Transition-Metal-Free C-C, C-O, and C-N Cross-Couplings Enabled by Light

Article abstract of DOI:10.1021/jacs.9b02684

Transition-metal-catalyzed cross-couplings to construct C-C, C-O, and C-N bonds have revolutionized chemical science. Despite great achievements, these metal catalysts also raise certain issues including their high cost, requirement of specialized ligands, sensitivity to air and moisture, and so-called "transition-metal-residue issue". Complementary strategy, which does not rely on the well-established oxidative addition, transmetalation, and reductive elimination mechanistic paradigm, would potentially eliminate all of these metal-related issues. Herein, we show that aryl triflates can be coupled with potassium aryl trifluoroborates, aliphatic alcohols, and nitriles without the assistance of metal catalysts empowered by photoenergy. Control experiments reveal that among all common aryl electrophiles only aryl triflates are competent in these couplings whereas aryl iodides and bromides cannot serve as the coupling partners. DFT calculation reveals that once converted to the aryl radical cation, aryl triflate would be more favorable to ipso substitution. Fluorescence spectroscopy and cyclic voltammetry investigations suggest that the interaction between excited acetone and aryl triflate is essential to these couplings. The results in this report are anticipated to provide new opportunities to perform cross-couplings.

Full text of DOI:10.1021/jacs.9b02684

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Article
Transition-metal-free C-C, C-O and C-N cross-couplings enabled by light
Wenbo Liu, Jianbin LI, Pierre Querard, and Chao-Jun Li
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 01 Apr 2019
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Transition-metal-free C-C, C-O and C-N cross-couplings enabled by
light
Wenbo Liu, Jianbin Li, Pierre Querard, and Chao-Jun Li*
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Department of Chemistry and FRQNT Centre for Green Chemistry and Catalysis, McGill University, 801 Sherbrooke St. W.,
Montreal, Quebec H3A 0B8, Canada.
Supporting Information Placeholder
ABSTRACT: Transition-metal catalyzed cross-couplings to construct C-C, C-O and C-N bonds have revolutionized chemical science.
Despite the great achievements, these metal-catalysts also raise certain issues including their high cost, requirement of specialized ligands,
the sensitivity to air and moisture and the so-called “transition-metal-residue issue”. Complementary strategy, which does not rely on the
well-established “oxidative addition, transmetalation and reductive elimination” mechanistic paradigm, would potentially eliminate all these
metal-related issues. Herein, we show that aryl triflates can be coupled with potassium aryl trifluoroborates, aliphatic alcohols and nitriles
without the assistance of metal catalysts empowered by photo energy. Control experiments reveal that among all common aryl electrophiles,
only aryl triflates are competent in these couplings whereas aryl iodides and bromides can not serve as the coupling partners. DFT calculation
reveals that once converted to aryl radical cation, aryl triflate would be more favorable to undergo ipso substitution. Fluorescence spectros-
copy and cyclic voltammetry investigations suggest that the interaction between excited acetone and aryl triflate is essential for these cou-
plings. The results in this report are anticipated to open up new opportunities to perform cross-couplings.
1. Introduction
couple aryl halides or related electrophiles with nucleophiles. Un-
fortunately, compared to the well-established metal-catalyzed
cross-couplings, the metal-free versions are less explored (Figure
1b).8-9
Transition-metal catalyzed cross-coupling reactions between
aryl halides and various nucleophiles are of great significance in
chemical science.1 Benefited from several decades’ polishing, a
wide range of transition-metal catalyzed coupling reactions to forge
C-C, C-O and C-N bonds have become indispensable to many
chemical fields.2 Due to their importance and efficiency, the devel-
opment of transition-metal catalyzed cross-couplings is generally
considered to be one of the most important scientific discoveries in
20th century, exemplified by the 2010 Nobel Prize in chemistry.
