Ir-Catalyzed Regiocontrolled Allylic Amination of Di-/Trienyl Allylic Alcohols with Secondary Amines

Rhodium-Catalyzed Asymmetric N2-C5 Allylation of Indazoles with Dienyl Allylic Alcohols

The development of site-selective and regio- and enantioselective reactions of substrates with multiple active sites is an important topic and remains a substantial challenge in synthetic chemistry. Here, we describe a rhodium-catalyzed asymmetric N2-C5 allylation of indazoles with dienyl allylic alcohols under mild conditions. In the presence of a Rh/(P/olefin) catalyst and formic acid, chiral N2-C5 allylic indazoles were formed in good yields with excellent enantioselectivities (up to 97% ee). The mechanism proceeds through an elusive intermediate Int B, which represents a challenging task on asymmetric allylic substitution (AAS) of dienyl substrates.

Iridium-Catalyzed Regio- and Enantioselective N-Allylation of Pyrazoles with Dienyl/Monoallylic Alcohols

Here we report the first example of iridium-catalyzed asymmetric N-allylation of pyrazoles with dienyl allylic alcohols under mild conditions with broad functional group tolerance, exhibiting excellent N1/C3-site selectivities and enantioselectivities (up to >99% ee). In addition to pyrazoles, other nucleophiles including benzotriazole, triazole, and pyrazole precursors (aryl vinyldiazos) are also suitable in this method. Notably, with the use of Sc(OTf)3 as additive and reactions performed at 30 °C for 24 h, the N1-C5 or N1-C1 selective alkylated pyrazoles are also obtained.

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 1] Cycloaddition of Hydroxyallyl Anilines with Sulfoxonium Ylides

We present here an iridium-catalyzed diastereo- and enantioselective [4 + 1] cycloaddition reaction of hydroxyallyl anilines with sulfoxonium ylides under mild reaction conditions, leading to 3-vinyl indolines in moderate to good yields with excellent enantioselectivities. Control experiments disclosed a plausible reaction mechanism.

Access to Chiral Tetrahydroquinazolines/1,3-Benzoxazines via Iridium-Catalyzed Asymmetric [4 + 2] Cycloaddition

An iridium-catalyzed asymmetric [4 + 2] cycloaddition of 1,3,5-triazinanes with 2-(1-hydroxyallyl)anilines/2-(1-hydroxyallyl)phenols has been developed, providing a straightforward and efficient approach to a wide range of tetrahydroquinazolines in good yields and excellent enantioselectivities (up to >99% ee). Typically, chiral 1,3-benzoxazines, which are challenging substrates in asymmetric [4 + 2] cycloaddition, could be obtained in excellent enantioselectivities via this protocol.

Boron-catalysed transition-metal-free arylation and alkenylation of allylic alcohols with boronic acids

The development of efficient catalytic reactions with excellent atom and step economy employing sustainable catalysts is highly sought-after in chemical synthesis to reduce the negative effects on the environment. The most commonly-used strategy to construct allylic compounds relies on the transition-metal-catalysed nucleophilic substitution reaction of allylic alcohol derivatives. These syntheses exhibit good yield and selectivity, albeit at the expense of toxic and expensive catalysts and extra steps. In this paper, we report a transition-metal-free arylation and alkenylation reaction between unprotected allylic alcohols and boronic acids. The reactions were performed with B(C₆F₅)₃ as the catalyst in toluene, and corresponding products were obtained in 23-92% yields. The reaction has mild conditions, scalability, excellent atom and step economy.

Dehydrative Cross-Coupling for C-N Bond Construction under Transition-Metal-Free Conditions

A transition-metal-free catalytic system was designed to address the dehydrative cross-coupling of unactivated primary/secondary alcohols with amines/amides under environmentally benign conditions. Mg²⁺ and counteranion (PF₆^-) worked synergistically to realize C-OH bond cleavage and concomitant C-N bond formation. A wide range of allylic alcohols and amines/amides were tolerated well in this transformation, which allowed C-N bond construction with high efficiency.

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.

