Selective Coupling of 1,2‐Bis‐Boronic Esters at the more Substituted Site through Visible‐Light Activation of Electron Donor–Acceptor Complexes

Abstract

1,2‐Bis‐boronic esters are useful synthetic intermediates particularly as the two boronic esters can be selectively functionalized. Usually, the less hindered primary boronic ester reacts, but herein, we report a coupling reaction that enables the reversal of this selectivity. This is achieved through the formation of a boronate complex with an electron‐rich aryllithium which, in the presence of an electron‐deficient aryl nitrile, leads to the formation of an electron donor–acceptor complex. Following visible‐light photoinduced electron transfer, a primary radical is generated which isomerizes to the more stable secondary radical before radical‐radical coupling with the arene radical‐anion, giving β‐aryl primary boronic ester products. The reactions proceed under catalyst‐free conditions. This method also allows stereodivergent coupling of cyclic cis‐1,2‐bis‐boronic esters to provide trans‐substituted products, complementing the selectivity observed in the Suzuki–Miyaura reaction.

Photocatalyzed Regioselective Hydrosilylation for the Divergent Synthesis of Geminal and Vicinal Borosilanes

“…β-Aryl-substituted alkenyl boronates were then investigated, and the hydrosilylation products were formed with exclusive α-selectivity (33-46, 41-81% yields), regardless of electron-withdrawing or electron-donating groups on the aryl substituent. Some sensitive functionalities in transition-metal catalyzed hydrosilylation or hydroborylation, including aryl chlorides (41) and aryl bromides (44) were well-tolerated with our protocol.…”

“…The extremely low reactivity of isopropenylboronic acid pinacol ester (29) suggests that a completely different strategy is required to synthesize vicinal borosilanes. We were inspired by the studies on 1,2-boron radical migration [42][43][44][45][46] and found that the switch of alkene substrates to α-substituted allyl boronates readily delivered vicinal borosilanes (Table 2). This strategy worked well for α,α-dialkyl-substituted allyl boronates (63-69, 52-84% yields), and the presence of a methyl group at the γ position was tolerated (69).…”

“…As for the allyl boronates, a silyl radical could selectively add to the sterically more accessible γ-position of the allyl boronic ester to give a β-boryl carbon-centered radical intermediate. At this stage, a 1,2-boron migration process in uenced by the α-substituents on the allyl boronates took place [42][43][44][45] . The migration was steered by thermodynamic effects to generate a more stable carbon radical.…”

A unified approach to access β-branched organoboron through coupling of boron-stabilized organozinc with alkyl halides

“…Furthermore, the Aggarwal group had reported the photoinduced coupling of 1,2-bis-boronic esters with aryl nitriles, aryl halides or electron-de cient alkenes, providing the β-tertiary and quaternary boronic ester products through a radical 1,2-boron shift process. [69][70][71] The Liu group had developed a visible-light-induced alkynylation of 1,2-bis-boronic esters with 4CzIPN as the organic photocatalyst to achieve the corresponding β-branched organoboron. 72 Despite important progress have been made, present methods for the synthesis of β-branched organoboron often suffer from poor substrate scope, elusive regioselectivity and/or multi-step synthesis.…”

Catalyst-Free C(sp2)-H Borylation through Aryl Radical Generation from Thiophenium Salts via Electron Donor–Acceptor Complex Formation

“…Notably, the hypervalent thiophenative aryl salts provided the possibility for photochemical reactions. In recent years, electron donor–acceptor (EDA) complex activation through photocatalysis has been demonstrated for radical transformations. − Procter reported one such route using a catalytic amount of triphenylamine as an effective EDA donor for thiophenative aryl salt activation. Also, Molander disclosed that sulfides and thioether could act as effective electron donors to form an EDA complex with thianthrenium salts that allow the reaction to take place under irradiation.…”