Electronic Asymmetry of an Annelated Pyridyl-Mesoionic Carbene Scaffold: Application in Pd(II)-Catalyzed Wacker-Type Oxidation of Olefins

Article abstract of DOI:10.1021/acscatal.0c02729

The two donor modules of an annelated pyridyl-mesoionic carbene ligand (aPmic) have different σ- and π-bonding characteristics leading to its electronic asymmetry. A Pd(II) complex 1 featuring aPmic catalyzes the oxidation of a wide range of terminal olefins to the corresponding methyl ketones in good to excellent yields in acetonitrile. The catalytic reaction is proposed to proceed via syn-peroxypalladation and a subsequent rate-limiting 1,2-hydride shift, which is supported by kinetic studies. The electronic asymmetry of aPmic renders a well-defined coordination sphere at Pd. The favored arrangement of reactants on the metal center features an olefin trans to the pyridyl module and a tbutylperoxide trans to the carbene. This arrangement gains added stability by the π-delocalization paved by the compatible orbitals on Pd, the pyridyl module, and the olefin that is perpendicular to the Pd(aPmic) plane. The π-interactions are absent in an alternate arrangement wherein the olefin is trans to the carbene. Density functional theory studies reveal the matching orbital overlaps responsible for the preferred arrangement over the other. This work provides an orbital description for the electronic asymmetry of aPmic.

Full text of DOI:10.1021/acscatal.0c02729

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Article
Electronic Asymmetry of an Annellated Pyridyl–Mesoionic Carbene
Scaffold: Application in Pd(II) Catalyzed Wacker#type Oxidation of Olefins
Sayantani Saha, suman yadav, Noor U Din Reshi, Indranil Dutta, Sooraj Kunnikuruvan, and Jitendra K. Bera
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c02729 • Publication Date (Web): 08 Sep 2020
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Electronic Asymmetry of an Annellated Pyridyl–
Mesoionic Carbene Scaffold: Application in Pd(II)
Catalyzed Wacker‒type Oxidation of Olefins
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Sayantani Saha, Suman Yadav, Noor U Din Reshi, Indranil Dutta, Sooraj Kunnikuruvan and
Jitendra K. Bera*
*Department of Chemistry and Center for Environmental Science, Indian Institute of Technology
Kanpur, Kanpur, 208016, India
ABSTRACT
The two donor modules of an annellated pyridyl–mesoionic carbene ligand (aPmic) have
different – and –bonding characteristics leading to its electronic asymmetry. A Pd(II) complex
1
featuring aPmic catalyzes the oxidation of a wide range of terminal olefins to the
corresponding methyl ketones in good to excellent yields in acetonitrile. The catalytic reaction is
proposed to proceed via syn–peroxypalladation and subsequent rate limiting 1,2–hydride shift,
which is supported by kinetic studies. The electronic asymmetry of aPmic renders a well–defined
coordination sphere at Pd. The favored arrangement of reactants on the metal center features an
t
olefin trans to the pyridyl module and a butylperoxide trans to the carbene. This arrangement
gains added stability by the ‒delocalization paved by the compatible orbitals on Pd, the pyridyl
module and the olefin that is perpendicular to the Pd(aPmic) plane. The ‒interactions are absent
in an alternate arrangement wherein the olefin is trans to the carbene. DFT studies reveal the
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matching orbital overlaps responsible for the preferred arrangement over the other. This work
provides an orbital description for the electronic asymmetry of aPmic.
KEYWORDS: Electronically asymmetric ligands, annellated carbenes, mesoionic carbenes,
Wacker–type oxidation, Catalysis.
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INTRODUCTION
Electronically asymmetric ligands wherein the donor modules differ in their electronic
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characteristics have received significant attention in recent years. The difference in electronic
characteristics of the donor modules results in significant variation in the nature and strength of
metal–ligand interactions. Electronic asymmetry of the pyridine–oxazoline ligand (Pyrox–A)
2
(Scheme 1a) has been implicated in stabilizing low–valent Co(I) intermediates. The electronic
disparity also plays a key role in many transformations catalyzed by transition metal compounds
by controlling the catalyst coordination sphere. For example, Pyrox–B (Scheme 1a) imparts site–
selectivity in palladium–catalyzed 1,4–difunctionalization of isoprene by controlling the
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migratory insertion of alkene into the Pd–alkenyl bond. Mechanistic studies suggested that the
directed insertion is the result of the unique electronic asymmetry and steric properties of the
ligand. Electronic asymmetry has also been found to influence the chiral induction in
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enantioselective transformations. Based on density functional theory (DFT) calculations, Stahl
and coworkers suggested that the electronic asymmetry of Pyrox–C (Scheme 1b) complements
the steric asymmetry and thereby controls the stereochemical outcome of the amidopalladation
step in the Pd(II)–catalyzed enantioselective oxidative amidation of alkenes.4a
Scheme 1. Electronic asymmetric ligands.
