Oxidative Addition to Palladium(0) Made Easy through Photoexcited-State Metal Catalysis: Experiment and Computation

Article abstract of DOI:10.1002/anie.201811439

Visible-light induced, palladium catalyzed alkylations of α,β-unsaturated acids with unactivated alkyl bromides are described. A variety of primary, secondary, and tertiary alkyl bromides are activated by the photoexcited palladium metal catalyst to provide a series of olefins at room temperature under mild reaction conditions. Mechanistic investigations and density functional theory (DFT) studies suggest that a photoinduced inner-sphere mechanism is operative in which a barrierless, single-electron transfer oxidative addition of the alkyl halide to Pd⁰ is key for the efficient transformation.

Full text of DOI:10.1002/anie.201811439

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Accepted Article
Title: Oxidative Addition to Palladium(0) Made Easy through
Photoexcited-State Metal Catalysis: Experiment and Computation
Authors: Magnus Rueping
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811439
Angew. Chem. 10.1002/ange.201811439
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10.1002/anie.201811439
Angewandte Chemie International Edition
COMMUNICATION
Oxidative Addition to Palladium(0) Made Easy through
Photoexcited-State Metal Catalysis: Experiment and Computation
Rajesh Kancherla+, Krishnamoorthy Muralirajan+, Bholanath Maity, Chen Zhu, Patricia E. Krach, Luigi
Cavallo*, and Magnus Rueping*
Abstract: Visible-light induced, palladium catalyzed alkylations of
α,β-unsaturated acids with unactivated alkyl bromides are described.
A variety of primary, secondary, and tertiary alkyl bromides are
activated by the photoexcited palladium metal catalyst to provide a
series of olefins at room temperature under mild reaction conditions.
Mechanistic investigations and density functional theory (DFT)
studies suggest that a photoinduced inner-sphere mechanism is
operative in which a barrierless, single electron oxidative addition of
the alkyl halide to Pd(0) is key for the efficient transformation.
complexes as both photosensitizer and cross-coupling
catalyst.[5] This allows the oxidative addition barrier to be
lowered resulting in an alkyl radical and Pd(I) intermediate
(Figure 1) which can add to an α,β-unsaturated acids.
Subsequent carbon dioxide elimination provides the akylated
olefin and the regenerated catalyst. To the best of our
knowledge,
a
visible light induced palladium catalyzed
decarboxylative C(sp3)–C(sp2) cross coupling alkylation has not
been reported.
Lowering the oxidative addition (O.A.) barrier
using excited-state Pd
Cross-coupling reactions are one of the most important
synthesis tools for the formation of C–C or C–heteroatom bonds.
Conventionally, three key steps are involved in the cross-
coupling reaction: oxidative addition, transmetalation, and
reductive elimination. Often challenges arise in the individual
steps, depending on the coupling partners, metal catalysts or
reaction conditions employed. In recent decades, numerous
catalysts, as well as ligands, have been developed in order to
reduce the energy barrier in the bond-forming and breaking
steps. More recently, it has been shown that visible-light
barrierless
hv
ΔGǂ = 41.6 kcal/mol
enhances
cross-couplings
employing
radicals.[1]
This
Conventional O.A. to ground-state Pd0
high activation barrier
-
methodology allows single electron transfer (SET) to replace the
conventional two electron transfer process by using photoredox
and transition metal dual-catalysis systems, thus significantly
reducing the barrier for each step in cross-coupling reactions.[2]
Despite developments made in cross-coupling chemistry, the
conventional oxidative addition of alkyl electrophiles to Pd(0)
remains sluggish, even at elevated temperature, due to the
electron-rich nature of the alkyl halide bond.[3] In fact,
computational studies have revealed that the alkyl bromide
oxidative addition to Pd(0) is endothermic with a high energy
barrier of 41.6 kcal/mol. Further, alkyl-palladium(II) species that
form via a two-electron transfer are considerably less stable due
to the lack of -electrons interacting with the empty d-orbitals of
metal. This results in fast β-hydride (β–H) elimination from -
alkyl-palladium(II) intermediates which outcompetes the required
olefin insertion in decarboxylative alkylation cross-coupling
reactions.[3, 4]
Figure 1. Visible-light assisted decarboxylative alkylation.
