Picolinamide Assisted Oxidation of CH2 Groups Bound to Organic and Organometallic Compounds Using Ferrocene as a Catalyst

Article abstract of DOI:10.1021/acs.organomet.9b00085

Picolinamide group assisted sp³ C-H bond oxidation of methylene groups to the corresponding carbonyl compounds has been achieved by using simple bottle ferrocene as catalyst and Cu(OAc)₂ or tert-butyl peroxybenzoate (TBPB) as oxidant under mild conditions. This method is applicable for picolinamide bound organic as well as organometallic compounds with yields in the range of 46-82%. Control experiments and mechanistic studies indicate that a radical mechanism is responsible for these oxidative transformations in which ferrocene acts as a catalyst.

Full text of DOI:10.1021/acs.organomet.9b00085

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Picolinamide Assisted Oxidation of CH2 Groups Bound to Organic
and Organometallic Compounds Using Ferrocene as a Catalyst
Pritam Dolui, Susanta Hazra, Mayukh Deb, and Anil J. Elias*
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India
S
* Supporting Information
ABSTRACT: Picolinamide group assisted sp3 C−H bond
oxidation of methylene groups to the corresponding carbonyl
compounds has been achieved by using simple bottle
ferrocene as catalyst and Cu(OAc)2 or tert-butyl peroxyben-
zoate (TBPB) as oxidant under mild conditions. This method
is applicable for picolinamide bound organic as well as
organometallic compounds with yields in the range of 46−
82%. Control experiments and mechanistic studies indicate
that a radical mechanism is responsible for these oxidative
transformations in which ferrocene acts as a catalyst.
conductors, conducting polymers, and charge storage materials
have also been reported.15,16 However, only a handful of
reports exist indicating the use of nonfunctionalized ferrocene
as a catalyst for organic transformations.17−19 Mao and co-
workers have reported ferrocene catalyzed decarboxylative
cross coupling with toluene.17 Ferrocene has also been used in
catalytic amounts for the borylation of diazonium salts.18 Baran
and co-workers have reported ferrocene catalyzed C−H
imidation of arenes.19 Recently, we have reported oxidation
of primary amines to imines using ferrocinium hexafluor-
ophosphate.20 However, to the best of our knowledge simple
ferrocene or functionalized ferrocene has never been used as
catalyst for such oxidation reactions.
Herein, we report ferrocene catalyzed oxidation of
picolinamide bound CH2 groups of organic and organometallic
compounds using TBPB or Cu(OAc)2 as the oxidant (Scheme
1). To the best of our knowledge, this represents an
unprecedented use of a picolinamide directing group as well
as ferrocene for the oxidation of sp3 CH2 units bound to the
phenyl ring or Cp ring of metal sandwich compounds.
INTRODUCTION
■
Oxidation reactions are one of the most important tools for
transformation of functional groups in organic synthesis.1
Among all the oxidized feedstocks, aldehydes and carboxylic
acids are needed as bulk chemicals in large amounts in various
fields including fine chemicals, polymers, and many commer-
cial products.2 Direct oxidation of alcohols to the correspond-
ing carbonyl compounds such as ketones, aldehydes, and
carboxylic acids have been achieved using various transition
metal catalysts as well as metal free catalysts.3−8 However,
direct oxidation of primary amines to carbonyl compounds
have rarely been reported.9 This is due to fact that the direct
oxidation of primary amines inherently produces imines during
the oxidation process.10 Recently, Beller and co-workers have
reported direct oxidation of benzyl amines to benzamide
derivatives using FeCl3/ZnCl2 as the catalyst and tert-butyl
hydroperoxide (TBHP) as the oxidant (Scheme 1A) .9a The
mechanistic studies on this reaction indicated that the free
amine or N-protected amine could not be removed from the
parent molecules, and as a result the reaction ended up with
amide derivatives. Therefore, synthesis of aldehydes and
carboxylic acids are not possible using this approach. Recently,
we have developed a directing group assisted Ru-catalyzed C−
H bond oxidation to the corresponding aldehydes (Scheme
1B).11 However, this method was found to be applicable only
on methylene units bound to sandwich compounds and
required the relatively expensive organometallic compound
[(RuCl2(p-cym)]2 as the catalyst.
