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 [R(1)C(O)CH2{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 MHBEt3; two are reduced to chiral β-alcohol carbene-boranes R(1)CH(OH)CH2{C(BEt3)[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(OBEt3)CH2{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)CH2{C(BEt3)[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(OBEt2)CH2{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)CH2{CH[NC(BEt3)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.

An Unexpected Boron Rearrangement Leads to a Fluorogenic and Colorimetric BODIPY Probe

“…More recently Arnold observed a net trialkyl borane (BR 3 ) transfer in the reduction of β‐ketoimidazolium (Rki) using NaHBEt 3 or KHBEt 3 . [ 8 ] In this reaction the formed boronic intermediate (Rki‐O‐BEt 3 ) undergoes a migration of a triethylboron (BEt 3 ) group to either the imidazole carbon (C2 or C3) or nitrogen, (i.e., C‐BEt 3 or N‐BEt 3 type bond formations) depending on the N‐substituent and temperature, yielding “abnormal” carbene–borane adducts. In 2017, Aggarwal reported a net boron pinacolate migration in the reaction of ortho‐lithiated benzylic amines with boronic esters.…”