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Phosphoric Acid Catalysis
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
Covalent catalysis involves the formation of transient covalent bond between substrate and catalyst and subsequent breakage of the bond to regenerate the catalyst. In theory, the rigid and highly directional covalent linkage should enhance the interactions between substrate and its surrounding chiral elements, and hence, it helps to maximize the asymmetric induction from the catalyst. One of the major challenges in developing covalent catalysis is to identify a catalyst that is nucleophilic enough to form the covalent bond with substrate, but also possesses properties of a good leaving group to facilitate the turnover of the catalyst. The formation of covalently linked with CPA intermediates had not been invoked to rationalize the CPA-catalyzed reactions before 2011 until the seminal study by the Toste group demonstrates the feasibility of such intermediates.63 Since then, several other reports have described related covalent intermediates, and a growing number of such transformations indicate that this is a rather general class of reactions promoted by CPAs.64–67
New Asymmetric Heterogeneous Catalytic Hydrogenation Reactions
Published in Mike G. Scaros, Michael L. Prunier, Catalysis of Organic Reactions, 2017
A. Tungle, T. Tarnai, T. Máthé, G. Vidra, J. Petró, R.A. Sheldon
Recently we reported the enantioselective hydrogenation of isophorone and acetophenone using palladium-on-carbon catalyst in the presence of (S)-proline as a chiral auxiliary.4–9 We are presently investigating possible common features in the various systems. In the first two systems mentioned above, the asymmetric induction results from modification of the catalyst by the chiral ligand. In our system on the other hand, the chiral auxiliary [(S)-proline] reacts with the substrate in solution and asymmetric induction results from diastereoseiective hydrogenation of this adduct as shown.10
Organocatalysis Induced by the Anion of an Ionic Liquid
Published in Pedro Lozano, Sustainable Catalysis in Ionic Liquids, 2018
Andrea R. Schmitzer, Vincent Gauchot
To the best of our knowledge, this example is the first report of anionic IL catalysis where the chirality is brought by the supramolecular inclusion of the cation. ILs using organocatalytic achiral anions and chiral countercations have, however, yet to be developed, and with respect to the advances made regarding asymmetric ion-pair catalysis, it is reasonable to think that such systems could be extremely efficient in terms of asymmetric induction.
Reactions between lithiated 1,3-dithiane oxides and trialkylboranes
Published in Journal of Sulfur Chemistry, 2021
Basil A. Saleh, Keith Smith, Mark C. Elliott, Gamal A. El-Hiti
Reactions of trialkylboranes with various trisubstituted methanes such as chloroform (CHCl3), dichlorofluoromethane (CHCl2F), chlorodifluoromethane (CHClF2) and 1,1-dichloromethyl methyl ether (DCME) in the presence of a strong base result in the transfer of all three alkyl groups from boron-to-carbon in a single process [9,10]. Even trialkylboranes having a tertiary alkyl group, such as a tert-butyl or thexyl moiety, on reaction with DCME and lithium triethylcarboxide at 25°C, transfer all three groups successfully [9,10]. A reagent with three different leaving groups attached to a central carbon atom could, in principle, be used as an alternative to DCME, opening up possibilities for asymmetric induction to generate enantiomerically enriched chiral tertiary alcohols. Compounds having two sulfur-containing leaving groups have been used successfully to perform up to two 1,2-boron to carbon migrations [11–15], and in principle, a third leaving group could be incorporated to allow a third migration. A potential advantage of using sulfur-based leaving groups might be that stereoselectivity could be controlled, as it can, for example, in reactions of various electrophiles with metalated 1,3-dithiane oxides [16–21]. Such reagents might be able to offer possibilities for the generation of appropriately substituted chiral reagents for reactions with trialkylboranes. However, some basic studies are needed in order to underpin such possibilities.