Radical-Polar Crossover Reactions of Dienylboronate Complexes: Synthesis of Functionalized Allylboronic Esters

Abstract

Radical-polar crossover reactions of dienylboronate complexes are applied to the synthesis of functionalized secondary and tertiary allylboronic esters. The transition-metal-free three-component coupling uses readily accessible dienylboronate esters as substrates in combination with various sp3/sp2 carbon nucleophiles and commercial alkyl iodides as radical precursors. In the visible light-initiated radical chain process, two new C–C bonds are formed, and the E-double bond geometry in the product allylboronic esters is controlled with good to excellent selectivity.

Radical‐Induced 1,2‐Migrations of Boron Ate Complexes

“…Inspired by the radical polar crossover reaction of “simple” vinyl boron ate complexes (see Section 2.1), the Studer group investigated dienyl boron ate complexes as radical acceptors . If the addition of the transient C‐radical selectively occurs at the δ‐position of the boron ate complex A , an allyl radical intermediate B is generated.…”

“…Likely, polar effects play a key role in the selective α‐H‐abstraction of these anionic boron ate complexes by the electrophilic CF 3 radical (see below). Moreover, it is well established that CF 3 I is an excellent terminal oxidant for intermediates of type B , rendering this cheap iodide to be the reagent of choice to realize this interesting sequence . Ir(ppy) 3 was employed as a smart initiator upon irradiation with blue LEDs (Φ=8.8).…”

Generating Skeletal Diversity and Complexity from Boron‐Substituted 1,3‐Dienes and Enophiles

“…Melting points were measured on a Kofler bench Reichert‐Jung Heizbank or a melting point apparatus Stuart SMP10 and are uncorrected. Boron substituted 1,3‐dienes 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1'a ,[3b] 1''a , benzylhydroxycarbamate, 2‐nitrosopyridine, were prepared as previously reported.…”

Recent Advances in Radical Addition to Alkenylboron Compounds

“…In recent studies, Studer has shown that 1,2 migration of the cationic intermediate 20 (Scheme ) is stereoretentive at the migrating carbon and this allows conversion of enantiomerically‐enriched organoboronic esters into ketones bearing a configurationally defined sterogenic center at the α‐carbon (Scheme , 21) . The same group has also extended the radical‐polar crossover reaction to the construction of allylboronic esters such as 22 from simple dienylboronic ester starting materials …”