Umpolung Reaction Research Articles

Reactivity Umpolung of Tertiary Amide Enabled by Catalytic Reductive Stannylation

AbstractReactivity umpolung is an important concept in organic chemistry. Established reactivity umpolung mainly focuses on the aldehyde and umpolung of amide carbonyl group is not known. In this report, we describe a process to obtain the umpolung reactivity of tertiary amide. This process hinges on the efficient reductive stannylation catalyzed by Ir/silane and facile Sn−Li exchange. By leveraging this umpolung reactivity, drug Fluoxetine was derivatized to 12 different analogues via reacting with various electrophiles and four biologically active molecules were prepared concisely. This unlocked umpolung reactivity of tertiary amide is expected to find applications to synthesize complex amines from amides.

Solvent-Induced Umpolung Reaction from Dioxygenation to C-S Coupling in Bis(2-phenylquinoline) Iridium(III) Thiolate Complexes.

The construction of C-S bonds is of great importance in the field of synthetic and medicinal chemistry. Herein, solvent-induced umpolung reactions from dioxygenation to interligand C-S cross-coupling in bis(cyclometalated) Ir(III) thiolate complexes are reported in good to excellent yields at room temperature. Specifically, the reaction of rac-[Ir(pq)2(aet)] (where pq is 2-phenylquinoline and aet is 2-aminoethanethiolate) can be selectively switched to dioxygenation in acetonitrile solution in the presence of O2, resulting in the formation of a sulfinato complex rac-[Ir(pq)2(aes)] (where aes is 2-aminoethanesulfinato). Alternatively, the reaction in trifluoroethanol solution leads to interligand C-S cross-coupling, affording a rac-[Ir(pq)(pqaet)](PF6) [where pqaet is 2-((2-phenylquinolin-8-yl)thio)ethan-1-amine] complex, which generates a new tetradentate ligand in situ. Mechanistically, the formation of electrophilic metal thiyl radicals is proposed as a key intermediate in the interligand C-S coupling reaction. Furthermore, the sequential oxidation of a thioether complex into a sulfoxide complex is also observed at room temperature using H2O2 as an oxidant. Additionally, a new approach for the synthesis of a hexadentate ligand is developed through sequential C-S and C-N interligand coupling of metal thiolate complexes in situ under visible light irradiation in the presence of O2.

Pd-Catalyzed Chemodivergent Synthesis of 1-Indanones via Diethyl Zinc-Mediated Precise Switching of Umpolung Reactivity

Pd-Catalyzed Chemodivergent Synthesis of 1-Indanones via Diethyl Zinc-Mediated Precise Switching of Umpolung Reactivity

Organophosphorus-Catalyzed "Dual-Substrate Deoxygenation" Strategy for C-S Bond Formation from Sulfonyl Chlorides and Alcohols/Acids.

A green method to construct C-S bonds using sulfonyl chlorides and alcohols/acids via a PIII/PV═O catalytic system is reported. The organophosphorus-catalyzed umpolung reaction promotes us to propose the "dual-substrate deoxygenation" strategy. Herein, we adopt the "dual-substrate deoxygenation" strategy, which achieves the deoxygenation of sulfonyl chlorides and alcohols/acids to synthesize thioethers/thioesters driven by PIII/PV═O redox cycling. The catalytic method represents an operationally simple approach using stable phosphine oxide as a precatalyst and shows broad functional group tolerance. The potential application of this protocol is demonstrated by the late-stage diversification of drug analogues.

Aryl-, Akynyl-, and Alkenylbenziodoxoles: Synthesis and Synthetic Applications.

Hypervalent iodine reagents are in high current demand due to their exceptional reactivity in oxidative transformations, as well as in diverse umpolung functionalization reactions. Cyclic hypervalent iodine compounds, known under the general name of benziodoxoles, possess improved thermal stability and synthetic versatility in comparison with their acyclic analogs. Aryl-, alkenyl-, and alkynylbenziodoxoles have recently received wide synthetic applications as efficient reagents for direct arylation, alkenylation, and alkynylation under mild reaction conditions, including transition metal-free conditions as well as photoredox and transition metal catalysis. Using these reagents, a plethora of valuable, hard-to-reach, and structurally diverse complex products can be synthesized by convenient procedures. The review covers the main aspects of the chemistry of benziodoxole-based aryl-, alkynyl-, and alkenyl- transfer reagents, including preparation and synthetic applications.

