Ringöffnung von N‐(Tetraalkylamidinio)pyridinium‐Salzen durch Anionen methylenaktiver Verbindungen

Abstract

Die N-Carbeniopyridinium-Salze 2, 4, 14 und 18 bieten fur den Angriff eines Nucleophils drei Moglichkeiten, die sich in Abhangigkeit von den Reaktionspartnern alle realisieren lassen. Mit Anionen methylenaktivcr Verbindungen fuhrt der u-Angriff am Pyridinium-Ring unter Ringoffnung zu Azahexamethinneutrocyaninen (8, 15, 23). In einigen Fallen bilden sich kinetisch kontrolliert 1,4-Dihydropyridine, die sich thermisch zu den 1,2-Dihydropyridinen bzw. deren ringgeoffnetcn Folgeprodukten umlagern. SchlieDlich ist auch der nucleophile Angriff am kationischen Substituenten der N-Carbeniopyridinium-Salze moglich, wie die Bildung der Push-pull-Olefine 11, 16 und des Enolethers 12 zeigt. Ring Opening of N-(Tetraa1kylamidinio)pyridinium Salts by Anions of CH-Acidic Methylene CompoundsN-Carbeniopyridinium salts 2,4,14, and 18 offer three sites to nucleophilic attack; depending on the reactants, all three modes have been realized. With anions of CH-acidic methylene compounds, u-attack at the pyridinium ring followed by ring opening leads to azahexamethine merocyanines (8, 15, 23). In some cases, kinetically controlled reactions yield 1,4-dihydropyridines which isomerize thermally to give 1,2-dihydropyridines which undergo ring opening spontaneously. Also, nucleophilic attack at the cationic substituent of the N-carbeniopyridinium salts is possible as is indicated by the formation of push-pull olefins 11, 16 and of enol ether 12.Nucleophile Ringoffnungen quartarer Pyridiniumsalze (Schema 1) gehoren zu den lange bekannten Reaktionen der Organischen Chemie '**). Ohne auf den genauen, im Detail gut untersuchten Reaktionsablauf (Bildung eines 1,2-Dihydropyridins, disrotatorische elektrocyclische Ringoffnung und Isomerisierung zum meistens isolierten all-trans-Trien) einzugehen, sei hier nur erwihnt, daB als Nucleophile in der Regel OH-und sekundare Amine verwendet werden, wodurch man im ersten Fall direkt, im zweiten erst nach Hydrolyse zu 5-Amino-2,4-pentadienalen gelangt ',*I. Seltener wurden die Anionen aktiver Methylenverbindungen 3-7) und Phosphorylide8) zur nucleophilen Ringoffnung herangezogen.

Synthesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodium(III) Hydrides

“…Both the literature and other experiments are consistent with the implication that the 1,4-dihydropyridine 6b is more stable than its 1,2 isomer 6a . Fowler measured in 1972 a positive Δ G , 2.3 kcal/mol, for the equilibrium (established by base in DMSO at 91.6 °C) between the N -methyl dihydropyridines 10a and 10b in eq .…”

Property Tuning in Charge‐Transfer Chromophores by Systematic Modulation of the Spacer between Donor and Acceptor

“…Synthesis of donor–acceptor chromophores with different spacers : The preparation of compounds 1 a ,6 1 b, 4b and 2 a 7 has been previously described. The synthesis of 2 b started from ketone 7 (Scheme ) 8.…”

“…4‐Ethynyl‐ N,N ‐dimethylaniline was prepared from 4‐iodo‐ N,N ‐dimethylaniline and trimethylsilylacetylene by Sonogashira cross‐coupling reaction and final Me 3 Si group removal (K 2 CO 3 , MeOH) in 92 % overall yield. Compounds 1 a ,6 1 b ,4b 2 a ,7 and 7 8 were prepared according to literature procedure.…”

Formal Ring‐Opening/Cross‐Coupling Reactions of 2‐Pyrones: Iron‐Catalyzed Entry into Stereodefined Dienyl Carboxylates

“…Though potentially very useful, such transformations have surprisingly little precedent in the prolific cross-coupling arena altogether. [15,16] We became aware of this unusual reaction format while analyzing the results of the seemingly routine cross-coupling of the triflate 1 with various Grignard reagents in the presence of [Fe(acac) 3 ] (5 mol %, Scheme 1). [17,18] Although iron-catalyzed CÀC bond-forming reactions of other 2-pyrone derivatives had previously met with limited success, [19] the alkylations of 1 proceeded smoothly, thus furnishing compounds 2 in appreciable yields.…”