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

Polly L. Arnold*, Nicola L. Bell, Isobel H. Marr, Siyi She, Jonathan Hamilton, Craig Fraser, Kai Wang

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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.

Original languageEnglish
Pages (from-to)15419-15428
JournalDalton Transactions
Volume43
Issue number41
Early online date4 Sep 2014
DOIs
Publication statusPublished - 7 Nov 2014

Keywords

  • N-HETEROCYCLIC CARBENE
  • DIHYDROGEN ACTIVATION
  • TRIETHYLBORANE ADDUCT
  • LEWIS PAIR
  • COMPLEXES
  • REACTIVITY
  • HYDROGEN
  • LIGANDS
  • ALKENES
  • KETONES

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