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Catalytic transformations that involve two electronically different reactants are best suited to
evaluate the effects of electronic asymmetry of a ligand. In such transformations, the
electronically asymmetric ligand directs the binding of the reactants (inner–sphere) in a specific
arrangement. The most well–studied example is the catalyst–controlled Wacker–type oxidation,
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t
which involves the coordination of an olefin and a butylperoxide to a Pd center. The Wacker
oxidation is a synthetically important process for converting a terminal olefin to a methyl
ketone.5,6,7 Under Tsuji–Wacker conditions, this reaction is substrate controlled, which can result
in a mixture of aldehyde and ketone products, especially when the alkene bears a proximal
heteroatom.1,8 Sigman and coworkers reported a Pd(II) compound featuring quinoline–2–
oxazoline (Quinox) (Scheme 1c) for catalyzing the efficient and selective oxidation of difficult
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a
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substrates such as allylic alcohols, internal olefins, protected allylic amines, and homoallylic
alcohols9d employing butylhydroperoxide (TBHP) as the oxidant. The excellent activity and
selectivity of this catalyst were credited to the electronic disparity of the two coordinating
modules of Quinox, featuring electron–rich oxazoline and electron–deficient quinoline rings.10
t
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Sigman hypothesized that butylperoxide preferentially binds trans to oxazoline ring whereas
alkene binds trans to quinoline ring (Scheme 2a). Hammett analysis of a series of substituted
Quinox ligands revealed that the substitution by electron–withdrawing groups on quinoline ring
enhanced the rate of the catalytic reactions.10a Similar inferences were also drawn by Hammett
analysis of electronically diverse substrates. Muldoon group later examined 2–(2–
pyridyl)benzoxazole (Pyrbox) (Scheme 1d) for the Pd catalyzed oxidation of terminal olefins to
1
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methyl ketones. Notably, electronically symmetric ligand 2,2–bipyridine was ineffective in the
catalytic oxidation of allylic acetate9a,10a and related catalytic reactions.12,13,14
One class of ligands which are particularly suitable to endure harsh conditions in oxidation
chemistry is mesoionic carbenes (MICs).15 Moreover, annellated carbenes exhibit unique
stereoelectronic properties which can result in improved catalytic activity and selectivity of their
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metal complexes. An annellated pyridyl–MIC (aPmic) ligand (Scheme 1e) was reported from
our group for Ru(II) catalyzed selective C=C bond scission of olefins to aldehydes using NaIO4
1
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as oxidant at room temperature. We now unravel the electronic asymmetric nature of aPmic in
Pd–catalyzed Wacker–type oxidations. The two coordinating modules of this ligand differ in
their relative ability for σ–donation/π–bonding. Whereas a carbene moiety is a strong σ–donor,15
1
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pyridyl unit exhibits π–bonding character besides acting as a σ–donor. We reasoned that aPmic
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would impose a preferential binding of olefin and peroxide anion to Pd with respect to its
electronically differentiated donor components (Scheme 2b). To validate this hypothesis, the
catalytic activity of the Pd(II) complex 1 (Scheme 3) in Wacker‒type oxidation of olefins to
methyl ketones using TBHP as oxidant was examined. Very good efficiency and selectivity were
observed for a wide range of olefins under optimized conditions. The catalytic reaction is
proposed to proceed via syn–peroxypalladation and subsequent rate limiting 1,2–hydride shift,
which is supported by kinetic studies. DFT calculations reveal that the key catalytically active
species has a well–defined coordination of olefin and peroxide at Pd governed by the electronic
asymmetry of aPmic. A delocalized π–bonding interaction is identified for the preferred
orientation of olefin and peroxide anion with respect to the aPmic.
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Scheme 2. Possible coordination models of Pd compounds featuring Quinox (a) and aPmic (b).
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RESULT AND DISCUSSION
Synthesis of 1. Compound [Pd(aPmic)(CH CN) ][BF ] (1) was obtained via the oxidative
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cleavage of the Pd–Pd bond in [Pd (CH CN) ][BF ] with precursor salt in a
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dichloromethane/acetonitrile mixture at room temperature (Scheme 3).19
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Scheme 3. Synthesis of compound 1.
