Access to 3-Oxindoles from Allylic Alcohols and Indoles

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Enantioselective Alkynylation of 2-Aryl-3H-indol-3-ones via Cooperative Catalysis of Copper/Chiral Phosphoric Acid

A facile enantioselective alkynylation of cyclic ketimines attached to a neutral functional group utilizing the dual Cu(I)-CPA catalysis is described. The strategy of the alkynylation of 2-aryl-3H-indol-3-one directly to chiral propargylic amines containing indolin-3-one moiety in good yields and enantioselectivities. Moreover, gram-scale synthesis of chiral propargylamines based C2-quaternary indolin-3-ones was performed. The synthetic applications were confirmed by transformations of the products with no decrease in the yield and enantioselectivity.

Peptide-based metalloporphyrin catalysts: unveiling the role of the metal ion in indole oxidation

Over the last decades, much effort has been devoted to the construction of protein and peptide-based metalloporphyrin catalysts capable of promoting difficult transformations with high selectivity. In this context, mechanistic studies are fundamental to elucidate all the factors that contribute to catalytic performances and product selectivity. In our previous work, we selected the synthetic peptide-porphyrin conjugate MnMC6*a as a proficient catalyst for indole oxidation, promoting the formation of a 3-oxindole derivative with unprecedented selectivity. In this work, we have evaluated the role of the metal ion in affecting reaction outcome, by replacing manganese with iron in the MC6*a scaffold. Even though product selectivity is not altered upon metal substitution, FeMC6*a shows a lower substrate conversion and prolonged reaction times with respect to its manganese analogue. Experimental and theoretical studies have enabled us to delineate the reaction free energy profiles for both catalysts, indicating different thermodynamic limiting steps, depending on the nature of the metal ion.

Organocatalyzed Enantioselective Aza-Morita-Baylis-Hillman Reaction of Cyclic Ketimine with α,β-Unsaturated γ-Butyrolactam

The enantioselective aza-MBH reaction is an efficient strategy for constructing novel carbon-carbon bonds, providing access to multitudinous chiral densely functionalized MBH products. However, the enantioselective aza-MBH reaction of cyclic-ketimines that would generate a versatile synthon is still missing and challenging. Herein, we developed a challenging direct organocatalytic asymmetric aza-MBH reaction involving cyclic ketimines attached to a neutral functional group. Moreover, the α,β-unsaturated γ-butyrolactam was utilized as a rare nucleophile alkene in this work. The reactions provide enantiomerically enriched 2-alkenyl-2-phenyl-1,2-dihydro-3H-indol-3-ones, bearing with a tetra-substituted stereogenic center. Moreover, this reaction features high α-selectivities, high enantioselectivities (up to 99% ee), and good yields (up to 80%).

Asymmetric Ruthenium Catalysis Enables Fluorophores with Point Chirality Displaying CPL Properties

A novel enantiopure π-allylruthenium(IV) precatalyst allowed the enantioselective and stereospecific allylations of indoles and gave access to indolin-3-ones, containing vicinal stereogenic centers. Facile separation of diastereoisomers exhibiting opposite circularly polarized luminescence (CPL) activities in diverse solvents, including water, demonstrated the potential of these sustainable transformations and of the newly prepared molecules.

Multicomponent Assembly of Complex Oxindoles by Enantioselective Cooperative Catalysis

Chiral oxindoles are important chemical scaffolds found in many natural products, and their enantioselective synthesis thus attracts considerable attention. Highly diastereo- and enantioselective synthetic methods for constructing C3 quaternary oxindoles have been well-developed. However, the efficient synthesis of chiral 3-substituted tertiary oxindoles has been rarely reported due to the ease of racemization of the tertiary stereocenter via enolization. Therefore, we herein report on the multicomponent assembly (from N-aryl diazoamides, aldehydes, and enamines/indoles) of complex oxindoles by enantioselective cooperative catalysis. These reactions proceed under mild conditions and show broad substrate scope, affording the desired coupling products (>90 examples) with good to excellent stereocontrol. Additionally, this research also demonstrates the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of Speradine A.

Chiral Amines via Enantioselective π-Allyliridium-C,O-Benzoate-Catalyzed Allylic Alkylation: Student Training via Industrial-Academic Collaboration

ConspectusCyclometalated π-allyliridium-C,O-benzoate complexes discovered in the Krische laboratory display unique amphiphilic properties, catalyzing both nucleophilic carbonyl allylation and electrophilic allylation of diverse amines as well as nitronates. Given the importance of chiral amines in FDA-approved small-molecule drugs, a collaboration with medicinal chemists at Genentech that included on-site graduate student internships was undertaken to explore and expand the scope of π-allyliridium-C,O-benzoate-catalyzed allylic amination and related processes. As described in this Account, our collective experimental studies have unlocked asymmetric allylic aminations of exceptionally broad utility and scope. Specifically, using racemic branched alkyl-substituted allylic acetate proelectrophiles, primary and secondary aliphatic or aromatic amines, including indoles, engage in highly regio- and enantioselective allylic amination. Additionally, unactivated nitronates were found to be competent nucleophilic partners for regio- and enantioselective allylic alkylation, enabling entry to β-stereogenic α-quaternary primary amines. Notably, these π-allyliridium-C,O-benzoate-catalyzed allylic substitutions, which display complete branched regioselectivity in reactions of alkyl-substituted allyl electrophiles, complement the scope of corresponding iridium phosphoramidite-catalyzed allylic aminations, which require aryl-substituted allyl electrophiles to promote high levels of branched regioselectivity. Computational, kinetic, ESI-CID-MS, and isotopic labeling studies were undertaken to understand the mechanism of these processes, including the origins of regio- and enantioselectivity. Isotopic labeling studies suggest that C-N bond formation occurs through outer-sphere addition to the π-allyl. DFT calculations corroborate C-N bond formation via outer-sphere addition and suggest that early transition states and distinct trans effects of diastereomeric chiral-at-iridium π-allyl complexes render the reaction less sensitive to steric effects, accounting for complete levels of branched regioselectivity in reactions of hindered amine and nitronate nucleophiles. Reaction progress kinetic analysis (RPKA) reveals a zero-order dependence on allyl acetate, a first-order dependence on the catalyst, and a fractional-order dependence on the amine. As corroborated by ESI-CID-MS analysis, the 0.4 kinetic order dependence on the amine may reflect the intervention of cesium-bridged amine dimers, which dissociate to form monomeric cesium amide nucleophiles. Hence, the requirement of cesium carbonate (vs lower alkali metal carbonates) in these processes may reside in cesium's capacity for Lewis acid-enhanced Brønsted acidification of the amine pronucleophile. Beyond the development of catalytic processes for the synthesis of novel chiral amines, the present research was conducted by graduate students who benefited from career development experiences associated with training in both academic and industrial laboratories.

