Computational Assessment of the Electronic Structure of 1‐Azacyclohexa‐2,3,5‐triene (3δ²‐1H‐Pyridine) and Its Benzo Derivative (3δ²‐1H‐Quinoline) as well as Generation and Interception of 1‐Methyl‐3δ²‐1H‐quinoline

Abstract

Treatment of a solution of 3-bromo-1-methyl-1,2-dihydroquinoline (9) and [18]crown-6 in furan or styrene with KOtBu followed by hydrolysis afforded a mixture of 1-methyl-1,2-dihydroquinoline (10) and 1-methyl-2-quinolone (11). If the reaction was performed in [D(8)]THF and the mixture was immediately analysed by NMR spectroscopy, 2-tert-butoxy-1-methyl-1,2-dihydroquinoline (17) was shown to be the precursor of 10 and 11. The structure of 17 is evidence for the title cycloallene 7, which arises from 9 by beta elimination of hydrogen bromide and is trapped by KOtBu to give 17 so fast that cycloadditions of 7 with furan or styrene cannot compete. Since this reactivity is unusual compared to the large majority of the known six-membered cyclic allenes, we performed quantum-chemical calculations on 8, which is the parent compound of 7, and the corresponding isopyridine 6 to assess the electronic nature of these species. The ground state of 6 was no longer an allene (6 a) but the zwitterion 6 b. In the case of 8, the allene structure 8 a is more stable than the zwitterionic form 8 b by only approximately 1 kcal mol(-1). These results suggest a high reactivity of 6 and 8 towards nucleophiles and explains the behaviour of 7. In addition to the ground states, the low-lying excited states of 6 and 8 were considered, which are represented by the diradicals 6 c and 8 c and, as singlets, lie above 6 b and 8 a by 19.1-24.8 and 14.4-17.7 kcal mol(-1), respectively.

Are Allenes with Zwitterionic Character Still Allenes? Of Course!

“…Until our communication appeared, Christl and Engels seemed to agree with our usage of nomenclature, since the title of one of their papers10 refers to compound 3 as “1‐azacyclohexa‐2,3,5‐triene”, implying allene 3 a , despite the fact that the ground state of this transient compound was already known to be that of 3‐dehydropyridinium ylide 3 b 11 In other work by Engels et al.,11c a scheme, which is entitled “Possible Electronic Structures of Cyclic Allenes”, shows allene, zwitterion, and diradical forms of a compound.…”

Are Allenes with Zwitterionic Character Still Allenes? Of Course!

“…Until our communication appeared, Christl and Engels seemed to agree with our usage of nomenclature, since the Correspondence title of one of their papers [10] refers to compound 3 as "1-azacyclohexa-2,3,5triene", implying allene 3 a, despite the fact that the ground state of this transient compound was already known to be that of 3-dehydropyridinium ylide 3 b. [11] In other work by Engels et al, [11c] a scheme, which is entitled "Possible Electronic Structures of Cyclic Allenes", shows allene, zwitterion, and diradical forms of a compound.…”

Stable Five‐Membered‐Ring Allenes with Second‐Row Elements Only: Not Allenes, But Zwitterions

“…The calculation of the structures 4 b and 5 b indicated this very effect by conspicuously small bond angles of the carbanionic centers. [6,7,9] If 1 c were the correct structure instead of 1 b, the consequence would be a much larger angle, probably as large as that of 2 a, which has been calculated to be 1148. [4] The contribution of the resonance structure 1 d of 1 b to the ground state of the species can only be determined by valence-bond calculations.…”

“…[8] As more extensive studies confirmed, the allene structure 5 a does not represent an energy minimum. [7,9] If the results of the calculations of 4 and 5 are extrapolated, the coplanar arrangement of the three carbon atoms of the alleged allene system of formula 1 a and its two nitrogen and oxygen atoms has to be expected. According to the X-ray structure analysis performed by Lavallo, Dyker, Donnadieu, and Bertrand, this is perfectly true.…”