Cycloaddition



A Cycloaddition is a σ bonds are gained [1] the resulting reaction is a cyclization reaction.

Cycloadditions are usually described by the backbone size of the participants. This would make the Diels-Alder a addition reaction.

Reaction mechanism

Thermal cycloadditions usually have 4n + 2 π electrons participating in the starting material. These occur, for reasons of transition state.

Cycloadditions in which 4n π electrons participate can also occur as a result of antibonding). Orbital symmetry is then such that the reaction can proceed in a suprafacial-suprafacial manner. One example is the photochemical dimerization of cinnamic acid [2] [3]:

Note that not all photochemical (2+2) cyclizations are cycloadditions; some are known to operate by radical mechanisms.

Some cycloadditions instead of π bonds operate through strained cyclopropane rings; as these have significant π character. For example, an analog for the Diels-Alder reaction is the quadricyclane-DMAD reaction:


In the (i+j+...) cycloaddition notation i and j refer to the number of atoms involved in the cycloaddition. In this notation a Diels-Alder reaction is a (4+2)cycloaddition and a 1,3-dipolar addition such as the first step in alkyne is a [2+2+2]cycloaddition.

Types of cycloaddition

Two major cycloaddition reactions are :

Formal cycloadditions

Cycloadditions often have metal-catalyzed and stepwise radical analogs, however these are not strictly speaking pericyclic reactions. When in a cycloaddition charged or radical intermediates are involved or when the cycloaddition result is obtained in a series of reaction steps they are sometimes called formal cycloadditions to make the distinction with true pericyclic cycloadditions.

One example of a formal [3+3]cycloaddition between a cyclic cascade reaction [4]:

References

  1. ^ IUPAC Gold Book definition
  2. ^ An Exploration of a Photochemical Pericyclic Reaction Using NMR Data Sara M. Hein Journal of Chemical Education Vol. 83 No. 6 June 940 2006
  3. ^ The two trans isomer is called truxillic acid
  4. ^ Stereoselective Intermolecular Formal [3+3] Cycloaddition Reaction of Cyclic Enamines and Enones Mohammad Movassaghi, Prof. Dr. *, Bin Chen, Dr. Angew. Chem. Int. Ed. 2007, 46, 565 –568 doi:10.1002/anie.200603302
 
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