ic5009126_si_007.cif (65.75 kB)
An Al/P-Based Frustrated Lewis Pair as an Efficient Ambiphilic Ligand: Coordination of Boron Trihalides, Rearrangement, and Formation of HBX2 Complexes (X = Br, I)
dataset
posted on 2014-09-02, 00:00 authored by Werner Uhl, Christian Appelt, Agnes Wollschläger, Alexander Hepp, Ernst-Ulrich WürthweinThe Al/P-based frustrated Lewis pair (FLP) Mes2PC(CHPh)Al(CMe3)2 (1) reacted with boron halides
BX3 (X = F, Cl, Br, I) as an ambiphilic ligand to form
complexes (2–5) in which the boron
atoms were coordinated to phosphorus and one of the halogen atoms
to aluminum. Nonplanar five-membered heterocycles resulted that had
five different ring atoms (AlCPBX). The distance of the bridging halogen
atoms to the AlCPB plane increased steadily with the radius of the
halogen atoms. Only the BF3 adduct showed a dynamic behavior
in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases,
the rigid conformation led to the magnetic inequivalence of the substituents
at Al and P with well-resolved signals for each group. The BBr3 and BI3 complexes underwent in solution at room
temperature a spontaneous stereoselective rearrangement with the concomitant
release of isobutene. The obtained products, Mes2P(μ-CCHPh)(μ-HBX2)AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP,
Mes2PC(CHPh)AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The
trapped boranes are either completely unknown (X = I) or unstable
in the free form. Quantum–chemical calculations suggest an
ionic rearrangement mechanism via the formation of a borenium cation,
β-hydride elimination, and hydride transfer. The bromine migration
from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic
rearrangement.