Chromium‐Catalyzed Selective Dimerization/Hydroboration of Allenes to Access Boryl‐Functionalized Skipped ( E , Z )‐Dienes

Regio- and Stereoselective Carbon-Boron Bond Formation via Heterogeneous Palladium-Catalyzed Hydroboration of Enallenes

A highly efficient regio- and stereoselective heterogeneous palladium-catalyzed hydroboration reaction of enallenes was developed. Nanopalladium immobilized on microcrystalline cellulose (MCC) was successfully employed as an efficient catalyst for the enallene hydroboration reaction. The nanopalladium particles were shown by HAADF-STEM to have an average size of 2.4 nm. The cellulose-supported palladium catalyst exhibits high stability and provides vinyl boron products in good to high isolated yields (up to 90 %). The nanopalladium catalyst can be efficiently recycled and it was demonstrated that the catalyst can be used in 7 runs with a maintained high yield (>80 %). The vinylboron compounds prepared from enallenes are important synthetic intermediates that can be used in various organic synthetic transformations.

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.

1,2-Carboboration of Arylallenes by In Situ Generated Alkenylboranes for the Synthesis of 1,4-Dienes

We herein report a novel method for the coupling of unactivated alkynes and arylallenes, which relies on an unprecedented and regioselective 1,2‐carboboration of the allene by an alkenylborane. The alkenylborane is conveniently prepared in situ by hydroboration of an alkyne with Piers’ borane, i. e., HB(C₆F₅)₂. The boryl‐substituted 1,4‐dienes that are formed by this carboboration are well‐suited for a subsequent Suzuki‐Miyaura coupling with aryl iodides. This allowed us to develop a three‐step, one‐pot protocol for the synthesis of aryl‐substituted 1,4‐dienes. The generality of the reaction was demonstrated by the synthesis of twenty dienes with modular variations of all three reaction partners. The mechanism of the new 1,2‐carboboration was investigated using dispersion corrected double‐hybrid DFT computations that allowed us to rationalize the chemo‐ and regioselectivity of this key step.

Silaborative Assembly of Allenamides and Alkynes: Highly Regio- and Stereocontrolled Access to Bi- or Trimetallic Skipped Dienes

A highly stereo- and regiocontrolled multicomponent approach to skipped 1,4-dienes decorated with one boryl and two silyl functionalities is described. This Pd-catalyzed atom-economical union of allenamides, alkynes, and Me₂ PhSiBpin (or Et₃ SiBpin) proceeds without the use of phosphine ligands, instead relying on chelation through the internal amide group of the allenamide sulfonyl. A variety of alkynes, including those derived from complex bioactive molecules, can be efficiently coupled with allenamides and Me₂ PhSiBpin in good yields and with excellent selectivity. The synthetic potential was demonstrated through multiple valuable chemoselective transformations, establishing new disconnections for functionalized dienes. Density functional theory calculations revealed that the reaction first proceeded through borylation of the allenamide, followed by silylation of the alkyne and then reductive elimination, which convergently assemble the skipped 1,4-diene.