Continuous-Flow Direct Azidation of Alcohols and Peroxides for the Synthesis of Quinoxalinone, Benzooxazinone, and Triazole Derivatives

Continuous-flow reactors provide an ideal tool for the synthesis of potentially explosive but synthetically useful organic substances like organic azides due to their intrinsically small volume leading to very effective collision and highly controlled reaction conditions. Herein, we report the continuous-flow direct azidation of various alcohols by using TMSN₃ as an azide transfer reagent in the presence of Amberlyst-15 as a recyclable catalyst. Numerous 3-hydroxy-2-oxindoles effectively undergo azide transfer to afford azide-functionalized quaternary stereocenters in a continuous-flow module. Interestingly, peroxyoxindole undergoes sequential skeletal rearrangement to generate a carbocation followed by nucleophilic azidation to afford a library of substituted 2-azido-2H-benzo[b][1,4]oxazin-3(4H)-one derivatives under continuous flow. Furthermore, a continuous-flow Cu-catalyzed click reaction afforded triazole-functionalized deivatives. Next, reduction of azide in the presence of PPh₃ affords the amine derivatives in good yields. The continuous-flow application was extended further for the thermolytic skeletal rearrangement of 3-azide-2-oxindole for the synthesis of biologically important quinoxalin-2(1H)-ones without any reagents. Furthermore, this continuous-flow direct azidation reaction is scaled up to 6.144 g of azides with a turnover number of 9.24 under safer conditions.

Montmorillonite clay-based heterogenous catalyst for the synthesis of nitrogen heterocycle organic moieties: a review

The use of montmorillonite clay as solid catalyst has grabbed much attention in the liquid phase reactions for organic synthesis. In recent years, there has been a lot of interest in organic synthesis using montmorillonite-based composites, especially in the synthesis of heterogeneous nanoparticles. Due to the robust and green nature of montmorillonite-based nanocatalysts, it has been widely used in N-heterocyclic reactions. In this review, we have concentrated on the reports pertaining the use of montmorillonite-based nanocatalyst in the synthesis of N-heterocycles, a category of organic compounds with excellent biological properties. This manuscript is arranged by the types of N-containing heterocycles synthesized using montmorillonite-based composite as catalysts including polycyclic spirooxindoles, heterocyclic propargylamine, indole-based heterocycles, quinoline and its derivatives, six-membered N-heterocyclic-based compounds and five-membered N-heterocyclic-based compounds. Special attention was given to the structural stability under experimental parameters of the montmorillonite-based composite with the incidence of metal leaching and reusability. Finally, along with recent developments, new findings in heterogeneous montmorillonite (Mt)-based catalysis have also been addressed.

Direct Alkylation of Benzene at Lower Temperatures in the Liquid Phase: Catalysis by Montmorillonites as Noble-Metal-Free Solid Acids

Alkylated benzenes are widely used as raw materials for the production of a variety of chemical compounds. Conventionally, they are obtained by the Friedel-Crafts reaction between alkyl halides and benzene. In this study, the synthesis of halogen-free alkylated benzenes was made possible by the direct alkylation of benzene with alkanes using montmorillonites as noble-metal-free solid acid catalysts. The direct alkylation of benzene with n-heptane was performed at 150 °C. Aluminum-exchanged montmorillonite showed the highest yield of the target C-7 alkylated products (Ph-C7) compared with other homogeneous and heterogeneous acid catalysts: 1.8 % conversion of benzene with 58 % selectivity in 16 h. The montmorillonite catalyst system was applied to other linear and cyclic alkanes to give the corresponding alkylated products with good selectivities.

A Low-Cost, Well-Designed Catalytic System Derived from Household Waste "Egg Shell": Applications in Organic Transformations

A waste feedstock-derived economical basic alternative catalyst is described in this review. Eggshell is one of the household wastes created in tons of weight daily. Therefore, in order to reduce the environmental pollution-related problems, its use in heterogeneous catalysis can be attributed as a great contribution for the chemical and material science society to carry out several known reactions and for the much-needed energy alternative biodiesel production as low-cost catalytic system. Keeping green chemistry in mind, industrial use of these catalysts may also reduce the use of other traditionally used high-cost chemical catalytic systems.

Highly selective direct azidation of alcohols over a heterogeneous povidone–phosphotungstic solid acid catalyst

A heterogeneous povidone–phosphotungstic acid catalyzed direct selective azidation of alcohols gave excellent product yields at room temperature.