Benzyl Borane NHC Adducts: Beyond B-C Bond Scission

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B-C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B-C bond are poorly investigated although several other reactive sites in benzyl-boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr-BH₂ Bn [IiPr=:C{N(iPr)CH}₂ , Bn=CH₂ C₆ H₅ ] is demonstrated. Dihalogenation of the BH₂ entity is observed with BCl₃ and BBr₃ , whereas BI₃ either affords IiPr-BHI₂ or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH₂ group was demonstrated with elemental bromine Br₂ . The brominated product IiPr-BBr₂ -CHBr-C₆ H₅ was borylated at the benzylic carbon atom in an umpolung event with BI₃ to afford the zwitterion IiPr-BI-CH(BI₃ )-C₆ H₅ .