Electrophiles cyclopropane/cyclobutane reactions

The calculated energies again confirm that cyclopropane is much more easily attacked by electrophiles than is cyclobutane, and this accounts for the common observation that cyclobutanes are much less reactive toward electrophiles than are cyclopropanes, despite the similar strain energy relief for these compounds.55 The reactions of cyclopropane with other electrophiles, such as mercuric ion,65 and metal radical cations,67 have also been studied. 

Many of the unique properties of cyclopropanes, and to a lesser extent, cyclobutanes, are derived from the formation of bent bonds. They may act in a fashion similar to 7r-bonds in interacting with electron-deficient centers, and are more easily cleaved thermally via electrophilic attack than are ordinary C-C bonds. The strain energy associated with bond angle deformation is also an important quantity, especially when considering thermal reactions. 

In marked contrast to those of cyclopropanes, the addition reactions of cyclobutanes usually require drastic conditions and activating substituents. Notwithstanding these shortcomings, the cleavage of cyclobutanes by addition reactions has been widely employed in the synthesis of organic molecules of moderate complexity. In this section, electrophilic addition, as well as nucleophilic addition, of cyclobutanc derivatives are reviewed with the aim of demonstrating their synthetic profile. 

Similarly, cyclopropane reacts readily with electrophiles such as bromine, whereas cyclobutane and the larger carbocycles are inert. This carries over to the corresponding bicyclic compounds. Bicyclobutane is extraordinarily reactive towards electrophiles, bicyclo[2.1.0]pentane has high reactivity, but bicyclo[2.2.0]hexane is almost inert". This does not correspond to differences in driving force for the reactions, in fact a reverse correlation is found ... 

The relation between strain and reactivity may be illustrated with electrophilic reactions of cyclopropane and cyclobutane (218). The decrease in strain is essentially equal in both compounds, but cyclopropane is highly reactive, whereas cyclobutane is essentially inert. Acid-catalyzed ring opening, for example, is controlled by the basicity of the two cycloalkanes. The latter are only slightly deformed by protonation and little strain is therefore released. 

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