Structural effects on valence tautomerism

Umpolung of tropone: the reaction of 2-halogenocycloheptadienone enolates with tropylium cations and several other cationic electrophiles. Preparation of novel 2-substituted and 2,7-disubstituted tropones

“…It is reported that the equilibrium between cycloheptatriene and norcaradiene can be shifted to the norcaradiene side by the introduction of a t-butyl group into the C-1, C-2, or C-3 position of 7-alkyl-7-cyanocycloheptatriene 22*23 and into the C-2 or C-3 position of 7-cyano~ycloheptatriene.~~ These findings have been ascribed to the steric effect, which destabilizes the cycloheptatriene form rather than the norcaradiene form. 23 The present result seems to be consistent with these facts. Furthermore, several cationic electrophiles reacted conveniently with 2-chlorocycloheptadienone enolate (4a), which also derived from (3a) and LiAlH(OBu'),, to give the corresponding 2-substituted tropones (21H27) in high to 27), all of which are new compounds, were assigned on the basis of their spectroscopic and analytical data except for thermally unstable high boiling oil (22).…”

“…After the reaction mixture was quenched by H20 (10 ml), the mixture was extracted with ethyl acetate and the extract was dried (Na2S04). After evaporation of the solvent, the residue was separated by TLC on silica gel using hexane-thy1 acetate (3: 1-8: 1) as a developer to give the products (21)(22)(23)(24)(25)(26)(27). The results are summarized in Table 5.…”

Synthesis and Dynamic Properties of Cycloheptatrienyl(dipropyl)borane. Equilibrium with 7-Dipropylborylnorcaradiene

“…The temperature-dependent spectra correspond to an equilibrium between cycloheptatrienylborane 2a ( endo and exo forms) [the eqilibrium between the exo and endo forms of 2a is fast on the NMR time scale in the studied temperature range; only at 153 K can some indications of this process be detected, viz., the broadening of the signals of the propyl groups in the 13 C NMR spectrum, the other signals remaining relatively narrow] and its valence tautomer 7-(dipropylboryl)norcaradiene ( 2b ) (Scheme ) . However, the 1 H and 13 C NMR spectra measured at the lowest temperature (153 K, Figure ) display only one set of signals easily assigned to the norcaradienyl 2b .…”

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

“…Even at temperatures as low as −150°C, no signals for the norcaradiene were observed, however Rubin did succeed in directly observing the norcaradiene 3N at low temperature (77 K) using UV, which allowed him to provide an insight into the kinetics of its interconversion to 3T. 18 Hannemann has reported that the chemical shifts of the 1,6 H and C atoms of pure cycloheptatriene 3N lie in the region δ H 5.2-5.3 and δ C 120-130 respectively, whereas the corresponding signals in typical norcaradienes 3T resonate at δ H 2.8-3.5 and δ C 35-45. 2) as a fluxional system, with only 3% NCD observed at low temperature ( 13 C NMR at −132.5°C and 1 H NMR at −150°C).…”

“…17 Okamoto has described extensive variable-temperature NMR work in the study of the position of the norcaradiene-cycloheptatriene equilibrium of various substituted cycloheptatrienes. 18 Hannemann has reported that the chemical shifts of the 1,6 H and C atoms of pure cycloheptatriene 3N lie in the region δ H 5.2-5.3 and δ C 120-130 respectively, whereas the corresponding signals in typical norcaradienes 3T resonate at δ H 2.8-3.5 and δ C 35-45. 19 This tautomeric system is influenced by the steric and electronic effects of the substituents on the azulenone framework.…”