Regioselective C-H Hydroarylation of Internal Alkynes with Arenecarboxylates: Carboxylates as Deciduous Directing Groups

Article abstract of DOI:10.1002/anie.201600894

In the presence of catalytic [Ru(p-cym)I₂]₂and the base guanidine carbonate, benzoic acids react with internal alkynes to give the corresponding 2-vinylbenzoic acids. This alkyne hydroarylation is generally applicable to diversely substituted electron-rich and electron-poor benzoic and acrylic acids. Aryl(alkyl)acetylenes react regioselectively with formation of the alkyl-branched hydroarylation products, and propargylic alcohols are converted into γ-alkylidene-δ-lactones. The hydroarylation can also be conducted decarboxylatively with a different choice of catalyst and reaction conditions. This reaction variant, which does not proceed via intermediate formation of 2-vinylbenzoic acids, opens up a regioselective, waste-minimized synthetic entry to vinylarenes. Make the switch: A simple ruthenium(II) complex catalyzes the regioselective hydroarylation of internal alkynes with benzoic acids. The conditions can be tuned to switch from a non-decarboxylative to a decarboxylative pathway. Aryl(alkyl)acetylenes react regioselectively with formation of the alkyl-branched 2-vinylbenzoic acids, and propargylic alcohols cyclize to γ-alkylidene-δ-lactones.

Full text of DOI:10.1002/anie.201600894

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International Edition: DOI: 10.1002/anie.201600894
German Edition: DOI: 10.1002/ange.201600894
À
C H Activation
À
Regioselective C H Hydroarylation of Internal Alkynes with
Arenecarboxylates: Carboxylates as Deciduous Directing Groups
Liangbin Huang, Agostino Biafora, Guodong Zhang, Valentina Bragoni, and Lukas J. Gooßen*
Dedicated to K. Peter C. Vollhardt on the occasion of his 70th birthday
À
Abstract: In the presence of catalytic [Ru(p-cym)I2]2 and the
base guanidine carbonate, benzoic acids react with internal
alkynes to give the corresponding 2-vinylbenzoic acids. This
alkyne hydroarylation is generally applicable to diversely
substituted electron-rich and electron-poor benzoic and acrylic
acids. Aryl(alkyl)acetylenes react regioselectively with forma-
tion of the alkyl-branched hydroarylation products, and
propargylic alcohols are converted into g-alkylidene-d-lac-
tones. The hydroarylation can also be conducted decarbox-
ylatively with a different choice of catalyst and reaction
conditions. This reaction variant, which does not proceed via
intermediate formation of 2-vinylbenzoic acids, opens up
a regioselective, waste-minimized synthetic entry to vinyl-
arenes.
in the development of regiospecific C H-activating processes.
In recent years, substantial advances in carboxylate-directed
[13]
À
C H activation have been made,
for example, by the
groups of Yu,[14] Miura,[15] Ackermann,[16] and Larrosa,[12b,c] as
well as our own group.[12d,17]
In this context, oxidative couplings of benzoic acids with
alkynes to form isocoumarins, naphthalenes, and other cyclic
structures have intensively been studied. These reactions
À
involve carboxylate-directed C H activation to give vinyl-
metal intermediates, which immediately undergo cyclization
steps before either reductive elimination or protonolysis can
occur (Scheme 1a).[15,18] Moreover, in the presence of elec-
tron-deficient transition-metal catalysts or ruthenium(II),
alkynes preferentially react with the nucleophilic carboxylate,
to form enol esters, rather than with the C H moiety.[19] This
reactivity points to the challenges associated with developing
selective C H hydroarylations in the proximity of a reactive
À
G
iven the prevalence of vinylarene moieties in functional
materials, pharmaceuticals, and natural products,[1] efficient
methods for the construction of this structural motif are
constantly sought. Established synthetic approaches include
Mizoroki–Heck[2] and Fujiwara–Moritani reactions,[3] as well
as various catalytic cross-couplings of organometallic
reagents with alkynes.[4]
À
carboxylate.[20]
In continuation of our research on the use of carboxylic
acids as substrates in transition-metal catalysis,[11,12d,17] we
explored whether carboxylate groups could be utilized as
directing groups in redox-neutral intermolecular hydroaryla-
À
C H hydroarylations of alkynes are advantageous over
these processes, because they require neither prefunctional-
ized substrates nor oxidants. Since the pioneering studies by
Murai and co-workers,[5] various metals have been found to
efficiently catalyze the hydroarylation of alkynes, for example
Ru,[6] Rh,[7] Re,[8] Co,[9] and others.[10] However, these C H
À
functionalizations are highly regioselective only when
directed by ketone, pyridine, amide, sulfoxide, or other
strong directing groups, groups which need to be synthesized
in additional reaction steps and are not easily removed.
