Propargyl-like structure

It was found that the azirine-nitrile ylide isomerization was a completely reversible process. The unlabeled nitrile ylide showed a prominent band at 1926 cm-1 that underwent a 66-cm I shift with I5N substitution. This shift was interpreted as being consistent with an allene-like skeleton (8) rather than the alternative propargyl-like structure (9). This conclusion was supported by the spectra from the l3C- and 2H-labeled variants. Warming the nitrile ylide in a xenon matrix from 12 to 82 K provided no new absorptions suggesting that the allene-like structure may also be adopted in solution. Some absorption spectra for benzonitrilio benzylide (DPNY) and some substituted benzonitrilio methylides obtained via pulsed-laser photolysis of azirines are given in Table 7.1 (5). 

The mobile Jt-electrons of unsaturated systems, responsible for the stabilization of the carbocations, provide equally efficient stabilization for carbanions. Consequently, a retrosynthetic cleavage of a benzylic, allylic, or propargylic C-C bond has additional merits since the resulting fragments can be visualized as either an electrophile or a nucleophile. The dual synthetic value of the allylic moiety has been extensively utilized in the synthesis of a large number of natural acyclic isoprenoids. The structures of many of these compounds look like they were purposely tailored for this type of retrosynthetic analysis. In fact, the 1,5-diene system, usually present in their structure (Scheme 2.19), immediately suggests the cleavage of its central C-3-C-4 bond, which leads to two allylic precursors. 

The other reactions which are observed in this system are due to the CsHs and CHs- radicals. The duality in the structure of the CsHs-radical was pointed out by Collin and Lossing. They concluded that the radical behaved more like CH=C—CH2 (propargyl) than CH2=C=CH (allenyl) at low pressure since it reacted with methyl to give 1-butyne rather than 1,2-butadiene. 

Benzene, however, is an entirely different compound than this, i.e., it is isomeric with di-propargyl. It does not act like an unsaturated compound and, therefore, can not be represented by a structural formula like the above. Some of the characteristic differences between these two isomeric compounds are shown by the following reactions. 

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. 

Benzyl halides and allyl (propargyl) halides are structurally similar but have drastically different chemical reactivities in the aqueous Barbier-Grignard-type reactions. Although tribenzyl and dibenzyltin derivatives have been prepared in aqueous conditions since the 1960s, they do not add onto carbonyls, most likely because it is not possible to form a six-membered cyclic transition state with the carbonyl group in a two-componenf fashion. Still, zinc-mediated benzylation of carbonyl compounds in aqueous media was reported by Bieber et al. recently. The benzylation of 4-nitrobenzaldehyde could be controlled chemoselectively by using various phase transfer catalysts and metal reductants in water (Eq. 4.41). 2... 

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