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

Abstract

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)5WC(CH3)(OEt) (5a), besides the pyridinium ylide complex (CO)5W-−C(H)(CH3)(pyridine)+ (7a), the dihydropyridinium complex (CO)5W-−C(H)(CH3)(2,5-dihydropyridine)+ (8a) was also isolated. The intermediate tungstate (CO)5W-−C(H)(CH3)(OEt)(CH3NC5H5)+ 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.

Zwitterionic Organometallates

“…Such complexes are produced by the reaction of (CO) 5 MϭC(OEt)R (M ϭ Cr, W) with 1,4-dihydropyridine, formed in situ from 1-methoxycarbonyl-1,2-dihydropyridine and methyllithium (Scheme 11). [48] The stability of the zwitterionic complex might be ascribed to the aromaticity of the pyridinium product. However, nonaromatic 2,5-dihydropyridine analogues 30 have also been structurally characterized, demonstrating the presence of 2,5-dihydropyridine derivatives along with 1,4-dihydropyridine in the in situ formed reactant (Scheme 11).…”

Linear Coupling of Three CO Ligands of Chromium Hexacarbonyl Leading to Functionalized Butenolides via Fischer Carbene Complexes

“…[2] The lack of knowledge about these phenomena means that a rational a priori design of novel molecular sieves is still impossible and, consequently to date, the synthesis of such materials requires a systematic and intelligent screening of the n-dimensional reaction. Recently, the concepts of combinatorial chemistry [3] and experimental design [4] have been successfully explored in order to speed up this screening process. Clearly, a more detailed understanding of the processess occurring during the synthesis of these materials is required, which can lead to a more rational approach towards zeolite syntheses.…”

Bis(trimethylsilyl)ketene Acetals as C,O‐Dinucleophiles: One‐Pot Formation of Polycyclic γ‐ and δ‐Lactones from Pyridines and Pyrazines

“…agents, [12][13][14] such compounds could also be used for the biomimetic reduction of Fischer carbene complexes. [15][16][17] The second was the dinucleophilic addition reactions, since we had discovered that bis(trimethylsilyl)ketene acetals 7 constitute an original class of ketene acetals among nucleophiles, [18][19][20][21] as they can behave as 1,3-carbon,oxygen dinucleophiles 8 in one-pot reactions, as the result of successive cleavage of both oxygen-silicon bonds (Scheme 2). pyridines through the use of bis(trimethylsilyl)ketene acetals had not been described to the best of our knowledge, although an early example of the addition of the bis(trimethylsilyl)ketene acetal 7a derived from acetic acid and giving the corresponding ester has been outlined.…”