Catalytic Studies
Reaction Optimization. Initial experiment using styrene (1 mmol), TBHP(decane) (5 mmol) and
catalyst 1 (0.05 mmol) in acetonitrile at 45 °C afforded a full conversion (100%) to acetophenone
(entry 1, Table 1). The reduction of the catalyst loading and the amount of TBHP(decane) to 0.02
mmol and 3 mmol, respectively, did not alter the product amount and the selectivity for the
model reaction (entry 2, Table 1). Further decrease in the amount of catalyst or oxidant, or
lowering the temperature diminished the conversion as well as the selectivity. Screening of
solvents and oxidants was carried out with the model reaction. Oxidants H O , NaIO and O
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were less efficient in terms of conversion and selectivity. Dichloromethane, tetrahydrofuran,
dimethyl sulfoxide, toluene and methanol were ineffective.20
A control reaction was carried out (styrene/1/TBHP(decane) = 1 mmol/0.02 mmol/3 mmol; 45
1
8
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°C, CH CN = 4 mL) using 5 mmol of H O . GC–MS analysis revealed no O incorporation in
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the product acetophenone confirming that the source of the oxygen atom is TBHP. Initial rates of
the model reaction (styrene/1/TBHP(decane) = 1 mmol/0.05 mmol/5 mmol; 45 °C) were studied
by adding 2 to 50 mmol of H O externally. It was observed that the rate of the reaction decreases
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after 12 h with 80% conversion and a black solid, presumably that of metallic Pd, was observed
in the reaction vessel.
Table 1. Reaction optimization.
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(5 mol%), Oxidant (5 equiv.)
Solvent (3 mL), 45 °C, 12 h
O
+
+
A
B
C
(1 mmol)
Entry
Oxidant
Solvent
Conv. (%)a
A:B:C
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TBHP(decane)
TBHP(decane)
H O (aq.)
CH CN
100
98
90
99.99:0.01:0
99.99:0.01:0
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b
CH CN
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CH CN
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CH CN/H O
(2/1)
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NaIO4
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70:30:0
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O2c
CH CN
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TBHP(decane)
TBHP(decane)
CH Cl
2
2
THF
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TBHP(decane)
TBHP(decane)
TBHP(decane)
DMSO
Toluene
MeOH
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49:34:17
56:44:0
76:24:0
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100
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Determined by GC–MS analysis using n–dodecane as an internal standard; 0.02 mmol of 1 and 3
mmol of oxidant used. Schlenk vial was connected to a balloon containing molecular oxygen.
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Substrate Scope. The substrate scope for catalyst 1 was explored under the following optimized
reaction conditions: alkene/TBHP(decane)/1 = 1 mmol/3 mmol/0.02 mmol; acetonitrile (4 mL);
45 °C (Table 2). Styrene, 4–methylstyrene, 3–methylstyrene and 2,4–dimethylstyrene afforded
quantitative conversions to the corresponding methyl ketones (1–4). The presence of electron–
donating groups –OMe/–SMe on the phenyl ring (4–methoxystyrene, 3,4–dimethoxystyrene and
4
–methylthiostyrene) increased the reaction rate and full conversions to the corresponding
products (5–7) were obtained within 4 h. The presence of electron–withdrawing groups, for
examples, 4–fluorostyrene (72%, 30 h), 2–fluorostyrene (70%, 30 h) and 4–chlorostyrene (68%,
3
0 h) gave lower conversions to the corresponding methyl ketones (8–10). 2–vinylpyridine gave
no product (11) presumably due to catalyst deactivation by coordination of the pyridine nitrogen
to the metal center. Other heteroaromatic alkenes 1‒methyl‒2‒vinyl‒1H‒pyrrole, 2‒vinylfuran
and 2,5‒dimethyl‒3‒vinylthiophene gave the corresponding methyl ketones (12–14) in good
yields. The oxidation of substrates having C=C bond attached to different polycyclic aromatic
rings, such as, 2–vinylnaphthalene, 9–vinylanthracene and 1–vinylpyrene, afforded the
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corresponding methyl ketones (15–17) with 90–96% yields after 12 h. Vinylferrocene gave
acetylferrocene (18) in 95% yield after 12 h.
Due to lower reactivity of the aliphatic alkenes in comparison to the aromatic congeners, the
reactions of aliphatic alkenes were carried out with higher catalyst loading (5 mol%) and 5 equiv.
of TBHP(decane) at 45 °C. The oxidation of aliphatic terminal alkenes afforded ~55%
conversion after 36 h. However, increasing the temperature to 70°C led to considerably higher
yields. Excellent yields of the corresponding products 19–22 were obtained for 2–
allylisoindoline–1,3–dione (80%), vinylcyclohexane (90%), 4,4–dimethoxybut–1–ene (95%) and
ethyl acrylate (85%) within 24 h.
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Internal alkenes trans–stilbene and cis–stilbene afforded the product 23 in 50% and 56%
conversions, respectively, after 36 h with ~5–10% benzaldehyde as the side product. At elevated
temperature (70°C), trans/cis–stilbenes result in 82% and 85% conversions, respectively, after
24 h without any trace of benzaldehyde. Oxidation of trans–β–methylstyrene gave a mixture of
products propiophenone (24) and phenylacetone (25) (70:30 ratio) after 24 h. Indene afforded 2–
indanone (26) with 52% conversion after 36 h. Internal aliphatic alkene, cis–cyclooctene, also
yielded 50% of cyclooctenone (27) after 36 h. Indene and cis–cyclooctene afforded 95% and
90% conversions, respectively, after 24 h at 70 °C.
