Triazole-assisted ruthenium-catalyzed C-H arylation of aromatic amides

Article abstract of DOI:10.1002/chem.201403019

Site-selective ruthenium(II)-catalyzed direct arylation of amides was achieved through C-H cleavages with modular auxiliaries, derived from easily accessible 1,2,3-triazoles. The triazolyldimethylmethyl (TAM) bidentate directing group was prepared in a highly modular fashion through copper(I)-catalyzed 1,3-dipolar cycloaddition and allowed for ruthenium-catalyzed C-H arylations on arenes and heteroarenes, as well as alkenes, by using easy-to-handle aryl bromides as the arylating reagents. The triazole-assisted C-H activation strategy was found to be widely applicable, to occur under mild reaction conditions, and the catalytic system was tolerant of important electrophilic functionalities. Notably, the flexible triazole-based auxiliary proved to be a more potent directing group for the optimized ruthenium(II)-catalyzed direct arylations, compared with pyridyl-substituted amides or substrates derived from 8-aminoquinoline. Assisting activation: Ruthenium(II)-catalyzed C-H arylations of (hetero)arenes and alkenes have been achieved with aryl halides through removable bidentate auxiliaries derived from modular 1,2,3-triazoles (see scheme; TAM=triazolyldimethylmethyl).

Full text of DOI:10.1002/chem.201403019

DOI: 10.1002/chem.201403019
Full Paper
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CÀH Activation
Triazole-Assisted Ruthenium-Catalyzed CÀH Arylation of Aromatic
Amides
Hamad H. Al Mamari, Emelyne Diers, and Lutz Ackermann*[a]
Abstract: Site-selective ruthenium(II)-catalyzed direct aryla-
tion of amides was achieved through CÀH cleavages with
modular auxiliaries, derived from easily accessible 1,2,3-tria-
zoles. The triazolyldimethylmethyl (TAM) bidentate directing
group was prepared in a highly modular fashion through
copper(I)-catalyzed 1,3-dipolar cycloaddition and allowed for
ruthenium-catalyzed CÀH arylations on arenes and heteroar-
enes, as well as alkenes, by using easy-to-handle aryl bro-
mides as the arylating reagents. The triazole-assisted CÀH
activation strategy was found to be widely applicable, to
occur under mild reaction conditions, and the catalytic
system was tolerant of important electrophilic functionali-
ties. Notably, the flexible triazole-based auxiliary proved to
be a more potent directing group for the optimized ruthe-
nium(II)-catalyzed direct arylations, compared with pyridyl-
substituted amides or substrates derived from 8-aminoqui-
noline.
Introduction
modifications of this auxiliary are particularly challenging in
a modular fashion.[4] In consideration of these major limita-
tions, we recently introduced a novel family of highly flexible
bidentate directing groups derived from modular 1,2,3-tria-
zoles[22–24] for iron-catalyzed CÀH activation.[25] Thus, a triazolyl-
dimethylmethyl (TAM) amide allowed for efficient direct aryla-
tions in a site-selective fashion through chelation assistance.
Unfortunately, these iron-catalyzed CÀH bond transformations
were accompanied by the use of an expensive diphosphine
ligand, stoichiometric amounts of sacrificial oxidants, as well as
highly reactive Grignard reagents as the arylating reagent
(Scheme 1a). Because organomagnesium reagents are required
for these CÀH functionalizations, valuable electrophilic func-
tional groups were inherently not tolerated by the catalytic
system. To address these limitations, we set out to devise com-
Methods for the activation and subsequent functionalization of
otherwise inert CÀH bonds are particularly attractive because
they avoid the use of prefunctionalized starting materials, and
thereby improve the step-economy of the synthetic route.[1]
Typical organic substrates contain a variety of CÀH bonds with
comparable dissociation energies.[1] As a consequence, achiev-
ing site selectivity is a prerequisite for the design of syntheti-
cally useful CÀH-functionalization protocols. Arguably, the
most powerful strategies make use of chelation assistance
through reversible precoordination.[2] In this context, mono-
dentate Lewis basic entities within the substrate have proven
instrumental for advancing the research area of CÀH activation.
In contrast, Daugulis et al. illustrated the utility of bidentate di-
recting groups derived from 8-aminoquinoline for CÀH func-
tionalizations.[3,4] Hence, palladium-[5,6] or copper-catalyzed[7–9]
CÀH transformations were viable.[10] Chatani et al. very recently
reported on useful ruthenium-catalyzed direct CÀC forma-
tions,[11–13] as well as nickel-catalyzed CÀH transformations.[14–16]
Likewise, the practical importance of the 8-aminoquinoline di-
recting group was also highlighted by iron-catalyzed direct
functionalizations,[17–19] as well as step-economical syntheses of
bioactive natural products.[4,20,21]
Despite the remarkable recent advances, this approach is
mostly restricted to the 8-aminoisoquinoline, and structural
[a] Dr. H. H. Al Mamari, Dr. E. Diers, Prof. Dr. L. Ackermann
Institut fꢀr Organische und Biomolekulare Chemie
Georg-August-Universitꢁt
Tammannstrasse 2, 37077 Gçttingen (Germany)
Fax: (+49)551-39-6777
E-mail: lutz.ackermann@chemie.uni-goettingen.de
Scheme 1. TAM-assisted CÀH functionalizations on amides 1 (dppe=1,2-
bis(diphenylphosphino)ethane; TMEDA=N,N,N’,N’-tetramethylethylenedi-
amine; DCIB= 1,2-dichloroisobutane).
