Direct Catalytic Enantioselective Vinylogous Aldol Reaction of α‑Branched Enals with Isatins

The direct vinylogous aldol reaction of α-substituted α,β-unsaturated aldehydes with isatins is described. The chemistry provides easy access to valuable 3-substituted 3-hydroxyoxindole derivatives with high stereocontrol and perfect γ-site selectivity. Preliminary mechanistic studies suggest that, depending on the nature of the α-branched enal substituents, two divergent reaction mechanisms can be operating, leading to different products and stereochemical outcomes

Mechanism of the Stereoselective α‑Alkylation of Aldehydes Driven by the Photochemical Activity of Enamines

Herein we describe our efforts to elucidate the key mechanistic aspects of the previously reported enantioselective photochemical α-alkylation of aldehydes with electron-poor organic halides. The chemistry exploits the potential of chiral enamines, key organocatalytic intermediates in thermal asymmetric processes, to directly participate in the photoexcitation of substrates either by forming a photoactive electron donor–acceptor complex or by directly reaching an electronically excited state upon light absorption. These photochemical mechanisms generate radicals from closed-shell precursors under mild conditions. At the same time, the ground-state chiral enamines provide effective stereochemical control over the enantioselective radical-trapping process. We use a combination of conventional photophysical investigations, nuclear magnetic resonance spectroscopy, and kinetic studies to gain a better understanding of the factors governing these enantioselective photochemical catalytic processes. Measurements of the quantum yield reveal that a radical chain mechanism is operative, while reaction-profile analysis and rate-order assessment indicate the trapping of the carbon-centered radical by the enamine, to form the carbon–carbon bond, as rate-determining. Our kinetic studies unveil the existence of a delicate interplay between the light-triggered initiation step and the radical chain propagation manifold, both mediated by the chiral enamines

Direct Catalytic Enantioselective Vinylogous Aldol Reaction of α‑Branched Enals with Isatins

The direct vinylogous aldol reaction of α-substituted α,β-unsaturated aldehydes with isatins is described. The chemistry provides easy access to valuable 3-substituted 3-hydroxyoxindole derivatives with high stereocontrol and perfect γ-site selectivity. Preliminary mechanistic studies suggest that, depending on the nature of the α-branched enal substituents, two divergent reaction mechanisms can be operating, leading to different products and stereochemical outcomes

Direct Catalytic Enantioselective Vinylogous Aldol Reaction of α‑Branched Enals with Isatins

The direct vinylogous aldol reaction of α-substituted α,β-unsaturated aldehydes with isatins is described. The chemistry provides easy access to valuable 3-substituted 3-hydroxyoxindole derivatives with high stereocontrol and perfect γ-site selectivity. Preliminary mechanistic studies suggest that, depending on the nature of the α-branched enal substituents, two divergent reaction mechanisms can be operating, leading to different products and stereochemical outcomes

Direct Catalytic Enantioselective Vinylogous Aldol Reaction of α‑Branched Enals with Isatins

The direct vinylogous aldol reaction of α-substituted α,β-unsaturated aldehydes with isatins is described. The chemistry provides easy access to valuable 3-substituted 3-hydroxyoxindole derivatives with high stereocontrol and perfect γ-site selectivity. Preliminary mechanistic studies suggest that, depending on the nature of the α-branched enal substituents, two divergent reaction mechanisms can be operating, leading to different products and stereochemical outcomes

Direct Catalytic Enantioselective Vinylogous Aldol Reaction of α‑Branched Enals with Isatins

The direct vinylogous aldol reaction of α-substituted α,β-unsaturated aldehydes with isatins is described. The chemistry provides easy access to valuable 3-substituted 3-hydroxyoxindole derivatives with high stereocontrol and perfect γ-site selectivity. Preliminary mechanistic studies suggest that, depending on the nature of the α-branched enal substituents, two divergent reaction mechanisms can be operating, leading to different products and stereochemical outcomes

Asymmetric Photocatalytic C–H Functionalization of Toluene and Derivatives

Reported herein is a visible-light-mediated organocatalytic direct C–H functionalization of toluene derivatives to afford enantioenriched β-benzylated aldehydes from the corresponding enals. The process combines the oxidative power of a chiral excited-state iminium ion and the basic nature of its counteranion to trigger the generation of benzylic radicals by means of a sequential multisite proton-coupled electron transfer mechanism. This study shows that feedstock chemicals generally used as solvents, such as toluene and xylene derivatives, can be used as substrates for making chiral molecules with high enantioselectivity

A Mechanistic Rationale for the 9‑Amino(9-deoxy)<i>epi</i> Cinchona Alkaloids Catalyzed Asymmetric Reactions via Iminium Ion Activation of Enones

The 9-amino­(9-deoxy)<i>epi</i> cinchona alkaloids have expanded the synthetic potential of asymmetric aminocatalysis, enabling the highly stereoselective functionalization of a variety of sterically hindered carbonyl compounds. However, there is a lack of basic understanding of the mechanisms of cinchona-based primary aminocatalysis. Herein, we describe how a combination of experimental and theoretical mechanistic studies has revealed the origin of the stereoselectivity of the Friedel–Crafts alkylation of indoles with α,β-unsaturated ketones catalyzed by 9-amino­(9-deoxy)<i>epi</i> quinine. An essential role for the achiral acid cocatalyst is uncovered: upon condensation of the cinchona catalyst with the enone, the resulting covalent imine intermediate and the acid interact to build-up a well-structured ion-pair supramolecular catalytic assembly, which is stabilized by multiple attractive noncovalent interactions. All the components of the assembly cooperatively participate in the stereocontrolling event, with the anion of the achiral acid being the structural element responsible for the π-facial discrimination of the iminium ion intermediate

Multicatalytic Asymmetric Synthesis of Complex Tetrahydrocarbazoles via a Diels–Alder/Benzoin Reaction Sequence

Expanding upon the recently developed aminocatalytic asymmetric indole-2,3-quinodimethane strategy, a straightforward synthesis of structurally and stereochemically complex tetrahydrocarbazoles has been devised. The chemistry’s complexity-generating power was further harnessed by designing a multicatalytic, one-pot Diels–Alder/benzoin reaction sequence to stereoselectively access <i>trans</i>-fused tetracyclic indole-based compounds having four stereogenic centers with very high fidelity

Computational Study with DFT and Kinetic Models on the Mechanism of Photoinitiated Aromatic Perfluoroalkylations

A combination of DFT calculations and kinetic models is applied to fully elucidate the seemingly complex reactivity of α-cyano arylacetates toward metal-free photoinitiated aromatic perfluoroalkylation. The resulting mechanistic framework rationalizes the observed quantum yield as well as the differences in reactivity and/or selectivity of seemingly similar substrates. The use of a kinetic model for the chemical interpretation of the DFT-computed reaction constants is shown to be critical