O-Protected NH-free hydroxylamines: emerging electrophilic aminating reagents for organic synthesis

Ortho-Selective amination of arene carboxylic acids via rearrangement of acyl O-hydroxylamines

Direct amination of arene C-H bonds is an attractive disconnection to form aniline-derived building blocks. This transformation presents significant practical challenges; classical methods for ortho-selective amination require strongly acidic or forcing conditions, while contemporary catalytic processes often require bespoke directing groups and/or precious metal catalysis. We report a mild and procedurally straightforward ortho-selective amination of arene carboxylic acids, arising from a facile rearrangement of acyl O-hydroxylamines without requiring precious metal catalysts. A broad scope of benzoic acid substrates are compatible and the reaction can be applied to longer chain arene carboxylic acids. Mechanistic studies probe the specific requirement for trifluoroacetic acid in generating the active aminating agent, and suggest that two separate mechanisms may be operating in parallel in the presence of an iron catalyst: (i) an iron-nitrenoid intermediate and (ii) a radical chain pathway. Regardless of which mechanism is followed, high ortho selectivity is obtained, proposed to arise from the directivity (first) or attractive interactions (second) arising with the carboxylic acid motif.

From Cycloalkanols to Heterocycles via Nitrogen Insertion

A variety of cyclic alcohols are found to undergo nitrogen insertion by subjection to O-mesitylsulfonylhydroxylamine. Critical to a successful process is the use of fluorinated alcoholic solvents, which ensures sufficient substrate activation to allow engagement with the ambiphilic aminating agent. This transition-metal-free nitrogen insertion provides access to a variety of medicinally relevant heterocycles such as pyrrolidenes, quinolines, and benzazepines (24 examples). Furthermore, combination with a photochemical Norrish-Yang-type cyclization allows an unprecedented access to indoles from ortho-substituted acetophenones.

The advent of electrophilic hydroxylamine-derived reagents for the direct preparation of unprotected amines

Electrophilic aminating reagents have seen a renaissance in recent years as effective nitrogen sources for the synthesis of unprotected amino functionalities. Based on their reactivity, several noble and non-noble transition metal catalysed amination reactions have been developed. These include the aziridination and difunctionalisation of alkenes, the amination of arenes as well as the synthesis of aminated sulfur compounds. In particular, the use of hydroxylamine-derived (N-O) reagents, such as PONT (PivONH3OTf), has enabled the introduction of unprotected amino groups on various different feedstock compounds, such as alkenes, arenes and thiols. This strategy obviates undesired protecting-group manipulations and thus improves step efficiency and atom economy. Overall, this feature article gives a recent update on several reactions that have been unlocked by employing versatile hydroxylamine-derived aminating reagents, which facilitate the generation of unprotected primary, secondary and tertiary amino groups.

An Aminative Rearrangement of O-(Arenesulfonyl)hydroxylamines: Facile Access to ortho-Sulfonyl Anilines

Ortho‐sulfonyl anilines are important building blocks for a range of applications. We report the discovery of an aromatic rearrangement reaction of O‐(arenesulfonyl)hydroxylamines which leads directly to ortho‐sulfonyl anilines through formation of a new C−N bond with excellent levels of regiocontrol for the ortho position(s) over all others. We establish that the rearrangement is proceeding through an intermolecular mechanism and propose that the regiocontrol observed is the result of attractive non‐covalent interactions occurring during the C−N bond‐forming step. Importantly, this method is complementary to classical aniline sulfonation in terms of the variously substituted regioisomers that can be obtained and it is also applicable to O‐(benzylsulfonyl) hydroxylamines.

Metal-free synthesis of secondary amides using N-Boc-O-tosylhydroxylamine as nitrogen source via Beckmann rearrangement

An efficient methodology for the direct synthesis of secondary amides from ketones has been developed by using N-Boc-O-tosylhydroxylamine (TsONHBoc).