Radical Monofluoroalkylative Alkynylation of Olefins by a Docking–Migration Strategy

A radical‐mediated monofluoroalkylative alkynylation of alkenes is disclosed for the first time. The reaction demonstrates a remarkably broad substrate scope in which both activated and unactivated alkenes are suitable starting materials. The concurrent addition of an alkynyl and a monofluoroalkyl group onto an alkene proceeds through a docking–migration sequence, affording a vast array of valuable fluoroalkyl‐substituted alkynes. Many complex natural products and drug derivatives are readily functionalized, demonstrating that this method can be used for late‐stage alkynylation.     

Non-stabilized transition metal carbenes as intermediates in intramolecular reactions of alkynes with alkenes

In this tutorial review we summarize the two major pathways followed in the reaction of alkenes with alkynes catalysed by electrophilic transition metals. If the metal coordinates simultaneously to the alkyne and the alkene, an oxidative cyclometallation can ensue to give a metallacyclopentene, which usually evolves by [small beta]-hydrogen elimination to give Alder-ene cycloisomerisation derivatives. On the other hand, coordination of the metal to the alkyne promotes the attack of the alkene to give metal cyclopropyl carbenes.     

Synthesis of Spirocyclic Ethers by Enantioselective Copper‐Catalyzed Carboetherification of Alkenols

Spirocyclic ethers can be found in bioactive compounds. This copper‐catalyzed enantioselective alkene carboetherification provides 5,5‐, 5,6‐ and 6,6‐spirocyclic products containing fully substituted chiral carbon centers with up to 99 % enantiomeric excess. This reaction features the formation of two rings from acyclic substrates, 1,1‐disubstituted alkenols functionalized with either arenes, alkenes, or alkynes, and clearly constitutes a powerful way to synthesize chiral spirocyclic ethers.     

Rhenium(I)‐Catalyzed ortho‐C−H Addition to Bicyclic Alkenes

Hydroarylation of bicyclic alkenes has been developed using a low‐valent Re^I‐catalyzed, directing group‐assisted C?H bond activation strategy. The addition of sodium acetate significantly improves the reaction efficiency; moreover, bicyclic alkenes such as 7‐oxa and aza benzonorbornadienes worked efficiently under this reaction condition. Preliminary mechanistic studies suggest that, after the alkene insertion, the rhenacycle preferentially undergoes protonolysis rather than reductive elimination.     

Catalytic,Enantioselective α‐Alkylation of Azlactones with Nonconjugated Alkenes by Directed Nucleopalladation

A palladium(II)‐catalyzed enantioselective α‐alkylation of azlactones with nonconjugated alkenes is described. The reaction employs a chiral BINOL‐derived phosphoric acid as the source of stereoinduction, and a cleavable bidentate directing group appended to the alkene to control the regioselectivity and stabilize the nucleopalladated alkylpalladium(II) intermediate in the catalytic cycle. A wide range of azlactones were found to be compatible under the optimal reaction conditions to afford products bearing α,α‐disubstituted α‐amino‐acid derivatives with high yields and high enantioselectivity.     

Alkenyl complexes of platinum group metals: a platform for diverse reactivity patterns

Intermediates for many catalysis reactions reported in the literature are metal‐alkyl and metal‐alkenyl, including metallacycloalkane species. Synthesis and reactions of metal‐alkyl, alkenyl and metallacyle complexes have shown a great deal of development during the past few decades. This review summarizes the significant contributions reported on metal‐alkenyl compounds, specifically those containing at least a carbon chain with pendant alkene group [M―CH₂CH₂CH?CH₂]. Although metal‐alkenyl complexes are stable with strong chelating diphosphines and with a decrease in the ligand donor strength, the complexes can decompose without any ambiguity. For example, platinum‐dialkenyl complexes react readily via β‐hydrogen elimination and reductive elimination promoted by the nature of the ligand, solvent and length of carbon chains. These complexes can also undergo intramolecular irreversible isomerization and this leads to the selective catalytic isomerization of 1‐alkenes to 2‐alkenes in the presence of platinum‐dialkenyl complexes as catalysts. Perhaps the most striking manifestations of flexibility are the facile and complete intramolecular and intermolecular alkene metathesis to yield the corresponding metallacycloalkenes in the presence of Grubbs’ catalysts. The diverse chemical reactivity of these complexes demonstrates both the scope and complexity of metal‐alkenyl chemistry depending on the nature of ligand and metal. Copyright © 2014 John Wiley & Sons, Ltd.     

