1,3‐Anionische Cycloadditionen, XVII. Synthese von Imidazolinen und Imidazolen durch Cycloaddition von 2‐Azaallyllithium‐Verbindungen an aromatische Nitrile

Abstract

1,3-Diphenyl-2-azaallyllithium (lc) reagiert mit aromatischen Nitrilen unter Cycloaddition und anschlieRender Dehydrierung zu lithiierten Imidazolen (3 b). 1 ,l-Diphenyl-2-azaallyllithium ( l a ) bildet dagegen unter entsprechenden Bedingungen entweder durch Cycloaddition lithiierte Imidazoline (2) oder durch normale Addition lithiierte offenkettige Enamine (5a, b). Die Enamine (4-Amino-2-azabutadiene) sind trotz ihrer primaren Aminogruppe isolierbar. -Mit aliphatischen Nitrilen waren Cycloadditionen in praparativ brauchbarer Ausbeute nicht moglich. 1,3-Anionic Cycloadditions, XVII 1) Synthesis of Imidazolines and Imidazoles by Cycloaddition of 2-Azaallyllithium Compounds to Aromatic Nitriles1,3-Diphenyl-2-azaallyllithium (1 c) reacts with aromatic nitriles by cycloaddition and subsequent dehydration to give lithiated imidazoles (3 b). 1 ,l-Diphenyl-2-azaallyllithium ( l a ) under corresponding conditions gives either by cycloaddition lithiated imidazolines (2) or by normal addition lithiated open-chain enamines (5a, b). The enamines formed (4-amino-2-azabutadienes) are isolable in spite of their primary amino group. -Cycloaddition in preparatively useful yield was not possible with aliphatic nitriles. A) Synthese von Imidazolinen und ImidazolenWie von uns gefunden, cycloaddieren sich 2-Azaallyllithium-Verbindungen an aromatische Nitrile2s3). Mit den Reagenzien 1 ,l-Di~henyl-~) (la) und 1,1,3-Triphenyl-2-a~aallyllithium~) (1 b) entstanden Imidazoline 2, mit 1 ,3-Diphenyl-2-azaallyllithium6) (lc) dagegen infolge Dehydrierung des Primarprodukts Imidazole 4 (Tab. 1). Es ist anzunehmen, darj die zu Imidazolen fuhrende Dehydrierung durch Lithiumhydrid-Eliminierung aus einem Zwischenprodukt des Typs 3a eingeleitet wird. Vermutlich verbleibt das Lithiumhydrid in unmittelbarer Nahe zum cyclischen Fragmentierungsprodukt und bewirkt dessen Deprotonierung zu 3b. Eine analoge Dehydrierung wurde bei der Einwirkung von 1,3-Dipheny1-2-azaallyllithium auf Tolan ') sowie auf Dehydrobenzol') beobachtet. Bekannte Analogien zur Lithiumhydrid-Eliminierung aus den Cycloaddukten 3a sind die Metallhydrid-Eliminierungen aus den Addukten des Pyridins mit Natriumamid') oder Phenyllithium').

Mechanism of Anionic [3 + 2] Cycloadditions. An ab Initio Computational Study on the Cycloaddition of Allyl-, 2-Borylallyl-, and 2-Azaallyllithium to Ethylene

“…There are many examples involving olefins with aromatic and/or electron- withdrawing substituents. , Alkenes bearing organoelement groups containing Si, Ge, P, As, S, and Se have proved to be very useful extensions, 9bb,cc, since such substituents can easily be replaced by a large number of other functionalities. Cycloadditions with some dienes and alkynes, ,9bb, aromatic nitriles, and with CN, CS, and NN double bonds 9r,13 also have been reported.…”

“…An equally elegant access to pyrrolidine ring systems is provided by the cycloaddition of 2-azaallyl anions and olefins. Numerous examples of both reactions have been reported in the literature. − Frontier orbital considerations (the Woodward−Hoffmann rules) 15 predict that, for these reactions, a concerted 4π s + 2π s cycloaddition mechanism is symmetry-allowed (route A in Scheme ). Alternatively, a two-step pathway may be followed, involving pentenyl or azapentenyl anion intermediates (route B).…”

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

“…The reaction between semistabilized 2-azaallyl anions and aromatic nitriles proved to be more complex than other π-electrophiles; the outcome of the reactions, after aqueous workup, were highly dependent on the nature of the 2-azaallyl anion and the aryl nitrile (Scheme ). , For example, 1,1-diphenyl-2-azaallyllithium ( 6b ) performed direct nucleophilic addition and double-bond isomerization, instead of cycloaddition, with either benzonitrile or picolinonitrile to afford enamines 55a and 55b , respectively, in moderate to poor yield. The same 2-azaallyl anion ( 6b ) did undergo formal [3 + 2] cycloaddition, however, with more electron-rich aromatic nitriles to afford the corresponding 2,5-dihydro-1 H -imidazoles 56a – 56c in moderate yields.…”

Diversity-Oriented Synthesis of a Library of Star-Shaped 2H-Imidazolines

“…(4) The two ring nitrogens should improve aqueous solubility and allow potential hydrogen bonding and polar interactions. Surprisingly, the literature contains only a few reports describing the synthesis of 2 H -imidazolines starting from α-halogenoketones or α-aminoketones, by photochemical degradation of 4,5-diphenyl-2,2,6,6-tetramethyl-l,3-diazabicyclo[3.1.0]­hex-3-ene, by reduction of quaternary salts of 2 H -imidazoles, by cycloaddition of 2-azaallyllithium compounds with arylnitriles, or by electrochemical oxidation of ketones in ammoniacal methanol . To the best of our knowledge, no systematic effort has been reported to exploit this scaffold for library synthesis.…”