (0.25
mmol, 0.05 M) and 800 μL (10 mmol, 2 M) of thiophene in MeCN−
H2O 9:1 (5 mL). Purification by column chromatography (eluant:
cyclohexane/acetate from 99:1 to 9:1) afforded 37 mg of 4e (oil, 69%
yield). Compound 3e was likewise formed in 27% yield as evaluated
on the basis of GC calibration curves. The spectroscopic data of 4e
were in accordance with the literature.46 Anal. Calcd for C14H16S: C,
77.72; H, 7.45. Found: C, 77.7; H, 7.3.
2-(4-Methoxyphenyl)thiophene (4f): from 54 mg (0.25 mmol, 0.05
M) of 1f and 800 μL (10 mmol, 2 M) of thiophene in MeCN−H2O
9:1 (5 mL). Purification by column chromatography (eluant:
cyclohexane/acetate from 99:1 to 9:1) afforded 27 mg of 4f (pale
yellow solid, 57% yield, mp 99.9−101.1 °C, lit.47a 102−103 °C).
Compound 3f was likewise formed in 27% yield as evaluated on the
basis of GC calibration curves. The spectroscopic data of 4f were in
accordance with the literature.47b Anal. Calcd for C11H10OS: C, 69.44;
H, 5.30. Found: C, 69.7; H, 5.2.
4-(5-Methylthiophen-2-yl)benzonitrile (5a) and 4-(2-methylthio-
phen-3-yl)benzonitrile (5′a): from 52 mg (0.25 mmol, 0.05 M) of 1a
and 995 μL (10 mmol, 2 M) of 2-methylthiophene. Purification by
column chromatography (eluant: cyclohexane/acetate 95:5) gave 36
mg of a mixture containing 5a48a and 5′a48b in a 3:1 ratio (72% overall
1
yield). 5a (major isomer): H NMR (from the mixture, 300 MHz,
CDCl3) δ: 7.64 (s, 4H), 7.19−7.16 (d, J = 5.3 Hz, 1H), 6.80−6.78 (m,
1H), 2.58 (s, 3H); 13C NMR (from the mixture, 75 MHz, CDCl3) δ:
142.1, 139.5, 138.8, 132.6 (CH), 126.7 (CH), 125.4 (CH), 125.0
(CH), 118.9, 109.8, 15.4 (CH3). 5′a (minor isomer): 1H NMR (from
the mixture, 300 MHz, CDCl3) δ: 7.74−7.50 (AA′BB′, 4H), 7.20−
7.15 (d, J = 3 Hz, 1H), 7.10−7.05 (d, J = 3 Hz, 1H), 2.51 (s, 3H). 13C
NMR (from the mixture, 75 MHz, CDCl3) δ: 147.9, 144.2, 136.6,
135.9, 132.1 (CH), 129.0 (CH), 128.4 (CH), 122.3 (CH), 109.9, 14.1
(CH3).
2-(4-Cyanophenyl)pyrrole-1-carboxylic acid tert-butyl ester (6a):
from 21 mg (0.1 mmol) of 1a and 84 μL (0.5 mmol, 0.5 M) of N-Boc-
pyrrole in MeCN−H2O 9:1 (1 mL). Purification by column
chromatography (eluant: cyclohexane/acetate 9:1) gave 11.5 mg of
6a (pale yellow solid, 43% yield, mp 98.2−98.5 °C, lit.49 110−112 °C).
Compound 6a was obtained in 59% yield exposing the reaction vessel
to a 450 nm LED (1W) for 15 h. The spectroscopic data of 6a were in
accordance with the literature.11 Anal. Calcd for C16H16N2O2: C,
71.62; H, 6.01; N, 10.44. Found: C, 71.6; H, 6.1; N, 10.2.
2-(4-(tert-Butyl)phenyl)furan (2e): from 60 mg (0.25 mmol, 0.05
M) of 1e and 730 μL (10 mmol, 2 M) of furan in MeCN−H2O 9:1 (5
mL). Purification by column chromatography (eluant: cyclohexane/
ethyl acetate from 99:1 to 9:1) afforded 35.5 mg of 2e (oil, 71% yield).
The spectroscopic data of 2e were in accordance with the literature.40
Anal. Calcd for C14H16O: C, 83.96; H, 8.05. Found: C, 84.0; H, 7.9.
