Oxydifluoromethylation of Alkenes by Photoredox Catalysis: Simple Synthesis of CF2H‐Containing Alcohols

We have developed a novel and simple protocol for the direct incorporation of a difluoromethyl (CF₂H) group into alkenes by visible‐light‐driven photoredox catalysis. The use of fac‐[Ir(ppy)₃] (ppy=2‐pyridylphenyl) photocatalyst and shelf‐stable Hu's reagent, N‐tosyl‐S‐difluoromethyl‐S‐phenylsulfoximine, as a CF₂H source is the key to success. The well‐designed photoredox system achieves synthesis of not only β‐CF₂H‐substituted alcohols but also ethers and an ester from alkenes through solvolytic processes. The present method allows a single‐step and regioselective formation of C(sp³)–CF₂H and C(sp³)?O bonds from C=C moiety in alkenes, such as hydroxydifluoromethylation, regardless of terminal or internal alkenes. Moreover, this methodology tolerates a variety of functional groups.     

High Reactivity of Strained Seven‐Membered‐Ring trans‐Alkenes

trans‐Oxasilacycloheptenes are highly reactive strained alkenes. Competition reactions showed that these seven‐membered ring trans‐alkenes underwent [4+2] cycloaddition reactions faster than a trans‐cyclooctene. They also reacted with quinones and dimethyl acetylenedicarboxylate to form adducts with high diastereoselectivity. Kinetic studies showed that ring strain increases nucleophilicity by approximately 10⁹.     

Synthesis of Cyclobutenes and Allenes by Cobalt‐Catalyzed Cross‐Dimerization of Simple Alkenes with 1,3‐Enynes

Cobalt‐catalyzed cross‐dimerization of simple alkenes with 1,3‐enynes is reported. A [2+2] cycloaddition reaction occurred, with alkenes bearing no allylic hydrogen, by reductive elimination of a η³‐butadienyl cobaltacycle. On the other hand, aliphatic alkenes underwent 1,4‐hydroallylation by means of exo‐cyclic β‐H elimination. These reactions can provide cyclobutenes and allenes that were previously difficult to access, from simple substrates in a highly chemo‐ and regioselective manner.     

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.     

Photoredox‐Catalyzed Stereoselective Conversion of Alkynes into Tetrasubstituted Trifluoromethylated Alkenes

A regio‐ and stereoselective synthesis of trifluoromethylated alkenes bearing four different substituents has been developed. Stereocontrolled sulfonyloxytrifluoromethylation of unsymmetric internal alkynes with an electrophilic CF₃ reagent, namely the triflate salt of the Yagupol’skii–Umemoto reagent, in the presence of an Ir photoredox catalyst under visible‐light irradiation afforded trifluoromethylalkenyl triflates with well‐predictable stereochemistry resulting from anti addition of the trifluoromethyl and triflate groups. Subsequent palladium‐catalyzed cross‐couplings led to tetrasubstituted trifluoromethylated alkenes in a highly stereoselective manner. The present method is the first example of a facile one‐pot synthesis of tetrasubstituted trifluoromethylated alkenes from simple alkynes.     

An Alternative Mechanism for the Cobalt‐Catalyzed Isomerization of Terminal Alkenes to (Z)‐2‐Alkenes

The cobalt‐catalyzed selective isomerization of terminal alkenes to the thermodynamically less‐stable (Z)‐2‐alkenes at ambient temperatures takes place by a new mechanism involving the transfer of a hydrogen atom from a Ph₂PH ligand to the starting material and the formation of a phosphenium complex, which recycles the Ph₂PH complex through a 1,2‐H shift.     

Branched‐Selective Direct α‐Alkylation of Cyclic Ketones with Simple Alkenes

Herein, we describe an intermolecular direct branched‐selective α‐alkylation of cyclic ketones with simple alkenes as the alkylation agents. Through an enamine‐transition metal cooperative catalysis mode, the α‐alkylation is realized in an atom‐ and step‐economic manner with excellent branched selectivity for preparing β‐branched ketones. Employment of a pair of bulky Brønsted acid and base as additives is responsible for enhanced efficiency. Promising enantioselectivity (74 % ee) has been obtained. Experimental and computational mechanistic studies suggest that a pathway through alkene migratory insertion into the Ir?C bond followed by C?H reductive elimination is involved for the high branched selectivity.     