Although various nucleophiles can react with aryl electrophiles, a
generic reaction paradigm underling all these coupling reactions
comprises oxidative addition-transmetalation-reductive elimina-
tion cycle.2 Involved in all three elemental steps, transition-metal
catalysts are obviously essential (Figure 1a), among which palla-
dium, nickel and copper and many others have been extensively
investigated in a wide array of reactions.3 Besides transition metals,
another crucial factor to impact these reactions is the ligand.4-5 Over
the past several decades, various phosphine-, nitrogen- and NHC
(N-heterocyclic carbene)-based ligands with diverse structural
characters and different reactivities have been invented and found
broad applications.1 Through the modulation of their electronic and
steric properties, the ligands can dramatically increase the versatil-
ity of these metal-catalyzed cross-couplings.
The power of transition metals to couple aryl electrophiles with
nucleophiles relies on their property to readily exchange the oxida-
tion states. Due to this unique redox activity, transition metals can
efficiently activate the C-X bonds of aryl electrophiles, which ini-
tiates the oxidative addition in the well-recognized three-step cata-
lytic cycle. In contrast, another less explored and distinct concept
to activate C-X bond of aryl electrophile is based on single electron
transfer (SET) process, the so-called SRN1 reaction.10-13 Injecting
an electron to aryl electrophile to generate radical anion can weaken
the C-X bond when the extraneous electron is located at its * or-
bital. For example, to initiate an SRN1 reaction, aryl electrophiles
require to be reduced by external electron source.14 In this context,
we have recently reported reactions to prepare aryl iodides from
aryl chlorides, aryl bromides and aryl triflates in a transition metal-
free manner.15-16 Encouraged by these results and intrigued by the
significance of the Suzuki coupling and Ullmann coupling to con-
struct C-C, C-O and C-N bonds, we were motivated to develop their
metal-free counterparts. Unfortunately, these less reductive nucle-
ophiles such as organoboron reagents and alcohols fail to react with
aryl chlorides, bromides and triflates under previous conditions.
We posited that albeit nucleophilic, these non-reductive nucleo-
philes are less reactive than NaI to trigger the SRN1 reaction. Be-
sides, even in cases that aryl radical intermediates are indeed gen-
erated, they may not be productively trapped by weaker nucleo-
philes.17 Upon these challenges and inspired by the pioneering
works of Fukuzumi, Nicewicz, Wu and others, we proposed that
instead of injecting an electron to form aryl radical anion, removing
an electron to generate aryl radical cation may facilitate the nucle-
ophilic addition of these weaker nucleophiles (Figure 1c).18-27
Guided by this working model, herein, we wish to report the devel-
opment of metal-catalyst-free cross-couplings of aryl triflates with
less reductive nucleophiles i.e. aryl trifluoroborates, aliphatic alco-
hols and nitriles (Figure 1d).28
However, the cost of metal catalysts and ligands especially in the
cases requiring high catalyst loading raises serious economic con-
cerns, especially in large scale industrial processes. Besides, some
transition-metal complexes are extremely moisture and air sensi-
tive, which not only complicates the handling but also generates
safety issues. Furthermore, addressing “transition-metal residual is-
sue” in the pharmaceutic and electronic material industries neces-
sitates tedious endeavours and special equipment to decrease the
transition metal amount to the regulation level (e.g. < ppm level),
thereby depleting additional resources.6-7 Consequently, comple-
mentary protocols, which do not rely on transition metals and lig-
ands, are more desirable. Identifying efficient and transition-metal-
free protocols will eliminate metal-catalyst associated issues from
the beginning and more significantly, provide new strategies to
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in acetone because: (1) it is readily available as a common organic
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solvent; and (2) the triplet acetone has a higher energy state than
other ketones, which is beneficial for both electron transfer and en-
ergy transfer.44-45
Table 1. Comparison of various coupling partners on the metal-
free Suzuki coupling. a
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Entry
Different combinations
1a & 2a
Yield
84% (75%)
0%
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Figure 1. Metal-catalyzed coupling and metal-free coupling. (a)
Metal-catalyzed cross-couplings. (b) Metal-free cross-couplings.
(c) Research hypothesis to couple less reductive nucleophiles via
oxidative initiation. (d) Developed reactions to couple aryl triflates
with boron nucleophiles, aliphatic alcohols and nitriles.
