Enantioselective C(sp3)-H Functionalization of Oxacycles via Photo-HAT/Nickel Dual Catalysis

Enantioselective Alkyl-Acyl Radical Cross-Coupling Enabled by Metallaphotoredox Catalysis

Radical-radical cross-coupling (RCC) offers a promising approach for carbon-carbon bond formation in organic synthesis, particularly for creating complex, three-dimensional molecules. However, achieving both cross- and enantioselectivity in RCC reactions has remained a significant challenge. Here, we report a novel metallaphotoredox platform that enables highly enantioselective decarboxylative coupling of carboxylic acid derivatives with aldehydes. Our strategy leverages independent control over radical generation and subsequent enantioselective bond formation through fine-tuning of a common photocatalyst and a simple chiral bis(oxazoline) nickel catalyst. This redox-neutral protocol requires no exogenous oxidants or reductants and demonstrates broad substrate scope and functional group compatibility in the synthesis of enantioenriched α-aryl and α-amino ketones. The α-amino ketone products can be readily transformed into valuable β-amino alcohols, streamlining access to these important motifs. Furthermore, we showcase the potential of this approach for more challenging enantioselective C(sp3)-C(sp3) alkyl-alkyl RCC reactions. This unified platform for enantioselective alkyl-acyl radical cross-coupling represents a significant advance in asymmetric catalysis and underscores the potential for metallaphotoredox catalysis to exploit new mechanisms to solve long-standing synthetic problems.

Cobalt-Catalyzed Hydroglycosylation of Alkynes for the Synthesis of Vinyl α-C-Glycosides

Herein we report a cobalt-catalyzed hydroglycosylation of terminal alkynes, employing bench-stable ortho-iodobiphenyl (oIB) substituted sulfides as glycosyl donors. This reaction occurs with high stereo- and regioselectivity to afford E-configured vinyl α-C-glycosides, a class of compounds nontrivial to access by previous methods. The use of a bis(oxazoline) ligand with bulky side chains is critical for the high selectivities observed. Aryl iodides have not been used previously as surrogates for alkyl (glycosyl) electrophiles in Co-H catalyzed reactions. The transformation features mild reaction conditions, simple reagents, and broad substrate scope. This method can be applied to the derivatization of bioactive compounds and the preparation of naturally occurring products. Mechanistic studies suggest this stereoconvergent process involves glycosyl radicals as key reaction intermediates.

Direct stereoselective C(sp3)-H alkylation of saturated heterocycles using olefins

Despite cross-coupling strategies that enable the functionalization of aromatic heterocycles, the enantioselective C(sp3)-H alkylation of readily available saturated hydrocarbons to construct C(sp3)-C(sp3) bonds remains a formidable challenge. Here we describe a nickel-catalysed enantioselective C(sp3)-H alkylation of saturated heterocycles using olefins, providing an efficient strategy for the stereoselective construction of C(sp3)-C(sp3) bonds. Using readily available and stable olefins and simple saturated nitrogen and oxygen heterocycles as prochiral nucleophiles, the coupling reactions proceed under mild conditions and exhibit broad scope and high functional group tolerance. Furthermore, the enantio- and diastereoselective C(sp3)-H alkylation of saturated hydrocarbons with alkenyl boronates has been achieved, enabling the synthesis of versatile alkyl boronates containing 1,2-adjacent C(sp3) stereocentres. Application of this approach to the late-stage modification of natural products and drugs, as well as to the enantioselective synthesis of a range of chiral building blocks and natural products, is demonstrated.

