Rapid Access to β‐Trifluoromethyl‐Substituted Ketones: Harnessing Inductive Effects in Wacker‐Type Oxidations of Internal Alkenes

We present a practical trifluoromethyl‐directed Wacker‐type oxidation of internal alkenes that enables rapid access to β‐trifluoromethyl‐substituted ketones. Allylic trifluoromethyl‐substituted alkenes bearing a wide range of functional groups can be oxidized in high yield and regioselectivity. The distance dependence of the regioselectivity was established by systematic variation of the number of methylene units between the double bond and the trifluoromethyl group. The regioselectivity enforced by traditional directing groups could even be reversed by introduction of a competing trifluoromethyl group. Besides being a new powerful synthetic method to prepare fluorinated molecules, this work directly probes the role of inductive effects on nucleopalladation events.     

Defluorinative C(sp3)−P Bond Construction for the Synthesis of Phosphorylation gem‐Difluoroalkenes under Catalyst‐ and Oxidant‐Free Conditions

Defluorinative C(sp³)?P bond formation of α‐trifluoromethyl alkenes with phosphine oxides or phosphonates have been achieved under catalyst‐ and oxidant‐free conditions, giving phosphorylation gem‐difluoroalkenes as products. α‐Trifluoromethyl alkenes bearing various of aryl substituents such as halogen, cyano, ester and heterocyclic groups are available in this transformation. The results of control experiments demonstrated that the mechanism of dehydrogenative/defluorinative cross‐coupling reactions was not a radical route, but might be an S_N2′ process involving phosphine oxide anion.     

Achiral Pyridine Ligand‐Enabled Enantioselective Radical Oxytrifluoromethylation of Alkenes with Alcohols

A conceptually novel strategy with achiral pyridine as the ancillary ligand to stabilize high‐valent copper species for the first asymmetric radical oxytrifluoromethylation of alkenes with alcohols under Cu^I/phosphoric acid dual‐catalysis has been developed. The transformation features mild reaction conditions, a remarkably broad substrate scope and excellent functional group tolerance, offering an efficient approach to a wide range of trifluoromethyl‐substituted tetrahydrofurans bearing an α‐tertiary stereocenter with excellent enantioselectivity. Mechanistic studies support the presumed role of the achiral pyridine as a coordinative ligand on copper metal to stabilize the key transient reaction species involved in the asymmetric induction process.     

From C1 to C3: Copper‐Catalyzed gem‐Bis(trifluoromethyl)olefination of α‐Diazo Esters with TMSCF3

A Cu‐catalyzed gem‐bis(trifluoromethyl)olefination of α‐diazo esters, using TMSCF₃ as the only fluorocarbon source, has been developed and provides an exquisite method to access gem‐bis(trifluoromethyl)alkenes. This unprecedented olefination process involves a carbene migratory insertion into “CuCF₃” to generate the α‐CF₃‐substituted organocopper species, which then undergoes β‐fluoride elimination and two consecutive addition‐elimination processes to give the desired products. The key to this efficient one‐pot C₁‐to‐C₃ synthetic protocol lies in the controllable double (over single and triple) trifluoromethylations of the gem‐difluoroalkene intermediates.     

Synthesis of Functionalized Indoles with a Trifluoromethyl‐Substituted Stereogenic Tertiary Carbon Atom Through an Enantioselective Friedel–Crafts Alkylation with β‐Trifluoromethyl‐α,β‐enones

Chiral complexes of BINOL‐based ligands with zirconium tert‐butoxide catalyze the Friedel–Crafts alkylation reaction of indoles with β‐trifluoromethyl‐α,β‐unsaturated ketones to give functionalized indoles with an asymmetric tertiary carbon center attached to a trifluoromethyl group. The reaction can be applied to a large number of substituted α‐trifluoromethyl enones and substituted indoles. The expected products were obtained with good yields and ees of up to 99 %.     

Radical Cyclization Reactions via Manganese(III) Acetate Leading to 2‐Thienyl‐Substituted Dihydrofuran Compounds

The 2‐thienyl‐substituted 4,5‐dihydrofuran derivatives 3 – 8 were obtained by the radical cyclization reaction of 1,3‐dicarbonyl compounds 1a – 1f with 2‐thienyl‐substituted conjugated alkenes 2a – 2e by using [Mn(OAc)₃] (Tables 1–5). In this study, reactions of 1,3‐dicarbonyl compounds 1a – 1e with alkenes 2a – 2c gave 4,5‐dihydrofuran derivatives 3 – 5 in high yields (Tables 1–3). Also the cyclic alkenes 2d and 2e gave the dihydrobenzofuran compounds, i.e., 6 and 7 in good yields (Table 4). Interestingly, the reaction of benzoylacetone (=1‐phenylbutane‐1,3‐dione; 1f ) with some alkenes gave two products due to generation of two stable carbocation intermediates (Table 5).     

