A study of the norcaradiene–cycloheptatriene equilibrium in a series of azulenones by NMR spectroscopy; the impact of substitution on the position of equilibrium

Abstract

A systematic investigation of the influence of substitution at positions C-2 and C-3 on the azulenone skeleton, based on NMR characterisation, is discussed with particular focus on the impact of the steric and electronic characteristics of substituents on the position of the norcaradiene-cycloheptatriene (NCD-CHT) equilibrium. Variable temperature (VT) NMR studies, undertaken to enable the resolution of signals for the equilibrating valence tautomers revealed, in addition, interesting shifts in the equilibrium.

Cyclic and Acyclic N═N Bonds in Reactions with Some Alkenes and Dienes

“…The DA reaction rates of 1 with cycloheptatriene 5 and with cyclooctatetraene 7 are of particular interest. Hundreds of papers deal with the possible equilibrium and reactivity of the valence tautomers and . The very active singlet oxygen 1 О 2 demonstrates a low selectivity in the DA reaction with 5 , giving 95% of 1,4‐adduct with the structure similar to 18 , and about 5% with a structure similar to that of 15 , whereas the less active and more selective dienophile 1 gives only the adduct 15 (Schemes , ).…”

Visible and Reversible Restrict of Molecular Configuration by Copper Ion and Pyrophosphate

“…The remarkable fluorescence quenching effect induced by the Cu 2+ binding and the reversible recovery by P 2 O 7 4– raised an interesting question, what is the relationship between the fluorescent properties and the H-HPMO configuration? To answer this question, cryogenic 1 H NMR experiments were performed, , which is a favorable tool for the determination of molecular configuration and the capture of tautomer and has great application potential in synthetic chemistry . In this experiment, CDCl 3 was chosen as the solvent because both H-HPMO and Cu­(NO 3 ) 2 could dissolve in it.…”

“…To answer this question, cryogenic 1 H NMR experiments were performed, 31,32 which is a favorable tool for the determination of molecular configuration and the capture of tautomer and has great application potential in synthetic chemistry. 33 In this experiment, CDCl 3 was chosen as the solvent because both H-HPMO and Cu(NO 3 ) 2 could dissolve in it. By comparison, d 6 -DMSO and D 2 O were not chosen as solvents for the NMR tests because they freeze below 0 °C, while methanol-D 4 was not used since H-HPMO was slightly soluble in it.…”

Transition‐Metal‐Mediated Cleavage of C−C Bonds in Aromatic Rings

“…On the basis of these observations, the authors suggested that the insertion reaction occurs via initial formation of an η 2 ‐arene complex, which then undergoes ring expansion to relieve the ring strain and give the corresponding metallacycle 1 or 2 . The ring expansion here can proceed via a direct insertion of Pt into a weakened single C−C bond in a strained metallacylopropane or via electrocyclic ring opening as in norcaradiene‐cycloheptatriene isomerization, given isolobal analogy of PtL 2 and carbene fragment . Thus, the success of this arene ring C−C bond scission is the result of a combination of a metal induced ring strain that promoted C−C scission and the use of H‐free arene that helped to avoid competing C−H activation.…”

“…However,r eplacemento fP Me 3 for PEt 3 in the starting complex led to an h 2 -arene complex Pt(h 2 -C 6 (CF 3 ) 6 )(PEt 3 ) 2 ,w hich did not undergo CÀCc leavage even upon heating at 140 8C for 14 h. The XRD structure of Pt(h 2 -C 6 (CF 3 ) 6 )(PEt 3 ) 2 indicated that the complex can be better described as as ubstituted metallacyclopropaned ue to ac ombination of strong p-donating properties of the platinum center and p-accepting properties of the electron poor arene. [17] On the basis of these observations,t he authors suggested that the insertion reaction occurs via initial formation of an h 2 -arene complex, which then undergoes ring expansion to relieve the ring strain and give the corresponding metallacycle 1 or 2.T he ring expansion here can proceed via ad irect insertion of Pt into aw eakened single CÀCb ond in as trained metallacylopropane or via electrocyclicr ing openinga si nn orcaradiene-cycloheptatriene isomerization, [18] given isolobal analogy of PtL 2 and carbene fragment. [19] Thus, the successo ft his arene ring CÀCb ond scission is the result of ac ombination of am etal induced ring strain that promotedC ÀCs cission and the use of H-free arene that helpedt oavoid competingC ÀHa ctivation.…”