Dihydropyridines in Organometallic Synthesis. Formation of Pyridine and Dihydropyridine-Stabilized Alkylidene Complexes of Tungsten(0) and Chromium(0) from Fischer Carbene Complexes:  Structure and Reactivity

1,2- and 1,4-dihydropyridines react with alkoxycarbene complexes of chromium and tungsten to give, upon an unprecedented hydride transfer, alcohol elimination, and pyridine fixation on the carbene carbon, a new class of air-stable pyridinium ylide complexes. These pyridine-protected alkylidene complexes of chromium(0) and tungsten(0) were fully characterized by X-ray crystallography. In the case of (CO) 5 W C(CH 3 )(OEt) ( 5a ), besides the pyridinium ylide complex (CO) 5 W - −C(H)(CH 3 )(pyridine) + ( 7a ), the dihydropyridinium complex (CO) 5 W - −C(H)(CH 3 )(2,5-dihydropyridine) + ( 8a ) was also isolated. The intermediate tungstate (CO) 5 W - −C(H)(CH 3 )(OEt)(CH 3 NC 5 H 5 ) + could be easily obtained and characterized by using, as reducing agent, N -methyldihydropyridine. Whereas phenyl-substituted pyridinium complexes easily transferred the benzylidene moiety to alkenes, alkyl-substituted complexes appeared more reluctant to such a transfer: satisfactory results were observed in the case of nucleophilic olefins such as enol ethers. However, straightforward transfer of the tungsten(0) alkylidene group took place, even at room temperature, in the case of alkoxycarbene complexes tethered to alkenes, giving access, upon intramolecular cyclopropanation reactions, to polycyclic systems.