Transition-Metal-Free Synthesis of Biarylmethanes from Aryl Iodides and Benzylic Ketones

Article abstract of DOI:10.1002/chem.201804415

An original metal-free procedure for the synthesis of biarylmethanes is disclosed herein. The reactions occur with high selectivity starting from aryl iodides and benzylic ketones in the presence of superbasic media (CsOH/DMSO). This procedure allows a straightforward access to a wide range of biarylmethane derivatives substituted with electron-withdrawing and -donating substituents.

Full text of DOI:10.1002/chem.201804415

A Journal of
Accepted Article
Title: Transition-Metal-Free Synthesis of Biarylmethanes from Aryl
Iodides and Benzylic Ketones
Authors: Martin Pichette Drapeau, Anis Tlili, Yassir Zaid, Dounia
Toummini, Fouad Ouazzani Chahdi, Jean-MArc Sotitopoulos,
Thierry Ollevier, and Marc Taillefer
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To be cited as: Chem. Eur. J. 10.1002/chem.201804415
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Transition-Metal-Free Synthesis of Biarylmethanes from Aryl
Iodides and Benzylic Ketones
Martin Pichette Drapeau,[a,b] Anis Tlili,[b,c] Yassir Zaid,[b,d] Dounia Toummini,[b,d] Fouad Ouazzani
Chahdi,[d] Jean-Marc Sotiropoulos,[e] Thierry Ollevier,[a] and Marc Taillefer*[b]
Abstract: An original metal-free procedure for the synthesis of
biarylmethanes is disclosed herein. The reactions occur with high
selectivity starting from aryl iodides and benzylic ketones in the
presence of superbasic media (CsOH/DMSO). This procedure allows
a straightforward access to a wide range of biarylmethane derivatives
substituted with electron-withdrawing and -donating substituents.
Friedel-Crafts and Cu-catalyzed benzylic CH activation
processes have also been shown to yield BAM derivatives
(Scheme 1c).[6] Recently, Leadbeater disclosed a one-pot -
arylation/debenzoylation sequence of deoxybenzoin derivatives
under palladium catalysis (Scheme 1d).[7a] Alternatively, a one-pot
double -arylation/debenzoylation of acetophenone derivatives
yielding symmetrical BAM was disclosed by Zhang. [7b] Although
the aforementioned methodologies are interesting, novel methods
for straightforward transition-metal-free syntheses of BAM from
commonly available building blocks remain of great interest.
As part of our studies on the transition-metal-free CC bond
formation,[8] we now report a synthesis of biarylmethanes under
straightforward reaction conditions and with high selectivity
starting from aryl iodides and benzylic ketone derivatives
(Scheme 1e).
Transition-metal catalyzed construction of carbon-carbon bonds
is one of the most studied reactions in modern organic chemistry.
Among the many advances that have been made during the last
century,[1] the most important breakthroughs have been disclosed
under palladium catalysis with the pioneering work of Heck,
Negishi and Suzuki for the formation of C(sp2)C(sp2) and
C(sp2)C(sp3) bonds. In the last decades, impressive efforts have
been made in this field with regard to the synthesis and design of
new and more efficient catalysts.[2] Among the many classes of
compounds accessible by these methods, biarylmethanes (BAM)
occupy an essential place since they are finding a plethora of
applications in life sciences and materials.[3] Traditional syntheses
of BAM mostly rely on cross-coupling processes between aryl
(pseudo)halides and benzylic organometallic species or benzylic
(pseudo)halides and aryl metal species, reactions that are mainly
catalyzed by Pd or first row transition metals such as Ni, while
transition-metal-free procedures have recently been reported
(Scheme 1a).[4] Even if these procedures are efficient, a
significant drawback lies in the generation of over stoichiometric
metal waste. With the recent emergence of CH bond
functionalization processes, several groups have reported
transition metal catalyzed syntheses of BAM via direct
benzylation of arenes (Scheme 1b).[5] However, the necessity of
directing groups limits the reaction scope. It should be noted that
Scheme 1. State of the art and new approach for the synthesis of
biarylmethanes (BAM).
