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Stepwise application of the Pd-catalyzed SN2′ reaction and the desilylative SE2′ reaction to the ambivalent 2-bromo-1-silyl-1,3-dienes provides a novel route to the highly enantioselective construction of tertiary and quaternary propargylic stereogenic centers via axially chiral allenylsilanes. [ABSTRACT FROM AUTHOR]

Treatment of acetal-tethered (allenylmethyl)silanes, which were obtained from the corresponding 3-bromo-5-silyl-1,3-pentadienes by a Pd-catalyzed reaction with an acetal-tethered malonate, with TiCl4gave not only vinylcyclohexene derivatives via a standard SE2′ pathway but also unusual allenylcyclopentane species via cyclization at the δ-position. Deuterium-labeling experiments revealed participation of a 1,2-hydride shift in a carbocation intermediate for the formation of the latter products. [ABSTRACT FROM AUTHOR]

Palladium-catalyzed reactions of various 2-bromo-3-exo-methylenecycloalkenes with a stabilized nucleophile were examined. When the carbocycles were nine-membered or larger, the corresponding endocyclic allenes were isolated in excellent yields. In a reaction of the eight-membered cyclic substrate, initial formation of a cycloocta-1,2-diene derivative was detected; however, it dimerized slowly. The seven-membered carbocycle was inert to the reaction. Using a chiral Pd-catalyst, an axially chiral endocyclic allene was obtained in 65% ee. The cyclic allenes were applied to [2 + 2]cycloaddition with ketenes, and the stereoselectivity was studied. [ABSTRACT FROM AUTHOR]

The cationic exo-isopropylidene--allylpalladium complex Pd(3-CH2CMeCCMe2)(dppb)BArF4(1; ArFC6H3-3,5-(CF3)2), was prepared, and its solid-state structure was determined by X-ray crystallography. The CMe2plane of the exo-isopropylidene moiety and the 3-CH2CMeC plane are not coplanar, with a dihedral angle of 66.06°. A reaction between 1and a stabilized nucleophile, NaCMe(CO2Me)2, took place at the terminal methylene carbon of 3-CH2CMeC to give the corresponding allene exclusively. On the other hand, a reaction of 1with a Grignard reagent proceeded via an initial nucleophilic attack at the Pd center to give a conjugated diene. Complex 1showed good catalytic activity in the reactions between 2-bromo-1,3-dienes and stabilized nucleophiles, and a variety of allenes were obtained in good yields. The dynamic process of forming the alkylidene--allylpalladium complexes from 2-bromo-1,3-dienes and Pd(0) was also examined. [ABSTRACT FROM AUTHOR]