Aluminum alkoxide complexes from highly substituted 3,4‐epoxy alcohols: An NMR and computational study

Abstract

The organoaluminum mediated epoxide ring opening of epoxy alcohols is a key step in the oxirane-based approach for polypropionate synthesis. However, this reaction has shown unanticipated regioselectivities when applied to 2-methyl-3,4-epoxy alcohols. In order to gain mechanistic insight into the factors controlling the epoxide ring opening process, diastereomeric 2-methyl-3,4-epoxy alcohols were reacted with triethylaluminum in order to identify the aluminum complexes formed by these systems. Different epoxide-aluminum complexes were calculated using ab initio HF/[13s7p/11s5p] and B3PW91/6-31G** gauge-including atomic orbital (GIAO) methods and compared to the experimental NMR data. The calculated and experimental data correlates with the aluminum dimer complex (TIPSOCH 2 CH(OAlEt 2 )CH(CH) 3 CHCH(O)(AlEt 3 )) 2 (VIII) for the systems favoring the nucleophilic attack at the external C4 epoxide carbon, while an unusual trialuminum species TIPSOCH 2 CH(OAlEt 2 )CH(CH) 3 CHCH(O)(AlEt 3 ) 2 (X) is consistent with the systems favoring the internal C3 attack. The 27 Al NMR data established the tetracoordinated nature of the aluminum metal in all alkoxy aluminum intermediates, while the 13 C NMR data provided insight into the aluminum-oxygen coordination. The formation of the complexes was dictated by the stereochemical disposition of the substituents. These complexes are different from the generally accepted bidentate intermediates proposed for 2,3-epoxy alcohols and simpler 3,4-epoxy alcohols.