(1,3‐Dihydro‐1,3‐dimethyl‐2 H ‐imidazol‐2‐ylidene)trihydroboron (1)

Abstract

image [1211417‐77‐4] C 5 H 11 BN 2 (MW 109.97) InChI = 1S/C5H11BN2/c1‐7‐3‐4‐8(2)5(7)6/h3‐4H,1‐2,6H3 InChiKey = WDITXRGBBBKEET‐UHFFFAOYSA‐N (hydroboration, radical reduction; hydride reduction) Alternative Name(s) : diMe‐Imdg‐BH 3 , (1,3‐Dimethyl‐1H‐imidazol‐3‐ium‐2‐yl)trihydroborate, 1,3‐Dimethylimidazoyl‐2‐ylidene borane. 1 Physical Data : white solid, mp: 138–139 °C. 2 Solubility : 1 is soluble in several organic solvents such as dimethylsulfoxide, N , N ‐dimethylformamide, ethyl acetate, methanol, chloroform, dichloromethane, acetonitrile, mixtures acetonitrile/water up to 0.1 M. Form Supplied in : 1 is commercially available in the form of a white solid. Analysis of Reagent Purity : compound 1 is air‐ and moisture‐stable at 23 °C for at least 12 months, impurities derive from the synthesis and usually not from decomposition. The purity of 1 is conveniently determined via 1 H NMR (CDCl 3 ). 2 1 H NMR (CDCl 3 ) δ 6.80 (s, 2 H ), 3.74 (s, 6 H ), δ 1.49–0.53 (m, 3 H , J 11B–H = 86.5 Hz, J 10B–H = 29.0 Hz). 13 C NMR (CDCl 3 ) δ 172.0 (q, J B−C = 50 Hz), 120.0, 35.9. 11 B NMR (CDCl 3 ) δ −37.5 (q, J B–H = 86.4 Hz). Preparative Methods : the title compound is conveniently prepared via a two‐step procedure from N ‐methylimidazole (eq 1), up to a 50 mmol scale. 2 image Step 1: Methyl iodide (1.2 equiv) is added dropwise to a dichloromethane solution of the corresponding imidazole (5 M) over the course of 15 min in an ice bath (Caution: exothermic reaction!). After addition, the ice bath is removed and the reaction mixture is allowed to stir for 1 h, after which the mixture is concentrated and dried under vacuum to give 1,3‐dimethyl‐1 H ‐imidazol‐3‐ium iodide in yields ranging from 95% to quantitative. The purity of the latter compound can be checked via 1 H‐NMR (CD 3 OD) and is typically used in Step 2 without any further purification. 2 Step 2: 1,3‐dimethyl‐1 H ‐imidazol‐3‐ium iodide is suspended in toluene (1 mL mmol −1 ) and sodium borohydride (1.2 equiv) is added in one portion. The mixture is vigorously refluxed for 16 h. The hot reaction solvent was decanted from the insoluble mixture, and the remaining residue was extracted with hot toluene (2 × 1 reaction volume). Toluene is removed under pressure to give the title compound in a good purity (usually > 90%). In case further purification is needed, the title compound can be purified via flash chromatography or crystallization (vide infra). Alternative synthetic approaches rely on the deprotonation of 1,3‐dimethyl‐1 H ‐imidazol‐3‐ium iodide with a strong base such as sodium bis(trimethylsilyl)amide to generate the N‐heterocyclic carbene and subsequent quenching with BH 3 (as a tetrahydrofuran, 3 dimethylsulfide, 4 amine, 5 or phosphine 5 complex). Purification : the title compound can be purified via flash chromatography on silica gel (cyclohexane:ethyl acetate 20 : 80). 6 Recrystallization from H 2 O (14 mL g −1 ) gives the pure compound as fine white crystals (Cryst. yield: 73%). 2 Handling, Storage, and Precautions : the title compound can be stored at room temperature (20–23 °C) for at least 12 months, under air‐equilibrated conditions. The compound is not sensitive to ambient light. All possible precautions should be taken to avoid contact with eyes ( H319 ) and skin ( H315 ). All precautions should be taken to avoid inhalation ( H335 ) as well. 7