Monomeric Magnesium Catalyzed Alkene and Alkyne Hydroboration

In this work, two monomeric magnesium alkyl complexes ( 1 and 2 ) were prepared using bis(phosphino)carbazole framework and among them 1 has been used as a catalyst for hydroboration of alkenes and alkynes with pinacolborane (HBpin). A broad variety of aromatic and aliphatic alkenes and alkynes were efficiently reduced. Anti-Markovnikov regioselective hydroboration of alkenes and alkynes was achieved, which was confirmed by deuterium-labelling experiments. The work represents the first example of the use of magnesium in homogeneous catalytic hydroboration of alkene with broad substrate scope. Experimental mechanistic investigations and DFT calculations provided insights into the reaction mechanism. Finally, the hydroboration protocol was extended to terpenes.     

Stereoselective single (copper) or double (platinum) boronation of alkynes catalyzed by magnesia-supported copper oxide or platinum nanoparticles

Copper(II) oxide nanoparticles supported on magnesia have been prepared from Cu(II) supported on magnesia by hydrogen reduction at 400 °C followed by storage under ambient conditions. X-ray photoelectron spectroscopy of the material clearly shows that immediately after the reduction copper(0)-metal nanoparticles are present on the magnesia support, but they undergo fast oxidation to copper oxide upon contact with the ambient for a short time. TEM images show that the catalytically active CuO/MgO material is formed of well-dispersed copper oxide nanoparticles supported on fibrous MgO. CuO/MgO exhibits a remarkable catalytic activity for the monoborylation of aromatic, aliphatic, terminal, and internal alkynes, the products being formed with high regio- (borylation at the less substituted carbon) and stereoselectivity (trans-configured). CuO/MgO exhibits complete chemoselectivity towards the monoborylation of alkynes in the presence of alkenes. Other metal nanoparticles such as gold or palladium are inactive towards borylation, but undergo undesirable oligomerization or partial hydrogenation of the C≡C triple bond. In contrast, platinum, either supported on magnesia or on nanoparticulate ceria, efficiently promotes the stereoselective diborylation of alkynes to yield a cis-configured diboronate alkene. By using platinum as the catalyst we have developed a tandem diborylation/hydrogenation reaction that gives vic-diboronated alkanes from alkynes in one pot.     

Photoreactions of bicyclic aziridines with alkenes and alkynes: a novel synthetic methodology for 8-azabicyclo[3.2.1]octane derivatives

The photochemical C-C bond cleavage of bicyclic aziridines 7 and subsequent [3 + 2] cycloaddition with electron-deficient alkenes and alkynes afforded the novel head-to-head adducts selectively and efficiently. The adducts contain the naturally occurring 8-azabicyclo[3.2.1]octane skeleton (e.g. tropane alkaloids). The aziridine 8 fused with a 6-membered ring also afforded the cycloadducts but in poor yields. The methylaziridine 9 reacted with an electron-deficient alkene, affording the head-to-tail adduct 23 in addition to head-to-head adducts 22a and 22b. The photoreactions of bicyclic aziridines with alkenes and alkynes indicate a similar behavior to that of aziridines with a linear chain.     