We supposed that the synthesis of biarylmethanes could
occur
in
a
one-pot
procedure
through
an
-
arylation/deacetylation sequence in the absence of a transition
metal. The use of strong bases (MOtBu) could leverage the
orthogonality of transition metal catalysis.[8c] Initial attempts
focused on discovering suitable conditions. We were glad to
observe the formation of diphenylmethane 3aa in a 23% yield
when iodobenzene was reacted with phenylacetone in the
presence of potassium tert-butoxide and DMSO as base/solvent
mixture at 90 °C (Table 1, entry 1). A better yield of 41% was
obtained when sodium tert-butoxide was used (Table 1, entry 2).
Having in mind that superbasic media can be achieved when
alkali hydroxides are mixed with DMSO, we tested various
hydroxide bases. Fair yields of 42% and 51% were obtained by
using sodium and potassium hydroxides respectively (Table 1,
entries 3 and 4). The yield of diphenylmethane 3aa rose to 70%
when cesium hydroxide was used and reached an excellent 88%
yield by increasing the reaction temperature to 110 °C (Table 1,
entry 5). It should be mentioned that lowering the quantity of
CsOH to 2 equiv. furnished a fair yield (55%) of diphenylmethane.
[a]
[b]
Dr. M. Pichette Drapeau, Prof. Dr. T. Ollevier
Département de chimie, Pavillon Alexandre-Vachon Université Laval
1045 avenue de la Médecine, Québec (Qc) G1V 0A6 (Canada)
Dr. M. Pichette Drapeau, Dr. A. Tlili, Y. Zaid, Dr. D. Toummini,
Dr. M. Taillefer
Institut Charles Gerhardt Montpellier, ENSCM
8 rue de l’Ecole Normale, 34296 Montpellier (France)
E-mail: marc.taillefer@enscm.fr
[c]
Dr. A. Tlili
Institute of Chemistry and Biochemistry (ICBMS-UMR CNRS 5246)
Univ Lyon, Universitꢀ Lyon 1, CNRS
43 Bd du 11 Novembre 1918, 69622 Villeurbanne (France)
Prof. Fouad Ouazzani Chahdi
Laboratoire chimie organique appliquée LCOA, Faculté des Sciences
et Techniques, 30000 Fès, (Maroc)
Dr. J.-M. Sotiropoulos
[d]
[e]
CNRS/Univ. Pau & Pays Adour, Institut des Sciences Analytiques et
de Physico-Chimie pour l’Environnement et les Matꢀriaux, IPREM
UMR-CNRS 5254, Hélioparc 2 av. P. Angot, 64053 Pau (France)
Supporting information for this article is given via a link at the end of
the document.
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In contrast, the use of cesium or potassium carbonates did not
lead to significant amounts of 3aa (Table 1, entries 6 and 7) and
Although similar yields were obtained with phenylacetone and
deoxybenzoin, we decided to use the former from an atom
only marginal desired product formation was observed when DMF, economical perspective. Thereafter, the scope of the reaction with
1,4-dioxane and NMP were employed (Table 1, entries 810). In
the conditions of entry 5, chlorobenzene did not react and
bromobenzene only led to 20 % of biphenylmethane 3aa.
different phenylacetone derivatives was evaluated (Scheme 3). In
general, in the presence of iodobenzene, the reaction furnishes
the desired biarylmethanes in very good to excellent yields
whether the aryl ring of the phenylacetone is substituted with
electron-donating (R = Me, Et, OMe) or withdrawing groups (R =
F, CF3). In most of the examples the substituent is in para position
but a good yield was also obtained with a CF3 group in meta
position (3af, 72 %).
Table 1. Synthesis of diphenylmethane from iodobenzene and phenylacetone
using various solvents and bases.[a]
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
Base
KOtBu
NaOtBu
NaOH
KOH
Yield (%)[b]
1
2
23
41
3
42
4
51
5
CsOH
Cs2CO3
K2CO3
CsOH
CsOH
CsOH
70, 88[c,d]
6
5
7
<1
<1
5
8
9
1,4-Dioxane
NMP
Scheme 3. Synthesis of BAM from iodobenzene and various arylacetones.
Reaction conditions: 2 mmol of iodobenzene, 1 mmol of arylacetone derivative
and 3 mmol of CsOH at 110 °C. Yields of isolated products.
