Recent advances and applications of iridium-catalysed asymmetric allylic substitution

Enantioselective construction of C-chiral allylic sulfilimines via the iridium-catalyzed allylic amination with S,S-diphenylsulfilimine: asymmetric synthesis of primary allylic amines

We have devised a highly regio- and enantioselective iridium-catalyzed allylic amination reaction with the sulfur-stabilized aza-ylide, S,S-diphenylsulfilimine.

We have devised a highly regio- and enantioselective iridium-catalyzed allylic amination reaction with the sulfur-stabilized aza-ylide, S,S-diphenylsulfilimine. This process provides a robust and scalable method for the construction of aryl-, alkyl- and alkenyl-substituted C-chiral allylic sulfilimines, which are important functional groups for organic synthesis. Additionally, the combination of the allylic amination with an in situ deprotection of the sulfilimine constitutes a convenient one-pot protocol for the construction of chiral nonracemic primary allylic amines.

Transition-metal-catalyzed asymmetric allylic dearomatization reactions

Dearomatization reactions serve as powerful methods for the synthesis of highly functionalized, three-dimensional structures starting with simple planar aromatic compounds. Among processes of this type, catalytic asymmetric dearomatization (CADA) reactions are attractive owing to the large number of aromatic compounds that are readily available and the fact that they enable direct access to enantiopure polycycles and spirocycles, which frequently are key structural motifs in biologically active natural products and pharmaceuticals. However, as a consequence of their high stabilities, arenes only difficultly participate in dearomatization reactions that take place with high levels of enantioselectivity. Transition-metal-catalyzed asymmetric allylic substitution reactions have been demonstrated to be powerful methods for enantioselective formation of C-C and C-X (X = O, N, S, etc.) bonds. However, the scope of these processes has been explored mainly using soft carbon nucleophiles, some hard carbon nucleophiles such as enolates and preformed organometallic reagents, and heteroatom nucleophiles. Readily accessible aromatic compounds have been only rarely used directly as nucleophiles in these reactions. In this Account, we present the results of studies we have conducted aimed at the development of transition-metal-catalyzed asymmetric allylic dearomatization reactions. By utilizing this general process, we have devised methods for direct dearomatization of indoles, pyrroles, phenols, naphthols, pyridines, and pyrazines, which produce various highly functionalized structural motifs bearing all-carbon quaternary stereogenic centers in a straightforward manner. In mechanistic investigations of the dearomatization process, we found that the five-membered spiroindolenines serve as intermediates, which readily undergo stereospecific allylic migration to form corresponding tetrahydro-1H-carbazoles upon treatment with a catalytic amount of TsOH. It is worth noting that no notable loss of the enantiomeric excess of the spiroindolenine derivatives takes place during the rearrangement process as a consequence of the intervention of a "three-center-two-electron"-type transition state, a proposal that has gained support from the results of DFT calculations. Equally intriguing, upon tuning of the electronic nature of the tethers, pyrroles or indoles undergo unprecedented Ir or Ru catalyzed intramolecular allylic alkylation promoted dearomatization/migration reactions. The operation of this novel reaction pathway provides additional information leading to a greater mechanistic understanding of the transition-metal-catalyzed enantioselective intramolecular functionalizations of pyrroles and indoles. The combined results of this effort provide not only methods for the efficient synthesis of highly enantioenriched fused and spiro polycycles but also novel strategies in the field of asymmetric catalysis.

Iridium-catalyzed enantioselective allylic substitution of unstabilized enolates derived from α,β-unsaturated ketones

We report Ir-catalyzed, enantioselective allylic substitution reactions of unstabilized silyl enolates derived from α,β-unsaturated ketones. Asymmetric allylic substitution of a variety of allylic carbonates with silyl enolates gave allylated products in 62-94% yield with 90-98% ee and >20:1 branched-to-linear selectivity. The synthetic utility of this method was illustrated by the short synthesis of an anticancer agent, TEI-9826.

Enantioselective domino reaction of CO2, amines and allyl chlorides under iridium catalysis: formation of allyl carbamates

The enantioselective domino reaction between CO2 (1 atm), amines and linear allyl chlorides in the presence of an iridium complex, DABCO and toluene at 15 °C was realized, which gave branched allyl carbamates in acceptable to high yields with up to excellent regioselectivity (99/1) and 94% ee. This is the first example of the synthesis of chiral allyl carbamates through catalytic domino reactions using CO2.

