title: Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives.
abstract: 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.
authors: Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK. msh27@bath.ac.uk
authors: Michael S
authors: MS
authors: Hill
authors: Gabriele
authors: G
authors: Kociok-Köhn
authors: Dugald J
authors: DJ
authors: MacDougall
authors: Mary F
authors: MF
authors: Mahon
authors: Catherine
authors: C
authors: Weetman
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry*; D008968:Molecular Conformation; D011725:Pyridines / Q000737:chemistry*; D011804:Quinolines / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D056831:Coordination Complexes; D011725:Pyridines; D011804:Quinolines; C037219:quinoline; D008274:Magnesium; C023666:pyridine