Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

Reference:

Hill, M., Kociok-Kohn, G., MacDougall, D. J., Mahon, M. F. and Weetman, C., 2011. Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives. Dalton Transactions, 40 (46), pp. 12500-12509.

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Official URL:

http://dx.doi.org/10.1039/c1dt11235j

Abstract

Reactions of the -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 2- n2-n2-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.

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