A mechanochemical method for the preparation of synthetically useful 2-arylindoles is developed using Pd(II) as the catalyst in the absence of phosphine ligands in a ball-mill. The developed protocol is highly C-2 selective and tolerant of structural variations with electron-rich and electron-deficient substituents both in indoles and iodoarenes. Arylation is possible in both unprotected indoles and Nprotected indoles with the electron-donating group with the former substrate being relatively slower to react and little less yielding. Indoles with a deactivated five-membered ring could also take part in the reaction with ease. The scalability of the reaction was demonstrated by conducting the reaction in the gram scale. In general, the reactions were achieved in a shorter time than the conventional methods.
An efficient and environmentally
sustainable method for the synthesis
of imidazo[1,2-
a
]pyridine derivatives by domino A
3
-coupling reaction catalyzed by Cu(II)–ascorbate was
developed in aqueous micellar media in the presence of sodium dodecyl
sulfate (SDS). The catalyst, a dynamic combination of Cu(II)/Cu(I),
was generated in situ in the reaction mixture by mixing CuSO
4
with sodium ascorbate and aided a facile 5-
exo
-dig
cycloisomerization of alkynes with the condensation products of 2-aminopyridines
and aldehydes to afford a variety of imidazo[1,2-
a
]pyridines in good overall yields. A simple experimental setup, water
as the “green” medium, and inexpensive catalyst and
auxiliary are some of the merits of this protocol.
A new AIE-based fluorimetric probe (TPE-PMI) has been successfully developed utilizing Gabriel reaction for the selective detection of hydrazine in solid, liquid and vapour phases.
An efficient mechanochemical method for manganese-catalyzed regioselective C−H bond alkenylation of indoles with alkynes is developed. Recently, mechanochemical C−H bond activation has received due attention and the use of low-valent first-row transition metals in C−C cross-coupling via C−H activation under mechanochemical conditions is an additional feather to the cost-effectiveness of such useful synthetic protocols.The present method allows direct C-2 alkenylation of indoles in a mixer mill, employing a commercially available low-valent manganese catalyst, MnBr(CO) 5 , providing a sustainable route to hydroindolation on alkynes. The developed protocol is highly C-2-selective by the presence of a heteroaromatic N atom as a directing group (namely, pyridyl) and tolerant of structural variations with electron-rich and electron-deficient substituents both in the indoles and in the alkynes. Silica as the grinding media and the presence of a catalytic amount of acid and DIPEA as the base worked favorably to afford a variety of 2-alkenyl indoles in excellent yields at ambient conditions. The terminal alkynes offered better results than internal alkynes in terms of yields and reactivity. The scalability of the reaction was demonstrated by conducting the reactions in the gram scale. A short study indicated that a little tweak in conditions can be useful for double alkenylation to afford carbazole derivatives in moderate yields. A low E-factor along with a clean reaction profile, an easy experimental setup, the absence of an anhydrous condition, and being devoid of toxic organic solvents proclaims its advantage over the available conventional methods.
A green and efficient mechanochemical method for the synthesis of a series of 2‐arylimidazo[1,2‐a]pyridines was developed using an electrical grinder. I2 catalyzed mechanochemical grinding facilitates the cyclocondensation reaction between various aryl methyl ketones and 2‐aminopyridines to afford 2‐arylimidazo[1,2‐a]pyridines in good yields at ambient temperature. The method was successfully used for the gram‐scale synthesis of a marketed drug, zolimidine. The noticeable advantages of this environmentally sustainable protocol include mild conditions, simple instrumentation, inexpensive catalyst, atom economy, short reaction time etc.
Chiral polyheterocycles are one of the most frequently encountered scaffolds in natural products and in current drugs repertoire. A carbohydrate-based diversity oriented synthetic (DOS) approach has been employed for gaining access to many structurally diverse and stereochemically complex rigid polyheterocyclic molecules with multiple chiral hydroxyl groups to enhance aqueous solubility. Inexpensive chiral pool of D-Glucose has been judiciously exploited to get access of complex chiral polyheterocyclic structures using inexpensive, common achiral reagents and domino-Knoevenagel hetero-Diels-Alder (DKHDA) reaction as one of the key synthetic tools. Stereochemistry of newly generated stereocenters of polycyclic structures are unambiguously determined through NMR and X-ray crystallographic study. A chemoinformatic comparison (PCA and PMI) with 40 branded blockbuster drugs showed that newly generated polyheterocycles have good three-dimensional scaffold diversity and most of these pass the Lipinski filter of drug-likeness.
A simple electrical mortar–pestle was used for the development of a green and facile mechanochemical route for the catalyst-free halogenation of phenols and anilines via liquid-assisted grinding using PEG-400 as the grinding auxiliary. A series of mono-, di-, and tri-halogenated phenols and anilines was synthesized in good to excellent yields within 10–15 min in a chemoselective manner by controlling the stoichiometry of N-halosuccinimides (NXS, X = Br, I, and Cl). It was observed that PEG-400 plays a key role in controlling the reactivity of the substrates and to afford better regioselectivity. Almost exclusive para-selectivity was observed for the aromatic substrates with free o- and p-positions for mono- and dihalogenations. As known, the decarboxylation (or desulfonation) was observed in the case of salicylic acids and anthranilic acids (or sulfanilic acids) leading to 2,4,6-trihalogenated products when 3 equiv of NXS was used. Simple instrumentation, metal-free approach, cost-effectiveness, atom economy, short reaction time, and mild reaction conditions are a few noticeable merits of this environmentally sustainable mechanochemical protocol.
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