Novel Reactivity of Main Group Alkynylmetals and Its Application to Organic Synthesis

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1-Alkynyl organometallics of main group metals have been frequently used as efficient carbon nucleophiles in organic syntheses. They react with a variety of electrophilic reagents generally at the carbon α to the metal atom. In order to develop a convergent method for the construction of complex carbon frameworks, a hitherto unexplored nucleophilic reactivity of alkynylmetals at the β position is investigated. The study reveals that various alkynylmetals react at the β position especially in intramolecular reactions. Alkynylzincates bearing a remote leaving group undergo smooth exo-cyclization at the β position with simultaneous 1, 2-migration of a ligand on the zinc atom to give 1-(cycloalkylidene) alkylzinc reagents, which can be used in subsequent reactions with electrophiles. Alkynylmetals of lithium, sodium, and potassium also undergo exo-cyclization to give cycloalkylidene carbene. A method for the synthesis of polycyclic compounds is developed utilizing the exo-cyclization of alkynylmetals and intramolecular C-H insertion of the resulting carbenes.