Radicals: Reactive Intermediates with Translational Potential

Forging C(sp3)-C(sp3) Bonds with Carbon-Centered Radicals in the Synthesis of Complex Molecules

Radical fragment coupling reactions that unite intricate subunits have become an important class of transformations within the arena of complex molecule synthesis. This Perspective highlights some of the early contributions in this area, as well as more modern applications of radical fragment couplings in the preparation of natural products. Additionally, emphasis is placed on contemporary advances that allow for radical generation under mild conditions as a driving force for the implementation of radical fragment couplings in total synthesis.

Mn-Catalyzed Electrochemical Chloroalkylation of Alkenes

The heterodifunctionalization of alkenes is an efficient method for synthesizing highly functionalized organic molecules. In this report, we describe the use of anodically coupled electrolysis for the catalytic chloroalkylation of alkenes-a reaction that constructs vicinal C-C and C-Cl bonds in a single synthetic operation-from malononitriles or cyanoacetates and NaCl. Knowledge of the persistent radical effect guided the reaction design and development. A series of controlled experiments, including divided-cell electrolysis that compartmentalized the anodic and cathodic events, allowed us to identify the key radical intermediates and the pathway to their electrocatalytic formation. Cyclic voltammetry data further support the proposed mechanism entailing the parallel, Mn-mediated generation of two radical intermediates in an anodically coupled electrolysis followed by their selective addition to the alkene.

Chemoselective asymmetric dearomative [3 + 2] cycloaddition reactions of purines with aminocyclopropanes

Chemoselective asymmetric [3 + 2] cycloaddition reactions of purines with aminocyclopropanes for the dearomatization of purines have been successfully developed.

Recent advances of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) in photocatalytic transformations

In this review, the recent advances of the application of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a photoredox catalyst in the past three years (2016–2018) for various organic reactions are summarized.

Multi-component heteroarene couplings via polarity-reversed radical cascades

A multi-component radical addition strategy enables difunctionalization of alkenes with heteroarenes and a variety of radical precursors, including N3, P(O)R2, and CF3. This unified approach for coupling diverse classes of electrophilic radicals and heteroarenes to vinyl ethers allows for direct, vicinal C-C as well as C-N, C-P, and C-Rf bond formation.

A scalable electrochemical dehydrogenative cross-coupling of P(O)H compounds with RSH/ROH

A practical, scalable electrochemical dehydrogenative cross-coupling of P(O)H compounds with thiols, phenols and alcohols in both an undivided cell and a continuous-flow setup is disclosed. Its broad substrate scope (>50 examples), good functional-group tolerance and scalability (>10 g) show potential for practical synthesis. A preliminary mechanistic study suggests that the phosphorus radicals are involved in the catalytic cycle.

Metal-catalysed radical carbonylation reactions

A review of metal-catalysed radical carbonylation reactions is presented.

Photo- and dioxygen-enabled radical C(sp3)–N(sp2) cross-coupling between guanidines and perfluoroalkyl iodides

The utilization of visible light and dioxygen (air) as green and sustainable mediators in photocatalyzed synthetic chemistry is demonstrated.

Biaryl synthesis with arenediazonium salts: cross-coupling, CH-arylation and annulation reactions

The rich legacy of arenediazonium salts in the synthesis of unsymmetrical biaryls, built around the seminal works of Pschorr, Gomberg and Bachmann more than a century ago, continues to make important contributions at various evolutionary stages of modern biaryl synthesis. Based on in-depth mechanistic analysis and design of novel pathways and reaction conditions, the scope of biaryl synthesis with arenediazonium salts has enormously expanded in recent years through applications of transition metal/photoredox-catalysed cross-coupling, thermal/photosensitized radical chain CH-arylation of (hetero)arenes and arylative radical annulation reactions with alkynes. These recent developments have provided facile synthetic access to a wide variety of unsymmetrical biaryls of pharmaceutical, agrochemical and optoelectronic importance with green scale-up options and created opportunities for late-stage modification of peptides, nucleosides, carbon nanotubes and electrodes, the details of which are captured in this review.

