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1,3-Dipolar cycloaddition
The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition (this term is often used to specifically describe the 1,3-dipolar cycloaddition between an organic azide and an alkyne to generate 1,2,3-triazole). Currently, 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives.
==Mechanistic overview==
There were originally two proposals that describe the mechanism of the 1,3-dipolar cycloaddition: first, the concerted pericyclic cycloaddition mechanism, proposed by Rolf Huisgen; and second, the stepwise mechanism involving a diradical intermediate, proposed by Firestone. After much debate, the former proposal is now generally accepted—the 1,3-dipole reacts with the dipolarophile in a concerted, often asynchronous, and symmetry-allowed π4s + π2s fashion through a thermal six-electron Huckel aromatic transition state. Although, there are few examples of stepwise mechanism of the catalyst free 1,3-dipolar cycloaddition reactions for thiocarbonyl ylides and also for nitrile oxides
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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