2. Results and discussion
a. Please see supporting information for the details and the UV-Vis
spectrum of the “280 nm” optical filter.
2.1 Cross-coupling between aryl triflate and potassium aryl
trifluoroborate. Cross-coupling between organoboron compounds
and aryl electrophiles, known as the Suzuki coupling, is one of the
most important reactions in organic synthesis.29 Traditional Suzuki
coupling predominantly employs palladium as the catalyst alt-
hough other metals such as nickel or copper can also promote this
reaction.30-34 However, metal-free Suzuki-type coupling to prepare
biaryl compounds remains elusive.35 Guided by the hypothesis
shown in Figure 1c, we envisioned that the ideal conditions of a
metal-free Suzuki coupling underlying electron catalysis should
meet the following requirements: (1) the oxidant to generate aryl
radical cation should remove one electron from aryl electrophile
instead of the nucleophile, which causes a dilemma because the nu-
cleophiles are usually more electron rich than electrophiles; (2) the
leaving group from the aryl electrophile should be less nucleophilic
than the boron nucleophile; and (3) contrary to the metal-catalyzed
Suzuki reaction, this proposed reaction should be performed under
acidic conditions instead of basic conditions because the bases
could potentially serve as the reductants to interfere the desired re-
action. Moreover, triplet ketones, which can activate the substrates
either via the energy transfer or electron transfer mechanism,36-43
could potentially serve as the oxidant to generate the aryl radical
cation. Among the common ketones, we are particularly interested
With these concerns in mind, we performed the investigation by
combining a wide variety of aryl electrophiles (i.e. 4-tolyl aryl elec-
trophiles) and phenyl boron reagents (i.e. phenyl boron nucleo-
philes) as the coupling partners (Table 1). Following extensive ex-
ploration, it was identified that 4-methyl phenyl triflate (1a) can be
coupled with potassium phenyl trifluoroborate (2a) in 84% yield
under optimized conditions promoted by light without transition
metals (see supporting information for details). Besides the acetone
as the co-solvent, we found that water is an efficient solvent for this
reaction probably because potassium phenyl trifluoroborate can
dissolve well in this co-solvent mixture. Moreover, although water
can potentially serve as the nucleophile to react with the proposed
aryl radical cation, we did not observe any phenol by-product after
the reaction, which suggests that phenyl trifluoroborate is a much
more efficient nucleophile than water in this reaction. Notably, aryl
iodide (1b, entry 2), aryl bromide (1c, entry 3), phenol (1d, entry
4), aryl acetate (1e, entry 5) and aryl trifluoroacetate (1f, entry 6)
are not suitable substrates. Besides aryl triflate, aryl mesylate (1g,
entry 7) and aryl phosphate (1h, entry 8) can deliver the product in
lower yields.46 That only certain phenol derivatives participate in
this metal-free Suzuki coupling suggests that the electron-rich
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Figure 2. Scope of the metal-free Suzuki coupling. The regio-isomer ratio was determined by GC/MS via comparing with authentic samples.
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character of phenol derivatives might facilitate the generation of
proposed aryl radical cation. Moreover, among the feasible phenol
derivatives, the triflate is a better leaving group than phosphate and
methylate, which rationalizes that aryl triflate is the most reactive
substrate among all three. Regarding the boron nucleophiles, aryl
boronic acid (2b, entry 9), aryl boronic ester (2c, entry 10) and
MIDA boron ester (2d, entry 11) are all inferior to aryl trifluorob-
orate (2a), which is consistent with the nucleophilic strength of
these boron reagents.47 Although popular aryl bromide and iodide
are not viable substrates in this reaction, aryl triflate, which can be
prepared from the corresponding phenol, provides unique ad-
vantages regarding biomass utilization.48-50
the products in higher yields. One notable difference between tra-
ditional Pd-catalyzed Suzuki protocol and this metal-free protocol
is that substituted aryl trifluoroborates would generate regioiso-
mers. Since aryl trifluoroborates are electron-rich aryl nucleophiles
with multiple nucleophilic sites,51 following the working hypothe-
sis shown in Figure 1c, regioisomers from the nucleophilic attack
by different sites of aryl trifluoroborates can be generated. When
para-substituted aryl trifluoroborates (3w-3ad) were subjected to
the standard conditions, three regioisomers were observed, which
is consistent with the Friedel-Crafts aromatic electrophilic substi-
tution model. The generation of regioisomers in this metal-free Su-
zuki coupling can partially support the existence of aryl radical cat-
ion intermediate (Figure 1c). Among all three isomers, the major
isomers are produced by the nucleophilic attack of ortho-carbon in
aryl trifluoroborates. Regarding the generation of regioisomers in
cases substituted aryl trifluoroborate were used, besides the aro-
matic electrophilic substitution process, the protodeboronation52-53
process may also be involved (please see supporting information
for the stepwise mechanisms to produce all three regioisomers).