Radical Brook rearrangement: past, present, and future

The Brook rearrangement has emerged as one of the most pivotal transformations in organic chemistry, with broad applications spanning organic synthesis, drug design, and materials science. Since its discovery in the 1950s, the anion-mediated Brook rearrangement has been extensively studied, laying the groundwork for the development of numerous innovative reactions. In contrast, the radical Brook rearrangement has garnered comparatively less attention, primarily due to the challenges associated with the controlled generation of alkoxyl radicals under mild conditions. However, recent advancements in visible-light catalysis and transition-metal catalysis have positioned the radical Brook rearrangement as a promising alternative synthetic strategy in organic synthesis. Despite these developments, significant limitations and challenges remain, warranting further investigation. This review provides an overview of the radical Brook rearrangement, tracing its development from past to present, and offers perspectives on future directions in the field to inspire the creation of novel synthetic tools based on this transformation.

A modular approach to catalytic stereoselective synthesis of chiral 1,2-diols and 1,3-diols

Optically pure 1,2-diols and 1,3-diols are the most privileged structural motifs, widely present in natural products, pharmaceuticals and chiral auxiliaries or ligands. However, their synthesis relies on the use of toxic or expensive metal catalysts or suffer from low regioselectivity. Catalytic asymmetric synthesis of optically pure 1,n-diols from bulk chemicals in a highly stereoselective and atom-economical manner remains a formidable challenge. Here, we disclose a versatile and modular method for the synthesis of enantioenriched 1,2-diols and 1,3-diols from the high-production-volume chemicals ethane-1,2-diol (MEG) and 1,3-propanediol (PDO), respectively. The key to success is to temporarily mask the diol group as an acetonide, which imparts selectivity to the key step of C(sp3)-H functionalization. Additionally, 1,n-diols containing two stereogenic centers are also prepared through diastereoselective C(sp3)-H functionalization. The late-stage functionalization of biological active compounds and the expedient synthesis of chiral ligands and pharmaceutically relevant molecules further highlight the synthetic potential of this protocol.

Regio- and diastereoselective synthesis of cyclobutylated phenothiazines via [2 + 2] photocycloaddition: demonstrating wavelength-gated cycloreversion inside live cells

Herein, we unveiled a regio- and diastereoselective synthesis of cyclobutylated phenothiazines, a unique class of structural congeners of phenothiazines via visible-light-irradiated intermolecular [2 + 2]-cycloaddition reaction, from readily available naphthoquinones, 2-aminothiophenols, and styrenes, either in a two-step or three-component coupling process. By varying substitutions in all three coupling partners, a library of cyclobutylated phenothiazines, including late-stage derivatization with five commercial drugs, has been realized with up to 97% isolated yield. In contrast to the reported pathways, the developed [2 + 2]-photocycloaddition seems to proceed via a 'photoinduced-electron-transfer' (PET) mechanism, which is well corroborated with the experimental observations, Rehm-Weller equation, and computation studies. Delightfully, a wavelength-gated reversibility of the [2 + 2]-photocycloaddition reaction has been accomplished on the synthesized cyclobutylated phenothiazines. By monitoring the rate of the cycloreversion reactions for different derivatives, a structure-activity relationship has also been achieved. Interestingly, this phenomenon was further replicated inside living cells, which leads to turn-on emission and is applied for photoresponsive cell imaging. This marks the first report of a light-triggered [2 + 2]-cycloreversion phenomenon occurring inside a live cell, leading to cell imaging. Moreover, the synthesized drug derivatives were utilized for synchronous cell imaging as well as drug delivery through the developed [2 + 2]-photocycloreversion process, which demonstrated the potential applicability of this class of molecules.

Cross-Electrophile Coupling: Principles, Methods, and Applications in Synthesis

Cross-electrophile coupling (XEC), defined by us as the cross-coupling of two different σ-electrophiles that is driven by catalyst reduction, has seen rapid progression in recent years. As such, this review aims to summarize the field from its beginnings up until mid-2023 and to provide comprehensive coverage on synthetic methods and current state of mechanistic understanding. Chapters are split by type of bond formed, which include C(sp3)-C(sp3), C(sp2)-C(sp2), C(sp2)-C(sp3), and C(sp2)-C(sp) bond formation. Additional chapters include alkene difunctionalization, alkyne difunctionalization, and formation of carbon-heteroatom bonds. Each chapter is generally organized with an initial summary of mechanisms followed by detailed figures and notes on methodological developments and ending with application notes in synthesis. While XEC is becoming an increasingly utilized approach in synthesis, its early stage of development means that optimal catalysts, ligands, additives, and reductants are still in flux. This review has collected data on these and various other aspects of the reactions to capture the state of the field. Finally, the data collected on the papers in this review is offered as Supporting Information for readers.

