Aryloxide-Facilitated Catalyst Turnover in Enantioselective α,β-Unsaturated Acyl Ammonium Catalysis
An ew general concept for a,b-unsaturated acyl ammonium catalysis is reported that uses p-nitrophenoxide release from an a,b-unsaturated p-nitrophenyl ester substrate to facilitate catalyst turnover.T his method was used for the enantioselective isothiourea-catalyzed Michael addition of nitroalkanes to a,b-unsaturated p-nitrophenyl esters in generally good yield and with excellent enantioselectivity (27 examples,u pt o7 9% yield, 99:1 er). Mechanistic studies identified rapid and reversible
... lyst acylation by the a,bunsaturated p-nitrophenyl ester,a nd ar ecently reported variable-time normalization kinetic analysis method was used to delineate the complex reaction kinetics. Lewis base organocatalysis is aw idely studied field due to the diverse range of molecular frameworks that can be produced with high levels of regio-, chemo-and stereocontrol.  At the carboxylic acid oxidation level av ariety of ammonium intermediates with differing reactivity can be accessed from readily available substrates using tertiary amine Lewis bases (Scheme 1a). Acyl ammonium and ammonium enolate intermediates have been extensively studied and applied in enantioselective acyl transfer processes and formal cycloadditions,respectively. [2, 3] Aless studied but equally powerful reactivity mode is that of a,b-unsaturated acyl ammonium intermediates.  These species contain electrophilic centres at the C1 and C3 positions,a nd al atent nucleophilic centre at C2, providing new opportunities for reaction design to target previously inaccessible product architectures.  Seminal work by Fu first demonstrated the feasibility of this concept in af ormal [3+ +2] cycloaddition using a,bunsaturated acyl fluorides as the a,b-unsaturated acyl ammonium precursor (Scheme 1b).  Recent studies from ourselves,R omo,a nd Matsubara, has built on this precedent to achieve highly enantioselective Michael addition-annulation, formal cycloaddition and complex cascade methodologies.  These examples used a,b-unsaturated acid anhydrides or halides as the a,b-unsaturated acyl ammonium precursors.In addition, these methodologies require the reactive partner to contain two distinct nucleophilic functionalities to 1) undergo conjugate addition to the a,b-unsaturated acyl ammonium intermediate,a nd 2) enable turnover of the Lewis base catalyst (Scheme 1b). This requirement inherently limits a,b-unsaturated acyl ammonium catalysis and must be overcome to allow more diverse processes.I na ddition only preliminary experimental mechanistic work has been undertaken, with no kinetic analysis reported to date.  Here we report the development of anew general concept for a,b-unsaturated acyl ammonium catalysis.C atalyst turnover is not facilitated by the nucleophilic reaction partner,but by an aryloxide counterion released in situ during the reaction by using an a,b-unsaturated aryl ester as the a,b-unsaturated acyl ammonium precursor (Scheme 1c).    This allows the use of simple nucleophiles as reaction partners,p roviding enhanced potential for further advancement of the field. Mechanistic work including kinetic analysis,catalyst labeling and crossover studies are also reported to deliver af undamental understanding of this process. Scheme 1. Nomenclature, reactivity and applicationsofammonium intermediates in catalysis.