Generally, this metal-free Suzuki protocol is applicable to a wide
With the optimized conditions identified, we then explored the
scope of this metal-free Suzuki coupling. Figure 2 shows that vari-
ous aryl triflates can be coupled with aryl trifluoroborates in modest
to good yields. Aryl triflates with substituents at para and meta po-
sitions (3a-3j, 3l, 3n, 3o, 3p) produce higher yields than those with
ortho substituents (3k-3m, 3q, 3r) probably due to the steric effect.
Both electron-rich (3g, 3j, 3k, 3m, 3p, 3q) and electron-poor (3h,
3i) aryl triflates are tolerated and electron-rich aryl triflates deliver
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Figure 3. Scope of the coupling between aryl triflate and alcohol.
range of aryl triflates and aryl trifluoroborates with non-polar sub-
stituents. Unfortunately, compared to the well-established powerful
Pd-catalyzed Suzuki coupling, the scope of this current metal-free
Suzuki procedure is still limited. For example, in cases of substrates
with polar substituents such as amine, carboxylic acid, esters, am-
ides, sulfonamides and the heterocycles such as indoles, pyridines,
no desired coupling products can be observed. Finally, both vinyl
triflates and vinyl trifluoroborates are not viable substrates in this
reaction. We rationalized that the presence of the polar substituents
would increase the solubility of both reactants in the solvent mix-
ture to reduce the hydrophobic effect, thereby decreasing the inter-
action of aryl triflate and aryl trifluoroborate. Another possible rea-
son is that the heteroatom would compete with the oxygen atom in
triflate to coordinate with the boron atom in aryl trifluoroborate,
which also disrupted the productive interaction between aryl triflate
and aryl trifluoroborates. Efforts to expanding the scope of this
metal-free Suzuki coupling are undergoing in our laboratory and
will be reported in due course.
2.2 Cross-coupling between aryl triflate and aliphatic alco-
hol. Aryl alkyl ethers are ubiquitous in a wide range of compounds,
which are valuable for pharmaceutical, agrochemical and material
industries.54 Current protocols to prepare these ethers from aryl hal-
ides and alcohols predominantly utilize copper and palladium.55-56
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Although several transition-metal-free protocols are known, they
(7f, 7j, 7k) can be applied under this condition (Figure 4). This ar-
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usually undergo the benzyne mechanism, thus requiring strong ba-
ses and harsh conditions.57-58 Following the hypothesis shown in
Figure 1c and encouraged by the results of metal-free Suzuki cou-
pling as described in section 2.1, we wondered whether the alcohol
is an applicable nucleophile within this mechanistic model. To our
delight, by slightly modifying the previous conditions, various al-
cohols can be coupled with aryl triflates. Likewise, the unique fea-
ture of this protocol is that only aryl triflate is an efficient coupling
partner in presence of acetone as the key promoter whereas aryl
bromide and aryl iodide can not produce the desired ether product.
