Palladium-Catalyzed Selective Reduction of Carbonyl Compounds

Article abstract of DOI:10.1002/ejic.202000310

Two new examples of structurally characterized β-diketiminate analogues i.e., conjugated bis-guanidinate (CBG) supported palladium(II) complexes, [LPdX]₂; [L= {(ArHN)(ArN)–C=N–C=(NAr)(NHAr)}; Ar = 2,6-Et₂-C₆H₃], X = Cl (1), Br (2) have been reported. The synthesis of complexes 1–2 was achieved by two methods. Method A involves deprotonation of LH by nBuLi followed by the treatment of LLi (insitu formed) with PdCl₂ in THF, which afforded compound 1 in good yield (75 %). In Method B, the reaction between free LH and PdX₂ (X = Cl or Br) in THF allowed the formation of complexes 1 (Yield 73 %) and 2 (Yield 52 %), respectively. Moreover, these complexes were characterized thoroughly by several spectroscopic techniques (¹H, ¹³C NMR, UV/Vis, FT-IR, and HRMS), including single-crystal X-ray structural and elemental analyses. In addition, we tested the catalytic activity of these complexes 1–2 for the hydroboration of carbonyl compounds with pinacolborane (HBpin). We observed that compound 1 exhibits superior catalytic activity when compared to 2. Compound 1 efficiently catalyzes various aldehydes and ketones under solvent-free conditions. Furthermore, both inter- and intramolecular chemoselectivity hydroboration of aldehydes over other functionalities have been established.

Full text of DOI:10.1002/ejic.202000310

European Journal of Inorganic Chemistry
Accepted Article
Title: Palladium-Catalyzed Selective Reduction of Carbonyl
Compounds
Authors: Nabin Sarkar, Mamata Mahato, and Sharanappa Nembenna
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000310
Link to VoR: https://doi.org/10.1002/ejic.202000310
01/2020

10.1002/ejic.202000310
European Journal of Inorganic Chemistry
FULL PAPER
Palladium-Catalyzed Selective Reduction of Carbonyl
Compounds
Nabin Sarkar, [a] Mamata Mahato,[a] and Sharanappa Nembenna*[a]
Abstract: Two new examples of structurally characterized -
diketiminate analogues i.e., conjugated bis-guanidinate (CBG)
supported palladium (II) complexes, [LPdX]2; [L= {(ArHN)(ArN)–
C=N–C=(NAr)(NHAr)}; Ar = 2,6-Et2-C6H3], X = Cl (1), Br (2) have
been reported. The synthesis of complexes 1-2 was achieved by two
methods. Method A involves deprotonation of LH by n-BuLi followed
by the treatment of LLi (insitu formed) with PdCl2 in THF, which
afforded compound 1 in good yield (75%). In Method B, the reaction
between free LH and PdX2 (X = Cl or Br) in THF allowed the
Moreover, there have been few reports on metal- and solvent-
free hydroboration reactions of aldehyde and ketone, but they
require an excessive amount of HBpin and high-temperature
conditions.10 Surprisingly, there have been no reports on
palladium-catalyzed hydroboration of aldehydes and ketones,
except two illustrations, in which it is limited to CO2 substrate
(vide infra).11
As for as palladium-based N,N’-donor-supported molecular
complexes are concerned, Jin and co-workers synthesized
NacNac supported palladium alkyl complexes and efficiently
utilized as catalysts in the coupling reactions, for example,
Suzuki, Hiyama, Stille, Heck, Buchwald-Hartwig and
Sonogashira reactions (See, Figure 1A).12 In the context of
palladium-catalyzed hydroboration, Huang and co-workers
reported palladium-catalyzed regioselective hydroboration of aryl
formation of complexes
1 (Yield 73%) and 2 (Yield 52%),
respectively. Moreover, these complexes were characterized
thoroughly by several spectroscopic techniques (1H, 13C NMR, UV-
Vis, FT-IR, and HRMS), including single-crystal X-ray structural and
elemental analyses. In addition, we tested the catalytic activity of
these complexes 1-2 for the hydroboration of carbonyl compounds
with pinacolborane (HBpin). We observed that compound 1 exhibits
superior catalytic activity when compared to 2. Compound 1
efficiently catalyzes various aldehydes and ketones under solvent-
free conditions. Furthermore, both inter- and intramolecular
chemoselectivity hydroboration of aldehydes over other
functionalities have been established.
