Stevens rearrangement

Overview of the Stevens rearrangement

Example of a Stevens rearrangement with an ammonium salt. R denotes organic radicals, Z denotes an electron-withdrawing group. B denotes a base and HB the protonated base

Example of a Stevens rearrangement with a sulfonium salt. R denotes organic radicals, Z denotes an electron-withdrawing group. B denotes a base and HB the protonated base

The Stevens rearrangement is a name reaction in organic chemistry that was named after Thomas Stevens Stevens . In her, under the action of a base, an organic radical R ( alkyl , allyl , benzyl ) is shifted from a four-fold bonded nitrogen atom (a quaternary ammonium ion ) to an adjacent carbon atom, which has an electron-withdrawing radical such as. B. carries a ketone, an ester group or an aryl group (electron-withdrawing groups are often referred to as EWG "electron withdrawing group"). The same reaction takes place with sulfonium salts .

mechanism

The rearrangement proceeds with retention of the stereochemistry of the migrating substituents , which speaks against a concerted [1,2] migration, since this would have to result in an inversion of the stereochemistry of the migrating group. A radical mechanism is therefore assumed in which the two radicals arising from the homolytic dissociation of the bond remain close to one another in the “solvent cage”, since rapid recombination to the product takes place. In addition to the radical course, an ion pair mechanism is also discussed, which takes place via a heterolytic cleavage .

Mechanism of the Stevens rearrangement using the example of an ammonium salt, R denotes an organic residue, Z an electron-withdrawing residue.

The mechanism is analogous for ammonium and sulfonium salts, an ammonium salt is used here as an example . An amide anion serves as an example of the base. First, a quaternary ammonium salt with methylene group 1 is mixed with an amide anion, whereby the methyl group is deprotonated and molecule 2 is obtained. In the radical route (above), the bond between the nitrogen atom and a residue splits homolytically and radical 3 is created , which has two boundary structures due to the mesomerism . In addition, a radical residue is formed. This radical residue attaches itself to the carbon atom of radical 3 in such a way that the end product 5 is formed.

During the ionic reaction process (below) the charge on the carbon is rearranged in such a way that a double bond is formed between the carbon and the nitrogen and the bond between the nitrogen and one of its residues is split heterolytically. This creates an iminium ion, which can be represented as another mesomeric boundary structure as carbenium ion 4 . Furthermore, a negatively charged residue is formed. This residue is now attached to the carbon atom of the carbenium ion after a nucleophilic attack on it. Amine 5 is thus obtained .

Individual evidence

1. ^ Lecture notes organic chemistry IV, Uni-Duisburg-Essen (PDF; 165 kB).
2. ^ László Kürti , Barbara Czako: Strategic Applications of Named Reactions in Organic Synthesis. Elsevier, Amsterdam et al. 2009 , ISBN 978-0-12-369483-6 , pp. 434-435.
3. ↑ BP Mundy, MG Ellerd, FG Favaloro: Name Reactions and Reagents in Organic Synthesis , 2nd Edition, Wiley-Interscience, Hoboken, NJ 2005 , ISBN 978-0-471-22854-7 , S. 618th