Synthesis and Evaluation of a Functionalised Set of Highly Lewis Acidic η6-Arene Ru(II) α,α-Diimine Catalysts to Efficiently Promote Benchmark Diels-Alder Cycloadditions

In this research work, the synthesis and characterisation of a series of neutrally charged N,N-chelating α,α-diimine ligands is comprehensively described. These class of N,N-ligands were employed in expanding the library of known arene Ru(+2) α,α-diimine complexes but the new application as pre-catalysts for the formation of super Lewis acids, to promote the Diels-Alder class of [4+2] cycloaddition reactions. These in-situ activated dicationic catalysts were compared for their overall performance, particularly in regards to the rate of conversion and regio-selectivity towards the preferential formation of exo or endo products. All catalysts were compared in terms of literature standard benchmark cycloadditions involving acrolein or methacrolein as the dienophile and cyclopentadiene as the diene. This work examined several key aspects of the two types of ligands ( α,α-diimines and arenes), varying the substitution pattern on both to determine the influence of steric and electronic effects towards the Diels-Alder cycloaddition. The α,α-diimines were extensively functionalised, particular at the backbone component, examining the use of both rigid aromatic and three-dimensional aliphatic motifs, including acyclic and cyclic frameworks. Moreover, for some catalysts, additional parameters were examining to optimise the cycloaddition process, including temperature, catalyst loading and the type of auxiliary counterion employed.