Highly Enantioselective Iron‐Catalyzed cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide Oxidant via an FeIII‐OOH Reactive Intermediate

The development of environmentally benign catalysts for highly enantioselective asymmetric cis‐dihydroxylation (AD) of alkenes with broad substrate scope remains a challenge. By employing [Fe^II(L)(OTf)₂] (L=N,N′‐dimethyl‐N,N′‐bis(2‐methyl‐8‐quinolyl)‐cyclohexane‐1,2‐diamine) as a catalyst, cis‐diols in up to 99.8 % ee with 85 % isolated yield have been achieved in AD of alkenes with H₂O₂ as an oxidant and alkenes in a limiting amount. This “[Fe^II(L)(OTf)₂]+H₂O₂” method is applicable to both (E)‐alkenes and terminal alkenes (24 examples >80 % ee, up to 1 g scale). Mechanistic studies, including ¹⁸O‐labeling, UV/Vis, EPR, ESI‐MS analyses, and DFT calculations lend evidence for the involvement of chiral Fe^III‐OOH active species in enantioselective formation of the two C?O bonds.     

Cobalt‐Catalyzed Isomerization of 1‐Alkenes to (E)‐2‐Alkenes with Dimethylphenylsilylmethylmagnesium Chloride and Its Application to the Stereoselective Synthesis of (E)‐Alkenylsilanes

“Co”axing selectivity into isomerization : Treatment of 1‐alkenes with dimethylphenylsilylmethylmagnesium chloride in the presence of a cobalt‐NHC complex in dioxane at 50 °C or higher provides the corresponding (E)‐2‐alkenes selectively. The isomerization is applicable to the stereoselective synthesis of (E)‐crotylsilanes and (E)‐1‐propenylsilanes from the corresponding homoallylsilanes and allylsilanes, respectively.

     

Gold‐Catalyzed 1,2‐Diarylation of Alkenes

Herein, we disclose the gold‐catalyzed 1,2‐diarylation of alkenes through the interplay of ligand‐enabled Au^I/Au^III catalysis with the idiosyncratic π‐activation mode of gold complexes. Unlike the classical migratory‐insertion‐based approach to 1,2‐diarylation, the present approach not only circumvents the formation of direct Ar?Ar′ coupling and Heck‐type side products but more intriguingly demonstrates reactivity and selectivity complementary to those of previously known metal catalysis (Pd, Ni, or Cu). Detailed investigations to underpin the mechanistic scenario revealed oxidative addition of aryl iodides to an Au^I complex to be the rate‐limiting step owing to the non‐innocent nature of the aryl alkene.     

High‐Resolution Photoelectron Imaging of IrB3−: Observation of a π‐Aromatic B3+ Ring Coordinated to a Transition Metal

In a high‐resolution photoelectron imaging and theoretical study of the IrB₃^? cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B₃ ring coordinated with the Ir atom. The isomer with the higher EA consists of a B₃ ring with a bridge‐bonded Ir atom (C_s , ²A′), and the second isomer features a tetrahedral structure (C_3v , ²A₁). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C_3v isomer involves significant covalent Ir?B bonding and weak ionic bonding with charge transfer from B₃ to Ir, and can be viewed as an Ir–(η³‐B₃⁺) complex. This study provides the first example of a boron‐to‐metal charge‐transfer complex and evidence of a π‐aromatic B₃⁺ ring coordinated to a transition metal.     