alkenes
with
B2pin2.13
Further,
palladium-catalyzed
hydroboration can be seen only for CO2, alkene, allene, and
alkyne functionalities (Figure1B).11, 13-14
A. NacNac supportedPd complexcatalyzed coupling reactions
Introduction
The reduction of carbonyl compounds to the corresponding
alcohols is one of the most significant transformations in organic
synthesis.1 Besides, chemoselective reduction of aldehyde in
choice of other reducible functionalities such as ketone, ester,
amide, and alkene, is another synthetically valuable organic
conversion. The reduction of carbonyl compounds usually
demands the use of stoichiometric amounts of metal hydride
reducing agents, such as LiAlH4, or NaBH4, which in turn offer
some disadvantages, including safety issues, poor functional
group tolerance, production of a large amount of waste and
laborious isolation and purification steps.2 Metal catalyzed
hydrogenation of carbonyl compounds has been developed as
an excellent method; nevertheless, it needs the use of
flammable hydrogen gas and harsh reaction conditions.3Thus,
due to the drawbacks as mentioned above, metal-catalyzed
hydroboration has become an essential organic transformation
because organoborates are stable toward air and moisture and
non-toxic.4 To date, there have been numerous articles on the
-Diketiminatophosphanepalladium
complex
B. Reported Pd catalyzed hydroborations
C. This work
main
group,5
and
transition6
or
lanthanide7metal-
catalyzedhydroboration reactions.8 Also, a few organocatalyzed
hydroboration of carbonyl compounds is known in the literature.9
[a]
Mr. N. Sarkar, Dr. M. Mahato, Dr. S. Nembenna
School of Chemical Sciences, National Institute of Science Education
and Research (NISER), Homi Bhabha National Institute (HBNI),
Bhubaneswar, 752050, India
Fig1. N, N’ -donor-supported Pd complexes or Pd reagents in coupling and
hydroboration reactions.
http://www.niser.ac.in/scs/professor/snembenna
E-mail: snembenna@niser.ac.in Twitter: SharanNembenna
1
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10.1002/ejic.202000310
European Journal of Inorganic Chemistry
FULL PAPER
It is noteworthy that metal complexes of guanidinate anions in
catalysis have attracted consideration only recently.15 Previously
our group reported guanidinate supported alkaline earth metal
as catalysts for the hydroaminationand Tishchenko reactions.16
Herein, we report well-defined palladium(II) halide complexes
bearing a new class of guanidines, i.e., conjugated bis-
guanidinate (CBG) ligand. These molecules could be considered
as analogues of β-diketiminate (NacNac) palladium complexes.
Moreover, these palladium heterocycles have been tested for
catalytic activity in hydroboration reaction. Compound 1 exhibits
excellent catalytic activity in the hydroboration of aldehydes and
ketones. To our knowledge, this is the first report on the
palladium-catalyzed hydroboration of aldehydes and ketones
with HBpin.
ppm, respectively, which are good in concurrence with the
reported guanidines (148-160 ppm).18 It is worth mentioning that
these resonances are in the upfield region when compared to
tetra-substituted four-membered N,N'-chelating gunidinate metal
complexes (163-168.5 ppm).18c The IR spectra of compounds 1
and 2 display medium-intensity bands at 1593 cm-1 and 1592
cm-1 for the C=N stretching frequencies, and bands at 3405 cm-1
and 3404 cm-1 for the N-H stretches, respectively. The UV-vis
spectra of compounds 1 and 2 in dichloromethane display high-
intensity bands at 248 nm and 316 nm and 248 nm and 316 nm
for ligand (due to -n-transitions) and weak-intensity
bands at 405 nm and 412 nm for metal-centered d-d transitions,
respectively.
