Stereoconvergent Approach to the Enantioselective Construction of α-Quaternary Alcohols by Radical Epoxide Allylation

We describe a highly stereoconvergent radical epoxide allylation towards diastereomerically and enantiomerically enriched α-quaternary alcohols in two steps from olefins. Our approach combines the stereospecifity and enantioselectivity of the Shi epoxidation with the unprecedented Ti(III)-promoted intramolecular radical group transfer allylation of epoxides. A directional isomerization step via configurationally labile radical intermediates enables the selective preparation of all-carbon quaternary stereocenters in a unique fashion.

Synthesis of Azabicyclo[3.1.1]heptenes Enabled by Catalyst-Controlled Annulations of Bicyclo[1.1.0]butanes with Vinyl Azides

Bridged bicyclic scaffolds are emerging bioisosteres of planar aromatic rings under the concept of "escape from flatland". However, adopting this concept into the exploration of bioisosteres of pyridines remains elusive due to the challenge of incorporating a N atom into such bridged bicyclic structures. Herein, we report practical routes for the divergent synthesis of 2- and 3-azabicyclo[3.1.1]heptenes (aza-BCHepes) as potential bioisosteres of pyridines from the readily accessible vinyl azides and bicyclo[1.1.0]butanes (BCBs) via two distinct catalytic annulations. The reactivity of vinyl azides tailored with BCBs is the key to achieving divergent transformations. TiIII-catalyzed single-electron reductive generation of C-radicals from BCBs allows a concise (3 + 3) annulation with vinyl azides, affording novel 2-aza-BCHepe scaffolds. In contrast, scandium catalysis enables an efficient dipolar (3 + 2) annulation with vinyl azides to generate 2-azidobicyclo[2.1.1]hexanes, which subsequently undergo a chemoselective rearrangement to construct 3-aza-BCHepes. Both approaches efficiently deliver unique azabicyclo[3.1.1]heptene scaffolds with a high functional group tolerance. The synthetic utility has been further demonstrated by scale-up reactions and diverse postcatalytic transformations, providing valuable azabicyclics including 2- and 3-azabicyclo[3.1.1]heptanes and rigid bicyclic amino esters. In addition, the related sp2-hybridized nitrogen atom and the similar geometric property between pyridines and corresponding aza-BCHepes indicate that they are promising bioisosteres of pyridines.

Boosting Energy-Transfer Processes via Dispersion Interactions

The generation of open-shell intermediates under mild conditions has opened broad synthetic opportunities during this century. However, these reactive species often require a case specific and tailored tuning of experimental parameters in order to efficiently convert substrates into products. We report a general approach that can overcome these ubiquitous limitations for several visible-light promoted energy-transfer processes. The use of either naphthalene (5-20 equiv.) or simple binaphthyl derivatives (10-30 mol %) greatly increases their efficiency, giving rise to a new strategy for catalysis. The trend is consistent among different media, photocatalysts, light sources and substrates, allowing one to improve existing methods, to more easily optimize conditions for new ones, and, moreover, to disclose otherwise inaccessible reaction pathways.

Multiplicative enhancement of stereoenrichment by a single catalyst for deracemization of alcohols

Stereochemical enrichment of a racemic mixture by deracemization must overcome unfavorable entropic effects as well as the principle of microscopic reversibility; recently, photochemical reaction pathways unveiled by the energetic input of light have led to innovations toward this end, most often by ablation of a stereogenic C(sp3)-H bond. We report a photochemically driven deracemization protocol in which a single chiral catalyst effects two mechanistically different steps, C-C bond cleavage and C-C bond formation, to achieve multiplicative enhancement of stereoinduction, which leads to high levels of stereoselectivity. Ligand-to-metal charge transfer excitation of a titanium catalyst coordinated by a chiral phosphoric acid or bisoxazoline efficiently enriches racemic alcohols that feature adjacent and fully substituted stereogenic centers to enantiomeric ratios up to 99:1. Mechanistic investigations support a pathway of sequential radical-mediated bond scission and bond formation through a common prochiral intermediate and reveal that, although the overall stereoenrichment is high, the selectivity in each individual step is moderate.