Utilization of a Tris(carbene)borate Ligand for Umpolung Reactivity of a Nucleophilic Tin(II) Cation Salt.

We show that a tris(carbene)borate (TCB) ligand, namely [PhB(tBuIm)3]- ([PhB(tBuIm)3]- = phenyltris(3-tert-butylimidazol-2-ylidene)borato), is capable of stabilizing an unprecedented nucleophilic Sn(II) cation salt. Unlike known Sn(II) cations, the strong electron-donating ability of [PhB(tBuIm)3]- makes the cationic tin atom electron-rich, σ-donating yet slightly π-accepting, which allows for the ensuing facile oxidation with o-chloranil and S8 as well as coordination with coinage metals. The former oxidations give the Sn(IV) cation salts, while the latter reactions produce the metal complexes. The electronic structures of these species are thoroughly probed by quantum chemical computations. These results uncover an added role for TCB ligands in isolating unprecedented p-block species.

Organobase-Catalyzed Umpolung of Amides: The Generation and Transfer of Carbamoyl Anion

A novel organobase-catalyzed umpolung reaction of amides was disclosed. This method provides an efficient method to generate and transfer carbamoyl anions. In this transformation, some of the inherent disadvantages of carbamoyl metal were avoided. The mechanistic analysis revealed that the reaction proceeds through polarity inversion of amide, and various carbamoyl anions were applied in the reaction. Moreover, a wide range of substrates was achieved with moderate to excellent yield.

Gold-Catalyzed Cascade Reaction of 2-Alkynyl Aryl Azides with Enecarbamates for Direct Synthesis of α-(3-Indolyl)ketones

α-(3-Indolyl)ketones are essential building blocks for the generation of biologically active molecules. We described a new method for the direct assembly of α-(3-indolyl)ketones through the cascade reaction of 2-alkynyl aryl azides with enecarbamates, in which the in situ generated α-imino gold carbene intermediate was trapped by enecarbamate to achieve umpolung reactivity of indole at the 3-position.

Discovering the Site-Selective Umpolung of Ketones Triggered by Hypervalent Fluoro-Iodanes – Why Investigating Side Reactions Matters!

AbstractIn this account, we describe our journey leading to the discovery of a generally applicable umpolung method for the α-functionalization of ketones. Central to this reaction is the cyclic hypervalent fluoro-iodane, which is mostly known for various alkene functionalizations enabling, for example, the synthesis of fluoro-benzoxazepines, indoles, and ketones. During this work, we encountered α-functionalized ketones as minor side products. This observation prompted us to further investigate this reactivity, thus revealing a directed umpolung of pyridyl ketones by the fluoro-iodane. The key to the success was the unexpected non-covalent interaction between the nucleophile, substrate, and iodane.1 Introduction2 Cyclizations Triggered by the Fluorination of Styrenes3 Umpolung Reactions Facilitated by Hypervalent Iodanes4 Discovering and Evolving a Fluoro-Iodane-Triggered Regioselective α-Functionalization of Carbonyl Compounds5 First Investigations on the Nitrogen-Directed Umpolung6 Conclusion

Mechanistic Exploration of Umpolung Guided Stetter‐Aldol Reaction and Its Dependence on the Choice of Imine

AbstractNHC‐catalyzed Stetter‐aldol reaction for the synthesis of hydroxyindanone takes place via umpolung reactions of phthalaldehyde followed by intramolecular aldol reaction when N‐tert‐butoxycarbonyl (Boc) imine is used and this leads to the formation of [4+1] annulation product. However, this mechanism doesn't follow similar sequence in case of N‐tosyl‐α,β‐unsaturated ketimine and [4+2]‐annulation product is found as the sole product for the latter case through formation of aza‐“Berslow intermediate”. In this study we have used state of the art computational techniques to find out the possible reason for the difference in product and the detailed mechanistic pathway for the above two reactions. Thus, we explain the mechanism for obtaining the selective annulation product ([4+1] vs [4+2]) that can help chemists to design and synthesize a new series of compounds.

