Ring opening cyclopropanes, bromine

The mechanism for bromination of semibullvalene has been proposed, based on quantum chemical calculations. The reaction pathway involves concerted bromine addition and cyclopropane ring opening to form an allylic cation, without the intermediacy of a bromonium or a cyclopropylcarbinyl cation.18... 

Analogous ionic ring-openings have been described for polycyclic gem-dibromo-cyclopropanes. For example, when solvolyzed in 50% aqueous acetone in the presence of triethylamine, the already mentioned dibromocarbene adduct of benzvalene loses its endo-bromine substituent and opens in a disrotatory fashion to a tricycloheptenyl cation which may be intercepted by water to provide the bromoalcohol shown [175]. 

Reaction of cyclopropane with bromine in the presence of a Lewis acid as catalyst (FeBrs, AICI3, AlBra) gives a mixture of ring-opened dibromides (1,1 1,2 1,3) (Eq. (35)) which are best accounted for by postulating the intermediacy of protonated cyclopropanes >, although... 

The yield of BrCH2CH2CH Br found experimentally in bromine-scavenged cyclopropane is less than 1 % of the CHf Br yield. Thus ring-opening has some influence on this system but not to any significant extent. 

The electrophilic bromine cation can also be generated from A -bromosuccinimide (NBS). When 2-methylene-6,6-dimethylbicyclo[3.1.0]hexane with an ester function at the bridgehead position was treated with NBS in diethyl ether the electrophilic attack took place at the terminal position of the C—C double bond and the cyclopropane ring was opened to give a cyclopentene derivative. Due to the lack of a reactive nucleophile, a proton was eliminated. The reduction of the ester function did not change the course of this reaction. With tert-hvAy hypochlorite, the corresponding chloro product was obtained. ... 

Deuterated cyclopropane has been brominated with bromine in order to determine the mechanism of electrophilic halogenation. The observed stereochemistry indicates initial edge halogenation with retention to form a four-membered bromonium ion, followed by nucleophilic ring opening with inversion194 l95. 

Whereas photosensitized bromination of cyclopropane affords 1,3-dibromo-propane, bromination with NBS gives cyclopropyl bromide. Thus, whereas Br reacts exclusively by attack on carbon to give ring-opening, the succinimidyl radical abstracts hydrogen to give a cycloprop radical, which rapidly combines with a bromine atom. ... 

Cheap and readily available cyclopentadecanone is alpha-brominated to (30) and ketalised with (29), and the resulting a-bromoketal (31) subjected to base induced elimination of HBr, giving (32). The key asymmetric step is now at hand. The Simmons-Smith cyclopropanation proceeds with very high facial selectivity, giving almost entirely the diastereomer of (33) shown. The result is rationalised on the basis of zinc chelation by the oxygens of the ketal. The synthesis is completed by acid hydrolysis of the ketal (the chiral diol may be recovered for reuse), reductive opening of the cyclopropane ring under Birch conditions, and re-oxidation of the alcohol back to the ketone. 

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