A Titanium(III)-Catalyzed Reductive Umpolung Reaction for the Synthesis of 1,1-Disubstituted Tetrahydroisoquinolines

Total synthesis of (±)-3-demethoxyerythratidinone via Tf2O-promoted cascade reaction of enaminone

The tetracyclic rings with chiral quaternary center represent a formidable synthetic challenge for Erythrina alkaloids. We present a 6-step synthesis of the Erythrina alkaloid 3-demethoxyerythratidinone with a 16% overall yield. Our synthesis highlights a cascade reaction initiated by Tf2O-induced activation of an enaminone substrate, yielding an iminium species and an enol triflate, followed by a Pictet-Spengler reaction. This method efficiently constructs the tetracyclic core skeleton, featuring an N-substituted quaternary center. It exhibits versatility across diverse (hetero)arenes and enaminone structures, providing a general strategy for the rapid synthesis of fused or spiro ring systems including the core structure of homoerythrina alkaloids.

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

The 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 Introduction

2 Active Titanium(III) Species and Additives

3 Ketone-Nitrile Couplings

4 Further Reductive Umpolung Reactions

5 Catalytic Homolytic C–CN and C–SO2R Cleavage

6 Conclusion

Concise synthesis of α-cyano tetrahydroisoquinolines with a quaternary center via Strecker reaction

A concise synthesis of α-cyano tetrahydroisoquinolines with a quaternary center via the Strecker reaction was successfully realized by employing TMSCN as cyano source and KF as fluoride source, furnishing the products with up to 99% yield. An isomerization of α-cyano tetrahydroisoquinoline was observed under alkaline conditions to give the isomer via [1,3]-H shift.

Concise synthesis of α-cyano tetrahydroisoquinolines with a quaternary center was successfully realized, furnishing products with up to 99% yield and an isomerization was observed under alkaline conditions, giving the isomer via [1,3]-H shift.

Fluorescent palladium(II) and platinum(II) NHC/1,2,3-triazole complexes: antiproliferative activity and selectivity against cancer cells

Fluorescent Pd(ii) and Pt(ii) complexes bearing 4-methylene-7-methoxycoumarin (MMC) and 2,6-diispropylphenyl (Dipp) substituted NHC/1,2,3-triazole hybrid ligands are described. Depending on the reaction conditions two different ligand coordination modes are observed, i.e., bidentate solely coordinating via NHCs or tetradentate coordinating via NHCs and 1,2,3-triazoles. All Dipp substituted complexes show antiproliferative activity against cervix (HeLa) and breast (MCF-7) human carcinoma cells. The activity significantly depends on the coordination mode, with the tetradentate motif being notably more effective (HeLa: IC50 = 3.9 μM to 4.7 μM; MCF-7: IC50 = 2.07 μM to 2.35 μM). Amongst the MMC series, only the Pd(ii) complex featuring the bidentate coordination mode is active against HeLa (IC50 = 6.1 μM). In contrast to its structurally related Dipp derivative (SI = 0.6), it shows a high selectivity for HeLa (SI > 16) compared to healthy skin cells (HaCaT). According to fluorescence microscopy, this compound is presumably located in late endosomes or lysosomes.

The Proven Versatility of Cp2TiCl

In the last two decades, titanocene monochloride has been postulated as a monoelectronic transfer reagent capable of catalyzing an important variety of chemical transformations. In this Perspective, our contributions to this growing field of research are summarized and analyzed. Especially known have been our contributions in C-C bond formation reactions, hydrogen-atom transfer from water to radicals, and isomerization reactions, as well as the development of a catalytic cycle that has subsequently allowed the preparation of a great variety of natural terpenes. It is also worth mentioning our contribution in the postulation of this single-electron transfer agent (SET) as a new green catalyst with a broad range of applications in organic and organometallic chemistry. The most significant catalytic processes developed by other research groups are also briefly described, with special emphasis on the reaction mechanisms involved. Finally, a reflection is made on the future trends in the research of this SET, aimed at consolidating this chemical as a new green reagent that will be widely used in fine chemistry, green chemistry, and industrial chemical processes.

