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

Arnold, P. L., Bell, N. L. , Marr, I. H., She, S., Hamilton, J., Fraser, C. and Wang, K. (2014) Protonolysis and thermolysis reactions of functionalised NHC-carbene boranes and borates. Dalton Transactions, 43(41), pp. 15419-15428. (doi: 10.1039/C4DT01464B) (PMID:25188908)

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.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Bell, Dr Nicola
• Authors: Arnold, P. L., Bell, N. L., Marr, I. H., She, S., Hamilton, J., Fraser, C., and Wang, K.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Dalton Transactions
• Publisher: Royal Society of Chemistry
• ISSN: 1477-9226
• ISSN (Online): 1477-9234
• Published Online: 04 September 2014