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, allylbenzene 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 positive reaction entropy. Van't Hoff analyses have allowed quantification of the binding events with . 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 complexes. 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 reaction network in which alkene dissociation and reformation of 1 is required for allylic C-H activation, a realisation that has important implications for the long-term goal of developing redox-based catalytic cycles with main group compounds.
Highlights
Since the turn of the 21st century, there has been a growing realisation that main group compounds can imitate the behaviour of transition metal complexes.[1,2,3,4] The identi cation of reversible redox processes involving substrate activation is widely believed to be the most important bottleneck for developing transition-metal-like catalytic cycles
In the p-allyl mechanism for alkene isomerization reversible alkene binding is coupled to the intramolecular activation of an allylic C–H bond, resulting in the transposition of the unsaturated bond to a new position in the carbon chain.[16]
We have previously communicated that the aluminium(I) complex 1, known to activate a series of small molecules,[34,35,36] reacts reversibly with norbornene to form the metallocyclopropane complex 2a (Scheme 1).[37]
Summary
Since the turn of the 21st century, there has been a growing realisation that main group compounds can imitate the behaviour of transition metal complexes.[1,2,3,4] The identi cation of reversible redox processes involving substrate activation is widely believed to be the most important bottleneck for developing transition-metal-like catalytic cycles. We have previously communicated that the aluminium(I) complex 1, known to activate a series of small molecules,[34,35,36] reacts reversibly with norbornene to form the metallocyclopropane complex 2a (Scheme 1).[37] A preliminary analysis of the bonding within this complex allowed assignment of 2a as an genuine aluminium(III) complex and alkene binding as a redox process involving reversible oxidative addition and reductive elimination steps.
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