Reaction of Azole Heterocycles with Tris(dimethylamino)borane, a New Method for the Construction of Tripodal Borate-Centred Ligands
✍ Scribed by Philip J. Bailey; Daniel Lorono-Gonzales; Chiara McCormack; Frances Millican; Simon Parsons; Robert Pfeifer; Pedro P. Pinho; Felix Rudolphi; Alejandro Sanchez Perucha
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 250 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0947-6539
No coin nor oath required. For personal study only.
✦ Synopsis
Abstract
Reaction of 2‐mercapto‐1‐methylimidazole (methimazole) with tris(dimethylamino)borane, B(NMe~2~)~3~, provides the tetrahedral dimethylamine adduct of tris(methimazolyl)borane, [(Me~2~HN)B(methimazolyl)~3~]. By contrast, imidazole, 2‐methylimidazole, 2‐chloroimidazole and benzimidazole provide the homoleptic tetra‐azolyl systems H[B(azolyl)~4~], and the same product is obtained even when a substoichiometric quantity of the heterocyle is employed. The change in reaction outcome is correlated with the variation of basic p__K__~a~ for the heterocycles. A simple acid‐base reaction with elimination of HNMe~2~ is proposed for the reaction with the weakly basic, but more strongly acidic, methimazole. However, for the more strongly basic imidazoles, initial coordination of the heterocycle imine nitrogen to the weakly Lewis acidic boron centre in B(NMe~2~)~3~ to form the tetrahedral adduct [(azole)B(NMe~2~)~3~] is proposed. The greater availability of the NMe~2~ lone pairs in this species results in increased basicity and a rapid reaction with further heterocycle to provide the observed H[B(azolyl)~4~] products. For 2‐nitroimidazole, the low basicity (and increased NH acidity) results in the formation of [(HNMe~2~)B(2‐nitroimidazolyl)~3~] on reaction with B(NMe~2~)~3~, analogous to the product formed with methimazole. Both [(HNMe~2~)B(methimazolyl)~3~] and H[B(benzimidazolyl)~4~] have been structurally characterised by single crystal X‐ray crystallography. This chemistry has been exploited to provide a new synthesis of borate‐centred tripod ligands, whereby N‐methylimidazole is used to activate B(NMe~2~)~3~ to reaction with methimazole to form the new ligand [(N‐methylimidazole)B(methimazolyl)~3~] in good yield and a complex of this ligand with Ru^II^ has been structurally characterised.