Stieglitz rearrangement

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The Stieglitz rearrangement is an organic chemistry reaction named after Julius Stieglitz. It describes a 1,2-shift of a aryl group from a saturated amine derivative to nitrogen, forming a triaryl imine (a Schiff base). It was first studied around 1913 by Stieglitz and Leech. The rearrangement is closely related to the Beckmann rearrangement, which also involves a carbon-to-nitrogen shift, but starts from a different kind of substrate.

In simple terms, after activating a leaving group on nitrogen, the migrating aryl group moves to nitrogen and an iminium ion forms. The molecule then becomes neutral by losing a proton. If there are no N–H protons to remove, a reducing agent (for example sodium borohydride) can reduce the iminium to the corresponding saturated amine.

The classic example involves triaryl hydroxylamines. When dehydrated and activated (with reagents such as phosphorus pentachloride, phosphorus pentoxide, or boron trifluoride), the reaction causes the phenyl group to migrate and the product is the triaryl imine. Variations also work with N-hydroxy or N-alkoxy trityl amines, but these often give lower yields.

The reaction also works with other leaving groups. N-sulfonated amines (for example, N-tosylated amines) can rearrange readily, and some substrates like bridged bicyclic N-sulfonated amines do so under mild conditions, sometimes at room temperature in water–dioxane. In some cases the resulting imine is not formed because of strain (a Bredt-like rule), and the leaving group can instead add to the iminium ion.

Stieglitz-type rearrangements can also occur on organic azides, with nitrogen gas being released in the process. These azide reactions are helped by protonation or heating and may proceed through nitrene-like intermediates. N-haloamines (such as N-chloro or N-bromo derivatives) can rearrange as well, often needing a base, though there are base-free examples using reagents like AgBF4.

In addition, reagents such as lead tetraacetate can promote the rearrangement by activating the amine derivative, after which the aryl group migrates and the product forms.

Overall, the Stieglitz rearrangement covers a family of related reactions in which a carbon-to-nitrogen migration occurs after activating a leaving group on nitrogen, typically giving imines from saturated, aryl-bearing amines, with many variations depending on the substrate and reagents used.