Branch-selective alkene hydroarylation by cooperative destabilization: Iridium-catalyzed ortho-alkylation of acetanilides

Article abstract of DOI:10.1002/anie.201506581

An iridium(I) catalyst system, modified with the wide-bite-angle and electron-deficient bisphosphine d^Fppb (1,4-bis(di(pentafluorophenyl)phosphino)butane) promotes highly branch-selective hydroarylation reactions between diverse acetanilides and aryl- or alkyl-substituted alkenes. This provides direct and ortho-selective access to synthetically challenging anilines, and addresses long-standing issues associated with related Friedel-Crafts alkylations. An iridium(I) catalyst system modified with the wide-bite-angle and electron-deficient bisphosphine d^Fppb (1,4-bis(di(pentafluorophenyl)phosphino)butane) promotes highly branch-selective hydroarylation reactions between diverse acetanilides and aryl- or alkyl-substituted alkenes. This provides direct and ortho-selective access to synthetically challenging anilines.

Full text of DOI:10.1002/anie.201506581

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International Edition: DOI: 10.1002/anie.201506581
German Edition: DOI: 10.1002/ange.201506581
Hydroarylation
Branch-Selective Alkene Hydroarylation by Cooperative
Destabilization: Iridium-Catalyzed ortho-Alkylation of Acetanilides
Giacomo E. M. Crisenza, Olga O. Sokolova, and John F. Bower*
Abstract: An iridium(I) catalyst system, modified with the
wide-bite-angle and electron-deficient bisphosphine dFppb
(1,4-bis(di(pentafluorophenyl)phosphino)butane) promotes
highly branch-selective hydroarylation reactions between
diverse acetanilides and aryl- or alkyl-substituted alkenes.
This provides direct and ortho-selective access to synthetically
challenging anilines, and addresses long-standing issues asso-
ciated with related Friedel–Crafts alkylations.
A
nilines are privileged building blocks for medicinal
chemistry and materials science,[1] and many methods have
been developed to access substituted derivatives.[2–7] How-
ever, a long-standing deficiency resides in the lack of general
procedures for the ortho-selective introduction of branched
alkyl substituents. Palladium-catalyzed cross-couplings of
secondary alkyl organometallics are not well suited to this
task, partially because of competitive isomerization after
transmetalation, which can lead to linear adducts.[8] ortho-
Selective Friedel–Crafts reactions are an appealing approach,
which, in practice, is effective only in certain simple cases.[9]
Well established problems associated with controlling the site
and extent of alkylation usually predominate, and competitive
coordination of the acid catalyst to the aniline nitrogen atom
means that, where feasible, harsh reaction conditions are
required.[9] Consequently, a method that addresses these
issues by providing direct and mild access to ortho-branched
anilines is likely to have widespread application.
Scheme 1. Branch-selective hydroarylation by “cooperative destabiliza-
tion” and outline of this work. BARF=tetrakis(3,5-bis(trifluoromethyl)-
phenyl)borate, cod=1,5-cyclooctadiene, dFppb=1,4-bis(di(penta-
fluorophenyl)phosphino)butane.
