Trimethylsilyl-Protected Alkynes as Selective Cross-Coupling Partners in Titanium-Catalyzed [2+2+1] Pyrrole Synthesis

Transition-Metal-Catalyzed Transformations for the Synthesis of Marine Drugs

Transition metal catalysis has contributed to the discovery of novel methodologies and the preparation of natural products, as well as new chances to increase the chemical space in drug discovery programs. In the case of marine drugs, this strategy has been used to achieve selective, sustainable and efficient transformations, which cannot be obtained otherwise. In this perspective, we aim to showcase how a variety of transition metals have provided fruitful couplings in a wide variety of marine drug-like scaffolds over the past few years, by accelerating the production of these valuable molecules.

Elevating pyrrole derivative synthesis: a three-component revolution

Pyrrole is an essential chemical with considerable relevance as a pharmaceutical framework for many biologically necessary medications. The growing demand for biologically active compounds calls for a simple one-pot method for generating novel pyrrole derivatives. Nots surprisingly, several multicomponent reactions (MCRs) aim to synthesize pyrrole derivatives. However, this review presents the three-component synthesis of pyrrole derivatives, highlighting the significance of multicomponent reaction in synthesizing eclectic multi-functionalised pyrrole covering the selected literature on the three-component synthesis of substituted pyrrole from 2016 to late 2023. Furthermore, this article classifies the reactions based on the starting material with functional groups involved in the pyrrole ring formation.

Progress on synthesis and structure-activity relationships of lamellarins over the past decade

Lamellarins are polyaromatic alkaloids isolated from marine organisms, including mollusks, tunicates, and sponges. Currently, over 60 structurally distinct natural lamellarins have been reported, and most of them exhibit promising biological activities, such as topoisomerase inhibition, mitochondrial function inhibition, multidrug resistance reversing, and anti-HIV activity. There has also been a significant progress on the synthetic study of lamellarins which has been regularly updated by numerous medicinal chemists as well. This review provides a detailed summary of the synthesis, pharmacology, and structural modification of lamellarins over the past decades.

Evaluation of Tungsten Catalysis among Early Transition Metals for N-Aryl-2,3,4,5-tetraarylpyrrole Synthesis: Modular Access to N-Doped π-Conjugated Material Precursors

Low-valent tungsten species generated from WCl6 and N,N'-bis(trimethylsilyl)-2,5-dimethyldihydropyrazine (Si-Me2-DHP) promotes the catalytic formation of N-phenyl-2,3,4,5-tetraarylpyrroles 3aa-ka from diarylacetylenes 1a-k and azobenzene (2a). An initial catalyst activation process is a three-electron reduction of WCl6 with Si-Me2-DHP to afford transient 'WCl3' species. Catalytically active bis(imido)tungsten(VI) species via successive one-electron reduction and N═N bond cleavage of 2a was revealed by isolating W(═NPh)2Cl2(PMe2Ph)2 from imidotungsten(V) trichloride and 2a in the presence of PMe2Ph. The superior catalytic activity of the tungsten catalyst was clarified by a density functional theory study: activation energies for the key three steps, [2 + 2]-cycloaddition of W═NPh and diarylacetylene to form (iminoalkylidene)tungsten species, enyne metathesis with second diarylacetylene, and C-N bond formation, are reasonable values for the catalytic reaction at 180 °C. In addition, this tungsten catalyst overcame two distinct deactivation processes: α-enediamido formation and aggregation of the low-valent species, both of which were observed for previously developed vanadium and titanium catalysts. We also demonstrated the synthetic utility of pentaarylpyrroles 3aa and 3ba as well as N-(2-bromophenyl)-2,3,4,5-tetraarylpyrrole 3ab by derivatizing their π-conjugated compounds 9aa, 10ba, and 11ab.