Copper-catalyzed intramolecular alkene carboetherification: synthesis of fused-ring and bridged-ring tetrahydrofurans

Fused-ring and bridged-ring tetrahydrofuran scaffolds are found in a number of natural products and biologically active compounds. A new copper-catalyzed intramolecular carboetherification of alkenes for the synthesis of bicyclic tetrahydrofurans is reported herein. The reaction involves Cu-catalyzed intramolecular addition of alcohols to unactivated alkenes and subsequent aryl C-H functionalization provides the C-C bond. Mechanistic studies indicate a primary carbon radical intermediate is involved and radical addition to the aryl ring is the likely C-C bond-forming mechanism. Preliminary catalytic enantioselective reactions are promising (up to 75% ee) and provide evidence that copper is involved in the alkene addition step, likely through a cis-oxycupration mechanism. Catalytic enantioselective alkene carboetherification reactions are rare and future development of this new method into a highly enantioselective process is promising. During the course of the mechanistic studies a protocol for alkene hydroetherification was also developed.     

N-heterocyclic carbene platinum complexes functionalized with a polyether chain and silyl group: Synthesis and application as a catalyst for hydrosilylation

The synthesis and characterization of new N-heterocyclic carbene platinum(II) complexes functionalized with a polyether chain and silyl group are described. In addition, their application towards the catalytic hydrosilylation of unsaturated carbon–carbon bonds, including alkenes, alkynes, vinyl ether, and unsaturated esters, is reported. These new complexes exhibit both excellent catalytic activity and selectivity for hydrosilylation. The catalytic system can be recycled >27 times.     

Catalytic,Enantioselective α‐Alkylation of Azlactones with Nonconjugated Alkenes by Directed Nucleopalladation

A palladium(II)‐catalyzed enantioselective α‐alkylation of azlactones with nonconjugated alkenes is described. The reaction employs a chiral BINOL‐derived phosphoric acid as the source of stereoinduction, and a cleavable bidentate directing group appended to the alkene to control the regioselectivity and stabilize the nucleopalladated alkylpalladium(II) intermediate in the catalytic cycle. A wide range of azlactones were found to be compatible under the optimal reaction conditions to afford products bearing α,α‐disubstituted α‐amino‐acid derivatives with high yields and high enantioselectivity.     

An Alkene‐Promoted Borane‐Catalyzed Highly Stereoselective Hydrogenation of Alkynes to Give Z‐ and E‐Alkenes

The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Z‐alkenes, the hydrogenation of alkynes to give E‐alkenes is still not well resolved. In particular, selective preparation of both Z‐ and E‐alkenes by the same catalytic hydrogenation system using molecular H₂ has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C₆F₅)₂‐catalyzed metal‐free hydrogenation of alkynes was adopted. Significantly, both Z‐ and E‐alkenes can be furnished by hydrogenation with molecular H₂ in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H₂ play an essential role in promoting the hydrogenolysis reaction.     

Formal and standard ruthenium-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,6-diynes to alkenes: a mechanistic density functional study

"Formal" and standard Ru(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,6-diynes 1 to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. The neutral Ru(II) catalyst was formed in situ by mixing equimolecular amounts of [Cp*Ru(CH3CN)3]PF6 and Et4NCl. Two isomeric bicyclic 1,3-cyclohexadienes 3 and 8 were obtained depending on the cyclic or acyclic nature of the alkene partner. Mechanistic studies on the Ru catalytic cycle revealed a clue for this difference: (a) when acyclic alkenes were used, linear coupling of 1,6-diynes with alkenes was observed giving 1,3,5-trienes 6 as the only initial reaction products, which after a thermal disrotatory 6e-pi electrocyclization led to the final 1,3-cyclohexadienes 3 as probed by NMR studies. This cascade process behaved as a formal Ru-catalyzed [2 + 2 + 2] cycloaddition. (b) With cyclic alkenes, the standard Ru-catalyzed [2 + 2 + 2] cycloaddition occurred, giving the bicyclic 1,3-cyclohexadienes 8 as reaction products. A complete catalytic cycle for the formal and standard Ru-catalyzed [2 + 2 + 2] cycloaddition of acetylene and cyclic and acyclic alkenes with the Cp*RuCl fragment has been proposed and discussed based on DFT/B3LYP calculations. The most likely mechanism for these processes would involve the formation of ruthenacycloheptadiene intermediates XXIII or XXVII depending on the alkene nature. From these complexes, two alternatives could be envisioned: (a) a reductive elimination in the case of cyclic alkenes 7 and (b) a beta-elimination followed by reductive elimination to give 1,3,5-hexatrienes 6 in the case of acyclic alkenes. Final 6e-pi electrocyclization of 6 gave 1,3-cyclohexadienes 3.     

