Pentacoordinate organoaluminum chemistry: Catalytic efficiency of Me3Al in the epoxide cleavage with alkynyllithiums

Article abstract of DOI:10.1021/ja9842464

A new and highly effective catalytic method for epoxide alkynylations has been developed that involves the chelation-controlled alkylation of heterosubstituted epoxides with Me₃Al via pentacoordinate organoaluminum complexes by taking advantage of the exceedingly high affinity of aluminum to oxygen. For example, reaction of epoxy ether, (1-benzyloxy)-3-butene oxide (1), in toluene with PhC?CLi under the influence of catalytic Me₃Al (10 mol%) proceeded smoothly at 0 °C for 5 h to furnish the alkynylation product, 1-(benzyloxy)-6-phenylhex-5-yn-3-ol, in 76% yield [cf. 3% without Me₃Al catalyst; 78% with stoichiometric Me₃Al under similar conditions]. This represents the first catalytic procedure for the amphiphilic alkylation of epoxides. The participation of pentacoordinate Me₃Al complexes of epoxy ethers of type 1 is emphasized by comparing the reactivity with the corresponding simple epoxide, 5-phenyl-1-pentene oxide, which was not susceptible to nucleophile attack of PhC?CLi with catalytic Me₃Al under similar conditions. The pentacoordinate complex formation of Me₃Al with epoxy ether 1 is characterized by low-temperature ¹³C and ²⁷Al NMR spectroscopy. This approach is also applicable to the selective alkynylation of tosyl aziridines with adjacent ether functionality, which provides a promising method for amino alcohol synthesis.