Fragmentation of Cyclic Structures

Cycloreversion Reactions A cycloreversion reaction is the reverse of a cycloaddition reaction and leads to the formation of the starting reactants through the cleavage of two bonds in the ring [18], A typical example is the formation of C2H4+ and neutral C2H4 from the cyclobutane radical cation. As shown in reaction (6.37), this reaction proceeds through the intermediacy of a distonic ion. The radical cations of a variety of other four-membered cyclic compounds, such as cyclobutanones (3), diketene (4), oxetane (5), cyclobutylamine (6), and thiocyclobutane (7), are known to participate in cycloreversion reactions [27]. 

This process is quite useM in the structure determination of various natural products, such as cyclic isoprenoids. 

Fragmentation of Cycloalkanes Similar to cyclobutane, higher-member cycloalkanes also exhibit the loss of an ethene molecule. As shown in reaction (6.40) for cyclohexane, this reaction requires cleavage of two bonds in the ring. Similarly, the origin of a series of ions at even-numbered m/z values in the spectrum of cyclodecane can be explained by the consecutive losses of alkene molecules from the initially formed distonic ion. 

The H-transfer to the radical site in the distonic ion, preferably via a six-membered transition state, followed by the loss of alkyl radicals and eventual formation of an allyl cation, also competes with the ethene loss [reaction (6.40)]. In alkyl-substituted cycloalkanes, loss of the alkyl side chain overwhelms other fragmentation processes due to branching at that site. For example, m/z 83 is the base peak in the spectra of methyl- and n-bntylcylohexanes. 

Fragmentation of Heteroatom-Containing Cyclic Ions The possible fragmentations of heteroatom-containing cyclic ions are shown in the following reaction which shows that the charged fragments are formed via three different reaction pathways  

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