Publication Date:
2016-06-11
Description:
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 stereoselective 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. Beilstein J. Org. Chem. 2016, 12, 1178–1184. doi:10.3762/bjoc.12.113
Keywords:
base-free dehydrobrominationcis/trans stereochemistryfive-membered ring conformationindenesNMR couplings
Electronic ISSN:
1860-5397
Topics:
Chemistry and Pharmacology
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