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.

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.

A small library of triazolylidene-boranes which differ only by the nature of the aryl group on the external nitrogen has been prepared. Their associated reactivity as B–H hydrogen atom donors, as well as that of the corresponding NHC-boryl radicals toward methyl acrylate and oxygen were investigated by laser flash photolysis (LFP), molecular orbital calculations and ESR-spin trapping experiments, and benchmarked relative to the already known dimethyl-triazolylidene-borane. The new NHC-boranes were also used as co-initiators for the Type II photopolymerization of acrylates. This allowed us to establish a structure/reactivity relationship with regard to the substitution pattern of the NHC and to probe the role of electronic effects in the reactivity of NHC-boryl radicals. Although their rate of addition to methyl acrylate depends on their electronegativity the radicals are all nucleophilic and good initiators for the photopolymerization reactions.

Nickela-2-oxazolidinones, formed by oxidative coupling of imines and CO 2 with Ni 0 , react with LiCl under mild conditions (4 °C, 1 atm) to afford vicinal diamines in up to 89% yield. The reaction is the first organometallic example of reductive imine coupling requiring CO 2 . In this system, CO 2 is participatory and is not incorporated in the reaction products. These results represent an important addition to our understanding of the reactivity of metallacycles derived from CO 2 and unsaturated compounds.

The synthesis and reactivity of mono‐ and bis‐S‐xanthyl NHC‐boranes is reported. The new NHC‐boranes are prepared through nucleophilic exchange at boron from either mono‐ or bis‐triflyl NHC‐boranes, themselves obtained by protolysis of the NHC‐BH3 starting compounds. The B−H bond of the S‐xanthyl NHC‐boranes can be cleaved both homolytically and heterolytically, albeit the latter is more synthetically useful. The S‐xanthyl NHC‐boranes can reduce both aldehydes and imines. The B−S bond can also be cleaved homolytically. Under UV irradiation, the S‐xanthyl NHC‐boranes generate NHC‐boryl radicals that can initiate radical polymerizations of acrylates.

B-Substituted NHC-boranes react with aryldisulfides to generate mono-thiolated compounds selectively under both ionic or radical conditions.

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