Cyclopropenecarboxylic acids

Semiempirical calculations on the Favorskii rearrangement of a-chlorocyclobutan-one to cyclopropenecarboxylic acid suggest that it proceeds via a stepwise semibenzilic acid pathway, both in solution and in vacuo, rather than by a cyclopropanone... 

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

The hydrogenation of cyclopropenecarboxylic acids leads to good yields of the saturated acid. Two elegant routes to cis-chrysanthemic acids have been reported based on this process. In the first, the ester (249) is reduced by either diimide or nickel boride and hydrogen 49) ... 

Tris(trimethylsilyl)cyclopropenylium hexachloroantimonate (13) was obtained by decarbonyla-tion of the corresponding cyclopropenecarboxylic acid chloride 12. 

A mixture of benzonitrile and 1.5 eqs. Mn02-on-silica gel in cyclohexane stirred gently at reflux for 3 h benzamide. Y 100%. Reaction is considerably faster than with other forms of active Mn02 (Synth. Meth. 21, 166), and no more than 1.5 molar eqs. of oxidant are required. The method may also be used for preparing a,P-ethylenecar-boxylic and cyclopropenecarboxylic acid amides, but phenylacetamide was partially oxidised. F.e.s. K.-T. Liu et al., Synthesis 1988, 715-7. 

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