10
14
[a] Reaction conditions: 2 mmol of PhI, 1 mmol of phenylacetone, 3 mmol of
base, 2 mL of solvent at 90 °C for 20 h. [b] NMR yield calculated with 1,3-
dimethoxybenzene as internal standard. [c]110 °C. [d]A 55% yield was obtained
when using only 2 equiv. of CsOH.
We next decided to investigate the scope of suitable aryl
iodides (Scheme 4). When the electrophile is substituted with an
electron-donating group such as a methyl in ortho, meta or para
positions, the desired products were isolated in good to very good
yields (up to 78%) as mixtures of two or three regioisomers. In the
case of 3-iodotoluene, the meta BAM (3ba) was mainly obtained.
In the presence of an ortho-methoxy substituent the major product
was the corresponding ortho BAM and starting from the meta
iodoanisole the reaction was selective as the corresponding meta
BAM was quite exclusively formed (3ea). We were also glad to
observe the formation of single regioisomers from 1-fluoro-4-
iodobenzene (3ga) and benzotrifluoride derivatives of
iodobenzene (meta (3ha) and ortho positions (3ia)) with good to
very good isolated yields. It should be noted that starting from 4-
iodobenzonitrile the reaction almost quantitatively furnishes
biphenylmethane 3aa.
With the best conditions in hand, iodobenzene (2 equiv.),
phenylacetone (1 equiv.), CsOH (3 equiv.) and DMSO at 110 °C,
we decided to first evaluate the use of various carbonyl
compounds (Scheme 2). Although the reaction with
phenylacetaldehyde furnishes diphenylmethane in a low yield, the
use of acetophenone allows the formation of the desired product
3aa in 87% yield, along with 13% of deoxybenzoin as detected by
1H NMR. It should be noted that
a double -arylation,
debenzoylation sequence is occuring under these conditions and
that deoxybenzoin is a reaction intermediate. Furthermore, benzyl
aryl ketones have also been evaluated under these conditions.
The desired product was formed in an excellent yield of 89%
starting from simple deoxybenzoin. Benzyl aryl ketone derivatives
substituted with electron-donating groups were also tolerated and
afforded products 3ab and 3ac in good to very good yields.
Scheme 2. Screening of suitable carbonyl compounds . Reaction conditions: 2
mmol of PhI, 1 mmol of carbonyl derivative and 3 mmol of CsOH at 110 °C. 1H
NMR yield calculated with 1,3-dimethoxybenzene as internal standard.
Scheme 4. Synthesis of BAM from various aryl iodides and phenylacetone.
Reaction conditions: 2 mmol of aryl iodide, 1 mmol of phenylacetone and 3
mmol of CsOH at 110 °C. Yields of isolated products. [a] 90 °C.
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Next, we decided to investigate the synthesis of
biarylmethanes substituted on both aromatic cycles by starting
from different aryl iodides and benzyl ketones derivatives
(Scheme 5). The reaction encompasses electron-withdrawing
and -donating groups on the aryl iodides as well as on the benzyl
ketones. The yields are good to excellent (up to 99%) and in
general high selectivities were observed, whatever the nature of
the substituent tested. The observed selectivity for molecules
described in Schemes 4 and 5 is difficult to predict as illustrated
for example for the synthesis of 3da and 3de for which the simple
presence of a fluorine substituent on the benzylic ketone tips the
p/m ratio from 99/1 to 1/1.
pathways are at work: an aryne pathway [9] (Scheme 8, eq. 1) and
a SRN1 pathway (Scheme 8, eq. 2).[10] In cases when single
regioisomers of BAM are observed, the ipso-selective SRN
1
pathway is expected to be the only one at work, while an equal
mixture of regioisomers could arise from an exclusive aryne
pathway. All other cases could result from a competition between
the two mechanisms, electronic factors dictating the selectivity
outcome. In fact, it is interesting to note that DFT calculations
show that, whatever the group carried by the iodoarene (electron-
withdrawing or –donating - See SI), the radical form obtained by
iodine removal is always favoured by ~9-13 kcal/mol compared to
the benzyne form. Thus, considering the energetic gap between
the two forms, from a thermodynamic point of view both
approaches cannot be excluded. Naturally, the kinetic aspects of
these reactions must be considered in order to refine the overall
understanding of the results.
