Dipolar propargyl-allenyl type

Organic azides are octet-stabilized 1,3 dipoles of the propargyl-allenyl type, la<- lb.15 The dipolar character of the azido group9,15 enables it to... 

FIGURE 2. Propargyl-allenyl type 1,3-Dipolar Cycloaddition. 

In 1893, Arthur Michael observed the formation of a 1,2,3-triazole derivative in the reaction of diazobenzolimid (i.e. Ph-Ns) with dimethyl acetylenedicarboxylate Michael - a future Harvard professor - worked with R. Bunsen and A.W. von Hofmann. Numerous cycloadditions of organic azides and HN3 to alkynes and alkenes were described in the sequel. In the general definition and classification azides belong to the 1,3-dipoles of propargyl-allenyl type (R. Huisgen, I960). 1,3-Dipolar cycloadditions share the 6 r-electron balance with Diels-Alder reactions - and the wide synthetic apphcation. Albert Padwa edited monographs on 1,3-dipolar cycloaddition chemistry in 1984 and 2003 -substantial chapters on azides were included. ... 

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. 

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

Sixteen-Electron. Those for which the dipolar canonical form has a double bond on the sextet atom and the other resonance structure has a triple bond. Examples are azides (R—N3), diazoalkanes (R2C=N=N), and nitriloxides (R—C= N—O). These have also been labeled as propargyl/allenyl anion type [270]. 

The synthesis of a variety of five-membered heterocycles involves the reaction of a neutral 471-electron three-atom system, the dipole, with a 271-electron system, the dipolarophile (Scheme 2). Table 3 illustrates 1,3-dipoles with a double bond and with internal octet stabilization, the propargyl-allenyl anion type. The application of 1,3-dipolar cycloadditions to the synthesis of heterocyles is discussed in several comprehensive reviews <2007T12247, 2007T3235, 2006H(68)2177>. 

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