Synthetic and mechanistic studies on Pd(0)-catalyzed diamination of conjugated dienes

Synergistic Palladium/Copper-Catalyzed 1,4-Difunctionalization of 1,3-Dienes for Stereodivergent Construction of 1,5-Nonadjacent Stereocenters

The construction of two distal stereocenters through a single catalytic process is of great interest in organic synthesis. While there are some successful reports regarding stereodivergent preparation of 1,3- or 1,4-stereocenters, the more challenged 1,5-nonadjacent stereocenters have never been achieved in a stereodivergent fashion. Herein we describe a synergistic palladium/copper catalysis for 1,4-difunctionalization reactions of 1,3-dienes, providing access to 1,5-nonadjacent quaternary stereocenters. Because each of the two catalysts separately controlled one of the newly formed stereocenters, stereodivergent synthesis of all four diastereomers of the products could readily be achieved simply by choosing an appropriate combination of chiral catalysts. Experimental and computational studies supported a mechanism involving a Heck/Tsuji-Trost cascade reaction, and the origins of the stereoselectivity were elucidated.

Cobalt-Promoted Photoredox 1,2-Amidoamination of Alkenes with N-Sulfonamidopyridin-1-ium Salts and Free Amines

A cobalt-promoted photoredox 1,2-amidoamination of alkenes with N-sulfonamidopyridin-1-ium salts and free amines for the synthesis of unsymmetrical vicinal diamines has been developed. The reaction handles N-(sulfonamido)pyridin-1-ium salts as the sulfonamidyl radical precursors and free amines as the nucleophilic terminating reagents to enable the formation of two new C(sp3)-N bonds in a single reaction step and offers a route to selectively producing unsymmetrical vicinal diamines with an exquisite selectivity and a good compatibility of functional groups.

Visible-Light-Driven Multicomponent Diamination and Oxyamination of Alkene

Herein, we describe an effective method for the synthesis of 2-alkoxyamides and 1,2-diamines through visible-light-mediated difunctionalization of alkenes. N-Aminopyridinium salts were employed as appropriate precursors to generate key amidyl radical intermediates via a photoinduced single-electron transfer (SET) process. The amidyl radicals would react with alkenes, followed by oxidation and nucleophilic addition. Excellent functional group tolerance and good yields demonstrate the synthetic potential of this transformation.

Transition-Metal-Catalyzed 1,2-Diaminations of Olefins: Synthetic Methodologies and Mechanistic Studies

1,2-Diamines are synthetically important motifs in organo-catalysis, natural products, and drug research. Continuous utilization of transition-metal based catalyst in direct 1,2-diamination of olefines, in contrast to metal-free transformations, with numerous impressive advances made in recent years (2015-2023). This review summarized contemporary research on the transition-metal catalyzed/mediated [e. g., Cu(II), Pd(II), Fe(II), Rh(III), Ir(III), and Co(II)] 1,2-diamination (asymmetric and non-asymmetric) especially emphasizing the recent synthetic methodologies and mechanistic understandings. Moreover, up-to-date discussion on (i) paramount role of oxidant and catalyst (ii) key achievements (iii) generality and uniqueness, (iv) synthetic limitations or future challenges, and (v) future opportunities are summarized related to this potential area.

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis

The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp² moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Catalytic Reactions Directed by a Structurally Well-Defined Aminomethyl Cyclopalladated Complex

