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Knorr, Rudolf; Stephenson, David S.; Lattke, Ernst; Böhrer, Petra; Ruhdorfer, Jakob (2016): Unusual traits of cis and trans-2,3-dibromo-1,1-dimethylindane on the way from 1,1-dimethylindene to 2-bromo-, 3-bromo-, and 2,3-dibromo-1,1-dimethylindene. In: Beilstein Journal of Organic Chemistry, Vol. 12
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Abstract

Do not rely on the widely accepted rule that vicinal, sp(3)-positioned protons in cyclopentene moieties should always have more positive (3)J NMR coupling constants for the cis than for the trans arrangement: Unrecognized exceptions might misguide one to wrong stereochemical assignments and thence to erroneous mechanistic conclusions. We show here that two structurally innocent-looking 2,3-dibromo-1,1-dimethylindanes violate the rule by means of their values of (3)J(cis) = 6.1 Hz and (3)J(trans) = 8.4 Hz. The stereo-selective formation of the trans diastereomer from 1,1-dimethylindene was improved with the tribromide anion (Br-3(-)) as the brominating agent in place of elemental bromine;the ensuing, regiospecific HBr elimination afforded 3-bromo-1,1-dimethylindene. The addition of elemental bromine to the latter compound, followed by thermal HBr elimination, furnished 2,3-dibromo-1,1-dimethylindene, whose Br/Li interchange reaction, precipitation, and subsequent protolysis yielded only 2-bromo-1,1-dimethylindene.