Reactions of Nitroalkenes with Nitroalkanes or Sulfur Ylides Catalyzed by Amine-Thiourea Bifunctional Polymeric Organocatalysts

Abstract

Non-cross-linked and cross-linked bifunctional polystyrenes bearing both amine and thiourea groups have been synthesized and used as organocatalysts in reactions between nitroalkenes and nitroalkanes or sulfur ylides. Control experiments using monofunctional polymers with only either amine or thiourea groups attached indicated that both functional groups were essential for efficient catalysis of the reactions studied. The non-cross-linked polystyrene was soluble in typical organic solvents and was used as a homogeneous catalyst, while the cross-linked polystyrene was used as a heterogeneous catalyst.

Key words

polymer-supported organocatalyst - polystyrene - ­JandaJel - nitroalkanes - nitroalkenes - sulfur ylides

References and Notes

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See Supporting Information for details.

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General Procedure for the Michael Addition Reactions of Nitroalkanes to Nitrostyrenes Catalyzed by Polymer 9
Nitrostyrene 13a-i (1 mmol) and catalyst 9 (0.05 mmol) were dissolved in 12 (2.6 mL, 30 mmol). The mixture was stirred at r.t. for 20 h, and then the reaction mixture was purified directly by column chromatography to afford the desired product14a-i as a mixture of stereoisomers. The syn/anti ratio was determined by ^¹H NMR analysis of the crude product mixture.

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General Procedure for the Cycloaddition Reactions Catalyzed by Polymer 9 Nitrostyrene 13a,c,d,f-l (0.5 mmol) and 9 (0.05 mmol) were dissolved in CHCl₃ (1 mL). Sulfur ylide 15a-c (0.6 mmol) in CHCl₃ (1 mL) was then added dropwise to the mixture. After stirring at r.t. for 24 h, the reaction mixture was purified directly by column chromatography to afford the desired products 16a-l. The anti/syn ratio was determined by ^¹H NMR analysis of the crude product mixture.

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General Procedure for the Michael Addition Reactions of Nitroalkanes to Nitrostyrenes Catalyzed by Polymer 18
To nitrostyrene 13a-i (1 mmol) in 12 (2.6 mL, 30 mmol) was added catalyst 18 (0.1 mmol). The mixture was stirred at r.t. for 15 h. The polymer was then removed by filtration, washed with THF, and the filtrate was concentrated in vacuo to remove the excess nitropropane. The syn/anti ratio was determined by ^¹H NMR analysis.

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General Procedure for the Reuse of Polymer 18
To nitrostyrene 13a (3.0 mmol) in 12 (90 mmol) was added catalyst 18 (0.3 mmol). Cycles 3-5 were performed on a 1.5 mmol scale and cycle 6 was performed on a 1.2 mmol scale. The mixture was stirred at r.t. for 15 h, and then the polymer was then removed by filtration, washed with THF, and dried. The filtrate was concentrated in vacuo to remove the excess nitropropane. The syn/anti ratio was determined by ^¹H NMR analysis to be 85:15 in all cycles.