Reactions of Dimeric Aluminium Hydride Compounds Containing Bidentate Dianionic Pyrrolyl Ligands and Their Applications in Ring‐Opening Polymerization of ϵ‐Caprolactone

Abstract

AbstractA series of dimeric aluminium compounds containing substituted bidentate dianionic pyrrolyl ligands have been synthesized and their reactivity and application in the ring‐opening polymerization of ϵ‐caprolactone have been studied. The reactions of [{C4H3N(2‐CH2NtBu)}AlH]2 (1) with 2 equiv. of 1‐indanone and 9‐fluorenone in dichloromethane generated [{C4H3N(2‐CH2NtBu)}Al(μ‐OC9H9)]2 (2) and [{C4H3N(2‐CH2NtBu)}Al{μ‐OCH(C12H8)}]2 (3), respectively, by hydroalumination. Similarly, the reactions of 1 with 2 equiv. of 2‐cyclohexen‐1‐one, 1‐(2,4,6‐trimethylphenyl)‐1‐ethanone, benzophenone, and 1,1‐diphenylacetone in dichloromethane afforded NtBu‐bridged dialuminium compounds 4–7, [{C4H3N(2‐CH2NtBu)}Al(OR)]2 [4, R = C6H9; 5, R = CH(Me)(C6H2‐2,4,6‐Me3); 6, R = CHPh2; 7, R = CH(Me)(CHPh2)] by insertion. A similar insertion occurred when 1 was treated with 2 equiv. of 2,4‐pentandione and dibenzoylmethane in dichloromethane to yield NtBu‐bridged dioxylate aluminium dimeric compounds 8 and 9, respectively. The Al atoms in compounds 2 and 4–7 possess a distorted tetrahedral geometry whereas the Al atoms in 8 and 9 have a square‐pyramidal environment. All the compounds have been well characterized by NMR spectroscopy and compounds 2 and 4–9 in the solid state were subjected to X‐ray diffraction analysis. A study of the polymerization of ϵ‐caprolactone revealed that the activity of the Al complexes is largely reliant on the steric nature of the substituents of their alkoxide groups.