Visible-light induced cross-electrophile coupling of imines and anhydrides to synthesize α-amino ketones.

α-Amino ketones are important motifs in synthetic and medicinal chemistry. Efficient methods to directly access these motifs from feasible precursors are, however, limited. Herein, a visible-light mediated reductive cross-electrophile coupling of readily available imines and anhydrides was developed. Under mild reaction conditions, the umpolung reactivity of diverse imines engaged with anhydrides gives a variety of α-amino ketones with good yields and a broad functional group compatibility. Primary mechanistic studies revealed that this transformation might proceed through a radical-radical cross coupling pathway dominantly.

Practical synthesis and biological screening of sulfonyl hydrazides.

A methodology for the synthesis of sulfonyl hydrazides mediated by hypervalent iodine is described. Taking advantage of the umpolung properties of hypervalent iodine reagents, the polarity of sodium sulfinate salts is reversed, and a key intermediate is generated and reacted with mono- and disubstituted hydrazines. To highlight the practical utility of this protocol, a diverse range of sulfonyl hydrazides were synthesized in yields up to 62%. Furthermore, a gram-scale reaction was performed, showing the robustness of the procedure. Mechanistic studies, including DFT calculations, were performed and the bioactivity of the generated compounds was evaluated.

Reductive Umpolung and Defunctionalization Reactions through Higher-Order Titanium(III) Catalysis

AbstractThe single-electron transfer from an in situ formed titanium(III) catalyst to ketones, imines, nitriles, Michael acceptors, and many other functions has enabled a large number of intra- and intermolecular reductive umpolung reactions. Likewise, it allows the homolytic cleavage of functional groups for selective defunctionalizations. These reactions often take place with the participation of two titanium(III) species, avoiding free-radical pathways and enabling high catalyst control of the reaction selectivity. This account discusses the development of the individual reactions together with the fundamental mechanistic discoveries that led to a better understanding of such titanium(III)-catalyzed processes in general.1 Introduction2 Active Titanium(III) Species and Additives3 Ketone-Nitrile Couplings4 Further Reductive Umpolung Reactions5 Catalytic Homolytic C–CN and C–SO2R Cleavage6 Conclusion

Umpolung Reactivity of Imine Ester: Visible-Light Mediated Transfer Hydrogenation of α-Aryl Imino Esters by Phenylsilane and Water.

A visible-light mediated chemoselective transfer hydrogenation of α-aryl imino esters was demonstrated. The methodology allowed the efficient and practical preparation of α-amino acid esters. The mechanism of the reaction was probed by DFT calculations, and deuteration experiments indicated deuterium was introduced into amino acid esters efficiently (up to 99 % D ratio), enabling a feasible way to obtain deuterated amino acids using D2 O as a cheap deuterium source.

First example of organocatalysis by cathodic N-heterocyclic carbene generation and accumulation using a divided electrochemical flow cell.

In this paper we present the first electrochemical generation of NHC carried out in a divided flow cell. The flow cell operated in the recycle mode. The need for a divided cell derived from the anodic electroactivity of the electrogenerated carbene. In order to have NHC accumulation in the catholyte, the Nafion membrane (cell separator) was pretreated with an alkaline solution. The formation of NHC was quantified as its reaction product with elemental sulfur. The NHC was successfully used as organocatalyst in two classical umpolung reactions of cinnamaldehyde: its cyclodimerization and its oxidative esterification.

