Unanticipated Silyl Transfer in Enantioselective α,β-Unsaturated Acyl Ammonium Catalysis Using Silyl Nitronates

Abstract

The use of silyl nitronates is reported for the isothiourea-catalyzed synthesis of -nitro-substituted silyl esters containing up to two contiguous stereocenters in good yield and with excellent enantioselectivity (up to 93% yield, 99:1 er). The serendipitously-discovered formation of silyl ester products in this reaction demonstrates a novel platform for catalyst turnover in α,β-unsaturated acyl ammonium catalysis.

Chemistry Evolves, Terms Evolve, but Phenomena Do Not Evolve: From Chalcogen–Chalcogen Interactions to Chalcogen Bonding

“…Asynchronous [2+ +2] cycloadditions of intermediate enolates with strongly activated ketones were realized using these catalytic systems, [28] in addition to other types of Michael additions. [29] Extensive structural investigations,b oth in solution using nuclear Overhauser effect spectroscopy and in the crystal using X-ray structural analyses,w ere able to demonstrate the importance of these oxygen-sulfur interactions for this catalysis. [30] Kinetic studies showed asignificantly higher catalytic activity for the analogous isoselenourea derivatives.C omputational studies,s uch as the first theoretical investigation on chalcogen-chalcogen interactions in 2006, [7,8] demonstrated the electron delocalization as n Ch !s* Ch-C interactions.…”

Chemie und Begriffe entwickeln sich, aber Phänomene nicht: Von Chalkogen‐Chalkogen‐Wechselwirkungen zu “Chalcogen Bonding”

“…Nach Protonierung des Enolats und Abspaltung des Katalysators wird das Additionsprodukt in hervorragenden Diastereomeren-und noch besseren Enantiomerenverhältnissen erhalten, womit sich der Katalysezyklus schließt. So wurden mit diesen Katalysatorsystemen neben anderen Ty pen von Michael-Additionen [28] beispielsweise asynchron verlaufende [2+ +2]-Cycloadditionen [29] von intermediäre rzeugten Enolaten mit stark aktivierten Ketonen verwirklicht. Umfangreiche strukturelle Untersuchungen, sowohl in Lçsung mittels Nuclear-Overhauser-Effekten als auch im Kristall über Rçntgenstrukturanalysen konnten die Bedeutung dieser Sauerstoff-Schwefel-Wechselwirkungen fürd iese Katalyse belegen.…”

Isothiourea-Catalyzed Enantioselective Michael Addition of Malonates to α,β-Unsaturated Aryl Esters

“…This PNP turnover strategy has previously been employed to promote the enantioselective nitronate addition to α,β-unsaturated PNP esters; 15a however, this process required nitroalkane to be used as a solvent or highly reactive silyl nitronates to be used as stoichiometric nucleophiles. 15b , 16 The use of dihydropyrazol-3-ones and 3-substituted oxindoles as N -heterocyclic enolates was also achieved through the aryloxide catalytic turnover. 4 Herein, we report the HyperBTM-catalyzed addition of simple malonates and related derivatives to α,β-unsaturated aryl esters possessing a variety of electron-withdrawing β substituents under mild reaction conditions.…”