Heterocyclische Ylide, II¹⁾ Ylide durch Decarboxylierung von N‐Methyl‐carbonsäurebetainen des Pyridins, Chinolins und Isochinolins

Abstract

Die Decarboxylierungsgeschwindigkeit der von Pyridin, Chinolin und Isochinolin abgeleiteten N-Methyl-carbonsaurebetaine hangt stark von deren Solvatation ab. Die Betaine 8-12 mit der Carboxylat-Gruppe in %-Stellung zum quartaren Stickstoff decarboxylieren in aprotischen Losungsmitteln bereits bei 60" (Tab. 2). Die dabei primar entstehenden reaktiven Zwischenstufen (Ylide vom Typ 2) lassen sich durch elektrophile Reagenzien wie DiazoniumIonen (13a), Diazo-Verbindungen (13b,f-h), Azide (13c-e) und Benzaldehyd abfangen (Schemata I und 2). Damit wird die a-Stellung quartarer Sechsring-Heterocyclen unter milden Bedingungen elektrophilen Substitutionen zuganglich. Der Vergleich der relativen Bildungstendenz der Ylide mit der kinetischen Aciditat analoger aromatischer Verbindungen zeigt, daB auch bei den Yliden 2 ein induktiver, elektrostatischer Effekt die Stabilitat bestimmt. Heterocyclic Ylides, I1 l ) . Ylides via Decarboxylation of N-Methylcarboxylates Derived from Pyridine, Quinoline, and IsoquinolineThe rate of decarboxylation of N-methylcarboxylates derived from pyridine, quinoline, and isoquinoline strongly depends on solvation. In aprotic solvents the zwitterions 8 -12 having the carboxylate group in the cr-position to the quaternary nitrogen readily lose carbon dioxide at 60" ( Table 2). The reactive intermediates (ylides of type 2) initially formed, can be trapped by electrophilic reagents (charts 1 and 2) such as diazonium ions (13a), diazo compounds (13b,f--h), azides (13c-e), and benzaldehyde. Thus the cr-position of quaternary six membered heterocycles is susceptible to electrophilic substitution under mild conditions. Comparison of the relative tendency of ylide formation with the kinetic acidity of analogous aromatic compounds, reveals that in the case of the ylides 2 as well, an inductive electrostatic effect determines the stability.Schon lange sind Reaktionen bekannt, bei denen ein Ringsubstituent aromatischer Heterocyclen durch ein Proton ersetzt wird, und zwar an einem C-Atom, bei dem die ubliche elektrophile aromatische Substitution uber eine tetraedrische Zwischenstufe versagt .

N‐Heterocyclic Carbenes of 5‐Haloindazoles Generated by Decarboxylation of 5‐Haloindazolium‐3‐carboxylates

“…Known pseudo‐cross‐conjugated heterocyclic mesomeric betaines, which form N‐heterocyclic carbenes by in situ decarboxylation, are presented in Figure 2. The alkaloid Homarine ( III , R 1 = R 2 = R 3 = H),25 and quinolinium ( III , R 1 = H, R 2 –R 3 = –CH=CH–CH=CH–), as well as isochinolinium derivatives ( III , R 1 –R 2 = –CH=CH–CH=CH–,R 3 = H),26 imidazolium‐2‐carboxylates IV ,27 pyrazolium‐3‐carboxylates V ,28 as well as indazolium‐3‐carboxylates V (R 1 –R 2 = –CH=CH–CH=CH–)29 were found to undergo thermal fragmentation to CO 2 and the correspondinghetarene ylidenes. Extrusions of isocyanates and isothiocyanates have been reported as well 29.…”

Recent advances in neutral and anionic N-heterocyclic carbene – betaine interconversions. Synthesis, characterization, and applications

“…117 In aprotic solvents, treatment of the pyridine-2-ylidene with aldehydes gave alcohols such as 165. 118,119 Sulfur converted 164 into pyridine-2-thione 166, CS 2 gave dithioacids 167, 120 azides converted the carbene into triazenes 168, and diazonium ions gave azo compounds 169. The decarboxylation of quinoline-2-carboxylic acid at elevated temperatures (Hammick reaction, 1937) also gave rise to the postulation of nucleophilic carbenes as highly reactive intermediates.…”

Formation of N‐Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2‐(Imidazolium‐1‐yl)phenolates

“…[54] By contrast, interconversions of pseudo-cross-conjugated heterocyclic mesomeric betaines( PCCMB) into nNHCs (Scheme 2e)p roved to be av ery versatile methodf or the in situ generation of heterocyclic carbenes. Numerous applications in synthesis as welli nc atalysis have been described to date, and imidazolium-2-carboxylates 9, [55][56][57][58][59][60][61][62][63] pyrazolium-3-carboxylates, [64][65][66] indazolium-3-carboxylates [67][68][69] and pyridinium-2-carboxylates [70][71][72] are examples of suitables tarting materials. In addition to the interconversions shown in Scheme 2, deprotonation of mesomeric betaines to form anionic NHCs has been studied.…”