Racemic Vinylallenes in Catalytic Enantioselective Multicomponent Processes: Rapid Generation of Complexity through 1,6‐Conjugate Additions

Umpolung Asymmetric 1,5-Conjugate Addition via Palladium Hydride Catalysis

Electronically matched nucleophilic 1,6-conjugate addition has been well studied and widely applied in synthetic areas. In contrast, nucleophilic 1,5-conjugate addition represents an electronically forbidden process and is considered unfeasible. Here, we describe modular protocols for 1,5-conjugate addition reactions via palladium hydride catalysis. Both palladium and synergistic Pd/organocatalyst systems are developed to catalyze 1,5-conjugate reaction, followed by inter- or intramolecular [3+2] cyclization. A migratory 1,5-addition protocol is established to corroborate the feasibility of this umpolung concept. The 1,5-addition products are conveniently transformed into a series of privileged enantioenriched motifs, including polysubstituted tetrahydrofuran, dihydrofuran, cyclopropane, cyclobutane, azetidine, oxetane, thietane, spirocycle and bridged rings. Preliminary mechanistic studies corroborate the involvement of palladium hydride catalysis.

Chemoselective Coupling of π-Systems to Access Metallated 1,4- or 1,5-Skipped Dienes in Multicomponent Reactions

Integrating distinct unsaturated C-C systems while simultaneously installing metallic groups has been significantly challenging to execute in a multicomponent reaction. Therefore, designing a suitable mechanistic pathway that provides the required reactivity and selectivity for target C-C bonds with metallic reagents to ensure successful coupling is the key to success. Copper-catalyzed borylallylation and silylallylation have emerged as the most efficient strategies for assembling borylated/silylated skipped (1,4 or 1,5) dienes by catalytically combining an organocopper intermediate with allyl electrophiles. However, reactions involving interelemental reagents (e. g., [Si]-[B]) to accomplish intermolecular atom-economic couplings have not been studied thoroughly. Therefore, to aid the development of new transformations in this research area, this article attempts to include all precedents, including recent studies by the authors. The present Concept article may be helpful for researchers working in this area as it provides a basic conceptual framework.

Photochemical Organocatalytic Regio- and Enantioselective Conjugate Addition of Allyl Groups to Enals

We report the first catalytic enantioselective conjugate addition of allyl groups to α,β‐unsaturated aldehydes. The chemistry exploits the visible‐light‐excitation of chiral iminium ions to activate allyl silanes towards the formation of allylic radicals, which are then intercepted stereoselectively. The underlying radical mechanism of this process overcomes the poor regio‐ and chemoselectivity that traditionally affects the conjugate allylation of enals proceeding via polar pathways. We also demonstrate how this organocatalytic strategy could selectively install a valuable prenyl fragment at the β‐carbon of enals.

Rhodium(I)-Catalyzed Three-Component [4+2+1] Cycloaddition of Two Vinylallenes and CO

Transition metal-catalyzed [4+2+1] reactions of dienes (or diene derivatives such as vinylallenes), alkynes/alkenes, and CO (or carbenes) are expected to be the most straightforward approach to synthesize challenging seven-membered ring compounds, but so far only limited successes have been realized. Here, an unexpected three-component [4+2+1] reaction between two vinylallenes and CO was discovered to give highly functionalized tropone derivatives under mild conditions, where one vinylallene acts as a C₄ synthon, the other vinylallene as a C₂ synthon, and CO as a C₁ synthon. It was proposed that this reaction occurred via oxidative cyclization of the diene part of one vinylallene molecule, followed by insertion of the terminal alkene part of the allene moiety in another vinylallene, into the Rh-C bond of five-membered rhodacycle. Then, CO insertion and reductive elimination gave the [4+2+1] cycloadduct. Further experimental exploration of why ene/yne-vinylallenes and CO gave monocyclic tropone derivatives instead of 6/7-bicyclic ring products were reported here.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.