Quaternary Stereocenters via Catalytic Enantioconvergent Allylation of Epoxides

The development of catalytic and enantioselective transformations for the synthesis of all-carbon quaternary stereocenters has long been recognized as a significant challenge in organic synthesis. While considerable progress has been made in asymmetric allylations, their potential to functionalize the commonly used synthon, epoxide, remains largely underexplored. Here we demonstrate the first highly regio- and enantioselective allylation of epoxides that delivers a range of quaternary stereocenters in the face of potentially problematic elimination and protonation reactions. The reaction proceeds via a radical approach under mild conditions and benefits from the use of earth-abundant titanium with a highly sophisticated salen ligand, which facilitates remarkable enantiocontrol and suppresses undesired side reactions. The resulting allylation products are multifunctional building blocks that can be elaborated chemo- and stereoselectively to a broad array of stereodefined structural motifs.

Enantio- and Diastereomerically Pure Titanocenes by Dynamic Conformational Locking

The synthesis of enantiomerically pure titanocenes is limited to cases with enantiomerically pure substituents at the cyclopentadienyl ligands and to ansa-titanocenes. For the latter complexes, the use of achiral ligands requires a resolution of enantiomers and frequently also a separation of the diastereoisomers obtained after metalation. Here, we introduce a new synthetic method that relies on the use of enantiomerically pure camphorsulfonate (CSA) ligands as control elements for the absolute and relative configuration of titanocene complexes. Starting from the conformationally flexible (RC5 H4 )2 TiCl2 , the desired conformationally locked and hence enantio- and diastereomerically pure complexes (RC5 H4 )2 Ti(CSA)2 are obtained in just two steps. According to X-ray crystallography the (RC5 H4 )2 Ti fragment is essentially C2 -symmetric and nuclear magnetic resonance displays overall C2 -symmetry. We applied density functional theory methods to unravel the dynamics of the complexes and the mechanisms and selectivities of their formation.

Formal Anti-Markovnikov Addition of Water to Olefins by Titanocene-Catalyzed Epoxide Hydrosilylation: From Stoichiometric to Sustainable Catalytic Reactions

Here, the evolution of the titanocene-catalyzed hydrosilylation of epoxides that yields the corresponding anti-Markovnikov alcohols is summarized. The study focuses on aspects of sustainability, efficient catalyst activation, and stereoselectivity. The latest variant of the reaction employs polymethylhydrosiloxane (PMHS), a waste product of the Müller-Rochow process as terminal reductant, features an efficient catalyst activation with benzylMgBr and the use of the bench stable Cp₂TiCl₂ as precatalyst. The combination of olefin epoxidation and epoxide hydrosilylation provides a uniquely efficient approach to the formal anti-Markovnikov addition of H₂O to olefins.

Visible Light-Induced Cascade Sulfonylation/Cyclization to Produce Quinoline-2,4-Diones under Metal-Free Conditions

A general visible light-induced sulfonylation/cyclization to produce quinoline-2,4-diones was achieved under photocatalyst-free conditions. The reactions were performed at room temperature, and various substituents (halogen, alkyl, aryl) and substituted products were obtained with 29 examples within 2 h. Large-scale synthesis and derivatization study via carbonyl reduction to produce easily modified hydroxyl groups and convenient N-Ts deprotection showed the potential utility of this strategy.

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

Electrochemical Aerobic Oxygenation and Nitrogenation of Cyclic Alkenes via C═C Bond Cleavage or Oxygenation and Azidation of Open-Chain Alkenes

An efficient strategy involving electrochemical C═C double-bond cleavage and functionalization of cyclic alkenes for the synthesis of ketonitriles is described. This transformation features environmentally friendly conditions and utilizes relatively safe TMSN₃ as the nitrogenation reagent and molecular oxygen as the oxidant. For the open-chain alkenes, the reaction gave 1,2-difunctionalized products. A wide range of cyclic alkenes and open-chain alkenes were found to be compatible, providing the corresponding ketonitriles and α-azido aromatic ketones in moderate to good yields.

Diastereoselective and enantioselective photoredox pinacol coupling promoted by titanium complexes with a red-absorbing organic dye

The pinacol coupling reaction, a reductive coupling of carbonyl compounds that proceeds through the formation of ketyl radicals in the presence of an electron donor, affords the corresponding 1,2-diols in one single step. The photoredox version of this transformation has been accomplished using different organic dyes or photoactive metal complexes in the presence of sacrificial donors such as tertiary amines or Hantzsch's ester. Normally, the homo-coupling of such reactive ketyl radicals is neither diastereo- nor enantio-selective. Herein, we report a highly diastereoselective pinacol coupling reaction of aromatic aldehydes promoted by 5 mol% of the non-toxic, inexpensive and available Cp₂TiCl₂ complex. The key feature that allows the complete control of diastereoselectivity is the employment of a red-absorbing organic dye in the presence of a redox-active titanium complex. Taking advantage of the well-tailored photoredox potential of this organic dye, the selective reduction of Ti(iv) to Ti(iii) is achieved. These conditions enable the formation of the d,l (syn) diastereoisomer as the favored product of the pinacol coupling (d.r. > 20 : 1 in most of the cases). Moreover, employing a simply prepared chiral SalenTi complex, the new photoredox reaction gave a complete diastereoselection for the d,l diastereoisomer, and high enantiocontrol (up to 92% of enantiomeric excess).

