Reaction, preferential bromine substituents

In the case of 2-alkyl derivatives, deprotonation occurs preferentially at the substituent a-position (91JOC3058), although in some cases this problem can be overcome by the use of consecutive bromination and halogen-metal exchange reactions, as was shown with 2-methyl-4-pheny-loxazole (Scheme 83)(680MS13). 

All these transformations are obtained in good yields and 5-bromo derivatives are often crystalline and easy to separate. By-products can be bromides or dibromides with halogen substituent at C-l and C-5, but the regioselectivity of photobromination at C-5 results from the easier formation of tertiary radicals. The a-bromination confirms the stereoselectivity of the substitution reaction with the preferential abstraction of axial H-5. 

The fused benzene ring in benzimidazole deactivates C-2 to electrophilic bromination by a factor of about 5000. However, most bromination occurs in the fused benzene ring with the preferential order in multiple brominations being 5 > 7 > 6, 4 > 2, an order which closely parallels that predicted by MO calculations. These reactions will be discussed in more detail as substituent reactions (Section 4.07.3.1). 

The exact nature of the reaction (oxidative vs. reductive) will depend on the redox properties of I ) and Q. The electron transfer process is a special case of exciplex formation favored in the strongly polar solvents, such as water. The involvement of an exciplex in a photochemical reaction is generally established by studying the effects of known exciplex quenchers such as amines on the exciplex fluorescence and the product formation. The heavy atom effect, due to the presence of substituents such as bromine or iodine intra- or intermolecularly, causes an exciplex to move to the triplet state preferentially, with a quenching of fluorescence. 

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