Catalytic Hydroarylation of Alkenes with Phenols using B(C6F5)3

Article abstract of DOI:10.1021/acs.organomet.8b00621

We demonstrate that tris(pentafluorophenyl)borane, B(C₆F₅)₃, is shown to be an effective catalyst for the hydroarylation of olefins to yield substituted phenols. This system features fast reaction times, mild conditions, and good yields for a select scope of olefinic substrates and various phenols, resulting in C-C bond formation. Experimental data support two possible mechanisms, where the Lewis acid can activate either the olefin or the phenol as the first step in the catalytic mechanism.

Full text of DOI:10.1021/acs.organomet.8b00621

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Catalytic Hydroarylation of Alkenes with Phenols using B(C6F5)3
Jordan N. Bentley and Christopher B. Caputo*
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada
S
* Supporting Information
ABSTRACT: We demonstrate that tris(pentafluorophenyl)borane, B-
(C6F5)3, is shown to be an effective catalyst for the hydroarylation of
olefins to yield substituted phenols. This system features fast reaction
times, mild conditions, and good yields for a select scope of olefinic
substrates and various phenols, resulting in C−C bond formation.
Experimental data support two possible mechanisms, where the Lewis
acid can activate either the olefin or the phenol as the first step in the catalytic mechanism.
et al. has shown that an anti-Bredt bis(amino)carbene cationic
Au(I) complex can tolerate Lewis bases, catalyzing the
hydroarylation of alkenes with diarylamines at >135 °C
(Scheme 1).4 However, the recent renaissance in main-group
chemistry and the discovery of frustrated Lewis pairs have
provided effective alternatives to transition-metal catalysts.5
Strong Lewis acids have been used independently, or in
frustrated Lewis pair (FLP) combinations, to promote C−C
bond formation.6
Nevertheless, the transition-metal-free hydroarylation of
olefins has been less studied. Niggemann and colleagues have
reported the calcium-catalyzed hydroarylations of alkenes using
Ca(NTf2)2.7 Heterogeneous aluminum chlorofluoride (ACF)
has also been shown to promote the hydroarylation of olefins
with arenes. The substrates described in this work did not
contain Lewis basic functionalities.8 Stephan and co-workers
have shown that highly reactive electrophilic phosphonium
cations can catalyze the hydroarylation of olefins or alkynes
with diarylamines under mild conditions (Scheme 1).9 In these
cases the steric bulk of the diarylamine prevents adduct
formation of the amine Lewis base with the Lewis acid catalyst,
allowing for activation of the olefin. In 2017, Werner and co-
workers illustrated the use of B(C6F5)3 as a catalyst for the
Michael reaction between arene C−H bonds and electron
deficient α,β-unsaturated carbonyl compounds at 80 °C.10 In
these reactions, the arenes were limited to N,N′-disubstituted
anilines, indoles, and furans. Very recently, Yuan, Yao, and co-
workers have utilized [Ph3C][B(C6F5)4] as a catalyst for the
hydroarylation of sterically encumbered anilines with a series
of olefins at elevated temperatures.11
he significant growth in main-group-catalyzed reactions
Thas brought focus on pushing the boundaries into new
reactivity. The formation of alkyl-substituted arenes can be
traditionally achieved through Friedel−Crafts substitution
reactions (electrophilic aromatic substitution).1 These reac-
tions generally exploit stoichiometric or catalytic amounts of a
Lewis acid to proceed. Another atom-economical route toward
the desired C−C bond formation is through the reductive
coupling of olefins with arenes via hydroarylation. This
reaction results in the addition of an aromatic C−H bond
across an unsaturated substrate, but Lewis acid catalysts limit
application to weakly Lewis basic substrates. Transition-metal
catalysts have been used to promote this reaction.2 For
example, Beller and co-workers described the use of FeCl3 as a
catalyst for the hydroarylation of styrenes at elevated
temperatures.3 Tunge et al. reported that catalytic TiCl4 can
promote the formation of dihydrocoumarins via a hydro-
arylation−lactonization reaction from phenols and benzylidene
malonic esters (Scheme 1).2c One recent example by Bertrand
Scheme 1. Examples of Lewis acid Catalyzed Hydroarylation
Alcohols, while less Lewis basic than amines, may still
interact with Lewis acids, and main-group-catalyzed hydro-
arylation reactions with phenol have been sparsely reported.