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Table 2. Substrate Scope.a
O
R'
t
1 (2 mol%), BuOOH (3 equiv.)
R'
tBuOH
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CH CN (3 mL), 45 °C
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Yield was determined by GC–MS analysis using n–dodecane as an internal standard; Traces of C=C
c
d
cleaved product also detected; catalyst 1 (0.05 mmol) and 5 mmol of TBHP(decane) used; Reaction
temperature: 70 C; Both trans and cis–stilbene gave the same product 1,2 diphenylethanone; Using
trans–Stilbene; ~5–10% benzaldehyde obtained; Using cis–stilbene; ~2–3% epoxide.
e
f
g
h
i
1
1
An important advantage of this catalyst is that it does not require Ag salt to be active. The
bidentate aPmic ligand provides access to incoming oxidant and olefin in cis orientation by the
removal of two acetonitriles. To investigate the fate of the catalyst, a stoichiometric reaction was
performed with equimolar (0.1 mmol) amount of catalyst 1, styrene and TBHP(decane) in
acetonitrile. After 4 h, the solvent was evaporated, and the solid residue was washed with (3 × 10
mL) diethyl ether. It was then dissolved in acetonitrile and diethyl ether was layered to obtain
crystals, which were collected, weighed (yield 72%) and identified to be the catalyst 1 by X‒ray
crystallography and ESI‒MS. Recovery of the catalyst suggests that its molecular integrity is
retained during the oxidation.
Ligand Asymmetry
t
‒
For the oxidation reaction to proceed, one molecule each of olefin and BuOO needs to bind
the metal. An electronic asymmetric ligand enables the metal to bind the precursors in a specific
t
‒
arrangement. Thus, the binding of two olefins or two BuOO to Pd center, which result in a
catalytically inactive species, is averted. In Sigman’s catalyst, the disparity in the binding of two
reactants is caused by the presence of electron–poor quinoline ring and electron–rich oxazoline.
t
‒
BuOO preferably binds trans to oxazoline and olefin binds the electrophilic site trans to
quinoline. We argued that aPmic ligand may display electronic disparity due to the difference in
the – and π–characteristics of its two donor modules (vide infra). Therefore, one could assume
an arrangement where π‒acid ligand olefin binds trans to the pyridyl unit and the –donor
t
‒
BuOO binds trans to the carbene carbon which is also a –donor (Scheme 4). This matching
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‒
arrangement is defined as ‘σσ/ππ’ whereas the reverse arrangement, where BuOO and olefin
bind metal trans to the pyridyl and the carbene, respectively, is identified as ‘σπ/σπ’.21
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‒
Scheme 4. Two possible arrangements for binding of BuOO and olefin.
Reaction Mechanism
A plausible mechanism for this TBHP–mediated Wacker oxidation, similar to the one proposed
1
0
22
t
by Sigman and Mimoun, is depicted in Scheme 5. Initially, 1 reacts with olefin and BuOOH
to form species A with  arrangement. This is followed by syn–peroxypalladation involving
t
‒
nucleophilic attack of the coordinated oxygen atom of BuOO to the olefin, providing an
intermediate B. To release the strain in the four–membered ring, intermediate B undergoes a
rapid rearrangement affording the five‒membered pseudo-palladacyclic species C. Heterolytic
cleavage of the O–O bond with a concomitant 1,2–hydride shift gives intermediate D. Finally,
t
the methyl ketone and BuOH are released to complete the cycle.
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Scheme 5. Proposed mechanism for olefin oxidation by 1 (σσ/ππ).
Kinetic Studies
Kinetic studies are carried out to validate the proposed mechanism. The initial rate of the
reaction was monitored (up to <10–15% conversion) to determine the reaction order with respect
to the catalyst 1, the alkene (styrene) and TBHP(decane). The initial rate varied linearly with [1]
and [styrene] (Figures 1(a) and 1(b)). Increasing [TBHP(decane)] results in a faster rate, until a
certain point. After that, increasing [TBHP] leads to no further acceleration of the reaction
(Figure 1(c)). The saturation kinetics for [TBHP(decane)] and the first order dependency with
respect to [styrene] indicates the coordination of TBHP before styrene.10a
The initial rates of the reactions of electronically varied para–substituted styrenes (p–
YC H CH=CH ; Y= OMe, Me, H, F, Cl) with ‘1 + TBHP(decane)’ were measured. The
6
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2
+
+
Hammett plot of log[initial rate] vs σ (Hammett constant) gave a straight line with a  value of
+
–2.99 (see, SI). A negative  value suggests the development of positive charge at the α–carbon
atom of styrene implying that the rate of the reaction should be accelerated by electron–donating
substituents. This is in agreement with the results discussed under substrate scope.