Mechanistic Interrogation of Photochemical Nickel-Catalyzed Tetrahydrofuran Arylation Leveraging Enantioinduction Data

This manuscript details the development of an asymmetric variant for the Ni-photoredox α-arylation of tetrahydrofuran (THF), which was originally reported in a racemic fashion by Doyle and Molander. Leveraging the enantioselectivity data that we obtained, a complex mechanistic scenario different from those originally proposed is uncovered. Specifically, an unexpected dependence of the product enantiomeric ratio was observed on both the halide identity (aryl chloride vs bromide substrates) and the Ni source. Stoichiometric experiments and time course analyses of the evolution of product enantioselectivity with time revealed a different initial behavior for reactions carried out with Ni(II) and Ni(0) precatalysts that later converge into a common mechanism. For studying the predominant pathway, this paper describes a rare example of the syntheses of chiral bisoxazoline Ni(II) aryl halide complexes, which proved essential for probing enantioselectivity via stochiometric experiments. These experiments identify the Ni(II) aryl halide complex as the primary species involved in the key THF radical trapping event. A multivariate linear regression model is presented that further validates the dominant mechanism and delineates structure-selectivity relationships between ligand properties and enantioselectivity. EPR analysis of Ni(0)/aryl halide mixtures highlights the fast access to a variety of Ni complexes in 0, +1, and +2 oxidation states that are proposed to be responsible for the initial divergence in mechanism observed when using Ni(0) precatalysts. More broadly, beyond advancing the mechanistic understanding of this THF arylation protocol, this work underscores the potential of leveraging enantioselectivity data to unravel intricate mechanistic manifolds within Ni-photoredox catalysis.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Metal-carbene-guided twofold cross-coupling of ethers with chromium catalysis

Coupling by metal-carbene transfer enables the formation of several different bonds at the carbenoid site, enabling prochiral Csp3 centers that are fundamental three-dimensional substructures for medicines to be forged with increased efficiency. However, strategies using bulk chemicals are rare because of the challenge of breaking two unactivated geminal bonds. Herein, we report the reactivity of ethers to form metal-carbene intermediate by cleavage of α-Csp3-H/Csp3-O bonds, which achieve selective coupling with arylmagnesium bromides and chlorosilanes. These couplings are catalysed by cyclic (alkyl)(amino)carbene-chromium complex and enable the one-step formation of 1,n-arylsilyl alcohols and α-arylated silanes. Mechanistic studies indicate that the in-situ formed low-valent Cr might react with iodobenzene to form phenyl radical species, which abstracts the α-H atom of ether in giving α-oxy radical. The latter combines with Cr by breaking α-Csp3-O bond to afford metal-carbene intermediate, which couples with aryl Grignard and chlorosilane to form two σ-bonds.

A catalytic collaboration: pairing transition metals and Lewis acids for applications in organic synthesis

The use of metal catalysts to accelerate an organic transformation has proven indispensable for access to structural motifs having applications across medicinal, polymer, materials chemistry, and more. Most catalytic approaches have cast transition metals in the "leading role"; these players mediate important reactions such as C-C cross coupling and the hydrogenation of unsaturated bonds. These catalysts may require collaboration, featuring Lewis acidic or basic additives to promote a desired reaction outcome. Lewis acids can serve to accelerate reactions by way of substrate stabilization and/or activation, and as such, are valuable in optimizing catalytic transformations. A burgeoning area of chemical research which unifies these concepts has thus sought to develop transition metal complexes having ambiphilic (containing a Lewis basic and acidic unit) ligands. This approach takes advantage of metal-ligand cooperativity to increase the efficiency of a given chemical transformation, leveraging intramolecular interactions between a transition metal and an adjacent secondary ligand site. While this has shown significant potential to facilitate challenging and important transformations, there remains unexplored depth for creativity and future advancement. This Frontier highlights inter- and intramolecular combinations of transition metals and Lewis acids that together, provide a collaborative platform for chemical synthesis.