Vinylic acids are readily available and stable, which makes
them versatile coupling partners in cross-coupling reactions
forming C–C bonds.[6] As a result, decarboxylative reactions
have been explored.[7] However, the use of pre-activated
substrates, stoichiometric amounts of sacrificial reagents,[8] high
temperature or the use of high energy UV irradiation may limit
their synthetic applicability.[9] Therefore, the development of a
cross-coupling alkylation of α,β-unsaturated acids with alkyl
halides under mild reaction conditions, without the need for
additional metal mediators or photocatalysts, would be
desirable goal.
With these considerations in mind, we started to examine a
new visible light induced, palladium catalyzed alkylation by
applying different catalysts and alkyl halides (Table 1).[10] The
reaction of cinnamic acid (1a) and cyclohexyl bromide (2d) in
In contrast to the photoredox dual-catalysis strategy which
promotes cross-coupling by lowering the energy barrier, we now
report the use of a mechanistically distinct, SET barrierless
oxidative addition strategy for alkylation of vinylic acids, using Pd
a
[*] Dr. R. Kancherla,[+] Dr. K. Muralirajan, [+] Dr. B. Maity, Prof. Dr. L.
Cavallo, Prof. Dr. M. Rueping,
presence
of
Pd(OAc)2,
PPh3
ligand
and
N,N-
KAUST Catalysis Center (KCC)
diisopropylethylamine (DIPEA) gave the substituted olefin 3d in
86% (isolated 82%) yield with a stereoselectivity of E/Z = 99:1
under irradiation with 34 W blue LEDs (entry 1). Among the
other palladium sources examined, Pd(PPh3)2Cl2 and Pd(PPh3)4
gave comparable yields of 87% and 76% respectively, whereas
PdCl2 was not reactive (entries 2–4). PPh3 was found to be a
better ligand compared to Xantphos and PCy3 (entries 5 and 6).
King Abdullah University of Science and Technology (KAUST)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: magnus.rueping@kaust.edu.sa
C. Zhu, P. Krach, Prof. Dr. M. Rueping,
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074, Aachen (Germany)
E-mail: Magnus.Rueping@rwth-aachen.de
[+] These authors contributed equally
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10.1002/anie.201811439
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Table 1. Optimization of the reaction conditions
Use of other organic and inorganic bases however did not
improve the yield (entries 7 and 8). Only traces of the product
was observed when the reaction was carried out at 100 °C
without visible light irradiation (entry 9). Control experiments
demonstrated that all components are necessary to promote the
reaction (entries 10 and 11). Among
a range of alkyl
Entry
1
Change in standard reaction conditions
none
Yield (3d, %)[a]
86 (82)
87
76
electrophiles tested, alkyl bromide was found to be more
reactive (Br > I >> Cl) when compared to alkyl iodide (56%, due
to possible side reactions) and alkyl chloride (traces, difficulty of
oxidative addition).
2
3
4
Pd(PPh3)2Cl2 instead of Pd(OAc)2
Pd(PPh3)4 instead of Pd(OAc)2
PdCl2 instead of Pd(OAc)2
0
5
6
7
8
9
10
11
Xantphos (10 mol%) instead of PPh3
Tricyclohexylphosphine (PCy3) instead of PPh3
using quinuclidine, DABCO
using K2CO3, K2HPO4, K3PO4
at 100 °C, in the absence of light
without irradiation (or) Pd (or) PPh3
without DIPEA
11
0
55, 58
18-72
traces
0
Using the optimized reaction conditions, the scope of the
reaction was first evaluated with respect to the alkyl bromide
(Table 2). A variety of tertiary, secondary, and primary alkyl
bromides were found to react with E-stereoselectivity, and a
reactivity pattern of 3° ≥ 2° > 1° was observed. Especially, the
reactivity and stereoselectivity of tertiary alkyl bromide with
eliminable β-H is noteworthy (3a and 3b). Secondary alkyl
bromides with different chain lengths reacted with good
selectivity and yields (3c–3o). A range of functional groups such
as ether (3n–3p), ester (3r), acetal (3s), and amide (3i and 3j)
groups were well tolerated.
