CYCLOPROPENE DIMERIZATION CATALYST

The choice of catalyst is important, for instance the use of tetrakis(triphenylphos-phane)palladium(O) complex results in the quantitative cyclotrimerization of 3,3-dimethylcy-elopropene.17 In similar fashion 3,3-dimethoxycyclopropene cyciodimerizes to 3 (R = OMe, 74%) using bis(dibenzylideneacetone)paUadium(0) [Pd(dba)2] complex.18 The trisubstituted cyclopropene 4 is transformed to the head-to-head dimer 5 in the presence of copper(I) iodide.19... 

Besides polymeric material, a mixture of three ene reaction products is obtained (18% combined yield) in the thermal reaction, which proceeds at 20°C within several minutes. Similar results are found when zeolites with a large pore diameter (5-13 A) are used as catalysts. After irreversible addition of the cyclopropene to the zeolite, only polymeric material is obtained. Zeolites with 3- or 4-A pores give rise to a product spectrum similar to that obtained with phosphane-free palladium(O) catalyst systems, i.e. 90-97% of the tricyclic dimers are formed from 1- or 3-methylcyclopropene at temperatures between — 35 and 35 C. The palladium(II) chloride catalyzed reaction almost quantitatively yields a 2 1 mixture of the isomeric 1,2- and l,4-dimethyl-evo-tricyclo[3.1.0.0 ]hexane (lA and IB) from 1-methylcyclopropene. The reaction is strongly exothermic and therefore must be carried out in dilute solutions with effective... 

The reaction of 3,3-disubstituted cyclopropenes with mono- and 1,2-disubstituted alkenes proceeds only with difficulty and leads to low yields of cyclopropanes. In the case of but-l-ene, an 8% yield, with hex-1-ene and hept-l-ene between 5 and 10% yield, and with cyclooctene about 10% of the cyclopropane product is formed. In these cases, the major product is the formal dimer of the intermediate ethenylcarbene complex, i.e. the corresponding (fj-hexatriene. When copper(I) chloride is used as catalyst rather than the copper halide/phosphane or phosphite system, about half the yield of the [2-f-1] cycloadduct is obtained along with an increased amount of the hexatriene. Mechanistically, these acyclic trienes could also be formed from an (alk-l-enyl)bicyclo[1.1.0]butane intermediate without any carbene being involved. Bicyclo[1.1.0]butanes are low yield (< 20%) byproducts of the thermal dimerization reaction of methyl 3,3-dimethylcyclopropenecarboxylate (1). On the other hand, bicyclo[l. 1. Ojbutanes, such as 3, are known to undergo isomerization to form 1,3-dienes. ... 

Unsubstituted cyclopropenes rapidly react with palladium(0) salts to give dimerization products. The palladacycles formed (Scheme 5-2) can be used as catalysts.f 1-Substituted cyclopropanes, in contrast, are stable under these conditions 1-Acetoxymethylcyclopropanes can serve as building blocks in allylic substitution... 

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