Rhodium(III)-catalysed, redox-neutral C(sp2)-H alkenylation using pivalimide as a directing group with internal alkynes

Article abstract of DOI:10.1016/j.tetlet.2016.11.039

In the presence of [RhCp^?Cl₂]₂, N-pivaloyl anilines react with internal alkynes to give the corresponding 2-alkenylpivalimides under redox neutral conditions through C-H activation. This redox neutral hydroarylation, which does not require an external organic acid, unlocks a regioselective synthetic route to 2-alkenyl anilines and is generally applicable to diversely substituted electron rich and electron poor pivalimides.

Full text of DOI:10.1016/j.tetlet.2016.11.039

Accepted Manuscript
Rhodium(III)-Catalyzed, Redox-Neutral C(sp2)-H Alkenylation Using Pivali-
mide as a Directing Group with Internal Alkynes
Subban Kathiravan, Ian A. Nicholls
PII:
S0040-4039(16)31495-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.039
TETL 48325
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Tetrahedron Letters
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8 November 2016
Please cite this article as: Kathiravan, S., Nicholls, I.A., Rhodium(III)-Catalyzed, Redox-Neutral C(sp2)-H
Alkenylation Using Pivalimide as a Directing Group with Internal Alkynes, Tetrahedron Letters (2016), doi: http://
dx.doi.org/10.1016/j.tetlet.2016.11.039
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Rhodium(III)-Catalyzed, Redox-Neutral
C(sp2)-H Alkenylation Using Pivalimide as a
Directing Group with Internal Alkynes
Subban Kathiravan a , Ian A. Nicholls a,b


1
Tetrahedron Letters
Rhodium(III)-Catalyzed, Redox-Neutral C(sp2)-H Alkenylation Using Pivalimide as a
Directing Group with Internal Alkynes
Subban Kathiravan a, Ian A. Nicholls a,b 
aBioorganic & Biophysical Chemistry Laboratory, Department of Chemistry & Biomedical Sciences and Linnaeus University Centre for Biomaterials Chemistry,
Linnaeus University, SE-391 82, Kalmar, Sweden.
bDepartment of Chemistry – BMC, Uppsala University, SE-75123, Uppsala, Sweden.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In the presence of RhCp*Cl22, N-pivaloyl anilines react with internal alkynes to give
the corresponding 2-alkenylpivalimides under redox neutral conditions through C-H
activation. This redox neutral hydroarylation, which does not require an external
organic acid, unlocks a regioselective synthetic route to 2-alkenyl anilines and is
generally applicable to diversely substituted electron rich and electron poor
pivalimides.
Available online
Keywords:
Hydroarylation
Internal alkynes
Redox-neutral
Rhodium catalysis
Pivalimide
2009 Elsevier Ltd. All rights reserved.
Transition metal catalyzed C-H transformations are a
promising emerging strategy to construct new C-C or C-X
(X=heteroatom) bonds which employ omnipresent C-H
bonds as a latent functional group.1 Aniline is one of the
most significant nitrogen-containing materials and is
extensively used in the manufacture of polymers, dyes,
agrochemicals, pharmaceuticals, and pigments.2 Diversely
substituted conjugated anilines are often used for the
synthesis of biologically active molecules and functional
materials.3 The transition metal catalysed alkenylation of
anilines is widely employed as an important tool for the
synthesis of alkenylanilines. Specifically, direct alkenylation
of the sp2 C-H bonds of anilines is a highly attractive
protocol from the viewpoint of atom- and step-economy.
Murai and co-workers reported the ruthenium catalysed,
carbonyl group directed C-H alkylation of a variety of
aromatic compounds with alkenes.4 Since this seminal report
significant effort has been devoted to the development of
direct oxidative alkenylations of (hetero)arene C-H bonds
since this would avoid the need for the synthesis of
prefunctionalized starting materials. A number of successful
examples of sp2 C-H alkenylation with alkenes possessing
heteroatom containing directing groups have been reported
using various catalysts.5
The hydroarylation of alkynes offers an attractive alternative
method to prepare functionalized alkenes from simple
(hetero)arenes and alkynes.6 Despite the high synthetic
efficacy of this transformation with ruthenium(II) catalysts,
high catalyst loadings, the requirement for an excess of acid
(AcOH, PivOH, Ad-COOH) to protonate the organometallic
species, and high temperatures, limit the functional group
tolerance and enhance the likelihood of detrimental side-
reactions.7 For example, Miura and co-workers developed
the ruthenium(II) catalysed hydroarylation of alkynes via
directed C-H functionalization and indicated that the
reaction involves amide directed ortho-metalation,
carbometalation of alkynes, and protonolysis by excessive
addition of acetic acid to form the hydroarylation product.8
Recently, Manikandan and co-workers described the
ruthenium catalysed hydroarylation of alkynes via a
deprotonation pathway.9 In contrast, the Rh(III) catalysed
sp2 C-H bond alkenylation with internal alkynes has been
developed in a limited number of examples.10 Fagnou and
co-workers reported the rhodium catalysed intermolecular
hydroarylation of alkynes.11
These reactions function best with substrates bearing
strongly coordinating phosphoryl, phosphine sulfide, phenyl
sulfone, and isoquinolone directing groups as elegantly
developed by the research groups of Glorius,12 Miurai,13 and
———
 Corresponding author. Tel.: +46-480-44-6230; e-mail: suppan.kathiravan@lnu.se
 Corresponding author. Tel.: +46-480-44-6258; e-mail: ian.nicholls@lnu.se