Catalytic reactions developed for these combinations have
predominately focused on the hydroalkoxylation reaction, the
addition of the O−H bond across the olefin.12 One report
from scientists at Bayer in 1957 reported the use of aluminum
Received: August 28, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.8b00621
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Communication
a
with phenol at >150 °C and high pressures, resulting in the
hydroarylation of alkenes, presumably through an Al(OPh)3
catalyst.13 The aforementioned examples of main-group Lewis
acid catalyzed hydroarylation reactions generally are done at
higher temperatures, where lower yields are reported at room
temperature, presumably attributed to adduct formation with
the Lewis basic arene. Still, it has been shown through
frustrated Lewis pair chemistry that an equilibrium exists
between Lewis acid/base adduct and free components, such as
ethers and B(C6F5)3, which can be exploited to activate small
molecules.14 This led us to believe that similar reactivity may
be possible with phenols. Herein, we describe the use of
B(C6F5)3 as a hydroarylation catalyst to achieve C−C bond
formation between olefins and phenols at room temperature.
The reaction between phenol and B(C6F5)3 in CDCl3
strongly implied that a weak, reversible adduct was being
formed. Using 10 mol % of B(C6F5)3, the 19F NMR spectrum
of the reaction mixture exhibited three resonances at −129.7,
−147.2, and −160.9 ppm with a para−meta gap of Δp-m =
13.7 ppm, indicating adduct formation with phenol (free
B(C6F5)3 Δp-m = 17.4 ppm).6c Nevertheless, only one species
Scheme 2. Substrate Scope for Catalytic Hydroarylation
1
was observed in the H NMR spectrum, implying that a rapid
1
and reversible exchange is occurring. A 1D H−19F HOESY
experiment was undertaken on the reaction mixture and
showed through-space correlation between the O−H and
ortho C−H protons on the phenol with the borane, supporting
that a reversible adduct was forming. No C6F5H was observed,
even after 48 h, implying that protodeboronation, a common
degradation pathway for B(C6F5)3, was not occurring. We also
observed analogous spectral features for mixtures of olefins and
a
Conversions were calculated by 1H NMR integration. Isolated yields
are reported in parentheses. A catalyst loading of 10 mol % was found
to be optimal; a lower catalyst loading resulted in a decrease in
conversion.
1
B(C6F5)3, where no difference was evident in the H NMR
spectrum but signals in the 19F NMR spectrum broadened and
shifted from those of the free Lewis acid (see the Supporting
Information). These data are corroborated by previous reports
that olefins can weakly interact with Lewis acidic boranes.15
The respective equilibria observed for both phenol and
olefins with B(C6F5)3 prompted us to investigate the feasibility
of the borane to activate the olefin for hydroarylation with
phenols. We initially probed the reaction of phenol with 1,1-
diphenylethylene in the presence of 10 mol % of B(C6F5)3.
This reaction rapidly produced the para-substituted hydro-
arylated product 4-(1,1-diphenylethyl)phenol (1) in 98% yield
after 3 h at 25 °C (Scheme 2), as evidenced by the
disappearance of the olefin resonance in the 1H NMR
spectrum and the appearance of two apparent doublets at
7.04 and 6.77 ppm, respectively, indicating a para-substituted
aromatic ring. Strong Lewis acids have previously been shown
to catalyze the Friedel−Crafts dimerization of 1,1-diphenyl-
ethylene.16 Nonetheless, in the presence of phenol this product
is not observed, suggesting that the weak phenol adduct with
the borane prevents dimerization.