Ring‐Opening Lithiation–Borylation of 2‐Trifluoromethyl Oxirane: A Route to Versatile Tertiary Trifluoromethyl Boronic Esters

Stereogenic trifluoromethyl‐substituted carbon centers are highly sought‐after moieties in pharmaceutical and agrochemical discovery. Here, we show that lithiation–borylation reactions of 2‐trifluoromethyl oxirane give densely functionalized and highly versatile trifluoromethyl‐substituted α‐tertiary boronic esters. The intermediate boronate complexes undergo the desired 1,2‐rearrangement of the carbon‐based group with complete retentive stereospecificity, a process that was only observed in non‐polar solvents in the presence of TESOTf. Although the trifluoromethyl group adversely affects subsequent transformations of the α‐boryl group, Zweifel olefinations provide trifluoromethyl‐bearing quaternary stereocenters substituted with alkenes, alkynes and ketones.     

Double CF Bond Activation through β‐Fluorine Elimination: Nickel‐Mediated [3+2] Cycloaddition of 2‐Trifluoromethyl‐1‐alkenes with Alkynes

The nickel‐mediated [3+2] cycloaddition of 2‐trifluoromethyl‐1‐alkenes with alkynes afforded fluorine‐containing multi‐substituted cyclopentadienes in a regioselective manner. This reaction involves the consecutive two C? F bond cleavage of a trifluoromethyl or a pentafluoroethyl group through β‐fluorine elimination.     

Radical‐Mediated Three‐Component Coupling of Alkenes

A tandem radical process involving conjugate addition to an activated alkene followed by allylation is reported. B‐Alkylcatecholboranes, easily available via hydroboration of the corresponding alkenes, were used to generate the initial radicals. These radicals add efficiently to electrophilic alkenes such as phenyl vinyl sulfone, N‐phenylmaleimide, and dialkyl fumarate. In the last step of this one‐pot process, the radical adducts react with the allylic sulfones. The whole process can be considered as a unique and selective coupling of three different alkenes.     

A Bulky Thiyl‐Radical Catalyst for the [3+2] Cyclization of N‐Tosyl Vinylaziridines and Alkenes

Thiyl‐radical‐catalyzed cyclization reactions of N‐tosyl vinylaziridines and alkenes were developed as a new synthetic method for the generation of substituted pyrrolidines. The key to making this process accessible to a broad range of substrates is the use of a sterically demanding thiyl radical, which prevents the undesired degradation of the catalyst.     

Trifluoromethylfluorosulfonylation of Unactivated Alkenes Using Readily Available Ag(O2CCF2SO2F) and N‐Fluorobenzenesulfonimide

Presented is a novel intermolecular radical trifluoromethylfluorosulfonylation of unactivated alkenes under mild reaction conditions with good functional‐group tolerance in the most atom‐economic manner by using readily available Ag(O₂CCF₂SO₂F) and N ‐fluorobenzenesulfonimide (NFSI). Both the trifluoromethyl and sulfonyl groups in the products originate from Ag(O₂CCF₂SO₂F).     

Titelbild: Double C?F Bond Activation through β‐Fluorine Elimination: Nickel‐Mediated [3+2] Cycloaddition of 2‐Trifluoromethyl‐1‐alkenes with Alkynes (Angew. Chem. 29/2014)

The nickel‐mediated [3+2] cycloaddition of 2‐trifluoromethyl‐1‐alkenes with alkynes afforded fluorine‐containing multi‐substituted cyclopentadienes in a regioselective manner. This reaction involves the consecutive two C F bond cleavage of a trifluoromethyl or a pentafluoroethyl group through β‐fluorine elimination.     

Copper‐Catalyzed Oxyboration of Unactivated Alkenes

The first regiodivergent oxyboration of unactivated terminal alkenes is reported, using copper alkoxide as a catalyst, bis(pinacolato)diboron [(Bpin)₂] as a boron source, and (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) as an oxygen source. The reaction is compatible with various functional groups. Two regioisomers are selectively produced by selecting the appropriate ligands on copper. The products may be used as a linchpin precursor for various other functionalizations, and net processes such as carbooxygenation, aminooxygenation, and dioxygenation of alkenes can be achieved after C?B bond transformations. Mechanistic studies indicate that the reaction involves the following steps: 1) Transmetalation between CuOtBu and (Bpin)₂ to generate a borylcopper species; 2) regiodivergent borylcupration of alkenes; 3) oxidation of the thus‐generated C?Cu bond to give an alkyl radical; 4) trapping of the resulting alkyl radical by TEMPO.     

Combining Photoredox‐Catalyzed Trifluoromethylation and Oxidation with DMSO: Facile Synthesis of α‐Trifluoromethylated Ketones from Aromatic Alkenes

Trifluoromethylated ketones are useful building blocks for organic compounds with a trifluoromethyl group. A new and facile synthesis of ketones with a trifluoromethyl substituent in the α‐position proceeds through a one‐pot photoredox‐catalyzed trifluoromethylation–oxidation sequence of aromatic alkenes. Dimethyl sulfoxide (DMSO) serves as a key and mild oxidant under these photocatalytic conditions. Furthermore, an iridium photocatalyst, fac[Ir(ppy)₃] (ppy=2‐phenylpyridine), turned out to be crucial for the present photoredox process.     

