Milan Dinda

Cross dehydrogenative coupling (CDC) of aldehydes with N-hydroxyimides by visible light photoredox catalysis

A photoinduced cross-dehydrogenative-coupling (CDC) reaction between aldehydes and N-hydroxyimides has been developed for the synthesis of ester derivatives. Using 2 mol% [Ru(bpy)3]Cl2 in dry acetonitrile at room temperature with an LED light bulb, we were able to synthesize N-hydroxyesters in good yields. The ester derivatives are very useful synthetic intermediates, which were transformed to amide and oxazole building blocks in excellent yields.

Solar photo-thermochemical syntheses of 4-bromo-2,5-substituted oxazoles from N-arylethylamides

Solar photo-thermochemical C(sp3)–H bromination, conducted efficiently in a specially designed reactor, was reported recently. In the present study, the more complex formation of 4-bromo-2,5-substituted oxazoles from N-arylethylamides using this approach was achieved. The one-pot syntheses were carried out with N-bromosuccinimide–dichloroethane over 6 h (10.00 am to 4.00 pm) on sunny days. The isolated yields were in the range 42–82%. Benzylic bromination, followed by O–C bond formation through intramolecular nucleophilic substitution, and a second benzylic bromination followed by HBr elimination, gave the oxazole ring. A third bromination of the ring yielded the final product. The feasibility of synthesizing thiazole derivatives using a similar approach was also demonstrated. During the course of the reactions, succinimide and HBr were co-generated in the aqueous phase. Treatment with NaBrO3 and additional acid returned the reagent in 57% isolated yield upon chilling. The overall methodology was greener as a result.

Clean synthesis of crystalline p-nitrobenzyl bromide from p-nitrotoluene with zero organic discharge

2 : 1 : 3 NaBr-NaBrO3-NaCl (obtainable as a low cost and eco-friendly reagent from an alkaline bromine intermediate) has been utilized previously in a number of bromination reactions. One such reaction is the conversion of p-nitrotoluene (PNT) to p-nitrobenzyl bromide (PNBBr) used widely for functional group protection. In the present work, selective cold (0–5 °C) crystallization of PNBBr from the reaction mixture containing ca. 2.5 M PNBBr along with 2.5–5.0 M PNT in ethylene dichloride (EDC) was found to be a winning move which led to gains in two fronts. It allowed the bromination reaction to be carried out cleanly and also enabled direct recycling of the mother liquor in the subsequent batch. Para NO2-Ph-CHBr2 impurity, which built up gradually in the reaction mixture over 8 batches, was converted into PNBBr/PNT through treatment of the mother liquor with NaBH4, thereby helping to recycle the liquor perpetually and eliminate organic waste. EDC was the sole solvent in the entire process and its losses were minimal. The combined yield of isolated and recoverable PNBBr was 98.30% with respect to PNT consumed. The reagent utilization efficiency was 98.26%.

Iodine catalysed intramolecular C(sp3)–H functionalization: synthesis of 2,5-disubstituted oxazoles from N-arylethylamides

Iodine catalyzed synthesis of 2,5-disubstituted oxazoles from N-arylethylamides through intramolecular C(sp3)–H functionalization under metal-free conditions is described. The method is tolerable to a wide range of substrates having a variety of functional groups with moderate to good yields of the products.

Solar driven uphill conversion of dicyclopentadiene to cyclopentadiene: an important synthon for energy systems and fine chemicals

The retro Diels–Alder conversion of endo-dicyclopentadiene to cyclopentadiene (Cp) – which is thermodynamically uphill under ambient conditions (ΔG = +9.7 kcal mol−1; values based on computation at 273 K following CBS-QB3 methodology) – was carried out at 175–190 °C in neat state using solar energy. The reaction is thermodynamically favorable at elevated temperature. Considering heat release from the reverse reaction (ΔH = −23.4 kcal mol−1), the energy storage efficiency was computed to be ca. 5.5% with respect to the IR component in concentrated solar radiation. Solar energy was further utilized for preparation of a model 2,5-norbornadiene derivative (75% isolated yield) through the cycloaddition reaction of Cp with 4-phenylbut-3-yn-2-one at 150–185 °C. The norbornadiene–quadricyclane system has been proposed for solar energy storage, and its solar assisted synthesis would help reduce its carbon footprint over its life cycle. Norbornadiene synthesis using solar energy may also be of interest for greener processing of fuels derived from this compound. Cooksons cage ketone, which too has been proposed as an energy storage medium, was additionally synthesized from the Diels–Alder adduct of Cp and p-benzoquinone employing concentrated solar photo-thermochemical conditions. The reaction proceeded rapidly (15 min) and gave the desired product in 96% isolated yield. Besides the above applications, Cp is an important synthon in the preparation of fine chemicals.