The introduction of N-containing moieties into feedstock molecules to build nitrogenated functional molecules has always been widely studied by the organic chemistry community. Progress in this field paves new roads to the synthesis of N-containing molecules, which are of significant importance in biological activities and play vital roles in pharmaceuticals and functional materials. Remarkable progress has been achieved in the field of transition metal-catalyzed C-N bond-forming reactions, typified by alkene hydroamination and the aza-Wacker reaction. However, the poisoning effect of electron-donating amine substrates on late transition metal catalysts presents a key impediment to these reactions, thus limiting the scope of amine substrates to electron-deficient amide derivatives. To address this problem, our group developed a palladium-aminomethyl complex with a three-membered palladacycle structure that allowed for the incorporation of electron-rich amine building blocks via C-C bond instead of C-N bond construction. This Account details the discovery of the well-defined aminomethyl cyclopalladated complex and recapitulates its applications for the catalysis of a series of aminomethylation reactions. We highlight how the understanding of the fundamental structural properties of the defined complex guided us toward tuning the reactivity of nucleophiles to initiate aminomethylation in different modes. Moreover, principles of designing and establishing further cascade reactions are also described.Aminomethyl cyclopalladated complexes can be prepared via the oxidative addition of aminals or N,O-acetals to Pd⁰ species. Thorough structural investigations by single-crystal X-ray diffraction analysis of the cyclopalladated complex suggest the presence of both aminomethylene-Pd^II (3-membered-ring) and Pd⁰-iminium (π-ligated) resonance forms, which indicates that both the palladium center and the methylene site are electrophilic. This is further verified by analysis of charge distribution. Two general types of reactions can be established, differing by the selective affinity of the nucleophiles to the two electrophilic positions, which is relevant to the "hardness suitability" of the nucleophiles with each electrophilic site. Softer nucleophiles such as alkenes prefer to attack the palladium center to initiate the reaction, mainly via migratory insertion into the Pd-C bond on the 3-membered ring with high strain. Through tandem β-hydride or reductive elimination, the Heck-type aminomethylation of styrenes, the aminomethylalkoxylation of electron-rich olefins, and even the aminomethylamination of allenes, dienes, enynes, and carbenoids with full atom-economy have been realized in line with this reaction mode. In contrast, harder nucleophiles tend to attack the harder electrophilic methylene site, leading to the aminomethylation of electron-deficient dienes. For secondary amines, a "C-N bond metathesis" process would be furnished through a reductive elimination, 1,3-proton transfer, and oxidative addition sequence. More intriguingly, when using appropriate "dinucleophile" substrates such as electron-rich amine-tethered dienes, sequential C-N bond metathesis and intramolecular insertion would occur to furnish Pd-catalyzed annulation reactions, which exhibits both the hard and soft nucleophile reactivities mentioned above. These transformations provide convenient methods for the preparation of N-containing molecules, such as amines, diamines, amino acetals, and multiple types of N-heterocycles.

Synthesis of N,O-Chelating Hydrazidopalladium Complexes from 1,2-Bis(trifluoroacetyl)hydrazine

N,O-chelating dicarbonylhydrazido-palladium complexes were synthesized by treatment of 1,2-bis(trifluoroacetyl)hydrazine with a mixture of a Pd(0) source, [Pd(dba)2] (DBA = dibenzylideneacetone), and four-electron donors including 1,3-bis(diphenylphosphino)propane (DPPP), N,N,N’,N’-tetramethylethylenediamine (TMEDA), and two equivalents of triphenylphosphine. The same products from DPPP and TMEDA could be obtained alternatively by using Pd(OAc)2 through deprotonation of the diacylhydrazine. The five-membered chelate structure was confirmed by NMR spectra and X-ray crystal structure determination. The X-ray structures indicate that the products are formally considered as Pd(II) complexes with a hydrazido(2–) ligand. In the case of the triphenylphosphine-coordinated complex, a fluxional behavior in dichloromethane-d2 was observed by variable temperature NMR experiments, possibly due to structural changes between the square planar and pseudo-tetrahedral geometries.

Regioselective Three-Component Synthesis of Vicinal Diamines via 1,2-Diamination of Styrenes

The vicinal diamine motif plays a significant role in natural products, drug design, and organic synthesis, and development of synthetic methods for the synthesis of diamines is a long-standing interest. Herein, we report a regioselective intermolecular three-component vicinal diamination of styrenes with acetonitrile and azodicarboxylates. The diamination products can be produced in moderate to excellent yields via the Ritter reaction. Synthetic applications and theoretical studies of this reaction have been conducted.

