Sites of successive bromination of the ambident nucleophiles 2-methoxy-, 2,4-dimethoxy- and 2,4,6-trimethoxy-benzylidenefluorenes and 2,2-dimesityl-1-tert-butylethenol

Abstract

The reaction of the 2-methoxy-2a, 2, 4-dimethoxy-2c and 2, 4, 6-trimethoxy-benzylidenefluorene 2d with bromine in AcOH has been investigated. With compound 2a the initial product is the α, 9′-dibromide 3a which, with excess of bromine, gives the α, 5, 9′-tribromo derivative 3c. In NaOAc–AcOH compound 3a gives both debromination to 2a and dehydrobromination to 1a and 3b gives the α, 5-dibromo derivative 5a. Compounds 2c and 2d give successively the 5-bromo- and 3-bromo-4b and 4c and the α, 5-dibromo- and α, 3-dibromo-5b and 5c derivatives and further bromination of compound 2d gives, first, the α, 3, 5-tribromo-6 and then the α, 3, 5, 2′, 7′-pentabromo-7 derivatives. Higher sensitivity to substituents of the α-aryl ring compared with the double bond towards electrophilic bromine and accompanying increased steric shielding of the double bond account for this order. Bromination of 2, 2dimesityl-1 -tert-butylethenol 11 gives mainly ring-brominated 2, 2-dimesityl-1 -tert-butylethanones 12–14 and several other derivatives.