Unusual room temperature activation of 1,2-dialkoxyalkanes by niobium and tantalum pentachlorides

Abstract

The outcome of the reactions of MCl₅ (M = Nb, 1a; M = Ta, 1b) with 1,2-dialkoxyalkanes [i.e. MeO(CH₂)₂OMe (dme), EtO(CH₂)₂OEt, MeOCH₂CH(Me)OMe, MeO(CH₂)₂OCH₂Cl, MeO(CH₂)₂O(CH₂)₂OMe (diglyme)] depends strictly on the stoichiometry. In the 1 : 1 molar ratio reactions, single C–O bond cleavage occurs, resulting in formation of alkyl chloride and of the complexes MCl₄[O(R)CH(R′)CH₂O] [M = Nb, R = Me, R′ = H, 3a; M = Ta, R = Me, R′ = H, 3b; M = Nb, R = Et, R′ = H, 5; M = Nb, R = R′ = Me, 6; M = Nb, R = CH₂Cl, R′ = H, 7; M = Nb, R = (CH₂)₂O(Me)NbCl₅, R′ = H, 8], which have been characterized spectroscopically. Moreover, minor amounts of the oxo-bridged adducts MOCl₃(dme)MCl₅ (M = Nb, 4a; M = Ta, 4b) have been isolated in the reactions involving dme. On the other hand, compounds 1 react with two (or more) equivalents of dme mainly via a multiple C–O bond cleavage process, affording MOCl₃(dme) (M = Nb, 2a; M = Ta, 2b), 1,4-dioxane and methyl chloride. The oxychloride compounds MOCl₃ (M = Nb, 11a; M = Ta, 11b) have been efficiently obtained by addition of TiCl₄ to 2. Compound 2a is reduced in high yield to the Nb(III) species NbCl₃(dme), 12, upon treatment with SnBu₃H. The oxychloride NbOCl₃(diglyme), 9, 1,4-dioxane, CH₃Cl and the hexachloroniobate salt [MeOCH₂CH₂OCH₂CH₂O(H)Me][NbCl₆], 10, have been identified as products of the reaction of 1a with two equivalents of diglyme. The 1 : 2 molar ratio reaction of 1a with MeOCH₂CH(Me)OMe gives 2,5-dimethyl-1,4-dioxane. Compound 1a reacts with two equivalents of EtO(CH₂)₂OEt or MeO(CH₂)₂OCH₂Cl yielding Cl(CH₂)₂OCH₂CH₃ or O(CH₂Cl)₂ and diglyme, respectively, but not dioxane, suggesting that fragmentation pathways different from that found for dme are operating. The X-ray molecular structures of 4a, 4b and 10 have been determined.