Recently, we reported a method that overturns the linear
selectivity of Murai-type hydroarylations[10,11] to provide
access to branched adducts 1b (Scheme 1a).[12,13] All steps
up to iridium(III)–alkyl intermediates 3a and 3b are fast and
reversible, with linear adduct 3a likely favored on steric
grounds. Our “cooperative destabilization” strategy employs
a novel bisphosphine, dFppb, with a wide bite angle to
increase bond angle y and compress angles xa/xb, thereby
enhancing steric destabilization of 3a/3b.[12] Destabilization is
most acute for 3b, as this has a bulkier secondary alkyl ligand,
and consequently, reductive elimination by path b is amplified
to provide branched products 1b at the expense of linear
isomers 1a.[14] This process employs weakly coordinating
carbonyl directing groups, and tolerates both aryl- and alkyl-
substituted alkenes. Related branch-selective hydroarylation
methods invariably require strongly coordinating N-based
directing groups and are limited to styrenes[15a–d] or, more
recently, enol ethers as the olefinic partner.[15e,16,17] In this
report, we extend our strategy to the branch-selective ortho-
alkylation of acetanilides (Scheme 1b).[18] Significantly, this
work expands our approach to encompass a) electron-rich
arenes, and b) inherently more demanding six-ring metalla-
cycles (2 vs. 4). Indeed, to the best of our knowledge, this
study outlines the first intermolecular branch-selective
Murai-type alkene hydroarylations that proceed via six-ring
chelates. In combination with earlier work,[12] these results
suggest that a unified approach to branch-selective alkene
hydroarylation is achievable and underpin ongoing efforts
towards enantioselective variants.
[*] G. E. M. Crisenza, O. O. Sokolova, Dr. J. F. Bower
School of Chemistry, University of Bristol
Bristol, BS8 1TS (United Kingdom)
E-mail: john.bower@bris.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506581.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
Preliminary studies involved exposing acetanilide 5a and
styrene to an IrI system derived from [Ir(cod)2]BARF and
dFppb (Table 1). At 1208C in dioxane, adduct 6a was formed
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Table 2: Aniline scope.[a]
Table 1: Selected optimization results.
Entry
R
X
y [mol%] Solvent T [8C] t [h] Yield [%][a]
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
BARF 200
BARF 200
BARF 200
BARF 200
dioxane 120
dioxane 100
24
24
24
72
24
24
24
24
24
24
24
24
34
17
dioxane
80
<5
dioxane 120
dioxane 120
dioxane 120
dioxane 120
1,2-DCB 120
1,2-DCB 120
22[b]
BF4
OTf
OTf
OTf
200
200
450
200
32
85
85
61
BARF 200
72
10
11
12
OTf
OTf
200
200
200
xylene
PhCl
120
120
58[b]
60[b]
CH3 OTf
dioxane 120
<5
[a] Yields of isolated products unless stated otherwise, >25:1 branched/
1
linear in all cases. [b] Determined by H NMR analysis with 1,3,5-
trimethoxybenzene as the internal standard. Tf =trifluoromethanesul-
fonyl.
in 34% yield and with complete branch selectivity (entry 1).
Lower reaction temperatures or longer reaction times
resulted in diminished yields of 6a (entries 2–4). However,
a strong dependence upon the Ir counterion was observed,
and increased yields were achieved using more strongly
associating variants. This resulted in the conditions in entry 6,
which deliver 6a in 85% yield and, importantly, with
complete branch selectivity and full selectivity for mono-
ortho-alkylation. Higher loadings of styrene offered no
appreciable benefit (entry 6 vs. entry 7). Solvents other than
dioxane can be employed, but marginally lower yields of 6a
were obtained (entries 8–11). Interestingly, for reactions run
in 1,2-dichlorobenzene (1,2-DCB), the BARF counterion was
superior to triflate (entry 8 vs. entry 9). The secondary
acetamide directing group of 5a is crucial to the process
and the corresponding N-methylated derivative (entry 12) as
well as carbamate-, tosyl-, formyl-, and pivaloyl-protected
substrates were all ineffective.[19]
The scope of the aniline component is outlined in Table 2.
Hydroarylation of styrene with ortho-substituted derivatives
5b–5e provided target compounds 6b–6e in high yield and
with complete branch selectivity in all cases. For optimal
efficiencies, fine-tuning of the styrene loading was required on
a case by case basis. For example, hydroarylation to afford 6b
occurred in only 54% yield with 200 mol% styrene, but
a 73% yield was achieved using 450 mol%. The tolerance to
ortho-substitution contrasts our earlier work with aryl
ketones and benzamides, where analogous processes were
not feasible.[12] Lower loadings of styrene can be used for
substrates with electron-donating groups at the C3 position.
