Stepwise Nucleophilic Substitution to Access Saturated N-heterocyclic Carbene Haloboranes with Boron–Methyl Bonds

Abstract

Compounds of boranes with N-heterocyclic carbenes are known, yet little attention has been paid to NHC compounds of boron bearing methyl and halogen moieties together. The reason can be attributed to the hazardous methyldichloroborane (MeBCl2), which ignites in air. We describe here convenient solution-phase access to SIDipp·MeBCl2 (SIDipp = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene) (3) by a salt metathesis reaction of SIDipp·BCl3 (2) with MeLi. Replacement of the chlorine atoms of 3 with stepwise addition of AgOTf led to the formation of SIDipp·MeBCl­(OTf) (4) and SIDipp·MeB­(OTf)2 (5). In the case of 4, all of the substituents on the boron atom are different. Subsequently, we extended our synthetic approach to the amidinate system and prepared PhC­(NtBu)2B­(Me)Cl (7) from the reaction of PhC­(NtBu)2BCl2 (6) with MeLi.

Nucleophilic Substitution at a Coordinatively Saturated Five-Membered NHC∙Haloborane Centre

“…The B−CNHC bond distance is 1.6661(10) Å, which is slightly longer i comparison to that in 1 due to the steric congestion of the phenyl group at the boro center. The B-Cl bonds are 1.8992(8) Å and 1.8698(8) Å, which match with the previousl reported B-Cl bond distance [8]. 2) Å, which is in good agreement with the B−O bond length in our previously reported 5-SIDipp•BMeOTfCl (B1−O3 1.503(3) Å) [8].…”

“…1 is crystalized in the monoclinic P21/n space group. The B-C bond length in 1 [1.628( 7) Å] is in good accordance with the previously reported 5-SIDipp•MeBCl2 [1.6261(19) Å], but considerably longer compared to that in the 5-SIDipp•BH3 [1.593(4) Å] [8]. The increase in the bond length can be ascribed to the enhancement of steric hindrance at the central boron atom.…”

“…In our previous work, we have reported the first carbene MeBCl2 adduct via salt metathesis procedure [8]. In this work, we have prepared the 5SIDipp•haloborane adducts and shown their reactivities towards nucleophilic substitution.…”

“…Braunschweig and coworkers recently broadened the range of Lewis base by demonstrating nucleophilic addition and substitution of cyclic alkyl amino carbene (cAAC) BH 3 adduct [7]. In our previous works, we have shown the nucleophilic substitution at 5-SIDipp•MeBCl 2 (5-SIDipp = 1,3,-bis-(2,6diisopropylphenyl)imidazolin-2-ylidine) [8]. Further, we have explored the substitution at 6-SIDipp•BH 3 (6-SIDpp = 1,3-di(2,6-diisopropylphenyl) tetrahydropyrimidine-2-ylidene) center and the introduction of rare functional groups such as -OTf, -NO 3 [9].…”

Substitution at sp3 boron of a six-membered NHC·BH3: convenient access to a dihydroxyborenium cation

“…13 Besides, Tamm and coworkers used 6-SIDipp as a Lewis base component in an FLP for the activation of dihydrogen. 14 Due to our current interest in boron chemistry, 15,16 we have studied the synthesis and reactivity of a saturated sixmembered N-heterocyclic carbene (6-SIDipp) borane (1) adduct. 1 is amenable to further functionalization via iodination, providing access to NHC mono and diboryl iodides (2 and 3), selectively, which undergo nucleophilic substitution reactions with AgOTf and AgNO 3 to give boron compounds with OTf (4 and 6) and ONO 2 (5 and 7) functionalities.…”