Access to Chiral Diamine Derivatives through Stereoselective Cu-Catalyzed Reductive Coupling of Imines and Allenamides

Chiral 1,2-diamino compounds are important building blocks in organic chemistry for biological applications and as asymmetric inducers in stereoselective synthesis that are challenging to prepare in a straightforward and stereoselective manner. Herein, we disclose a cost-effective and readily available Cu-catalyzed system for the reductive coupling of a chiral allenamide with N-alkyl substituted aldimines to access chiral 1,2-diamino synthons as single stereoisomers in high yields. The method shows broad reaction scope and high diastereoselectivity and can be easily scaled using standard Schlenk techniques. Mechanistic investigations by density functional theory calculations identified the mechanism and origin of stereoselectivity. In particular, the addition to the imine was shown to be reversible, which has implications toward development of catalyst-controlled stereoselective variants of the identified reductive coupling of imines and allenamides.

Palladium-Catalyzed [2 + 2 + 1] Annulation of Alkyne-Tethered Aryl Iodides with Diaziridinone: Synthesis of 3,4-Fused Tricyclic Indoles

A novel palladium-catalyzed [2 + 2 + 1] annulation of alkyne-tethered aryl iodides with diaziridinone was developed, leading to the formation of 3,4-fused tricyclic indoles. From a mechanistic standpoint, the formation of fused tricyclic indole scaffolds involved C,C-palladacycles, which were synthesized through the intramolecular reaction of aryl halides and alkynes. The cascade reaction described herein could be carried out with a broad range of substrates and provided various 3,4-fused tricyclic indoles with yields up to 98%.

Rhodium(iii)-catalyzed diamidation of olefins via amidorhodation and further amidation

Rh(iii)/Co(iii)-catalyzed diamidation of amide-tethered olefins has been realized using dioxazolones and arylsulfonamides as different classes of amidating reagents.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes

This review article provides an overview of progress in asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.

Cross-Coupling of Heteroatomic Electrophiles

At the advent of cross-coupling chemistry, carbon electrophiles based on halides or pseudohalides were the only suitable electrophilic coupling partners. Almost two decades passed before the first cross-coupling reaction of heteroatom-based electrophiles was reported. Early work by Murai and Tanaka initiated investigations into silicon electrophiles. Narasaka and Johnson pioneered the way in the use of nitrogen electrophiles, while Suginome began the exploration of boron electrophiles. The chemistry reviewed within provides perspective on the use of heteroatomic electrophiles, specifically silicon-, nitrogen-, boron-, oxygen-, and phosphorus-based electrophiles in transition-metal catalyzed cross-coupling. For the purposes of this review, a loose definition of cross-coupling is utilized; all reactions minimally proceed via an oxidative addition event. Although not cross-coupling in a traditional sense, we have also included catalyzed reactions that join a heteroatomic electrophile with an in situ generated nucleophile. However, for brevity, those involving hydroamination or C-H activation as a key step are largely excluded. This work includes primary references published up to and including October 2018.

Intermolecular alkene difunctionalizations for the synthesis of saturated heterocycles

Saturated heterocycles are important structural motifs in a range of pharmaceuticals and agrochemicals. As a result of their importance, syntheses of these molecules have been extensively investigated. Despite the progress in this area, the most adopted strategies are still often characterized with inefficiency or relying on functionalizations with specialized precursors and pre-existing cores. This review highlights a dynamic synthetic strategy for the direct synthesis of saturated heterocycles from intermolecular alkene difunctionalizations. These coupling processes are highly modular, and therefore, offer perhaps the most convenient means to prepare diverse heterocyclic structures in compound libraries for bioactivity evoluations.

Catalyst-free amination of α-cyanoarylacetates enabled by single-electron transfer

A radical benzylic amination of α-cyanoarylacetates without the assistance of any transition-metal catalysts or external photosensitizers was realized.

Mechanistic study of the solvent-controlled Pd(ii)-catalyzed chemoselective intermolecular 1,2-aminooxygenation and 1,2-oxyamination of conjugated dienes

The different coordination modes in MeCN and DMSO solvents could clarify the origin of chemoselectivity.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

Recent Advances in the Catalytic Synthesis of Imidazolidin-2-ones and Benzimidazolidin-2-ones

2-Imidazolidinone and its analogues are omnipresent structural motifs of pharmaceuticals, natural products, chiral auxiliaries, and intermediates in organic syntheses. Over the years, continuous efforts have been addressed to the development of sustainable and more efficient protocols for the synthesis of these heterocycles. This review gives a summary of the catalytic strategies to access imidazolidin-2-ones and benzimidazolidin-2-ones that have appeared in the literature from 2010 to 2018. Particularly important contributions beyond the timespan will be mentioned. The review is organized in four main chapters that identify the most common approaches to imidazolidin-2-one derivatives: (1) the direct incorporation of the carbonyl group into 1,2-diamines, (2) the diamination of olefins, (3) the intramolecular hydroamination of linear urea derivatives and (4) aziridine ring expansion. Methods not included in this classification will be addressed in the miscellaneous section.