For example, 6c and 6d were both generated in excellent
yield using just one equivalent of the alkene. Aniline
[a] In all cases, branched/linear >25:1.
to > 25:1 ortho-regioselectivity); the structures of 6h and iso-
6h were determined by single-crystal X-ray diffraction.[20] For
5i and 5j, hydroarylation was moderately selective for the
[21]
À
ortho C H bond adjacent to the heteroatom substituent.
para-Substituted anilines 5k and 5m participated smoothly,
and products 6k and 6m were formed in good yield.
Conversely, hydroarylation using para-trifluoromethyl deriv-
ative 5l was not efficient, and adduct 6l was formed in 21%
yield. Complete branch selectivity and complete selectivity
for mono-ortho-alkylation (> 95:5 mono/bis) were observed
for 6 f–6m.
We have also examined the scope of the alkene compo-
nent using acetanilide 5 f, and, again, complete branch
selectivity was achieved in all cases (Table 3). Electronically
diverse styrenes are well tolerated, and the target compounds
7a–7e were formed in moderate to quantitative yield, with
complete selectivity for mono-alkylation at the less hindered
ortho-site. Processes involving alkyl-substituted alkenes
required separate optimization. Changing the precatalyst
counterion from triflate to BARF and switching the solvent
from dioxane to 1,2-DCB provided a system that delivered
targets 7 f–7i in 33–99% yield, albeit with 600 mol% of the
alkene component. For 7g, propylene gas was delivered at
atmospheric pressure to introduce the ortho-isopropyl moiety
in 92% yield. Isopropyl groups are challenging to install using
Pd-catalyzed cross-couplings,[8a] and the present method
provides a direct and atom-economic alternative. Sterically
À
derivatives 5 f–5j have two different ortho C H bonds
available, and regioselectivity is strongly influenced by the
meta-substituent. For 5 f–5h, hydroarylation occurred prefer-
entially at the less hindered site to afford adducts 6 f–6h (4:1
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Table 3: Alkene scope.
outlined in the Supporting Information. The X-ray structures
of 6h and iso-6h[20] show that the secondary alkyl substituent
of the products causes the acetamide moiety to twist from the
plane of the arene, such that directed insertion of IrI into the
À
remaining ortho C H bond is challenging; consequently, bis-
ortho-alkylation is not observed. This effect must be finely
balanced given that ortho-substituted acetanilides 5b–5e
participate smoothly in the hydroarylation reaction.
A key feature of the processes described here is the use of
the wide-bite-angle and electron-deficient bisphosphine
ligand dFppb. The branched/linear selectivity for 5a to 6a/
iso-6a has been evaluated as a function of ligand bite angle
(Scheme 3).[22] A progression from low linear to complete
[a] In all cases, branched/linear and ortho-selectivity >25:1.
[b] 7.5 mol% of [Ir(cod)2]BARF and dFppb.
demanding alkenes are challenging, and the conversion of 5 f
into 7h occurred in only 33% yield; however, even here,
branch selectivity was maintained.
The mechanism of the hydroarylation process is likely
analogous to that outlined in our earlier work (Scheme 1a).[12]
Hydroarylation of [D2]-8 with aniline 5 f delivered deuterated
7c, in which deuterium incorporation at both the methyl and
methine positions indicates reversible alkene hydrometala-
Scheme 3. Ligand effects for the hydroarylation of styrene with 5a.
À
tion prior to product-determining C C bond formation
(Scheme 2). The lack of deuterium incorporation at the
branch selectivity is observed as the ligand is varied from
dFppm to dFppb. Although a strong bite-angle effect is
evident, a significant electronic influence is also operative.
