(2 + 2) cycloaddition

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A [2 + 2] cycloaddition (also 1,2-cycloaddition) is the photochemical formation of cyclobutane or cyclobutane derivatives from alkenes that have an activated double bond. These include ketenes , allenes , cinnamic acids , fluoro- or chlorofluoro ethylene . The stereochemistry can be predicted using the Woodward-Hoffmann rules via orbital symmetry. A fundamental distinction is made here between thermally permitted and photochemically permitted [2 + 2] cycloadditions.

Thermal [2 + 2] cycloaddition

Thermal cycloadditions take place in three ways: concerted, radical, or ionic . In the case of a radical or ionic reaction process, in contrast to the concerted reaction process, the orbital symmetry does not play a role, but this is determined by rules as they also apply to non-cycloadditions ( Cram's rule , Felkin-Anh rule , Bürgi-Dunitz angle and the Baldwin Rules ). In the meantime, the energy level of the SOMO (Single Occupied Molecular Orbital) involved or the first half-occupied orbital plays a role in the radical process , but only in the sense of the HSAB concept and not the symmetry. There are few examples of thermal concerted cycloadditions where orbital symmetry is maintained by antarafacial ring closure ([π2s + π2a]).

Photochemical [2 + 2] cycloadditions

Most concerted [2 + 2] cycloadditions are photochemically permitted, electrocyclic reactions and are described by the Woodward – Hoffmann rules . The stereochemistry can be predicted here. This is a [π2σ + π2σ] cycloaddition and the ring closure of the orbitals occurs suprafacially .

Examples

A name reaction , the mechanism of which can be described as a [2 + 2] cycloaddition, is the Paternò-Büchi reaction . In contrast, the reaction mechanism of the Wittig reaction is an ionic cycloaddition reaction.

literature

  • I. Fleming: Frontier orbitals and reactions of organic compounds . VCH Wiley, Weinheim 1988, ISBN 3-527-25792-6 .
  • NJ Turro: Modern Molecular Photochemistry . Benjamin / Cunnings Publishing Co., London 1978, ISBN 0-8053-9353-6 , pp. 419-465.
  • J. Ninomiya, T. Naito: Photochemical Synthesis . Academic Press, New York 1989, ISBN 0-12-519490-0 , pp. 59-109.
  • MT Cimmins, TL Reinhold: Org. React , 44, 1993, pp. 297-588.
  • K. Langer, J. Mattay, A. Heidbreder, M. Miller, Photochemical Key Steps in Organic Synthesis Liebigs Ann. Chem. 1992, 257-260.

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