Azomethine ylides, allenyl

Dipolar compounds (1,3-dipoles), first designated by Huisgen [1], are a class of 4tt-electron species that can be represented by zwitterionic (or ylide) forms with a separation of charge over three atoms. Basically, 1,3-dipoles are categorized into two subclasses, including the allyl type (carbonyl ylides, carbonyl imines, azomethine ylides, azomethine imines, and nitrones) and the propargyl/allenyl type (azides, diazoalkanes, nitrile ylides, nitrile imines, and nitrile oxides), as shown in Scheme 16.1. 

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