Indolizine, calculated electron densities

The diagram (Fig. 2) shows the calculated alteration of the one-jr-electron densities at the single positions in indolizine-8-carbonitrile (302a) going from HOMO to LUMO. 

Charge densities calculated by MNDO for indolizine and its 2-phospha derivative (11) and 1-phospha derivative (12) show the rr-electron excess in the five-membered ring to increase in this order. This result suggests that the phosphorus atom in (11) will be less likely to act as a donor than in (12) <91CB475>. As a dicoordinate phosphorus atom lowers the 7r-electron density at the adjacent carbon atoms, 2-phosphaindolizines are less reactive towards electrophiles in position 1 and 3 than indolizines. 

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. 

---