Abstract
Herein, we report that iron(II)/ammonium persulfate in aqueous acetonitrile mediates the Newman–Kwart rearrangement of O-aryl carbamothioates. Electron-rich substrates react rapidly under moderate heating to afford the rearranged products in excellent yields. The mild conditions, rapid reaction rates, and suitability for scale up offers immediate practical benefits to access functionalized thiophenols.
Highlights
The Newman−Kwart rearrangement (NKR) the transformation of O-aryl carbamothioates to the corresponding S-aryl carbamothioates gives access to thiophenols from their more readily available phenol counterparts.[1,2]
Renewed interest in the NKR has led to the discovery of several catalytic systems that favor electron-rich substrates, including a photoredox catalytic system[8] and, very recently, an electrochemical method,[9] as well as a chemical reaction involving single-electron oxidation of O-aryl carbamothioates with ceric ammonium nitrate (CAN) in dimethyl sulfoxide (DMSO).[10]
Successful on small scale, the thermal NKR proved operationally challenging to scale to multigram quantities, as inconsistent heating resulted in variable yields
Summary
The Newman−Kwart rearrangement (NKR) the transformation of O-aryl carbamothioates to the corresponding S-aryl carbamothioates gives access to thiophenols from their more readily available phenol counterparts.[1,2] The threestep sequence, which involves phenol protection with thiocarbamoyl chloride, NKR, and deprotection of the resulting carbamothioate, is appealing, as it avoids the need for highly reactive reagents or handling of foul-smelling chemicals. Attempted Pd-catalyzed NKR only afforded trace amounts of product 2a in agreement with the previously reported scope.[12] Inspired by the work of Anderson and Kochi on radical decarboxylation of carboxylic acids,[13] we attempted to use silver nitrate and ammonium persulfate (APS) as a single electron oxidant to mediate this transformation. Under these conditions (35 mol % AgNO3, 1 equiv of APS, CH3CN/H2O, 85 °C) 1a rearranged to target product 2a in 78% yield (Scheme 1).
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