2 2 Photocycloaddition

A [2 2] cycloaddition ( 1,2- cycloaddition ) is the photochemical formation of cyclobutane and cyclobutane derivatives from alkenes having an activated double bond. These include ketenes, allenes, cinnamic acids, fluorine or chlorine fluoroethylenes. The stereochemistry can be predicted using the Woodward -Hoffmann rules on the orbital symmetry. We distinguish in principle between thermally allowed and photochemically allowed [2 2 ] cycloaddition reactions.

Thermal [2 2 ] cycloaddition

Thermal cycloaddition reactions take place in three ways: concerts, radical or ionic. In a radical or ionic reaction course, the orbital symmetry plays in contrast to the concerted reaction pathway not matter but this is by rules as well as for non- cycloadditions are determined ( Cram's rule, Felkin -Anh rule Bürgi- Dunitz angle and Baldwin rules) for the ionic gradient. Although the radical pathway, the energy position of the participating SOMO (Single Occupied Molecular Orbital ) or first half occupied orbital Meanwhile plays a role, but only in terms of the HSAB concept and not symmetry. There are few examples of thermal concerted cycloaddition, where compliance with orbital symmetry by a antarafacial ring closure can be achieved ( [ π2σ π2a ] ).

Photochemical [2 2 ] cycloaddition

The most concerted [2 2] Cycloaddionen are photochemically allowed electrocyclic reaction and are described by the Woodward -Hoffmann rules. The stereochemistry can be here predict it. This is a [ π2σ π2σ ] cycloaddition, and ring closure of the orbitals done suprafacially.

Examples

A name whose reaction mechanism can be described as [2 2 ] cycloaddition is the Paternò - Büchi reaction. Reaction mechanism, however, the Wittig reaction is an ionic Cycloaddionsreaktion.