CO and C2H2 Loss from Quinones

Quinones [124] and aromatic ketones such as flavones, [116] fluorenone, anthra-quinone, and similar compounds [121] dissociate by competing and consecutive losses of CO and C2H2. Multiple CO losses may also occur subsequent to the RDA reaction of flavones. [116,125] As these molecules all have large 71-electron 

Example 1,4-Benzoquinone represents the perfect prototype of this fragmentation pattern. The subsequent eliminations of intact molecules causes a series comprising of odd-electron fragment ions only (Fig. 6.35). 

The molecular ions of arylmethylethers preferably dissociate by loss of a formaldehyde molecule or by loss of the alkyl group [88,113], thereby yielding an even-electron phenolic ion, CgHsO , m/z 93, that readily expels CO (Fig. 6.34) [114,115]  

Astonishingly, the study of the mechanism of formaldehyde loss from anisole revealed two different pathways for this process, one involving a four- and one a five-membered cyclic transition state (Fig. 6.35) [116]. The four-membered transition state conserves aromaticity in the ionic product, which therefore has the lower heat of formation. Prompted by the observation of a composite metastable peak, this rather unusual behavior could be uncovered by deconvolution of two different values of kinetic energy release with the help of metastable peak shape analysis (Chap. 2.8). 

In case of longer-chain alkyl substituents, alkene loss can compete with the McLafferty rearrangement as is observed from suitably substituted phenylalkanes (Chap. 6.7.3), e.g., ethene loss from phenetole and its derivatives (Fig. 6.36) [113]. 

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