RESULTS AND DISCUSSION
■
Picolinamide is a well-known bidentate directing group for
transition metal catalyzed C−H functionalization in organic
synthesis.21a,d,e The picolinamide group, in the pioneering
report by Daugulis in 2005, demonstrated excellent directing
abilities that enable various types of functionalization including
arylation, alkylation and alkenylation of aromatic and aliphatic
substrates. In addition, picolinamide directed C−C, C−Se, C−
Ge bond formation has also been reported.21,22 Recently, we
have reported picolinamide directed sp3 C−H bond oxidation
Ferrocene is one of the most stable, readily available, and
inexpensive organometallic compounds. Functionalized ferro-
cenes have attracted the attention of chemists of various
disciplines due to their applications as pharmaceuticals,
biosensors, fuel additives, and most importantly as ligands in
asymmetric synthesis.12−14 Recently, use of ferrocene deriva-
tives in the field of electroactive materials such as semi-
Received: February 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00085
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to find out the possibility of using ferrocene as catalyst for this
oxidation.
Scheme 1. Comparison of Reported Methods for C−H
Bond Oxidation to Carbonyl Compounds of Organic and
Organometallic Sandwich Compounds with the Present
Study
After detailed studies (Table S1−S2 in the Supporting
Information) it was found that 20 mol % ferrocene brings out
this sp3 C−H bond oxidation most effectively.
We performed sp2 C−H bond activation reactions with the
ferrocene picolinamides and sp3 C−H bond oxidation in a
stepwise manner and also as a one pot method. This reaction
worked well with both α,α-bis-alkyl and α,α-bis-aryl sub-
stituted picolinamides (Scheme 3, 2a−2c).
Scheme 3. Substrate Scope for sp2 C−H Bond
Functionalization and sp3 C−H Bond Oxidation on
Ferrocene
We were keen to explore this method for the oxidation of
CH2-units bound to an organic substrate. First we tried out the
oxidation reactions of benzylic picolinamides under identical
reaction conditions (Scheme 3). However, this reaction did
not yield the target product and we recovered the benzyl
picolinamide. We assumed that oxidant may be playing a
significant role in this transformation. Therefore, we varied the
oxidants to attempt oxidation of the CH2 unit of the
picolinamide. We chose 4-methoxybenzyl picolinamide as the
model compound for optimization studies, and after several
reactions, it was found that ferrocene (20 mol %) and tert-butyl
peroxybenzoate (TBPB) work well for this transformation
(Scheme 4). However, in this case we obtained 4-methoxy
of metal sandwich compounds using ruthenium para-cymene
as a catalyst and Cu(OAc)2 as an oxidant.11
During these studies, we observed that ferrocene derived
picolinamides were undergoing oxidation even in the absence
of the Ru-catalyst (Scheme 2A). However, there was no such
oxidation when the picolinamide derived cobalt sandwich
compound [η5-(CH2R)C5H4]Co(η4-C4Ph4) (R = Picolinami-
do) was used along with Cu(OAc)2 (Scheme 2B). This
observation indicated that ferrocene unit has some role to play
in this oxidation. Therefore, detailed studies were undertaken
Scheme 2. Differences in the Cu(OAc)2 Oxidation of
Picolinamide Derived Iron and Cobalt Sandwich
Compound
Scheme 4. Optimized Reaction Conditions for Oxidation of
4-Methoxy Benzyl Picolinamide to Its Carboxylic Acid
benzoic acid as the final product which may have resulted due
to the reactivity of corresponding benzaldehyde under this
reaction condition. The effect of reaction parameters were also
optimized (Table S3−S7 in the Supporting Information). On
the basis of isolated yields, it was observed that DCE was the
most appropriate solvent for the reaction with TBPB as
oxidant and ferrocene as the catalyst at 80 °C (Scheme 4).
