The reaction of six-membered saturated NHC [1,3-di(2,6-diisopropylphenyl)
tetrahydropyrimidine-2-ylidene; henceforth abbreviated as 6-SIDipp]
with PhBCl2 yields a Lewis base adduct, 6-SIDipp·PhBCl2 (1), which readily undergoes nucleophilic substitution
reaction with AgNO3, leading to the single (2) and double (3) substitution of both chlorides with
ONO2 moieties at the boron atom. The reaction of 1 with 1 equiv of AlCl3 resulted in a borenium
cation of composition [6-SIDipp·B(Ph)Cl]+ (4) with AlCl4
– as the counteranion. Although
borenium cations with different substituents on boron have been reported,
a structurally characterized phenylchloroborenium cation remains unknown.
Similarly, the reaction of 1 with triflic acid provides
the first representative of a new class of borenium cations bearing
one hydroxyl and one phenyl group on boron (5), a cationic
analogue of borinic acid.
The combination of 6-SIDipp•AlH3 (1) and 5-IDipp resulted in the ring expansion of 6-NHC, while the five-membered NHC remained unchanged; which was subsequently explained by DFT studies. Besides, the substitution...
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