Cation control of diastereoselectivity in iridium-catalyzed allylic substitutions. Formation of enantioenriched tertiary alcohols and thioethers by allylation of 5H-oxazol-4-ones and 5H-thiazol-4-ones

We report highly diastereo- and enantioselective allylations of substituted 5H-oxazol-4-ones and 5H-thiazol-4-ones catalyzed by a metallacyclic iridium complex. Enantioselective Ir-catalyzed allylation of substituted 5H-oxazol-4-ones occurs with high diastereoselectivity by employing the corresponding zinc enolates; enantioselective Ir-catalyzed allylation of substituted 5H-thiazol-4-ones occurs with the corresponding magnesium enolates with high diastereoselectivity. The allylation of substituted 5H-oxazol-4-ones provides rapid access to enantioenriched tertiary α-hydroxy acid derivatives unavailable through Mo-catalyzed allylic substitution. The allylation of substituted 5H-thiazol-4-ones provides a novel method to synthesize enantioenriched tertiary thiols and thioethers. The observed cation effect implies a novel method to control the diastereoselectivity in Ir-catalyzed allylic substitution.

Enantioselective functionalization of allylic C-H bonds following a strategy of functionalization and diversification

We report the enantioselective functionalization of allylic C-H bonds in terminal alkenes by a strategy involving the installation of a temporary functional group at the terminal carbon atom by C-H bond functionalization, followed by the catalytic diversification of this intermediate with a broad scope of reagents. The method consists of a one-pot sequence of palladium-catalyzed allylic C-H bond oxidation under neutral conditions to form linear allyl benzoates, followed by iridium-catalyzed allylic substitution. This overall transformation forms a variety of chiral products containing a new C-N, C-O, C-S, or C-C bond at the allylic position in good yield with a high branched-to-linear selectivity and excellent enantioselectivity (ee ≤97%). The broad scope of the overall process results from separating the oxidation and functionalization steps; by doing so, the scope of nucleophile encompasses those sensitive to direct oxidative functionalization. The high enantioselectivity of the overall process is achieved by developing an allylic oxidation that occurs without acid to form the linear isomer with high selectivity. These allylic functionalization processes are amenable to an iterative sequence leading to (1,n)-functionalized products with catalyst-controlled diastereo- and enantioselectivity. The utility of the method in the synthesis of biologically active molecules has been demonstrated.

Enantio-, diastereo-, and regioselective iridium-catalyzed asymmetric allylic alkylation of acyclic β-ketoesters

The first regio-, diastereo-, and enantioselective allylic alkylation of acyclic β-ketoesters to form vicinal tertiary and all-carbon quaternary stereocenters is reported. Critical to the successful development of this method was the employment of iridium catalysis in concert with N-aryl-phosphoramidite ligands. Broad functional group tolerance is observed at the keto-, ester-, and α-positions of the nucleophile. Various transformations demonstrating the utility of this method for rapidly accessing complex enantioenriched compounds are reported.

Construction of vicinal tertiary and all-carbon quaternary stereocenters via Ir-catalyzed regio-, diastereo-, and enantioselective allylic alkylation and applications in sequential Pd catalysis

Highly congested vicinal stereocenters comprised of tertiary and all-carbon quaternary centers were generated via Ir-catalyzed asymmetric allylic alkylation of β-ketoesters. These catalytic reactions proceed in excellent yields with a broad scope on either reaction partner and with outstanding regio-, diastereo-, and enantiocontrol. Implementation of a subsequent Pd-catalyzed alkylation affords dialkylated products with pinpoint stereochemical control of both chiral centers.

Enantioselective functionalization of indoles and pyrroles via an in situ-formed spiro intermediate

Herein we report a highly enantioselective synthesis of polycyclic indoles and pyrroles with up to 99% ee by an iridium catalyst system consisting of a commercially available iridium precursor and a readily accessible ligand. Investigation of the reaction mechanism led to the discovery of an unprecedented dearomatized spiro intermediate and its in situ migration phenomenon. The new reaction mode features switching of the substituent from the indole C-3 position to the C-2 position (from the C-2 position to the C-3 position in the case of pyrrole) without loss of the enantiomeric purity, providing a novel concept in designing the asymmetric construction of enantiopure polycyclic indoles and pyrroles.