9
[a]Yields determined by GC analysis using an internal standard. DIPEA = N,N-
diisopropylethylamine,
dimethylxanthene, DABCO = 1,4-Diazabicyclo[2.2.2]octane.
Xantphos
=
4,5-Bis(diphenylphosphino)-9,9-
Table 2. Reaction scope with respect to alkyl bromides.[a]
Table 3. Reaction scope with respect to vinyl carboxylic acid.[a]
[a]Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), 34 W blue LEDs, Ar, 24 h.
Yields of isolated products. Ratio of stereoisomers was determined by 1H NMR
analysis. Temperature (T = 28 ± 2 °C). See supporting information.
[a]Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), 34 W blue LEDs, Ar, 24 h.
Yields of isolated products. Ratio of stereoisomers was determined by 1H NMR
analysis. Temperature (T = 28 ± 2 °C). [b]Pd(PPh3)4 (10 mol%, 0.02 mmol) is
used instead of Pd(OAc)2 and PPh3. See supporting information.
Although decarboxylative alkylation using tertiary (3a and 3b)
and secondary (3c–3o) alkyl bromides gave the alkylated
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product in good yield using in situ formed Pd(0) species, primary
alkyl bromides displayed poor reactivity due to the predominant
ester formation. However, use of Pd(0) source (Pd(PPh3)4)
instead of in situ formed Pd(0) species helped to minimize the
ester formation resulting in the formation of the olefinated
products 3p–3t.
conditions, α-fluorocinnamic acid was found to be reactive to
give (Z)-(2-cyclohexyl-2-fluorovinyl)benzene (4w).
Next, radical clock and TEMPO trapping experiments were
conducted to support the proposed light-mediated SET oxidative
addition mechanism. Experiments with (bromomethyl)-
cyclopropane and 6-bromohex-1-ene were performed which
resulted in the formation of radical rearranged products 4y and
4z exclusively. TEMPO trapping experiments under standard
conditions did not yield product 3d; instead, a TEMPO-alkyl
adduct was observed also supporting the radical mechanism.[10]
Further, to exclude the possibility of a radical chain process, light
on-off experiments were carried out. During the light off-cycles
no reaction was observed which confirms that light does not
Subsequently, the scope of the reaction with respect to
substituted vinyl carboxylic acids was studied (Table 3). Aryl and
heteroaryl acrylic acids with both electron-rich and electron-
deficient groups at ortho- (4a-4e), meta- (4f-4h), and para- (4i-
4m) positions are amenable substrates. Additionally, a variety of
functional groups including CF3- (4c and 4h), HO- (4d, 4g, and
4p), halide- (4e and 4l), amino- (4k), aldehyde- (4m), and acetal
groups (4r) were well tolerated. Typically, heterocyclic
substrates with nitrogen and sulfur atoms can cause palladium
deactivation due to the strong coordination of the heteroatom
with a metal catalyst.[11] Notably, pyridinyl and thiophenyl acrylic
acids showed good reactivity (4s-4v), even at room temperature.
In addition to the 3-aryl acrylic acids, 3,3-diphenylacrylic acid is
also an amenable substrate (4x). Reactions with α-substituted
cinnamic acid or β-substituted styrenes to give (2,2-disubstituted
vinyl)benzene are rare. However, under the optimized reaction
initiate
a
chain-reaction.[10] The competition experiments
between primary, secondary and tertiary alkyl bromides and the
increased reactivity of secondary and tertiary alkyl bromides
compared to primary alkyl bromides, clearly demonstrate that on
photoirradiation, alkyl halides give thermodynamically more
stable alkyl radicals that will undergo fast oxidative addition via a
SET pathway rather than an SN2 type oxidative addition to a
phosphine-ligated Pd(0) complex.[12]
‡
‡
δ
B(T1)
C1-D1
48.1
2c
C + C1-D1
36.1
δ
23.4
- L
D
hν
G-H
1a
C + C1
16.9
D + D1
19.8
G
10.8
B
13.1
F
C + D1
3.7
0.0
6.1
L
E
C
A (S0)
3c
-15.6
I
2 L
-21.8
H
C1
D1
-20.1
J
-29.5
L = PPh3
B = iPr2NEt
CO2
+
A(S0)
B H+/Br –
A (S0) + 2c + 1a + B
A (S0) + 3c + CO2 + B H /Br
(-29.5)
(0.0)
Figure 2. Computed energy profile for the decarboxylative alkylation of α,β-unsaturated acids. Free energies in solution (in kcal/mol) at the M06(SMD)/SDD/Def2-
TZVP//PBE0/SDD/Def2-SVP level are displayed.
