Synthesis of Stereodefined Borylated Dendralenes through Copper-Catalyzed Allylboration of Alkynes

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.

Enantioselective oxa-Diels-Alder Sequences of Dendralenes

Diene-transmissive hetero-Diels-Alder sequences involving carbonyl dienophiles are reported for the first time. High enantioselectivities are achieved in the reaction of phenylglyoxal with a broad range of dendralene structures, through the optimization of a Pd²⁺ catalyst system. The initial catalyst-controlled enantioselective oxa-Diels-Alder (ODA) cycloaddition to a [3]dendralene generates a dihydropyran carrying a semicyclic diene. This participates in a subsequent catalyst or substrate-controlled Diels-Alder reaction to generate sp³ -rich fused polycyclic systems containing both heterocycles and carbocycles. Computational investigations reveal a concerted asynchronous mechanism. π-Complexation of a diene C=C bond to Pd²⁺ occurs in both the pre-transition state (TS) complex and in cycloaddition TSs, controlling stereoselectivity. A formal enantioselective [4+2]cycloaddition of a CO₂ dienophile is demonstrated.

Site‐Selective ( Z )‐α‐Borylalkenyl Copper Systems for Nucleophilic Stereodefined Allylic Coupling

1,1‐Diborylalkenes can be transformed into (Z)‐skipped dienes through Cu^I‐phosphine catalyzed allylic coupling reactions. The energetically preferred formation of (Z)‐α‐borylalkenyl copper (I) species and the subsequent nucleophilic attack, explains the stereoselective nucleophilic substitution with allyl bromides. The eventual treatment of (Z)‐skipped dienes with NaO^tBu promotes cyclization/aromatization patterns via enyne intermediates.

C-Boron Enolates Enable Palladium Catalyzed Carboboration of Internal 1,3-Enynes

A new family of carbon-bound boron enolates, generated by a kinetically controlled halogen exchange between chlorocatecholborane and silylketene acetals, is described. These C-boron enolates are demonstrated to activate 1,3-enyne substrates in the presence of a Pd⁰ /Senphos ligand complex, resulting in the first examples of a carboboration reaction of an alkyne with enolate-equivalent nucleophiles. Highly substituted dienyl boron building blocks are produced in excellent site-, regio-, and diastereoselectivity by the described catalytic cis-carboboration reaction.

Stereoselective Synthesis of Highly Substituted 1,3-Dienes via "à la carte" Multifunctionalization of Borylated Dendralenes

Despite the high relevance of 1,3-dienes, stereoselective methods to access tetrasubstituted conjugated dienes are still scarce. We here report an efficient and modular approach that provides access to multifunctional tetrasubstituted 1,3-dienes with excellent levels of regio- and stereoselectivity. This methodology is based on a tetracomponent reaction between a borylated dendralene, an organolithium reagent and two different electrophiles. Mechanistic studies reveal that this transformation proceeds through a regio- and stereoselective carbolithiation/electrophilic trapping of an in situ formed dendralenic boron-ate complex, followed by a stereoretentive halodeborylation. The ease in which complex structural dienes can be accessed and their synthetic versatility highlight the importance and utility of this method.

[n]Dendralenes as a Platform for Selective Catalysis: Ligand-Controlled Cu-Catalyzed Chemo-, Regio-, and Enantioselective Borylations

We report the development of two complementary methods for the Cu-catalyzed anti-Markovnikov borylation of one specific olefin in 2-substituted [n]dendralenes (n = 3-6). The first protocol operates with a bisphosphine ligand and occurs with high regio- and chemoselectivity for the terminal double bond, independently of the number of cross-conjugated alkenes. We show that the use of a chiral phosphanamine ligand enables the highly chemo-, regio-, and enantioselective borylation of the alkene cross-conjugated with the terminal olefin in [n]dendralenes.