Radical site initiation

Note The term a-cleavage for this widespread radical-site initiated process with charge retention can be misleading, because the bond cleaved is not directly attached to the radical site, but to the next neighboring atom. 

The alpha bond to the radical cation site can be broken by a radical-site-initiated reaction, that is by a transfer of the unpaired electron to form a new bond to an adjacent atom (a atom) with concomitant cleavage of another bond of this atom. The new bond compensates energetically for the cleaved bond. In this case we speak about a radical-site-initiated a fragmentation ... 

Other examples are shown in Figure 7.18. f-Butylamineand n-butylamine yield only one intense fragment, the ion corresponding to the radical-site-initiated fragmentation. These... 

DBT, 2-DBT (2-methyldibenzo[3]thiophene), and 4,6-DBT (4,6-dimethyldibenzo[3]thiophene) in ESI tandem mass spectra have a a consistent 32 Da neutral loss, which is believed to be sulfur. Based on the tandem mass spectrum of the PASH compounds, the mechanism of fragmentation is considered to be a charge site-initiated reaction followed by a radical site-initiated fragmentation. Taking the example of DBT, Scheme 4 shows a possible mechanism. 

The presence of heteroatoms with non-bonding n electrons favors the localization of charges. When the radical and/or the charge are localized, this influences the fragmentation and the reaction mechanisms can be classified as sigma electron ionizations, localized charge initiations, radical site initiations, and rearrangements. 

For the radical site initiation (a cleavage), the tendency for electron pairing drives the formation of a new bond to an adjacent atom to the radical site and the cleavage of a bond to an a atom. 

Aliphatic Ethers The abundance of the molecular ion from aliphatic ethers is usually low but higher than that of the corresponding alcohols. These compounds exhibit radical site- and charge site-initiated fragmentation reactions. Radical site-initiated reactions prodnce resonance-stabilized oxyninm ions (e.g., of m/z 45,59,73,. ..). As exemplified in the reaction... 

As with aromatic ethers, the radical site-initiated reaction in phenyl alkyl ketones cleaves the bond P to the ring to form a structurally diagnostic ion of m/z 105 (CgH5CO+). Further loss of CO from this ion prodnces the phenyl cation of m/z 77 (see Figure 1.3). Similar to aliphatic ketones, the aromatic ketones also undergo McLafferty rearrangement when the alkyl chain is >Cs. 

Oxygen is intermediate in its ability to influence either aliphatic ether attraction of an electron pair initiated by the localized positive charge on the oxygen can form the alkyl ion and the alkoxyl radical in this case the charge site is moved (Equation 4.22 see Figure 3.9). Note that the bond cleaved (R —CH2—OR) is not that cleaved by the radical-site initiation... 

Radical-site initiation (one bond cleaved without rearrangement)... 

Inasmuch as this chapter covers the cyclization of hydroxy-biradicals, it is important to remember that excited-state hydrogen abstraction forms a biradical with its two radical sites initially very close to each other. Unless some of the connecting bonds rotate very rapidly, disproportionation back to ketone would be aU that happens. One would think that rather simple conformational changes are all that is required for cyclization to take place. In order to predict cyclization quantum efficiency, the challenge is to understand the kinetics of those bond rotations as well as other conformational changes that lead to competing disproportionations and cleavage (only 1,4-biradicals). 

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