Methods of Reactivity Umpolung

Chemoselective Intermolecular Cross-Enolate-Type Coupling of Amides

A new approach for the synthesis of 1,4-dicarbonyl compounds is reported. Chemoselective activation of amide carbonyl functionality and subsequent umpolung viaN-oxide addition generates an electrophilic enolonium species that can be coupled with a wide range of nucleophilic enolates. The method conveys broad functional group tolerance on both components, does not suffer from formation of homocoupling byproducts and avoids the use of transition metal catalysts.

α-Carbonyl Cations in Sulfoxide-Driven Oxidative Cyclizations

The selective, metal-free generation of α-carbonyl cations from simple internal alkynes was accomplished by the addition of a sulfoxide to a densely substituted vinyl cation. The high reactivity of the α-carbonyl cations was found to efficiently induce hydrogen and even carbon shift reactions with unusual selecivities. Complex compounds with highly congested tertiary and all-carbon-substituted quartenary carbon centers can thus be accessed in a single step from simple precursors. Mechanistic analysis strongly supports the intermediacy of the title compounds and provides a simple predictive scheme for the migratory aptitude of different substituents.

Nitro-enabled catalytic enantioselective formal umpolung alkenylation of β-ketoesters

A formal umpolung strategy is presented for the enantioselective installation of an alkenyl group with a terminal double bond at a tertiary center. This one-pot two-step sequence relies on the unique features of the nitro group, which after inverting the polarity of the alkenylating agent toward the desired bond formation, itself serves as a leaving group. The application of this protocol to cyclic β-ketoesters results in densely functionalized products, bearing an all-carbon quaternary stereocenter including an alkenyl substituent with a terminal double bond, in high yields with excellent enantioselectivities.

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

Regio- and stereospecific rhodium-catalyzed allylic alkylation with an acyl anion equivalent: an approach to acyclic α-ternary β,γ-unsaturated aryl ketones

The regio- and stereospecific rhodium-catalyzed allylic alkylation of secondary allylic carbonates with cyanohydrin pronucleophiles facilitates the direct construction of acyclic α-ternary β,γ-unsaturated aryl ketones. Interestingly, this study illustrates the impact of deaggregating agents on regiocontrol and the electronic nature of the aryl component to suppress olefin isomerization. In addition, we demonstrate that the dimethylamino substituent, which modulates the pKa of the α-ternary β,γ-unsaturated aryl ketone, provides a useful synthetic handle for further functionalization via Kumada cross-coupling of the aryl trimethylammonium salt. Finally, the stereospecific alkylation of a chiral nonracemic secondary allylic carbonate affords the enantioenriched α-ternary aryl ketone, which was employed in a formal synthesis of trichostatic acid to illustrate that the allylic alkylation proceeds with net retention of configuration.

The Enzymology of Organic Transformations: A Survey of Name Reactions in Biological Systems

Chemical reactions that are named in honor of their true, or at least perceived, discoverers are known as "name reactions". This Review is a collection of biological representatives of named chemical reactions. Emphasis is placed on reaction types and catalytic mechanisms that showcase both the chemical diversity in natural product biosynthesis as well as the parallels with synthetic organic chemistry. An attempt has been made, whenever possible, to describe the enzymatic mechanisms of catalysis within the context of their synthetic counterparts and to discuss the mechanistic hypotheses for those reactions that are currently active areas of investigation. This Review has been categorized by reaction type, for example condensation, nucleophilic addition, reduction and oxidation, substitution, carboxylation, radical-mediated, and rearrangements, which are subdivided by name reactions.

Utilization of electron-donating α,β-unsaturated oximes: regioselective inverse 1,3-dipolar cycloaddition of nitrones

The cycloaddition of nitrones with α,β-unsaturated carbonyl compounds (enones) afforded predominantly 4-acylisoxazolidines, whereas the cycloaddition of the corresponding oximes afforded 5-iminoisoxazolidines. This inverse regioselection is due to HOMO activation by the oxime functionality.