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403019.
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plementary protocols for TAM-assisted CÀH activation that
would allow for the use of user-friendly organic electrophiles
as the arylating reagents and, thus, avoid the use of terminal
oxidants. To this end, we developed highly step- and atom-
economical ruthenium-catalyzed CÀH arylations, with aryl hal-
ides,[26–30] of arenes, heteroarenes, and alkenes (Scheme 1b), re-
ported herein. Notable features of our findings include the use
of a highly modular removable triazole-based auxiliary, chemo-
selective direct arylations with easily accessible aryl bromides,
and ample substrate scope.
Results and Discussion
Optimization
We initiated our studies by preparing a representative set of
novel benzamides, 1, displaying substituted 1,2,3-triazoles, by
means of the copper-catalyzed 1,3-dipolar Huisgen cycloaddi-
tion under mild reaction conditions (see the Supporting Infor-
mation). With diversely decorated substrates, 1, in hand, we
subsequently probed different reaction conditions for the de-
sired CÀH arylation of TAM-amide 1a with aryl bromide 2a
(Table 1 and Tables S1–S3 in the Supporting Information). Pre-
Scheme 2. Variation of the auxiliary substitution pattern. [a] NMR conversion
with CH2Br2 as internal standard.
With the optimized reaction conditions in hand, we subse-
quently explored the effect exerted by the substituents of the
auxiliary on the performance of the ruthenium catalyst
(Scheme 2). A gem-dimethyl substitution pattern in the amide
backbone of substrate 1a was found to be beneficial for ensur-
ing high catalytic efficacy (3aa versus 3ba and 3ca). Moreover,
electron-donating N-substituents on the 1,2,3-triazole proved
to be suitable, whereas N-aryl-substituted derivatives 1e and
1 f did not deliver the desired products. With these less-elec-
tron-donating N-aryl substituents on the triazole moiety only
the starting materials, 1e and 1 f, were isolated. It is particular-
ly noteworthy that significantly inferior results were obtained
when using the hitherto “gold standard”, namely, a pyridyl-
substituted amide or the substrate derived from 8-aminoqui-
noline (4aa and 5aa, respectively). This observation is a strong
testament to the superior directing-group power of our tria-
zole auxiliary for ruthenium(II)-catalyzed CÀH functionalization.
Thereafter, we put the proposed bidentate binding motif of
the triazolyl-substituted amides, 1, to the test (Scheme 3). Terti-
ary amide 1g failed to deliver desired arylated product 3ga,
highlighting the importance of the acidic NH-free functionality.
In accordance with this finding, the corresponding ester, 1h,
was not a viable substrate for the ruthenium(II)-catalyzed
direct arylation. Likewise, simple primary or secondary amides
1i–1k, being devoid of the second Lewis basic nitrogen, were
not converted by the ruthenium catalyst.
Table 1. Optimization of ruthenium-catalyzed CÀH arylation of amide
1a.[a]
Entry
Variation from standard conditions
Yield [%]
1
2
3
4
5
6
7
–
98
–
–
PCy3, X-Phos, or dppf instead of PPh3
HIPrCl instead of PPh3
P(p-Tol)3 or P(4-FC6H4)3 instead of PPh3
[Ru3(CO)12] instead of [RuCl2(p-cymene)]2
1408C
86 or 91
–
96
99
[RuCl2(PPh3)3] instead of [RuCl2(p-cymene)]2/PPh3
[a] Reaction conditions: 1a (0.50 mmol), 2a (0.60 mmol), base
(0.75 mmol), [RuCl2(p-cymene)]2 (2.5 mol%), catalyst, ligand, o-xylene
(2.0 mL), 22 h, 120–1408C. Cy = cyclohexane, dppf = 1,1’-bis(diphenyl-
phosphino)ferrocene; Tol = tolyl.
liminary experiments identified ruthenium(II) complex [RuCl2(p-
cymene)]2 as a highly effective catalyst, particularly when
being coordinated by phosphine ligands (Table 1, entries 1–5).
However, well-defined ruthenium(II) complex [RuCl2(PPh3)3]
outperformed the in situ generated catalytic system (Table 1,
entries 1, and 7). Among a variety of bases (K2CO3, K3PO4,
KOAc, NaOAc, or CsOPiv; Piv= pivaloyl), Na2CO3 furnished op-
timal results. Furthermore, ortho-xylene allowed for more effi-
cient catalysis compared with reactions being conducted with
H2O[29,31–33] or in toluene, N-methyl-2-pyrrolidone (NMP), or
DMF as the solvent.