Catalytic cascade reactions by radical relay

Catalytic radical relays are an attractive tool for the rapid construction of complex molecular architectures.

Convergent Synthesis of Taxol Skeleton via Decarbonylative Radical Coupling Reaction

The highly oxygenated 6/8/6-membered ABC-ring 2 of taxol was assembled in a convergent fashion. A decarbonylative radical reaction between α-alkoxyacyl telluride 4 and cyanocyclohexenone 5 linked the A- and C-rings and stereoselectively installed the C2- and C3-tertiary carbon centers of 3. After the C8-quaternary stereocenter was constructed, the C9-methyl ketone and the C11-vinyl triflate of 30 participated in Pd(0)-promoted cyclization of the eight-membered B-ring, giving rise to the taxol skeleton 2.

Concise Synthesis of Polysubstituted Carbohelicenes by a C-H Activation/Radical Reaction/C-H Activation Sequence

Disclosed herein is the merging of C-H activation and radical chemistry, enabling rapid access to a structurally diverse family of fused carbohelicenes through the fusion of α-acetylnaphthalenes with alkynes under oxidative conditions. This cascade process exhibits exquisite chemoselectivity and regioselectivity. The reaction pathway was analyzed by intermediate separations, control experiments, radical trapping, EPR, MALDI-TOF-MS, and ESI-HRMS experiments, and shown to involve a C2-H activation/radical reaction/C8-H activation relay.

Studies toward the Total Synthesis of Caribbean Ciguatoxin C-CTX-1: Synthesis of the LMN-Ring Fragment through Reductive Olefin Cross-Coupling

Synthesis of the LMN-ring fragment of Caribbean ciguatoxin C-CTX-1, the principal causative toxin for ciguatera fish poisoning around the Caribbean Sea areas, is described. The key feature of the synthesis is the stereoselective introduction of an angular methyl group on the sterically encumbered seven-membered M-ring by the application of a hydrogen atom transfer-based reductive olefin coupling.

Radical Hydroarylation of Functionalized Olefins and Mechanistic Investigation of Photocatalytic Pyridyl Radical Reactions

We report the photoredox alkylation of halopyridines using functionalized alkene and alkyne building blocks. Selective single-electron reduction of the halogenated pyridines provides the corresponding heteroaryl radicals, which undergo anti-Markovnikov addition to the alkene substrates. The system is shown to be mild and tolerant of a variety of alkene and alkyne subtypes. A combination of computational and experimental studies support a mechanism involving proton-coupled electron transfer followed by medium-dependent alkene addition and rapid hydrogen atom transfer mediated by a polarity-reversal catalyst.

A General Amino Acid Synthesis Enabled by Innate Radical Cross-Coupling

The direct union of primary, secondary, and tertiary carboxylic acids with a chiral glyoxylate-derived sulfinimine provides rapid access into a variety of enantiomerically pure α-amino acids (>85 examples). Characterized by operational simplicity, this radical-based reaction enables the modular assembly of exotic α-amino acids, including both unprecedented structures and those of established industrial value. The described method performs well in high-throughput library synthesis, and has already been implemented in three distinct medicinal chemistry campaigns.

Easy access to elusive radical reactions

Two mild approaches generate radical intermediates from masked aldehydes

Twelve-Step Asymmetric Synthesis of (-)-Nodulisporic Acid C

A short, enantioselective synthesis of (-)-nodulisporic acid C is described. The route features two highly diastereoselective polycyclizations en route to the terpenoid core and the indenopyran fragment and a highly convergent assembly of a challenging indole moiety. Application of this chemistry allows for a 12-step synthesis of the target indoloterpenoid from commercially available material.

Effect of Natural Food Antioxidants against LDL and DNA Oxidative Changes

Radical oxygen species formed in human tissue cells by many endogenous and exogenous pathways cause extensive oxidative damage which has been linked to various human diseases. This review paper provides an overview of lipid peroxidation and focuses on the free radicals-initiated processes of low-density lipoprotein (LDL) oxidative modification and DNA oxidative damage, which are widely associated with the initiation and development of atherosclerosis and carcinogenesis, respectively. The article subsequently provides an overview of the recent human trials or even in vitro investigations on the potential of natural antioxidant compounds (such as carotenoids; vitamins C and E) to monitor LDL and DNA oxidative changes.