Molecular Editing of Pyrroles via a Skeletal Recasting Strategy

Heterocyclic scaffolds are commonly found in numerous biologically active molecules, therapeutic agents, and agrochemicals. To probe chemical space around heterocycles, many powerful molecular editing strategies have been devised. Versatile C-H functionalization strategies allow for peripheral modifications of heterocyclic motifs, often being specific and taking place at multiple sites. The past few years have seen the quick emergence of exciting "single-atom skeletal editing" strategies, through one-atom deletion or addition, enabling ring contraction/expansion and structural diversification, as well as scaffold hopping. The construction of heterocycles via deconstruction of simple heterocycles is unknown. Herein, we disclose a new molecular editing method which we name the skeletal recasting strategy. Specifically, by tapping on the 1,3-dipolar property of azoalkenes, we recast simple pyrroles to fully substituted pyrroles, through a simple phosphoric acid-promoted one-pot reaction consisting of dearomative deconstruction and rearomative reconstruction steps. The reaction allows for easy access to synthetically challenging tetra-substituted pyrroles which are otherwise difficult to synthesize. Furthermore, we construct N-N axial chirality on our pyrrole products, as well as accomplish a facile synthesis of the anticancer drug, Sutent. The potential application of this method to other heterocycles has also been demonstrated.

Synthesis of Ti Complexes Supported by an ortho-terphenoxide Ligand and their Applications in Alkyne Hydroamination Catalysis

The synthesis of a series of Ti complexes of an aryl-linked bis-phenoxide ligand, 3,3''-di-tert-butyl-5,5''-dimethyl-[1,1':2',1''-terphenyl]-2,2''-bis(olate), (TPO)H2, is reported. This ortho-linked terphenyl ligand builds on previously reported meta- and para- linked terphenyl based ligands, completing the isomeric series of terphenoxide ligands. The 4-coordinate (TPO)Ti(NMe2)2 is an active catalyst for alkyne hydroamination with a variety of arylamines, revealing good regioselectivity in reactions with unsymmetric alkynes. Terminal alkynes such as phenylacetylene undergo additional insertion reactions with the key azatitanacyclobutene intermediates, providing further evidence that Ti aryloxide complexes are susceptible to this further reactivity.

Fused diethynylbenzenes and phenanthrenes via arynes with alkynylsilanes

A method for the hydroalkynylation and catalytic cyclization reactions of hexadehydro-Diels-Alder-derived benzynes is described. Diethynylbenzene derivatives are generated in a one-step reaction via trimethylsilyl-alkyne groups with benzyne formed by heating the appropriate tetrayne substrate. Trimethyl(phenylethynyl)silane loses TMS and binds to the electron-deficient site on HDDA-derived benzynes, and then phenanthrene was synthesized under mild reaction conditions by transition-metal-free, base promoted intramolecular cyclization.

Regiocontrolled halogen dance of 2,5-dibromopyrroles using equilibrium between dibromopyrrolyllithiums

A regiocontrolled halogen dance reaction of 2,5-dibromopyrroles is described. An N,N-dimethylsulfamoyl group on the pyrrole nitrogen was especially effective for facilitating interconversion of the resulting 2,3- and 2,4-dibromopyrrolyllithiums, rendering the smooth halogen dance reaction. This method was applicable to the formal synthesis of atorvastatin.

Expanding the series of alkali metal plumbolyl complexes to Na and K

Herein we detail the straightforward and scalable synthesis of sodium and potassium complexes of the 2,5-bis(tert-butyldimethylsilyl)-3,4-diphenylplumbolyl dianion (Pbl^TBS,Ph). Their solid-state structures were found to comprise either monomeric solvates or coordination polymers depending on the alkali metal ion and crystallization medium. These complexes were readily prepared with high yields and purity compared to known routes to the dilithium congener of Pbl^TBS,Ph and are well-positioned to serve as convenient precursors for salt metathesis-type reactions leading to metal complexes of the understudied Pbl^TBS,Ph ligand.

Synthesis of Pyrrole via Formal Cycloaddition of Allyl Ketone and Amine under Metal-Free Conditions

A new metal-free synthesis of pyrrole from allyl ketone and amine has been established. The reaction proceeds via an thiolative activation of the C-C double bond with dimethyl(methylthio)sulfonium trifluoromethanesulfonate, followed by a nucleophilic ring-opening addition of primary amine to the generated episulfonium intermediate, and then an internal condensation and aromatization. This mild procedure provides a novel strategy to the construction of substituted pyrroles through a formal [4 + 1] cycloaddition reaction.