Unlike other aryl radical cation involved aromatic substitutions,
only one ipso substitution product can be detected.
omatic Ritter-type reaction represents a unique strategy to convert
phenol derivatives to aniline derivatives (Scheme 1).61 Considering
the significance of aniline type compounds and ready availability
of phenols, we expect that this simple protocol would find further
synthetic applications to prepare aniline derivatives. Although ali-
phatic nitriles are competent coupling partners in this aromatic Rit-
ter reaction, attempts to expand this protocol to aromatic nitriles
proved futile probably due to the energy/transfer interference
caused by the chromophore in aromatic nitriles. To expand the
scope of nitrogen nucleophiles, our current efforts focus on: 1) to
identify a more efficient leaving group instead of triflate ion to
change the redox property of aryl electrophiles and 2) to identify a
better photo redox catalyst instead of the triplet acetone as the elec-
tron shuttle to generate the radical cation.
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To examine the synthetic utility of this metal-free Ullmann pro-
tocol, the scope of aryl triflates and aliphatic alcohols were inves-
tigated (Figure 3). Regarding the scope of aryl triflates, a wide
range of aryl triflates can be tolerated regardless of the steric and
electronic effect and the yields are generally moderate to good (5a-
5m). To our delight, aryl triflates derived from complex molecules
such as sterol (5n), vitamin (5o), amino acid (5p), dipeptide (5q)
and ribose (5r) are compatible to the current conditions. Subse-
quently, we investigated the scope of the alcohols. Both primary
and secondary alcohols can be successfully coupled with aryl tri-
flate but tertiary alcohols such as t-amyl alcohol and t-butanol can
not produce any coupling product (5s-5ae). Considering that in
metal-catalyzed coupling protocols, coupling secondary alcohols is
especially challenging because of the competitive -H elimination,
this metal-free protocol shows particularly advantages since we did
not observe any oxidative ketone products. Moreover, this reaction
can be easily scaled up to gram scale without significantly decreas-
ing the yields (5a, 5s). During the scope investigation, besides the
desired ether product, another by-product that we always observed
is the reduction product of aryl triflate (i.e. ArOTf to ArH) with
various amounts depending on the substrates. In contrast, this re-
duction by-product was not detected in the C-C and C-N formation.
We rationalized that besides the generation of aryl radical cation,
aryl triflate can also be converted to the aryl radical under the cur-
rent conditions.16 The alcohols, as efficient hydrogen donors, can
reduce the aryl radical to arenes; whereas water and nitrile are not
good hydrogen donors for aryl radical. Although this procedure is
efficient for alcohols as the nucleophile to synthesize aryl alkyl
ether, it is not applicable to phenols to prepare biaryl ethers proba-
bly because the phenols can also absorb the light, which will inter-
fere with the productive energy/electron transfer between acetone
and aryl triflate (vide infra).
Figure 4. Scope of the aromatic Ritter reaction to convert phenol
derivatives to aniline derivatives.
Scheme 1. A new strategy to convert phenol to aniline.
2.3 Cross-coupling between aryl triflate, aliphatic nitrile and
water. Encouraged by the coupling of alcohols and aryl triflates,
we attempted to develop a similar protocol to construct C-N bonds
under metal-free conditions because the amination of aryl halides
is one of the most widely used reactions in pursuit of drug candi-
dates in pharmaceutical industry.59 However, developing C-N cou-
pling is more challenging than C-O coupling because nitrogen nu-
cleophiles are usually reactive reductants, which are not compatible
with oxidative conditions. Therefore, we hypothesized that less re-
ductive nitrogen nucleophiles such as azoles and nitriles might be
potentially competent in this reaction.21 Various nitrogen nucleo-
philes, including anilines, amides, imides, sulfonamides, aliphatic
amines, imidazoles, indoles, pyrroles, benzotriazoles, purines and
nitriles were evaluated. Among all these nitrogen compounds, we
found that only nitriles can react with aryl triflates under light irra-
diation to produce the C-N bond in presence of water, which is rem-
iniscent of the Ritter reaction.60 It is notable that we did not observe
any phenol by-product, thereby implying that water is not a reactive
nucleophile to react with the proposed aryl radical cation. Scope
evaluation suggests that numerous aryl triflates (7a-7i) and nitriles