Mechanism of Ligand‐Controlled Regioselectivity‐Switchable Copper‐Catalyzed Alkylboration of Alkenes

Cu‐catalyzed alkylboration of alkenes with bis(pinacolato)diboron ((Bpin)₂) and alkyl halides provides a ligand‐controlled regioselectivity‐switchable method for the construction of complex boron‐containing compounds. Here, we employed DFT methods to elucidate the mechanistic details of this reaction and the origin of the different regioselectivity induced by Xantphos and Cy‐Xantphos. The calculation results reveal that the catalytic cycle mainly proceeds through the migratory insertion of alkenes on Cu‐Bpin complex, the oxidative addition of alkyl halides, and the reductive elimination of a C?C bond. Meanwhile, the rate‐ determining step is the oxidative addition of alkyl halides and the regioselectivity‐determining step is the migratory insertion of alkenes. The bulky cyclohexyl group of Cy‐Xantphos facilitates the approach of the substituents of alkenes to Bpin in the migratory insertion step and thus leads to the Markovnikov products. The less bulky phenyl group on Xantphos prefers keeping the substituents of alkenes away from the Bpin moiety in the migratory insertion step and thus results in anti‐Markovnikov products.     

Iron‐Catalyzed Highly Enantioselective cis‐Dihydroxylation of Trisubstituted Alkenes with Aqueous H2O2

Reliable methods for enantioselective cis‐dihydroxylation of trisubstituted alkenes are scarce. The iron(II) complex cis‐α‐[Fe^II(2‐Me₂‐BQPN)(OTf)₂], which bears a tetradentate N₄ ligand (Me₂‐BQPN=(R,R)‐N,N′‐dimethyl‐N,N′‐bis(2‐methylquinolin‐8‐yl)‐1,2‐diphenylethane‐1,2‐diamine), was prepared and characterized. With this complex as the catalyst, a broad range of trisubstituted electron‐deficient alkenes were efficiently oxidized to chiral cis‐diols in yields of up to 98 % and up to 99.9 % ee when using hydrogen peroxide (H₂O₂) as oxidant under mild conditions. Experimental studies (including ¹⁸O‐labeling, ESI‐MS, NMR, EPR, and UV/Vis analyses) and DFT calculations were performed to gain mechanistic insight, which suggested possible involvement of a chiral cis‐Fe^V(O)₂ reaction intermediate as an active oxidant. This cis‐[Fe^II(chiral N₄ ligand)]²⁺/H₂O₂ method could be a viable green alternative/complement to the existing OsO₄‐based methods for asymmetric alkene dihydroxylation reactions.     

Copper‐Catalyzed 1,2‐Methoxy Methoxycarbonylation of Alkenes with Methyl Formate

Reported here is a copper‐catalyzed 1,2‐methoxy methoxycarbonylation of alkenes by an unprecedented use of methyl formate as a source of both the methoxy and the methoxycarbonyl groups. This reaction transforms styrene and its derivatives into value‐added β‐methoxy alkanoates and cinnamates, as well as medicinally important five‐membered heterocycles, such as functionalized tetrahydrofurans, γ‐lactones, and pyrrolidines. A ternary β‐diketiminato‐Cu^I‐styrene complex, fully characterized by NMR spectroscopy and X‐ray crystallographic analysis, is capable of catalyzing the same transformation. These findings suggest that pre‐coordination of electron‐rich alkenes to copper might play an important role in accelerating the addition of nucleophilic radicals to electron‐rich alkenes, and could have general implications in the design of novel radical‐based transformations.     

Enantioselective Radical‐Polar Crossover Reactions of Indanonecarboxamides with Alkenes

Highly efficient asymmetric intermolecular radical‐polar crossover reactions were realized by combining a chiral N,N′‐dioxide/Ni^II complex catalyst with Ag₂O under mild reaction conditions. Various terminal alkenes and indanonecarboxamides/esters underwent radical addition/cyclization reactions to afford spiro‐iminolactones and spirolactones with good to excellent yields (up to 99 %) and enantioselectivities (up to 97 % ee). Furthermore, a range of different radical‐mediated oxidation/elimination or epoxide ring‐opening products were obtained under mild reaction conditions. The Lewis acid catalysts exhibited excellent performance and precluded the strong background reaction.     