B
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Scheme 6. Control Experiments for Ferrocene Catalyzed sp3
C−H Bond Oxidation
After optimizing the reaction conditions, we were keen to
explore the substrate scope for this reaction. The reaction was
found to work well with para as well as meta-substituted
benzylamines (Scheme 5).
Scheme 5. Substrate Scope for the Oxidation of Benzyl
Picolinamide to Corresponding Carboxylic Acids
Scheme 7. Proposed Reaction Mechanism for Ferrocene
Catalyzed sp3 C−H Bond Oxidation of Metal Sandwich
Compounds
We observed that yields are slightly better for para
substituted products. Both electron rich and electron poor
benzylamines gave good to moderate yields of benzoic acids.
The study was extended to a few amines of higher complexity
as well. However, ortho-substituted benzyl picolinamides were
unreactive under this optimized reaction conditions possibly
due to steric constraints. Compounds substituted at the
benzylic position were also unresponsive under this optimized
reaction condition.
Proposed Mechanism for sp3 C−H Bond Oxidation.
To investigate the possible reaction mechanism for the sp3 C−
H bond oxidation, we have carried out some control
experiments (Scheme 6).
Since, single electron transfer (SET) can be a possible
mechanistic pathway for the sp3 C−H bond oxidation reaction,
we have carried out this reaction in the presence of radical
inhibitors. When performed in the presence of TEMPO
((2,2,6,6-tetramethylpiperidine-1-yl)oxyl) as additive (1−2
equiv), 1 reacted to give aldehyde 2 in 30% and 25% yields,
respectively (Scheme 6B). We have also performed the
reaction with 1 and 2 equiv of BHT (butylated hydrox-
ytoluene), which gave aldehyde 2 in 20% and 10% yields,
respectively. Reaction of 1 with 1 equiv of L-ascorbic acid gave
10% yield of the aldehyde 2. This study indicates that a free
radical process is involved in the oxidation of picolinamide to
aldehyde. Picolinamide group does play an important role in
this oxidation, as changing the picolinamide directing group
with aminomethyl group or the benzamide derivative of metal
sandwich compounds, the oxidation reactions are unresponsive
(Scheme 6A). On the basis of our observation and related
studies, we have proposed a possible catalytic cycle as shown in
Scheme 7. At the outset Cu2+ oxidizes ferrocene to ferrocinium
and itself gets reduced to Cu1+ in a redox reaction, which was
confirmed by UV studies (Supporting Information Page S7).
Then Cu1+ abstracts a proton from C−H bond of ferrocene
picolinamide to form the radical intermediate IM1. Afterward
IM1 is converted to an iminium intermediate IM2, in the
presence of FeCp2 by a single electron transfer (SET)
reaction. Thereafter IM2 undergoes hydrolysis to form the
aldehyde.
Reaction of 5, an organic substrate with equivalent amount
of TEMPO and BHT gave 10% and 8% yields of
corresponding acid respectively (Scheme 6C). A sharp
decrease in yield indicated that the reaction mechanism goes
through a radical process. We have carried out a UV study
(Supporting Information Page S7−S8) of the reaction mixture,
we observed the presence of the ferrocinium ion from its
characteristic peak. On the basis of our observations and
control experiments, we have proposed a possible mechanism
(Scheme 8).
+
C
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H), 3.82 (s, 6H, OMe-H). 13C NMR (75 MHz, CDCl3) δ 163.44,
158.40, 149.88, 148.04, 137.17, 130.13, 129.51, 125.98, 122.02,
113.66, 89.05, 80.57, 71.46, 68.55, 55.27. HRMS m/z 555.1342, calcd
for C31H28FeN2NaO3 [M + Na]+ 555.1347.
