Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 40; no. 46; pp. 12500 - 12509
• Main Authors: Hill, Michael S., Kociok-Köhn, Gabriele, MacDougall, Dugald J., Mahon, Mary F., Weetman, Catherine
• Format: Journal Article
• Language: English
• Published: England 01.01.2011
• Subjects: Coordination Complexes - chemistry; Crystallography, X-Ray; Derivatives; Diffraction; Hydrides; Kinetics; Ligands; Magnesium; Magnesium - chemistry; Molecular Conformation; Pyridines; Pyridines - chemistry; Quinoline; Quinolines - chemistry; Thermodynamics; X-rays
• ISSN: 1477-9226; 1477-9234; 1477-9234
• DOI: 10.1039/c1dt11235j

Reactions of the β-diketiminato n-butyl magnesium complex, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg(n)Bu], with a range of substituted pyridines and fused-ring quinolines in the presence of PhSiH(3) 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 μ(2)-η(2)-η(2)-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.