Convenient Syntheses of 2‐Nitrophenylsulfen‐(NPS‐)imines by Oxidation of NPS‐Protected Amines and Amino Acid Derivatives

Abstract

Abstract2‐Nitrophenylsulfenimines 2 of large structural variety are easily available from 2‐nitrophenylsulfenamides 1, by direct and indirect electrochemical oxidation or by using stoichiometric amounts of a triarylamine radical cation salt. In most cases the direct electrochemical method is to be preferred, because of the convenient reaction procedure and the simple workup of the products. Inter‐ and intramolecular addition of nucleophiles to the C  N double bond is possible as shown by two examples.

Mechanistic Aspects of the Electrochemical Oxidation of Aliphatic Amines and Aniline Derivatives

“…The oxidation products benzaldehyde, acetaldehyde, and N-methylamine were obtained in yields of 65%, 68%, and 87%, respectively [ 90 ]. A similar mechanism and product distribution were observed for related alkanolamines, confirming the proposed mechanism [ 79 , 91 , 92 ]. …”

“…The formal potential for the oxidation of the p -substituted triphenylamines depends on the electron-donative or electron-withdrawing nature of the p -substituent, covering the potential range of 0.6 to 1.8 V vs. SCE [ 25 , 76 , 77 , 78 ]. For example, the electrochemical oxidation of N-ethyl- S -(2-nitrophenyl)thiohydroxylamine was performed at 0.69 V vs. SCE in dichloromethane containing 0.1 M [Bu 4 N][ClO 4 ] as the supporting electrolyte in the presence of N 1 , N 1 , N 4 , N 4 -tetrakis(4-bromophenyl)benzene-1,4-diamine as the catalyst (Equation (19)) [ 79 ]. The respective sulfenimine product was isolated in a 72% yield.…”

Free-Radical Reactions in the Synthesis of α-Amino Acids and Derivatives

“…It is possible to form an α-carbon-centered amino acid radical by either method, and in some cases there may be ambiguity about which mechanism is involved. The electron-transfer process may be accomplished either using chemical reagents or electrochemically, and a variety of N -protected amino acid derivatives react in this manner to give the corresponding imines. − Depending on the reaction conditions, these may react in situ to give α-methoxy 145,147-150 and α-hydroxy 149 amino acid derivatives. The products have the potential to react in similar ways to those described above for other α-functionalized amino acid derivatives.…”

Tris(4-bromophenyl)aminium Hexachloroantimonate

“…The oxidation of 2-nitrophenylsulfenamide protected amines or amino acids to the corresponding sulfenimine is readily brought about by (1) in the presence of a base such as 2,6-Lutidine (eq 9). 17 Other methods for this conversion have included electrochemical oxidation. Other Reactions.…”