2-(4-Methoxyphenyl)furan (2f): from 54 mg (0.25 mmol., 0.05 M)
of 1f and 730 μL (10 mmol, 2 M) of furan in MeCN−H2O 9:1 (5
mL). Purification by column chromatography (eluant: cyclohexane/
ethyl acetate from 99:1 to 9:1) gave 42.8 mg of 2f (pale gray solid,
96% yield, mp 48.6−49.0 °C, lit.41a 49.7−51.2 °C). The spectroscopic
data of 2f were in accordance with the literature.41b Anal. Calcd for
C11H10O2: C, 75.84; H, 5.79. Found: C, 75.8; H, 5.4.
F
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
1,1′-Biphenyl-4-carbonitrile (7a): from 52 mg of 1a (0.25 mmol,
0.05 M), and 890 μL (10 mmol, 2 M) of benzene in MeCN−H2O 9:1
(5 mL). Purification by column chromatography (eluant: cyclohexane/
acetate from 99:1 to 9:1) gave 33 mg of 7a (colorless solid, 74% yield,
mp 84.1−85.7 °C, lit.50a 83−84 °C). Compound 3a was likewise
formed in 14% yield as evaluated on the basis of GC calibration curves.
Compound 7a was obtained in 70% yield when exposing the reaction
vessel to natural sunlight for 3 days (8 h a day). Compound 7a was
isolated in 72% yield when carrying out the reaction in neat benzene.
The spectroscopic data of 7a were in accordance with the literature.50b
Anal. Calcd for C13H9N: C, 87.12; H, 5.06; N, 7.82. Found: C, 87.1;
H, 5.1; N, 7.9.
4-(Acetyl)-1,1′-biphenyl (7b): from 57 mg of 1b (0.25 mmol, 0.05
M) and 890 μL (10 mmol, 2 M) of benzene in MeCN−H2O 9:1 (5
mL). Purification by column chromatography (eluant: cyclohexane/
acetate 99:1) afforded 22.5 mg of 7b (colorless solid, 46% yield, mp
117.7−118.2 °C, lit.51a 117−118 °C). Compound 3b was likewise
formed in 43% yield as evaluated on the basis of GC calibration curves.
Compound 7b was found in 52% yield when carrying out the reaction
in neat benzene. The spectroscopic data of 7b were in accordance with
the literature.51b Anal. Calcd for C14H12O: C, 85.68; H, 6.16. Found:
C, 85.7; H, 6.3.
4-(tert-Butyl)-1,1′-biphenyl (7e): from 60 mg of 1e (0.25 mmol,
0.05 M) and 890 μL (10 mmol, 2 M) of benzene in MeCN−H2O 9:1
(5 mL). Purification by column chromatography (eluant: cyclohexane/
acetate 99:1) gave 30.5 mg of 7e (colorless solid, 58% yield, mp 45.1−
46.7 °C, lit.52a 48−49 °C). Compounds 3e (21% yield) and 9e (13%
yield) were also determined on the basis of GC calibration curves.
Compound 7e was isolated in 53% yield when carrying out the
reaction in neat benzene. The spectroscopic data of 7e were in
accordance with the literature.52b Anal. Calcd for C16H18: C, 91.37; H,
8.63. Found: C, 91.5; H, 8.4
4-Methoxy-1,1′-biphenyl (7f): from 54 mg of 1f (0.25 mmol, 0.05
M) and 890 μL (10 mmol, 2 M) of benzene in MeCN (5 mL).
Purification by column chromatography (eluant: cyclohexane/acetate
99:1) afforded 32 mg of 7f (colorless solid, 70% yield, mp 83.6−85.9
°C, lit.53a 84−85 °C). Compound 3f was likewise formed in 12% yield
as evaluated on the basis of GC calibration curves. Compound 7f was
found in 51% yield when carrying out the reaction in neat benzene.
