HBF4- and AgBF4-Catalyzed ortho-Alkylation of Diarylamines and Phenols

Article abstract of DOI:10.1021/acs.orglett.9b02470

A silver-tetrafluoroborate- or HBF₄-catalyzed ortho-alkylation reaction of phenols and diarylamines with styrenes has been explored. A broad substrate scope is presented as well as mechanistic experiments and discussion.

Full text of DOI:10.1021/acs.orglett.9b02470

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
HBF4- and AgBF4‑Catalyzed ortho-Alkylation of Diarylamines and
Phenols
̈
̈
Christian K. Rank, Bunyamin Ozkaya, and Frederic W. Patureau*
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
S
* Supporting Information
ABSTRACT: A silver-tetrafluoroborate- or HBF4-catalyzed ortho-alkylation reaction of phenols and diarylamines with styrenes
has been explored. A broad substrate scope is presented as well as mechanistic experiments and discussion.
example,3a−c elegant methods were very recently reported by
odern hydroarylation methods are increasingly popular
Mfor the construction of C−C bonds. Indeed, some Caputo3a and independently by Li,3b which demonstrate the
use of a powerful and increasingly popular Lewis acid catalyst,
tris(perfluorophenyl)borane (Scheme 1, eq 2). We therefore
contemplated whether a redox approach might provide a
superior strategy, in particular, in terms of the ortho selectivity,
a persistent problem. We thus turned our attention to Ag(I)
salts as prospective catalysts.4 We considered, in particular,
elegant strategies have recently appeared that allow excellent
Markovnikov or anti-Markovnikov regioselectivity and broad
functional group tolerance.1 In 1999, Beller et al. reported the
case of a Rh(I)/HBF4 cocatalyzed system for the ortho-
alkylation of primary electron-rich anilines with styrene. For
the most electron-rich anilines (pKa of the corresponding
ammonium >5), it was even found that the reaction could
proceed without the Rh catalyst (Scheme 1, eq 1).2 This
brought us to wonder what it would take to bring this very
simple HBF4-catalyzed hydroarylation system to both lower
reaction temperatures and especially to broader and less
reactive substrate classes (lower basicity of the substrate; pKa
of the corresponding ammonium <2). With phenols, for
4
AgBF4 for poorly O- or N-basic phenol and diarylamine
substrates. Indeed, we anticipated that radical mechanisms5
might improve the reactivity and regioselectivity while
providing a cheaper and operationally simpler synthetic
method compared with perfluoro organo-boron Lewis acidic
catalysts (Scheme 1, eq 3). In parallel, we also re-explored
Beller’s control HBF4-catalyzed approach, without the rhodium
catalyst, to evaluate the impact of the redox-active Ag(I)
component. To our surprise, and in contrast with the
literature,2a we found that the considerably cheaper HBF4
catalyst (Scheme 1, eq 3) also performs admirably well in the
catalytic alkylation of anilines and phenols, with only small
differences. This study is therefore focused on both AgBF4 and
HBF4 catalysts and on related mechanistic considerations.
Phenothiazine was selected as a first convenient nonbasic
diarylamine test substrate, a compound known to easily
undergo radical oxidation.6 Phenothiazines are, moreover,
interesting scaffolds in some fields of organic materials7 as well
as essential bioactive compounds.8 Some optimization
elements are shown in Table 1. (See the SI for other
parameters such as solvent and temperature.) Importantly, it
was found that the reaction proceeds well in a number of very
diverse conditions, whether potentially radical (Table 1, entry
1), Brønsted-acid-catalyzed (entry 23), or Lewis-acid-catalyzed
(entry 24). For the phenothiazine test substrate, the AgBF4
catalyst (entry 1) delivered the highest yield of desired
Scheme 1. Introduction
Received: July 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02470
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Table 1. Reaction Optimization
Scheme 2. Phenothiazine Scope, Isolated Yields
a
catalyst
loading
1a/2a (mmol)
yield (%)
b
1
2
3
4
5
6
7
8
9
AgBF4
AgBF4
NaBF4
Ag2O
AgNO3
AgOAc
AgF
AgCl
AgBr
AgI
AgOTf
AgSbF6
AgSbF6
AgSbF6
CuCl2
AuCl3
PPh3AuCl
AgBF4
AgBF4
AgBF4
AgBF4
AgBF4
HBF4Et2O
PPh3AuX
10 mol %
5 mol %
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/0.75
0.5/1.00
0.5/1.00
0.5/1.00
0.5/1.00
0.5/0.5
90 (84)
77
0
0
trace
0
trace
0
0
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
5 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
10 mol %
20 mol %
10 mol %
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0
54
64 (63)
57
54
0
8
0
46
70
55
80
82
65
48
0.5/1
0.75/0.5
1/0.5
3/0.5
0.5/0.75
0.5/0.75
protonated or oxidized than phenothiazines, however,
necessarily implying higher activation energies and potentially
shorter-lived radical intermediates. Fortunately, simply increas-
ing the reaction temperature to, respectively, 80 and 100 °C
allowed the hydroarylation reaction to proceed under
otherwise altered starting material ratios. Elements of the
substrate scope are presented in Schemes 3 and 4, again with
very high ortho-alkylation selectivity.
