Mono- vs. dialkylation of carbanions. Effects of absolute and relative acidity of the conjugate carbon acids in selectivity control
作者:Luděk Ridvan、Jiří Závada
DOI:10.1016/s0040-4020(97)00989-7
日期:1997.10
in the reaction of the dibromide 1 with carbanions 2a – 2g covering a range greater than 15 pK units in DMSO. It was found that the bis(monoalkylated) product 3 arises exclusively or predominantly from the carbanions 2d – 2g derived from the less acidic carbon acids 7d – 7g whereas the cyclic product of dialkylation 4 prevails in the reaction of the carbanions 2a – 2c derived from the more acidic carbon
A mechanistic study on the alkylation of pentane-2,4-dione through its Co(II) complex in concentrated chloroform solutions indicates the operation of a “non carbon radical” chain mechanism based in the ability of cobalt to undergo redox processes between the Co(III) and Co(I) states, including oxidative addition and reductive elimination steps. This process is initiated by inner sphere electron transfer
α-(1-adamantyl)-β-diketones are now accessible by the alkylation of Co(II) complexes of β-diketones with 1-bromoadamantane. The intermediacy of adamantyl cation is proposed. Co(II), Co(III) and Zn(II) complexes of pentane-2,4-dione react with alkyl halides precursors of stabilized carbeniumions, to give 3-alkylpentane-2,4-diones.
oxidative coupling protocol. Highly selective formation of mono-enol silyl ethers of 3,3-disubstituted acetylacetones was achieved using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base. Subsequent silver(i) oxide mediated coupling reactions provided tetraketones. Unique substituent dependence was found for the terminal-selective silylation of tetraketones. Finally, octaketones (tetramers of acetylacetone