Indolizines spectra

The chemical shifts and coupling constants for compounds 1-3 are shown in Table III. In this table a uniform numbering is adopted for all compounds, as shown in the formula of indolizine at the head of the table. No spectrum is available for compound 4, although that of compound 5 is reported.204 From the figures available for the simple derivatives 142 and 143, chemical shifts and coupling constants for compound 4 can be included in Table III. 

The PMR spectrum of 85a agrees well with theoretical considerations. It can be seen from the vicinal coupling constants that the seven-membered ring is planar, containing localized bonds. A paratropic effect of 1.7 ppm in the five- and six-membered rings and one of at least 2.7 ppm in the seven-membered ring follows from comparison with suitable reference compounds. The different paratropism can be explained by the superposition of two ring systems, a diatropic indolizine and a paratropic [ 12]annulene moiety.106,107... 

Feitelson obtained good agreement between the ultraviolet spectrum of indolizine calculated from MO theory and the observed spectrum.95 Mason studied the electronic spectra of indolizine and many of its... 

Some 13C NMR spectra have been studied in a similar way,138 whereas the 14N NMR spectrum of indolizine has been recorded among those of a large number of other N-containing compounds.127... 

Mason and Smith (1969) found that for a series of mono- and bicyclic aromatic hydrocarbons the changes in the fluorescence spectrum with acidity reflected the ground state protonation reaction. The p Sj )-values calculated for benzene, toluene, naphthalene, azulene, and indolizine do not correspond to observable processes since the rate of protonation is too slow to compete with deactivation of the Sj state. Photochemical deuterium and tritium exchange experiments in 1 mole dm-3 perchloric acid indicate that the radiative deactivation rate of an electronically excited aromatic hydrocarbon is faster than the rate of protonation by a factor >10s. 

The band of longest wavelength absorption in the spectrum of indolizine which shows some relation in shape to that of azulene corresponds to the n ir transition... 

Earlier, Mason266 studied the aza analogs of indolizine and indole and found that the electronic spectra of the former series conformed more closely to the spectrum of the indenyl anion, from which it was concluded that 7r-electron delocalization is more pronounced in this series, a result which appears to be contrary to the conclusions from MO calculations. Proton resonances in the PMR spectrum of 215 and of various aza-indolizines428 429 show that these compounds sustain a ring current, and Black, Brown, and Hcfferman,429 in a study designed to correlate electron density and chemical shift data, commented that the data also support Mason s view that jr-electron delocalization is more extensive in these compounds than in the indole series. 

Several sets of theoretical calculations have been performed on the parent ring system. HMO calculations of total rr-electron densities and frontier electron densities successfully predicted that the nucleus would undergo electrophilic substitution at the 6- and 8-positions. Two groups, " have compared the electronic structure of indolizine and various aza derivatives using the SCF or semiempirical antisymmetric configuration interaction method. The results allowed interpretations of the electronic spectrum to be made which were in good agreement with experiment. 

The ultraviolet spectrum of the parent heterocycle in water shows three main peaks, at 221.5, 282, and 302 nm. The similarity of this spectrum to that of indolizine is apparently due to the fortuitous cancellation of the effects of the introduction of successive nitrogen atoms into indolizine. UV data are available for several quaternary derivatives of this heterocycle and their V-oxides." 2,3-Dihydroimidazo[l,2-a]pyr izine (5) has only two peaks, at 258 and 394 nm, in its ultraviolet spectrum in water. Both this compound and the unsubstituted aromatic heterocycle undergo large hypsochromic shifts on protonation. This contrasts with the bathochromic shift observed on protonation of aminopyrazines and is consistent with the hypsochromic effect obtained on protonation of the analogous imino compounds. These observations confirm the conclusion that protonation occurs at the 1-position in this heterocycle to give the cation 6. 

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