Cyclopropenes small ring compounds

Strained alkenes, such as cyclopropene or norbomcne (5.5), bind unusually strongly to metals because the rehybridization on binding leads to relief of strain. Much of the strain in a small ring compound arises because the real... 

The idea behind bond localization in annelated benzenes is exemplified by compound 66, tricyclopropabenzene. Of the two standard Kekule structures of 66, one places the double bonds within the three-member ring (endo) and the other places the double bonds outside (exo) of the small ring. Since cyclopropene is much more strained than cyclopropane, the avoidance of strain energy suggests that the double bonds might be localized into the exo positions. Compounds 66 and 67 are the prime test subjects for this idea of strain-induced bond localization (SIBL). ... 

Carbene chemistry constitutes a particular but challenging field in organic synthesis. Carbenes offer a straightforward access to small rings (cyclopropanes, cyclopropenes) as well as to cycloheptatriene derivatives (the Buchner reaction) from cheap raw material (olefins, acetylenes, benzenic compounds, etc.). 

Most workers have preferred to prepare stable derivatives prior to analysis. For example, cyclopropene fatty acids can be subjected to hydrogenation, or reaction with silver nitrate [449] or methanethiol [746]. Silver nitrate in anhydrous methanol reacts with cyclopropene rings in about 2 hours at 30 C to form predominantly methoxy ether but with some enonic derivatives, which appear as twin peaks (because of reaction on either side of the ring) on analysis by GC [99,241,281]. An application of this procedure to the analysis of kapok seed oil is illustrated in Figure 5.14. Alternatively, a brief reaction with hydrazine will selectively reduce the cyclopropene compounds to the more stable cyclopropanes by examination by GC before and after the reaction, the small amounts of natural cyclopropane components can also be identified [194]. 

Cyclopropane and Cyclopropene Adds.— The undesirable physiological properties of cyclopropene acids such as malvalic acid (20 R = H) and sterculic acid (21 R = H) and their presence, albeit in small amount, in cottonseed oil has aroused interest in these compounds. The content of cyclopropene acids is reduced by heating with palladium catalysts, though not with nickel or platinum catalysts. Palladium treatment cleaves the cyclopropene ring and produces methyl- and methylene-substituted acids. Hydrogenolysis of cyclopropane acids is reported to give methyl-substituted fatty acids which can be identified by their g.l.c. behaviour on capillary columns. The chemical reactions of cyclopropene acids (malvalic and sterculic) shown in Scheme 5 have been reported. ... 

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