The spectroscopic data of 7f were in accordance with the literature.53b
Anal. Calcd for C13H12O: C, 84.75; H, 6.57. Found: C, 84.8; H, 6.4
2′,4′,6′-Trimethyl-[1,1′-biphenyl]-4-carbonitrile (8a): from 52 mg
of 1a (0.25 mmol, 0.05 M) and 1390 μL (10 mmol, 2 M) of
mesitylene in MeCN−H2O 9:1 (5 mL). Purification by column
chromatography (eluant: cyclohexane/acetate 99:1) gave 31 mg of 8a
(colorless solid, 56% yield, mp 58.4−59.6 °C). Compound 8a was
likewise formed in 71% yield (as evaluated on the basis of GC
calibration curves) by irradiating at 366 nm. The spectroscopic data of
8a were in accordance with the literature.54 Anal. Calcd for C16H15N:
C, 86.84; H, 6.83; N, 6.33. Found: C, 86.9; H, 6.5; N, 6.6.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: prottistefano@gmail.com.
*E-mail: fagnoni@unipv.it.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Cariplo Foundation, Italy, project 2015-0756
“Visible Light Generation of Reactive Intermediates from
Azosulfones”.
REFERENCES
■
(1) (a) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev.
2011, 40, 102−113. (b) Prier, C. K.; Rankic, D. A.; MacMillan, D. W.
C. Chem. Rev. 2013, 113, 5322−5363. (c) Yoon, T. P. ACS Catal.
2013, 3, 895−902. (d) Xuan, J.; Xiao, W.-J. Angew. Chem., Int. Ed.
2012, 51, 6828−6838. (e) Nicewicz, D. A.; Nguyen, T. M. ACS Catal.
2014, 4, 355−360. (f) Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc.
Rev. 2013, 42, 97−113.
(2) Protti, S.; Fagnoni, M.; Ravelli, D. ChemCatChem 2015, 7, 1516−
1523.
(3) Yoshida, J.-I.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev.
2008, 108, 2265−2299.
(4) Albini, A.; Fagnoni, M. Photochemically-Generated Intermediates in
Synthesis; John Wiley & Sons: Hoboken, 2013.
(5) Saraiva, M. F.; Couri, M. R. C.; Le Hyaric, M.; de Almeida, M. V.
Tetrahedron 2009, 65, 3563−3572.
(6) (a) Ni, S.; Cao, J.; Mei, H.; Han, J.; Li, S.; Pan, Y. Green Chem.
2016, 18, 3935−3939. (b) Tan, H.; Li, H.; Ji, W.; Wang, L. Angew.
Chem., Int. Ed. 2015, 54, 8374−8377. (c) Protti, S.; Artioli, G. A.;
Capitani, F.; Marini, C.; Dore, P.; Postorino, P.; Malavasi, L.; Fagnoni,
M. RSC Adv. 2015, 5, 27470−27475.
(7) (a) Lima, C. G. S.; de M. Lima, T.; Duarte, M.; Jurberg, I. D.;
Paixao, M. W. ACS Catal. 2016, 6, 1389−1407. (b) Arceo, E.; Jurberg,
̃
́
I. D.; Alvarez-Fernan
́
dez, A.; Melchiorre, P. Nat. Chem. 2013, 5, 750−
756.
(8) Kamigata, N.; Kobayashi, M. Sulfur Rep. 1982, 2, 87−128.
(9) (a) Kice, J. L.; Gabrielsen, R. S. J. Org. Chem. 1970, 35, 1004−
1009. (b) Kobayashi, M.; Gotoh, M.; Minato, H. J. Org. Chem. 1975,
40, 140−142. (c) Kice, J. L.; Gabrielsen, R. S. J. Org. Chem. 1970, 35,
1010−1015. (d) Rosini, G.; Ranza, R. J. Org. Chem. 1971, 36, 1915−
1918. (e) Yoshida, M.; Furuta, N.; Kobayashi, M. Bull. Chem. Soc. Jpn.
1981, 54, 2356−2359. (f) Evers, M. J.; Christiaens, L. E.; Guillaume,
M. R.; Renson, M. J. J. Org. Chem. 1985, 50, 1779−1780. (g) Evers, M.
J.; Christiaens, L. E.; Renson, M. J. J. Org. Chem. 1986, 51, 5196−5198.
(h) Sapountzis, I.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 897−
900. (i) Zhang, Q.; Meng, L.-G.; Wang, K.; Wang, L. Org. Lett. 2015,
17, 872−875.
