Thiolation of NHC-boranes: influence of the substitution at boron.

Several N-Heterocyclic Carbene (NHC)-boryl sulfides with B-substituents were prepared. The added steric hindrance leads to much improved selectivities as only the NHC-boryl mono-sulfides were obtained. The B-substituted NHC-boranes are also conducive to more selective S to N NHC-boryl shift, provided that the NHC used is not too sterically demanding.

Influence of Electronic Effects on the Reactivity of Triazolylidene-Boryl Radicals: Consequences for the use of N-Heterocyclic Carbene Boranes in Organic and Polymer Synthesis.

A small library of triazolylidene-boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen-atom donors, as well as that of the corresponding N-heterocyclic carbene (NHC)-boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin-trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene-borane. The new NHC-boranes were also used as co-initiators for the Type I photopolymerization of acrylates. This allowed a structure-reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC-boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions.

Formation of N-heterocyclic carbene-boryl radicals through electrochemical and photochemical cleavage of the B-S bond in N-heterocyclic carbene-boryl sulfides.

The B-S bond in N-heterocyclic carbene (NHC)-boryl sulfides can be cleaved homolytically to NHC-boryl or NHC-thioboryl and thiyl radicals using light, either directly around 300 nm or with a sensitizer at a longer wavelength (>340 nm). In contrast, the electrochemical reductive cleavage of the B-S bond is difficult. This easy photolytic cleavage makes the NHC-boryl sulfides good type I photopolymerization initiators for the polymerization of acrylates under air.

Mechanistic and preparative studies of radical chain homolytic substitution reactions of N-heterocyclic carbene boranes and disulfides.

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.