The 1,3 migration of a methyl group in reactions of the trisyl iodide (Me3Si)3CSi(CD3)2I and related iodides with silver salts in 2,2,2-trifluoroethanol. Refinement of the mechanism

Abstract

In the reactions of the isotopically labelled iodide (Me₃Si)₃CSi(CD₃)₂I with silver salts AgX the mixtures of unrearranged (Me₃Si)₃CSi(CD₃)₂X and rearranged (Me₃Si)₂[(CD₃)₂MeSi]CSiMe₂X are formed in lower yields in CF₃CH₂OH than in EtOH but the proportion of the rearranged isomer is markedly larger in the former alcohol, sometimes approaching 50%. The yields of the trifluoroethoxides (Me₃Si)₃CSi(CD₃)₂OCH₂CF₃ and (Me₃Si)₂[(CD₃)₂MeSi]CSiMe₂OCH₂CF₃ are correspondingly smaller than those of the ethoxides in EtOH, but the percentage of the rearranged product is markedly higher, at 63 ± 5, the first observations of percentages of rearranged alkoxide products of > 50%. In some reactions in which not all the iodide has reacted that remaining contains significant amounts of the rearranged iodide (Me₃Si)₂[(CD₃)₂MeSi]CSiMe₂I. Formation from this of (Me₃Si)₂[(CD₃)₂MeSi]CSiMe₂X and (Me₃Si)₂[(CD₃)₂MeSi]CSiMe₂OCH₂CF₃ must contribute to the unusually high proportions of rearranged products in this medium. It is suggested that the initial reaction of the (Me₃Si)₃CSi(CD₃)₂I with AgX produces an ion pair containing the Me-bridged cation [(Me₃Si)₂Me₂SiCSi(CD₃)₂IMe]⁺ and the argentate ion [AgXI]⁻ and that attack on the cation by the solvent at either the α-Si or γ-Si ends of the bridge to give trifluoroethoxides competes with release of an X⁻ or an I⁻ ion from the argentate ion to the α-site and also with migration of the argentate ion to the γ-site. The cation in the new ion pair formed by this migration can then in turn be attacked by the solvent or abstract X⁻ or I⁻ from the argentate ion. Such migration of argentate ion is assumed to occur also in the corresponding reactions in inert and other hydroxylic media.

Reaction of (Me₃Si)₃CSiEt₂I with AgO₃SMe in CF₃CH₂OH gives the expected (Me₃Si)₃CSiEt₂OSO₂Me, (Me₃Si)₂(Et₂MeSi)CSiMe₂OSO₂Me, (Me₃Si)₃CSiEt₂OCH₂CF₃, and (Me₃Si)₂(Et₂MeSi)CSiMe₂OSO₂Me, together with a little (Me₃Si)₂(EtMe₂Si)CSiMeEtOCH₂CF₃. The presence of this last product is attributed to the generation of some of the rearranged iodide (Me₃Si)₂(Et₂MeSi)CSiMe₂I and formation from that of an Et-bridged cation. The iodide (Me₃Si)₃CSi(CD₃)₂I also undergoes solvolysis slowly in refluxing CF₃CH₂OH alone to give the rearranged and unrearranged trifluoroethoxides, but only in roughly 1 ∶ 4 ratio.