c
d
a
Yields were determined by GC using n-dodecane as the standard
b
(isolated yield in parentheses). +15% of a mixture of bis-alkylated
products. +31% of a mixture of bis-alkylated products. X =
[N(CF3SO2)2].
c
d
There, too, we could not find or identify any para-
monoalkylated byproducts. In the case of product 5a, much
(monoalkylated) product. We moreover screened numerous
counterions (Table 1, entries 4−14), thereby demonstrating
the clear superiority of the tetrafluoroborate anion.
a
Scheme 3. Diarylamine Scope, Isolated Yields
With the AgBF4-catalyzed optimized conditions in hand
(Table 1, entry 1), we then screened a number of
phenothiazines and styrenes (Scheme 2). Interestingly, the
branched (Markovnikov) ortho (C1) alkylated product is
typically by far the major product. In some cases, small
amounts of bis-alkylated products can be observed (i.e., Table
1, entry 1); however, the first alkylation step seems to
consistently occur in the ortho position to the X−H functional
group (Scheme 2). This is moreover a synthetically interesting
regioselectivity outcome in light of the usual preference of
phenothiazine for C3-(para-) electrophilic aromatic substitu-
tion.9 This strong preference for the ortho-branched alkylated
product is in good agreement with the concerted mechanism
of Scheme 1. Even 1,1-and 1,2-disubstituted styrenes were
found to be competent hydroarylation substrates, albeit in
lower yields (3i, 43%; 3j, 38%). Acrylates, however, or
heterocyclic olefins such as vinylpyridines, did not afford any
hydroarylation product (Scheme 2).
With this first set of phenothiazine examples in hand, we
wondered whether noncyclic diarylamines and phenols (all
with lower basicity than the primary anilines of Beller)2 would
also be applicable. Diarylamines and phenols are less easily
a
Numbers in black are the yields with 10 mol % AgBF4; numbers in
red are the yields with 20 mol % HBF4.
B
DOI: 10.1021/acs.orglett.9b02470
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a
Scheme 4. Phenol Scope, Isolated Yields
Scheme 5. Mechanistic Experiments, Isolated Yields
a
Numbers in black are the yields with 10 mol % AgBF4; numbers in
red are the yields with 20 mol % HBF4.
of the excess of the diarylamine substrate 4a could be
recovered and reisolated (1.97 mmol; see the SI), which seems
to be a general trend when examining the various crude
products presented herein. In contrast, none of the limiting
coupling partners is ever reisolated, indicating the full
conversion and probable decomposition of the missing mass
balance. Importantly, we noted a superior isolated yield with
the simple Brønsted HBF4 catalyst in almost all diarylamine
cases (Scheme 3, red yields in parentheses).