Arguably, the most advantageous directing groups in
ortho-functionalizations are carboxylates, because benzoic
acids are widely available in great structural diversity and at
low cost, and can subsequently be derivatized further, utilized
as leaving groups in decarboxylative couplings,[11] or removed
tracelessly by protodecarboxylation.[12] However, the weak
coordinating ability of this group poses additional challenges
[*] Dr. L. Huang, A. Biafora, G. Zhang, V. Bragoni, Prof. Dr. L. J. Gooßen
FB Chemie-Organische Chemie
Technische Universität Kaiserslautern
Erwin-Schrodinger-Str. Geb. 54, 67663 Kaiserslautern (Germany)
E-mail: goossen@chemie.uni-kl.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201600894.
À
Scheme 1. Carboxylate-directed C H activation and coupling with
alkynes. DG=directing group, FG=functional group.
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tions of alkynes (Scheme 1b). The desired process would have
ability of guanidine to form stable ruthenium(II) com-
plexes.[21] A thorough solvent screening revealed that a 10:1
mixture of AmOH and H2O gives the highest yields and
selectivities (entry 10; see the Supporting Information).
Substrates 1a and 2a were best employed in a 1:1.5 ratio
(entries 11 and 12).
À
to be initiated by a carboxylate-directed ortho-C H alkyne
t
insertion step. The resulting vinyl–metal species would then
need to be forced towards a reductive elimination step to
yield alkenylbenzoic acids, despite the abundance of facile
pathways leading to cyclized products.
To probe the feasibility of this concept, we investigated
the reaction of 2-methylbenzoic acid (1a) with 1-phenyl-1-
propyne (2a) in the presence of various metal catalysts
(Table 1). Many complexes known for their ability to mediate
The scope of the hydroarylation reaction with regard to
the acid component was investigated using 1-phenyl-1-pro-
pyne (2a) as the coupling partner (Table 2). Benzoic acids
Table 2: Substrate scope of the directed hydroarylation.[a]
Table 1: Optimization of the directed hydroarylation conditions.[a]
Entry [M] (mol%)
Base
Yield [%]
(3aa/
3aa’)[b]
1
2
3
4
5
6
7
8
9
[Ru(p-cym)Cl2]2 (4) 0.3 equiv K2CO3
[Ru(p-cym)I2]2 (4) 0.3 equiv K2CO3
[Ru(p-cym)I2]2 (2) 0.3 equiv K2CO3
[Ru(p-cym)I2]2 (2) 0.1 equiv K2CO3
[Ru(p-cym)I2]2 (2) 0.5 equiv K2CO3
[Ru(p-cym)I2]2 (2) 1.0 equiv K2CO3
[Ru(p-cym)I2]2 (2) 0.5 equiv Cs2CO3
[Ru(p-cym)I2]2 (2) 0.5 equiv Li2CO3
43:5
52:10
60:6
39:7
46:trace
n.d.
53:14
44:9
[Ru(p-cym)I2]2 (2) 0.5 equiv guanidine carbonate 68:7
10[c] [Ru(p-cym)I2]2 (2) 0.5 equiv guanidine carbonate 74:5
11[c,d] [Ru(p-cym)I2]2 (2) 0.5 equiv guanidine carbonate 73:6
12[c,e] [Ru(p-cym)I2]2 (2) 0.5 equiv guanidine carbonate 90 (93):5
[a] Reaction conditions: 1a–l (0.5 mmol), 2a–f (0.75 mmol), [Ru(p-
cym)l2]2 (2 mol%), guanidine carbonate (0.5 equiv.), tAmOH/
H2O=10:1 (1.1 mL), 1008C, 12–24 h. Yields of the corresponding
methyl ester isolated after derivatization with MeI. [b] 2 f (0.5 mmol).