A general approach to 2,2-disubstituted indoxyls: total synthesis of brevianamide A and trigonoliimine C

The indoxyl unit is a common structural motif in alkaloid natural products and bioactive compounds. Here, we report a general method that transforms readily available 2-substituted indoles into 2,2-disubstituted indoxyls via nucleophile coupling with a 2-alkoxyindoxyl intermediate and showcase its utility in short total syntheses of the alkaloids brevianamide A (7 steps) and trigonoliimine C (6 steps). The developed method is operationally simple and demonstrates broad scope in terms of nucleophile identity and indole substitution, tolerating 2-alkyl substituents and free indole N-H groups, elements beyond the scope of most prior approaches. Spirocyclic indoxyl products are also accessible via intramolecular nucleophilic trapping.

Multi-component synthesis of dihydro-1,3-azaborinine derived oxindole isosteres

We report on the synthesis of the first examples of 1,3-azaborinine derived oxindole systems, the BN-isosteres of the important compound class of the oxindoles. Hydroboration of terminal aryl acetylenes with FmesBH2, followed by treatment with 2 equiv. of a glycine ester derived isonitrile gave a small series of 1,3-azaborinine derived oxindoles. The new BN-oxindoles show interesting photophysical behavior.

An Apparent Umpolung Reactivity of Indole through [Au]-Catalysed Cyclisation and Lewis-Acid-Mediated Allylation

The sequential functionalization of indole C2 and C3 in an umpolung fashion was executed with a predesigned substrate and choice of reagents. The developed method comprises gold-catalysed alkynol cycloisomerisation/intramolecular addition of C2 of indole and subsequent BF₃ ⋅OEt₂ -mediated regioselective C3 allylation, resulting in the synthesis of the functionalized indoloisoquinolinone scaffold. The reaction involves 5-endo-alkynol cycloisomerisation and the dearomative addition of indole C2 to the intermediate oxocarbenium cation, which results in two equilibrating fused and spiropentacyclic intermediates, which upon treatment with allyl silane in the presence of BF₃ ⋅OEt₂ , undergo selective indole C3 allylation. Other nucleophiles, such as hydride, azide and indole, were also found to be compatible with this process.

Insights into Substrate Self-Assisted Activation of Allylic Alcohols Guiding to Mild Allylic Substitution of Tautomerizable Heteroarenes

A substrate self-assisted activation of allylic alcohols by tautomerizable heteroarenes via hydrogen bonding was disclosed by various NMR techniques, including variable-temperature ¹H NMR, Job plot, and ¹H NMR titration. Guided by these finding, a much milder allylic substitution of tautomerizable heteroarenes with allylic alcohols was developed, affording the target products in high yields.

Substrate Self-Assisted Secondary Bond Activation of Allylic Alcohol in a Tsuji-Trost Reaction Revealed by NMR Methods

The first experimental evidence for the palladium-catalyzed secondary bond activation of allylic alcohols in a Tsuji-Trost reaction was provided by NMR methods, such as variable-temperature ¹H NMR, diffusion-ordered spectroscopy (DOSY), Job's method, ¹H NMR titration, and nuclear Overhauser enhancement spectroscopy (NOESY). The experimental results revealed that the substrate self-assisted activation of allylic alcohols is probably performed via a 1:1 binding six-membered-ring complex, which are formed by the formation of the secondary bonds, the hydrogen bond and P···O noncovalent bond between allylic alcohol and phosphonium ylide.

Regio- and Enantioselective Iridium-Catalyzed N-Allylation of Indoles and Related Azoles with Racemic Branched Alkyl-Substituted Allylic Acetates

Cyclometallated π-allyliridium C,O-benzoates modified with (S)-tol-BINAP, which are stable to air, water, and SiO₂ , catalyze highly enantioselective N-allylations of indoles and related azoles. This reaction complements previously reported metal-catalyzed indole allylations in that complete levels of N versus C3 and branched versus linear regioselectivity are observed.

Modulation of emission properties of phosphine-sulfonate ligand containing copper complexes: playing with solvato-, thermo-, and mechanochromism

A series of Cuprous and Cuprate(i) complexes have been selectively obtained by addition of solvents or metal salts.

Palladium-catalyzed N1-selective allylation of indoles with allylic alcohols promoted by titanium tetraisopropoxide

The N1-selective allylation of indoles with allylic alcohols has been accomplished by synergistic functions of palladium catalysts and titanium tetraisopropoxide.