Dual-Hydrogen-Bond Donor and Brønsted Acid Cocatalysis Enables Highly Enantioselective Protio-Semipinacol Rearrangement Reactions

Organocatalytic asymmetric electrophilic tandem selenylation semipinacol rearrangement of 1-(1-arylvinyl)cyclobutanols

Chiral BINAM-derived sulfide and boron trifluoride etherate cocatalyzed enantioselective electrophilic selenylation/semipinacol rearrangement of 1-(1-arylvinyl)cyclobutanols was developed for the first time. Various selenium-containing cyclopentanones were obtained in moderate to excellent yields with good enantioselectivities. Moreover, a chiral quaternary carbon center was efficiently constructed via this reaction.

Chiral sulfide and achiral sulfonic acid cocatalyzed enantioselective electrophilic tandem selenylation semipinacol rearrangement of allenols

A highly enantioselective electrophilic selenylation/semipinacol rearrangement of allenols has been developed, which is enabled by the cooperative catalysis of a chiral sulfide and an achiral sulfonic acid. The designed and synthesized chiral sulfide catalyst and selenylating reagent play a crucial role in enhancing both enantioselectivity and reactivity. This approach exhibits excellent regio-, chemo-, and enantioselectivity, providing access to diverse enantioenriched cyclopentanones featuring an arylselenovinyl-substituted quaternary carbon stereocenter. Furthermore, these products can be transformed into synthetically valuable alkyne, vinyl bromide, and aniline derivatives. Mechanistic studies reveal that the combination of a chiral sulfide and an achiral sulfonic acid not only facilitates the formation of catalytically active species, but also governs the enantioselectivity of the reaction. Meanwhile, density functional theory calculations disclose that four hydrogen bond interactions and a π‧‧‧π interaction are responsible for the observed enantioselectivity.

Recent advances in asymmetric synthesis via cyclopropanol intermediates

Cyclopropanols have attracted significant attention in organic synthesis as versatile three-carbon synthons, as this readily available class of donor-activated cyclopropanes undergoes miscellaneous transformations, either via ring-opening or with retention of the cyclopropane ring. This review summarizes stereoselective and stereoretentive transformations suitable for asymmetric synthesis. The utility of cyclopropanols is discussed for two main strategies: (i) substrate-controlled transformations using enantiomerically enriched cyclopropanol intermediates through a traditional approach, and (ii) the use of nonchiral or racemic cyclopropanols, where asymmetric induction is achieved through a chiral catalyst, representing a direction that has recently emerged.

Enantioselective Synthesis of α-Aryl Ketones by a Cobalt-Catalyzed Semipinacol Rearrangement

A highly enantioselective cobalt-catalyzed semipinacol rearrangement of symmetric α,α-diarylallylic alcohols is disclosed. A chiral cobalt-salen catalyst generates a highly electrophilic carbocation surrogate following hydrogen atom transfer and radical-polar crossover steps. This methodology provides access to enantioenriched α-aryl ketones through invertive displacement of a cobalt(IV) complex during 1,2-aryl migration. A combination of readily available reagents, silane and N-fluoropyridinium oxidant, are used to confer this type of reactivity. An exploration into the effect of aryl substitution revealed the reaction tolerates para- and meta-halogenated, mildly electron-rich and electron-poor aromatic rings with excellent enantioselectivities and yields. The yield of the rearrangement diminished with highly electron-rich aryl rings whereas very electron-deficient and ortho-substituted arenes led to poor enantiocontrol. A Hammett analysis demonstrated the migratory preference for electron-rich aromatic rings, which is consistent with the intermediacy of a phenonium cation.

