DIHYDROPYRIDINES IN ORGANOMETALLIC SYNTHESIS - FORMATION OF PYRIDINE AND DIHYDROPYRIDINE-STABILIZED ALKYLIDENE COMPLEXES OF TUNGSTEN(0) ANDCHROMIUM(0) FROM FISCHER CARBENE COMPLEXES - STRUCTURE AND REACTIVITY
H. Rudler et al., DIHYDROPYRIDINES IN ORGANOMETALLIC SYNTHESIS - FORMATION OF PYRIDINE AND DIHYDROPYRIDINE-STABILIZED ALKYLIDENE COMPLEXES OF TUNGSTEN(0) ANDCHROMIUM(0) FROM FISCHER CARBENE COMPLEXES - STRUCTURE AND REACTIVITY, Journal of the American Chemical Society, 118(48), 1996, pp. 12045-12058
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.