ppm and the formation of three new resonances corresponding
to the reduced acenaphthene C2H3Ar fragment at 5.16, 3.98,
and 3.39 ppm. All three peaks have complex multiplicities,
indicating significant coupling to protons on the aromatic rings
and the diastereotopic nature of the CH2 protons (see the
Supporting Information). A control experiment with 10 mol %
of B(C6F5)3 and acenaphthylene resulted in immediate
1
broadening of all peaks in the H NMR, yet free B(C6F5)3
was observed in the 19F NMR spectrum. The sample was
analyzed by variable- temperature NMR to determine whether
or not this was a rapid equilibrium; however, even at −40 °C,
no evidence of coalescence was observed. Finally, norbornene
also underwent hydroarylation with phenol, resulting in a
mixture of para- and ortho-substituted products 11a,b in 58%
and 39% yields, respectively, after 3 h. Interestingly, trans-
stilbene does not show any reactivity, nor do more highly
substituted aryl olefins, as observed with triphenylethylene.
Simple aliphatic alkenes such as cyclohexene and 1-hexene also
do not react under these conditions or at elevated temper-
atures.
We endeavored to explore the olefin substrate scope, and in
a similar fashion, phenol reacts with α-methylstyrene, affording
the para-substituted phenols and a minor amount of
polymerized products. The hydroarylated product could be
isolated in good yield (90%; 2). Chlorinated styrenes also
undergo reactivity, with the hydroarylated product 4-(2-(4-
chlorophenyl)propan-2-yl)phenol (5) isolated in 88% yield.
Strained aromatic substrates also undergo hydroarylation
reactivity. Acenaphthylene will rapidly react with phenol
under these conditions to yield the hydroarylated product 6
To further investigate the scope of the reaction, we explored
a series of para-substituted phenols. 4-Methylphenol, 4-tert-
butylphenol, and 4-methoxyphenol undergo catalytic hydro-
arylation under these conditions, resulting in products 3, 4, 7,
10, 12, and 15 (Scheme 2). Unexpectedly, phenols containing
a deactivating fluorine or bromine substituent still underwent
hydroarylation with acenaphthylene or norbornene with
varying yields (19−69%), resulting in their respective ortho-
substituted products 8, 9, 13, and 14 (Scheme 2).
1
in 76% isolated yield. This is very evident in the H NMR
spectrum through depletion of the olefinic resonance at 7.07
B
DOI: 10.1021/acs.organomet.8b00621
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The selective hydroarylation of olefins with phenols was
initially surprising because B(C6F5)3 is an effective catalyst for
hydroelementation reactions, including hydrosilylation, hydro-
amination, and hydrothiolation.17 Thus, we sought out further
studies to explore the mechanism of the hydroarylation
reaction. We hypothesized that the reaction could proceed
via two possible mechanisms (Scheme 3). The Lewis acidic
shown to readily undergo intramolecular hydroalkoxylation,
resulting in the cyclized dihydrobenzofuran.21 Nevertheless,
under our reaction conditions no appreciable yield of the
cyclized product was observed (Scheme 4, top). Additionally,
Scheme 4. Hydroalkoxylation Attempts Using 2-Allylphenol
(Top) and 1-Naphthol Isomerization Mediated by B(C6F5)3
(Bottom)
Scheme 3. Proposed Potential Hydroalkoxylation (Top)
and Observed Hydroarylation (Bottom) Mechanisms
we screened the ability of triflic acid to act as a catalyst for the
model reaction between norbornene and p-methoxyphenol,
and these results confirmed that B(C6F5)3 and Brønsted acid
catalysts produce different products (see the Supporting
Information).
Finally, we explored the use of phenol-d in the reaction. The
preparation of phenol-d also results in the partial deuteration
2
of the para and ortho positions, as observed in the H NMR
spectrum.22 The deuterated analogue of 1 was prepared from
phenol-d, showing substitution at only the para position of the
1
2
phenol as evidenced by the H, H, and 13C NMR spectra.
2
Only two resonances were observed in the H NMR at 6.72
and 2.09 ppm, corresponding to the o-C−D and the CH2D
resonances, respectively (see the Supporting Information).
