Generation of α‐Boryl Radicals by H^. Transfer and their Use in Cycloisomerizations

Abstract

Carbon-centered radicals can be stabilized by delocalization of their spin density into the vacant p orbital of a boron substituent. a-Vinyl boronates, in particular pinacol (Bpin) derivatives, are excellent hydrogen atom acceptors. Under H 2 , in the presence of a cobaloxime catalyst, they generate a-boryl radicals; these species can undergo 5-exo radical cyclizations if appropriate double bond acceptors are present, leading to densely functionalized heterocycles with tertiary substituents on Bpin. The reaction shows good functional group tolerance with wide scope, and the resulting boronate products can be converted into other useful functionalities.

Hydrogen Atom Transfer (HAT)-Mediated Remote Desaturation Enabled by Fe/Cr–H Cooperative Catalysis

“…C­(sp 3 )–H bonds that adjoin radicals are relatively weak, around 35 kcal/mol, whereas metal–hydrogen bonds (M–H) range from 55 to 83 kcal/mol, making H• abstraction by metal radicals M• thermodynamically favorable. In recent years, the combination of photoredox catalysis and biomimetic cobaloxime complexes has emerged as a promising option in this field. − The Norton group has examined many applications of transition-metal hydrides to unsaturated systems (the kinetics of H• transfer, and hydrogenation, hydrofunctionalization, cyclization, and isomerization reactions). − Our previous report on the epoxide hydrogenation has proven the feasibility of H• abstraction by CpCr­(CO) 3 • from C­(sp 3 )–H bonds α to radicals …”

Borylated Cyclopentanes via Atom Transfer Radical [3+2] Annulation

“…[42] Nucleophilic radicals also add efficiently to vinylboronic esters as demonstrated by Carboni. [43] Similarly, hydroalkylation involving either hydrogen atom transfer (HAT) from Hantzsch esters [44,45] or reductive [46][47][48][49] and oxidative [50,51] termination processes have been reported. For instance, Aggarwal reported the addition of nucleophilic α-aminoalkyl radicals generated under photoredox conditions from the corresponding αaminoacids to vinylboronic esters.…”

Generation and Application of Homoallylic α,α‐Diboryl Radicals via Diboron‐Promoted Ring‐Opening of Vinyl Cyclopropanes: cis‐Diastereoselective Borylative Cycloaddition**

“…[3][4][5] The ability of boron atom to modulate the formation and reactivity of a radical at an adjacent carbon provides unique opportunities to expand the applications of organoborons in new directions. Previous approaches [1][2] (Scheme 1a-b) to access α-boryl radicals have employed one of the following strategies: i) halogen atom abstraction of α-halo borons, [6][7][8] ii) H-atom abstraction of secondary alkyl boronates, [9] iii) CÀ S bond cleavage of boryl xanthates, [10] iv) radical addition/ HAT to vinyl boronates [1,[11][12][13][14][15][16] and v) CÀ B bond cleavage of unsymmetrical geminal diborons. [17] The ensuing α-boryl stabilized carbon-centered radicals were used in addition reaction with alkenes, [6][7]10,17] CÀ X bond formation [9] or SET/proton capture.…”