4NO2C6H4SO2

4NO2C6H4SO2 denotes the para-nitrobenzenesulfonyl group, a common aryl sulfonyl fragment used as a protecting group in organic synthesis. In practice, this fragment is most often encountered as the nosyl group (abbreviated Ns or Nos), derived from 4-nitrobenzenesulfonyl.

Structure and naming

The group consists of a benzene ring bearing a nitro substituent at the para position relative to a sulfonyl group (SO2) attached to the ring. The formal formula for the nosyl fragment is C6H4NO4S, with the para-nitro substitution providing electron-withdrawing character that facilitates leaving-group behavior in certain reactions.

Applications

The 4-nitrobenzenesulfonyl group is widely used to protect amines during multi-step organic syntheses. Protection is typically achieved by reacting an amine with 4-nitrobenzenesulfonyl chloride to form a Nos-protected amine. The Nos group is valued for its stability under many reaction conditions and its ability to be selectively removed later in a synthesis sequence.

Removal and deprotection

Desulfonylation of Nos-protected amines can be accomplished under various conditions, depending on the substrate and the desired chemoselectivity. Common approaches include nucleophilic cleavage using thiols (for example, thiophenol) in the presence of base, or reductive methods under appropriate conditions. These strategies aim to cleave the S–N bond and regenerate the free amine while converting the Nos fragment into a sulfinyl or related byproduct.

Related reagents

A closely related reagent is 4-nitrobenzenesulfonyl chloride (NosCl), which is used to install the Nos protecting group on amines. The Nos group is one of several para-nitrobenzenesulfonyl derivatives employed in protecting-group chemistry.