Electron densities bridgehead systems

A CNDO study on benzo[l,2-c 4,5-c ]dithiophene predicted that it would be more polar than thieno[3,4-c]thiophene, and that excess electron density would be distributed at the bridgehead carbon atoms. A singlet ground state was also predicted <75ACR139>. The heats of atomization, charge densities, and resonance energies of a series of benzodithiophenes were calculated <80T27il>. A [c] fusion led to less resonance stabilization of a system than a [b] fusion. 

The results of various electron density calculations for pyrazolo[l,5-a]pyridine are collected in Table 1. All successfully predict the observed electrophilic attack at C(3). The CNDO/2 values ( ) lead to a theoretical dipole moment of 2.6 D and those of C ) to 2.5 D. These compare favourably with the experimental value (2.18 D). The values are also consistent with delocalization of the bridgehead nitrogen lone pair over the lOir-electron system. 

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