Arnold Polly L, Bell Nicola L, Marr Isobel H, She Siyi, Hamilton Jonathan, Fraser Craig, Wang Kai
EaStCHEM School of Chemistry, University of Edinburgh, The King's Buildings, Edinburgh, EH9 3JJ, UK.
Dalton Trans. 2014 Nov 7;43(41):15419-28. doi: 10.1039/c4dt01464b.
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.
通式为[R(1)C(O)CH2{CH[NCR(3)CR(3)N(R(2))]}]X（R(1) = (叔丁基)、萘基；R(2) = (异丙基)、均三甲苯基、(叔丁基)；R(3) = 氢、甲基、(氢)2；X = 氯、溴）的一组β - 酮咪唑鎓盐和β - 酮咪唑啉鎓盐与超氢化物碱MHBEt3显示出截然不同的反应活性；其中两种被还原为手性β - 醇卡宾硼烷R(1)CH(OH)CH2{C(BEt3)[NCR(3)CR(3)N(R(2))]} 2（R(1) = (叔丁基)；R(2) = (异丙基)、均三甲苯基；R(3) = 氢），两种带有庞大R(2)取代基的被还原为手性β - 硼酸咪唑鎓盐[R(1)CH(OBEt3)CH2{CH[NCR(3)CR(3)N(R(2))]}]X 3（R(1) = (叔丁基)、萘基；R(2) = 均三甲苯基、(叔丁基)；R(3) = 氢、甲基；X = 氯、溴），并且两种饱和杂环衍生物未被还原但形成了卡宾硼烷加合物R(1)C(O)CH2{C(BEt3)[NCR(3)CR(3)N(R(2))]} 4（R(1) = (叔丁基)、萘基；R(2) = 均三甲苯基；R(3) = (氢)2）。加热咪唑硼酸酯3的溶液会导致乙烷消除，这是有机硼酸盐作为布朗斯特碱并形成卡宾硼烷R(1)CH(OBEt2)CH2{C[NCR(3)CR(3)N(R(2))]} 5（R(1) = 萘基；R(2) = 均三甲苯基；R(3) = 甲基）的首个例子。形式为2 R(1)CH(OH)CH2{CH[NC(BEt3)CR(3)N(R(2))]}（R(1) = (叔丁基)；R(2) = (叔丁基)；R(3) = 氢）的“异常”卡宾硼烷也可通过3的热解得到，这表明卡宾 - 硼烷醇比两性离子咪唑硼酸酯是一种热力学上更稳定的组合。高温热解还可导致醇臂完全裂解，消除叔丁基环氧乙烷并形成B - N键合的咪唑硼酸酯7。讨论了反应产物对每个咪唑前体分子的空间和电子性质的强烈依赖性。
