Intramolecular substitution reaction of geminal dibromo alkenes proceeds to afford indenes, dihydronaphthalenes, dihydrofurans, and dihydropyran via in situ generated lithium alkylidene carbenoids, which have a carbon or oxygen nucleophilic moiety. This reaction provides a regioselective method for the preparation of polysubstituted indenes.
When 4,4-dibromo-3-alkenols are treated with butyllithium, an intramolecular substitution reaction with alkoxide moiety occurs at the lithium alkylidene carbenoid center to give dihydrofurans. The reaction mechanism of this intramolecular substitution reaction is studied by B3LYP density functional calculations with the 6-31+G(d) basis set, and the substitution is found to proceed in a concerted manner. This substitution reaction is applied to the regioselective preparation of indene derivatives. That is, treatment of 3-(o-bromophenyl)-1,1-dibromopropene derivatives 23 with butyllithium results in the formation of intramolecular substitution intermediates, 3-indenyllithiums D, which are trapped with electrophiles to afford substituted indenes regioselectively.
Intramolecular Substitution Reaction of Lithium Alkylidene Carbenoids. Regioselective Synthesis of Indenes. -Treatment of various gem-dibromoalkenes with BuLi gives 5-and 6-membered rings (dihydrofurans, indenes, dihydronaphthalenes) by the title reaction. Experimental and theoretical studies reveal that this reaction proceeds in a concerted manner. The intermediately formed indenyl-and naphthyllithiums can be trapped with electrophiles to afford the corresponding substituted derivatives regioselectively. -(YANAGISAWA, H.; MIURA, K.; KITAMURA, M.; NARASAKA*, K.; ANDO, K.; Bull. Chem.
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