RESUMO
A new synthetic concept was suggested in the chemistry of substituted methylidenemalonates that enables their utilization as 1,2-zwitterionic synthons. This strategy is to generate liquid ionic Ga complexes from methylidenemalonates and GaHal3 with a strict 3/4 composition and then use them in further synthesis. A number of complexes with different metal halides have been synthesized and studied in detail. The unique properties of gallium among all metals have been demonstrated and explained. On the basis of the discovered new class of gallium complexes of methylidenemalonates, a number of novel reactions with acetylenes have been elaborated, which are unknown in the conventional chemistry of methylidenemalonates. The main demonstrated process is a three-component addition to a triple bond involving halide anions, leading to the formation of polyfunctional vinyl halides with high E-selectivity. The mechanism has been studied experimentally in fine detail. Application of specially optimized 71Ga NMR spectroscopy makes it possible to take an in-depth look into the gallium chemistry in a new light. In particular, the key participation of GaHal4- anions in the occurring transformations has been established.
RESUMO
A new process of "inverted" formal [3 + 2]-cycloaddition of donor-acceptor cyclopropanes (DACs) to allylic systems to give polyfunctionalized cyclopentanes has been developed. Unlike the classical version of formal [3 + 2]-cycloaddition, a DAC acts in this process as a two-carbon synthon (1,2-zwitterionic reactivity type), while an alkene acts as a three-carbon synthon. Tetramethylethylene, allylbenzenes, homoallylbenzene, terminal and internal aliphatic alkenes, and methylenecyclobutane were successfully used as alkenes. Furthermore, in order to suppress annulation at the aromatic ring in 2-arylcyclopropane-1,1-dicarboxylates and enhance the selectivity of the reaction with the allylic system of alkenes, we suggested a new approach for managing the reactivity of DACs involving substitution at both ortho positions of the aromatic ring. Multinuclear NMR spectroscopy was used to study the structure of the 1,2-zwitterionic gallium complexes generated and their properties and to examine the mechanisms of the occurring reactions.
RESUMO
A new cascade process for reactions of donor-acceptor cyclopropanes (DACs) with spiro[cyclopropanepyrazolines] in the presence of EtAlCl2 or Ga halides is reported. The action of a Lewis acid results in DAC activation and addition of the carbocationic intermediate to the azocyclopropane system of the pyrazoline with opening of the second three-membered ring and addition of a halide anion from the Lewis acid. A specific feature of this process is that one activated cyclopropane ring activates another one, and depending on the component ratio, the process can involve two DAC molecules and one pyrazoline molecule or one DAC molecule and two pyrazoline molecules. The process is tolerant to various functional groups and occurs with a wide range of substrates to give polyfunctionalized structures based on a 2-pyrazoline moiety.
RESUMO
A new process for (4 + 2)-annulation of donor-acceptor cyclopropanes (DACs) with unsaturated compounds in the presence of anhydrous GaCl3 has been developed. In this process, DACs act as sources of formal 1,2- and 1,4-dipoles to give polysubstituted tetralins in high yields and with high regio- and diastereoselectivity. Alkenes with both aryl and alkyl substituents at the double bond undergo this reaction equally readily. A most likely mechanism of the reaction has been proposed. It involves preliminary generation of a key 1,2-dipolar gallium complex and its subsequent participation in annulation with an alkene.