Iron-Nickel Dual-Catalysis: A New Engine for Olefin Functionalization and the Formation of Quaternary Centers

Alkene hydroarylation forms carbon-carbon bonds between two foundational building blocks of organic chemistry: olefins and aromatic rings. In the absence of electronic bias or directing groups, only the Friedel-Crafts reaction allows arenes to engage alkenes with Markovnikov selectivity to generate quaternary carbons. However, the intermediacy of carbocations precludes the use of electron-deficient arenes, including Lewis basic heterocycles. Here we report a highly Markovnikov-selective, dual-catalytic olefin hydroarylation that tolerates arenes and heteroarenes of any electronic character. Hydrogen atom transfer controls the formation of branched products and arene halogenation specifies attachment points on the aromatic ring. Mono-, di-, tri-, and tetra-substituted alkenes yield Markovnikov products including quaternary carbons within nonstrained rings.

Metal-Free and Redox-Neutral Conversion of Organotrifluoroborates into Radicals Enabled by Visible Light

Converting organoboron compounds into the corresponding radicals has broad synthetic applications in organic chemistry. To achieve these transformations, various strong oxidants such as Mn(OAc)₃ , AgNO₃ /K₂ S₂ O₈ , and Cu(OAc)₂ , in stoichiometric amounts are required, proceeding by a single-electron transfer mechanism. Established herein is a distinct strategy for generating both aryl and alkyl radicals from organotrifluoroborates through an S_H 2 process. This strategy is enabled by using water as the solvent, visible light as the energy input, and diacetyl as the promoter in the absence of any metal catalyst or redox reagent, thereby eliminating metal waste. To demonstrate its synthetic utility, an efficient acetylation to prepare valuable aryl (alkyl) methyl ketones is described and applications to construct C-C, C-I, C-Br, and C-S bonds are also feasible. Experimental evidence suggests that triplet diacetyl serves as the key intermediate in this process.

Copper-Catalyzed Synthesis of Hindered Ethers from α-Bromo Carbonyl Compounds

A catalytic method for the synthesis of sterically hindered ethers and thioethers from α-bromo carbonyl compounds and the corresponding nucleophiles using an inexpensive Cu(I) catalytic system is reported. This facile transformation takes place at ambient temperature and does not require the exclusion of air or moisture; thus, it is well-suited for the functionalization and derivatization of complex organic molecules.

Photo-Organocatalytic Enantioselective Radical Cascade Reactions of Unactivated Olefins

Radical cascade processes are invaluable for their ability to rapidly construct complex chiral molecules from simple substrates. However, implementing catalytic asymmetric variants is difficult. Reported herein is a visible‐light‐mediated organocatalytic strategy that exploits the excited‐state reactivity of chiral iminium ions to trigger radical cascade reactions with high enantioselectivity. By combining two sequential radical‐based bond‐forming events, the method converts unactivated olefins and α,β‐unsaturated aldehydes into chiral adducts in a single step. The implementation of an asymmetric three‐component radical cascade further demonstrates the complexity‐generating power of this photochemical strategy.

