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Issue 46, 2011
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Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

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Reactions of the β-diketiminato n-butyl magnesium complex, [HC{(Me)CN(2,6-iPr2C6H3)}2MgnBu], with a range of substituted pyridines and fused-ring quinolines in the presence of PhSiH3 has been found to result in dearomatisation of the N-heterocyclic compounds. This reaction is proposed to occur through the formation of an unobserved N-heterocycle-coordinated magnesium hydride and subsequent hydride transfer via the C2-position of the heterocycle prior to hydride transfer to the C4-position and formation of thermodynamically-favoured magnesium 1,4-dihydropyridides. This reaction is kinetically suppressed for 2,6-dimethylpyridine while the kinetic product, the 1,2-dihydropyridide derivative, was isolated through reaction with 4-methylpyridine (4-methylpyridine), in which case the formation of the 1,4-dihyropyridide is prevented by the presence of the 4-methyl substituent. X-ray structures of the products of these reactions with 4-methylpyridine, 3,5-dimethylpyridine and iso-quinoline comprise a pseudo-tetrahedral magnesium centre while the regiochemistry of the particular dearomatisation reaction is determined by the substitution pattern of the N-heterocycle under observation. The compounds are all air-sensitive and exposure of the magnesium derivatives of dearomatised pyridine and 4-dimethylaminopyridine (DMAP) to air resulted in ligand rearomatisation and the formation of dimeric μ222-peroxomagnesium compounds which have also been subject to analysis by single crystal X-ray diffraction analysis. An unsuccessful extension of this chemistry to N-heterocycle hydrosilylation is suggested to be a consequence of the low basicity of the silane reagent in comparison to the pyridine substrates which effectively impedes any further interaction with the magnesium centres.

Graphical abstract: Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

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The article was received on 28 Jun 2011, accepted on 31 Aug 2011 and first published on 11 Oct 2011

Article type: Paper
DOI: 10.1039/C1DT11235J
Dalton Trans., 2011,40, 12500-12509

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    Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

    M. S. Hill, G. Kociok-Köhn, D. J. MacDougall, M. F. Mahon and C. Weetman, Dalton Trans., 2011, 40, 12500
    DOI: 10.1039/C1DT11235J

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