Solvent Effects and Internal Functions Control Molecular Recognition of Neutral Substrates in Functionalized Self-Assembled Cages

A suite of internally functionalized Fe4L6 cage complexes has been synthesized with lipophilic end groups to allow dissolution in varied solvent mixtures, and the scope of their molecular recognition of a series of neutral, nonpolar guests has been analyzed. The lipophilic end groups confer cage solubility in solvents with a wide range of polarities, from hexafluoroisopropanol (HFIP) to tetrahydrofuran, and the hosts show micromolar affinities for neutral guests, despite having no flat panels enclosing the cavity. These hosts allow interrogation of the effects of an internal functional group on guest binding properties, as well as solvent-based driving forces for recognition. Introducing polar effects to the interior of the cavity enhances guest binding affinity in nonpolar solvents; adding space-filling aliphatic groups reduces affinity in all cases. While high dielectric solvents such as acetonitrile strongly favor guest binding, "low dielectric, high polarity" solvents such as HFIP strongly occupy the cavity and prevent guest recognition. Analysis of the cage optical transitions shows that the guests interact with the central ligand cores and reside in close proximity to the internal functions. These results have implications for supramolecular catalysis: balancing directed host:guest interactions (e.g., H-bonds) with entropic effects from solvent displacement is essential for reactions in these (and related) biomimetic hosts.

Ring-Enlargement of in Situ Generated Cyclopropanones by the Reaction with Sulfonium Ylides: One-Pot Synthesis of Cyclobutanones

In this report, we describe a simple method for the synthesis of 2-aryl-2-vinyl-cyclobutanones through the reaction of in situ generated cyclopropanones and cinnamylsulfonium ylides, representing an example of a formal carbene insertion into these three-membered rings. The cyclobutanones thus obtained are ideal substrates for palladium-catalyzed coupling reactions upon enol triflate formation, thereby providing access to densely functionalized cyclobutenes. A mechanistic proposal for the ring-enlargement is presented based on experimental evidence.

Cobalt-Catalyzed Photo-Semipinacol Rearrangement of Unactivated Allylic Alcohols

We report a photochemical method for the semipinacol rearrangement of unactivated allylic alcohols. Aliphatic as well as aromatic groups participate as migrating groups, yielding a variety of α,α-disubstituted ketones. The reaction proceeds under mild conditions and is compatible with ethers, esters, halides, nitriles, carbamates, and substituted arenes. The operationally simple and fully catalytic conditions prescribe 1 mol % benzothiazinoquinoxaline as organophotocatalyst, 0.5 mol % Co-salen, and 10 mol % lutidinium triflate and, importantly, display reactivity complementary to procedures employing Brønsted acid. We showcase the utility of the protocol in late-stage drug diversifications.

Catalytic asymmetric intramolecular propargylation of cyclopropanols to access the cuparane core

The catalytic asymmetric propargylation of enol(ate) intermediates is a well-established method for the synthesis of α-propargyl-substituted carbonyl compounds. However, the propargylation of homo-enol(ate) or its equivalents for the synthesis of β-propargyl-substituted carbonyl compounds remains underdeveloped. A catalytic enantioselective decarboxylative intramolecular propargylation of cyclopropanols has been developed using a PyBox-complexed copper catalyst. This reaction offers an effective approach to assemble a cyclopentanone skeleton bearing an all-carbon quaternary stereogenic center and an adjacent quaternary gem-dimethyl carbon center, which is the core scaffold of the naturally occurring cuparenoids. Key to the success of this protocol is the use of a new structurally optimized PyBox ligand. This study represents the first example of catalytic asymmetric intramolecular propargylation of cyclopropanols.

Catalytic asymmetric cationic shifts of aliphatic hydrocarbons

Asymmetric catalysis is an advanced area of chemical synthesis, but the handling of abundantly available, purely aliphatic hydrocarbons has proven to be challenging. Typically, heteroatoms or aromatic substructures are required in the substrates and reagents to facilitate an efficient interaction with the chiral catalyst. Confined acids have recently been introduced as tools for homogenous asymmetric catalysis, specifically to enable the processing of small unbiased substrates1. However, asymmetric reactions in which both substrate and product are purely aliphatic hydrocarbons have not previously been catalysed by such super strong and confined acids. We describe here an imidodiphosphorimidate-catalysed asymmetric Wagner-Meerwein shift of aliphatic alkenyl cycloalkanes to cycloalkenes with excellent regio- and enantioselectivity. Despite their long history and high relevance for chemical synthesis and biosynthesis, Wagner-Meerwein reactions utilizing purely aliphatic hydrocarbons, such as those originally reported by Wagner and Meerwein, had previously eluded asymmetric catalysis.