Mechanism and Origins of Enantio- and Regioselectivities in Catalytic Asymmetric Minisci-Type Addition to Heteroarenes

Activation Model and Origins of Selectivity for Chiral Phosphoric Acid Catalyzed Diradical Reactions

To develop new radical synthesis strategies, a profound understanding of the electronic transfer mechanism is critical. An activation model called relayed proton-coupled electron transfer (relayed-PCET) was developed and investigated for chiral phosphoric acid-catalyzed diradical reactions by density functional theory (DFT). The driving force of electron transfer from the nucleophile to the electrophile is the proton transfer that occurs via the chiral phosphoric acid (CPA) catalyst to the electrophile. Moreover, the origins of the selectivity can be explained by distortion of the catalyst, favorable hydrogen bonding, and strong interactions of the substrates with substituents of the CPAs.

New insights into the mechanism of synergetic photoredox/copper(i)-catalyzed carbocyanation of 1,3-dienes: a DFT study

This work presents a DFT-based computational study to understand the mechanism, and regio- and enantioselectivities in the synergetic photoredox/copper(i)-catalyzed carbocyanation of 1,3-dienes with alkyl redox-active esters. The calculated results show an unprecedented copper catalytic mechanism, where the reaction follows a catalytic cycle involving CuI-only catalysis, instead of a Cu(i)/Cu(ii)/Cu(iii)/Cu(i) cycle as proposed in the experimental study. Moreover, it is found that the critical step involves the reaction of the cyanocopper(i) species with an allyl cation rather than the cyanocopper(ii) species reacting with an allyl radical as proposed in the experiment, and that the photocatalyst is regenerated via single electron transfer from the allyl radical to the oxidized photocatalyst. In the newly proposed photoredox/copper(i) catalysis, the reaction consists of four stages: (i) generation of the copper(i) active catalyst, (ii) formation of an allyl radical with oxidative quenching of the photoexcited species, (iii) generation of an allylcopper complex accompanied by the regeneration of the photocatalyst, and (iv) formation of the allyl cyanide product with the regeneration of the copper(i) active catalyst. The cyanation of the allyl cation is calculated to be the regio- and enantioselectivity-determining step. The excellent regio- and stereoselectivities are attributed to the favorable CH-π interaction between the substrate and catalyst as well as the small distortion of the substrate.

Role of Noncovalent Interactions in Inducing High Enantioselectivity in an Alcohol Reductive Deoxygenation Reaction Involving a Planar Carbocationic Intermediate

The involvement of planar carbocation intermediates is generally considered undesirable in asymmetric catalysis due to the difficulty in gaining facial control and their intrinsic stability issues. Recently, suitably designed chiral catalyst(s) have enabled a guided approach of nucleophiles to one of the prochiral faces of carbocations affording high enantiocontrol. Herein, we present the vital mechanistic insights from our comprehensive density functional theory (B3LYP-D3) study on a chiral Ir-phosphoramidite-catalyzed asymmetric reductive deoxygenation of racemic tertiary α-substituted allenylic alcohols. The catalytic transformation relies on the synergistic action of a phosphoramidite-modified Ir catalyst and Bi(OTf)₃, first leading to the formation of an Ir-π-allenyl carbocation intermediate through a turn-over-determining S_N1 ionization, followed by a face-selective hydride transfer from a Hantzsch ester analogue to yield an enantioenriched product. Bi(OTf)₃ was found to promote a significant number of ionic interactions as well as noncovalent interactions (NCIs) with the catalyst and the substrates (allenylic alcohol and Hantzsch ester), thus providing access to a lower energy route as compared to the pathways devoid of Bi(OTf)₃. In the nucleophilic addition, the chiral induction was found to depend on the number and efficacy of such key NCIs. The curious case of reversal of enantioselectivity, when the α-substituent of the allenyl alcohol is changed from methyl to cyclopropyl, was identified to originate from a change in mechanism from an enantioconvergent pathway (α-methyl) to a dynamic kinetic asymmetric transformation (α-cyclopropyl). These molecular insights could lead to newer strategies to tame tertiary carbocations in enantioselective reactions using suitable combinations of catalysts and additives.