Chiral Brønsted Acid-Catalyzed Intramolecular Asymmetric Allylic Alkylation of Indoles with Primary Alcohols

Herein, chiral Brønsted acid-catalyzed intramolecular asymmetric allylic alkylation of indoles with allylic primary alcohols is described. The allyl alcohols were directly employed as the allylic precursors in this metal-free protocol, without preactivation or any additional activating reagents. This method provides the convenient synthesis of a broad range of functionalized tetrahydrocarbazoles in excellent yields (≤97%) with good enantioselectivity (≤93% ee). The optimal conditions are compatible for gram-scale reaction.

Kinetic, ESI-CID-MS and Computational Studies of π-Allyliridium C,O-Benzoate-Catalyzed Allylic Amination: Understanding the Effect of Cesium Ion

The mechanism of π-allyliridium C,O-benzoate-catalyzed allylic amination was studied by (a) reaction progress kinetic analysis (RPKA), (b) tandem ESI-MS analysis, and (c) computational studies involving density functional theory (DFT) calculations. Reaction progress kinetic analysis (RPKA) reveals a zero-order dependence on allyl acetate, first-order dependence on catalyst and fractional-order dependence on amine. These data corroborate rapid ionization of the allylic acetate followed by turnover limiting C-N bond formation. To illuminate the origins of the 0.4 kinetic order dependence on amine, ESI-MS analyses of quaternary ammonium-labelled piperazine with multistage collision induced dissociation (CID) were conducted that corroborate intervention of cesium-bridged amine dimers that dissociate to form monomeric cesium amide nucleophiles. Computational data align with RPKA and ESI-CID-MS analyses and suggest early transition states mitigate the impact of steric factors, thus enabling formation of highly substituted C-N bonds with complete levels of branched regioselectivity. Specifically, trans-effects of the iridium complex facilitate nucleophilic attack at the more substituted allyl terminus trans to phosphorus with enantioselectivity governed by steric repulsions between the chiral bisphosphine ligand and the π-allyl of a dominant diastereomer of the stereogenic-at-metal complex. Beyond defining aspects of the mechanism of π-allyliridium C,O-benzoate-catalyzed allylic amination, these data reveal that a key feature of cesium carbonate not only lies in its enhanced basicity, but also its capacity for Lewis-acid enhanced Brønsted acidification of amines.

Regio- and Enantioselective Iridium-Catalyzed Amination of Alkyl-Substituted Allylic Acetates with Secondary Amines

Robust air-stable cyclometalated π-allyliridium C,O-benzoates modified by (S)-tol-BINAP catalyze the reaction of secondary aliphatic amines with racemic alkyl-substituted allylic acetates to furnish products of allylic amination with high levels of enantioselectivity. Complete branched regioselectivities were observed despite the formation of more highly substituted C-N bonds.

Recent advances in iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands

This minireview describes the recent progress of iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands realizing highly enantioselective carbon–carbon and carbon–heteroatom bond formation.

Iridium-catalyzed regio- and enantioselective allylic esterification of secondary allylic alcohols with carboxylic acids

We report herein an iridium-catalyzed asymmetric allylic esterification of racemic secondary allylic alcohols using free carboxylic acids as nucleophiles under mild conditions with broad functional group tolerance, exhibiting excellent regio- and enantioselectivity .

Modern Synthetic Methods for the Stereoselective Construction of 1,3-Dienes

The 1,3-butadiene motif is widely found in many natural products and drug candidates with relevant biological activities. Moreover, dienes are important targets for synthetic chemists, due to their ability to give access to a wide range of functional group transformations, including a broad range of C-C bond-forming processes. Therefore, the stereoselective preparation of dienes have attracted much attention over the past decades, and the search for new synthetic protocols continues unabated. The aim of this review is to give an overview of the diverse methodologies that have emerged in the last decade, with a focus on the synthetic processes that meet the requirements of efficiency and sustainability of modern organic chemistry.

Homoleptic Bis(trimethylsilyl)amides of Yttrium Complexes Catalyzed Hydroboration Reduction of Amides to Amines

Homoleptic lanthanide complex Y[N(TMS)₂]₃ is an efficient homogeneous catalyst for the hydroboration reduction of secondary amides and tertiary amides to corresponding amines. A series of amides containing different functional groups such as cyano, nitro, and vinyl groups were found to be well-tolerated. This transformation has also been nicely applied to the synthesis of indoles and piribedil. Detailed isotopic labeling experiments, control experiments, and kinetic studies provided cumulative evidence to elucidate the reaction mechanism.