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The proposed mechanism involves two key steps‒ syn‒peroxypalladation and 1,2–hydride
shift. A Lewis-acid/Lewis-base interaction between the cationic palladium and alkene in the
formation for species A would be favored by an electron-donating group, which is in line with
the experimental results.10a Further, palladation results in the development of positive charge at
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olefin and should be slowed by electron–withdrawing substituents. For 1,2–hydride shift as
well, an electron–withdrawing substituent would result in a slower rate due to decrease in
electron density at the α-C atom. This is consistent with the Hammett studies.
The effect of temperature on the rate of the reaction of 1 with styrene was also examined. The
activation parameters were determined from the plot of ln(k/T) versus 1/T, which was linear over
‡
the temperature range studied (308–333 K). The estimated entropy of activation (ΔS ) is –22.39
–
1
−1
‡
−1 24
±
0.48 cal mol K and the enthalpy of activation (ΔH ) is 11.76 ± 0.09 kcal mol . A negative
#
ΔS value is indicative of an organized transition state in the rate‒limiting step.
(c)
(a)
(b)
Figure 1. Initial rates vs (a) [1], (b) [alkene]; (c) [TBHP(decane)].
DFT Calculations
DFT calculations were carried out to distinguish between the catalytic cycles starting with
species having σσ/ππ and σπ/σπ arrangements. The optimized structures of all intermediates and
transition states along the match (σσ/ππ) and mismatch (σπ/σπ) pathways are provided in Figure
2
(σσ/ππ) and Figure S8 (σπ/σπ), respectively. The intermediates and transition states for σσ/ππ
path are denoted by alphabets whereas the corresponding structures for σπ/σπ path are
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represented by the same alphabets with a prime symbol (′). A comparative energy profile is given
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5
in Figure 3. The calculations were carried at the M06 level of theory. The basis set of Hay and
2
6
Wadt (LanL2DZ) with effective core potential (ECP) was used for Pd. Other atoms were
2
7
described using the 6‒31+G(d,p) basis set. The technical details of the calculations are given in
1
1
1
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the SI.
N1
C1
C1
C1
C4
N1
N1
N1
Pd
C1
C2
C2
Pd
Pd
Pd
C3
C3
H1
C2
C3 C2
H1
H1
C4
C4
C3
C4
H1
A
TSAB
B
TSBC
C1
N1
N1
C1
C2
N1
C1
Pd
Pd
H1
Pd
C2
C3
C4
C3
C4
C2
C3
H1
C4
D
TSCD
C
Figure 2. DFT optimized structures of all intermediates and transition states for olefin oxidation
t
by 1 (σσ/ππ). Ph substituent on the ligand, H atoms (except on C2, C3) and Me groups of Bu are
removed for clarity.
Figure 3. Free energy profiles of σσ/ππ (blue) and σπ/σπ (red) pathways are shown.
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A careful comparison of the bond parameters of the computed structures reveal interesting
features. The most notable difference in A and A′ is the orientation of the olefin. The olefin is
o
perpendicular to the Pd(aPmic) plane in A (Figure 4, the interplanar angle  = 70 ), whereas it
o
lies in the Pd(aPmic) plane in A′ ( = 8 ). The structure of A is akin to well–known Zeise’s salt
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K[PtCl (C H )] where the olefin is disposed perpendicular to the PtCl plane. There are
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3
significant differences in the metrical parameters of A and A′. The olefin trans to pyridyl unit is
more strongly bound to Pd (Pd–C2/C3 = 2.204/2.267 Å) in A than to the carbene (Pd–C2/C3 =
2
0
.364/2.540 Å) in A′. The corresponding Pd–C2/C3 bond orders are 0.336/0.285 and
.207/0.146, respectively. The C2–C3 distance is longer in A (1.380 Å) as compared to A′
(1.355 Å). Accordingly, the C2–C3 bond order in A (1.567) is lesser than in A′ (1.730).
Interestingly, Pd–N1 distance (2.187 Å) is significantly shorter in A than in A′ (2.255 Å),
tallying with the calculated bond orders (0.299, 0.270). Pd–O1 distances in A (2.028 Å) and A′
(
2.011 Å) are similar. Pd–C1 distance in A (2.062 Å) differs slightly as compared to A′ (2.013
-1
Å). The favorable bonding situation in A aids to its stability by 5.5 kcal mol than A′ (vide
infra).
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Figure 4. Structures of A and A′ with selected bond distances (Å) and interplanar angle  ()
t
given below. Ph, hydrogens (except on C2 and C3) and Me groups of Bu are removed for
clarity. Interplanar angle is the angle between Pd‒C2‒C3 and N1‒Pd‒C1 planes.
