Abstract
The use of nitroarenes as amino sources in synthesis is challenging. Herein is reported an unusual, straightforward, and transition metal-free method for the net [3 + 2]-cycloaddition reaction of 2-azaallyl anions with nitroarenes. The products of this reaction are diverse 2,5-dihydro-1,2,4-oxadiazoles (>40 examples, up to 95% yield). This method does not require an external reductant to reduce nitroarenes, nor does it employ nitrosoarenes, which are often used in N–O cycloadditions. Instead, it is proposed that the 2-azaallyl anions, which behave as super electron donors (SEDs), deliver an electron to the nitroarene to generate a nitroarene radical anion. A downstream 2-azaallyl radical coupling with a newly formed nitrosoarene is followed by ring closure to afford the observed products. This proposed reaction pathway is supported by computational studies and experimental evidence. Overall, this method uses readily available materials, is green, and exhibits a broad scope.
Highlights
IntroductionIs reported an unusual, straightforward, and transition metal-free method for the net [3 + 2]-cycloaddition reaction of 2-azaallyl anions with nitroarenes
The use of nitroarenes as amino sources in synthesis is challenging
We have been interested in the fascinating chemistry of N-benzyl ketimines, which undergo deprotonation to give 2-azaallyl anions that exhibit umpolung reactivity[46]
Summary
Is reported an unusual, straightforward, and transition metal-free method for the net [3 + 2]-cycloaddition reaction of 2-azaallyl anions with nitroarenes. The products of this reaction are diverse 2,5-dihydro-1,2,4-oxadiazoles (>40 examples, up to 95% yield). A downstream 2-azaallyl radical coupling with a newly formed nitrosoarene is followed by ring closure to afford the observed products This proposed reaction pathway is supported by computational studies and experimental evidence. We report a transition-metal-free net [3 + 2]-cycloaddition of nitroarenes with 2-azaallyl anions under basic conditions to afford 2,5-dihydro-1,2,4-oxadiazoles in good to excellent yields (Fig. 2c). The structure of 3aa was confirmed by X-ray diffraction (CCDC 2032323)
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