2
Tetrahedron
Li.14 Certainly, there are no examples of functionalization
replaced with
16-17).
(p-cymene)RuCl22 or Cp*IrCl22 (Entries
for substrates bearing moderate to weak directing groups
such as pivalimides. Pivalimides are versatile compounds in
organic synthesis, with applications in ligand-controlled
para-selective C-H arylation,15 cobalt-catalyzed C-H
amidation,16 and copper catalyzed meta-selective C-H
arylation.17 To this end, herein we present a rhodium(III)
catalyst for alkyne hydroarylation utilising weak oxygen-
coordination, with excellent substrate scope. Moreover, this
could provide an approach for achieving the alkenylation of
anilines that avoids competitive cyclization, and provide a
valuable complement for the elaboration of anilines.
Table 2. Scope of pivalimide in hydroarylation reaction
Table 1. Optimization of the rhodium catalyzed alkyne
hydroarylation
aReaction conditions: 1a-t (0.2 mmol), 2a (1 equiv.), RhCp*Cl22 (1
mol%), AgSbF6 (8 mol%), Cu(OAc)2•H2O (10 mol%), 1,2-DCE (1
mL) 80 °C, 12 h. bIsolated yield.
With the optimized conditions in hand, we examined the scope of
the reaction of pivalimides 1a-t with alkyne 2a. The alkenylation
of 1a provided the corresponding product 3a in 83% yield with
an E/Z ratio of 99:1. The electron donating methyl (1b-c)
methoxy (1d) and ethyl (1e) substituents afforded the
corresponding products 3b-e in 72-90% yield. Interestingly, this
alkenylation protocol also displayed good reactivity towards
substrates with electron withdrawing substituents 1f-j (Cl, Br, F,
CN, COOMe, and NO2), furnishing the corresponding products
3f-j in good to moderate yields (59-87%). The disubstituted
pivalimides 1m-p were also reactive, affording the desired
products 3m-p in good yields. Moreover, pivalimides substituted
with CF3 (1q) and OMe (1r) groups at the ortho-position gave
products 3q-r in good yields (61-66%). Unfortunately, pyridine
derivatives (1s-t) were not amenable to reaction under the
optimized conditions.
aReaction conditions: 1 (0.2 mmol), 2 (1 equiv.), Rh(III) (1 mol%),
silver salt (8 mol%), Cu(OAc)2•H2O (10 mol%), 1,2-DCE (1 mL), 80
°C, 12 h. Isolated yield. Cp*=Pentamethylcyclopentadiene. NR= no
b
reaction
At the outset of our investigation, to evaluate the proposed
strategy, we employed the reaction of the model substrate N-
pivaloyl aniline 1a with diphenyl acetylene 2a under
different catalytic conditions in various solvents at 120 °C
for 12 h. The use of the pivaloyl protecting group gave the
desired product while other directing groups were
ineffective (ESI). Initial experiments were carried out using
the rhodium catalyst derived from RhCp*Cl22 and AgNTf2,
We next analysed the scope of the reaction with regard to the
unsymmetrical alkynes 4a-s (Table 3). Gratifyingly, a variety of
different substituted internal alkynes were well tolerated. For
example 1-phenyl-1-propyne 4a, 1-phenyl-1-butyne 4b, 1-
phenyl-1-pentyne 4c, 1-phenyl-1-hexyne 4d and 1-phenyl-1-
cyclopropane 4e afforded the desired products 5a-e in good to
excellent yields. Notably, methyl 4f and ethyl phenylpropiolate
4g gave the corresponding mixtures of products 5f-g, in good to
moderate yields. Additionally, 2-cyclopentylethynyl benzene 4h
furnished 5h in good yield, highlighting the tolerance of this
strategy to relatively sterically crowded alkynes. Interestingly,
the bisthiophene-substituted alkyne 4i underwent facile
hydroarylation, furnishing product 5i in 75% yield.
and Cu(OAc)2•H2O (10 mol%) as the additive in 1,2-DCE
for 12 h at 120 °C, which resulted in a 15% yield of the
desired product (Table 1, entry 1). AgBF4 provided a similar
result (Entry 2), while the use of AgPF6, AgOTf, AgOCF3,
KPF6, or AgF, gave the alkene in lower yields (Entries 3-7).
It was found that AgSbF6 was the optimal additive for this
C-H activation reaction (Entry 8). Changing the additive
from Cu(OAc)2•H2O to Cu(OPiv)2, led to a decreased yield
(Entry 9). Conducting the reaction in other solvents gave the
expected product in trace amounts, confirming that DCE
was the best solvent for this transformation (Entries 10-15).
Poor conversion was observed when
RhCp*Cl22 was