A metallaphotoredox, diastereoselective and enantioselective pinacol coupling reaction promoted by titanium complexes with the use of a red-absorbing organic dye was developed.

Deciphering Reactivity and Selectivity Patterns in Aliphatic C-H Bond Oxygenation of Cyclopentane and Cyclohexane Derivatives

A kinetic, product, and computational study on the reactions of the cumyloxyl radical with monosubstituted cyclopentanes and cyclohexanes has been carried out. HAT rates, site-selectivities for C-H bond oxidation, and DFT computations provide quantitative information and theoretical models to explain the observed patterns. Cyclopentanes functionalize predominantly at C-1, and tertiary C-H bond activation barriers decrease on going from methyl- and tert-butylcyclopentane to phenylcyclopentane, in line with the computed C-H BDEs. With cyclohexanes, the relative importance of HAT from C-1 decreases on going from methyl- and phenylcyclohexane to ethyl-, isopropyl-, and tert-butylcyclohexane. Deactivation is also observed at C-2 with site-selectivity that progressively shifts to C-3 and C-4 with increasing substituent steric bulk. The site-selectivities observed in the corresponding oxidations promoted by ethyl(trifluoromethyl)dioxirane support this mechanistic picture. Comparison of these results with those obtained previously for C-H bond azidation and functionalizations promoted by the PINO radical of phenyl and tert-butylcyclohexane, together with new calculations, provides a mechanistic framework for understanding C-H bond functionalization of cycloalkanes. The nature of the HAT reagent, C-H bond strengths, and torsional effects are important determinants of site-selectivity, with the latter effects that play a major role in the reactions of oxygen-centered HAT reagents with monosubstituted cyclohexanes.

Radical Annulation of 2-Cyanoaryl Acrylamides via C═C Double Bond Cleavage: Access to Amino-Substituted 2-Quinolones

A novel annulation of 2-cyanoaryl acrylamides via C═C double bond cleavage has been developed for facile and efficient access to a broad spectrum of functionalized 4-amino-2-quinolones, which are important N-heterocycles. In this transformation, the solvent THF is demonstrated to play a crucial role, and the addition of alkyl radicals to nitrile, 1,5-hydride shift, and cleavage of the C-C bond are involved in the mechanism.

Catalytic Reductive Cross-Coupling between Aromatic Aldehydes and Arylnitriles

A reductive cross-coupling reaction between aromatic aldehydes and arylnitriles using a copper catalyst and a silylboronate as a reductant is reported. This protocol represents an unprecedented approach to the chemoselective synthesis of α-hydroxy ketones by electrophile-electrophile cross-coupling.

Recent advances in the chemistry of ketyl radicals

Ketyl radicals are valuable reactive intermediates for synthesis and are used extensively to construct complex, functionalized products from carbonyl substrates. Single electron transfer (SET) reduction of the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O bond of aldehydes and ketones is the classical approach for the formation of ketyl radicals and metal reductants are the archetypal reagents employed. The past decade has, however, witnessed significant advances in the generation and harnessing of ketyl radicals. This tutorial review highlights recent, exciting developments in the chemistry of ketyl radicals by comparing the varied contemporary – for example, using photoredox catalysts – and more classical approaches for the generation and use of ketyl radicals. The review will focus on different strategies for ketyl radical generation, their creative use in new synthetic protocols, strategies for the control of enantioselectivity, and detailed mechanisms where appropriate.

Ketyl radicals are valuable reactive intermediates for synthesis. This review highlights exciting recent developments in the chemistry of ketyl radicals by comparing contemporary and more classical approaches for their generation and use.

C-C Bond Acylation of Oxime Ethers via NHC Catalysis

An N-heterocyclic carbene (NHC)-catalyzed strategy has been developed to address the issue of using toxic transitional metals in the field of C-C bond activation. The novel reaction mode enables an efficient docking between the cyanoalkyl from the cycloketone oxime derivative and the acyl group from the aldehyde, affording ketonitrile in moderate to good yields, which is one kind of useful building block for synthesizing nitrogen-containing pharmacophores.

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.

