Polystyryl‐BEMP as an Efficient Recyclable Catalyst for the Nucleophilic Addition of Nitroalkanes to α,β‐Unsaturated Carbonyl Compounds under Solvent‐Free Conditions

Abstract

2-tert-Butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine supported on polystyrene (PS-BEMP) is an efficient catalyst for the addition of nitroalkanes (1-1.5 equiv.) to a,b-unsaturated carbonyl compounds (1.0 equiv.) in the absence of a reaction medium (solvent-free conditions). The corresponding g-nitro carbonyl compounds have been isolated in excellent yields but the catalyst can be satisfactorily recovered and used for only 3 times due to the magnetic stirring which caused crunching of the catalyst beads thus hampering its complete recovery. To optimize the catalysts reuse and improve the environmental efficacy of solvent-free conditions, the first solvent-free cyclic continuous-flow reactor has been set up. This reactor has allowed the product to be isolated in an almost quantitative yield by using a very small amount of organic solvent, making the recovery and reuse of the catalyst efficient and reproducible.

Continuous Flow Organophosphorus Chemistry

“…[163] Vaccaro and co-workers employed the same catalyst for the nucleophilic addition of nitroalkanes to α, -unsaturated ketone and ester derivatives in batch and continuous flow procedures under neat conditions. [164] Using PS-BEMP in a continuous flow fashion avoided the grinding of the polymeric beads due to the mechanical stirring. The grinding of the catalyst typically led to a reduced activity and hampered recyclability.…”

“…Cyclic continuous flow reactor used for the preparation of a nitroalkane derivative under solvent-free conditions through a nucleophilic addition. [164] The same group later reported another use of PS-BEMP for the ring-opening of epoxides using aryl alcohols under neat conditions. [165] The procedure was established in both batch and flow modes, and the authors eventually optimized a continuous flow reactor with a recycling loop to push the conversion.…”

Sequential Michael addition/retro-Claisen condensation of 1,3-diarylpropan-1,3-diones with nitrostyrenes: one-step synthesis of 4-nitro-1,3-diarylbutan-1-ones

“…As one of the main classes of ketone compounds, γnitro ketones are widely used as intermediates for the synthesis of various functionalized carbocycles. [3][4][5][6][7][8] The reported typical synthetically viable procedures for the construction of γ -nitro ketones include: i) the conjugate addition of nitromethane to chalcone; [9][10][11][12][13] ii) the detrifluoroacetylative Michael addition of 1-trifluoromethyl-1,3-diketones to conjugated nitroalkenes; 14 iii) the amine-catalyzed Michael addition of ketones to nitrostyrene; [15][16][17][18][19][20][21][22][23][24][25] iv) the nickel-catalyzed decarboxylative Michael addition of β-ketoacids to nitrostyrenes 26,27 (Scheme 1). However, some drawbacks, such as low yield, limited diversity and expensive substrates, still remains in the existing methods.…”

Definition of green synthetic tools based on safer reaction media, heterogeneous catalysis, and flow technology

“…A representative example of our approach on the use of flow conditions to minimize waste production has been disclosed for the Michael addition reactions of nitrocompounds to α,β-unsaturated compounds (Scheme 6) [72].…”

“…We have used flow technology as an effective mixing technology in the combination of heterogeneous catalysts (organic and organometallic) and green reaction media such as azeotropes, water and also solventfree conditions proving that with this approach the chemical efficiency as well as the greenness of a synthetic methodology can be effectively maximized and some examples will be described below [66,[69][70][71][72][73][74][75][76][77][78].…”