Non-fluorinated ligands, namely dppm, dppe, dppp, and dppb,
show the same bite-angle trend, but provide lower branch
selectivities. One explanation is that secondary alkyl ligands
are better able to stabilize a more electron-deficient Ir center,
and so the equilibrium branched/linear ratio of the alkyl–IrIII
intermediates (see 3b vs. 3a) increases when fluorinated
ligands are used.[23,24] Another option is that electron-
deficient ligands a) shorten the iridium–alkyl bond by
enhancing s-donation, and b) shorten the iridium–phosphine
bonds by increasing p-backbonding.[25] This results in a con-
traction of the coordination sphere to provide a more
congested environment, such that steric destabilization of
the branched alkyl–IrIII intermediate is amplified further, and
its propensity for reductive elimination increases.
Scheme 2. Deuterium labeling and exchange experiments.
À
C6 position suggests that C H insertion of the Ir catalyst is, in
this case, selective for the more sterically accessible ortho
À
The substituted aniline products enable access to a wide
range of challenging bicyclic heteroaromatic compounds
(Scheme 4). Pd-catalyzed ortho-bromination of 6a, which
was prepared on gram scale, delivered 10 in 88% yield and
excellent selectivity.[26] Pd-catalyzed reaction of 10 with ortho-
toluidine provided benzimidazole 11 in 89% yield.[27] In this
C H bond. Indeed, exposure of aniline 5 f to the Ir system in
the absence of the alkene, but in the presence of D2O, resulted
in 92% deuterium incorporation at the C6 position and in
< 5% at the C2 position; further exchange experiments are
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[2] For a recent review on the Buchwald – Hartwig amination, see:
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[5] For a review that encompasses C H amination approaches to
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À
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Scheme 4. Product derivatizations. dba=dibenzylideneacetone,
CAN=cerium ammonium nitrate, NBS=N-bromosuccinimide,
p-TSA=para-toluenesulfonic acid.
[8] Examples that circumvent this problem are not applicable to
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approach, the N-acetyl group is incorporated into the
heteroaromatic target. Other transformations required con-
version into aniline 12.[28] This intermediate underwent
condensation with ethyl acetoacetate, and subsequent Pd-
catalyzed oxidative cyclization delivered indole 13 in 52%
yield (over 2 steps).[29] Classical methods towards heteroar-
omatic compounds are also effective. For example, the Cohn
variant of the Skraup quinoline synthesis delivered 14 in 68%
yield.[30] The processes in Scheme 4 validate concise and
diversifiable entries to heteroaromatic targets that might be
difficult to prepare by other means.
To conclude, we outline a direct and controlled approach
to ortho-branched aniline derivatives, which addresses long-
standing issues associated with related Friedel–Crafts alkyla-
tions. More fundamentally, this work extends our “coopera-
tive destabilization” strategy[12] to include processes that
involve electron-rich arenes and proceed via six-ring metal-
lacycles. Both aspects represent a significant expansion to the
emerging area of branch-selective Murai-type hydroaryla-
tions.[12,15] The catalyst design features used here will guide
efforts in our laboratory aimed at developing a general and
enantioselective alkene hydroarylation method.
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Acknowledgements
[13] The structures of 3a and 3b have not been confirmed.
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G.E.M.C. thanks the Bristol Chemical Synthesis CDT
(EPSRC; EP/G036764/1) for a studentship. J.F.B. thanks the
Royal Society for a URF.
Keywords: acetanilides · branch selectivity · hydroarylation ·
iridium · phosphine ligands
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14866–14870
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[19] The acetamide NH group may facilitate a Friedel–Crafts-type
À
metalation of the ortho C H bond, perhaps under the action of
general base catalysis. Such an interpretation would be consis-
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Lewis acidity of the Ir center is a key factor.
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[20] CCDC 1413026, 1413027, 1413028, and 1413029 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre..
[21] The difference in ortho-regioselectivity for 6i versus 6g may
reflect the reduced steric demands of the 1,3-dioxole moiety of
5i versus the methoxy group of 5g.
Received: July 17, 2015
Revised: August 27, 2015
Published online: October 22, 2015
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