Pd(ii)-Catalyzed aerobic 1,2-difunctionalization of conjugated dienes: efficient synthesis of morpholines and 2-morpholones

A novel and efficient methodology concerning the Pd(ii)-catalyzed intermolecular difunctionalization of conjugated dienes is reported to synthesize a series of functionalized morpholines and 2-morpholones. Widely distributed and easily obtained β-amino alcohols and α-amino acids, as starting nitrogen and oxygen sources, are successfully applied in the difunctionalization of conjugated dienes respectively. The majority of the desired products were obtained in moderate to excellent yields. Oxygen was successfully employed as a terminal oxidant. Further transformation of the generated products allowed for the expansion of structural diversity.

Copper-Catalyzed Ligand-Free Diazidation of Olefins with TMSN3 in CH3CN or in H2O

An environmentally benign, copper-catalyzed diazidation of a broad range of olefins, including vinylarenes, unactivated alkenes, allene, and dienes, under mild conditions with TMSN₃ (trimethylazidosilane) as azido source, has been developed. This reaction can be carried out in organic solvent or in aqueous solution where water is the sole solvent. The functional group compatibility of this reaction is good, which is proved by late-stage functionalizations of complex substrates.

Enantioselective Synthesis of 1,2-Diamines Containing Tertiary and Quaternary Centers through Rhodium-Catalyzed DYKAT of Racemic Allylic Trichloroacetimidates

The amination of racemic secondary and tertiary allylic trichloroacetimidates possessing β-nitrogen substituents and proximal nitrogen-containing heterocycles, via chiral diene-ligated rhodium-catalyzed dynamic kinetic asymmetric transformations (DYKAT), provides branched allylic 1,2-diamines with high enantioselectivity. The catalytic system can be applied to the synthesis of 1,2-diamines possessing two contiguous stereocenters with excellent diastereoselectivity. Furthermore, the nitrogen-containing heterocycles suppress competing vinyl azirdine formation, allowing for the high enantioselective syntheses of 1,2-diamines possessing tertiary and quaternary centers.

Palladium-catalyzed dearomatizing 2,5-alkoxyarylation of furan rings: diastereospecific access to spirooxindoles

A protocol for Pd-catalyzed intra- and intermolecular 2,5-alkoxyarylation reactions of furans to diastereospecifically synthesize two series of spirooxindoles is reported.

Diastereoselective synthesis of substituted diaziridines from simple ketones and aldehydes

Diastereopure substituted diaziridines from simple ketones, aldehydes and amines are here reported. These important chemical scaffolds are obtained in the presence of a weak inorganic base and hydroxylamine O-sulfonic acid (HOSA). This method introduces three stereocenters in one step to provide a wide variety of substituted diaziridines with high yields and diastereoselectivities.

Manganese-Mediated Coupling Reaction of Vinylarenes and Aliphatic Alcohols

Alcohols and alkenes are the most abundant and commonly used organic building blocks in the large-scale chemical synthesis. Herein, this is the first time to report a novel and operationally simple coupling reaction of vinylarenes and aliphatic alcohols catalyzed by manganese in the presence of TBHP (tert-butyl hydroperoxide). This coupling reaction provides the oxyalkylated products of vinylarenes with good regioselectivity and accomplishes with the principles of step-economies. A possible reaction mechanism has also been proposed.

A convenient access to β-iodo sulfone by the iodine-mediated iodosulfonylation of alkenes

A novel iodine-mediated intermolecular iodosulfonylation reaction of alkenes for dual C–S and C–I bond formation was achieved.