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), [M] (4 mol%),
base, 1,4-dioxane:H2O (10:1, 1.1 mL), 1008C, 12 h. [b] Yields of
corresponding methyl esters determined by GC after esterification with
K2CO3 (2 equiv) and MeI (5 equiv) in MeCN using n-tetradecane as the
internal standard. Yields of isolated products are given within paren-
theses. [c] tAmOH/H2O=10:1 as solvent. [d] 0.6 mmol 1a.
[e] 0.75 mmol 2a. n.d.=not determined. cym=cymene.
bearing various functional groups, including halides (1b–c),
electron-withdrawing groups such as CF3, Ac, CO2Me (1d–
e,j), or electron-donating moieties (1a,f–g) all gave good to
moderate yields. Multisubstituted benzoic acids (1i,k,l)were
also suitable substrates for this transformation. Next, several
alkynes were evaluated as coupling partners in combination
with toluic acid (1a). All gave reasonable yields, with best
results being obtained with diphenylacetylene (2 f). Unpro-
tected hydroxy groups remained intact when at a distance
À
C H functionalizations, including Pd(OAc)2, [{IrCp*Cl2}2],
[{Ir(cod)2Cl}2], and [{Rh(cod)2Cl}2] were investigated, but
none of them gave the hydroarylation product 3aa in the
desired selectivity (see the Supporting Information for
details). However, the simple ruthenium complex [Ru(p-
cym)Cl2]2, usually an efficient hydroacyloxylation catalyst,[19]
surprisingly furnished 3aa in an encouraging 48% yield with
a high 7:1 regioselectivity in favor of the methyl-branched
stilbene derivative (entry 1). The iodine-bridged analogue
[Ru(p-cym)I2]2 proved to be an even more active and
selective catalyst (entries 2 and 3). The process requires the
presence of substoichiometric amounts of base, ideally
50 mol%, thus giving the best balance between conversion
and selectivity. If stoichiometric amounts of base are present,
the reaction is completely suppressed, thus indicating that
protons are required in the overall process (entries 4–6).
Carbonate bases, and guanidinium carbonate in particular,
were found to be most effective (entries 7–9). The remarkable
reactivity of the guanidinium base may result from the known
À
from the C C triple bond (2e).
À
With propargylic alcohols (2g–k) as substrates, the C H
hydroarylation was followed by intramolecular lactonization,
so that g-alkylidene-d-lactones were formed in high yields
À
(Table 3). This reaction nicely complements the oxidative C
H functionalizations/lactonizations, which lead to endocyclic
À
C C double bonds (Scheme 1). The broad scope spans from
electron-rich to electron-deficient benzoic acids bearing
a wealth of functional groups in the para-, ortho-, or meta-
position (Table 3; 1m–v). Various propargylic alcohols were
smoothly converted into the corresponding lactones (2h–k).
Not only benzoic acid, but also methacrylic acid (1w) was
successfully converted. During the optimization of the
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Table 3: Substrate scope of benzoic acids for hydroarylation and
Table 4: Scope with respect to benzoic acids for decarboxylative hydro-
arylation.[a]
annulation.[a]
[a] Reaction conditions: 1a–ab (0.5 mmol), 2 f (0.5 mmol), [Ru(p-
cym)Cl2]2 (4 mol%), guanidine carbonate (0.2 equiv), 2-picoline
(0.2 equiv), HOAc (1.0 equiv), toluene (2 mL), 1208C, 24 h.
ucts are not formed by a hydroarylation/protodecarboxyla-
tion sequence, but by an alternative mechanistic pathway. The
carboxylate group may thus be considered to act as a decid-
uous directing group, remaining in place for as long as it is
[a] Reaction conditions: 1a–w (0.5 mmol), 2g–k (0.75 mmol), [Ru(p-
cym)I2]2 (2 mol%), guanidine carbonate (0.5 equiv), HOAc (1.0 equiv),
tAmOH (1 mL), 1008C, 12–24 h.
