Inherent Stability Limits of Intramolecular Boron Nitrogen Lewis Acid-Base Pairs

The reaction of (C6F5)2BH (1) with N,N-dimethylallylamine (2), N,N-diethylallylamine (3) and 1-allylpiperidine (4) afforded the five-membered ring systems (C6F5)2B(CH2)3NR2 (R=Me (5), Et (6)) and (C6F5)2B(CH2)3N(CH2)5 (7) with an intramolecular dative B?N bond. A different product was obtained from the reaction of (C6F5)2BH (1) with N,N-diisopropylallylamine (8), which afforded the seven-membered ring system (C6F5)2B(CH2)3N(iPr)CH(Me)CH2 (9) under extrusion of dihydrogen. All compounds were characterised by elemental analysis, NMR spectroscopy and single-crystal X-ray diffraction experiments. Density functional theory (DFT) studies were performed to rationalise the different reaction mechanism for the formation of products 6 and 9. The bonding situation of compound 9 was analysed in terms of its electron density topology to describe the delocalised nature of a borane enamine adduct.