Mechanistic and Preparative Studies of Radical Chain Homolytic Substitution Reactions of N-Heterocyclic Carbene Boranes and Disulfides

Abstract

Reactions of 1,3-dimethylimidazol-2-ylidene-borane (diMe-Imd-BH3) and related NHC-boranes with diaryl and diheteroaryl disulfides provide diverse NHC-boryl monosulfides (diMe-Imd-BH2SAr) and NHC-boryl disulfides (diMe-Imd-BH(SAr)2). Heating in the dark with 1 equiv of disulfide favors monosulfide formation, while irradiation with 2 equiv disulfide favors disulfide formation. With heteroaryl disulfides, the NHC-borane in the primary NHC-boryl sulfide product migrates from sulfur to nitrogen to give new products with a thioamide substructure. Most substitution reactions are thought to proceed through radical chains in which homolytic substitution of a disulfide by an NHC-boryl radical is a key step. However, with electrophilic disulfides under dark conditions, a competing ionic path may also be possible.

Boron chemistry in a new light

“…Let us first consider the work devoted to the reduction of the latter compounds and which led to the synthesis of the first NHC-boryl mono- and disulfide derivatives. 39 …”

Visible‐Light‐Promoted Photocatalytic B−C Coupling via a Boron‐Centered Carboranyl Radical: Facile Synthesis of B(3)‐Arylated o‐Carboranes

“…[1] Them ajor work in this area focuses on the isolation and characterization of such reactive species by overcoming daunting synthetic difficulties. [2f, 4] Fori nstance,N -heterocyclic carbene (NHC) stabilized boryl radicals have been used in various organic radical reactions,s uch as radical deoxygenation of xanthates, [2f, 4c, 5] radical reductions of alkyl halides, [6] radical chain homolytic substitution reactions, [7] and reductive decyanation of organic nitriles. [2f, 4] Fori nstance,N -heterocyclic carbene (NHC) stabilized boryl radicals have been used in various organic radical reactions,s uch as radical deoxygenation of xanthates, [2f, 4c, 5] radical reductions of alkyl halides, [6] radical chain homolytic substitution reactions, [7] and reductive decyanation of organic nitriles.…”

Reactivity of B‐Xanthyl N‐Heterocyclic Carbene‐Boranes

“…We became interested in how the established reactivity was affected by substitution at boron. Installing a sulfur (an arylsulfide) proved rewarding since the corresponding B−S bond could be photolyzed to generate NHC‐boryl radicals, which initiated radical polymerizations . Alternatively, the Curran group demonstrated that the thiyl substituent could become a nucleophile, leading to sulfides .…”

“…Alternatively, the Curran group demonstrated that the thiyl substituent could become a nucleophile, leading to sulfides . However the same NHC‐boryl sulfides could also undergo H‐atom abstraction or hydride transfer, leading to bis‐thiyl derivatives . This means that not only their B−S bond, but also their B−H bonds could be activated.…”