Enantioselective approach to quinolizidines: total synthesis of cermizine D and formal syntheses of senepodine G and cermizine C

The formal syntheses of C5-epi-senepodine G and C5-epi-cermizine C have been accomplished through a novel diastereoselective, intramolecular amide Michael addition process. The total synthesis of cermizine D has been achieved through use of an organocatalyzed, heteroatom Michael addition to access a common intermediate. Additional key steps of this sequence include a matched, diastereoselective alkylation with an iodomethylphenyl sulfide and sulfone-aldehyde coupling/reductive desulfurization sequence to combine the major subunits. The utility of a Hartwig-style C-N coupling has been explored on functionally dense coupling partners. Diastereoselective conjugate additions to α,β-unsaturated sulfones have been investigated, which provided the key sulfone intermediate in just six steps from commercially available starting materials. The formal syntheses of senepodine G and cermizine C have been accomplished through an intramolecular cyclization process of a N-Boc-protected piperidine sulfone.

Control of diastereoselectivity for iridium-catalyzed allylation of a prochiral nucleophile with a phosphate counterion

We report a highly diastereo- and enantioselective allylation of azlactones catalyzed by the combination of a metallacyclic iridium complex and an optically inactive phosphate anion. The process demonstrates an approach for conducting diastereoselective reactions with prochiral nucleophiles in the presence of metallacyclic allyliridium complexes. The reaction provides access to an array of enantioenriched allylated azlactones containing adjacent tertiary and quaternary carbon centers. Preliminary mechanistic studies suggest that the phosphate and methyl carbonate anions together induce the unusually high diastereoselectivity.

Computational protein engineering: bridging the gap between rational design and laboratory evolution

Enzymes are tremendously proficient catalysts, which can be used as extracellular catalysts for a whole host of processes, from chemical synthesis to the generation of novel biofuels. For them to be more amenable to the needs of biotechnology, however, it is often necessary to be able to manipulate their physico-chemical properties in an efficient and streamlined manner, and, ideally, to be able to train them to catalyze completely new reactions. Recent years have seen an explosion of interest in different approaches to achieve this, both in the laboratory, and in silico. There remains, however, a gap between current approaches to computational enzyme design, which have primarily focused on the early stages of the design process, and laboratory evolution, which is an extremely powerful tool for enzyme redesign, but will always be limited by the vastness of sequence space combined with the low frequency for desirable mutations. This review discusses different approaches towards computational enzyme design and demonstrates how combining newly developed screening approaches that can rapidly predict potential mutation “hotspots” with approaches that can quantitatively and reliably dissect the catalytic step can bridge the gap that currently exists between computational enzyme design and laboratory evolution studies.

Iridium-catalyzed allylic substitutions with cyclometalated phosphoramidite complexes bearing a dibenzocyclooctatetraene ligand: preparation of (π-allyl)Ir complexes and computational and NMR spectroscopic studies

(π-Allyl)Ir complexes derived from dibenzocyclooctatetraene and phosphoramidites by cyclometalation are effective catalysts for allylic substitution reactions of linear monosubstituted allylic carbonates. These catalysts provide exceptionally high degrees of regioselectivity and allow the reactions to be run under aerobic conditions. A series of (π-allyl)Ir complexes were prepared and characterized by X-ray crystal structure analyses. An allylic amination with aniline displayed different resting states depending on the presence of a strong base. DFT calculations were carried out on the mechanistic aspects of these reactions. The results suggest that for the (π-allyl)Ir complexes, the formation and reactions with nucleophiles proceed with comparable rates.

Iridium-catalyzed regioselective and enantioselective allylation of trimethylsiloxyfuran

We report the regio- and enantioselective allylation of an ester enolate, trimethylsiloxyfuran. This enolate reacts at the 3-position with linear aromatic allylic carbonates or aliphatic allylic benzoates to form the branched substitution products in the presence of a metallacyclic iridium catalyst. This process provides access to synthetically important 3-substituted butenolides in enantioenriched form. Stoichiometric reactions of the allyliridium intermediate suggest that the trimethylsiloxyfuran is activated by the carboxylate leaving group.

Enantioselective allylic thioetherification: the effect of phosphoric acid diester on iridium-catalyzed enantioconvergent transformations

You'll take the high road and I'll take the low road: Enantioenriched allylic thioethers have been synthesized from chiral racemic allylic alcohols. The combination of an Ir-(P,alkene) complex and dibutyl phosphoric acid are required to attain high selectivities. Mechanistic studies uncover an enantioconvergent transformation in which substrate enantiomers react along different pathways to give the same product (see scheme; cod=1,5-cyclooctadiene).