Enantioselective [2 + 2] photocycloaddition of quinolone using a C1-symmetric chiral phosphoric acid as a visible-light photocatalyst

The enantioselective intra- and intermolecular [2 + 2] photocycloaddition of quinolone using a C₁-symmetric chiral phosphoric acid as a visible-light photocatalyst is developed. The thioxanthone chromophore on phosphoric acid plays an important role in both phototransformation and enantioselectivity.

Catalytic Asymmetric Construction of Axially and Centrally Chiral Heterobiaryls by Minisci Reaction

Axially chiral biaryls and heterobiaryls constitute the most represented subclass of atropisomers with prevalence in natural products, bioactive compounds, privileged chiral ligand/catalysts, and optically pure materials. Despite many ionic protocols for their construction, radical-based variants represent another highly desirable and intriguing strategy but are far less developed. Moreover, efficient synthesis of axially chiral heterobiaryl molecules, especially ones having multiple heteroatoms and other types of chiral elements, through radical routes remains extremely limited. We herein disclose the first catalytic asymmetric, metal-free construction of axially and centrally chiral heterobiaryls by Minisci reaction of 5-arylpyrimidines and α-amino acid-derived redox-active esters. This is enabled by the use of 4CzIPN as an organic photoredox catalyst in conjunction with a chiral phosphoric acid catalyst. The reaction achieved a variety of interesting 5-arylpyrimidines featuring the union of an axially chiral heterobiaryl and a centrally chiral α-branched amine with generally excellent regio-, diastereo-, and enantioselectivity (up to 82% yield; >19:1 dr; >99% ee). This finding also builds up a new platform for the development of desymmetrization methods via radical-involved atroposelective functionalization at heteroarene of prochiral heterobiaryls.

A Computational and Experimental Investigation of the Origin of Selectivity in the Chiral Phosphoric Acid Catalyzed Enantioselective Minisci Reaction

The Minisci reaction is one of the most valuable methods for directly functionalizing basic heteroarenes to form carbon–carbon bonds. Use of prochiral, heteroatom-substituted radicals results in stereocenters being formed adjacent to the heteroaromatic system, generating motifs which are valuable in medicinal chemistry and chiral ligand design. Recently a highly enantioselective and regioselective protocol for the Minisci reaction was developed, using chiral phosphoric acid catalysis. However, the precise mechanism by which this process operated and the origin of selectivity remained unclear, making it challenging to develop the reaction more generally. Herein we report further experimental mechanistic studies which feed into detailed DFT calculations that probe the precise nature of the stereochemistry-determining step. Computational and experimental evidence together support Curtin–Hammett control in this reaction, with initial radical addition being quick and reversible, and enantioselectivity being achieved in the subsequent slower, irreversible deprotonation. A detailed survey via DFT calculations assessed a number of different possibilities for selectivity-determining deprotonation of the radical cation intermediate. Computations point to a clear preference for an initially unexpected mode of internal deprotonation enacted by the amide group, which is a crucial structural feature of the radical precursor, with the assistance of the associated chiral phosphate. This unconventional stereodetermining step underpins the high enantioselectivities and regioselectivities observed. The mechanistic model was further validated by applying it to a test set of substrates possessing varied structural features.

Enantioselective intramolecular [2 + 2] photocycloaddition using phosphoric acid as a chiral template

The enantioselective intramolecular [2 + 2] photocycloaddition of 4-bishomoally-2-quinolone (quinolinone) using phosphoric acid as a chiral template has been developed. Mechanistic studies using several NMR measurement techniques and density functional theory (DFT) calculations indicate that π-π interactions between the phenyl ring on phosphoric acid and quinolinone play important roles in the enantioselectivity.