Scheme 8. Plausible reaction mechanisms illustrated in the case of meta
substituted aryl iodides: The aryl radical is favoured by ~9-13 kcal/mol in regards
to the benzyne form.
Scheme 5. Synthesis of BAM from various aryl iodides and arylacetones.
Reaction conditions: 2 mmol of aryl iodide, 1 mmol of arylacetone derivative and
3 mmol of CsOH at 110 °C. Yields of isolated products.
In conclusion, we demonstrated herein that the transition-metal-
free synthesis of biarylmethane derivatives occurs in the
presence of superbasic media (CsOH/DMSO) starting from aryl
iodides and benzyl ketones. The desired products are formed in
general in good to excellent yields with high selectivity (up to
>99/1). Preliminary mechanistic investigations suggest the
intervention of aryl radicals during the reaction process. Further
mechanistic investigations as well as the application of this
methodology to the synthesis of other interesting building blocks
are under way in our laboratory and will be reported in due course.
In order to confirm that deacetylation is the last step of the
BAM synthesis procedure, we investigated the reactivity of 1,1-
diphenylpropan-2-one in the presence of the superbase
(CsOH/DMSO) at 110 °C. Diphenylmethane was formed in nearly
quantitative yield (Scheme 6).
Experimental Section
Scheme 6. Deacetylation of 1,1-diphenylpropan-2-one with CsOH/DMSO.
Synthesis of diphenylmethane 3aa: To a flame-dried Schlenk flask was
added CsOH (504 mg, 3.0 mmol, 3.0 equiv). The flask was evacuated and
back-filled with argon 3 times, then dry DMSO (2 mL) was added via
syringe. Afterwards, iodobenzene (408 mg, 224 L, 2.0 mmol, 2.0 equiv),
and phenylacetone (134 mg, 133 L, 1.0 mmol, 1.0 equiv) were
successively added. The reaction mixture was stirred and heated at 110 oC
for 20 h. After allowing the reaction to cool to room temperature, 1N HCl
(2 mL) was added and the mixture was allowed to stir for 10 minutes at
room temperature. The resulting mixture was diluted with ethyl acetate (10
mL) and distilled water (5 mL). Following phase separation, the aqueous
layer was extrated with ethyl acetate (3 x 5 mL). The resulting organic layer
was washed with distilled water (1 x 2 mL) and brine (1 x 2 mL). The
organic layer was dried over anhydrous MgSO4 and evaporated under
reduced pressure (rotary evaporator). The residue was purified by column
chromatography (eluent: hexanes) to give diphenylmethane in 86%
isolated yield.
Furthermore, the effect of radical scavengers on the reaction
outcome has been evaluated (Scheme 7). When TEMPO
(2,2,6,6-tetramethylpiperidin-1-yl)oxy and Galvinoxyl were used,
the formation of diphenylmethane was completely suppressed.
Interestingly, the presence of a TEMPO adduct could be detected
in mass spectrometry but efforts to confirm its structure have
unfortunately failed.
Scheme 7. Effect of radical scavengers on the metal-free synthesis of
diphenylmethane.
Since we have observed varying ratios of regioisomers, it
seems plausible that two competing and complementary reaction
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The authors are thankful to the CNRS, ENSCM, ANR and NSERC
for financial support. M.P.D. thanks FRQNT and CGCC for
doctoral scholarships. We are grateful to Dr. Grégory Danoun and
Dr. Julien Bergès for preliminary results.
Keywords: CC coupling • Metal-Free • Biarylmethanes •
Superbase • Ketones
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COMMUNICATION
M. Pichette Drapeau, A. Tlili, Y. Zaid, D.
Toummini, F. Ouazzani Chahdi, J.-M.
Sotiropoulos, T. Ollevier, M. Taillefer *
Page No. – Page No.
Transition-Metal-Free Synthesis of
Biarylmethanes from Aryl Iodides and
Benzylic Ketones
Super Base for CC Coupling: The synthesis of biarylmethane derivatives is
performed herein under transition-metal-free conditions starting from a wide range of
aryl iodides and benzylic ketones. The key to success is the employment of the super
base (CsOH/DMSO). The desired products are obtained in very good yield and
selectivity.
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