Electrophilic Cleavage of Cyclopropanes

4 Reactions of Three- and Four-membered Carbocyclic Compounds With ElectrophUes.—Electrophilic Cleavage of Cyclopropanes Hydrogen chloride in dichloromethane at — 70 to — 80 °C preferentially attacks the cyclopropane of (209) rather than the double bond. With stronger acids such as HSO3F at - 80 °C, however, initial protonation appears to be at the double bond, and hydrogen chloride at — 60 °C also gives some products by 

Silvestri, S. Gambino, G. Filardo, M. Guainazzi, and R. Ercoli, Gazzetta, 1972, 102, 818. F. Toda and T. Kataoka, Chem. Letters, 1972, 173. 

Electrophilic addition of bromine at — 60°C to cis- and trans-1,2-dimethyl-cyclopropanes has been shown to open the C-1—C-3 bond preferentially in a non-stereospecific manner. This rules out predominant reaction through a 1,3-bromonium ion. The principal difference between the cis- and transisomers is that the former gives 33 % of a 1,2-dibromide by 1,2-hydride shift within an intermediate carbonium ion. It has been reported that oxymercura-tion of optically active trans-1,2-dimethylcyclopropane occurs with at least 85 % inversion of configuration at the site of nucleophilic attack in a reaction which also cleaves the less substituted cyclopropane bond. Under the influ- 

Tanaka, T. Kategiri, K. Takabe, and T. Sakamoto, Bull. Chem. Soc. Japan, 1972, 45, 3147. 

As the ring which is fused to the cyclopropane becomes larger, the proportion of external bond fission increases, and this is the exclusive reaction in acid-catalysed solvolyses of bicyclo[6,l,0]nonanes. The effects of the oxygens in the formolyses of oxabicyclic compounds (217)—(219) have been investigated. From (217) there is preferential C—O cleavage and formation of 

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Mechanistic aspects of the electrophilic cleavage of cyclopropanes by metal salts have received considerable attention. Studies of the stereochemistry of reactions of cyclopropanes with mercuric trifluoroacetate lead to the conclusions that the electrophilic mercuric ion attacks the least strained ring bond (for equally substituted bonds, attack is preferred at a cis-substituted bond over a trans-substituted one). Ring-opening occurs in the direction of the more stable carbonium ion, which can lead... 

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