DIHYDROPYRIDINES IN ORGANOMETALLIC SYNTHESIS - FORMATION OF PYRIDINE AND DIHYDROPYRIDINE-STABILIZED ALKYLIDENE COMPLEXES OF TUNGSTEN(0) ANDCHROMIUM(0) FROM FISCHER CARBENE COMPLEXES - STRUCTURE AND REACTIVITY

1,2- and 1,4-dihydropyridines react with alkoxycarbene complexes of ch romium and tungsten to give, upon an unprecedented hydride transfer, a lcohol elimination, and pyridine fixation on the carbene carbon, a new class of air-stable pyridinium ylide complexes. These pyridine-protec ted alkylidene complexes of chromium(0) and tungsten(0) were fully cha racterized by X-ray crystallography. In the case of (CO)(5)W=C(CH3)(OE t) (5a), besides the pyridinium ylide complex (COW--C(H)(CH3)(pyridine )(+) (7a), the dihydropyridinium complex (COW--C(H)(CH3)(2,5-dihydropy ridine)(+) (8a) was also isolated. The intermediate tungstate (COW--C( H)(CH3)(OEt)(CH3NC5H5)(+) could be easily obtained and characterized b y using, as reducing agent, N-methyldihydropyridine. Whereas phenyl-su bstituted pyridinium complexes easily transferred the benzylidene moie ty to alkenes, alkyl-substituted complexes appeared more reluctant to such a transfer: satisfactory results were observed in the case of nuc leophilic olefins such as enol ethers. However, straightforward transf er of the tungsten(0) alkylidene group took place, even at room temper ature, in the case of alkoxycarbene complexes tethered to alkenes, giv ing access, upon intramolecular cyclopropanation reactions, to polycyc lic systems.