Radical Retrosynthesis

In The Logic of Chemical Synthesis, E. J. Corey stated that the key to retrosynthetic analysis was a "wise choice of appropriate simplifying transforms" ( Corey , E. J. ; Cheng , X.-M. The Logic of Chemical Synthesis ; John Wiley : New York , 1989 ). Through the lens of "ideality", chemists can identify opportunities that can lead to more practical, scalable, and sustainable synthesis. The percent ideality of a synthesis is defined as [(no. of construction rxns) + (no. of strategic redox rxns)]/(total no. of steps) × 100. A direct consequence of designing "wise" or "ideal" plans is that new transformations often need invention. For example, if functional group interconversions are to be avoided, one is faced with the prospect of directly functionalizing C-H bonds ( Gutekunst , W. R. ; Baran , P. S. Chem. Soc. Rev. 2011 , 40 , 1976 ; Brückl , T. ; et al. Acc. Chem. Res. 2012 , 45 , 826 ). If protecting groups are minimized, methods testing the limits of chemoselectivity require invention ( Baran , P. S. ; et al. Nature 2007 , 446 , 404 ; Young , I. S. ; Baran , P. S. Nat. Chem. 2009 , 1 , 193 ). Finally, if extraneous redox manipulations are to be eliminated, methods directly generating key skeletal bonds result ( Burns , N. Z. ; et al. Angew. Chem., Int. Ed. 2009 , 48 , 2854 ). Such analyses applied to total synthesis have seen an explosion of interest in recent years. Thus, it is the interplay of aspirational strategic demands with the limits of available methods that can influence and inspire ingenuity. E. J. Corey's sage advice holds true when endeavoring in complex molecule synthesis, but together with the tenets of the "ideal" synthesis, avoiding concession steps leads to the most strategically and tactically optimal route ( Hendrickson , J. B. J. Am. Chem. Soc. 1975 , 97 , 5784 ; Gaich , T. ; Baran , P. S. J. Org. Chem. 2010 , 75 , 4657 ). Polar disconnections are intuitive and underlie much of retrosynthetic logic. Undergraduates exposed to multistep synthesis are often taught to assemble organic molecules through the combination of positively and negatively charged synthons because, after all, opposites attract. Indeed, the most employed two-electron C-C bond forming reactions today are those based upon either classical cross-coupling reactions (e.g., Suzuki, Negishi, or Heck) or polar additions (aldol, Michael, or Grignard). These reactions are the mainstay of modern synthesis and have revolutionized the way molecules are constructed due to their robust and predictable nature. In contrast, radical chemistry is sparsely covered beyond the basic principles of radical chain processes (i.e., radical halogenation). The historical perception of radicals as somewhat uncontrollable species does not help the situation. As a result, synthetic chemists are not prone to make radical-based strategic bond disconnections during first-pass retrosynthetic analyses. Recent interest in the use of one-electron radical cross-coupling (RCC) methods has been fueled by the realization of their uniquely chemoselective profiles and the opportunities they uncover for dramatically simplifying synthesis. In general, such couplings can proceed by relying on the innate preferences of a substrate (innate RCC) or through interception with a mediator (usually a transition metal) to achieve programmed RCC. This Account presents a series of case studies illustrating the inherent strategic and tactical advantages of employing both types of radical-based cross-couplings in a variety of disparate settings. Thematically, it is clear that one-electron disconnections, while not considered to be intuitive, can serve to enable syntheses that are more direct and feature a minimal use of protecting group chemistry, functional group interconversions, and nonstrategic redox fluctuations.

Metal-free alcohol-directed regioselective heteroarylation of remote unactivated C(sp3)-H bonds

Construction of C–C bonds via alkoxy radical-mediated remote C(sp³)–H functionalization is largely unexplored, as it is a formidable challenge to directly generate alkoxy radicals from alcohols due to the high bond dissociation energy (BDE) of O–H bonds. Disclosed herein is a practical and elusive metal-free alcohol-directed heteroarylation of remote unactivated C(sp³)–H bonds. Phenyliodine bis(trifluoroacetate) (PIFA) is used as the only reagent to enable the coupling of alcohols and heteroaryls. Alkoxy radicals are readily generated from free alcohols under the irradiation of visible light, which trigger the regioselective hydrogen-atom transfer (HAT). A wide range of functional groups are compatible with the mild reaction conditions. Two unactivated C–H bonds are cleaved and one new C–C bond is constructed during the reaction. This protocol provides an efficient strategy for the late-stage functionalization of alcohols and heteroaryls.

Direct remote C–H functionalization of aliphatic alcohols via alkoxy radicals is largely unexplored. Here, the authors report the C(sp³)-heteroaryl bond formation in aliphatic alcohols mediated by alkoxy radicals formed with a hypervalent iodine reagent.