Further, the molecular structures of compounds 1 and 2 were
established by single-crystal X-ray diffraction studies. The
molecular structures, selected bond lengths, and bond angles
are shown in Figure 2 (for crystal data and structure refinement,
see Table S1). Compounds 1 and 2 were crystallized in the
triclinic system with the P1 space group. The compound [LPdCl]2
(1) is isostructural with compound [LPdBr]2 (2). Both compounds
1 and 2 exist as dimers in the solid-state. The Pd–N (N, N’-CBG)
bond distances in compound 1 of Pd1–N1, 2.000(3) Å, Pd1–
N31.999(3) Å, Pd2–N6 2.002(3) Å and Pd2–N8 1.996(3) Å are
almost compliment to Pd–N (N, N’-CBG) bond distances of
compound 2, Pd1–N1 2.021(4) Å, Pd1–N3 2.007(5) Å. In both
cases, the palladium metal centers adopt a typical four-
coordinate square planar geometry bonded to one N,N’-chelated,
monoanionic CBG, and two bridged halide moieties. The bond
lengths in compound 1 Pd1–Cl1 2.3327(8) Å and Pd1–Cl2
2.3398(9) Å, are comparable with related -diketiminate
palladium chloride complexes [{Pd(Ph2NacNac)Cl2};19 Pd1–Cl1
2.343(1) Å] and shorter than that of Pd–Cl bond distance
(2.474(4)Å) in Pd2(hpp)4Cl220 and longer than that of Pd-Cl bond
distances in the compound [trans-{Pd(Ph3Guan)2Cl2}]21 (Pd–Cl1
2.312 (3)Å Pd–Cl2 2.308(3) Å).
Results and Discussion
Synthesis and Characterization of Conjugated Bis-
Guanidinate (CBG) Stabilized Palladium Complexes.
Deprotonation of LH17 [L= {(ArNH)(ArN)–C=N–C=(NAr)(NHAr)};
Ar =2,6-Et2-C6H3] with n-BuLi at –78 oC
The N1–Pd1–N3 bond angles in compounds 1 and 2 90.69(12)o
and N1–Pd1–N3 89.46(19)o, respectively, are slightly acute than
the corresponding bond angle observed in [{Pd(Ph2NacNac)Cl2};
N1–Pd1–N2 91.4 (1)o]19. Overall, the structures of 1 and 2 are
similar to those described by Song and coworkers for the closely
related -diketiminate palladium chloride complex. Complex 2
possessing half of the molecule in the asymmetric unit and
contains a crystallographically imposed inversion center in the
middle. As expected, the Pd–Br bond distance in compound 2,
i.e., Pd1–Br1 2.4464(6)Å longer than that of Pd–Cl bond
distances (Pd1-Cl1 2.3327(8)Å, Pd1-Cl2 2.3398(9)Å) in
compound 1(summation of covalent radii of Pd–Br = 2.59 (9) Å
and Pd–Cl = 2.41(10)Å).22
Scheme 1. Synthesis of complexes 1 and 2.
and subsequent reaction with PdCl2 led to a dinuclear palladium
complex [LPdCl]2 (1). Alternatively, compound
prepared by the direct addition of a free ligand, LH, with PdCl2 in
THF. Similarly, the reaction of LH with PdBr2 in THF afforded the
[LPdBr]2 (2) (Scheme 1).
1 can be
Compounds 1 and 2 are air- and moisture-stable and are soluble
in methanol, acetonitrile, dichloromethane, chloroform, and THF.
Both compounds 1 and 2 are thermally stable and melt in the
o
o
1
range of 232-234 C and 260-262 C, respectively. The H and
13C NMR spectra of compounds 1 and 2 (Figures S3-S6 in the
Supporting Information) are well in agreement with the
symmetrical structures established in the solid-state. The 1H
NMR spectra of compounds 1 and 2 display singlet resonances
at 4.70 ppm and 4.71 ppm in CDCl3, respectively, corresponding
to four protons of the side arms Ar-NH moieties of the ligand.