Selective propargylic C(sp3)-H activation of methyl-substituted alkynes versus [2 + 2] cycloaddition at a titanium imido template

The reaction of the titanium imido complex 1b with 2-butyne leads to the formation of the titanium azadiene complex 2a at ambient temperature instead of yielding the archetypical [2 + 2] cycloaddition product (titanaazacyclobutene) which is usually obtained by combining titanium imido complexes and internal alkynes. The formation of 2a is presumably caused by an initial propargylic C(sp³)–H activation step and quantum chemical calculations suggest that the outcome of this unexpected reactivity is thermodynamically favored. The previously reported titanaazacyclobutene I (which is obtained by reacting 1b with 1-phenyl-1-propyne) undergoes a rearrangement reaction at elevated temperature to give the corresponding five-membered titanium azadiene complex 2b.

An unexpected reactivity between a titanium imido complex and internal alkynes was unveiled yielding titanaazacyclobutenes instead of the expected [2 + 2] cycloaddition products.

Redox chemistry of discrete low-valent titanium complexes and low-valent titanium synthons

Titanium is a versatile metal that has important applications in practical synthesis, though this is typically limited to stoichiometric reactions or Lewis acid catalysis. Recently, interest has grown in using titanium and other early-metals for redox catalysis; however, notable limitations exist due to the thermodynamic preference of these metals to adopt high oxidation states. Nonetheless, discrete low-valent titanium (LVT) complexes and their synthons (titanium complexes which chemically behave as LVT sources) are known. Here, we detail the various ligand platforms that are capable of stabilizing LVT compounds and present the redox chemistry of these systems. This includes a discussion of recent developments in the use of LVT synthons for accessing fully reversible oxidative-addition/reductive-elimination reactions.

Marine Pyrrole Alkaloids

Nitrogen heterocycles are essential parts of the chemical machinery of life and often reveal intriguing structures. They are not only widespread in terrestrial habitats but can also frequently be found as natural products in the marine environment. This review highlights the important class of marine pyrrole alkaloids, well-known for their diverse biological activities. A broad overview of the marine pyrrole alkaloids with a focus on their isolation, biological activities, chemical synthesis, and derivatization covering the decade from 2010 to 2020 is provided. With relevant structural subclasses categorized, this review shall provide a clear and timely synopsis of this area.

Ti-Catalyzed and -Mediated Oxidative Amination Reactions

Titanium is an attractive metal for catalytic reaction development: it is earth-abundant, inexpensive, and generally nontoxic. However-like most early transition metals-catalytic redox reactions with Ti are difficult because of the stability of the high-valent Ti^IV state. Understanding the fundamental mechanisms behind Ti redox processes is key for making progress toward potential catalytic applications. This Account details recent progress in Ti-catalyzed (and -mediated) oxidative amination reactions that proceed through formally Ti^II/Ti^IV catalytic cycles.This class of reactions is built on our initial discovery of Ti-catalyzed [2 + 2 + 1] pyrrole synthesis from alkynes and azobenzene, where detailed mechanistic studies have revealed important factors that allow for catalytic turnover despite the inherent difficulty of Ti redox. Two important conclusions from mechanistic studies are that (1) low-valent Ti intermediates in catalysis can be stabilized through coordination of π-acceptor substrates or products, where they can act as "redox-noninnocent" ligands through metal-to-ligand π back-donation, and (2) reductive elimination processes with Ti proceed through π-type electrocyclic (or pericyclic) reaction mechanisms rather than direct σ-bond coupling.The key reactive species in Ti-catalyzed oxidative amination reactions are Ti imidos (Ti≡NR), which can be generated from either aryl diazenes (RN═NR) or organic azides (RN₃). These Ti imidos can then undergo [2 + 2] cycloadditions with alkynes, resulting in intermediates that can be coupled to an array of other unsaturated functional groups, including alkynes, alkenes, nitriles, and nitrosos. This basic reactivity pattern has been extended into a broad range of catalytic and stoichiometric oxidative multicomponent coupling reactions of alkynes and other reactive small molecules, leading to multicomponent syntheses of various heterocycles and aminated building blocks.For example, catalytic oxidative coupling of Ti imidos with two different alkynes leads to pyrroles, while stoichiometric oxidative coupling with alkynes and nitriles leads to pyrazoles. These heterocycle syntheses often yield substitution patterns that are complementary to those of classical condensation routes and provide access to new electron-rich, highly substituted heteroaromatic scaffolds. Furthermore, catalytic oxidative alkyne carboamination reactions can be accomplished via reaction of Ti imidos with alkynes and alkenes, yielding α,β-unsaturated imine or cyclopropylimine building blocks. New catalytic and stoichiometric oxidative amination methods such as alkyne α-diimination, isocyanide imination, and ring-opening oxidative amination of strained alkenes are continuously emerging as a result of better mechanistic understanding of Ti redox catalysis.Ultimately, these Ti-catalyzed and -mediated oxidative amination methods demonstrate the importance of examining often-overlooked elements like the early transition metals through the lens of modern catalysis: rather than a lack of utility, these elements frequently have undiscovered potential for new transformations with orthogonal or complementary selectivity to their late transition metal counterparts.