1-(2′,4′,6′-Trimethyl-[1,1′-biphenyl]-4-yl)ethan-1-one (8b): from
57 mg of 1b (0.25 mmol, 0.05 M) and 1390 μL (10 mmol, 2 M) of
mesitylene in MeCN−H2O 9:1 (5 mL). Purification by column
chromatography (eluant: cyclohexane/acetate 99:1) gave 19 mg of 8b
(colorless solid, 32% yield, mp 96.3−96.8 °C, lit.55a 95−96 °C).
Compound 8b was likewise formed in 73% yield (as evaluated on the
basis of GC calibration curves) by irradiating at 366 nm. The
spectroscopic data of 8b were in accordance with the literature.55b
Anal. Calcd for C17H18O: C, 85.67; H, 7.61. Found: C, 85.7; H, 7.9.
(10) Kobayashi, M.; Fujii, S.; Minato, H. Bull. Chem. Soc. Jpn. 1972,
45, 2039−2042.
(11) Dunkin, I. R.; Gittinger, A.; Sherrington, D. C.; Whittaker, P. J.
Chem. Soc., Perkin Trans. 2 1996, 1837−1842.
(12) Eastoe, J.; Sanchez-Dominguez, M.; Cumber, H.; Burnett, G.;
Wyatt, P.; Heenan, R. K. Langmuir 2003, 19, 6579−6581.
(13) (a) Hari, D. P.; Schroll, P.; Konig, B. J. Am. Chem. Soc. 2012,
̈
134, 2958−2961. (b) Hopkinson, M. N.; Sahoo, B.; Glorius, F. Adv.
Synth. Catal. 2014, 356, 2794−2800. (c) Guo, W.; Lu, L.-Q.; Wang, Y.;
Wang, Y.-N.; Chen, J.-R.; Xiao, W.-J. Angew. Chem., Int. Ed. 2015, 54,
2265−2269. (d) Majek, M.; Jacobi von Wangelin, A. Angew. Chem., Int.
Ed. 2015, 54, 2270−2274. (e) Zoller, J.; Fabry, D. C.; Rueping, M.
ACS Catal. 2015, 5, 3900−3904. (f) Zhang, J.; Chen, J.; Zhang, X.; Lei,
X. J. Org. Chem. 2014, 79, 10682−10688. (g) Gomes, F.; Narbonne,
V.; Blanchard, F.; Maestri, G.; Malacria, M. Org. Chem. Front. 2015, 2,
464−469.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.6b01619.
Full experimental details and characterization of the
compounds and selected 1H and 13C NMR spectra
(PDF)
(14) Milanesi, S.; Fagnoni, M.; Albini, A. J. Org. Chem. 2005, 70,
603−610.
G
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(15) Ullman, E. F.; Singh, P. J. Am. Chem. Soc. 1972, 94, 5077−5088.
(16) (a) Garden, S. J.; Avila, D. V.; Beckwith, A. L. J.; Bowry, V. W.;
Ingold, K. U.; Lusztyk, J. J. Org. Chem. 1996, 61, 805−809. (b) Galli,
C. Chem. Rev. 1988, 88, 765−792.
(42) Yabe, Y.; Maegawa, T.; Monguchi, Y.; Sajiki, H. Tetrahedron
2010, 66, 8654−8660.
(43) Wu, D.; Wang, Z.-X. Org. Biomol. Chem. 2014, 12, 6414−6424.
(44) (a) Seggio, A.; Jutand, A.; Priem, G.; Mongin, F. Synlett 2008,
2955−2960. (b) Manolikakes, G.; Knochel, P. Angew. Chem., Int. Ed.
2009, 48, 205−209.
(45) (a) Kourounakis, A. P.; Charitos, C.; Rekka, E. A.; Kourounakis,
P. N. J. Med. Chem. 2008, 51, 5861−5865. (b) Sase, S.; Jaric, M.;
Metzger, A.; Malakhov, V.; Knochel, P. J. Org. Chem. 2008, 73, 7380−
7382.
(46) Johansson Seechurn, C. C. C.; Parisel, S. L.; Colacot, T. J. J. Org.
Chem. 2011, 76, 7918−7932.
(47) (a) Denmark, S. E.; Baird, J. D.; Regens, C. S. J. Org. Chem.
2008, 73, 1440−1455. (b) Stefani, H. A.; Pena, J. M.; Manarin, F.;
Ando, R. A.; Leal, D. M.; Petragnani, N. Tetrahedron Lett. 2011, 52,
4398−4401.
