Chiral Propargylic Cations as Intermediates in S<sub>N</sub>1-Type Reactions: Substitution Pattern, Nuclear Magnetic Resonance Studies, and Origin of the Diastereoselectivity
作者:Dominik Nitsch、Stefan M. Huber、Alexander Pöthig、Arjun Narayanan、George A. Olah、G. K. Surya Prakash、Thorsten Bach
DOI:10.1021/ja411772n
日期:2014.2.19
substituents at the cationic center, with the nucleophile approaching the electrophilic center opposite to R(2). Transition states calculated for the reaction of allyltrimethylsilane with two representative cations support this hypothesis. Tertiary propargylic cations with a stereogenic center (-C*HXR(2)) in the α position were generated by ionization of the respective alcohol precursors with FSO3H in
Cooperative organocatalysis of Mukaiyama-type aldol reactions by thioureas and nitro compounds
作者:Konstantin V. Bukhryakov、Victor G. Desyatkin、Valentin O. Rodionov
DOI:10.1039/c6cc01984f
日期:——
A unique organocatalytic system for Mukaiyama-type aldol reactions based on the cooperative action of nitrocompounds and thioureas has been identified. This system is compatible with a wide range of...
Mukaiyama–Michael addition of a ketenesilylacetal on a cyclic α,β-unsaturated ketone, followed by the addition of a glyoxylic, aromatic or heteroaromatic imine. According to the nature of the silyl group the adducts resulting from this tandem process are isolated as ketones or as enoxysilanes. The presence of a coordinating group on the imine increases the rate of the reaction.
Kinetics of the reactions of nine symmetrically substituted 1,3-diarylallyl cations with different nucleophiles were studied photometrically in dichloromethane, acetonitrile, and DMSO solutions. The second-order rateconstants k2 were found to follow the correlation log k2 = sN(N + E). The electrophilicity parameters E of the title cations were derived, using the known values of sN and N of the nucleophilic
rate-determining C−C bond formation. The nucleophilic reactivities span over a range of 108 from the vinylethers 1a,x as the least reactivecompounds (comparable to allylsilanes) to the highly nucleophilic silyl ketene acetal 1u (comparable to enamines). Linear free enthalpy relationships are used to compare the reactivities of these compounds with those of other aliphatic and aromatic π-nucleophiles.