An Indenone Synthesis Involving a New Aminotransfer Reaction and Its Application to Dibenzopentalene Synthesis

Abstract o ‐(Phenylethynyl)phenyllithiums, generated by treatment of o ‐(phenylethynyl)bromobenzenes with t BuLi, were quenched with N , N ‐dimethylcarboamides to afford various 2‐phenylindenone derivatives in moderate to good yields upon acidic workup. The choice of lithiation reagents was critical. When n ‐butyllithium was used the yields were reduced. The N ‐alkyl groups in the carboamides were also important: quenching with N , N ‐diethylbenzamides also afforded 2,3‐diphenylindenones as the sole isolated products, but quenching with N ‐benzoylpiperidine led to 2‐benzoyltolane in good yield. Control experiments revealed that the reactions proceed through a multi‐step reaction sequence involving a novel intramolecular transamination to the α‐acetylenic carbon of the alkynyl group. The method was applicable to the synthesis of dibenzopentalene derivatives as well as various indenone derivatives. A mechanism for the reaction is discussed.