Scheme 8. Proposed Reaction Mechanism for Ferrocene
Catalyzed sp3 C−H Bond Oxidation of Aryl Picolinamide
[2,5-(4-BrC6H4)2-η5-(CH2R)C5H2]Fe(η5-C5H5) (R = Picolinamido)
(1b). Red orange semisolid; yield 75%. 1H NMR (300 MHz, CDCl3)
δ 8.43−8.44 (s, 1H), 8.15−8.18 (s, 1H), 7.84−7.85 (m, 2H), 7.41−
7.48 (s, 8H), 7.37−7.38 (s, 1H), 4.60−4.62 (s, 4H), 4.24 (s, 5H, Cp-
H).13C NMR (75 MHz, CDCl3) δ 163.46, 149.57, 148.02, 137.33,
136.46, 131.30, 130.57, 126.18, 122.14, 120.62, 88.25, 80.81, 71.79,
70.21, 36.60. HRMS m/z 650.9341, calcd for C29H22FeBr2N2NaO [M
+ Na]+ 650.9346.
General Procedure for Bis-Alkylation of [η5-(CH2R)C5H4]Fe-
(η5-C5H5) (R = Picolinamido). A 15 mL screw capped vial was
charged with a magnetic bead. One equivalent of ferrocenyl
picolinamide (0.03 g, 0.10 mmol), Pd(OAc)2 (20 mol %), Cs2CO3
(2 equiv), and alkyl iodide (2.5 equiv) were dissolved in tert-amyl
alcohol (5 mL). The reaction mixture was heated at 80−90 °C for 5−
10 h. The reaction was monitored by TLC. After completion, the
reaction mixture was dried under a vacuum, and the crude product
was purified through column chromatography using silica and
hexane/ethyl acetate (90/10) as eluent.
[2,5Bis(methyl)η5(CH2R)C5H2]Fe(η5C5H5) (R = Picolinamido) (1c).
1
Yellow-orange semisolid; yield 74%. H NMR (300 MHz, CDCl3) δ
8.51 (s, 1H, N-H), 8.21−8.23 (dd, 2H), 7.81−7.85 (m, 1H), 7.37−
7.40 (d, 1H), 4.43 (d, 2H, CH2−H), 3.99−4.07 (m, 7H, Cp-H), 1.99
(s, 6H, CH3-H). 13C NMR (75 MHz, CDCl3) δ 163.65, 150.17,
148.21, 137.45, 126.20, 122.40, 83.59, 82.69, 69.94, 67.67, 35.88,
13.38. HRMS m/z 371.0817, calcd for C19H20FeN2NaO [M+ Na]+
371.0823.
General Procedure for Synthesis of α,α-Bis-Substituted
Aldehydes of Ferrocene. To a 150 mL flask equipped with stirrer
bars, bis-aryl or bis-alkyl picolinamides were dissolved in DCE (10
mL) and stirred at room temperature for 15 min. Afterward ferrocene
(20 mol %) and Cu(OAc)2 (1 equiv) were added to the solution, and
this mixture was then heated at 80 °C for 10−16 h. The reaction was
monitored by TLC, and after completion, the mixture was dried over
a vacuum, and the crude product was purified through column
chromatography on silica gel using hexane/ethyl acetate (80/20) as
eluent.
At first TBPB oxidizes ferrocene to ferrocinium and cleaves
itself to form benzoate anion and tert-butyl radical. Then the
tert-butyl radical abstracts a proton from the C−H bond of the
organic picolinamide to form the radical intermediate IM3 and
converts itself to tert-butanol. Afterward a single electron
transfer takes place to generate an iminium ion intermediate
IM4, which subsequently goes through a hydrolysis followed
by oxidation to form the corresponding acid.
CONCLUSIONS
■
In conclusion, we report for the first time ferrocene catalyzed
picolinamide group directed oxidation of sp3 C−H bond to
carbonyl compounds both for organic and metal-sandwich
compounds. Both electron-donating and electron-withdrawing
groups were found to be suitable for this reaction.
Picolinamides having para and meta substituted aryl groups
underwent oxidation effectively under these reaction con-
ditions, but ortho substituted substrate groups were unre-
sponsive. We have also demonstrated sp3 C−H bond oxidation
after sp2 C−H functionalization on the ferrocene backbone.
