Transition-Metal-Free Aryl–Aryl Cross-Coupling: C?H Arylation of 2-Naphthols with Diaryliodonium Salts

Article abstract of DOI:10.1002/chem.201902204

Transition-metal-free regioselecitive C?H arylation of 2-naphthols with diaryliodonium salts has been developed. The reaction proceeds under very simple experimental conditions and affords a range of products with various substitution patterns. The method allows for the incorporation of electron-deficient aryls, which complements well currently existing metal-free aryl–aryl cross-couplings of phenols that have been so far restricted to the introduction of electron-rich aryl moieties. The mechanism of the reaction was studied by means of DFT calculations, demonstrating that the C?C bond formation occurs via a dearomatization of 2-naphthol substrate, followed by a subsequent rearomatization by tautomerization. The computations show that the use of a low polarity solvent and an insoluble inorganic base is key to securing the high selectivity of the C?C coupling over a competing C?O arylation pathway, by preventing the incipient deprotonation of 2-naphthol.

Full text of DOI:10.1002/chem.201902204

A Journal of
Accepted Article
Title: Transition Metal-Free Aryl-Aryl Cross-Coupling: C–H Arylation of
2-Naphthols with Diaryliodonium Salts
Authors: Manoj Kumar Ghosh, Jan Rzymkowski, and Marcin Kalek
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To be cited as: Chem. Eur. J. 10.1002/chem.201902204
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Transition Metal-Free Aryl-Aryl Cross-Coupling: C–H Arylation of
2-Naphthols with Diaryliodonium Salts
Manoj K. Ghosh,[a] Jan Rzymkowski,[a],[b] and Marcin Kalek*[a]
Abstract: Transition metal-free regioselecitve C–H arylation of 2-
naphthols with diaryliodonium salts has been developed. The reaction
proceeds under very simple experimental conditions and affords a
range of products with various substitution patterns. The method
allows for the incorporation of electron-deficient aryls, which
complements well currently existing metal-free aryl-aryl cross-
couplings of phenols that have been so far restricted to the
introduction of electron-rich aryl moieties. The mechanism of the
reaction was studied by means of DFT calculations, demonstrating
that the C–C bond-formation occurs via a dearomatization of 2-
naphthol substrate, followed by a subsequent rearomatization by
tautomerization. The computations show that the use of a low polarity
solvent and an insoluble inorganic base is key to securing the high
selectivity of the C–C coupling over a competing C–O arylation
pathway, by preventing the incipient deprotonation of 2-naphthol.
distinct approaches that enable the efficient introduction of an aryl
substituent into the structure of a phenol (Scheme 1). The first one
relies on an initial oxidation of the phenol, electrochemically or
with the use of a chemical oxidant, which promotes a subsequent
Fridel-Crafts type reaction with an electron-rich arene (Scheme
1a).[9] In the second strategy, Brønsted acids are used to effect a
coupling of quinone monoacetals or quinones with phenols
(Scheme 1b).[10] Because the reaction proceeds via a temporary
dearomatization of the phenolic ring, its scope is limited to
naphthols in the majority of cases. A great advantage of this
method is the possibility to employ chiral Brønsted acid catalysts,
thus enabling the enantioselective couplings. The last approach
makes use of aryl sulfoxides or aryliodine(III) diacetates as the
coupling partners, and it involves a [3,3]-rearrangement of
sulfonium or iodonium intermediates (Scheme 1c).[11] In this
instance, a transient dearomatization of both rings is required,
therefore, both the phenol and the introduced aryl must have a
reasonably low resonance energy.