Scope
Having identified the secondary N-benzyl-substituted TAM-
amide as the most effective auxiliary, we explored the scope of
the ruthenium(II)-catalyzed CÀH arylation with diversely deco-
rated substrates 1 (Scheme 4). The direct arylation efficiently
proceeded with the parent substrate 1l, as well as with differ-
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Scheme 3. Evaluation of the bidentate binding motif.
Scheme 5. Scope of CÀH arylation with (hetero)aryl bromides 2.
ies, were well tolerated by the catalyst, and electron-deficient,
as well as electron-rich, aryl bromides, 2, were found to be
viable electrophiles.[34] For instance, the reaction scope includ-
ed a thiophene (3af) as well as an NH-free indole (3ag), which
chemoselectively underwent the desired CÀH arylation.
Mechanistic Studies
In consideration of the high efficacy of our TAM-assisted ruthe-
nium-catalyzed CÀH activation, we conducted mechanistic
studies to delineate the mode of action. To this end, direct ary-
lations in the presence of deuterated cosolvents indicated a sig-
nificant H/D exchange solely occurring in the ortho-position of
the amide (Scheme S1 and S2 in the Supporting Information),
both in the presence and the absence of the organic electro-
philic arylating reagent. These findings provided strong sup-
port for the elementary step of CÀH bond metalation being re-
versible in nature.
Moreover, intermolecular competition experiments between
differently decorated benzamides 1q and 1r revealed elec-
tron-deficient substrates to be inherently more reactive, there-
by exclusively furnishing amide 3ra as the sole product
(Scheme 6). This observation suggests that a simple electro-
philic-type arene activation is unlikely to be operating.
Scheme 4. CÀH arylations of (hetero)arenes 1 and alkene 6. [a] Yield of iso-
lated diarylated product.
ent para-substituted amides 1m–1o. Intramolecular competi-
tion experiments with meta-substituted substrates 1p–1u oc-
curred with excellent site-selectivity at the less-sterically-con-
gested CÀH bonds. More-hindered ortho-substituted starting
materials 1v–1x also delivered the desired products in excel-
lent yields. Notably, the ruthenium(II)-catalyzed CÀH activation
was not restricted to arenes, but also proved viable to hetero-
arenes, as well as alkenes, illustrated by the preparation of ary-
lated products 3ya and 7aa.
Based on these observations, as well as previous find-
ings,[11,12] we propose the ruthenium-catalyzed CÀH arylation
to proceed by an initial base-assisted[35] cyclometalation, with
The ruthenium(II) catalyst proved to be widely applicable to
CÀH bond arylations with a variety of substituted aryl bro-
mides, 2, (Scheme 5). Indeed, important electrophilic functional
groups, such as enolizable ketones, or heteroaromatic moiet-
Scheme 6. Intermolecular competition experiment.
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a subsequent formal oxidative addition of aryl bromides 2. The
latter might occur by a single-electron transfer (SET)-type pro-
cess. Finally, reductive elimination and proto-demetalation pro-
vide the desired products 3, and, at the same time, regenerate
the catalytically active ruthenium complex (see Scheme 7).
Keywords: amides · aryl halides · catalysis · CÀH activation ·
ruthenium · triazoles
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In summary, we have disclosed the unprecedented triazole-as-
sisted direct CÀH arylation of unactivated amides by rutheni-
um catalysis. In contrast to the iron-catalyzed CÀH-functionali-
zation protocol,[25] the ruthenium(II) catalyst did not require
any sacrificial oxidants and was applicable to user-friendly aryl
bromides as the arylating reagents. The mild reaction condi-
tions[36] allowed chemoselective CÀH arylations of TAM amides
in the presence of useful electrophilic functional groups. The
TAM auxiliaries were shown to be modular in nature and oper-
ated as monoanionic bidentate directing groups, which were
removable in a traceless fashion.[25] The triazole auxiliary
proved to be superior under the optimized reaction conditions
compared with the hitherto-employed bidentate directing
groups in CÀH activation, that is, amides derived from pyri-
dines or 8-aminoquinoline. Therefore, the ruthenium-catalyzed
CÀH arylation of benzamides was viable for arenes and al-
kenes, with excellent chemo-, site- and diastereoselectivity, as
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Generous support by the European Research Council under
the European Community’s Seventh Framework Program (FP7
2007–2013)/ERC Grant agreement no. 307535, and the CaSuS
PhD program is gratefully acknowledged.
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Received: April 9, 2014
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FULL PAPER
&
CÀH Activation
H. H. Al Mamari, E. Diers, L. Ackermann*
&& – &&
Triazole-Assisted Ruthenium-Catalyzed
CÀH Arylation of Aromatic Amides
Assisting activation: Ruthenium(II)-cat-
alyzed CÀH arylations of (hetero)arenes
and alkenes have been achieved with
aryl halides through removable biden-
tate auxiliaries derived from modular
1,2,3-triazoles (see scheme; TAM=tri-
azolyldimethylmethyl).
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Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!