Moreover, a complete disappearance of peak at 12.68 ppm (N-
H··N) in compounds 1 and 2, which indicated the formation of
Fig 2. Molecular structures of 1 and 2. The thermal ellipsoids are shown at
50% probability, and all hydrogen atoms are omitted (except N-H's) for clarity.
Selected bond lengths (Å) and bond angles (o); 1: Pd1–N1 2.000(3), Pd1–N3
1.999(3), Pd2–N6 2.002(3), Pd2–N8 1.996(3), Pd1–Cl1 2.3327(8), Pd1–Cl2
2.3398(9), Pd2–Cl1 2.3519(9), Pd2–Cl2 2.3408(8); N1–Pd1–N3 90.69(12),
N3–Pd1–Cl1 175.87(9), N1–Pd1–Cl1 93.29(9),N3–Pd1–Cl2 93.62(9), N1–
Pd1–Cl2 175.44(9), Cl1–Pd1–Cl2 82.43(3), N6–Pd2–N8 90.12(12); 2: Pd1–N1
2.021(4), Pd1–N3 2.007(5), Pd1–Br1 2.4464(6), Pd1–Br1' 2.4583(7); N1–
1
the desired products. The H NMR spectra of compounds 1 and
2 exhibit two sets of triplet resonances and four sets of quartet
resonances corresponding to ArCH2CH3 and ArCH2CH3 of
diethyl-Ar groups attached to the nitrogen atom of the ligand
framework. The 13C NMR spectra of compounds 1 and 2 show a
significant peak for the N3C carbon atom at 149.8 and 148.5
2
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10.1002/ejic.202000310
European Journal of Inorganic Chemistry
FULL PAPER
Pd1–N389.46(19), N1–Pd1–Br1 94.63(13);N1–Pd1–Br1'176.22(13), N3–Pd1–
Br1 94.31(13), N3–Pd1–Br1' 175.91(13), Br1–Pd1–Br1' 81.60(2).
donating (methyl, methoxy, amino)and withdrawing substituents
(chloro, bromo, nitro, nitrile, ester)were converted into the
corresponding hydroborated products.
Catalytic Hydroboration of Aldehydes and Ketones.
Palladium-based catalysts have been widely employed as
catalysts in coupling (C-C and C-N, etc.) reactions.23
Table 1. Palladium-catalyzed hydroboration of benzaldehydea.
Entries
Catalyst
Mol %
Temp (oC)
Solvent
Conv
(%)b
1
2
3
4
5
6
7
1
1
1
1
2
2
rt
CDCl3
40
2
4
4
4
4
4
60
60
60
60
60
60
CDCl3
CDCl3
Neat
96
>99
>99
90
CDCl3
CDCl3
CDCl3
PdCl2
PdBr2
70
Scheme 2. Substrate scope for aldehydes hydroboration catalyzed by 1a
60
aReaction Conditions: benzaldehyde (0.3 mmol), pinacolborane (0.3 mmol),
and Pd based catalyst (x mol %), 60 oC, 12 h. bConversion was based upon 1H
NMR spectroscopy.
aReaction Conditions: aldehyde (1 mmol), HBpin (1 mmol) and catalyst
[LPdCl]2 (1) (4 mol %) at 60 C for 12 h under N2. Isolated yields were
determined after column chromatography.
o
b
Further, aldehydes of heterocycles, aromatic and aliphatic
alkenes, aliphatic alkyne, and biphenyls were also converted
into reduced products. Please note that this exotic substrate
scope includes few intramolecular chemoselectivities (e.g., nitrile,
ester, alkene, alkyne, and amine).1H and 13C NMR spectra for
boronate esters (3a-3s) and alcohols (4a-4s) are attached in the
supporting information.
Surprisingly, palladium (molecular or reagent)-catalyzed
hydroboration reactions remain scarce.11,13-14 Hence, we
determined to examine the catalytic activity of these complexes
1-2 for the hydroboration of carbonyl compounds. Initial
evaluations of the catalytic studies of compounds 1-2, ligand-
free palladium salts, PdCl2, and PdBr2 were undertaken for the
hydroboration of benzaldehyde with HBpin (Table 1).
