Dynamic Exchange in Intramolecular Lewis Pairs with Multiple Lewis-Acidic Functions

Hydroboration of
allyldimethylamine, diallylmethylamine, triallylamine,
and diallylmethylphosphane
with dimeric 9-BBN yielded the corresponding singly, doubly, and triply
Lewis-acid-functionalized intramolecular Lewis pairs. For the singly
Lewis-acid-functionalized derivative Me2N­(CH2)3-9-BBN no evidence for the existence of an equilibrium
involving an open-chain form was found in solution. For the doubly
and triply Lewis-acid-functionalized compounds Me3–xE­[(CH2)3-9-BBN]x [E = N (x = 2, 3), P (x = 2)] a dynamic exchange of the free Lewis-acid functions with an
intramolecular Lewis acid base complex was observed and investigated
by variable-temperature NMR spectroscopy. The free energies of activation
of the exchange processes were determined by the coalescence method
and found to be lower for the Lewis-base component nitrogen than for
phosphorus. To further understand the exchange process of the Lewis
acids at the central Lewis base, transition states for two different
exchange mechanisms were considered and searched for.