2
9
Metal-bound olefin is susceptible to the nucleophilic attack. The syn–peroxypalladation step
t
‒
involves the nucleophilic attack of the coordinated oxygen atom of BuOO to the olefin
-1
providing B. The free energy barrier for the formation of B (10.9 kcal mol ) is slightly less as
-1
compared to B′ (12.2 kcal mol ). The Pd–C1(carbene) and Pd–C2(olefin) distances are shorter
in B (1.994 and 2.036 Å) as compared to the same in B′ (2.156 and 2.060 Å). B′ is destabilized
-1
-1
compared to B by 11.5 kcal mol . A near barrier–less (0.7 kcal mol ) conversion of B to C
releases the ring strain. The energy difference between C and C′ was found practically identical
-1
(
11.6 kcal mol ) to that of B and B′. The structural parameters of C and C′ reflect the same
pattern as that in B and B′. The next step involves heterolytic O–O bond cleavage with a
concomitant 1,2–hydride shift that affords the product. The activation energy for this step is
-1
higher (20.6 kcal mol ) for σσ/ππ pathway compared to the alternate σπ/σπ pathway (8.7 kcal
-1
mol ).
The free energy difference between A and A′ is reflective of the different ligand arrangements
around the metal. Since A is stable than A′, the σσ/ππ pathway appears to be operational here.
Furthermore, the free energy barriers show that 1,2–hydride shift (C→D) should be the rate–
limiting step for the σσ/ππ pathway. Along the σπ/σπ pathway, the transition state corresponding
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to the peroxypalladation (TSA′B′) is located at the highest point in the energy landscape which
contradicts with 1,2–hydride shift being the rate limiting step. Thus, the σπ/σπ cycle can be ruled
out.
The preferential binding of reactants could be understood from a schematic orbital interaction
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diagram given in Figure 5. The pyridine module guides the orientation of the olefin in A. A
perpendicular arrangement of olefin to the Pd–pyridyl (aPmic) plane allows the overlap of p
orbital of N1, d of Pd and * of olefin establishing a –corridor (Figure 5a). The Pd→C2–C3

* (p –p ) back–donation is enhanced by the filled-filled interaction of N1(p ) with Pd (d.
z
z
y
Although pyridine is a weak –donor, annellation with an electron–rich aromatic system likely
improves the –donicity of pyridyl module in aPmic. The enhanced Pd→*(C2‒C3) back–
31
donation in A than A′ is revealed by the natural bond orbital (NBO) analysis. The second–order
(2)
perturbation energy (E ) corresponding to Pd→*(C2‒C3) is significantly higher in A (25.6
-1
-1
kcal mol ) than A′ (8.7 kcal mol ).
2
2
A higher Pd→olefin back–bonding can augment σ–donation from N1(sp )→Pd(dz ) which is
(2)
responsible for shorter Pd–N1 distance. This is also reflected in E corresponding to N1→Pd
-1
-1
interaction which is higher in A (43.3 kcal mol ) than A′ (27.4 kcal mol ). The –donation from
(2)
C2‒C3→Pd is also stronger in A than A′ as revealed by the corresponding E values (46.9 and
-1
9
.7 kcal mol , respectively). Clearly, a –corridor cannot be established if the C2–C3 axis lies in
the Pd(aPmic) plane as N1(p ) and C2–C3 * would be mutually orthogonal. The efforts to
y
optimize a related structure where C2–C3 axis resides in the Pd(aPmic) plane failed, instead, the
structure of A was invariably obtained.
The aPmic ligand imposes a preference for olefin to bind the metal trans to pyridyl module
and perpendicular to the Pd(aPmic) plane to pave the way for the –corridor. The mutually trans
tbutylperoxide and carbene constitute a σ–framework (Figure 5b). The Pd‒O1 distances are
similar irrespective of its trans partners in A and A′ indicating similar interactions of
tbutylperoxide with the metal.
Overall, the matching arrangement (A) has significant –type interactions between p of N1,
d of Pd and * of olefin. In contrast, the pyridyl and olefin are not mutually trans and the –
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type interactions are absent in A′. Hence a perpendicular disposition of olefin is not warranted in
A′. The cis Pd–N1 and Pd–C2/C3 interactions are thus weaker in comparison. The additional –
t
interactions in A incentivize the preference of the olefin and butylperoxide anion to bind in a
particular arrangement.
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Figure 5. Schematic representations of the π‒ and σ‒interactions involving N1(p ), Pd(d ) and
y
yz
2
2
2
2
(
C2‒C3)(p –p )* orbitals (a) and O1(sp ), Pd(d ‒ ) and C1(sp ) orbitals (b), respectively, in
z z x y
matching arrangement A.
