Gas-phase reactions of 2-benzyl- and 2-benzoyl-phenoxyl radicals, and of 2-phenoxybenzyl radicals: examples of new hydrogen-transfer processes
Generation of the 2-benzylphenoxyl radical 23 or the 2-phenoxybenzyl radical 24 by flash vacuum pyrolysis of the ethers 8 or 9, or the oxalate 19, respectively, leads to fluoren-1-ol 22 together with 2-benzylphenol 7 and a low yield of xanthene 21. Pyrolysis of the para-substituted derivatives 11 and 20 gives an analogous distribution of products, including two isomeric methylxanthenes 28 and 29 formed via the spirodienyl 27. The reactions of the corresponding 2-benzoylphenoxyl radicals give information on the mechanisms of these processes. Thus the formation of the fluorenones 37 and 43 provides evidence for the hydrogen-abstraction mechanism (Scheme 4) of fluorene formation. Secondly, a detailed study of the ratios of xanthones 41 and 42 under a variety of pyrolysis conditions suggests that such 6-membered-ring products are formed by sigmatropic shifts in the spirodienyl, rather than direct cyclisation of the phenoxyl or benzoyl radicals.