In the general context, it is difficult to achieve the alkylation
reaction since the majority of alkyl bromides possess lower
reduction potentials (E1/2 ≥ -2.0 V vs. SCE) than *[Pd(PPh3)4]
the alkyl-bromide 2c, due to the very high energy barrier
required to reach transition state J-K (∆G‡ = 41.6 kcal/mol,
Figure S1). Having excluded the possibility of oxidative addition
via a concerted 3−center reaction pathway, we explored a
reaction pathway involving photoexcitation of Pd0.[15,16] These
calculations allowed us to identify the triplet state B(T1) of
Pd0(PPh3)3, lying 48.1 kcal/mol higher than A(S0), as the active
catalytic species (Figure S2, Table S1 for detail). The triplet
B(T1) furnishes an open coordination site for the bromoalkane
2c association leading to an inner sphere photoinduced electron
red
(E(M+/M*) = -1.72 V vs SCE in THF). Hence, we performed DFT
calculations to unravel the mechanism of the newly developed
decarboxylative alkylation of α,β-unsaturated acid (1) with
cyclopentyl bromide (2c).[13] The in-situ generated Pd0(PPh3)4
complex A is considered to be the reference structure for the
calculations. Differently from the oxidative addition of aryl-
halides within Mizoroki-Heck coupling,[14] the catalytically active
Pd0(PPh3)2 species J is reluctant to undergo oxidative addition of
transfer (PET) with 2c,
a barrierless and exergonic step
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10.1002/anie.201811439
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according to our computational protocol (Figure S3, Table S2),
leading to the PdI-complex C with the liberation of cyclopentyl
radical C1. Complex C is in equilibrium with the three-
coordinated complex D via the slightly endergonic dissociation of
a phosphine. Instead, radical C1 can attack cinnamic acid 1a at
the Cα atom via transition state C1-D1, with an activation free
energy of 12.7 kcal/mol (Figures 2 and S4). The resulting D1
radical is 12.6 kcal/mol more stable than C1. At this point, the
two doublet species D and D1 can collapse into the PdII
intermediate E, with the release of 9.4 kcal/mol. Control
calculations indicated that attack of C1 at the Cβ atom of 1a is
unfavored by 2.1 kcal/mol (Figure S5). Further progress in the
reaction involves deprotonation of E by DIPEA, which leads to
the zwitterionic complex F, lying 2.4 kcal/mol above E and ready
to decarboxylate. The decarboxylation step is facilitated by the
endergonic rearrangement of DIPEA-H+ from the side of the
carboxylate group, near to the Br moiety, structure G. Although
intermediate G is 10.8 kcal/mol less stable than intermediate F,
proximity between DIPEA-H+ and the Br atom helps the
concerted elimination of CO2 and of DIPEA-H+/Br ‒ via transition
state G-H (Figure S4), with a moderate energy barrier of 16.1
kcal/mol from intermediate E. In addition, forward evolution of
transition state G-H leads to liberation of product 3c and the
Pd0(PPh3)2 complex. Coordination of two phosphine to
Pd0(PPh3)2 regenerates A(S0) for the next catalytic cycle, with a
thermochemistry of the reaction 1a + 2c + DIPEA → 3c +
DIPEA-H+/Br- + CO2 equal to -29.5 kcal/mol. Thus, the reaction
can be considered to be composed of three fundamental steps
(Figure 2): i) photoexcitation of a Pd0 species to an active state
promoting SET with the alkyl bromide to generate a PdI species
and an alkyl radical; ii) addition of the alkyl radical to the α,β-
unsaturated acid and coupling with the PdI species to generate a
PdII species; iii) decarboxylation of the PdII species to liberate
the product while regenerating the Pd0 species.