Carbonyl–carbonyl and carbonyl–π interactions as directing motifs in the supramolecular structure of carbonyl(3‐oxopenta‐1,4‐diene‐1,5‐diyl)[tris(3,5‐dimethyl‐1H‐pyrazol‐1‐yl‐κN2)methane]iridium(III) trifluoromethanesulfonate

In the title complex salt, [Ir(C₅H₄O)(C₁₆H₂₂N₆)(CO)](CF₃O₃S), the Ir^III centre adopts a distorted octahedral geometry with a facial coordination of the tris(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane ligand. The C—C distances of the iridacycle are in agreement with its iridacyclohexa‐2,5‐dien‐4‐one nature, which presents a nonsymmetric boat‐like conformation with the C—Ir—C vertex more bent than the C—C(=O)—C vertex. The supramolecular architecture is mainly directed by CO...CO and CO...π and Csp³—H...O interactions, the arrangement of which depends on the anion.     

Synthesis of trans‐Disubstituted Alkenes by Cobalt‐Catalyzed Reductive Coupling of Terminal Alkynes with Activated Alkenes

A cobalt‐catalyzed reductive coupling of terminal alkynes, RC?CH, with activated alkenes, R′CH?CH₂, in the presence of zinc and water to give functionalized trans‐disubstituted alkenes, RCH?CHCH₂CH₂R′, is described. A variety of aromatic terminal alkynes underwent reductive coupling with activated alkenes including enones, acrylates, acrylonitrile, and vinyl sulfones in the presence of a CoCl₂/P(OMe)₃/Zn catalyst system to afford 1,2‐trans‐disubstituted alkenes with high regio‐ and stereoselectivity. Similarly, aliphatic terminal alkynes also efficiently participated in the coupling reaction with acrylates, enones, and vinyl sulfone, in the presence of the CoCl₂/P(OPh)₃/Zn system providing a mixture of 1,2‐trans‐ and 1,1‐disubstituted functionalized terminal alkene products in high yields. The scope of the reaction was also extended by the coupling of 1,3‐enynes and acetylene gas with alkenes. Furthermore, a phosphine‐free cobalt‐catalyzed reductive coupling of terminal alkynes with enones, affording 1,2‐trans‐disubstituted alkenes as the major products in a high regioisomeric ratio, is demonstrated. In the reactions, less expensive and air‐stable cobalt complexes, a mild reducing agent (Zn) and a simple hydrogen source (water) were used. A possible reaction mechanism involving a cobaltacyclopentene as the key intermediate is proposed.     

Copper‐Catalyzed Alkylarylation of Unactivated Alkenes: Synthesis of 3‐Alkyl Indolines from N‐Allyl Anilines and Alkanes

A rare example of C(sp³)?H functionalization of simple alkanes with unactivated alkenes is presented. In the presence of a copper salt and di‐tert‐butyl peroxide (DTBP), N‐allyl anilines underwent exo‐selective alkylation/cyclization cascade with unactivated alkenic bonds as radical acceptors and simple alkanes as radical precursors, providing a direct access to 3‐alkyl indolines. The present protocol features simple operation, broad substrate scope and great exo selectivity.     

Highly soluble green‐emitting Ir(III) complexes with 9‐(6‐phenyl‐pyridin‐3‐ylmethyl)‐9H‐carbazole ligands and their application to polymer light‐emitting diodes

Highly organic soluble Ir(III) complexes with 9‐(6‐phenyl‐pyridin‐3‐ylmethyl)‐9H‐carbazole were simply synthesized, and the solubility of the new complex was significantly improved when compared with the conventional green‐emitting Ir(ppy)₃. Since a carbazole group is tethered through a nonconjugated methylene spacer, the photophysical properties of new complexes are almost identical with those of conventional Ir(ppy)₃. The pure complexes were utilized to prepare electrophosphorescent polymer light‐emitting diodes (PLEDs). The device performances were observed to be relatively better or comparable with those of Ir(ppy)₃ based poly(N‐vinylcarbazole) systems. The integration of rigid hole‐transporting carbazole and phosphorescent complex provides a new route to design highly efficient solution‐processable complex for electrophosphorescent PLED applications. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7419–7428, 2008     

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.