In recent years, the development of cross-coupling reactions that
do not employ transition metals has gained a considerable
momentum.[1] The interest in such methods stems from their
environmental and economic advantages over the corresponding
metal-catalyzed counterparts. The avoidance of transition metals
is desired in many applications, for example in the pharmaceutical
industry, wherein the aryl-aryl couplings are one of the most
widely applied transformations nowadays and the levels of metal
contaminants need to be strictly controlled.[2] Several approaches
for achieving the transition metal-free formation of aryl-aryl bonds
exist, often exploiting the reactivities of p-block elements, such as
phosphorus, sulfur, or iodine.[3] These cross-couplings include a
number of reactions proceeding via the direct functionalization of
C–H bonds, which is the most desirable variant from the green
chemistry and atom economy viewpoints.[3a-c,4]
Aryl-substituted phenols, and in particular aryl-substituted
naphthols, are prevalent in biologically active molecules.[5] They
are also extensively applied as versatile reactants,[6] building
blocks for important molecular motifs,[7] and scaffolds for ligands
and organocatalysts.[8] Not surprisingly, the arylation of phenols
have for long been an important synthetic objective. As far as the
transition metal-free methods are concerned, there exist three
Scheme 1. Existing transition metal-free methods for the arylation of phenols.
The above methods possess multiple advantages, foremost
they are C–H/C–H cross-couplings. On the downside, however,
they display inherent limitations with regard to the aryl groups that
can be attached to the phenolic moiety. Namely, in all of the
reactions only electron-rich aryl moieties can be introduced. Also,
the incorporated aryls will by necessity contain functional groups,
whose presence may, but equally well may not, be desired. In the
reaction depicted in Scheme 1a, these are the electron-donating
substituents, securing the efficiency and regioselectivity of the
coupling, while for the reactions shown in Schemes 1b and 1c,
the OH, OR, SMe, or I groups, originating from the specific
substrates required. With the aim of alleviating this constraint in
[a]
[b]
Dr. M. K. Ghosh, J. Rzymkowski, Dr. M. Kalek
Centre of New Technologies
University of Warsaw
S. Banacha 2C, 02-097 Warsaw, Poland
E-mail: m.kalek@cent.uw.edu.pl
J. Rzymkowski
Faculty of Chemistry
University of Warsaw
L. Pasteura 1, 02-093 Warsaw, Poland
Supporting information for this article is given via a link at the end of
the document
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the scope of the existing transition metal-free arylations of
phenols, we turned our attention to the work by Quideau and co-
workers from 2005. That report, focusing on the arylative
dearomatization of substituted phenols using diaryliodonium salts,
contains a single example, wherein (unsubstituted) 2-naphthol is
treated with diphenyliodonium chloride (Scheme 2).[12] The
reaction provides a mixture of three compounds in low yields,
being the products of O-arylation, C1-arylation, and the double
arylation of the starting material.[13] In the following years, the
groups of Olofsson and Gaunt have optimized and developed
efficient protocols for the O-arylation of phenols with
diaryliodonium salts.[14] Herein, we focus on the second pathway
demonstrated in Quideau’s paper and describe our work on a
novel, transition metal-free, C–H arylation of 2-naphthols with
diaryliodonium salts.[15]
Entry
1
Change from the Standard Conditions
Yield [%][a]
98
none
2
no base
<1
3
Li2CO3, instead of Na2CO3
K2CO3, instead of Na2CO3
Cs2CO3, instead of Na2CO3
t-BuOK, instead of Na2CO3
Et3N, instead of Na2CO3
43
4
79
5
22
6
62
7
67
8
pyridine, instead of Na2CO3
toluene, instead of cyclohexane
CH2Cl2, instead of cyclohexane
THF, instead of cyclohexane
DMSO, instead of cyclohexane
MeOH, instead of cyclohexane
X = BF4 (2b), instead of X = OTf
X = OTs (2c), instead of X = OTf
X = CF3CO2 (2d), instead of X = OTf
X = AsF6 (2e), instead of X = OTf
14
9
78
10
11
12
13
14
15
16
17
85
71
61
62
90
93
90
95
[a] Determined through analysis by 19F NMR spectroscopy.
