The Stevens rearrangement is an organic reaction in which nitrogen and sulfur ylides perform an intramolecular alkylation. The process usually begins with a persubstituted ammonium or sulfonium cation, in which case the net reaction is a 1,2-rearrangement as follows:

History Thomas S. Stevens' original 1928 publication concerned the reaction of 1-phenyl-2-(N,N-dimethylamino)ethanone with benzyl bromide. The resulting ammonium salt reacted with aqueous sodium hydroxide to the rearranged amine:

A 1932 publication described the corresponding sulfur reaction.

Reaction mechanism The Stevens rearrangement's mechanism is one of the most controversial in organic chemistry. Certain is an initial deprotonation by a strong base: this yields an ylide stabilized by the electron-withdrawing substituent.M B Smith, J March. March's Advanced Organic Chemistry (Wiley, 2001) ()Strategic Applications of Named Reactions in Organic Synthesis Laszlo Kurti, Barbara Czako Academic Press (4 March, 2005)

The difficulty with the subsequent migration is that it exhibits retention of configuration. Retention is inconsistent with a concerted reaction, which would require an antarafacial transition state. But it is also inconsistent with diradical (3a) or ionic (3b) intermediates, unless those intermediates are trapped in a solvent cage:

Scope Cationic reactants can be obtained through alkylation of the corresponding amines and sulfides. The active ylide intermediate can be formed without deprotonation through rhodium(II)-acetate–catalyzed coordination of a diazo carbenoid:Macrocycle Ring Expansion by Double Stevens RearrangementKeisha K. Ellis-Holder, Brian P. Peppers, Andrei Yu. Kovalevsky, and Steven T. Diver Org. Lett.; 2006; 8(12) pp. 2511–2514; (Letter)

Competing reactions are the Sommelet-Hauser rearrangement and Hofmann elimination.

Enzymatic reaction γ-butyrobetaine hydroxylase, an enzyme on the human carnitine biosynthesis pathway, catalyzes a Stevens-like C-C bond formation on meldonium.

See also Sommelet–Hauser rearrangement Pummerer rearrangement, which may be thought of as a specific example of the Stevens rearrangement for the case of sulfonium acetates γ-Butyrobetaine hydroxylase

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