Rhodium(I)‐Catalyzed Cycloisomerization of Homopropargylallene‐Alkynes through C(sp3)−C(sp) Bond Activation

Upon exposure to a catalytic amount of [RhCl(CO)₂]₂ in 1,4‐dioxane, homopropargylallene‐alkynes underwent a novel cycloisomerization accompanied by the migration of the alkyne moiety of the homopropargyl functional group to produce six/five/five tricyclic compounds in good yields. A plausible mechanism was proposed on the basis of an experiment with ¹³C‐labeled substrate. The resulting tricyclic derivatives were further converted into the corresponding bicyclo[3.3.0] skeletons with vicinal cis dihydroxy groups.     

Organophosphate‐accelerated copper‐catalyzed C(sp2)–C(sp) Sonogashira‐type cross couplings

A dramatic acceleration in copper‐catalyzed Sonogashira‐type reactions was observed when an organophosphate was used as additive. The catalyst systems featuring low copper loading (0.5 mol% < Cu < 5 mol%) gave Sonogashira‐type products with a broad scope of aromatic and aliphatic terminal alkynes as well as aryl iodides in good to excellent yields. Among the organophosphate/copper catalytic systems, that of 4 mol% Cu(OTf)₂/10 mol% (R)‐(?)‐1,1′‐binaphthyl‐2,2′‐diyl hydrogenphosphate exhibited the highest catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Enantioselective Construction of α‐Chiral Silanes by Nickel‐Catalyzed C(sp3)−C(sp3) Cross‐Coupling

An enantioselective C(sp³)?C(sp³) cross‐coupling of racemic α‐silylated alkyl iodides and alkylzinc reagents is reported. The reaction is catalyzed by NiCl₂/(S,S)‐Bn‐Pybox and yields α‐chiral silanes with high enantiocontrol. The catalyst system does not promote the cross‐coupling of the corresponding carbon analogue, corroborating the stabilizing effect of the silyl group on the alkyl radical intermediate (α‐silicon effect). Both coupling partners can be, but do not need to be, functionalized, and hence, even α‐chiral silanes with no functional group in direct proximity of the asymmetrically substituted carbon atom become accessible. This distinguishes the new method from established approaches for the synthesis of α‐chiral silanes.     

Base‐Mediated Defluorosilylation of C(sp2)−F and C(sp3)−F Bonds

The ability to selectively forge C–heteroatom bonds by C?F scission is typically accomplished by metal catalysts, specialized ligands and/or harsh reaction conditions. Described herein is a base‐mediated defluorosilylation of unactivated C(sp²)?F and C(sp³)?F bonds that obviates the need for metal catalysts. This protocol is characterized by its simplicity, mild reaction conditions, and wide scope, even within the context of late‐stage functionalization, constituting a complementary approach to existing C?Si bond‐forming protocols.     

Highly stable magnetically separable copper nanocatalyst as an efficient catalyst for C(sp2)–C(sp) and C(sp2)–C(sp2) cross‐coupling reactions

A copper catalyst has been explored as an efficient and recyclable catalyst to effect Sonogashira and Suzuki cross‐coupling reactions. After modification of 2‐(((piperazin‐1‐ylmethyl)imino)methyl)phenol (PP) on the surface of amorphous silica‐coated iron oxide (Fe₃O₄@SiO₂@Cl) magnetic core–shell nanocomposite, copper(II) chloride was employed to synthesize the Fe₃O₄@SiO₂@PP‐Cu catalyst, affording a copper loading of 1.52 mmol g⁻¹. High yield, low reaction times, non‐toxicity and recyclability of the catalyst are the main merits of this protocol. The catalyst was characterized using Fourier transform infrared, X‐ray photoelectron, energy‐dispersive X‐ray and inductively coupled plasma optical emission spectroscopies, X‐ray diffraction, scanning and transmission electron microscopies, and vibrating sample magnetometry.     

Rhodium‐Catalyzed C(sp2)‐ or C(sp3)−H Bond Functionalization Assisted by Removable Directing Groups

In recent years, transition‐metal‐catalyzed C?H activation has become a key strategy in the field of organic synthesis. Rhodium complexes are widely used as catalysts in a variety of C?H functionalization reactions because of their high reactivity and selectivity. The availability of a number of rhodium complexes in various oxidation states enables diverse reaction patterns to be obtained. Regioselectivity, an important issue in C?H activation chemistry, can be accomplished by using a directing group to assist the reaction. However, to obtain the target functionalized compounds, it is also necessary to use a directing group that can be easily removed. A wide range of directed C?H functionalization reactions catalyzed by rhodium complexes have been reported to date. In this Review, we discuss Rh‐catalyzed C?H functionalization reactions that are aided by the use of a removable directing group such as phenol, amine, aldehyde, ketones, ester, acid, sulfonic acid, and N‐heteroaromatic derivatives.     