Further, the hydroboration scope was expanded to the ketones.
Proceeding with the optimized condition for aldehydes resulted
in lesser conversion in the case of the representative ketone, i.e.,
acetophenone as expected (Table 2, entry 1). An increase in
At first, the reaction of equimolar amounts of benzaldehyde and
HBpin with catalyst 1 (2 mol %) was performed in CDCl3 at room
1
temperature. H NMR confirms the formation of PhCH2OBpin in
o
40% yield. The same reaction was performed at 60 C, which
catalyst loading up to
6 mol % led almost quantitative
produced the product in a 90% yield. When the catalyst loading
was increased to 4 mol%, the product 3a was obtained
quantitatively in 12 h (entry 3). Interestingly, when the same
reaction was performed under neat condition, a quantitative
conversation of benzaldehyde into product 3a was noticed.
Further, ligand free-Pd salts such as PdCl2 and PdBr2 were
tested for the catalytic hydroboration of benzaldehyde; we
noticed lesser conversion when compared to catalysts 1 and 2
(Entries 6 and 7).
Inspired by the fact of the high reactivity of catalyst 1 towards
hydroboration of benzaldehyde, further substrates scope was
broadened. Various aldehydes were smoothly converted to their
corresponding boronate esters in quantitative yields under
solvent-free conditions (3a-3s). Further, hydrolysis of the
boronate esters (3a-3s) upon heating with silica gel and
methanol at 60 oC yielded the corresponding alcohols (4a-4s)
with good to excellent yields (4a-4s 70-95%; except 4l-4m 46%)
(Scheme 2). Aromatic aldehydes containing both electron-
transformation for acetophenone as substrate at 60 oC for
compound 1 (entry 3). At this optimized condition, complex 2
shows lower catalytic activity (entry 4). The use of LH17 and
PdCl2 together resulted in almost quantitative conversion (entry
5). With the bare salts PdCl2 and PdBr2, the only partial
conversion was observed (entries 6 and 7).
Data corresponding to the boronate esters of the ketone is
shown in the supporting information (Figure S88-S134). Later,
corresponding secondary alcohols were isolated after workup
followed by column chromatography. The versatile substrate
scope is depicted in Scheme 3, which is made up of the
hydrolyzed products (Figure S135-S176) after hydroboration of
ketones containing electron-donating functionalities, e.g., methyl,
methoxy (6b-6f), electron-withdrawing functional groups, e.g.,
3
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10.1002/ejic.202000310
European Journal of Inorganic Chemistry
FULL PAPER
During substrate screening, intramolecular functional group
tolerance is also observed in the case of methoxy, halide, nitrile,
and heteroatom functionalities. In all cases, secondary alcohols,
6a-6s, were isolated with good yield, 70-85% after workup of
boronated compounds.
Table 2. Palladium-catalyzed hydroboration of acetophenonea
Further, palladium-catalyzed intermolecular chemoselective
reduction of benzaldehyde with HBpin over other reducible
functionalities such as ketone, ester, amide, and alkene was
examined (Scheme 4).
Acetophenone, benzyl benzoate, benzamide, and styrene were
independently reacted with an equimolar amount of
benzaldehyde, HBpin, and catalyst 1 (4 mol %) in neat condition
at 60 oC for 12 h.
Entries
Catalyst
Mol %
Temp
(oC)
Solvent
Conv (%)b
1
2
3
4
5
1
1
1
2
4
6
6
6
6
60
60
60
60
60
CDCl3
CDCl3
Neat
85
99
>99
75
CDCl3
CDCl3
LH+
>99
PdCl2
6
7
PdCl2
PdBr2
6
6
60
60
CDCl3
CDCl3
68
55
aReaction conditions: acetophenone (0.3 mmol), pinacolborane (0.3 mmol)
and Pd based catalyst (X mol %), 60 oC, 12 h. bConversion was based upon
1H NMR spectroscopy.
chloro, bromo, iodo, trifluoromethyl, nitrile (6g-6l), heterocycles
(6m-6n), and fused systems with ring size variation (6o-6q),
aromatic disubstituted ketones (6r-6s) and fused systems (6t-
6u).
