Two-fold extrusion reactions
Twofold extrusion reactions involve the loss of two small, typically inorganic fragments bridging two atoms with the formation of a double bond between the atoms (most commonly, an alkene or imine). These reactions are particularly useful for the introduction of hindered double bonds, as extrusion can often be initiated thermally without the need for added reagents.
Twofold extrusion reactions involve the loss of small, inorganic fragments from a heterocyclic ring system with the formation of a double bond between carbons in the ring (Eq. 1). This suite of reactions has been applied for the synthesis of hindered alkenes, for which other methods such as the Wittig reaction are not successful. Synthetic routes to molecular machines have also involved twofold extrusion reactions. The scopes of these reactions are limited primarily by the availability of precursors to the required cyclic starting materials. At high temperatures or in the presence of a reductant such as a phosphine, extrusion is commonly spontaneous.(1)
Precursors to the ring systems that undergo twofold extrusion include thiones, hydrogen sulfide, selones, diazo compounds, and azines. Although multiple mechanisms are known to access the required heterocycles, the extrusion process itself can be described using a single mechanistic paradigm.
Mechanism and Stereochemistry
Scope and Limitations
Comparison to Other Methods
Experimental Conditions and Procedure