2.4. Mechanistic discussion. The data described above provide
compelling evidences that aryl triflates offer unique reactivities
among the common aryl electrophiles at least regarding these light-
enabled metal-free cross-couplings. The hypothesis shown in Fig-
ure 1c implied that a key radical cation intermediate of aryl elec-
trophile is responsible for the nucleophilic attack to construct the
C-C, C-O and C-N bonds. In the context of aryl radical cation ini-
tiated aromatic substitution, the regiospecificity of aryl triflate
showcased herein i.e. ipso-substitution is different from the exam-
ples reported by Nicewicz,21, 62-63 most of which are ortho and para
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substitutions. To gain a better mechanistic understanding of this re-
To provide further mechanistic insight, we carried out the iso-
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giospecificity, we performed the natural population analysis (NPA)
by selecting phenyl triflate as the model substrate both on the
ground state and the radical cation state.64 The NPA analysis re-
vealed that: 1) only considering the charge distribution in the radi-
cal cation of aryl triflate, the positive charge is solely localized on
C1 (+0.456), which will exclusively react with the nucleophiles to
produce the ipso-substitution products;19 2) considering the charge
change between radical cation and neutral molecule, although phe-
nyl triflate would build up the positive charge at the ipso (C1)，
meta (C3) and para (C4) positions from the ground state to the aryl
radical cation, the accumulated cation charge density on C1 is still
higher than those of C3 and C4 (Table 2). Another possible reason
for the ipso specificity is that nucleophilic substitution at the ipso
position (C1) would be much easier to generate the aromatic prod-
uct than at the meta (C3) and para position (C4) since the triflate is
a much more potent leaving group than the hydride under the cur-
rent conditions. Similar reactions have also been reported by Nice-
wicz with -OMe as the leaving group20 and by Fukuzumi with -F
as the leaving group.19
topic labelling studies about the metal-free coupling of aryl triflate
with the alcohol. Regarding the C-C and C-N formation, there is no
ambiguity that Csp2-O bond was cleaved during the reactions.
However, in the C-O formation reaction, besides the proposed path-
way to rationalize the metal-free aryl ether formation (part A in
Scheme 2.1), another possible mechanism is a cascade triflate ex-
change/SN2 reaction (part B in Scheme 2.1).65 These two pathways
would generate two products which contain the oxygen atoms from
alcohol and aryl triflate respectively. Therefore, the information on
which molecule provides the oxygen atom in the product will re-
veal conclusive evidence to rule out one of the two mechanistic hy-
potheses. To identify the oxygen source, the 18O labelled aryl tri-
flate was prepared. By subjecting this 18O labelled aryl triflate and
regular ethanol (16O) to the standard conditions, we found that only
the regular oxygen atom (16O) was introduced into the ether prod-
uct, thereby confirming that the oxygen in the final product origi-
nates from the alcohol instead of aryl triflate (Scheme 2.2). This
isotopic labelling experiment suggests that during the coupling, the
C-O bond of the aryl triflate is cleaved, which corroborates hypoth-
esis A and rules out hypothesis B. Therefore, all three reactions fol-
low the same mechanistic paradigm that Csp2-O bond of aryl tri-
flate was cleaved during the reactions.
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Table 2. NPA analysis of phenyl triflate at ground state and
radical cation state.
Atom
Neutral
Cation

It was proposed that SET between aryl triflate and the photosen-
sitizer is important for these coupling reactions (Figure 1c). To un-
derstand the possible electron transfer process, we measured the
oxidative potentials of the representative aryl compounds including
aryl triflate, potassium aryl trifluoroborate, biphenyl, aryl alkyl
ether and acetanilide involved in this study. The cyclic voltamme-
try (CV) data revealed that among the investigated aryl compounds,
aryl trifluoroborate (Ep = -0.1 V versus Ag/AgCl) is the most read-
ily oxidized whereas other compounds share the comparable oxi-
dative potentials (Ep ~ -0.7 V versus Ag/AgCl) (Figure 5). Alt-
hough the conditions to collect these CV data (CH3CN as the sol-
vent) are different from those to perform the reactions, they still
imply that it is less likely to directly oxidize aryl triflate via SET in
presence of potassium aryl trifluoroborate as well as other products.
