Cycloaddition

Cycloaddition

A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity."[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 reaction a [4 + 2]cycloaddition, the 1,3-dipolar cycloaddition a [3 + 2]cycloaddition and cyclopropanation of a carbene with an alkene a [2+1]cycloaddition. This type of reaction is non-polar addition reaction.

Contents

Reaction mechanism

Thermal cycloadditions usually have (4n + 2) π electrons participating in the starting material, for some integer n. These occur, for reasons of orbital symmetry, in a suprafacial-suprafacial or antarafacial-antarafacial manner (rare). There are a few examples of thermal cycloadditions which have 4n π electrons (for example the [2 + 2] cycloaddition); these proceed in a suprafacial-antarafacial sense, such as the dimerisation of ketene, in which the orthogonal set of p orbitals allows the reaction to proceed via a crossed transition state.

Cycloadditions in which 4n π electrons participate can also occur as a result of photochemical activation. Here, one component has an electron promoted from the HOMO (π bonding) to the LUMO (π* antibonding). Orbital symmetry is then such that the reaction can proceed in a suprafacial-suprafacial manner. An example is the DeMayo reaction. Another example is shown below, the photochemical dimerization of cinnamic acid.[2][3]

Cinnamic Acid CycloAddition

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:

Qcane.png

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 ozonolysis is a (3+2)cycloaddition. The IUPAC preferred notation however, with [i+j+...] takes electrons into account and not atoms. In this notation the DA reaction and the dipolar reaction both become a [4+2]cycloaddition. The reaction between norbornadiene and an activated alkyne is a [2+2+2]cycloaddition.

Types of cycloaddition

Diels-Alder reactions

The Diels-Alder reaction is a [4+2]cycloaddition reaction.

Diels-Alder reaction

Huisgen cycloadditions

The Huisgen cycloaddition reaction is a [2+3]cycloaddition.

1,3-cycloaddition

Nitrone-olefin cycloaddition

The Nitrone-olefin cycloaddition is a [3+2]cycloaddition.

Nitrone olefin cycloaddition

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 enone and an enamine catalyzed by n-butyllithium is a Stork enamine / 1,2-addition cascade reaction:[4]

Intermolecular Formal [3+3] Cycloaddition Reaction

See also

  • [4+3] cycloaddition
  • [6+4] cycloaddition

References

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Cycloaddition".
  2. ^ Hein, Sara M. (June 2006). "An Exploration of a Photochemical Pericyclic Reaction Using NMR Data". Journal of Chemical Education 83 (6): 940–942. doi:10.1021/ed083p940. 
  3. ^ The two trans alkenes react head-to-tail, and the isolated isomer is called truxillic acid
  4. ^ Movassaghi, Mohammad; Bin Chen (2007). "Stereoselective Intermolecular Formal [3+3] Cycloaddition Reaction of Cyclic Enamines and Enones". Angew. Chem. Int. Ed. 46 (4): 565–568. doi:10.1002/anie.200603302. PMID 17146819. 

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См. также в других словарях:

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  • Cycloaddition — Une cycloaddition est une réaction chimique de cyclisation au cours de laquelle « deux molécules (ou parties d une même molécule) insaturées ou plus se combinent pour former un adduit cyclique dans lequel il y a une réduction de la… …   Wikipédia en Français

  • Cycloaddition — Cy|c|lo|ad|di|ti|on: in einem oder mehreren Schritten ablaufende Vereinigung von zwei oder mehr Molekülen unter Bildung eines Ringes, z. B. bei Diensynthesen, 1,3 dipolaren C., cheletropen C. oder Cyclooligomerisationen. Die Zahl der bei der C.… …   Universal-Lexikon

  • cycloaddition — ciklinis jungimasis statusas T sritis chemija apibrėžtis Reakcija, kurios metu nesočiojo junginio molekulės susijungia į ciklinę molekulę. atitikmenys: angl. cycloaddition rus. циклоприсоединение …   Chemijos terminų aiškinamasis žodynas

  • cycloaddition — noun Date: 1963 a chemical reaction leading to ring formation in a compound …   New Collegiate Dictionary

  • cycloaddition — /suy kloh euh dish euhn, sik loh /, n. Chem. the formation of a cyclic compound by the addition reaction of unsaturated molecules. [1960 65; CYCLO + ADDITION] * * * …   Universalium

  • cycloaddition — noun An addition reaction that leads to the formation of a cyclic product …   Wiktionary

  • cycloaddition — cy·clo·addition …   English syllables

  • cycloaddition — ¦sī(ˌ)klōˌ, also ¦si(ˌ)klōˌ noun Etymology: cycl + addition : a chemical reaction leading to ring formation in a compound …   Useful english dictionary

  • Azide alkyne Huisgen cycloaddition — The Azide Alkyne Huisgen Cycloaddition is a 1,3 dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3 triazole. Rolf Huisgen[1] was the first to understand the scope of this organic reaction. American chemist K …   Wikipedia


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