CONCLUSION
Herein we revealed the electronic asymmetry of bidentate annellated pyridyl–mesoionic
carbene ligand (aPmic). The electronic asymmetry of aPmic originates from the difference in the
σ– and π–bonding characteristics of its two donor modules. The aPmic ligand guides olefin to
bind the metal trans to pyridyl module and perpendicular to the Pd(aPmic) plane to allow –
delocalization involving compatible orbitals on pyridyl module and olefin. The added stability
t
associated with the –delocalization imposes a preference for olefin, and thereby butylperoxide,
to bind the metal center in a particular arrangement. The rational based on the σ– and π–bonding
properties of the donor modules outlined here provides an orbital perspective to Sigman’s
approach to electronic asymmetry. The aPmic was applied in the Pd(II) catalyzed Wacker type
oxidation of terminal olefins as a proof of concept of its electronic asymmetry. This ligand may
have wider utility in catalytic reactions involving electronically disparate substrates. We are
currently working in this direction.
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ASSOCIATED CONTENT
Supporting Information. Detailed experimental procedures, kinetic plots, supporting figures
and computational details (PDF). The Supporting Information is available free of charge at
https://pubs.acs.org/doi……...
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AUTHOR INFORMATION
Corresponding Author
*
jbera@iitk.ac.in
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the anonymous reviewers for insightful comments. We also thank Prof. Nisanth N.
Nair and Prof. Ayan Datta for their valuable suggestions on this work. Financial support from the
Science and Engineering Research Board (SERB), the Department of Atomic Energy (DAE),
and the Indo−French Centre for the Promotion of Advanced Research (IFCPAR) are gratefully
acknowledged. J.K.B. thanks SERB, India, for a J. C. Bose fellowship. S.S. and S.Y. thank the
UGC, India, I.D. thanks the CSIR, India, for fellowships. N.U.D.R. thanks IIT Kanpur for a
post–doctoral fellowship.
REFERENCES
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Catalyzed Alkenyl C–H Bond Functionalization Reactions. Acc. Chem. Res. 2012, 45, 874–
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84.
(2) Hickey, D. P.; Sandford, C.; Rhodes, Z.; Gensch, T.; Fries, L. R.; Sigman, M. S.; Minteer, S.
D. Investigating the Role of Ligand Electronics on Stabilizing Electrocatalytically Relevant
Low–Valent Co (I) Intermediates. J. Am. Chem. Soc. 2019, 141, 1382–1392.
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(
3) McCammant, M. S.; Sigman, M. S. Development and investigation of a site selective
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Chem. Sci. 2015, 6, 1355–1361.
(
4) (a) McDonald, R. I.; White, P. B.; Weinstein, A. B.; Tam, C. P.; Stahl, S. S. Enantioselective
Pd(II)–Catalyzed Aerobic Oxidative Amidation of Alkenes and Insights into the Role of
Electronic Asymmetry in Pyridine–Oxazoline Ligands. Org. Lett. 2011, 13, 2830–2833. (b)
Xu, J. Influence of the electronic effect of catalysts on the enantioselectivity: applicability
and complexity. Curr. Org. Synth. 2010, 7, 650–676. (c) Armstrong, P. B.; Dembicer, E. A.;
DesBois, A. J.; Fitzgerald, J. T.; Gehrmann, J. K.; Nelson, N. C.; Noble, A. L.; Bunt, R. C.
Investigation of the Electronic Origin of Asymmetric Induction in Palladium–Catalyzed
Allylic Substitutions with Phosphinooxazoline (PHOX) Ligands by Hammett and Swain–
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478.
(
16) Reshi, N. D.; Bera, J. K. Recent advances in annellated NHCs and their metal complexes.
Coord. Chem. Rev. 2020, 422, 213334.
(17) Daw, P.; Petakamsetty, R.; Sarbajna, A.; Laha, S.; Ramapanicker, R.; Bera, J. K. A Highly
Efficient Catalyst for Selective Oxidative Scission of Olefins to Aldehydes: Abnormal–
NHC–Ru(II) Complex in Oxidation Chemistry. J. Am. Chem. Soc. 2014, 136, 13987–13990.
(18) (a) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. How Important Is Metal–Ligand
Back–Bonding toward YX Ligands (Y = N, P, C, Si)? An NBO Analysis. Organometallics
3
2
007, 26, 2637–2645. (b) Shepherd, R. E.; Proctor, A.; Henderson, W. W.; Myser, T. K.
Assessment of the π–Acceptor Capability of Selected Ligands Based on the Photoelectron
Spectra of Ruthenium Ammine Complexes. Inorg. Chem. 1987, 26, 2440–2444. (c) Nelson,
S. M.; Shepherd, T. M. Metal–to–Ligand π–Bonding in Some Nickel(II) Pseudo–Halide
Complexes with Heterocyclic Aromatic Amines. Inorg. Chem. 1965, 4, 813–817.
