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

Abstract

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 Me 2 N(CH 2 ) 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 Me 3– x E[(CH 2 ) 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.