Visible‐Light‐Promoted Iron‐Catalyzed C(sp2)–C(sp3) Kumada Cross‐Coupling in Flow

A continuous‐flow, visible‐light‐promoted method has been developed to overcome the limitations of iron‐catalyzed Kumada–Corriu cross‐coupling reactions. A variety of strongly electron rich aryl chlorides, previously hardly reactive, could be efficiently coupled with aliphatic Grignard reagents at room temperature in high yields and within a few minutes’ residence time, considerably enhancing the applicability of this iron‐catalyzed reaction. The robustness of this protocol was demonstrated on a multigram scale, thus providing the potential for future pharmaceutical application.     

Rhodium(I)‐Catalyzed [4 + 2] Cycloaddition Reactions of 2‐Alkylenecyclo‐butanols with Alkynes and (E)‐2‐Nitroethenylbenzene through C(sp2)—C(sp3) Bond Cleavage

An intermolecular [4 + 2] cycloaddition was realized through C—C bond cleavage in the presence of Rh(I) catalyst. The selective ring opening of 2‐alkylenecyclobutanols enables the generation of active alkenylrhodium species, which underwent smooth cross addition over alkynes and (E)‐2‐nitroethenylbenzene, leading to highly substituted all‐carbon six‐membered rings in a single step and in a complete atom economy.     

Nitrimines as Reagents for Metal‐Free Formal C(sp2)–C(sp2) Cross‐Coupling Reactions

Nitrimines are employed as powerful reagents for metal‐free formal C(sp²)–C(sp²) cross‐coupling reactions. The new chemical process is tolerant of a wide array of nitrimine and heterocyclic coupling partners giving rise to the corresponding di‐ or trisubstituted alkenes, typically in high yield and with high stereoselectivity. This method is ideal for the metal‐free construction of heterocycle‐containing drug targets, such as phenprocoumon.     

Cu(I)‐Catalyzed Asymmetric Cross‐Coupling of N‐Tosylhydrazones and Trialkylsilylethynes: Enantioselective Construction of C(sp)—C(sp3) Bonds

The first catalytic enantioselective C(sp)―C(sp³) cross‐coupling reaction between N‐tosylhydrazones and trialkylsilylethynes in the presence of Cu(I) salts and chiral phosphoramidite ligands was developed. A series of synthetically interesting, functionalized alkynes were obtained with moderate to good enantioselectivities (up to 83% ee). Cu(II) carbene migratory insertion is proposed to be the enantio‐determining step.     

Copper‐catalyzed C(sp2)–C(sp) Sonogashira‐type cross‐coupling reactions accelerated by polycyclic aromatic hydrocarbons

Copper‐catalyzed Sonogashira‐type reactions were dramatically accelerated by introducing a catalytic amount of polycyclic aromatic hydrocarbon additive. This novel catalytic system features low copper loading (0.5 mol% < Cu < 5 mol%), broad reaction scope and remarkable substrate tolerance. Both aromatic and aliphatic terminal alkynes as well as diverse aryl iodides were employed in this transformation, affording respectable yields of the desired products. The novel Cu(OTf)₂/pyrene system was subsequently employed to synthesize phenylacetylene‐based fluorescent compounds. Copyright © 2015 John Wiley & Sons, Ltd.     

Rhodium(I)‐Catalyzed Sequential C(sp)C(sp3) and C(sp3)C(sp3) Bond Formation through Migratory Carbene Insertion

A Rh^I‐catalyzed three‐component reaction of tert‐propargyl alcohol, diazoester, and alkyl halide has been developed. This reaction can be considered as a carbene‐involving sequential alkyl and alkynyl coupling, in which C(sp)? C(sp³) and C(sp³)? C(sp³) bonds are built successively on the carbenic carbon atom. The Rh^I‐carbene migratory insertion of an alkynyl moiety and subsequent alkylation are proposed to account for the two separate C? C bond formations. This reaction provides an efficient and tunable method for the construction of all‐carbon quaternary center.