More interestingly, for the first time, we report a novel
methodology to convert primary amines to acids by removing
the directing group under oxidation conditions. A possible
mechanism has been proposed for the C−H oxidation. This
novel finding has the potential to convert many aminomethyl
derivatives to the corresponding aldehydes or acids.
[2,5-(4-OMeC6H4)2-η5-(CHO)C5H2]Fe(η5-C5H5) (2a). Red solid;
yield 52%. Anal. Found: C, 70.41; H, 5.17; N, 0.00. Calcd for
C25H22O3Fe: C, 70.44; H, 5.20, N, 0.00. 1H NMR (300 MHz,
CDCl3) δ 10.20 (s, 1H, CHO-H), 7.46−7.49 (d, 4H, Ph-H), 6.81−
6.84 (d, 4H, Ph-H), 4.77 (s, 2H, Cp-H), 4.20 (s, 5H, Cp-H), 3.77 (s,
6H, OMe-H). 13C NMR (75 MHz, CDCl3) δ 193.00, 158.96, 131.16,
130.77, 127.97, 113.83, 92.67, 71.01, 69.65, 55.33. HRMS m/z
449.0796, calcd for C25H22FeNaO3 [M + Na]+ 449.0811.
[2,5-(4-BrC6H4)2-η5-(CHO)C5H2]Fe(η5-C5H5) (2b). Red orange solid;
yield 48%. Anal. Found: C, 52.66; H, 3.18; N, 0.00. Calcd for
1
C23H16OBr2Fe: C, 52.72; H, 3.08, N, 0.00. . H NMR (300 MHz,
CDCl3) δ 10.24 (s, 1H, CHO-H), 7.48 (s, 8H, Ph-H), 4.92 (s, 2H,
Cp-H), 4.23 (s, 5H, Cp-H). 13C NMR (75 MHz, CDCl3) δ 193.44,
158.96, 131.16, 130.77, 127.97, 113.44, 92.09, 79.39, 76.72, 73.22,
69.67. HRMS m/z 544.8810, calcd for C23H16FeBr2NaO [M + Na]+
544.8815.
EXPERIMENTAL SECTION
■
General Procedure for Bis-Arylation of [η5-(CH2R)C5H4]Fe-
(η5-C5H5) (R = Picolinamido). A 15 mL screw capped vial was
charged with a magnetic bead. One equivalent of ferrocenyl
picolinamide (0.03 g, 0.10 mmol), Pd(OAc)2 (20 mol %), Cs2CO3
(2 equiv), and aryl iodide (2.5 equiv) were dissolved in tert-amyl
alcohol (5 mL). The reaction mixture was heated at 80−110 °C for
16−24 h. The reaction was monitored by TLC, and after completion,
the reaction mixture was dried under a vacuum, and the crude product
was purified through column chromatography using silica and
hexane/ethyl acetate (70/30) as eluent.
[2,5-Bis(methyl)-η5-(CHO)C5H2]Fe(η5-C5H5) (2c). Red solid; yield
46%. Anal. Found: C, 64.62; H, 5.75; N, 0.00. Calcd for C13H14OFe:
1
C, 64.50; H, 5.83, N, 0.00. H NMR (300 MHz, CDCl3) δ 10.29 (s,
1H, CHO-H), 4.34 (s, 2H, Cp-H), 4.12 (s, 5H, Cp-H), 2.21 (s, 6H,
CH3-H). 13C NMR (75 MHz, CDCl3) δ 194.63, 87.03, 72.51, 70.74,
13.69. HRMS m/z 265.0286, calcd for C13H14FeNaO [M + Na]+
265.0292.
[η5-(CHO)C5H4]Fe(η5-C5H5) (2d). Red solid; yield 50%. Anal.