À
required to direct C H functionalization, but being shed
tracelessly within the catalytic cycle.[12] The key advantage of
the concept of deciduous directing groups is that the
À
reaction conditions, we had occasionally observed the for-
mation of decarboxylation products.
unwanted formation of byproducts resulting from C H
functionalization on both sides of the directing group is
impossible, because the directing group is removed in course
Intrigued by this observation, we optimized the catalyst
and reaction conditions using the model reaction of 2 f with
1a until the decarboxylative reaction pathway became pre-
dominant (see the Supporting Information). In the presence
of [Ru(p-cym)Cl2]2 (4 mol%), guanidine carbonate
(0.2 equiv), 2-picoline (0.2 equiv), HOAc (1.0 equiv), and in
the nonpolar solvent toluene (2 mL), the decarboxylative
coupling product 5af was finally obtained in 73% yield at
1208C (Table 4). In this decarboxylative reaction variant,
various functional groups are tolerated including halides,
methoxy, and alkyl groups (Table 4). It also extends to
heterocyclic substrates. The tolerance of chloro and even
iodo groups demonstrates the orthogonality of the present
transformation into traditional cross-coupling processes.
However, this prototype protocol does not presently allow
a high-yielding coupling of alkyl-substituted alkynes.
À
of the first C H functionalization, and is no longer in place to
À
activate a second C H bond.
Based on the above findings and mechanistic investiga-
tions for related processes,[18] a tentative catalytic cycle for the
À
ruthenium-catalyzed C H hydroarylation of alkynes can be
outlined (Scheme 3). It starts with formation of the cyclo-
metallated ruthenium complex I, which coordinates to the
alkyne substrate. Migratory insertion affords the seven-
membered alkenyl–ruthenium species II. In pathway a,
which predominates in polar solvents, protonolysis occurs
(or reductive elimination, if the proton resides at the
ruthenium), thus releasing the hydroarylation product 3aa.
The alternative pathway b, leading to the decarboxylated
product 5acf, becomes more favorable at higher temper-
atures, when protonolysis is slower, that is, in less polar
solvents, and when coordinating chloride ions are present.
These factors contribute to facilitate extrusion of CO2 from II.
In-depth mechanistic studies are underway to fully clarify the
reaction pathways of this intriguing transformation.
In a control experiment, we submitted the products 3af,zf
of the non-decarboxylative hydroarylation to the decarbox-
ylative hydroarylation conditions (Scheme 2). These did not
decarboxylate, which suggests that the decarboxylated prod-
À
In conclusion, the carboxylate-directed C H hydroaryla-
tion of internal alkynes with benzoic or acrylic acids catalyzed
by the inexpensive, easy-to-handle [Ru(p-cym)I2]2 complex
opens up a convenient and waste-free entry to a wide variety
of 2-vinylbenzoic acids or aromatic d-lactones from abundant
precursors. In a less-polar solvent mixture and at higher
temperatures, the carboxylate group is removed directly
within the hydroarylation process. Beyond being removable,
the carboxylates thus become deciduous directing groups,
Scheme 2. Protodecarboxylation experiment under standard reaction
conditions.
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Scheme 3. Proposed mechanism for the ruthenium-catalyzed (decar-
À
boxylative) C H hydroarylation of alkynes. ESI-MS results provided for
compounds where R=Me, R’=Ph.
À
intrinsically preventing disubstitution in this directed C H
functionalization.
Acknowledgements
We thank Umicore for donating chemicals, the DFG (SFB/
TRR-88, “3MET”), the Alexander von Humboldt Founda-
tion (fellowship to L.H.), and the CSC (fellowship to G.Z.) for
financial support. We thank Joachim Hewer for ESI-MS
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À
Keywords: alkynes · C H activation · decarboxylation ·
ruthenium · synthetic methods
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6933–6937
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Received: January 26, 2016
Published online: April 26, 2016
Angew. Chem. Int. Ed. 2016, 55, 6933 –6937
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6937