278
Li et al. for the first-time developed rhodium(I)-catalysed oxidative dimerization
reaction of aromatic acids to obtain 2,2′-diaryl acids by two direct C–H bond activations in water (Scheme 35) [162]. Practical application of the developed procedure
has demonstrated for the construction of the antitumor natural product ellagic acid
in an effective and regiospecific manner.
Hu et al. developed Pd-catalysed β-C–H arylation reaction of ketones and arylboronic acids in water with very mild reaction conditions. Arylboronic acids used as
nucleophilic aryl sources and o-iodoxybenzoic acid as the oxidant have been used
in this direct β-C–H arylation reaction (Scheme 36) [163]. This method offers a succinct pathway for the production of β-arylated ketones and direct asymmetric β-C–H
arylation reaction of ketones could be performed.
Pu et al. explored the C–H alkylation reaction of maleimides and aromatic acids
in water and reaction showed varying chemoselectivity with substituted benzoic
acid (Scheme 37) [164]. In this reaction, the carboxyl group played an important
role of a classical or traceless directing group and thereby generated two different
products, i.e. 2-alkyl substituted benzoic acids and alkyl-substituted benzenes.
Recently in 2019, studies for Pd(II)-catalysed C(sp
2
)–H arylation and methylation of methyl, aryl, heteroaryl iodides, and sp
2
carbons both at β- and γ-positions
were performed by Mitra et al. (Scheme 38) [165]. Synthesis of pyridine and arylated acid synthons was carried out with the use of 3-amino-1-methyl-1H-pyridin-2one as bidentate N,O-directing group (DG) in the aqueous medium.
Maji et al. for the first-time developed synthesis of chromones by means of
Ir-catalysed C–H functionalization and annulation of salicylaldehyde and diazoketones (Scheme 39) [166]. This protocol afforded the C-3 substituted chromones
in good yield by one-pot decarboxylation and employing tert-butyl diazoester.
O
R 2
Cp*Rh(MeCN)3
(4 mol%)
K 2 CO 3
Air, H 2O, 110
o C
N
H
R 3
R 4
R 1
O
R 2
N
R 1
R 4
R 3
BF 4 2
Scheme 34 Rh-catalysed C–H activation of substituted benzamides
MnO 2 (3 equiv)
H 2O, 150
o
C, air, 24h
CO 2 H
R
R
CO 2 H
HO 2 C
R
Rh(nbd)Cl 2 ( 5 mol%)
Scheme 35 Rhodium(I)-catalysed oxidative dimerization reaction of aromatic acids
D. S. Deshmukh et al.
Li et al. for the first-time developed rhodium(I)-catalysed oxidative dimerization
reaction of aromatic acids to obtain 2,2′-diaryl acids by two direct C–H bond activations in water (Scheme 35) [162]. Practical application of the developed procedure
has demonstrated for the construction of the antitumor natural product ellagic acid
in an effective and regiospecific manner.
Hu et al. developed Pd-catalysed β-C–H arylation reaction of ketones and arylboronic acids in water with very mild reaction conditions. Arylboronic acids used as
nucleophilic aryl sources and o-iodoxybenzoic acid as the oxidant have been used
in this direct β-C–H arylation reaction (Scheme 36) [163]. This method offers a succinct pathway for the production of β-arylated ketones and direct asymmetric β-C–H
arylation reaction of ketones could be performed.
Pu et al. explored the C–H alkylation reaction of maleimides and aromatic acids
in water and reaction showed varying chemoselectivity with substituted benzoic
acid (Scheme 37) [164]. In this reaction, the carboxyl group played an important
role of a classical or traceless directing group and thereby generated two different
products, i.e. 2-alkyl substituted benzoic acids and alkyl-substituted benzenes.
Recently in 2019, studies for Pd(II)-catalysed C(sp
2
)–H arylation and methylation of methyl, aryl, heteroaryl iodides, and sp
2
carbons both at β- and γ-positions
were performed by Mitra et al. (Scheme 38) [165]. Synthesis of pyridine and arylated acid synthons was carried out with the use of 3-amino-1-methyl-1H-pyridin-2one as bidentate N,O-directing group (DG) in the aqueous medium.
Maji et al. for the first-time developed synthesis of chromones by means of
Ir-catalysed C–H functionalization and annulation of salicylaldehyde and diazoketones (Scheme 39) [166]. This protocol afforded the C-3 substituted chromones
in good yield by one-pot decarboxylation and employing tert-butyl diazoester.
O
R 2
Cp*Rh(MeCN)3
(4 mol%)
K 2 CO 3
Air, H 2O, 110
o C
N
H
R 3
R 4
R 1
O
R 2
N
R 1
R 4
R 3
BF 4 2
Scheme 34 Rh-catalysed C–H activation of substituted benzamides
MnO 2 (3 equiv)
H 2O, 150
o
C, air, 24h
CO 2 H
R
R
CO 2 H
HO 2 C
R
Rh(nbd)Cl 2 ( 5 mol%)
Scheme 35 Rhodium(I)-catalysed oxidative dimerization reaction of aromatic acids
D. S. Deshmukh et al.