Reversible Alkene Binding and Allylic C–H Activation with an Aluminium(I) Complex

DOI: 10.14469/hpc/4978 Metadata

Created: 2019-01-21 15:32

Last modified: 2019-01-21 17:06

Author: Clare Bakewell

License: Creative Commons: Public Domain Dedication 1.0

Funding: (none given)

ABSTRACT: The monomeric molecular aluminium(I) complex 1 [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl) reacts with a series of terminal and strained alkenes including ethylene, propylene, allylben-zene and norbornene to form alkene bound products. Remarkably all these reactions are reversible under mild conditions (298–353 K) with alkene binding being disfavoured at higher temperatures due to the posi-tive reaction entropy. Van’t Hoff analyses have allowed quantification of the binding events with ∆Gº298K = –4 to –8 kcal mol-1. Calculations and single crystal X-ray diffraction studies are consistent with the alkene bound species being metallocyclopropane complexes. Alkene binding involves a reversible redox process with changes from the +1 to +3 aluminium oxidation state. Under more forcing conditions the metallocyclopropane complexes undergo non-reversible allylic C–H bond activation to generate aluminium(III) allyl hydride com-plexes. This represents a rare example of redox-based main group reactivity in which reversible substrate binding is followed by a further productive bond breaking event. Analysis of the mechanism reveals a reac-tion network in which alkene dissociation and reformation of 1 is required for allylic C–H activation, a realisa-tion that has important implications for the long-term goal of developing redox-based catalytic cycles with main group compounds.

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10.14469/hpc/4979 NMR data

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