Having clarified the reaction mechanism, we focused on the
origin of the substantially barrierless SET between B(T1) and 2c.
The molecular orbitals diagram of Figure 3 shows that the
SOMO of B(T1) corresponds to a singly occupied MO mainly
delocalized on the PPh3 ligands with a minor Pd contribution,
while the LUMO of 2c corresponds to the σ* orbital of the Br–C
bond. Analysis of an intermediate structure halfway along the
PES connecting B(T1) + 2c to C + C1 indicates that during SET
the SOMO of B(T1) localizes on Pd and gets stabilized by mixing
with the LUMO of 2c (Figures 3 and S6), driving the reaction
towards the formation of the Pd–Br bond and the rupture of the
Br–C bond. The SOMO-1, which corresponds to a Pd centered
MO (Figure 3), is not involved in the SET and its energy is
marginally modified during the reaction. The energy difference
∆E‡ between this intermediate structure and free reactants, the
putative activation barrier, was decomposed into ∆E‡ = ∆Estrain
+
∆Eint, where ∆Estrain is the energy required to deform the free
reactants to the geometry they have in the intermediate structure,
and ∆Eint is the interaction energy between the deformed
reactants in the intermediate structure.[17] The calculated ∆Estrain
and ∆Eint of 6.0 and -10.6 kcal/mol indicate that even at early
stages of the reaction the interaction between the Pd SOMO and
the Br–C bond σ* orbital is strong enough to overcome the
positive deformation energy and result in a negative (barrierless)
∆E‡ of -4.6 kcal/mol.
In summary, by overcoming the challenges associated with
the reactivity of ground-state palladium for the stereoselective
decarboxylative alkylation of α,β-unsaturated acids, we
discovered a low energy pathway using photo-excitation of
palladium that allows a wide variety of primary, secondary, and
tertiary alkyl halides to be applied in the decarboxylation reaction.
In agreement with the experimental evidence, DFT calculations
show that the oxidative addition of the alkyl-bromide to ground
state Pd° is energy demanding. However, the PdL3 species in its
triplet state, generated by photoexcitation, can easily react with
the alkyl-bromide via SET. The formed alkyl radical attacks the
double bond of the α,β-unsaturated acid, leading to
decarboxylation and product formation via a neutral pathway. In
contrast to the outer sphere mechanisms observed in Ir and Ru
based polypyridyl photocatalysts, the decarboxylative alkylation
operates via a photoinduced inner sphere mechanism in which a
barrierless, single electron oxidative addition of the alkyl halide
to the metal is key for an efficient transformation.
α-SOMO
-2.33
-2.91
-3.94
SET
-4.66
-4.84
-4.95
Acknowledgements
P.E.K. thanks the Deutsche Bundesstiftung Umwelt for
a
doctoral stipend. The research leading to these results has
received funding from the European Research Council under the
European Union's Seventh Framework Programme (FP/2007-
2013) / ERC Grant Agreement no. 617044 (SunCatChem). B.M.
C.Z. and L.C. acknowledge King Abdullah University of Science
and Technology (KAUST) for computational resources of the
supercomputer Shaheen II.
α-SOMO-1
B(T1)
(PPh3)3Pd····Br····Alkyl
(PPh3)3Pd—Br····Alkyl
Figure 3. Molecular orbitals diagram showing the interaction between B(T1)
and 2c during the SET step. Orbital energies are in eV. Phenyl groups are
omitted for clarity.
Keywords: Palladium • Alkyl halide • Decarboxylation • Excited-
state • Barrierless • DFT
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10.1002/anie.201811439
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Excited-state palladium catalyzed
decarboxylative alkylation of α,β-
unsaturated acids is achieved by a
barrierless approach under visible
light irradiation at room temperature
with good stereoselectivity. Detailed
DFT calculations with experimental
evidence suggests a neutral
Rajesh Kancherla+, Krishnamoorthy
Muralirajan+, Bholanath Maity, Chen
Zhu, Patricia E. Krach, Luigi Cavallo*,
and Magnus Rueping*
Page No. – Page No.
Oxidative Addition to Palladium(0)
Made Easy through Photoexcited-
State Metal Catalysis
decarboxylative pathway.
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