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

• Alkylmagnesium amides with the bulky TMP anion display unusual bonding features.
• Multinuclear and DOSY NMR studies reveal a rather complex (and synthetically important) solution chemistry.
• The TMP anion can be cleaved in a similar manner as ethers.

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.

Two new magnesium amide complexes have been synthesised. The first is a dimeric alkylmagnesium amide which contains 2,2,6,6-tetramethylpiperidide (TMP) and (trimethylsilyl)methyl ions; the second is a surprising and highly unusual tetranuclear triheteroanionic amide-alkoxide-amidoalkene. Notably, the amidoalkene appears to be produced by ring opening of a TMP anion.Figure optionsDownload as PowerPoint slide