Cyclopropenes, oligomerization

Cyclopropenes are highly strained and thus very reactive molecules. Cyclopropene itself tends to polymerize even below 0 C and 1-methylcyclopropene dimerizes within minutes at room temperature via an ene reaction. In contrast, 3,3-dimethylcyclopropene does not undergo an ene reaction even at 100 C, but it readily oligomerizes under the influence of nickel(O) or cyclodimerizes to 3,3,6,6-tetramethyltricy-clo[3.1.0.0 ]hexane (80% yield) in the presence of palladium(O). 1,2-Diphenylcyclopropene is converted to 1,2,4,5-tetraphenylcyclohexa-1,4-diene in the presence of palladium(O) in high yield (equation 32). 4... 

Most of the systematic work in the field of catalytic di- and oligomerization of cyclopropenes has been performed using 3,3-dimethylcyclopropene (3), the most stable of all the... 

The non-hydrocarbon substrates most often employed in dimerization or oligomerization reactions are cyclopropen one acetals, due to their ease of preparation.54 3,3-Dimethoxycyclo-propene has been treated with various palladium(O) complexes. In the absence of phosphane, bis(dibenzylideneacetone)palladiumleads to cyclodimerization, with 3,3,6,6-tetramethoxy-cxo-tricyclo[3.1.0.02,4]hexane (30) being formed in 74% yield. No cyclotetramers, as in the reaction of 3,3-dialkyl- or 3,3-diarylcyclopropenes, are formed.55... 

The modest yields may be ascribed to a number of side reactions. Allylic halides are very prone to nucleophilic attack, which in this case will give rise to the formation of an allylic amine. One could also imagine a nucleophilic attack of the intermediary carbenoid on the allylic halide, or metallation of the cyclopropene by alkali amide and subsequent coupling of the cyclopropenylide with the allylic halide. The relatively high temperatures in the preparations of the cyclopropenes will certainly favour oligomerization, which may be a merely thermal one or an ionic process (similar to that observed by Dutch investigators [16] in the case of the parent hydrocarbon). Such reactions afford products which are considerably less volatile than the cyclopropenes and therefore will remain in the reaction mixture. One obvious subsequent reaction is the irreversible base-induced isomerization to methylenecyclopropane, which cannot be separated from the desired product [79] ... 

Structurally novel cyclopropane-fused cycloalkanes can be constructed by oligomerization of cyclopropenes. Cycloaddition across the double bond reduces ring strain in cyclopropenes enormously. When catalysed by transition metals, dimethyl-cyclopropene tri- or tetramerizes stereoselectively to novel polycycloalkanes (equation... 

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