Isoselenocyanates with Sterically Encumbered Substituents─Synthesis, Structures, and Spectroscopic Properties

A series of isoselenocyanates, SeCNR, spanning over a variety of organic residues in terms of steric encumbrance and electronic effects, was synthesized by reactions of corresponding isocyanides with elemental selenium. The residues range from tertiary butyl ($^t$Bu), phenyl (Ph), 2,4,6-trimethylphenyl (Mesityl), 2,6-diisopropylphenyl ($^{i‑Prop2}$Ph), 2,6-di(2,4,6-trimethylphenyl)-phenyl (Ar$^{Mes2}$), 2,6-di(2,6-diisopropylphenyl)phenyl (Ar$^{Dipp2}$), 2,6-di(2,4,6-triisopropylphenyl)phenyl (Ar$^{Tripp2}$), and 4-fluoro-phenyl (PhF) to 2,6-di{3,5-di(trifluoromethyl)phenyl}4-fluoro-phenyl (p-FAr$^{DarF2}$). They were formed as crystalline solids or viscous oils in medium to good yields, which recommends this synthetic approach as generally suitable. The products were studied by X-ray diffraction and spectroscopic methods, including $^{77}$Se NMR spectroscopy. The influence of the organic substituents on the $^{77}$Se NMR chemical shifts of the isoselenocyanates is in good accordance with DFT-modeled values. Experimental $^{77}$Se−$^{13}$C couplings of 280 Hz could be derived for Se$^{13}$CNAr$^{Dipp2}$ prepared from a sample of $^{13}$CNAr$^{Dipp2}$ (isotopic enrichment: 99%). A small amount of a selenourea-type product, (i-Prop)$_2$NC(Se)NHp-FAr$^{DarF2}$, was isolated from a reaction mixture of CNp-FAr$^{DarF2}$ with selenium and diisopropylamine as the supporting base, which indicates an influence of the supporting base used for the course of the reaction.