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4-8 transition metal catalysts

In this Perspective, we discuss recent syntheses of 5- and 6-membered aromatic heterocycles via multicomponent reactions (MCRs) that are catalyzed by group 4-8 transition metals. These MCRs can be categorized based on the substrate components used to generate the cyclized product, as well as on common mechanistic features between the catalyst systems. These particular groupings are intended to highlight mechanistic and strategic similarities between otherwise disparate transition metals and to encourage future work exploring related systems with otherwise-overlooked elements. Importantly, in many cases these early- to mid-transition metal catalysts have been shown to be as effective for heterocycle syntheses as the later (and more commonly implemented) group 9-11 metals.

Base-Promoted Three-Component Cascade Reaction of α-Hydroxy Ketones, Malonodinitrile, and Alcohols: Direct Access to Tetrasubstituted NH-Pyrroles

A base-promoted three-component cascade reaction of α-hydroxy ketones, malonodinitrile, and alcohols has been developed, providing a direct and efficient route to a range of structurally diverse and synthetically useful 2-alkyloxy-1H-pyrrole-3-carbonitrile derivatives. The reaction involved three different bond (C-C, C-O, and C-N) formations in a single step, and its regioselectivity was depended on the structure of the α-hydroxy ketones employed. The use of easily available starting materials, wide substrate scope, good functional group tolerance, operational simplicity, and high atom economy are attractive features of the new method.

AgI -Catalyzed Reaction of Enol Diazoacetates and Imino Ethers: Synthesis of Highly Functionalized Pyrroles

An unprecedented Ag^I -catalyzed efficient method for the coupling of imino ethers and enol diazoacetates through a [3+2]-cycloaddition/C-O bond cleavage/[1,5]-proton transfer cascade process is reported. The general class of imino ethers that includes oxazolines, benzoxazoles and benzimidates are applicable substrates for these reactions that provide direct access to fully substituted pyrroles with uniformly high chemo- and regioselectivity. High variability in substitution at the pyrrole 2-, 5- and N-positions characterizes this methodology that also presents an entry point for further pyrrole diversification via facile modification of resulting N-functional pyrroles.

Iron-Catalyzed Vinylsilane Dimerization and Cross-Cycloadditions with 1,3-Dienes: Probing the Origins of Chemo- and Regioselectivity

The selective, intermolecular, homodimerization and cross-cycloaddition of vinylsilanes with unbiased 1,3-dienes, catalyzed by a pyridine-2,6-diimine (PDI) iron complex is described. In the absence of a diene coupling partner, vinylsilane hydroalkenylation products were obtained chemoselectively with unusual head-to-head regioselectivity (up to >98% purity, 98:2 E/Z). In the presence of a 4- or 2-substituted diene coupling partner, under otherwise identical reaction conditions, formation of value-added [2+2]- and [4+2]-cycloadducts, respectively, was observed. The chemoselectivity profile was distinct from that observed for analogous α-olefin dimerization and cross-reactions with 1,3-dienes. Mechanistic studies conducted with well-defined, single-component precatalysts (MePDI)Fe(L2) (where MePDI = 2,6-(2,6-Me2-C6H3N═CMe)2C5H3N; L2 = butadiene or 2(N2)) provided insights into the kinetic and thermodynamic factors contributing to the substrate-controlled regioselectivity for both the homodimerization and cross cycloadditions. Diamagnetic iron diene and paramagnetic iron olefin complexes were identified as catalyst resting states, were characterized by in situ NMR and Mössbauer spectroscopic studies, and were corroborated with DFT calculations. Stoichiometric reactions and computational models provided evidence for a common mechanistic regime where competing steric and orbital-symmetry requirements dictate the regioselectivity of oxidative cyclization. Although distinct chemoselectivity profiles were observed in cross-cycloadditions with the vinylsilane congeners of α-olefins, these products arose from metallacycles with the same connectivity. The silyl substituents ultimately governed the relative rates of β-H elimination and C-C reductive elimination to dictate final product formation.