C1
C2
C3
C4
O5
0.261
-0.236
-0.238
-0.212
-0.600
0.456
-0.242
-0.197
-0.044
-0.594
0.195
-0.006
0.041
0.168
0.006
Scheme 2. Isotopic investigation of metal-free Ullmann cou-
pling. (1) Two possible mechanisms. (2) Isotopic labelling inves-
tigation.
(A)
(B)
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Figure 5. Cyclic voltammetry data of representative aryl sub-
nucleophile to construct the C-C bond with aryl triflate. A prelimi-
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strates in this study. (A) CV of two starting materials aryl triflate
and potassium phenyl trifluoroborate. (B) CV of three products
including biphenyl, aryl alkyl ether and acetanilide.
nary investigation suggested that in presence of tetrabutylammo-
nium fluoride hydrate (TBAF٠3H2O), trimethoxyphenylsilane can
react with aryl triflate to generate the desired biaryl coupling prod-
uct in 5% yield. In contrary, this biaryl coupling product can not be
formed without TBAF (Figure 8). The success of this reaction sup-
ports the proposed mechanism shown in Figure 7. Further optimi-
zation effort on this Hiyama-type coupling is undergoing in the la-
boratory.
These CV data suggest that it is less likely for aryl triflate to di-
rectly undergo SET under the reaction conditions. On the other
hand, it is known that once excited, the organic molecule would be
more reactive because the energy of HOMO is increased and the
energy of LUMO is decreased.66 Therefore, we proposed that the
excited aryl triflate might undergo SET to generate the aryl radical
cation. This is consistent with our control experiments, in which
other common visible-light active sensitizers to initiate SET includ-
ing diacetyl, eosine, xanthanone, Ru(bpy)3(PF6)2, and Fukuzumi’s
photocatalyst (9-mesityl-10-phenylacridinium tetrafluoroborate)
under the visible light irradiation ( > 400 nm) could not deliver
the desired coupling product. Since aryl triflate can only be excited
by the light with the wavelength  < 350 nm, under the conditions
of the control experiments, the photo-sensitizers can be excited but
not the aryl triflate. These negative controls imply that the excita-
tion of aryl triflate is required for the couplings to proceed. More-
over, to evaluate the interaction between excited aryl triflate and
acetone, we performed the fluorescence quenching experiment of
aryl triflate by acetone (Figure 6). The Stern-Volmer plot suggests
that strong interaction between excited aryl triflate and acetone ex-
ists. We proposed that it is the triplet acetone instead of ground state
acetone that interacts with excited aryl triflate because of two rea-
sons: (1) acetone and aryl triflate have similar UV-Vis absorption
spectra (see supporting information for both spectra), thereby indi-
cating that they can be both excited simultaneously; (2) triplet ace-
tone has a very high energy state (ET = 332 kJ/mol),45 which im-
plies that it is unlikely for triplet aryl triflate to sensitize ground
state acetone to its triplet state. Therefore, these experiments sup-
port that a SET process may be possible between excited aryl tri-
flate and triplet acetone.
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Figure 7. A proposed mechanism which is consistent with the
current experimental data.
Figure 8. Preliminary investigation about aryl silane as the nucle-
ophile.
3. Conclusion
In summary, we have established a general protocol to couple
aryl triflate with aryl trifluoroborate, aliphatic alcohol and nitrile in
absence of any transition metal catalyst promoted by light at mild
temperature (15 °C). Among the common aryl electrophiles, this
protocol is exclusively efficient to aryl triflate, whereas aryl iodide
and bromide can not be coupled. This unique reactivity of aryl tri-
flate probably hinges upon its exceptional leaving ability of triflate
ion in presence of the Lewis acid and its interaction with triplet ac-
etone. Contrary to the previous work to prepare aryl iodide from
aryl bromide and aryl triflate, in which NaI initiates the SRN1 reac-
tion, this current protocol is postulated to be triggered by an elec-
tron hole. The proposed key radical cation intermediate can be
trapped by the weak nucleophiles including aryl trifluoroborates,
alcohols and nitriles to form C-C, C-O and C-N bonds. Considering
the simplicity and efficiency of these reactions, it is anticipated that
these results would open up new approaches to execute the classical
coupling reactions.