Found: C, 61.67; H, 4.65; N, 0.00. Calcd for C11H10OFe: C, 61.73;
1
[2,5-(4-OMeC6H4)2-η5-(CH2R)C5H2]Fe(η5-C5H5) (R = Picolinamido)
(1a). Red semisolid; yield 77%. 1H NMR (300 MHz, CDCl3) δ 8.41−
8.46 (m, 2H), 8.19−8.21 (d, 1H), 7.80−7.85 (m, 1H), 7.50−7.53 (m,
5H), 6.83−6.86 (d, 4H, Ph-H), 4.58−4.64 (m, 4H), 4.22 (s, 5H, Cp-
H, 4.73, N, 0.00. H NMR (300 MHz, CDCl3) δ 9.97 (s, 1H, CHO-
H), 4.81−4.82 (s, 2H, Cp-H), 4.62−4.63 (s, 2H, Cp-H), 4.29 (s, 5H,
Cp-H), 13C NMR (75 MHz, CDCl3) δ 193.47,73.19, 69.65. HRMS
m/z 215.0154, calcd for C11H11FeO [M + H]+ 215.0159.
D
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General Procedure for Synthesis of Picolinamide of Benzyl
Amines. Picolinamide of benzylamines were prepared according to
literature procedure as follows. To a 150 mL flask charged with a
magnetic bead, picolinic acid (0.92 g, 7.50 mmol) was dissolved in dry
DCM (50 mL). The solution was cooled in an ice bath followed by
addition of oxalyl chloride (1.16 g, 9.50 mmol) and two drops of
DMF. The solution was then stirred for 5 min in an ice bath and then
at room temperature for 1 h. Amine (5 mmol) was dissolved in dry
DCM and acid chloride was added dropwise using a syringe. The
solution was warmed to room temperature and stirred for 12 h.
Afterward the solution was concentrated under a vacuum, and the
crude product was purified through column chromatography on
neutral alumina using hexane/ethyl acetate (70:30) as eluant. The
experimental data matched with reported values.
129.31, 129.24, 128.52, 127.13. 19F-NMR (DMSO-d6, 282 MHz,
ppm) −61.48.
3-Br(C6H4-COOH) (15a). White solid, yield 66%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.40 (br, 1H, COOH-H), 7.92−
7.93 (d, 2H,Ph-H), 7.70−7.72 (d, 1H, Ph-H), 7.53−7.59 (dt, 1H, Ph-
H); 13C NMR (DMSO-d6, 75 MHz, ppm) δ 166.52, 133.79, 133.35,
131.10, 131.02, 129.30, 128.35.
3-Cl(C6H4-COOH) (16a). White solid, yield 67%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.36 (s, 1H, COOH-H), 7.77 (s,
2H, Ph-H), 7.70 (d, 1H, Ph-H), 7.55−7.57 (dt, 1H, Ph-H); 13C NMR
(CDCl3, 75 MHz, ppm) δ 171.01, 134.72, 133.94, 130.94, 130.28,
129.86, 128.33.
COOH-C6H4-COOH (17a). White solid, yield 53%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.33 (br, 2H, COOH-H), 8.05−8.11
(s, 4H, Ph-H); 13C NMR (DMSO-d6, 75 MHz, ppm) δ 167.13,
134.85, 129.84.
General Procedure for Picolinamide Group Assisted sp3 C−
H bond Oxidation of Benzyl Amines. A 15 mL screw capped vial
was charged with a magnetic bead and organic derived picolinamide
(1 mmol) was dissolved in 5 mL of DCE and stirred at room
temperature for 10 min. Then ferrocene (0.2 mmol, 20 mol %) was
added to it followed by addition of tert-butyl peroxybenzoate (TBPB)
(4 mmol). The mixture was heated at 80 °C for 12 h. Afterward, the
solution was concentrated under a vacuum, and the crude product
was purified through column chromatography on silica gel using
hexane/ethyl acetate (90:10) as eluent.