Ti(OiPr)4-Facilitated Formal Deoxygenative Annulation of Alkynyl 1,2-Diketones for the Synthesis of Highly Functionalized Furans

A unique deoxygenative cyclodimerization of alkynyl 1,2-diketones facilitated by Ti(OⁱPr)₄ is achieved, affording a series of highly functionalized furan products. An unusual C-C bond and C═O bond cleavage of the substrates is observed, and Ti(OⁱPr)₄ plays triplicate roles in the reaction. Furthermore, the products show uncommon fluorescent emission in the solid state, indicating the potential practical applications of this work.

Cu(i)- and Au(i)-catalyzed regioselective oxidation of diynes: divergent synthesis of N-heterocycles

The efficient and divergent construction of two types of valuable N-heterocycle is achieved easily, with the first example of the generation of α-oxo copper carbenes via copper-catalyzed oxidation of non-polarized alkynes.

A Mononuclear and High-Spin Tetrahedral TiII Complex

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me- = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti═E(Cl)] (with E2- = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

Synthesis of pentasubstituted 2-aryl pyrroles from boryl and stannyl alkynes via one-pot sequential Ti-catalyzed [2 + 2 + 1] pyrrole synthesis/cross coupling reactions

Multisubstituted pyrroles are commonly found in many bioactive small molecule scaffolds, yet the synthesis of highly-substituted pyrrole cores remains challenging. Herein, we report an efficient catalytic synthesis of 2-heteroatom-substituted (9-BBN or SnR3) pyrroles via Ti-catalyzed [2 + 2 + 1] heterocoupling of heteroatom-substituted alkynes. In particular, the 9-BBN-alkyne coupling reactions were found to be very sensitive to Lewis basic ligands in the reaction: exchange of pyridine ligands from Ti to B inhibited catalysis, as evidenced by in situ 11B NMR studies. The resulting 2-boryl substituted pyrroles can then be used in Suzuki reactions in a one-pot sequential fashion, resulting in pentasubstituted 2-aryl pyrroles that are inaccessible via previous [2 + 2 + 1] heterocoupling strategies. This reaction provides a complementary approach to previous [2 + 2 + 1] heterocouplings of TMS-substituted alkynes, which could be further functionalized via electrophilic aromatic substitution.

Ti-catalyzed ring-opening oxidative amination of methylenecyclopropanes with diazenes

The ring-opening oxidative amination of methylenecyclopropanes (MCPs) with diazenes catalyzed by py3TiCl2(NR) complexes is reported. This reaction selectively generates branched α-methylene imines as opposed to linear α,β-unsaturated imines, which are difficult to access via other methods. Products can be isolated as the imine or hydrolyzed to the corresponding ketone in good yields. Mechanistic investigation via density functional theory suggests that the regioselectivity of these products results from a Curtin-Hammett kinetic scenario, where reversible β-carbon elimination of a spirocyclic [2 + 2] azatitanacyclobutene intermediate is followed by selectivity-determining β-hydrogen elimination of the resulting metallacycle. Further functionalizations of these branched α-methylene imine products are explored, demonstrating their utility as building blocks.

Synthesis of Pyrroles through the CuH-Catalyzed Coupling of Enynes and Nitriles

Herein, we describe an efficient method to prepare polysubstituted pyrroles via a copper hydride (CuH)-catalyzed enyne-nitrile coupling reaction. This protocol accommodates both aromatic and aliphatic substituents and a broad range of functional groups, providing a variety of N-H pyrroles in good yields and with high regioselectivity. We propose that the Cu-based catalyst promotes both the initial reductive coupling and subsequent cyclization steps. Density functional theory (DFT) calculations were performed to elucidate the reaction mechanism.

Regiodivergent Synthesis of Penta-Substituted Pyrroles through a Cascade [3 + 2] Cyclization of C-Acylimines with Activated Alkynes and Aromatic Nucleophiles

Highly robust one-pot, four-component cascade cyclization reaction of α-keto aldehydes, anilines, activated alkynes, and aromatic nucleophiles is developed to synthesize a diverse range of pharmaceutically important penta-substituted pyrroles. The reaction proceeds through the cascade cyclization of acylimines (in situ formed) with activated alkynes and aromatic nucleophiles such as indoles, pyrroles, and naphthols at room temperature under calcium(II) catalysis with high yields and broad substrate diversity.