CoH-catalyzed asymmetric remote hydroalkylation of heterocyclic alkenes: a rapid approach to chiral five-membered S- and O-heterocycles

Saturated heterocycles, which incorporate S and O heteroatoms, serve as fundamental frameworks in a diverse array of natural products, bioactive compounds, and pharmaceuticals. Herein, we describe a unique cobalt-catalyzed approach integrated with a desymmetrization strategy, facilitating precise and enantioselective remote hydroalkylation of unactivated heterocyclic alkenes. This method delivers hydroalkylation products with high yields and excellent stereoselectivity, representing good efficiency in constructing alkyl chiral centers at remote C3-positions within five-membered S/O-heterocycles. Notably, the broad scope and good functional group tolerance of this asymmetric C(sp3)-C(sp3) coupling enhance its applicability.

Mechanism of Ni-Catalyzed Photochemical Halogen Atom-Mediated C(sp3)-H Arylation

Within the context of Ni photoredox catalysis, halogen atom photoelimination from Ni has emerged as a fruitful strategy for enabling hydrogen atom transfer (HAT)-mediated C(sp3)-H functionalization. Despite the numerous synthetic transformations invoking this paradigm, a unified mechanistic hypothesis that is consistent with experimental findings on the catalytic systems and accounts for halogen radical formation and facile C(sp2)-C(sp3) bond formation remains elusive. We employ kinetic analysis, organometallic synthesis, and computational investigations to decipher the mechanism of a prototypical Ni-catalyzed photochemical C(sp3)-H arylation reaction. Our findings revise the previous mechanistic proposals, first by examining the relevance of SET and EnT processes from Ni intermediates relevant to the HAT-based arylation reaction. Our investigation highlights the ability for blue light to promote efficient Ni-C(sp2) bond homolysis from cationic NiIII and C(sp2)-C(sp3) reductive elimination from bipyridine NiII complexes. However interesting, the rates and selectivities of these processes do not account for the productive catalytic pathway. Instead, our studies support a mechanism that involves halogen atom evolution from in situ generated NiII dihalide intermediates, radical capture by a NiII(aryl)(halide) resting state, and key C-C bond formation from NiIII. Oxidative addition to NiI, as opposed to Ni0, and rapid NiIII/NiI comproportionation play key roles in this process. The findings presented herein offer fundamental insight into the reactivity of Ni in the broader context of catalysis.

Nickel-catalysed enantioselective alkene dicarbofunctionalization enabled by photochemical aliphatic C-H bond activation

The development of novel strategies to rapidly construct complex chiral molecules from readily available feedstocks is a long-term pursuit in the chemistry community. Radical-mediated alkene difunctionalizations represent an excellent platform towards this goal. However, asymmetric versions remain highly challenging, and more importantly, examples featuring simple hydrocarbons as reaction partners are elusive. Here we report an asymmetric three-component alkene dicarbofunctionalization capitalizing on the direct activation of C(sp 3)-H bonds through the combination of photocatalysed hydrogen atom transfer and nickel catalysis. This protocol provides an efficient platform for installing two vicinal carbon-carbon bonds across alkenes in an atom-economic fashion, providing a wide array of high-value chiral α-aryl/alkenyl carbonyls and phosphonates, as well as 1,1-diarylalkanes from ubiquitous alkane, ether and alcohol feedstocks. This method exhibits operational simplicity, broad substrate scope and excellent regioselectivity, chemoselectivity and enantioselectivity. The compatibility with bioactive motifs and expedient synthesis of pharmaceutically relevant molecules highlight the synthetic potential of this protocol.

