Main Group Elements in Catalysis: Alkene/Alkyne Activation Using Organoaluminium(III)–Pyridyl Dimers

Abstract

Owing to its high natural abundance compared to the commonly used precious transition metals, as well as its high Lewis acidity and ability to change oxidation state, aluminium has been widely explored as the basis for a range of single-site catalysts. This work details the development of new cationic Al(III)-based catalysts for alkene/alkyne polymerisation containing two active centres, with the potential to act co-operatively in stereoselective assembly. The focus of this study was organoaluminium pyridyl dimers of the type [R2Al(2- py')]2 (R = Me, iBu; py'= substituted pyridyl group), which have been previously shown to be involved in a unique “pyridyl ring-flipping” (cis/trans isomerism) equilibrium. Five new dimers of the type [R2Al(2-py')]2 (R = Me, iBu; py' = substituted pyridyl group) with different substituents on the Al atoms and pyridyl rings were synthesised. The formation of the undesired cis isomers can be suppressed by the presence of substituents on the 6-position of the pyridyl ring due to steric congestion, with DFT calculations showing that the selection of the trans isomer is thermodynamically controlled. Demethylation by [Ph3C][B(C6F5)4] was used to obtain the catalytically active cationic species. All six dimeric cations (including that derived from the previously reported [Me2Al(2-py)]2 (py = C6H4N) successfully polymerised styrene to low molecular weight polystyrene, with diisobutyl-substituted Al centres inducing syndioselectivity in the polymeric products. Aliphatic and cycloalkenes, namely 1-hexene and cyclohexene, were successfully homopolymerised; and alkynes (phenylacetylene and tbutylacetylene) were selectively trimerised. DFT studies indicate that polymerisation proceeds via dimeric units (resulting from the initial oligomerisation of two alkene units), which “switch” between the two cationic Al centres available and combine to cause chain growth. In the last chapter, the coordination behaviour of tris-(2-Me-6-qyinolyl) and tris-(5-quinolyl) ligands containing Sb and Bi bridgehead atoms with a range of transition metals is explored, for assembly into supramolecular structures and as potential ligands for new catalytic systems.