Mechanistic Studies Inform Design of Improved Ti(salen) Catalysts for Enantioselective [3 + 2] Cycloaddition

Ti(salen) complexes catalyze the asymmetric [3 + 2] cycloaddition of cyclopropyl ketones with alkenes. While high enantioselectivities are achieved with electron-rich alkenes, electron-deficient alkenes are less selective. Herein, we describe mechanistic studies to understand the origins of catalyst and substrate trends in an effort to identify a more general catalyst. Density functional theory (DFT) calculations of the selectivity determining transition state revealed the origin of stereochemical control to be catalyst distortion, which is largely influenced by the chiral backbone and adamantyl groups on the salicylaldehyde moieties. While substitution of the adamantyl groups was detrimental to the enantioselectivity, mechanistic information guided the development of a set of eight new Ti(salen) catalysts with modified diamine backbones. These catalysts were evaluated with four electron-deficient alkenes to develop a three-parameter statistical model relating enantioselectivity to physical organic parameters. This statistical model is capable of quantitative prediction of enantioselectivity with structurally diverse alkenes. These mechanistic insights assisted the discovery of a new Ti(salen) catalyst, which substantially expanded the reaction scope and significantly improved the enantioselectivity of synthetically interesting building blocks.

Oxidative cascade cyclization of 2-cyano-3-arylaniline derived acrylamides with toluenes, ethers, aliphatic alcohols or simple alkanes

Oxidative radical cascade cyclization of 2-cyano-3-arylaniline derived acrylamides with a variety of hydrocarbons including toluenes, ethers, aliphatic alcohols and alkanes has been developed to construct alkyl substituted pyridophenanthridines.

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.

Syntheses of Pyrroles, Pyridines, and Ketonitriles via Catalytic Carbopalladation of Dinitriles

The first example of the Pd-catalyzed addition of organoboron reagents to dinitriles, as an efficient means of preparing 2,5-diarylpyrroles and 2,6-diarylpyridines, has been discussed here. Furthermore, the highly selective carbopalladation of dinitriles with organoboron reagents to give long-chain ketonitriles has been developed as well. Based on the broad scope of substrates, excellent functional group tolerance, and use of commercially available substrates, the Pd-catalyzed addition reaction of arylboronic acid and dinitriles is expected to be significant in future synthetic procedures.

Recent Advances in Titanium Radical Redox Catalysis

New catalytic strategies that leverage single-electron redox events have provided chemists with useful tools for solving synthetic problems. In this context, Ti offers opportunities that are complementary to late transition metals for reaction discovery. Following foundational work on epoxide reductive functionalization, recent methodological advances have significantly expanded the repertoire of Ti radical chemistry. This Synopsis summarizes recent developments in the burgeoning area of Ti radical catalysis with a focus on innovative catalytic strategies such as radical redox-relay and dual catalysis.

Bimetallic Radical Redox-Relay Catalysis for the Isomerization of Epoxides to Allylic Alcohols

Organic radicals are generally short-lived intermediates with exceptionally high reactivity. Strategically, achieving synthetically useful transformations mediated by organic radicals requires both efficient initiation and selective termination events. Here, we report a new catalytic strategy, namely, bimetallic radical redox-relay, in the regio- and stereoselective rearrangement of epoxides to allylic alcohols. This approach exploits the rich redox chemistry of Ti and Co complexes and merges reductive epoxide ring opening (initiation) with hydrogen atom transfer (termination). Critically, upon effecting key bond-forming and -breaking events, Ti and Co catalysts undergo proton transfer/electron transfer with one another to achieve turnover, thus constituting a truly synergistic dual catalytic system.

Stereoselective Barbier-Type Allylations and Propargylations Mediated by CpTiCl3

CpTiCl₂, prepared in situ by manganese reduction of CpTiCl₃, is an excellent new system for the Barbier-type allylation and propargylation of carbonyl compounds. It can be used in catalytic amounts when combined with Et₃N·HBr/TMSBr, which acts as a regenerating system. The high regio- and stereoselectivity shown by this system makes it useful for prenylation and crotylation processes in the synthesis of natural products.

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.

Hydrogen Atom Transfer-Mediated Cyclisations of Nitriles

Hydrogen atom transfer-mediated intramolecular C-C coupling reactions between alkenes and nitriles, using PhSiH₃ and catalytic Fe(acac)₃ , are described. This introduces a new strategic bond disconnection for ring-closing reactions, forming ketones via imine intermediates. Of note is the scope of the reaction, including formation of sterically hindered ketones, spirocycles and fused cyclic systems.

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.