Reactivity of terminal imido complexes of group 4-6 metals: stoichiometric and catalytic reactions involving cycloaddition with unsaturated organic molecules

Imido complexes of early transition metals are key intermediates in the synthesis of many nitrogen-containing organic compounds. The metal-nitrogen double bond of the imido moiety undergoes [2+2] cycloaddition reactions with various unsaturated organic molecules to form new nitrogen-carbon and nitrogen-heteroatom bonds. This review article focuses on reactivity of the terminal imido complexes of Group 4-6 metals, summarizing their stoichiometric reactions and catalytic applications for a variety of reactions including alkyne hydroamination, alkyne carboamination, pyrrole formation, imine metathesis, and condensation reactions of carbonyl compounds with isocyanates.

Multicomponent Pyrazole Synthesis from Alkynes, Nitriles, and Titanium Imido Complexes via Oxidatively Induced N-N Bond Coupling

Pyrazoles are an important class of heterocycles found in a wide range of bioactive compounds and pharmaceuticals. Pyrazole synthesis often requires hydrazine or related reagents where an intact N-N bond is conservatively installed into a pyrazole precursor fragment. Herein, we report the multicomponent oxidative coupling of alkynes, nitriles, and Ti imido complexes for the synthesis of multisubstituted pyrazoles. This modular method avoids potentially hazardous reagents like hydrazine, instead forming the N-N bond in the final step via oxidation-induced coupling on Ti. The mechanism of this transformation has been studied in-depth through stoichiometric reactions of the key diazatitanacyclohexadiene intermediate, which can be accessed via multicomponent coupling of Ti imidos with nitriles and alkynes, ring opening of 2-imino-2H-azirines, or direct metalation of 4-azadiene-1-amine derivatives. The critical transformation in this reaction is the 2-electron oxidation-induced N-N coupling on Ti. This is a rare example of formal N-N coupling on a metal center, which likely occurs through an electrocyclic mechanism analogous to a Nazarov cyclization. Conveniently, these 2-electron-oxidized diazatitanacyclohexadiene intermediates can be accessed via disproportionation of the 1-electron-oxidized species, which allows utilization of weak oxidants such as TEMPO.

Reversible oxidative-addition and reductive-elimination of thiophene from a titanium complex and its thermally-induced hydrodesulphurization chemistry

The masked Ti(ii) synthon (Ketguan)(η6-ImDippN)Ti (1) oxidatively adds across thiophene to give ring-opened (Ketguan)(ImDippN)Ti[κ2-S(CH)3CH] (2). Complex 2 is photosensitive, and upon exposure to light, reductively eliminates thiophene to regenerate 1 - a rare example of early-metal mediated oxidative-addition/reductive-elimination chemistry. DFT calculations indicate strong titanium π-backdonation to the thiophene π*-orbitals leads to the observed thiophene ring opening across titanium, while a proposed photoinduced LMCT promotes the reverse thiophene elimination from 2. Finally, pressurizing solutions of 2 with H2 (150 psi) at 80 °C leads to the hydrodesulphurization of thiophene to give the Ti(iv) sulphide (Ketguan)(ImDippN)Ti(S) (3) and butane.

Lamellarin alkaloids: Isolation, synthesis, and biological activity

Lamellarins are marine alkaloids containing fused 14-phenyl-6H-[1]benzopyrano[4',3':4,5]pyrrolo[2,1-a]isoquinoline or non-fused 3,4-diarylpyrrole-2-carboxylate ring systems. To date, more than 50 lamellarins have been isolated from a variety of marine organisms, such as mollusks, tunicates, and sponges. Many of them, especially fused type I lamellarins, exhibit impressive biological activity, such as potent cytotoxicity, topoisomerase I inhibition, protein kinases inhibition, and anti-HIV-1 activity. Due to their useful biological activity and limited availability from natural sources, a number of synthetic methods have been developed. In this chapter, we present an updated and comprehensive review on lamellarin alkaloids summarizing their isolation, synthesis, and biological activity.