Fe-Catalyzed C-C Bond Construction from Olefins via Radicals

This Article details the development of the iron-catalyzed conversion of olefins to radicals and their subsequent use in the construction of C–C bonds. Optimization of a reductive diene cyclization led to the development of an intermolecular cross-coupling of electronically-differentiated donor and acceptor olefins. Although the substitution on the donor olefins was initially limited to alkyl and aryl groups, additional efforts culminated in the expansion of the scope of the substitution to various heteroatom-based functionalities, providing a unified olefin reactivity. A vinyl sulfone acceptor olefin was developed, which allowed for the efficient synthesis of sulfone adducts that could be used as branch points for further diversification. Moreover, this reactivity was extended into an olefin-based Minisci reaction to functionalize heterocyclic scaffolds. Finally, mechanistic studies resulted in a more thorough understanding of the reaction, giving rise to the development of a more efficient second-generation set of olefin cross-coupling conditions.

Photoredox radical conjugate addition of dithiane-2-carboxylate promoted by an iridium(iii) phenyl-tetrazole complex: a formal radical methylation of Michael acceptors

A readily accessible iridium(iii) phenyl-tetrazole complex ([Ir(ptrz)2(tBu-bpy)]+, 2; Hptrz = 2-methyl-5-phenyl-tetrazole; tBu-bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) is shown to be a versatile catalyst for a new photocatalytic Michael reaction. Under light irradiation in the presence of 2, a dithiane 2-carboxylic acid, obtained by simple hydrolysis of a commercially available ethyl ester, generates a 1,3-dithiane radical capable of performing addition to a variety of Michael acceptors (e.g., unsaturated ketones, esters, amides and malonates). This broad scope reaction with high yields is a formal photo-redox addition of the elusive methyl radical and the adducts obtained can be starting materials for a variety of functionalized products. The excited-state oxidation potential of catalyst 2 allows selective formation of radicals only from α-heterosubstituted carboxylates. Chemical modification of this metal complex can tune the electrochemical properties, opening a route to new highly selective catalytic photo-oxidation reactions.

Stereodivergent Coupling of Aldehydes and Alkynes via Synergistic Catalysis Using Rh and Jacobsen's Amine

We report an enantioselective coupling between α-branched aldehydes and alkynes to generate vicinal quaternary and tertiary carbon stereocenters. The choice of Rh and organocatalyst combination allows for access to all possible stereoisomers with high enantio-, diastereo-, and regioselectivity. Our study highlights the power of catalysis to activate two common functional groups and provide access to divergent stereoisomers and constitutional structures.

Synthesis of functionalized imidazolidine-2-thiones via NHC/base-promoted aza-benzoin/aza-acetalization domino reactions

Benzylidene thioureas were generated in situ from α-sulfonylamines and used as novel umpolung acceptors in aza-benzoin/aza-acetalization domino reactions giving 5-hydroxy-imidazolidine-2-thiones.

Umpolung of protons from H2O: a metal-free chemoselective reduction of carbonyl compounds via B2pin2/H2O systems

H₂O is routinely described as a proton donor, however, in the presence of diboron compounds, the umpolung reaction of H₂O under metal-free conditions was successfully developed, which could afford hydride species, leading to a highly efficient and chemoselective reduction of CO bonds.

Direct catalytic enantioselective Mannich-type reaction of α,α-dithioacetonitriles with imines using chiral bis(imidazoline)–Pd complexes

The first highly enantioselective reaction of α,α-dithioacetonitriles with imines has been developed.

Decarboxylative Umpolung of conjugated enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde

Decarboxylative Umpolung converts enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde group with excellent regio- and diastereoselectivities.

Preparation of Aryne-Nickel Complexes from ortho-Borylaryl Triflates

A facile method for preparing diverse aryne-nickel complexes from readily synthesized ortho-borylaryl triflates is described. Exploratory synthetic applications, including the synthesis of 1,2-difunctionalized arenes, based on the nucleophilic character of the aryne-nickel complexes are also demonstrated.

Radicals: Reactive Intermediates with Translational Potential

This Perspective illustrates the defining characteristics of free radical chemistry, beginning with its rich and storied history. Studies from our laboratory are discussed along with recent developments emanating from others in this burgeoning area. The practicality and chemoselectivity of radical reactions enable rapid access to molecules of relevance to drug discovery, agrochemistry, material science, and other disciplines. Thus, these reactive intermediates possess inherent translational potential, as they can be widely used to expedite scientific endeavors for the betterment of humankind.