129215-72-1Relevant articles and documents
Chemistry of oxaziridines. 14. Asymmetric oxidation of ketone enolates using enantiomerically pure (camphorylsulfonyl)oxaziridine
Davis, Franklin A.,Sheppard, Aurelia C.,Chen, Bang-Chi,Haque, M. Serajul
, p. 6679 - 6690 (1990)
The reagent-controlled asymmetric oxidation of tri- and tetrasubstituted ketone enolate anions 4 and 8 by enantiomerically pure (camphorylsulfonyl)oxaziridine 2 has been investigated. The stereoselectivities for oxidation of trisubstituted enolates 4a-d are good to excellent, 60-95% ee, while those for tetrasubstituted enolates 4e and 8 are lower; i.e., 21-30% ee. Isolated chemical yields for both types of enolate anions are good to excellent. The sodium enolate anions of 4a-d, which could be oxidized at -78°C, gave both higher yields and stereoselectivities than the corresponding lithium or zinc enolates, which required warming to higher temperatures for complete oxidation. The presence of HMPA generally had a deleterious effect on the stereoinduction. However, for oxidation of (E)- and (Z)-4d the highest ee's were observed in the presence of this additive. Investigation of the stereoselective trends reveals that the enolate substitution pattern and the enolate solution structure are the most important stereocontrol elements. The role that the enolate geometry has in the stereoinduction is less clear although Z enolates seem to exhibit higher stereoselectivities than the E enolates. The results obtained in this study have been formulated into a mechanistic rational involving an SN2-type substitution of the enolate anion on oxaziridine 2 via an "open" transition state.
Kinetic and mechanistic studies on the formation and reactions of early-transition-metal-ketene complexes
Galante, Julienne M.,Bruno, Joseph W.,Hazin, Paulette N.,Folting, Kirsten,Huffman, John C.
, p. 1066 - 1073 (2008/10/08)
A series of complexes of vanadocene or molybdenocene with unsymmetrical ketenes were prepared, either by reaction of the various ketenes with vanadocene itself or by reaction with the molybdenocene phosphine complex (C5H5)2Mo(PEt3). All of the complexes exhibited the expected ketene C=O bonding mode, and all reactions were very specific in their formation of the facial isomer with metallocene fragment located on the side of the smaller ketene substituent. Kinetic studies were used to assess the sensitivity of the incoming vanadocene to steric and electronic effects, with the latter found to dominate. Kinetic studies and activation parameters for reaction of Cp2Mo(PEt3) with EtPhC=C=O indicated a second-order associative mechanism, proposed to involve a nucleophilic attack of the metal center on the ketene central carbon in the rate-limiting step. Lastly, reactions of the bound ketenes with nucleophiles (alkyllithiums or Grignard reagents) proceeded readily to either ketone or aldehyde enolates (the latter via transfer of a β-hydride from the alkyl); the clean production of only Z enolates from the unsymmetrical ketenes is indicative of a metal-mediated internal attack of nucleophile on the bound ketene.