Synthesis of 1,2,4-azadiphosphole derivatives based on vanadium-catalyzed [2+2+1] cycloaddition reactions of azobenzenes with phosphaalkynes

A new synthetic method is described to construct 1,2,4-azadiphosphole derivatives based on vanadium-catalyzed [2+2+1] cycloaddition reactions. Reactions of azobenzenes as nitrogen sources with phosphaalkynes as phosphorous counterparts in the presence of VCl₂(thf)₂ as a catalyst afford the corresponding 1,2,4-azadiphospholes.

Vanadium-catalyzed [2+2+1] cycloaddition reactions opened a new access to phosphorous-heterocycles.

Retracted Article: Metal-free [2+2+1] cycloaddition polymerization of alkynes, nitriles, and oxygen atoms to functional polyoxazoles

The metal-free [2+2+1] cycloaddition polymerization of alkynes, nitriles, and O-atoms for the regioselective assembly of highly substituted oxazole compounds has been achieved by the use of iodosobenzene (PhIO) with trifluoromethanesulfonic acid (TfOH). The present reaction could be applied to a facile synthesis of polyoxazoles. In this work, the cycloaddition polymerization of 4-cyano-4′-ethynylbiphenyl and PhIO was developed and modified polyoxazole was prepared. All experimental conditions such as polymerization solvent, temperature, catalyst and time were systematically studied. The structure of the obtained polyoxazole was characterized by GPC and NMR, and its thermal properties were studied by TGA. In addition, the good thermal stability of polyoxazoles with unreacted terminal alkynes and cyano groups makes them potentially useful for modifying resins.

The metal-free [2+2+1] cycloaddition polymerization of alkynes, nitriles, and O-atoms for the regioselective assembly of highly substituted oxazole compounds has been achieved by the use of iodosobenzene (PhIO) with trifluoromethanesulfonic acid (TfOH).

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

Carbodiimide Synthesis via Ti-Catalyzed Nitrene Transfer from Diazenes to Isocyanides

Simple Ti imido halide complexes such as [Br₂Ti(N ^t Bu)py₂]₂ are competent catalysts for the synthesis of unsymmetrical carbodiimides via Ti-catalyzed nitrene transfer from diazenes or azides to isocyanides. Both alkyl and aryl isocyanides are compatible with the reaction conditions, although product inhibition with sterically unencumbered substrates sometimes limits the yield when diazenes are employed as the oxidant. The reaction mechanism has been investigated both experimentally and computationally, wherein a key feature is that the product release is triggered by electron transfer from an η ²-carbodiimide to a Ti-bound azobenzene. This ligand-to-ligand redox buffering obviates the need for high-energy formally Ti^II intermediates and provides further evidence that substrate and product "redox noninnocence" can promote unusual Ti redox catalytic transformations.

Catalyst design insights from modelling a titanium-catalyzed multicomponent reaction

High oxidation state transition metal catalysis touches our daily lives through bulk chemical production, e.g. olefin polymerization, and through specialty chemical reactions common in organic synthesis, e.g. the Sharpless asymmetric epoxidation and olefin dihydroxylation. Our group has been expanding the reaction chemistry of titanium(iv) to produce a host of nitrogen-based heterocycles via multicomponent coupling reactions. One such multicomponent coupling reaction discovered in our laboratory is iminoamination, involving an amine, an alkyne, and an isonitrile. However, the experimental modeling of high oxidation state reactions lags far behind that of low oxidation state systems, where a great deal is known about ligands, their donor properties and how their structures affect catalysis. As a result, we have developed an experimental method for determining the donor abilities of anionic ligands on high oxidation state systems, which is based on the chromium(vi) nitride system NCr(NiPr2)2X, where X = the ligand being interrogated. The parameters obtained are simply called ligand donor parameters (LDP). In this contribution, a detailed optimization of the Ti(NMe2)2(dpm)-catalyzed iminoamination reaction was carried out, where dpm = 5,5-dimethyldipyrrolylmethane. During the course of these studies, dimeric {Ti(μ-N-tolyl)(dpm)}2 was isolated, which is proposed as the resting state of the catalyst. To destabilize this resting state, a more electron-rich bis(aryloxide) catalyst system was investigated. The more electron-rich system is somewhat more active for iminoamination under some conditions; however, the catalyst is prone to disproportionation. A study of heteroleptic titanium complexes revealed that the disproportionation equilibrium constant can be effectively modeled as a function of the square of the difference in LDP between the ligands, (ΔLDP)2. Using this methodology, one can estimate the stability of titanium complexes toward disproportionation.