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

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. Copyright © 2012 John Wiley & Sons, Ltd.