3
Unfortunately, the unsymmetrical internal alkynes 4j-k resulted
in lower yields. Symmetrical alkynes 4m-s were found to be
unreactive.
Table 3. Scope of symmetrical and unsymmetrical internal
alkynes
Scheme 2. Deuterium kinetic isotope effects for C-H
alkenylation.
aReaction conditions: 1a (0.2 mmol), 4a-s (1 equiv.),
RhCp*Cl22 (1 mol%), AgSbF6 (8 mol%), Cu(OAc)2•H2O (10
mol%), 1,2-DCE (1 mL) 80 °C, 12 h. bIsolated yield.
Cp*=Pentamethylcyclopentadiene
Scheme 3. Proposed reaction mechanism
Based on the observed reactivity and literature precedent,18
a
mechanism for alkene synthesis was proposed (Scheme 3).
Initially, C-H activation of the pivalimide likely involves
deprotonative C-H metalation (via a concerted metalation-
deprotonation path) by the rhodium complex to give the six
membered rhodacycle A. The exact role of copper is presently
unclear, but may be responsible for generation of the active
catalyst. Alkyne coordination to the Rh atom (B) followed by
regioselective insertion of the alkyne into the metal-carbon bond
generates the corresponding intermediate C. This organo-
rhodium intermediate C undergoes a protodemetalation reaction
with HOAc to afford the product through an in situ catalytic
protonolysis along with the formation of the Rh species to
complete the cycle. In addition to the redox-neutral pathway, it
could be possible that redox chemistry operates between Rh and
Cu with residual oxygen acting as a sacrificial oxidant during
recycling of the copper co-catalyst.
Finally, to assess the feasibility of pivalimide hydroarylation
through a deprotonation methodology, we examined the reaction
of N-arylpivalimide 1a with 2a in the presence of pivalic
acid/acetic acid. The reaction proceeded in trace yield, which
clearly displays the advantage of the redox neutral pathway
(Scheme 1).
Scheme 1. Hydroarylation through a deprotonation methodology
To help elucidate the reaction mechanism, experiments were
carried out with the isotopically labelled substrate (D5-1a). The
initial study suggested that an irreversible C-H metalation step
occurs during the C-H activation as reflected by the observed
KIE value of 3.17 (Scheme 2a). In a further study D5-1a was
reacted with 2b under the optimized conditions. After 20 min,
53% of deuterium was incorporated at the alkene (D4-4a) carbon
as observed by 1H NMR analysis, which implicates in situ
catalytic protonolysis in the reaction mechanism (Scheme 2b).
Moreover, when the reaction was performed without
Cu(OAc)2•H2O, the expected alkenylated product was not
obtained indicating that Cu(OAc)2•H2O plays a key role in the
first step of the catalytic cycle and also in in situ protonolysis by
generating acetic acid (Scheme 2c).
Conclusions
In summary, we describe a novel rhodium catalyzed
hydroarylation reaction that employs catalytic protonolysis for
the synthesis of alkenes from alkynes and N-arylpivalimides. The
versatile pivaloyl-directing group permits the selective formation
of sterically crowded and even highly conjugated alkenes in high
yields. Mechanistic studies provide support for the proposed
catalytic cycle developed for this C-H activation reaction. On-
going work is focused on further exploring the underlying
mechanisms and the scope with respect to various C-H
functionalization strategies.
Acknowledgments

4
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5
Highlights
-
Regioselective pivalimide directed
alkenylation
Redox-neutral
Broad scope (30 examples) and yields of up
to 90%
-
-