Synthesis of an alkylmagnesium amide and interception of a ring-opened isomer of the important utility amide 2,2,6,6-tetramethylpiperidide (TMP)

Kennedy, Alan and Klett, Jan and McGrath, Graeme and Mulvey, Robert and Robertson, Gemma and Robertson, Stuart and O'Hara, Charles (2014) Synthesis of an alkylmagnesium amide and interception of a ring-opened isomer of the important utility amide 2,2,6,6-tetramethylpiperidide (TMP). Inorganica Chimica Acta, 411. pp. 1-4. ISSN 0020-1693

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    Abstract

    Two new magnesium complexes containing the important utility amide 2,2,6,6-tetramethylpiperidide (TMP) have been synthesised. Treating the magnesium bis(alkyl) reagent (Me3SiCH2)2Mg with a molar equivalent of TMP(H) in hydrocarbon medium produces the dimeric alkylmagnesium amide complex [(Me3SiCH2)Mg(μ-TMP)]22, which was isolated in high yield. X-ray crystallography revealed that 2 was an unsymmetrical dimer as unusually the two TMP ligands adopt different conformations – one a chair, the other a twisted boat. Solution studies (multinuclear NMR and DOSY NMR spectroscopies) show that 2 undergoes a monomerisation and Schlenk equilibrium in d8-THF. When (Me3SiCH2)2Mg was reacted with two molar equivalents of TMP(H) in hydrocarbon medium [in an effort to prepare Mg(TMP)2] a crystalline sample of a surprising product, a tetranuclear triheteroanionic amide-alkoxide-amidoalkene [(TMP)Mg(μ-TMP){μ-N(H)C(Me)2CH2CH2CH2C(Me) = CH2}Mg(μ-OCH2SiMe3)]23 was obtained. Complex 3 contains two unexpected anions, namely the alkoxide produced via oxygen insertion into a Mg–C bond, and the primary amidoalkene which is produced via ring opening of the TMP anion.