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27.8.1.3.2 Variation 2: With Carbenium Ions

DOI: 10.1055/sos-SD-027-00291

Abbaspour Tehrani, K.; De Kimpe, N.Science of Synthesis, (200427323.

Iminium salts can be synthesized by hydride abstraction of tribenzylamine with aryldiazonium tetrafluoroborates.[‌49‌] Analogous abstractions from tertiary amines have been performed with triphenylmethyl tetrafluoroborate, -bromide, or -hexachloroantimonate[‌50‌] in chlorinated hydrocarbons, acetonitrile, excess amine at room temperature, or with dianisylmethyl perchlorate.[‌51‌] If an amine contains only α-hydrogens, the corresponding ternary iminium salt can be isolated. When β-hydrogen atoms are present, the intermediate ternary iminium salt reacts with the tertiary amine to produce an enamine and an ammonium salt. Enamines transfer hydride ions very rapidly to triphenylmethyl fluoroborate and triphenylmethyl bromide.[‌50‌] Attempts to prepare pure N,N-dimethylmethaniminium salt 29 from trimethylamine by this method always lead to a mixture of the iminium salt 29 together with the ammonium salt 30, resulting from nucleophilic addition of trimethylamine to 29 (Scheme 9). However, iminium salts derived from sterically hindered tertiary amines can be prepared in more than 90% yield (e.g., 3132, Scheme 9).[‌52‌]

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References


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