Scheme 4.Intermolecular chemoselective hydroboration of aldehydes
catalyzed by 1
In all instances, the complete conversion of benzaldehyde into
1
PhCH2OBpin (3a) was established by H and 13C NMR studies
(Figures S177-S148).
Scheme 3. Substrates scope for ketones hydroboration catalyzed by 1a
aReaction Conditions: ketones (1 mmol), HBpin (1 mmol) and catalyst
[LPdCl]2
Scheme 5. Proposed Mechanism for the Hydroboration of Carbonyl
Compounds
(1
) (6 mol%) at 60 oC for 12 h under N2. bIsolated yields were
determined after column chromatography.
4
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10.1002/ejic.202000310
European Journal of Inorganic Chemistry
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Celite and concentrated under high vacuum to obtain an orange-brown
powder of 1. Yield: 0.182 g, 75%.
As established by the previously published reports on the metal
halide/hydride catalyzed hydroboration reactions, herewith, we
are suggesting an insertion/σ-bond metathesis type mechanism
(See, Scheme 5). Initially, catalyst 1 reacts with HBpin that leads
to the formation of active catalyst palladium hydride, i.e., LPdH.
Next, palladium hydride reacts with carbonyl substrate (aldehyde
or ketone), in which the insertion of C=O into Pd-H happens that
leads to the production of LPdalkoxide (III) via intermediate II.
The palladium alkoxide upon reaction with HBpin afforded
boronate ester product and regeneration of the active LPdH
catalyst, via intermediate IV. It should be noted that CBG
supported palladium chloride acts as a pre-catalyst for the
hydroboration of a vast number of substrates in the absence of
an activator.
Method B: To a solution of LH (0.2 g, 0.32 mmol) [L= {(ArNH)(ArN)C=N–
C=(NAr)(NHAr)}; Ar = 2,6-Et2-C6H3] in THF (15 mL), anhydrous palladium
(II) chloride (0.056 g, 0.32 mmol) in THF (10 mL) was added and stirred
o
at 0 C for 5 h and then left for stirring at rt for 48 h. Then it was filtered
through Celite and concentrated under a high vacuum. Orange-brown
crystals of complex 1 were isolated through slow evaporation technique.
Yield: 0.180 g, 73%.
Mp 232–234 oC. IR (KBr pellets, cm-1) : 3405 (N–H), 3391 (N–H), 1593
(C=N). UV-Vis: 230 nm, 248 nm, 316 nm, 405 nm. 1H NMR (400 MHz,
3
CDCl3) δ6.98–6.94 (m, 4H, ArH), 6.90–6.88 (d, JHH = 7.4 Hz, 8H, ArH),
3
3
6.72 (t, JHH = 7.5 Hz, 4H, ArH), 6.50–6.48 (d, JHH = 7.6 Hz, 8H, ArH),
4.70 (s, 4H, NH), 3.13–3.07 (dd, 8H, CH2CH3), 2.59–2.49 (m, 8H,
CH2CH3), 2.27–2.16 (m, 8H, CH2CH3), 2.08–2.00 (m, 8H, CH2CH3), 1.37
3
3
(t, JHH = 7.5 Hz, 24H, CH2CH3), 0.82 (t, JHH = 7.6 Hz, 24H, CH2CH3).
13C{1H}NMR (100 MHz, CDCl3) δ 149.8 (N3C), 142.3 (ArC), 141.1 (ArC),
140.0 (ArC), 135.2 (ArC), 126.3 (ArC), 126.0 (ArC), 125.0 (ArC), 124.9
(ArC), 24.6 (CH2CH3), 23.7 (CH2CH3), 14.2 (CH2CH3), 13.2 (CH2CH3).
