Trapping of Isocyanates with Benzotriazole in situ - Preparation of Carbamoyl Benzotriazoles as an Isocyanate Alternative via Curtius Rearrangement

Abstract

N-Aryl and N-alkenyl carbamoyl benzotriazoles were prepared in good to excellent yields from acyl azides and benzotriazole via Curtius rearrangement, while N-alkylcarbamoyl benzotriazoles were formed from N-alkanoyl benzotriazoles and sodium azide in one pot. The carbamoyl benzotriazoles can be used as a stable isocyanate alternative, as has been demonstrated by its reaction with amines to synthesize ureas in excellent yields.

Key words

carbamoyl benzotriazoles - isocyanate alternative - acyl azides - Curtius rearrangement - ureas

References and Notes

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11

General Procedure for the Preparation of N -Aryl and N -Alkenyl Carbamoyl Benzotriazoles 2a-k: To the aroyl azide 1 (1 mmol) in toluene (3 mL), was added benzotriazole (1.1 mmol, 0.31 g). The mixture was stirred at 70-80 ˚C for the time indicated in Table  [¹] (reaction monitored by TLC). After cooling, the crude product precipitated out and was washed with cold EtOH to give pure N-aryl or N-alkenyl carbamoyl benzotriazoles 2a-k.

12

General Procedure for the Preparation of N -Alkyl-carbamoyl Benzotriazoles 2l-o: To N-alkanoyl benzotriazole 3 (1 mmol) in acetone (3 mL), was added a fresh solution of NaN₃ (3 mmol) dissolved in H₂O (2 mL), and FeCl₃˙6H₂O (0.2 mmol, 54.1 mg). The mixture was stirred at room temperature for 5 min, until the complete conversion of N-alkanoyl benzotriazole was observed (monitored by TLC). Additional benzotriazole (2 mmol, 0.238 g) was added and the reaction mixture was heated gently at 32-40 ˚C for the time indicated in Table  [²] . After being extracted with Et₂O (3 × 10 mL), the organic layer was washed successively with sat. Na₂CO₃ and brine, dried over anhydrous Na₂SO₄ and concentrated under reduced pressure. Purification by flash column chromatography [petroleum ether (60-90)-EtOAc, 8:1] gave pure N-alkylcarbamoyl benzotriazoles 2l-o.

14

General Procedure for the Preparation of Ureas 4a-e: The amine (1.1 mmol) was stirred with N-substituted carbamoyl benzotriazole 2 (1 mmol) in toluene (5 mL) at 70-80 ˚C for 0.5 h (reaction monitored by TLC). After cooling, the solid precipitated out and was washed with cold EtOH to give pure ureas 4a-e.