Hydrolysis of dihydroisoquinoline derivatives activated by sulfonyl or acyl chloride

A new, efficient, and mild strategy for hydrolysis of 3,4-dihydroisoquinoline imines activated by sulfonyl chloride or acyl chloride has been developed, by which method ketosulfonamides and ketoamides have been synthesized. This process tolerates broad scope with respect to both the sulfonyl chloride and acyl chloride with moderate to excellent yields. This protocol features a broad substrate scope for various kinds of 3,4-dihydroisoquinoline and mild reaction conditions without using strong acidic or basic conditions. These features show that this user-friendly and simple system could be applied in the future to the synthesis of a broader range of amino benzophenones.

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.

A Method for Bischler-Napieralski-Type Synthesis of 3,4-Dihydroisoquinolines

A new method for the Bischler-Napieralski-type synthesis of 3,4-dihydroisoquinolines was developed by a Tf₂O-promoted tandem annulation from phenylethanols and nitriles. Its success was mainly due to the fact that a phenonium ion was formed in the process and practically functioned as a stable and reactive primary phenylethyl carbocation.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

Understanding titanium-catalysed radical-radical reactions: a DFT study unravels the complex kinetics of ketone-nitrile couplings

The computational investigation of a titanium-catalysed reductive radical-radical coupling is reported. The results match the conclusions from an earlier experimental study and enable a further interpretation of the previously observed complex reaction kinetics. Furthermore, the interplay between neutral and cationic reaction pathways in titanium(iii)-catalysed reactions is investigated for the first time. The results show that hydrochloride additives and reaction byproducts play an important role in the respective equilibria. A full reaction profile is assembled and the computed activation barrier is found to be in reasonable agreement with the experiment. The conclusions are of fundamental importance to the field of low-valent titanium catalysis and the understanding of related catalytic radical-radical coupling reactions.

Cp2 TiX Complexes for Sustainable Catalysis in Single-Electron Steps

We present a combined electrochemical, kinetic, and synthetic study with a novel and easily accessible class of titanocene catalysts for catalysis in single-electron steps. The tailoring of the electronic properties of our Cp₂ TiX-catalysts that are prepared in situ from readily available Cp₂ TiX₂ is achieved by varying the anionic ligand X. Of the complexes investigated, Cp₂ TiOMs proved to be either equal or substantially superior to the best catalysts developed earlier. The kinetic and thermodynamic properties pertinent to catalysis have been determined. They allow a mechanistic understanding of the subtle interplay of properties required for an efficient oxidative addition and reduction. Therefore, our study highlights that efficient catalysts do not require the elaborate covalent modification of the cyclopentadienyl ligands.

A Short Synthesis of (±)-3-Demethoxyerythratidinone by Ligand-Controlled Selective Heck Cyclization of Equilibrating Enamines

A short, 5-step total synthesis of (±)-3-demethoxyerythratidinone from a simple pyrrole derivative is described. Features include the formation of gram quantities of a key tricylic aziridine from a challenging photochemical cascade reaction through the use of flow photochemistry. The final step involved a highly unusual Heck cyclization whereby ligand control enabled efficient formation of the natural product in 69 % yield from the minor isomer present in an equilibrating mixture of labile enamines.

Titanium tetrachloride-mediated synthesis of N-aryl-substituted azacycles from cyclic ethers

Five- and six-membered N-aryl-substituted azacycles were synthesized from amines and cyclic ethers under mild conditions while no expensive metal reagent was required.

Synthesis of substituted tetrahydroisoquinolines by lithiation then electrophilic quench

Lithiation then electrophilic quench of tetrahydroisoquinolines provides access to 1-substituted products. Removal of the N-Boc group allows rapid access to natural products such as (±)-crispine A.

Synthetic approaches towards alkaloids bearing α-tertiary amines

Alkaloids account for some of the most beautiful and biologically active natural products. Although they are usually classified along biosynthetic criteria, they can also be categorized according to certain structural motifs. Amongst these, the α-tertiary amine (ATA), i.e. a tetrasubstituted carbon atom surrounded by three carbons and one nitrogen, is particularly interesting. A limited number of methods have been described to access this functional group and fewer still are commonly used in synthesis. Herein, we review some approaches to asymmetrically access ATAs and provide an overview of alkaloid total syntheses where those have been employed.