Figure 6. Stern-Volmer plot of aryl triflate and acetone.
Combining the above DFT calculations as well as CV and fluo-
rescence investigations, we proposed a following mechanism to ra-
tionalize these metal-free coupling reactions (Figure 7). Under the
light irradiation, both acetone and aryl triflate can be excited to their
triplet states (I) and (II). Due to their high reactivity, a SET process
would occur to produce the radical cation (III) and radical anion
(IV). Following reacting with the nucleophile, aryl radical cation
would be converted to radical intermediate (V). Driven by aroma-
tization energy, another SET from intermediate IV to V with triflate
ion as the leaving group would deliver the desired aromatic substi-
tution product.
Philosophically, a viable mechanism should not only be con-
sistent with the known facts but can also predict new reactivity.
According to the proposed mechanism in Figure 7, it can be pre-
dicted that aryl silyl compound is also expected to be a feasible
ASSOCIATED CONTENT
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15 Li, L.; Liu, W.; Zeng, H.; Mu, X.; Cosa, G.; Mi, Z.; Li, C.-J.
Photo-induced Metal-Catalyst-Free Aromatic Finkelstein
Reaction. J. Am. Chem. Soc. 2015, 137, 8328-8331.
16 Liu, W.; Yang, X.; Gao, Y.; Li, C.-J. Simple and Efficient
Generation of Aryl Radicals from Aryl Triflates: Synthesis of Aryl
Boronates and Aryl Iodides at Room Temperature. J. Am. Chem.
Soc. 2017, 139, 8621-8627.
17 Please see supporting information for further discussion.
18 Zheng, Y.-W.; Chen, B.; Ye, P.; Feng, K.; Wang, W.; Meng, Q.-
Y.; Wu, L.-Z.; Tung, C.-H. Photocatalytic Hydrogen-Evolution
Cross-Couplings: Benzene C–H Amination and Hydroxylation. J.
Am. Chem. Soc. 2016, 138, 10080-10083.
Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website.
Experimental details, characterization data and copies of spectra
AUTHOR INFORMATION
Corresponding Author
*cj.li@mcgill.ca
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Notes
19 Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. Visible-Light-Induced
Oxygenation of Benzene by the Triplet Excited State of 2,3-
Dichloro-5,6-dicyano-p-benzoquinone. J. Am. Chem. Soc. 2013,
135, 5368-5371.
20 Tay, N. E. S.; Nicewicz, D. A. Cation Radical Accelerated
Nucleophilic Aromatic Substitution via Organic Photoredox
Catalysis. J. Am. Chem. Soc. 2017, 139, 16100-16104.
21 Romero, N. A.; Margrey, K. A.; Tay, N. E.; Nicewicz, D. A.
Site-selective arene C-H amination via photoredox catalysis.
Science 2015, 349, 1326-1330.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We are grateful to the Canada Research Chair (Tier 1) foundation,
FRQNT (CCVC), CFI and NSERC for supporting our research. We
are also grateful to Dr. Xiaobo Yang (McGill University) and Dr.
Peng Liu (McGill University) to prepare some substrates used in
this study. We also thank Siting Ni (Lennox group at McGill Uni-
versity) for assistance on UV-Visible and fluorescence spectra
measurements and Yuanjiao Li (Mauzeroll group at McGill Uni-
versity) for assistance on cyclic voltammetric measurements.
22 Kei, O.; Takaki, K.; Shunichi, F. Direct Oxygenation of
Benzene to Phenol Using Quinolinium Ions as Homogeneous
Photocatalysts. Angew. Chem. Int. Ed. 2011, 50, 8652-8655.
23 Albini, A.; Fasani, E.; Dalessandro, N. Radical Cations -
Generation
by
Photochemical
Electron-Transfer
and
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