Protonated cyclopropane Chemistry

Protonated cyclopropane has been reported in the gas phase" "" to be ca 8 kcal mol" in energy above the isopropyl cation. The bent bonds of the cyclopropane ring are susceptible to electrophilic attack leading to the expectation that cyclopropane will be more basic than saturated alkanes and that protonation will occur on the C—C bond, i.e. the edge-protonated isomer will have the lowest energy. There is, however, considerable evidence from solution chemistry that corner-protonated cyclopropanes exist as intermediates in 1,2-alkyl shifts in carbocations. There have been several reviews of protonated cyclopropanes " and, in the current work, only the very recent theoretical work will be reviewed. 

Similar studies on the gas phase chemistry of cyclopropyl thioethers were reported by Chizhov et The conclusions drawn in the investigation were, however, hampered by the fact that protonation seems to occur competitively at both the cyclopropane ring, the sulphur atom and the aryl ring. 

Bromine addition to cyclopropanes is slow in the absence of light unless the ring is heavily substituted . In the presence of light, reaction is rapid since halogen radical attack on substituted cyclopropanes is fast even at — 78°C. In the presence of light, HBr is produced and cyclopropane reacts more competitively with proton acids than with bromine. Addition reactions to cyclopropane are generally slow in the absence of acid catalysts and therefore the conjugate acid is probably involved in reaction. The more facile reaction of cyclopropane with HBr compared with bromine is in contrast with alkene chemistry where bromine addition is the more rapid ... 

As has been discussed briefly in Section III above, the ring strain of the cyclopropenes is accommodated, in part, by somewhat unusual bonding to the substituents. Structural and spectroscopic studies suggest that the vinylic C-H bonds have ca. 44% s character and that this is approximately constant in bonding to other substituents. Consequently aspects of the chemistry of the cyclopropenes are expected to reflect this with enhanced acidity of the vinylic proton. Cleavage of an exocyclic bond at C(3) will result in the cyclopropenyl cation, radical or anion with consequent interaction with the n bond and perturbation of the energy of the system. By comparison addition to the n-system to yield a cyclopropane relieves ring strain by ca. llOkJmol" ... 

The nitrosation reactions and the chemistry of diazoalkancs are also used in synthesizing just certain specific types of compounds, like cyclopropanes. The mechanisms here are more involved, however, and an understanding of their steps at this point is helpful because similar types of chemistry will be seen again later, in Chapter 22. As you plow through this, try to focus on the rclartonship of each mechanistic step to processes you ve seen before. Almost all of this chemistry is based on relatively fundamental sorts of events, like protonation and deprotonation. combined with elimination or addition... 

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