(
19) Saha, S.; Daw, P.; Bera, J. K. Oxidative Route to Abnormal NHC Compounds from Singly
Bonded [M–M] (M = Ru, Rh, Pd) Precursors. Organometallics 2015, 34, 5509–5512.
20) Catalyst 1 decomposes in halogenated solvents and has limited solubility in toluene and
THF. A related mixed aPmic/phosphine complex shows enhanced stability in
dichloromethane (manuscript under revision). The poor activity/selectivity obtained in these
solvents is likely due to the limited solubility and/or stability of the catalyst.
(
–
(
21) Carbene and tBuOO anion are –donors. Olefin is a –acceptor in addition to being a –
donor. Annellated pyridine module, although predominantly a –donor exhibits –bonding
t
–
character. Accordingly, the arranagement where BuOO and olefin bind trans to carbene
and pyridine modules is defined as 
(
22) (a) Roussel, M.; Mimoun, H. Palladium–Catalyzed Oxidation of Terminal Olefins to Methyl
Ketones by Hydrogen Peroxide. J. Org. Chem. 1980, 45, 5387–5390. (b) Mimoun, H.; Perez
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Machirant, M. M.; Seree de Roch, I. Activation of Molecular Oxygen: Rhodium–Catalyzed
Oxidation of Olefins. J. Am. Chem. Soc. 1978, 100, 5437–5444. (c) Igersheim, F.; Mimoun,
H. Activation and Transfer of Molecular Oxygen. Oxidation of Terminal Olefins to Methyl
Ketones by Rhodium Dioxygen Complexes. J. Chem. Soc., Chem. Commun. 1978, 559–560.
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(d) Nishimura, T.; Kakiuchi, N.; Onoue, T.; Ohe, K.; Uemura, S. Palladium(II)–Catalyzed
Oxidation of Terminal Alkenes to Methyl Ketones Using Molecular Oxygen. J. Chem. Soc.,
Perkin Trans. 1. 2000, 12, 1915–1918.
(
23) The development of positive charge at olefin moiety in TSAB was confirmed by charge
analysis (see, Table S2). In addition, DFT calculations with propene and 3-chloropropene
showed that an electron withdrawing substituent increases the free energy barrier for both
steps A→B and C→D by a similar margin (see, Tables S3).
(
24) The measured thermodynamic parameters are comparable to computed values (see, Table
S4).
(
25) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group
thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and
transition elements: two new functionals and systematic testing of four M06–class
functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215–241.
26) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations.
Potentials for K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299–
(
3
10.
(
27) (a) Hehre, W. J.; Ditchfield R.; Pople, J. A. Self–Consistent Molecular Orbital Methods. 12.
Further extensions of Gaussian–type basis sets for use in molecular–orbital studies of
organic–molecules. J. Chem. Phys. 1972, 56, 2257–2261. (b) Petersson, G. A.; Al–Laham,
M. A. A complete basis set model chemistry. II. Open–shell systems and the total energies
of the first–row atoms. J. Chem. Phys. 1991, 94, 6081–6090.
(
28) (a) Black, M.; Mais, R. H. B; Owston, P. G. The crystal and molecular structure of Zeise’s
salt K.PtCl .C H . Acata Cryst. 1969, B25, 1753–1759. (b) Love, R. A.; Koetzle, T. F.;
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Williams, G. J.; Andrews, L. C.; Bau, R. Neutron diffraction study of the structure of Zeise's
salt, KPtCl (C H ).H O. Inorg. Chem. 1975, 14, 2653–2657. (c) Hajime, K. The Electronic
3
2
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2
−
Structure of Zeise’s Salt, [PtCl C H ] . B. Chem. Soc. Jpn. 1971, 44, 348–354. (d) For
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translation of Zeise’s original paper, see Classics in Coordination Chemistry: Part 2;
Kauffman, G., Ed; Dover: New York, 1976; 21–37.
(
29) When coordinated to a metal, the olefin becomes susceptible to nucleophilic attack unlike
free olefin which undergoes electrophilic attack. For a further discussion on this reversal
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(umpolung) see: Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
Wiley: New York, 1988, 91.
(
30) To view the –corridor, N1–Pd axis (Z-axis) is kept horizontal. Pd(aPmic) is in XZ plane
perpendicular to olefin C2–C3 axis (Y-axis). For clarity, the –corridor is also shown
horizontally.
(31) (a) Reed, A. E., Curtiss, L. A.; Weinhold, F. Intermolecular interactions from a natural bond
orbital, donor–acceptor viewpoint. Chem. Rev. 1988, 88, 6, 899–926. (b) Weinhold, F.;
Landis C. Valency and Bonding: A Natural Bond Orbital Donor–Acceptor Perspective;
Cambridge University Press, 2005, pp 363-575.
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