Exploring the Synergistic Relationship between Organic Synthesis and Electrochemistry

From a personal perspective, it is a joy to watch the larger organic synthesis community embrace and impressively develop new electrochemical methods. Those efforts are teaching us a great deal about not only electrochemical methods, but also how to best take advantage of the opportunities those methods offer. In our group, we continue to contribute to this larger enterprise by examining the role of direct electrochemical oxidation in the exploration of new oxidative cyclization strategies and the radical cation intermediates that are central to their success. These efforts have recently led to new umpolung reactions involving imine groups that are both setting the stage for introducing new levels of selectivity into the formation of non-natural amino acid building blocks and triggering new cascade types of reactions.Interestingly, the relationship between organic synthesis and electrochemistry can also be viewed in the opposite direction with the new synthetic chemistry being developed in order to facilitate exciting opportunities to expand the scope of electrochemical experiments. For example, new synthetic methods that allow for the site-selective generation of chemical reagents have enabled the construction of complex molecular surfaces on addressable microelectrode arrays. The efforts set the stage for using the microelectrode arrays as analytical devices for rapidly screening the binding behavior of small molecule libraries with biological targets. The method offers a previously unknown level of quality control over the molecular library, and it enables the binding events to be monitored in "real-time" as they happen. However, more complex reactions leading to more complex addressable surfaces also requires better methods for characterizing what has been made and analyzing the biological interactions that arise.Not surprisingly, the exploration of these new electrochemical tools is beginning to “give back” to the synthetic arena, and a number of preparative scale synthetic applications have now arisen based on the concept of site-selective reagent generation. For example, we have recently discovered that the strategy developed for confinement of a chemical reagent to a specific site on a microelectrode array can also be used to introduce new selectivity into solution phase chemical reactions. In the talk to be given, the interplay between organic synthesis and electrochemistry will be highlighted with an emphasis on how that interplay enables the development of new solutions to challenging synthetic problems.

A Guide to Low‐Valent Titanocene Complexes as Tunable Single‐Electron Transfer Catalysts for Applications in Organic Chemistry

AbstractLow‐valent titanocene catalysts are a versatile tool for organic synthesis. They promote inter‐ and intramolecular reactions ranging from homolytic bond cleavages to reductive umpolung reactions to additions and cyclizations in single electron steps. These reactions heavily depend on the redox potential of an in situ formed titanium(III) center, which can be adjusted by the choice of appropriate ligands. We herein review various chiral and achiral ligand‐modified titanocene catalysts and their reduction potentials Ep/2 obtained via cyclic voltammetry. The latter are found to correlate with the Hammett parameters σp of the cyclopentadienyl substituents and to the pKa values of the corresponding acids of the Ti−X ligands. For selected examples, we further discuss how the adjustment of the redox properties through modifications of the titanocene ligands can lead to greatly improved reaction outcomes in titanium(III) catalyzed single‐electron transfer reactions.

Photocatalytic fluoroalkylations of (hetero)arenes enabled by the acid-triggered reactivity umpolung of acetoxime esters

The importance of trifluoromethyl (CF 3 ) group-containing compounds in pharmaceuticals, agrochemicals, and materials has spurred tremendous efforts devoted to the development of methods for incorporation of the trifluoromethyl group into molecular frameworks and discoveries of trifluoromethylating reagents. In this paper, we report a general photocatalytic method for regioselective C–H fluoroalkylation of (hetero)arenes and olefines that uses readily available fluoroalkyl carboxylic anhydrides as fluoroalkylating reagents in the presence of simple acetoxime as an activator. The success achieved in the development of such a photocatalytic C–H fluoroalkylation method is attributed to discovery of the unprecedented acid-triggered reactivity umpolung of acetoxime ester toward single-electron reduction-induced cleavage of N–O σ bond. Such a new reactivity mode of oxime esters will stimulate efforts to expand the synthetic applications of readily available oxime esters. • The unprecedented acid-triggered reactivity umpolung of acetoxime ester • Photocatalytic method for C–H fluoroalkylation of (hetero)arenes and olefines • Inexpensive and easily available fluoroalkyl carboxylic anhydrides • Simple acetoxime as an efficient activator to fluoroalkyl carboxylic anhydrides Over the past decades, considerable efforts have been devoted toward exploring readily available trifluoroacetic acid or its anhydride as a CF 3 source for trifluoromethylation reactions. Herein, a general photocatalytic method was developed for fluoroalkylation of arenes and aromatic heterocycles with fluoroalkyl carboxylic anhydrides using simple acetoxime as the activator. The established conditions were also applicable to oxytrifluoromethylation and intramolecular carbotrifluoromethylation of alkenes, and the modification of biologically active molecules and gram-scale synthesis further highlighted its potential utility. Compared with the fact that single-electron transfer (SET) reduction of oxime trifluoroacetate esters generates preferentially iminyl radical and stable trifluoroacetate anions, our work discovered the unprecedented acid-triggered reactivity umpolung of acetoxime ester toward delivering a CF 3 radical. An ongoing challenge in trifluoromethylation reaction is the use of less expensive and more practical trifluoromethyl sources. We describe herein a general photocatalytic method for regioselective C–H fluoroalkylation of (hetero)arenes and olefines that uses readily available fluoroalkyl carboxylic anhydrides as fluoroalkylating reagents in the presence of simple acetoxime as an activator. The success achieved in the development of such a method is attributed to the discovery of the unprecedented acid-triggered reactivity umpolung of acetoxime ester toward single-electron reduction-induced cleavage of N–O σ bond.