Synthesis of Indolyl Phenyl Diketones through Visible-Light-Promoted Ni-Catalyzed Intramolecular Cyclization/Oxidation Sequence of Ynones

The combination of visible light catalysis and Ni catalysis has enabled the synthesis of indolyl phenyl diketones through the cyclization/oxidation process of ynones. This reaction proceeded under mild and base-free conditions and showed a broad scope and feasibility for gram-scale synthesis. Several natural products and biologically interesting molecules could be readily postfunctionalized by this method.

Ni-catalyzed enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohols and aryl bromides

Transition metal-catalyzed enantioconvergent cross-coupling of an alkyl precursor presents a promising method for producing enantioenriched C(sp3) molecules. Because alkyl alcohol is a ubiquitous and abundant family of feedstock in nature, the direct reductive coupling of alkyl alcohol and aryl halide enables efficient access to valuable compounds. Although several strategies have been developed to overcome the high bond dissociation energy of the C - O bond, the asymmetric pattern remains unknown. In this report, we describe the realization of an enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohol (β-hydroxy ketone) and aryl bromide in the presence of an NHC activating agent. The approach can accommodate substituents of various sizes and functional groups, and its synthetic potency is demonstrated through a gram scale reaction and derivatizations into other compound families. Finally, we apply our convergent method to the efficient asymmetric synthesis of four β-aryl ketones that are natural products or bioactive compounds.

Dual Nickel/Photoredox-Catalyzed Asymmetric Carbamoylation of Benzylic C(sp3 )-H Bonds

Radical-mediated Hydrogen Atom Abstraction of Csp3 -H bonds has become a powerful tool for the asymmetric functionalization of organic feedstocks. Here, we present an asymmetric synthesis of α-aryl amides via carbamoylation of alkylarenes with isocyanates as electrophiles. The synergistic combination of a photoredox and a chiral nickel-catalyst, enables the use of readily available and neutral reagents under mild reaction conditions and provides straightforward access to pharmacologically relevant motifs in enantiomerically pure form.

Blue-Light Irradiated Mn(0)-Catalyzed Hydroxylation and C(sp3 )-H Functionalization of Unactivated Alkanes with C(sp2 )-H Bonds of Quinones for Alkylated Hydroxy Quinones and Parvaquone

Site-selective C(sp3 )-H functionalization of unreactive hydrocarbons is always challenging due to its inherited chemical inertness, slightly different reactivity of various C-H bonds, and intrinsically high bond dissociation energies. Here, a site-selective C-H alkylation of naphthoquinone with unactivated hydrocarbons using Mn2 (CO)10 as a catalyst under blue-light (457 nm) irradiation without any external acid or base and pre-functionalization is presented. The selective C-H functionalization of tertiary over secondary and secondary over primary C(sp3 )-H bonds in abundant chemical feedstocks was achieved, and hydroxylation of quinones was realized in situ by employing the developed methodology. This protocol provides a new catalytic system for the direct construction of high-value-added compounds, namely, parvaquone (a commercially available drug used to treat theileriosis) and its derivatives under ambient reaction conditions. Moreover, this operationally simple protocol applies to various linear-, branched-, and cyclo-alkanes with high degrees of site selectivity under blue-light irradiated conditions and could provide rapid and straightforward access to versatile methodologies for upgrading feedstock chemicals. Mechanistic insight by radical trapping, radical scavenging, EPR, and other controlled experiments well corroborated with DFT studies suggest that the reaction proceeds by a radical pathway.

