Heterogeneous Cu‐MnO Catalyzed Monoselective ortho‐Halogenation of Aromatic C−H Bonds under Visible Light

Abstract

Heterogeneous Cu-MnO-catalyzed selective monohalogenation of aromatic compounds with cheap Nhalosuccinimide (NXS) as ah alogenating agent in the presence of molecular oxygen asanoxidant, under irradiation with visible light is reported. Monochlorination, bromination, and iodinationsw ere achieved in ah ighly selective fashion. Our developed heterogeneous Cu-MnO catalyst without any additive and ligandsw orks well for halogenation with electron-rich substrates and also works selectively with electron poor substrates. Moreover, chlorination occurs efficientlyw itht he more challenging substrates benzoice sters. Theh eterogeneous nature of the reactionwas provenbyh ot filtration test.Aromatic halogenated compounds play av ery important role in synthetic organic chemistry.T hey are widely used as starting materials to build complex molecules via transition-metal-catalyzed cross-couplingr eactions [1] in the synthesis naturalp roducts and important pharmaceutical compounds, including several blockbuster drugs such as Zoloft, Plavix, etc. (Figure 1). [2] The development of efficient and selective methods for their preparation is an important challenge in organic synthesis. Aromatic halides are typicallys ynthesized by direct electrophilic halogenation, [3] directed ortho lithiation, [4] and Sandmeyer reaction. [5] However, these methodsh aved isadvantages such as poor regioselectivitya nd tedious and/or hazardous reaction procedures.Transition-metal-catalyzed direct CÀHf unctionalization with halogenating reagents has been developedi nr ecent years. [6][7][8][9][10][11] Pd-catalyzed directing group assisted CÀHb ond halogenations has attracted considerable attention in recent years.S ignificant advances in this area were reportedb yS anford, Shi, Yu and other groups. [6] To the best of our knowledge, so far few other transition metalsthan Au [7] Rh, [8] Ru, [9] Co [10] and Cu [11] are investigated for direct CÀHh alogenation reaction. Since copper is an economic,a bundant, andv ersatile catalysti th as attracted more attention in direct aryl CÀHh alogenation.Yu and co-workers first reported Cu(OAc) 2 -catalyzedh alogenations of aryl CÀHb onds directed by pyridylg roups using Cl 2 CHCHCl 2 as ac hlorinating agent. [11a] CuCl 2 -catalyzed oxychlorination of phenols was reported by Menini and Gusevskaya in which LiCl was used as ac hlorinating agent. [11b-c] Shen and co-workers reported LiBr/LiCl as ac hlorinating agent using Cu(NO 3 ) 2 ·3H 2 Oa sc atalysta t1 50 8C. [11f] Han and co-workers reported CuI-mediatedh alogenation using acetic acid as an additive. [11h] Halogenation of electron-rich arenes in the presence of CuX 2 /LiX (X = Cl, Br) with acetic acid as as olvent wasr eported by Stahl and co-workers. [11i-j] ortho-Halogenation of benzamides with the removable directing group 2-(pyridine-2-yl)isopropylamine (PIP) was reported by Shi and co-workers. [11k] Electrophilic aromatic halogenation using N-halosuccinimide (NXS) [12] with aBrønsted or Lewis acid or base catalyst is another method to prepare ...

Synthesis of 2‐Arylbenzoxazoles by Hetereogeneous γ‐MnO2 Catalyzed Redox Reaction of o‐Nitrophenol with Benzyl Alcohols or Benzyl Amines

“…Therefore, we are interested in the development of an efficient method using readily available and inexpensive substrates and catalysts, with the reduction of hazardous waste, which is the main objective of green and sustainable chemistry. On continuation with our ongoing research on heterogeneous catalysis, recently we have reported the synthesis of 2‐aminobenzoxazole with γ‐MnO 2 catalyst, which has both oxidising and reducing property (confirmed by TPR and XPS analysis), we believe that the presence of surface active Mn +3 species and labile lattice oxygen will play an important role in catalysis, this fact motivated us to explore the synthesis of 2‐aryl benzoxazole by redox condensation. Herein, we report a simple MnO 2 catalysed direct redox condensation of 2‐nitrophenols with benzyl alcohols or benzyl amines to synthesize 2‐arylbenzoxazole.…”

Modern Transition-Metal-Catalyzed Carbon–Halogen Bond Formation

“…Recently, Panda and Ghosh reported a heterogeneous Cu-MnO-catalyzed direct ortho C−H halogenation of similar substrates which used visible light a promoter. 235 Later that year Liu reported a novel approach to aryl fluoride synthesis via a Cu(0)-catalyzed cycloaddition of aryl enynes (Scheme 111). 236 This reaction showed good levels of selectivity for the monofluorinated polyaromatic compounds when MeCN was used as solvent under mild heating for 90 min.…”

Heterogeneous catalytic approaches in C–H activation reactions

“…Alternatively, monochlorination, -bromination and -iodination of aromatic compounds with N-halosuccinimides in the presence of molecular oxygen as an oxidant were achieved in a highly selective process using heterogeneous Cu-MnO catalyst under irradiation with visible light. 75 The reactions proceeded without any additive or ligands and proved to be efficient for halogenations of electron-rich as well as electron-poor substrates, including benzoates. The heterogeneous nature of this catalyst was confirmed by a hot-filtration test.…”