Electro‐Olefination—A Catalyst Free Stereoconvergent Strategy for the Functionalization of Alkenes

Abstract

Conventional methods carrying out C(sp 2 )−C(sp 2 ) bond formations are typically mediated by transition‐metal‐based catalysts. Herein, we conceptualize a complementary avenue to access such bonds by exploiting the potential of electrochemistry in combination with organoboron chemistry. We demonstrate a transition metal catalyst‐free electrocoupling between (hetero)aryls and alkenes through readily available alkenyl‐tri(hetero)aryl borate salts (ATBs) in a stereoconvergent fashion. This unprecedented transformation was investigated theoretically and experimentally and led to a library of functionalized alkenes. The concept was then carried further and applied to the synthesis of the natural product pinosylvin and the derivatization of the steroidal dehydroepiandrosterone (DHEA) scaffold.

Computational Insights into Intramolecular Cross-Coupling of Quaternary Borate Salts

“…(2) proceeds directly from A to C via σ-bond cleavage and was also proposed for aryl-vinyl coupling reactions. [6][7][8] Subsequently, the boron-carbon bond in C is broken homolytically, furnishing the final product and removing the radical character. As a side product, borinic acids BR 2 OH are formed.…”

“…In our previous work with a focus on the experimental side, we investigated substituted tetraphenylborates 6 and alkenyltriarylborates. 7 The oxidation of the borate salts was perfomed electrochemically with RVC electrodes in acetonitrile and in a subsequent work by means of a photocatalyst. 8 Similar reactions occur with chemical oxidants.…”

Photocatalyzed Transition-Metal-Free Oxidative Cross-Coupling Reactions of Tetraorganoborates

“…Model substrate 3a was employed for optimizations, assuming that the oxidation of the alkenyl substituent would be preferred over the remaining aryl groups. [14] Performing the reaction with 5 mol% of Acr-1 without oxidant afforded 4a in 35% yield and similar conditions to the ones used for biaryl formation only pushed the conversion to 65% (Table 2). Table 2.…”

“…We previously demonstrated that the first oxidation of a TOB salt preferentially occurs on the most electron rich unsaturated moiety. [13,14] As a logical consequence, structures 1 were designed to possess one electron-richer aryl group (Ar 1 ), surrounded by electron-poorer groups (Ar 2 ). With optimized conditions and setup in hands, we started evaluating the scope of the aryl-aryl photocoupling (Scheme 2).…”

“…Blue light irradiation (470 nm) allows for the acridinium to reach its active, excited state to abstract an electron from substrate 1a. The resulting oxidized species [1a . ] undergoes a pseudo 1,2-metallate rearrangement, [13,14] giving the cyclic intermediate 6, which further opens towards 7 through [TS] # .…”