(17) Terpolilli, M.; Merli, D.; Protti, S.; Dichiarante, V.; Fagnoni, M.;
Albini, A. Photochem. Photobiol. Sci. 2011, 10, 123−127.
(18) Hari, D. P.; Konig, B. Angew. Chem., Int. Ed. 2013, 52, 4734−
̈
4743.
(19) Donck, S.; Baroudi, A.; Fensterbank, L.; Goddard, J.-P.; Ollivier,
C. Adv. Synth. Catal. 2013, 355, 1477−1482.
́
(20) Crisostomo, F. P.; Martin, T.; Carrillo, R. Angew. Chem., Int. Ed.
2014, 53, 2181−2185.
(21) Bondarchuk, S. V.; Minaev, B. F. J. Mol. Struct.: THEOCHEM
2010, 952, 1−7.
(22) Dichiarante, V.; Fagnoni, M. Synlett 2008, 2008, 787−800.
(23) (a) Dichiarante, V.; Dondi, D.; Protti, S.; Fagnoni, M.; Albini, A.
J. Am. Chem. Soc. 2007, 129, 5605−5611; J. Am. Chem. Soc. 2007, 129,
11662. (b) Lazzaroni, S.; Dondi, D.; Fagnoni, M.; Albini, A. J. Org.
Chem. 2010, 75, 315−323.
(48) (a) Luo, B.-T.; Liu, H.; Lin, Z.-J.; Jiang, J.; Shen, D.-S.; Liu, R.-
Z.; Ke, Z.; Liu, F.-S. Organometallics 2015, 34, 4881−4894. (b) Lin,
M.-H.; Huang, Y.-C.; Kuo, C.-K.; Tsai, C.-H.; Li, Y.-S.; Hu, T.-C.;
Chuang, T.-H. J. Org. Chem. 2014, 79, 2751−2757.
(49) Molander, G. A.; Canturk, B.; Kennedy, L. E. J. Org. Chem.
2009, 74, 973−980.
(24) See for instance: (a) Ackermann, L.; Dell’Acqua, M.; Fenner, S.;
Vicente, R.; Sandmann, R. Org. Lett. 2011, 13, 2358−2360. (b) Wu, Y.;
Wong, S. M.; Mao, F.; Chan, T. L.; Kwong, F. Y. Org. Lett. 2012, 14,
(50) (a) Ishii, G.; Harigae, R.; Moriyama, K.; Togo, H. Tetrahedron
2013, 69, 1462−1469. (b) Domingo, V.; Prieto, C.; Castillo, A.; Silva,
L.; Quílez del Moral, J. F.; Barrero, A. F. Adv. Synth. Catal. 2015, 357,
3359−3364.
(51) (a) Mu, B.; Li, T.; Xu, W.; Zeng, G.; Liu, P.; Wu, Y. Tetrahedron
2007, 63, 11475−11488. (b) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F.
Org. Lett. 2004, 6, 4435−4438.
(52) (a) Fan, X.-H.; Yang, L. H. Eur. J. Org. Chem. 2010, 2010,
2457−2460. (b) Li, H.; Sun, C.-L.; Yu, M.; Yu, D.-G.; Li, B.-J.; Shi, Z.-
J. Chem. - Eur. J. 2011, 17, 3593−3597.
(53) (a) Riggleman, S.; DeShong, P. J. Org. Chem. 2003, 68, 8106−
8109. (b) Alacid, E.; Najera, C. Org. Lett. 2008, 10, 5011−5014.
(54) Liu, Z.; Dong, N.; Xu, M.; Sun, Z.; Tu, T. J. Org. Chem. 2013,
78, 7436−7444.
(55) (a) Zhu, L.; Duquette, J.; Zhang, M. J. Org. Chem. 2003, 68,
3729−3732. (b) Bolliger, J. L.; Frech, C. M. Adv. Synth. Catal. 2010,
352, 1075−1080.
́ ́
5306−5309. (c) Castro, S.; Fernandez, J. J.; Vicente, R.; Fananas, F. J.;
̃
Rodrìguez, F. Chem. Commun. 2012, 48, 9089−9091.
(25) Curran, D. P.; Keller, A. I. J. Am. Chem. Soc. 2006, 128, 13706−
13707.
