Browsing by Subject "forming transfer hydrogenation"
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Item Application of a High-Throughput Enantiomeric Excess Optical Assay Involving a Dynamic Covalent Assembly: Parallel Asymmetric Allylation and ee Sensing of Homoallylic Alcohols(2015-08) Jo, H. H.; Gao, X.; You, L.; Anslyn, E. V.; Krische, M. J.; Jo, H. H.; Gao, X.; Anslyn, E. V.; Krische, M. J.Asymmetric Ir-catalyzed C-C coupling of primary alcohols with allyl-acetates, as described by Krische, to form chiral secondary homo-allylic alcohols were performed in parallel as a means to optimize the ee values thereof. Specifically, approximately 400 examples of this reaction were performed by varying the catalyst, added acids and bases, and starting reactants, to form 4-phenyl-1-butene-4-ol (1). The ee values for the transformations were determined in a high-throughput fashion using a 4-component assembly that creates a circular dichroism signal indicative of the extent of asymmetric induction. Further, a parallel and rapid quantitative TLC method measures the yield of each reaction, revealing which reactions give reliable ee values in the CD-based assay. Overall, the nearly 200 reactions whose ee values were determined could be quantitated in under two hours. Using a combination of the TLC method to measure yield with the CD-assay to measure ee values, several trends in reaction conditions were revealed. For example, it was found that the cyclometalated iridium catalyst modified by BINAP and m-nitro-p-cyano-benzoic acid delivered adduct 1 with the highest levels of enantiomeric enrichment (94%), whereas the corresponding SEGPHOS-modified catalyst gave a comparable yield but lower ee (91%). Most importantly, this study shows that supramolecular assemblies can report hundreds of ee values in a rapid and reliable fashion to analyze parallel synthesis routines.Item Regiodivergent Reductive Coupling of 2-Substituted Dienes to Formaldehyde Employing Ruthenium or Nickel Catalyst: Hydrohydroxymethylation via Transfer Hydrogenation(2013-02) Kopfer, Alexander; Sam, Brannon; Breit, Bbernhard; Krische, Michael J.; Sam, Brannon; Breit, Bbernhard; Krische, Michael J.The regioselective reductive coupling of paraformaldehyde to 2-substituted dienes at positions C1, C2 and C3 was achieved using metal catalysts based on nickel, cationic ruthenium and neutral ruthenium, respectively. Whilst C1 adducts predominate using dienes, silicon- or tin-substituted dienes exhibit C4 regioselectivity - the first time that this regioselectivity has been observed for the coupling of 2-substituted butadienes to aldehydes. Both nickel- and ruthenium-catalyzed processes avoid pyrophoric or mass intensive reducing agents, using paraformaldehyde or isopropanol, respectively. These couplings may be viewed as alternatives to diene hydroformylation, for which regioselective formation of constitutionally isomeric products has not yet been achieved.