HRMS. Calcd: m/z 1541.6242, Found: 1541.6301 ([C84H108N10Cl2Pd2]+);
Calcd: m/z 774.1138, Found: 774.3191 ([C42H54N5ClPd]+);Calcd: m/z
Conclusions
In summary, a new class of guanidine ligands, i.e., conjugated
bis-guanidinate (CBG) supported, N,N’-chelating, six-membered
palladium metallacycles (1-2) have been established. Both
compounds 1and 2 exist as dimers in the solid-state. Further,
compounds 1-2 employed as catalysts for the hydroboration of
carbonyl compounds with HBpin. We observed that catalyst 1 is
superior when compared to catalyst 2. The catalyst 1 has shown
remarkable features, including a wide functional group tolerance
and operates at solvent-free conditions. The noticeable
736.3400,
Found:
736.3521([C42H54N5Pd]+).
Anal.
Calcd for
C84H108N10Cl2Pd2: C, 65.45; H, 7.06; N, 9.09. Found: C, 65.88; H, 7.93; N,
9.24.
Synthesis of [LPdBr]2 (2).
Method B:To a solution of LH (0.2 g, 0.32 mmol, 1.0 equiv.) in THF (15
mL), anhydrous palladium(II) bromide (0.085 g, 0.32 mmol, 1.0 equiv.), in
THF (10 mL) was added and stirred at 0 oC or 5h and then left for stirring
at room temperature for 48h followed by filtered through Celite. Then the
solution was concentrated. After 2 days it yielded 2 as brown single
chemoselectivity displayed by catalyst
1 may also find
applications in the other organic transformations.It should be
recalled that biguanides and their coordination chemistry are
known in the literature.However, thecoordination chemistry of
anions of tetra-aryl substituted CBGs is very limited. Thus, such
studies are ongoing in our laboratory.
o
crystals through slow evaporation. Yield: 0.138 g, 52%. Mp 260–262 C.
IR (KBr pellets, cm-1) : 3404 (N–H), 3392 (N–H), 1592 (C=N). UV-Vis:
252 nm, 343 nm, 412 nm. 1H NMR (400 MHz, CDCl3) δ 6..94 (m, 4H,
3
3
ArH), 6.906.88 (d, JHH = 7.4 Hz, 8H, ArH), 6.73 (t, JHH= 7.5 Hz, 4H,
ArH), 6.516.49 (d, J = 7.6 Hz, 8H, ArH), 4.70 (s, 4H, NH), 3.13–3.06 (dd,
8H, CH2CH3), 2.59–2.50 (dd, 8H, CH2CH3), 2.26–2.17 (dd, 8H, CH2CH3),
2.08–2.01 (m, 8H, CH2CH3), 1.37 (t, 3JHH = 7.5 Hz, 24H, CH2CH3), 0.82 (t,
3JHH = 7.6 Hz, 24H, CH2CH3). 13C {1H} NMR (100 MHz, CDCl3) δ 148.5
(N3C), 142.4 (ArC), 138.1 (ArC), 137.7 (ArC), 130.6 (ArC), 128.3 (ArC),
127.4 (ArC), 126.9 (ArC), 126.0 (ArC), 24.8 (CH2CH3), 23.4 (CH2CH3),
14.4 (CH2CH3), 13.6 (CH2CH3). HRMS: Calcd: m/z, 815.2692, Found
815.2757 ([C42H55N5Br1Pd1]+),Calcd: m/z 735.4864, Found 735.3502
([C42H54N5Pd]+). Anal. Calcd for C84H108N10Br2Pd2: C, 61.88; H, 6.68; N,
8.59. Found: C, 59.70; H, 7.48; N, 8.05.
Experimental Section
All reagents were purchased from Sigma-Aldrich and Merck and used
without further purification. Various solvents utilized for synthesis were
purchased from Merck and used after drying by conventional methods.
