Dipoles of the Propargyl-Allenyl type

7-electrocyclization of only three types of propargyl-allenyl 1,3-dipole has been reported, and each of these will be covered in detail. 

Substituent Directive Effects in the Cyclization of Nitrile Ylides 18 

Cullen and Sharp extended the study of the mechanism for this process 

It can also be seen that steric hindrance alone is preventing 1,7-electro-cyclization of 60 since no formation of the common intermediate 58 can occur otherwise, 1H-2,3-benzodiazepine 59 would be obtained.To account for these experimental observations Sharp [83JCS(CC)1003 84T3095] has pos- 

When the a,/3-bond is olefinic and the y, 8-bond is aromatic, then 1,5- or 1,7-electrocyclization can be observed [75JCS(P1)102 79JCS(P1)1433], depending upon the substituents present on the a,/3-bond. In the simple acyclic case, the diazo compound 64 reacted to give pyrazoles 65 exclusively 

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Tnfluoromethyl-substUuted 1,3-dipoles of the propargyl-allenyl type and trifluoromethyl-substituted nitrilium betaines. Tnfluoromethyl- [164, 765] and bis(trifluoromethy])-substituted [166, 167] nitrile ylides have been generated by different methods and trapped with various dipolarophiles to yield [3+2] [768] and [3+1] cycloadducts [769], respectively... 

All 1,3-dipoles contain an allyl anion type n system, i.e., four electrons delocalized over three parallel atomic n orbitals, but, in addition, 1,3-dipoles of the propargyl-allenyl type contain an additional n bond in the plane perpendicular to the allyl anion MO (Fig. 1). 

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... 

Diazoalkanes, like azides, are 1,3-dipoles of the propargyl-allenyl type (Scheme 87)15 and their reaction with imines provides a route for building the triazoline framework from the C—N—N and C—N fragments. Although diazomethane addition to the carbon-carbon double bond was achieved by von Pechmann in 1898,325 its reaction toward the carbon-nitrogen double bond was investigated only 50 years later. 

A 1,3-dipole as shown in Schemes 6-5 and 6-6 corresponds to a system with three parallel atomic p-orbitals, i.e., to an allyl anion, but without net charge. It is, therefore, called an allyl-type 1,3-dipole. The system may contain, however, an additional 7i-bond in the plane perpendicular to the allyl anion type molecular oribtal, and then belongs to the propargyl - allenyl type. Normally, 1,3-dipoles of this type are linear, whereas those of the allyl type are bent. The term 1,3 relates to the reactivity in these positions, not to formal charges. A series of theoretical studies (e. g., by Hiberty and Leforestier, 1978 Yamaguchi et al., 1980 see review of Houk and Yamaguchi, 1984) clearly show, however, that some of these 1,3-dipoles have considerable biradical character (e.g., O3 53% and CH2N2 28% in ab initio calculations at the 4-3IG level). We will return to biradicals in the mechanistic discussion of Sect. 6.3. 

Whereas in the propargyl-allenyl type 1,3-dipoles, with the existence of sp orbital, the reaction scheme will be generally shown in Figure 2. 

Huisgen classifled the 1,3 dipoles into two categories, the allyl type and the propargyl-allenyl type, and suggested that both types of 1,3 dipole should follow the concerted mechanism and that both types of 1,3 dipole and dipolarophiles should approach each other in two parallel planes, forming an envelope-like transition structure. 

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. 

Dipoles of the allyl type are bent, whereas the propargyl/allenyl types, which bear an additional rr-orbital orthogonal to the allyl type of molecular orbital, have a linear structure. 

Table 2 illustrates 1,3-dipoles with a double bond and with internal octet stabilization, commonly referred to as the propargyl-allenyl anion type. These are all reactive dipoles and a large number of five-membered heterocycles can be constructed from these readily available dipoles, especially when the dipolarophile is varied to include heterocumulenes, etc. 

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. ... 

In general, unsaturated heterocyclic compounds are generated from the reactions of allyl-type 1,3-dipole precursors with either alkynes or alkenes as well as the reactions of propargyl/allenyl-type 1,3-dipoles with alkenes, while aromatic compounds are often accessible by the cycloaddition reactions of propargyl/allenyl type 1,3-dipoles and alkynes (Scheme 16.2). To obtain aromatic compounds from the former type of cycloaddition reactions, the resulting unsaturated cycloadducts are required to undergo further aromatization, such as oxidation and elimination of small molecules (i.e., H2O, CH3COOH, CO2, etc.). 

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. 

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