Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher-Order Cycloaddition Reactions

Enantioselective Multifunctionalization with Rh Carbynoids

Multifunctionalization from the interception of active intermediates is an attractive synthetic strategy for the efficient construction of complex molecular scaffolds in an atom and step economic fashion. However, the design of reactions involving metal carbynoids that exhibit carbene/carbocation behavior is currently limited, and developing catalyst-controlled highly enantioselective versions poses significant challenges. In this study, we present the first asymmetric trifunctionalization reactions with rhodium carbynoids. This reaction unveils the distinctive reactivity of the carbynoid precursor, enabling it to react with simultaneously two nucleophiles and one electrophile. This process involves the formation of two distinct carbene ylides with the alcohol/carbamate and the trapping of one ylide with the imine, resulting in the formation of three new bonds. Furthermore, this strategy allows for the divergent synthesis of a wide array of β-amino esters in high yields and exceptional enantioselectivity.

Synthesis of Fluorescent Pyrrolo[1,2-a]pyrimidines from Fischer Carbene Complexes as Building Blocks

A novel synthetic methodology is reported for the synthesis of fluorescent pyrrolo[1,2-a]pyrimidines. Fischer carbene complexes served as the synthetic platform for (3+3) cyclization to form the heterocyclic moiety. The reaction process furnished two products, their ratio being modulated by the metal, base, and solvent. The selectivity exhibited was studied by analyzing the potential energy surface with density functional theory tools. The photophysical properties of absorption and emission were also evaluated. The dyes absorbed at wavelengths of 240-440 nm, depending on the substituents. The maximum emission wavelength was in the range of 470-513 nm, with quantum yields of 0.36-1.0 and a high Stokes shift range of 75-226 nm.

Carbene Radicals in Transition-Metal-Catalyzed Reactions

Discovered as organometallic curiosities in the 1970s, carbene radicals have become a staple in modern-day homogeneous catalysis. Carbene radicals exhibit nucleophilic radical-type reactivity orthogonal to classical electrophilic diamagnetic Fischer carbenes. Their successful catalytic application has led to the synthesis of a myriad of carbo- and heterocycles, ranging from simple cyclopropanes to more challenging eight-membered rings. The field has matured to employ densely functionalized chiral porphyrin-based platforms that exhibit high enantio-, regio-, and stereoselectivity. Thus far the focus has largely been on cobalt-based systems, but interest has been growing for the past few years to expand the application of carbene radicals to other transition metals. This Perspective covers the advances made since 2011 and gives an overview on the coordination chemistry, reactivity, and catalytic application of carbene radical species using transition metal complexes and catalysts.

Electron-Rich Oxycarbenes: New Synthetic and Catalytic Applications beyond Group 6 Fischer Carbene Complexes

Oxycarbenes have emerged as useful intermediates in synthetic chemistry. Compared to the widely studied oxycarbene metal complexes bearing Group 6 metals, the synthetic and catalytic applications of oxycarbenes beyond Group 6 Fischer carbene complexes are less explored because of the difficulty in controlling their reactivity and the need to use a stoichiometric amount of a presynthesized Group 6 metal carbene complex as the starting material. This Minireview summarizes early synthetic and catalytic applications of late-transition-metal oxycarbene complexes and highlights recent advances in free oxycarbene reactions and transition-metal-catalyzed reactions involving oxycarbenes. We hope this Minireview will inspire further developments in this emerging area.

High-order dipolar annulations with metal-containing reactive dipoles

Medium-sized heterocycles are widespread among a spectrum of structurally intriguing and biologically significant natural products and synthetic pharmaceuticals. Metal-catalyzed high-order dipolar annulations resembling reactions of metal-containing reactive dipoles with dipolarophiles constitute a highly efficient and flexible strategy for constructing medium-sized heterocycles. Mechanistically, these annulation reactions usually proceeding through stepwise pathways are different from the classic high-order pericyclic reactions that follow the Woodward-Hoffman rules. More significantly, asymmetric high-order dipolar annulations using chiral organometallic catalysts have been proven successful for constructing chiral medium-sized heterocycles with high enantio- and diastereoselectivity. This review highlights the impressive advances in this area and is focused on the reactivity, scope, mechanisms and applications of high-order dipolar annulation reactions.

Higher-order [10 + 2] cycloaddition of 2-alkylidene-1-indanones enables the dearomatization of 3-nitroindoles: access to polycyclic cyclopenta[b]indoline derivatives

The higher-order [10 + 2] cycloaddition of 3-nitroindoles and 2-alkylidene-1-indanones enables the dearomatization of 3-nitroindoles and affords a range of structurally diverse cyclopenta[b]indolines with excellent results.