These results imply that protonation of the olefin may be
occurring; however, the disappearance of the O−D resonance
at 5.64 ppm is not unprecedented. Erker et al. have reported
that B(C6F5)3 can promote the formation of the keto isomer of
1-naphthol (Scheme 4, bottom);23 thus, the O−D may be
scrambled into the phenol ring and this implies that the
mechanism may be more complicated than was initially
presumed.
In summary, we have shown that B(C6F5)3 is an effective
catalyst for the reductive hydroarylation of olefins with
phenols. Preliminary experiments indicate that the reaction
mechanism could proceed via activation of the olefin or phenol
by the Lewis acid with a subsequent Friedel−Crafts C−C bond
formation step. Ongoing efforts are underway in our laboratory
to further investigate this reaction to fully elucidate the
mechanism and expand the scope.
borane could activate the phenol (A) and protonation of the
olefin could occur (B) followed by a Friedel−Crafts C−C
bond formation (C) and proton migration (D) to release the
catalyst and product. Alternatively, the borane could activate
the olefin (E), allowing for rapid Friedel−Crafts C−C bond
formation (F), followed by proton migration to release the
product and regenerate the catalysts.
We investigated the reactions between 1,1-diphenylethylene
with both 2,4,6-trimethylphenol and anisole under standard
catalytic conditions. Unsurprisingly, no reaction was observed
with 2,4,6-trimethylphenol, as the ortho and para positions in
the aromatic ring are blocked to substitution. Interestingly, no
reaction occurred with anisole; only the Friedel−Crafts
cyclodimerized olefin was observed. This unexpected result
raised the possibility that the O−H functionality can act as a
Brønsted acid in the catalytic cycle. Further, Parkin et al. had
shown that the pKa values of aqua and alcohols increase upon
coordination to B(C6F5)3.18 However, Hartwig and He
reported that Brønsted acid catalysts mediate the hydro-
alkoxylation reaction between phenols and olefins.19
In order to further probe the role of the hydroxyl
functionality, a series of alcohols were explored under the
standard catalytic conditions with various olefins. Nevertheless,
no conversion was observed when aliphatic alcohols, such as
tert-butyl alcohol, benzyl alcohol, and 1,1-diphenylmethanol,
were used. Furthermore, hexafluoroisopropyl alcohol, which
has a pKa similar to that of phenol (9.3 vs 9.98),20 also does
not undergo any reaction. In these reactions, it was clear by
multinuclear NMR analysis that an adduct between the alcohol
and the borane was being formed. 2-Allylphenol has been
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00621.
■
S
Experimental details, characterization data of all
compounds, and multinuclear NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.B.C.: caputo@yorku.ca.
■
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DOI: 10.1021/acs.organomet.8b00621
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Organometallics
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(8) Calvo, B.; Wuttke, J.; Braun, T.; Kemnitz, E. Heterogeneous
Catalytic Hydroarylation of Olefins at a Nanoscopic Aluminum
Chlorofluoride. ChemCatChem 2016, 8, 1945−1950.
ORCID
Christopher B. Caputo: 0000-0002-7523-0324
Notes
The authors declare no competing financial interest.
́
(9) (a) Perez, M.; Mahdi, T.; Hounjet, L. J.; Stephan, D. W.
Electrophilic Phosphonium Cations Catalyze Hydroarylation and
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(b) LaFortune, J. H. W.; Bayne, J. M.; Johnstone, T. C.; Fan, L.;
Stephan, D. W. Catalytic Double Hydroarylation of Alkynes to 9,9-
Disubstituted 9,10-Dihydroacridine Derivatives by an Electrophilic
Phenoxyphosphonium Dication. Chem. Commun. 2017, 53, 13312−
13315.
ACKNOWLEDGMENTS
■
The authors dedicate this paper to Professor Douglas W.
Stephan in celebration of his 65th birthday. The authors
gratefully acknowledge financial support from the NSERC of
Canada and the Canadian Foundation for Innovation. C.B.C. is
grateful for the award of a Canada Research Chair. The authors
thank Professors Thomas Baumgartner and Barry Lever for the
use of laboratory space and equipment while our laboratory
was being constructed.
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