Scheme 2. A reaction of 2-naphthol with diaryliodonium chloride reported by
Quideau.
Having determined the optimal conditions, we explored the
scope of this transition metal-free C-arylation of 2-naphthols
(Scheme 3). In the case of the model bis(2-fluorophenyl)iodonium
triflate (2a), the reaction proceeds in good to excellent yields for a
variety of 2-naphthol substrates. Specifically, an array of
substituents in positions 6 and 7 is tolerated (3a–3i), including
alkyl and aryl groups, halogen, trimethylsilyl, and protected
hydroxyl. Also, 3-methoxy-2-naphthol furnishes the product in
good yield (3j). The scope with regard to 2-naphthols was re-
examined upon changing the iodonium salt to bis(4-
fluorophenyl)iodonium triflate. In this instance, the yields are
somewhat lower, but a multitude of groups in positions 6 and 7
can be present (3k–3x). Noteworthy, even a styryl-substituted 2-
naphthol proves to be a viable starting material (3y), but the
inclusion of a thiophene moiety has a detrimental effect on the
yield (3z). Unfortunately, neither 1-naphthols nor any of examined
phenols afford the desired product under these conditions.[16]
Regarding the scope of diaryliodonium salts, the transferred
aryl groups need clearly to be electron-poor to attain the efficient
cross-coupling. Hence, an unsubstituted diphenyliodonium salt
provides only 40% of the desired product (3ac), while even the
incorporation of a slightly electron-donating chlorine atom in the
para position leads to a strong decrease in yield (3ab). On the
After exploring an array of reaction conditions, we have been
able to establish a set of parameters that secures a selective
formation of the desired C-arylation product in excellent yield for
the model substrates (Table 1, entry 1). The reaction does not
proceed to any noticeable extent in the absence of a base (entry
2). Other carbonates, as well as t-BuOK and amine bases are
inferior to Na2CO3 in promoting the cross-coupling (entries 3–8).
As far as the reaction solvent is concerned, cyclohexane provides
the most efficient formation of the desired product (entries 9–13).
The decrease in the yield in entries 3–13 was always due to the
undesired side O-arylation reaction, which is completely
suppressed under the optimized conditions. Finally, application of
diaryliodonium salts bearing different counter-anions results
invariably in excellent yields irrespective of the anion present
(entries 14–17). Overall, the developed reaction conditions are
very simple, in particular no inert atmosphere is required.
Table 1. Effect of Reaction Parameters.
contrary,
electron-withdrawing
substituents
such
as
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trifluoromethyl (3ad–3af), chlorine in meta positon (3ag), and
fluorine (3aa, 3ah) secure good coupling efficiency. This limitation
is an obvious downside, but it renders the developed coupling
complementary
to
the
existing
transition
metal-free
methodologies, which are, conversely, restricted to introducing
only electron-rich aryl substituents (Scheme 1). The ability of the
method to provide fluorinated compounds is also beneficial from
the perspective of potential applications in the synthesis of
pharmaceuticals.[17]
Scheme 4. Synthesis of 2-methoxynaphtho[2,1-b]benzofuran from 3a.
In order to obtain insight into the mechanism of the C–H
arylation of 2-naphthols with diaryliodonium salts we have
performed a density functional theory (DFT) investigation. The
free energy profile for the computationally established mechanism
is shown in Figure 1. The reaction is initiated by the association
of 2-naphthol and diaryliodonium salt (INT1), followed by the
exchange of a ligand at iodine center (INT2). Upon a considerably
endergonic (by 26.2 kcal/mol), but feasible at 80 °C, dissociation
of free TfOH, intermediate INT3 is generated. TfOH is then
irreversibly neutralized by heterogeneous inorganic base (vide
infra). Subsequently, intermediate INT3 undergoes a very facile
C–C coupling (TS1, <1 kcal/mol barrier), yielding the
dearomatized ketone 5. This step is very exergonic due to the
associated reduction of iodine from +3 to +1 oxidation state and
the loss of hypervalency. Upon rearomatization by
tautomerization 5 converts into the final product 3b. The C–O
bond-formation from INT3 via TS2 leading to aryl ether 6 is
disfavored compared to the C-arylation by 3.3 kcal/mol, in line
with the high selectivity observed experimentally for iodonium salt
2a.[18] The formation of common intermediate INT3 is
corroborated by practically identical experimental results in terms
of C-/O-arylation selectivity obtained with the iodonium salts
bearing different counter-anions (Table 1, entries 14–17).
