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?BH₂Bn IiPr=:C{N(iPr)CH}₂, Bn=CH₂C₆H₅] 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?C₆H₅ was borylated at the benzylic carbon atom in an umpolung event with BI₃ to afford the zwitterion IiPr?BI?CH(BI₃)?C₆H₅.