A Copper(I)‐Catalyzed Enantioselective γ‐Boryl Substitution of Trifluoromethyl‐Substituted Alkenes: Synthesis of Enantioenriched γ,γ‐gem‐Difluoroallylboronates

The first catalytic enantioselective γ‐boryl substitution of CF₃‐substituted alkenes is reported. A series of CF₃‐substituted alkenes was treated with a diboron reagent in the presence of a copper(I)/Josiphos catalyst to afford the corresponding optically active γ,γ‐gem‐difluoroallylboronates in high enantioselectivity. The thus obtained products could be readily converted into the corresponding difluoromethylene‐containing homoallylic alcohols using highly stereospecific allylation reactions.     

A Photoredox‐Induced Stereoselective Dearomative Radical (4+2)‐Cyclization/1,4‐Addition Cascade for the Synthesis of Highly Functionalized Hexahydro‐1H‐carbazoles

A stereoselective synthesis of functionalized hexahydrocarbazoles was developed based on an unprecedented photoredox‐induced dearomative radical (4+2)‐cyclization/1,4‐addition cascade between 3‐(2‐iodoethyl)indoles and acceptor‐substituted alkenes. The title reaction simultaneously generates three C−C bonds and one C−H bond, along with three contiguous stereogenic centers. The hexahydro‐1H ‐carbazole products are highly valuable intermediates for the synthesis of novel antibiotics, as well as unnatural ring homologues of polycyclic indoline alkaloids.     

Asymmetric Diels–Alder Reaction of α‐Substituted and β,β‐Disubstituted α,β‐Enals via Diarylprolinol Silyl Ether for the Construction of All‐Carbon Quaternary Stereocenters

The asymmetric Diels–Alder reaction of α‐substituted acrolein proceeds in the presence of the trifluoroacetic acid salt of trifluoromethyl‐substituted diarylprolinol silyl ether to afford the exo‐isomer with both excellent diastereoselectivity and high enantioselectivity. In the Diels–Alder reaction of a β,β‐disubstituted α,β‐unsaturated aldehyde, good exo‐selectivity and excellent enantioselectivity was obtained when the perchloric acid salt of the bulky triisopropyl silyl ether of trifluoromethyl substituted diarylprolinol was employed as an organocatalyst in the presence of water. In both cases, all‐carbon quaternary stereocenters are constructed enantioselectively.     

Broadly Applicable Z‐ and Diastereoselective Ring‐Opening/Cross‐Metathesis Catalyzed by a Dithiolate Ru Complex

A broadly applicable Ru‐catalyzed protocol for Z‐selective ring‐opening/cross‐metathesis (ROCM) is disclosed. In addition to reactions relating to terminal alkenes of different sizes, the first examples of Z‐selective ROCM processes involving heteroaryl olefins, 1,3‐dienes, and O‐ and S‐substituted alkenes as well as allylic and homoallylic alcohols are reported. Z‐Selective transformations with an α‐substituted allylic alcohol are shown to afford congested Z alkenes with high diastereoselectivity. Transformations are performed in the presence of 2.0–5.0 mol % of a recently disclosed Ru‐based dithiolate complex that can be easily prepared in a single step from commercially available starting materials. Typically, transformations proceed at ambient temperature and are complete within eight hours; products are obtained in up to 97 % yield, >98:2 Z/E, and >98:2 diastereomeric ratio. The present investigations reveal a mechanistically significant attribute of the Ru‐based dithiolates that arises from electrostatic interactions with anionic S‐based ligands.     

Photocatalytic Dehydrogenative Cross‐Coupling of Alkenes with Alcohols or Azoles without External Oxidant

Direct cross‐coupling between alkenes/R‐H or alkenes/RXH is a dream reaction, especially without external oxidants. Inputting energy by photocatalysis and employing a cobalt catalyst as a two‐electron acceptor, a direct C−H/X−H cross‐coupling with H₂ evolution has been achieved for C−O and C−N bond formation. A new radical alkenylation using alkene as the redox compound is presented. A wide range of aliphatic alcohols—even long chain alcohols—are tolerated well in this system, providing a new route to multi‐substituted enol ether derivatives using simple alkenes. Additionally, this protocol can also be used for N ‐vinylazole synthesis. Mechanistic insights reveal that the cobalt catalyst oxidizes the photocatalyst to revive the photocatalytic cycle.     

Visible‐Light‐Driven Aza‐ortho‐quinone Methide Generation for the Synthesis of Indoles in a Multicomponent Reaction

A visible‐light‐driven radical‐mediated strategy for the in situ generation of aza‐ortho ‐quinone methides from 2‐vinyl‐substituted anilines and alkyl radical precursors is described. This process enables an efficient multicomponent reaction of 2‐vinylanilines, halides, and sulfur ylides, and has a wide substrate scope and good functional group tolerance. Treatment of the cycloaddition products with a base leads to densely functionalized indoles in a single‐flask operation.