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Huisgen Cycloaddition 1,3-Dipolar Cycloaddition

Lianyungang Klinechem Co.,Ltd | Updated: Dec 05, 2018

The Huisgen Cycloaddition is the reaction of a dipolarophile with a 1,3-dipolar compound that leads to 5-membered (hetero)cycles. Examples of dipolarophiles are alkenes and alkynes and molecules that possess related heteroatom functional groups (such as carbonyls and nitriles). 1,3-Dipolar compounds contain one or more heteroatoms and can be described as having at least one mesomeric structure that represents a charged dipole.

Examples of linear, propargyl-allenyl-type dipoles

An example of an allyl-type dipole. See: Ozonolysis

Mechanism of the Huisgen 1,3-Dipolar Cycloaddition

2 π-electrons of the dipolarophile and 4 electrons of the dipolar compound participate in a concerted, pericyclic shift. The addition is stereoconservative (suprafacial), and the reaction is therefore a [2s+4s] cycloaddition similar to the Diels-Alder Reaction. Attention: many authors still use "[2+3] cycloaddition", which counts the number of involved atoms but does not follow IUPAC recommendations (DOI). IUPAC recommends the use of "(2+3)" for the number of involved atoms instead.

A condition for such a reaction to take place is a certain similarity of the interacting HOMO and LUMO orbitals, depending on the relative orbital energies of both the dipolarophile and the dipole. Electron-withdrawing groups on the dipolarophile normally favour an interaction of the LUMO of the dipolarophile with the HOMO of the dipole that leads to the formation of the new bonds, whereas electron donating groups on the dipolarophile normally favour the inverse of this interaction. Diazomethane as an electron-rich dipolar compound therefore rapidly reacts with electron-poor alkenes, such as acrylates. Relative reactivity patterns may be found in the literature  (R. Huisgen, R. Grashey, J. Sauer in Chemistry of Alkenes, Interscience, New York, 1964, 806-877.).

The regioselectivity of the reaction depends on electronic and steric effects and is somewhat predictable. For example, the addition of alkynes to azides, which is an interesting reaction for the generation of 1,2,3-triazole libraries by the simple reaction of two molecules ("click chemistry"), leads to regioisomers:

V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless, Angew. Chem. Int. Ed.200241, 2596-2599.

The reaction has been modified to a more regioselective, copper-catalyzed stepwise process by the Sharpless and Fokin groups, which is no longer a classic Huisgen Cycloaddition (for a discussion of the nonconcerted mechanism see: click chemistry) . Another approach prefers the use of a directing electron withdrawing group, which is removable later:

D. Amantini, F. Fringuelli, O. Piermatti, F. Pizzo, E. Zunino, L. Vaccaro, J. Org. Chem. 200570, 6526-6529.

In summary, the 1,3-dipolar cycloaddition allows the production of various 5-membered heterocycles. Many reactions can be performed with high regioselectivity and even enantioselective transformations of prochiral substrates have been published. Some interesting examples may be found in the recent literature.

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