Protonolysis and thermolysis reactions of functionalised NHC–carbene boranes and borates

Abstract

A set of β-ketoimidazolium and β-ketoimidazolinium salts of the general formula [R¹C(O)CH₂{CH[NCR³CR³N(R²)]}]X (R¹ = ^tBu, naphth; R² = ⁱPr, Mes, ^tBu; R³ = H, Me, (H)₂; X = Cl, Br) show contrasting reactivity with superhydride bases MHBEt₃; two are reduced to chiral β-alcohol carbene–boranes R¹CH(OH)CH₂{C(BEt₃)[NCR³CR³N(R²)]} 2 (R¹ = ^tBu; R² = ⁱPr, Mes; R³ = H), two with bulky R² substituents are reduced to chiral β-borate imidazolium salts [R¹CH(OBEt₃)CH₂{CH[NCR³CR³N(R²)]}]X 3 (R¹ = ^tBu, naphth; R² = Mes, ^tBu; R³ = H, Me; X = Cl, Br), and the two saturated heterocycle derivatives remain unreduced but form carbene–borane adducts R¹C(O)CH₂{C(BEt₃)[NCR³CR³N(R²)]} 4 (R¹ = ^tBu, naphth; R² = Mes; R³ = (H)₂). Heating solutions of the imidazolium borates 3 results in the elimination of ethane, in the first example of organic borates functioning as Brønsted bases and forming carbene boranes R¹CH(OBEt₂)CH₂{C[NCR³CR³N(R²)]} 5 (R¹ = naphth; R² = Mes; R³ = Me). The ‘abnormal’ carbene borane of the form 2 R¹CH(OH)CH₂{CH[NC(BEt₃)CR³N(R²)]} (R¹ = ^tBu; R² = ^tBu; R³ = H), is also accessible by thermolysis of 3, suggesting that the carbene–borane alcohol is a more thermodynamically stable combination than the zwitterionic imidazolium borate. High-temperature thermolysis also can result in complete cleavage of the alcohol arm, eliminating tert-butyloxirane and forming the B–N bound imidazolium borate 7. The strong dependence of reaction products on the steric and electronic properties of each imidazole precursor molecule is discussed.