We then performed a series of mechanistic experiments to
probe some of the possible scenarios, in particular, with the
ambiguous AgBF4 catalyst. First, N-methyl-phenothiazine does
not provide any hydroarylated product (Scheme 5, eq 4), thus
confirming the requirement for a heteroproton ortho to the
functionalized C−H bond. This is strong evidence that the
concerted protonation/C−C bond-formation hypothesis
postulated by Beller (Scheme 1) is probably also important
with the AgBF4 catalyst. Second, the presence of TEMPO, a
typical radical scavenger, does not allow the reaction to
proceed (eq 5). TEMPO might either inhibit radical chains or
alternatively reduce the Ag(I) catalyst toward the piperidi-
nium-2,2,6,6-tetramethyl-1-oxo-tetrafluoroborate salt, which
would, in turn, no longer be a competent oxidant for initiating
the catalytic cycle. Furthermore, labeled phenol-d6 was
engaged in the hydroarylation reaction, yielding a 25% D-
enriched branched methyl group in the coupling product (eq
6). This corresponds to a 76% deuteron transfer efficiency and
therefore also supports the ortho-concerted mechanism of
Scheme 1. It could be noted that the deviation from the
theoretical 33% deuterium content at the methyl group (full
deuteron transfer efficiency) may come from either the
integration approximation of the corresponding 1H NMR
experimental profile or traces of water contamination in some
of the components, which might lead to rapid OD/OH
scrambling. We then compared the initial reaction rate
between labeled phenol-d6 and natural abundance phenol in
a competition experiment, yielding an initial kinetic isotope
effect (KIE) of 1.4 (eq 7). This may indicate that C−H bond
cleavage is not rate-limiting, in contrast with the prior
concerted C−C bond-formation step. Moreover, interestingly,
when measuring the initial KIE between phenol and phenol-d6
in two parallel reactions, a somewhat higher KIE of 2.4 was
observed under otherwise identical conditions. This suggests
that the cyclic concerted C−C bond-forming step and proton/
deuteron oxygen-to-carbon transfer may indeed be rate-
significant. Finally, to probe the suspected radical character
of the AgBF4-catalyzed reaction, we performed a final control
experiment in which the speculated catalytic electron hole is
generated by a nonmetallic single electron oxidant (eq 8). For
this purpose, we selected the NOBF4 salt as the nonmetallic
catalytic electron hole generator because it possesses the same
counterion as our AgBF4 precatalyst and because it is reputed
to possess a similar (slightly superior) redox potential as well.10
To our surprise, when we indeed replaced the catalytic
AgBF4 salt with the same catalytic amount of NOBF4 salt (10
mol %) in the alkylation of diphenylamine under otherwise
unaltered reaction conditions (Scheme 3), we isolated almost
exactly the same amount of hydroarylated product 5a (65 vs
66%, respectively, eq 8). This result, in combination with the
TEMPO poisoning experiment of eq 5, indicates that an
electron-hole-catalyzed pathway is possible in the case of
AgBF4. This is moreover in line with the usual observation of
shiny Ag0 particles in suspension in the crude product
mixtures. The fact that HBF4 and a cationic gold species are
also competent catalysts (Table 1, entries 23 and 24)
nevertheless suggests that the various mechanistic scenarios
C
DOI: 10.1021/acs.orglett.9b02470
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Letter
considered herein are not necessarily mutually exclusive,11
especially if partial in situ hydrolysis of the AgBF4 would take
place to generate active HBF4. These scenarios are summarized
in Scheme 6.
ACKNOWLEDGMENTS
■
The DFG-funded transregional collaborative research center
SFB/TRR 88 “Cooperative effects in homo and heterometallic
complexes” (http://3MET.de), DFG-funded project PA 2395/
2-1, and ERC project 716136: “2O2ACTIVATION” are
acknowledged for financial support. We also thank Philipp
Kramer and Prof. G. Manolikakes for technical help. This work
Scheme 6. Possible ortho-Selective Transition States
̈
was started at the Technische Universitat Kaiserslautern and
finished at the RWTH Aachen University.
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cheapest herein studied catalyst, HBF4, we scaled up the
synthesis of new compound 5a on a multigram level. We were
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Scheme 7).
Scheme 7. Scale-Up of 5a, Isolated Yield
In conclusion, we have developed a AgBF4- and HBF4-
catalyzed alkylation method of phenothiazines, diarylamines,
and phenols. These methods allow the alkylation of
considerably less basic anilines and phenols compared with
previous methods,2 with moreover excellent ortho-selectivity.
Several mechanistic pathways were identified depending on the
reaction conditions: Brønsted acid catalysis, Lewis acid
catalysis, and also electron hole catalysis. The proximal XH
functional group was found to be essential for reactivity and
ortho regioselectivity through a characteristic concerted
protonation/C−C bond-formation pathway. The herein
presented reactivity elements are expected to complement
the hydroarylation/alkylation toolbox.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02470.
Experimental procedures and characterization of new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Frederic.Patureau@rwth-aachen.de.
ORCID
Frederic W. Patureau: 0000-0002-4693-7240
Notes
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(b) Wang, G.; Gao, L.; Chen, H.; Liu, X.; Cao, J.; Chen, S.; Cheng,
The authors declare no competing financial interest.
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