Protonolysis and thermolysis reactions of functionalised NHC-carbene boranes and boratesElectronic supplementary information (ESI) available: Additional synthetic and crystallographic details for the complexes. CCDC 1003569-1003576. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01464b

A set of β-ketoimidazolium and β-ketoimidazolinium salts of the general formula [R 1 C(O)CH 2 {CH[NCR 3 CR 3 N(R 2 )]}]X (R 1 = t Bu, naphth; R 2 = i Pr, Mes, t Bu; R 3 = H, Me, (H) 2 ; X = Cl, Br) show contrasting reactivity with superhydride bases MHBEt 3 ; two are reduced to chiral β-alcohol carbene-boranes R 1 CH(OH)CH 2 {C(BEt 3 )[NCR 3 CR 3 N(R 2 )]} 2 (R 1 = t Bu; R 2 = i Pr, Mes; R 3 = H), two with bulky R 2 substituents are reduced to chiral β-borate imidazolium salts [R 1 CH(OBEt 3 )CH 2 {CH[NCR 3 CR 3 N(R 2 )]}]X 3 (R 1 = t Bu, naphth; R 2 = Mes, t Bu; R 3 = H, Me; X = Cl, Br), and the two saturated heterocycle derivatives remain unreduced but form carbene-borane adducts R 1 C(O)CH 2 {C(BEt 3 )[NCR 3 CR 3 N(R 2 )]} 4 (R 1 = t Bu, naphth; R 2 = Mes; R 3 = (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 R 1 CH(OBEt 2 )CH 2 {C[NCR 3 CR 3 N(R 2 )]} 5 (R 1 = naphth; R 2 = Mes; R 3 = Me). The 'abnormal' carbene borane of the form 2 R 1 CH(OH)CH 2 {CH[NC(BEt 3 )CR 3 N(R 2 )]} (R 1 = t Bu; R 2 = t Bu; R 3 = 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. β-ketoimidazol(in)ium salts show contrasting reactivity with MHBEt 3 due to subtle electronic and steric differences. Thermolysis of the chiral β-borate imidazolium salts formed from these reactions results in alkane elimination in the first example of organic borates functioning as Brønsted bases.