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General Concept

Cycloadditions allow the synthesis of cyclic compounds from typically alkenes in a concerted reaction step. According to the WOODWARD-HOFFMANN rules the possibilities of formation of cyclic compounds depend on the symmetry of the orbitals of the reactants. The reaction of reactants with n and m π-electrons can be initiated either thermally or photochemically.[1] Such cycloadditions can be classified by the following scheme: [mπ+nπ]. Reactant A possesses mπ-electrons and reactant B nπ-electrons. Reactions between reactants of the [2·nπ+nπ] type proceed thermally, while reactions of the [nπ+nπ] must be intitiated photochemically. Besides alkenes, π-bonds of heteroatoms can follow this reaction path as well. To illustrated the synthetic potential, [4+2]-cycloadditions and [2+2]-cycloadditions are introduced below.[1,2]


Figure 1: Reaction of 1,3-Butadiene with ethylene.[1]

[4+2]-Cycloadditions (DIELS-ALDER-Reactions)

[4+2]-cycloadditions (DIELS-ALDER-reactions) represent the prototype of cycloadditions of the [2·nπ+nπ] type. In this kind of reactions, a molecule with 2·nπ electrons (e.g. diene) reacts with a molecule with nπ electrons (e.g. dienophile) to a new cyclic product. The [4+2]-cycloaddition of butadiene with ethylene and the correlation diagram of molecule orbitals. In this case, each bonding level of the reactants correlates with a product bonding level and the ground state levels correlate directly. Therewith, there is no or only a low activation energy required for this thermally allowed process.

The reactions in which the electrons shift in a concerted step and form a cyclic ring are called an electrocyclic reactions or cycloadditions. The Diels-Alder reaction is special for [4+2]cycloadditions. In this reaction, the conjugated π-system (diene) provides four π-electrons to the reaction and the other partner (dieneophile) provides two electrons. The reaction and electron flow is shown in Figure 0.1.[3] The cycloaddition reactions are stereospecific because of the concerted reaction mechanism. That means, the final structure of product dependents on the structure of the reactants (see Figure 0.2). The reactivity of compounds depends on substituents. Donor substituents increase the energy of HOMO and LUMO, acceptor substituents decrease the energy of both orbitals. The presence of electron attracting substituent increase and electron donating substituents decrease the reactivity of an alkene. For dienes the situation is inverse, the reactivity increases with electron-rich substituents like carbonyl containing groups. Therewith, high reaction rates are observed for electron-rich dienes with electron-poor dienophiles.[4]


Figure 2: Reaction of 1,3-Butadiene with maleic acid dimethyl ester.[3]



Figure 3: General reaction mechanism of Paternò-Büchi reaction.



[2+2]-cycloadditions give access to four membered rings starting from two alkenes or one alkene and a carbonyl compound. The symmetry of the π orbitals prohibits thermally initiated cycloadditions, but the photochemically initiated pathway is permitted. This applies for all [nπ+nπ] reactions. Irradiation with light of appropriated wavelength excites one reactant to the LUMO level. This level possesses the correct orbital symmetry and allows the concerted reaction. shows the correlation diagram for the reaction of two ethene molecules.[1]

An important example of [2+2]-cycloaddition reactions is the PATERNÒ-BÜCHI reaction, in which a carbonyl group reacts with an alkene and forms an oxetane. This reaction is allowed only photochemically and thermally prohibited. The general procedure and reaction mechanism of PATERNÒ-BÜCHI reaction is shown in Figure 3. In most cases the reaction proceeds via a 1,4-biradical and regioisomerism is determined by the stability of the biradical. Hence, orbital symmetry is not necessarily maintained.



  1. WOODWARD, R. B.; HOFFMANN, R.: The Conservation of Orbital Symmetrya, Angewandte Chemie International Edition in English, 1969-November, 8 (11), 781–853, DOI: 10.1002/anie.196907811.
  2. MATTAY, J.; GRIESBECK, A. G.: Photochemical Key Steps in Organic Synthesis. An Experimental Course Book, 1994, Wiley-VCH, ISBN 978–3–5272–9214–1.
  3. Comprehensive Heterocyclic Chemistry, 2009, Elsevier, ISBN 978-0-08-096519-2.
  4.  VOLLHARDT, K. P. C.; SCHORE, N. E.: Organic Chemistry, 6th Edition, 2010, W. H. Freeman, ISBN 978-1-4292-0494-1.

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