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azomethine ylide
Azomethine ylides are nitrogen-based 1,3-dipoles, consisting of an iminium ion next to a carbanion. They are used in 1,3-dipolar cycloaddition reactions to form 5-membered heterocycles, including pyrrolidines and pyrrolines.〔 These reactions are highly stereo- and regioselective, and have the potential to form four new contiguous stereocenters. Azomethine ylides thus have high utility in total synthesis, and formation of chiral ligands and pharmaceuticals. Azomethine ylides can be generated from many sources, including aziridines, imines, and iminiums. They are often generated ''in situ'', and immediately reacted with dipolarophiles.
== Structure ==
The resonance structures below show the 1,3-dipole contribution, in which the two carbon atoms adjacent to the nitrogen have a negative or positive charge.〔 The most common representation of azomethine ylides is that in which the nitrogen is positively charged, and the negative charge is shared between the two carbons atoms. The relative contributions of the different resonance structures depend on the substituents on each atom. The carbon containing electron-withdrawing substituents will have a more partial negative charge, due to the ability of the nearby electron-withdrawing group to stabilize the negative charge.
Three different ylide shapes are possible, each leading to different stereochemistry in the products of 1,3-dipolar cycloaddition reactions. W-shaped, U-shaped, and S-shaped ylides are possible. The W- and U-shaped ylides, in which the R substituents are on the same side, result in syn cycloaddition products, whereas S-shaped ylides result in anti products. In the examples below, where the R3 substituent ends up in the product depends on the substituent's steric and electronic nature (see regioselectivity of 1,3 dipolar cycloadditions). The stereochemistry of R1 and R2 in the cycloaddition product is derived from the dipole. The stereochemistry of R3 is derived from the dipolarophile—-if the dipolarophile is more than mono-substituted (and prochiral), up to four new stereocenters can result in the product.
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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