Ligand-Controlled Cobalt-Catalyzed Regio-, Enantio-, and Diastereoselective Oxyheterocyclic Alkene Hydroalkylation

Metal-hydride-catalyzed alkene hydroalkylation has been developed as an efficient method for C(sp3)-C(sp3) coupling with broad substrate availability and high functional group compatibility. However, auxiliary groups, a conjugated group or a chelation-directing group, are commonly required to attain high regio- and enantioselectivities. Herein, we reported a ligand-controlled cobalt-hydride-catalyzed regio-, enantio-, and diastereoselective oxyheterocyclic alkene hydroalkylation without chelation-directing groups. This reaction enables the hydroalkylation of conjugated and unconjugated oxyheterocyclic alkenes to deliver C2- or C3-alkylated tetrahydrofuran or tetrahydropyran in uniformly good yields and with high regio- and enantioselectivities. In addition, hydroalkylation of C2-substituted 2,5-dihydrofuran resulted in the simultaneous construction of 1,3-distereocenters, providing convenient access to polysubstituted tetrahydrofuran with multiple enantioenriched C(sp3) centers.

Photocatalyzed Enantioselective Functionalization of C(sp3)-H Bonds

Owing to its diverse activation processes including single-electron transfer (SET) and hydrogen-atom transfer (HAT), visible-light photocatalysis has emerged as a sustainable and efficient platform for organic synthesis. These processes provide a powerful avenue for the direct functionalization of C(sp3)-H bonds under mild conditions. Over the past decade, there have been remarkable advances in the enantioselective functionalization of the C(sp3)-H bond via photocatalysis combined with conventional asymmetric catalysis. Herein, we summarize the advances in asymmetric C(sp3)-H functionalization involving visible-light photocatalysis and discuss two main pathways in this emerging field: (a) SET-driven carbocation intermediates are followed by stereospecific nucleophile attacks; and (b) photodriven alkyl radical intermediates are further enantioselectively captured by (i) chiral π-SOMOphile reagents, (ii) stereoselective transition-metal complexes, and (iii) another distinct stereoscopic radical species. We aim to summarize key advances in reaction design, catalyst development, and mechanistic understanding, to provide new insights into this rapidly evolving area of research.

Mechanistic Insights into the Ligand-Directed Divergent Synthesis of 2-Benzazepine Derivatives via Ni-Catalyzed Tunable Cyclization/Cross-Coupling: A DFT Study

The detailed mechanisms of Ni-catalyzed ligand-controlled cyclization/cross-coupling of o-bromobenzenesulfonyl acrylamide (1a) with trifluoromethyl alkene were investigated by DFT calculations. The computational results support a single-electron reduction of NiII precatalyst to give BrNiIL species, which would react with 1a via oxidative addition to afford the (Ar)NiIIILBr2 complex. The subsequent cyclizations did not proceed until (Ar)NiIIILBr2 was reduced to the key (Ar)NiIL complex. For the bpy-involving reaction, the subsequent steps include nucleophilic attack to the carbonyl carbon atom, N-C bond breaking, intramolecular migratory insertion, as well as concerted C-C cross-coupling and β-F elimination. While the ligand of terpyridine promotes the 7-endocyclization followed by stepwise migratory insertion and β-F elimination to afford 2-benzazepine 2,5-dione. For both reactions, a theoretical study implied that the most favorable mechanism involved a NiI-NiIII-NiI catalytic cycle. The origins of the chemoselectivity, coupled with the factors responsible, were addressed.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

The mechanism of visible light-induced C-C cross-coupling by Csp3-H bond activation

Csp3-C cross-coupling by activating Csp3-H bonds is a dream reaction for the chemical community, and visible light-induced transition metal-catalysis under mild reaction conditions is considered a powerful tool to achieve it. Advancement of this research area is still in its infancy because of the chemical and technical complexity of this catalysis. Mechanistic studies illuminating the operative reaction pathways can rationalize the increasing amount of experimental catalysis data and provide the knowledge allowing faster and rational advances in the field. This goal requires complementary experimental and theoretical mechanistic studies, as each of them is unfit to clarify the operative mechanisms alone. In this tutorial review we summarize representative experimental and computational mechanistic studies, highlighting weaknesses, strengths, and synergies between the two approaches.