Charge-induced cleavage

The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

In these compounds, the intermediate arylmethyl radical is long-lived. However, further excitation of the radicals in benzene results in production of the bromine atom-benzene charge transfer complex ( max 550 nm), an indication of photo-induced cleavage of the remaining C-Br bond. This is further supported by observation of the corresponding vinyl products. 

The bond adjacent to a heteroatom can be broken by a charge-site-initiated reaction, that is by attraction of an electron pair from this bond, and we talk about an induced cleavage (i) ... 

Electron impact induced cleavages of heteroatom-carbon bonds with elimination of radicals and charge localization on the heteroatom function require considerable energy and therefore provide intense fragment ions only in exceptional cases44,45. At suitably substituted 5-membered heterocycles, for instance 4346 9 one al-... 

FIGURE 9.38 The O cleavage followed by charge-induced elimination of pentene. 

FIGURE 9.39 Charge-induced heterolytic cleavage of a o bond and consecutive elimination of carbon monoxide. 

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