Benzoquinoline basicity

Scheme 52 explains the [(Cp )Rh(MeCN)3]2+-assisted regioselective hydrogenation of pyridines, benzoquinolines, acridines as well as indoles and benzothiophene.258 The relative hydrogenation rates were attributed to both electronic and steric effects, the rate generally decreasing with increasing basicity and steric hindrance at the nitrogen atom. 

Prior work has been concentrated mostly on the isolation of distinct NSO compound classes from soluble organic matter. Often, selective extraction methods are applied as described for the isolation of, for example, phenols, [39,40] fatty acids [41] and basic nitrogen compounds such benzoquinolines... 

The o values of rings annelated to pyridine in systems such as quinoline, isoquinoline, benzoquinolines, acridine, and phenanthridine and determined by the basicity of the ring nitrogen atom are predicted186 and found to be small.33,34 This is an important point stressed and elucidated previously,181 and demonstrating the validity of the basic assumptions of the Hammett equation. 

Nitrogen species are also divided through acidic extraction into basic species (such as aniline, quinoline, benzoquinoline, and their derivatives) and nonbasic species (such as indole, carbazole, and their derivatives) (see Table 3). 

Ring-opened benzoquinoline isomers 1011 and 1012 (Fig. 13.25) were also observed to form under basic or neutral conditions, and somewhat more slowly under acidic conditions. It is likely that these result from the inherent instability of the isoxazolidinone core (by either E2 elimination or a hydrolysis/elimination sequence). 

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