Organocatalyzed Cross-Nucleophile Couplings: Umpolung of Catalytic Enamines.

The concept of umpolung, or polarity reversal, introduced by Seebach and Corey nearly half a century ago, ushered a new paradigm into synthetic chemistry. Novel connections were able to be forged among functional groups that were typically inaccessible. Conceptually, an umpolung reaction is identified only upon retrosynthetic analysis. Stoichiometric examples have served as a platform to develop and refine elegant methodologies into catalytic processes. The advent of these unconventional arrangements of canonical synthons into new points of diversity has expanded the repertoire of the synthetic toolbox. Within this context, asymmetric organocatalyzed methodologies remain rare, and there are even fewer aminocatalyzed variants.Recent years have witnessed a renaissance in α-functionalizations of aldehydes, specifically in the context of oxidative umpolung strategies. Unlike previous open-shell approaches, application of a quinone-based oxidant in conjunction with an aminocatalyst leads to a discrete, substitutionally labile quinone adduct. These have proven to be valuable building blocks toward polar reactivity─auguring the advent of new avenues to construct tetrasubstituted tertiary stereocenters through the application of conventional nucleophiles to form C-C, C-N, C-O, and C-S bonds through an organocatalyzed cross-nucleophile coupling (organo-CNC) reaction. The resulting nonepimerizable stereocenter demonstrates high optical fidelity and provides a significant advancement in many applications that suffer from racemization, such as in vivo studies.This strategy harnesses a trifunctional aminocatalyst to promote an unusual SN2 reaction at a highly congested center. The selection of the quinone oxidant and nucleophile converges to a continuum of reactivity ranging from enantioselective oxidation to stereoselective substitution. A remarkable aspect of these developments is the identification of an asymmetric SN2 dynamic kinetic resolution (SN2-DKR) manifold. These organo-CNC reactions are highly modular and demonstrate complete stereocontrol from the catalyst with minimal influence from incoming chiral nucleophiles. Leveraging this facet, these technologies have been extended to peptidic bioconjugations bearing bio-orthogonoal linker molecules.This Account aims to highlight the progress, from an internal perspective, toward directing the initial result into established methodologies. Within this construct, the underlying principles of each reaction will be disseminated with specific content on inherent challenges and opportunity. Combined, these will serve as an instructive tool to stimulate applications in cross-disciplinary interfaces.

Reactivity Umpolung of the C═N Bond in Quinoxaline Scaffold Enabling Direct Nucleophilic Attack of Alkyl Grignard Reagents at the N-Terminus.

The reactivity umpolung of the C═N bond in the quinoxaline scaffold has been successfully realized for the first time by introduction of a formyl or an acyl group adjacent to the C-position of the C═N moiety. The reversed reactivity of the C═N bond thus enabled direct nucleophilic attack of alkyl Grignard reagents at the N-terminus rather than the C-terminus, thereby providing an unprecedented and efficient method for the synthesis of quinoxalin-2(1H)-one derivatives involving a tandem N-alkylation/C─C bond cleavage process.