An alternative mechanistic pathway, wherein 2-naphthol is
deprotonated first, was also considered (see Figure S2 in the SI
for details). The calculations show that, if generated, the free 2-
naphtholate can facilely directly attack the aryl ring in the
diaryliodonium salt (<10 kcal/mol barrier), without the prior
formation of INT3. Importantly, the C–O coupling is preferred to
the C–C coupling in this scenario. This is in agreement with the
experimental result that the application of more soluble bases or
more polar solvents, promoting the initial deprotonation of 2-
naphthol, leads to the deterioration of yield due to the undesired
side O-arylation (Table 1, entries 3–13). Conversely, under the
optimized conditions, low polarity solvent (cyclohexane) in the
combination with an inorganic insoluble base (Na2CO3) prevents
the initial deprotonation of 2-naphthol, rendering the mechanism
shown in Figure 1 most plausible and, thus, predominant,
securing the selective formation of the desired C-arylation
product.[19]
Scheme 3. Substrate scope of the C–H arylation of 2-naphthols with
diaryliodonium salts. Yields of isolated products are given. [a] NMR yield. [b]
Obtained using tetrafluoroborate salt. [c] Obtained using unsymmetric (1,3-
dimethyluracil-5-yl)-pentafluorophenyliodonium salt.
In conclusion, we have developed a transition metal-free C–H
arylation of 2-naphthols using diaryliodonium salts as aryl-donors.
The reaction displays a complete regioselectivity with regard to
both of the coupling partners, providing ready access to an
important biaryl scaffold with many possible substitution patterns.
Conspicuously, unlike the other transition metal-free cross-
couplings of phenols reported up to date, the method enables the
introduction of electron-poor aryl rings. It is particularly suited for
the preparation of fluorinated products, further adding to its
potential for the synthesis of pharmaceutically relevant
On a gram scale, the developed procedure yielded compound
3a with a somewhat reduced, albeit synthetically acceptable yield
of 75% (1.20 g of product).The arylated 2-naphthols are of high
synthetic value and can be used in further transformations into
other interesting compounds. For example, upon intramolecular
cyclization under basic conditions 3a affords π-extended
dibenzofurane derivative 4 (Scheme 4).
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compounds. The mechanistic computational investigation
demonstrates that the overall C(sp2)–C(sp2) coupling occurs via a
dearomatization-rearomatization sequence, capitalizing on the
low resonance energy of 2-naphthols and the high oxidative
power of diaryliodonium salts. Further studies on the
derivatization of phenols with hypervalent iodine reagents are
underway.
Figure 1. Free energy profile for the reaction of 2-naphthol with bis(2-fluorophenyl)iodonium triflate (2a) calculated at the B3LYP-D3BJ/Def2-
TZVPPD(SMD)//B3LYP-D3BJ/Def2-SVP level of theory. The energy for the conversion of TfOH + Na2CO3 into NaOTf + NaHCO3 (dashed line) was calculated on
isolated species and is only a coarse estimation.
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We acknowledge the financial support from the National Science
Centre Poland (2016/22/E/ST5/00566). Computer time was
generously provided by the Interdisciplinary Centre for
Mathematical and Computational Modelling of the University of
Warsaw (G75-0).
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Keywords: C–H functionalization
•
DFT calculations
•
diaryliodonium salts • 2-naphthols • transition metal-free cross-
coupling
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