The precursor LH17[L = {(ArHN)(ArN)C=N
̶ C=(NAr)(NHAr)}; Ar = 2, 6-Et2
- C6H3] was prepared by using method developed in our laboratory.NMR
spectroscopic data were recorded on a Bruker AV 400 MHz instrument
{1H (400 MHz), and13C{1H} (100 MHz)}. Dry deuterated benzene (C6D6),
chloroform (CDCl3) solvents were used for NMR measurements;chemical
shift values (δ) were reported in parts per million (ppm) relatives to the
residual signals of their respective solvents.24 High-resolution mass
spectra (HRMS) were recorded on the BrukermicrOTOF-Q II
spectrometer. IR spectra were recorded on the Perkin-Elmer FTIR
spectrometer. Elemental analysis for compounds 1 and 2 was performed
byEuroEA3000-CHNS-Analyzer. UV-Visible spectra were recorded on
the Jasco V-730 spectrophotometer. The melting point of the respective
complexes was measured from Stuart SMP 10 instrument.
General Procedure for the Catalytic Hydroboration of Aldehydes
and Ketones.Aldehyde or ketone (1.0 mmol), pinacolborane (1.0 mmol),
catalyst 1 (4 mol% or 6 mol%) were charged in a screw cap vial inside
the glove box. The reaction mixture was allowed to heat at 60 oC for 12 h.
The progress of the reaction was checked by 1H NMR spectroscopy. The
1H NMR spectrum explains the complete disappearance of the starting
material and the appearance of a new CH2 or CH peak. Upon completion
of the reaction, the resulted boronate ester residue was charged with
o
silica gel and methanol and heated for 6 h at 60 C. Further, the solution
was evaporated under vacuum and extracted with ethyl acetate followed
by purified through column chromatography over silica gel using ethyl
acetate/hexane (1:5) mixture as eluent to obtain the pure primary
alcohols.
Synthesis of [LPdCl]2 (1).
Method A: A solution of 0.16 mLofn-BuLi, 2 M in cyclohexane was
added to a solution of LH (0.2 g, 0.32 mmol) [L= {(ArNH)(ArN)C=N–
C=(NAr)(NHAr)}; Ar = 2,6-Et2-C6H3] in THF (15 mL) at -78 oC and allowed
to warm to room temperature followed by stirring for 16 h. This solution
was added dropwise through a cannula to another Schlenk tube
containing palladium (II) chloride (0.028 g, 0.16 mmol) in anhydrous THF
(10 mL) and stirred for 12 h. After the desired time, it was filtered through
General Procedure for the Intermolecular Chemoselective Catalytic
Hydroboration by HBpin.In a sealed vial, 18.4 mg (0.03 mmol, 4 mol%)
of catalyst 1, 0.3 mmol of pinacolborane, and 1.0 mmol of reactants were
added successively. CDCl3 was then added. Progress of the reaction
was examined by 1H NMR analyses, which indicated the complete
5
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10.1002/ejic.202000310
European Journal of Inorganic Chemistry
FULL PAPER
hydroboration of aldehyde over unreacted ketone, ester, amide and
carboxylic acid (final spectra are provided) in individual cases.
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X-Ray Crystallography
Single crystals of 1-2 were grown from THF, mounted on a loop, and
attached to
a goniometer head on a RigakuOxford, four-circle
diffractometer equipped with graphite-monochromated Cu-Kradiation (k
= 1.54184 Å) and a hybrid photon counting detector (DECTRIS Pilatus
200K). The full data sets were recorded, and the images processed using
the CrysAlis Pro. Structure solution by direct methods was achieved
through the use of the SHELXT program, and the structural model
refined by full-matrix least-squares on F2using SHELXLby using the
Olex2 software.25-27 CCDC 1956754-1956755 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax:+ 44 1223 336033
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Acknowledgments
The authors thank the National Institute of Science Education
and Research (NISER), Homi Bhabha National Institute(HBNI),
Bhubaneswar, Department of Atomic Energy (DAE), Govt. of
India.
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Keywords:guanidine•hydroboration•N-donor
ligand•palladium•reduction
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Entry for the Table of Contents
Hydroboration of Carbonyls
The first examples of well-defined dinuclear palladium(II) complexes bearing N,N'-chelated conjugated-bis-guanidinate (CBG), and
halide ligands have been reported. Further, compound 1 catalyzed hydroboration of a wide range of aldehydes and ketones has been
investigated. Besides, palladium-catalyzed chemoselective hydroboration of aldehydes in the presence of other reducible functional
groups has been established.
8
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