1
C4H3S-COOH (18a). White solid, yield 50%. H NMR (DMSO-d6,
300 MHz, ppm) δ 13.09 (br, 1H, COOH-H), 7.88−7.90 (s, 1H),
7.75−7.77 (s, 1H), 7.18−7.22(s, 1H); 13C NMR (DMSO-d6, 75
MHz, ppm) δ 162.88, 134.63, 133.14, 128.14.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00085.
■
S
4-OMe(C6H4-COOH) (5a). White solid; yield 82%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 12.65 (s, 1H, COOH-H), 7.90−7.93
(d, 2H, Ph-H), 7.01−7.04 (d, 2H, Ph-H), 3.83 (s, 3H); 13C NMR
(DMSO-d6, 75 MHz, ppm) δ 166.99, 162.78, 131.67, 122.96, 113.66,
55.24.
General information; Experimental section; UV−visible
spectroscopic studies; 1H NMR and 13C{1H} NMR
spectra; References (PDF)
(C6H5-COOH) (6a). White solid; yield 75%. 1H NMR (CDCl3, 300
MHz, ppm) δ 12.71 (s, 1H, COOH-H), 8.17−8.20 (m, 2H, Ph-H),
7.63−7.78 (t, 1H, Ph-H), 7.37−7.49 (t, 2H, Ph-H); 13C NMR
(CDCl3, 75 MHz, ppm) δ 172.72, 133.87, 130.27, 129.96, 128.52.
AUTHOR INFORMATION
Corresponding Author
*E-mail: eliasanil@gmail.com.
■
1
4-NO2(C6H4-COOH) (7a). Pale yellow solid; yield 68%. H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.62 (s, 1H, COOH-H), 8.30−8.32
(d, 2H, Ph-H), 8.16−8.19 (d, 2H, Ph-H); 13C NMR (DMSO-d6, 75
MHz, ppm) δ 166.16, 150.36, 136.81, 131.04, 123.79.
ORCID
1
Anil J. Elias: 0000-0001-9980-5076
4-CF3(C6H4-COOH) (8a). Pale yellow solid; yield 68%. H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.46 (s, 1H, COOH-H), 8.12−8.15
(d, 2H, Ph-H), 7.86−7.89 (d, 2H, Ph-H); 13C NMR (DMSO-d6, 100
MHz, ppm) δ 166.64, 135.05, 130.52, 125.99, 125.95, 79.60. 19F-
NMR (DMSO-d6, 282 MHz, ppm) −62.55.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
4-F(C6H4-COOH) (9a). Pale yellow solid; yield 69%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 12.93 (s, 1H, COOH-H), 7.92−8.03
(d, 2H, Ph-H), 7.43−7.49 (d, 2H, Ph-H); 13C NMR (DMSO-d6, 75
MHz, ppm) δ 167.78, 133.25, 132.61, 131.26, 127.84. 19F-NMR
(DMSO-d6, 282 MHz, ppm) −106.91.
■
The authors thank the DST of India for financial assistance in
the form of research grant to A.J.E. (DST EMR/2015/
000285). S.H. and M.D. thank UGC, India, for a senior
research fellowship, while P.D. thanks the Indian Institute of
Technology Delhi for a senior research fellowship. We thank
the DST-FIST and IIT D for funding the HRMS facilities at
IIT Delhi.
4-Br(C6H4-COOH) (10a). White solid; yield 70%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.20 (s, 1H, COOH-H), 7.87−7.90
(d, 2H, Ph-H), 7.71−7.74 (d, 2H, Ph-H,); 13C NMR (DMSO-d6, 75
MHz, ppm) δ 167.05, 132.14, 131.74, 130.48, 127.32.
4-Cl(C6H4-COOH) (11a). White solid; yield 71%. 1H NMR
(DMSO-d6, 300 MHz, ppm) δ 13.20 (s, 1H, COOH-H), 7.84−7.87
(d, 2H, Ph-H), 7.67−7.71 (d, 2H, Ph-H); 13C NMR (DMSO-d6, 75
MHz, ppm) δ 166.06, 132.16, 131.74, 130.48, 127.32.
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