(26) (a) Qiu, Y.; Liu, Y.; Yang, K.; Hong, W.; Li, Z.; Wang, Z.; Yao,
Z.; Jiang, S. Org. Lett. 2011, 13, 3556−3559. (b) Dewanji, A.; Murarka,
S.; Curran, D. P.; Studer, A. Org. Lett. 2013, 15, 6102−6105. (c) Zhao,
H.; Shen, J.; Guo, J.; Ye, R.; Zeng, H. Chem. Commun. 2013, 49,
2323−2325.
(27) (a) Maity, P.; Kundu, D.; Ranu, B. C. Eur. J. Org. Chem. 2015,
2015, 1727−1734. (b) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; Konig, B.
̈
Science 2014, 346, 725−728. (c) Cantillo, D.; Mateos, C.; Rincon, J.
A.; de Frutos, O.; Kappe, C. O. Chem. - Eur. J. 2015, 21, 12894−
12898. (d) Mfuh, A. M.; Doyle, J. D.; Chhetri, B.; Arman, H. D.;
Larionov, O. V. J. Am. Chem. Soc. 2016, 138, 2985−2988.
(28) Li, Y.; Xie, W.; Jiang, X. Chem. - Eur. J. 2015, 21, 16059−16065.
(29) Kim, S.; Rojas-Martin, J.; Toste, D. Chem. Sci. 2016, 7, 85−88.
(30) Shin, K.; Park, S.-W.; Chang, S. J. Am. Chem. Soc. 2015, 137,
8584−8592.
(31) Guo, R.; Zhang, Z.; Shi, F.; Tang, P. Org. Lett. 2016, 18, 1008−
1011.
(32) Liu, J. B.; Chen, F.-J.; Liu, E.; Li, J.-H.; Qiu, G. New J. Chem.
2015, 39, 7773−7776.
(33) Lorris, M. E.; Abramovitch, R. A.; Marquet, J.; Moreno-Manas,
̃
M. Tetrahedron 1992, 48, 6909−6916.
(34) Kindt, S.; Wicht, K.; Heinrich, M. R. Org. Lett. 2015, 17, 6122−
6125.
(35) Colleville, A. P.; Horan, R. A. J.; Tomkinson, N. C. O. Org.
Process Res. Dev. 2014, 18, 1128−1136.
́
(36) Snegaroff, K.; Komagawa, S.; Chevallier, F.; Gros, P. C.; Golhen,
S.; Roisnel, T.; Uchiyama, M.; Mongin, F. Chem. - Eur. J. 2010, 16,
8191−8201.
(37) Friebe, N.; Schreiter, K.; Kubel, J.; Dietzek, J.; Moszner, N.;
̈
Burtscher, P.; Oehlke, A.; Spange, S. New J. Chem. 2015, 39, 5171−
5179.
́
(38) (a) Snegaroff, K.; L’Helgoual’ch, J. M.; Bentabed-Ababsa, G.;
Nguyen, T. T.; Chevallier, F.; Yonehara, F.; Uchiyama, M.; Derdour,
A.; Mongin, F. Chem. - Eur. J. 2009, 15, 10280−10290. (b) Cella, R.;
Cunha, R. L. O. R.; Reis, A. E. S.; Pimenta, D. C.; Klitzke, C. F.;
Stefani, H. A. J. Org. Chem. 2006, 71, 244−250.
(39) Alickmann, D.; Frohlich, R.; Maulitz, A. H.; Wurthwein, E.-U.
Eur. J. Org. Chem. 2002, 2002, 1523−1537.
(40) Su, W.; Urgaonkar, S.; McLaughlin, P. A.; Verkade, J. G. J. Am.
Chem. Soc. 2004, 126, 16433−16439.
(41) (a) Qian, Y. Y.; Wong, K. L.; Zhang, M. W.; Kwok, T. S.; To, C.
T.; Chan, K. S. Tetrahedron Lett. 2012, 53, 1571−1575. (b) Pauli, L.;
Tannert, R.; Scheil, R.; Pfaltz, A. Chem. - Eur. J. 2015, 21, 1482−1487.
H
DOI: 10.1021/acs.joc.6b01619
J. Org. Chem. XXXX, XXX, XXX−XXX