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.     

Electrophilic activation of alkenes by platinum(II): so much more than a slow version of palladium(II)

The electrophilic activation of alkenes by transition-metal catalysts is a fundamental step in a rapidly growing number of catalytic processes. Although palladium is the best known metal for this purpose, the special properties of its third-row cousin platinum (strong metal-ligand bonds and slow substitution kinetics) have enabled the development of transformations that are initiated by addition to the C=C bonds by protic carbon, nitrogen, oxygen, and phosphorus nucleophiles, as well as alkene or arene nucleophiles. Additionally, reactivity profiles, which are often unique to platinum, provide wholly new reaction products. This Review concerns platinum-catalyzed electrophilic alkene activation reactions, with a special emphasis on the mechanistic properties of known systems, on the differences between platinum and palladium catalysts, and on the prospects for the development of new systems.     

Rh-catalyzed addition of boronic acids to alkynes for the synthesis of trisubstituted alkenes in a biphasic system - Mechanistic study and recycling of the Rh/m-TPPTC catalyst

The versatile preparation of trisubstituted alkenes via selective Rh-catalyzed arylation of alkynes is described in water and in a water/toluene biphasic system. For hydrophobic alkyl alkynes, the reaction afforded either alkenes or dienes depending on the temperature and the solvent conditions. Aryl, heteroaryl, silylated and alkyl substituted alkynes reacted equally well with various boronic acids, leading regioselectively to functionalized alkenyl derivatives in high yields (65-99%). The mechanism was investigated in toluene/water mixture or water and involves a vinylrhodium complex. The efficient recycling of the Rh/m-TPPTC system is disclosed with excellent yield (92-96%) and purity of the alkene.     

Cationic palladium-catalyzed hydrosilylative cross-coupling of alkynes with alkenes

A cationic palladium complex-catalyzed cross-coupling of alkynes with alkenes is presented, which occurs selectively under the hydrosilylation conditions using trichlorosilane. The unique reaction might be well understood in terms of an initial hydropalladation of a given 1-alkyne to form regioselectively a 1-alkenylpalladium species, which, in turn, undergoes easily and specifically an alkene insertion. The resulting homoallylic organopalladium species terminates one catalytic cycle by substituting the palladium center with a trichlorosilyl group to give product(s).     

Model Studies towards the Synthesis of the Right‐Hand Part of Pederin

Bicyclic acetal derivatives of the type 3 were prepared based either on a dihydroxyaldehyde 5 or an oxiranebutanal 6 (cf. Scheme 2). Lewis acid catalyzed reaction of the bicyclic acetal with allyltrimethylsilane introduces the side chain (as yet unfunctionalized) and sets the stereogenic centers at the tetrahydro‐2H‐pyran ring of the pederin moiety.     

Nickel‐Catalyzed Allylmethylation of Alkynes with Allylic Alcohols and AlMe3: Facile Access to Skipped Dienes and Trienes

We present herein an unprecedented allylative dicarbofunctionalization of alkynes with allylic alcohols. This simple catalytic procedure utilizes commercially available Ni(COD)₂, triphenylphosphine, and inexpensive reagents, and delivers valuable skipped dienes and trienes with an all‐carbon tetrasubstituted alkene unit in a highly stereoselective fashion. Preliminary mechanistic studies support the reaction pathway of allylnickelation followed by transmetalation in this dicarbofunctionalization of alkynes.