Substitution, electrophilic predicting product distributions

It will be seen that the 7r-electron distribution seems indeed to be uneven, the nitrogen being estimated to have 1.8 n electrons and the boron but 0.2. Electrophilic substitution takes place very much more readily in the 6- and 8-positions than anywhere else the 2-position is the third most reactive. The 8-position is somewhat more reactive than the 6-position but the difference is very much less than that between the 6- and 2-positions. It will be seen that the simple PMO estimates of 6E give a very good account of substitution in this system, given that the 4-position, like that in phenanthrene, must be sterically hindered. These calculations correctly predicted the orientation of the products for us before we had established this experimentally indeed we were assisted in devising chemical proofs of their structures by this foreknowledge. 

Pyrrolo[l,2-a]quinoxaline undergoes smooth electrophilic substitution. The distribution of products obtained depends on the size of the incoming electrophile, as with bulky reagents steric interactions tend to inhibit reaction at the most electron-rich carbon-1 atom." Theoretical calculations predict that the most favored positions for electrophilic substitution, in decreasing order of susceptibility, are 1>3>6>2. Reports have appeared describing electrophilic substitution at the 1,3- and 2-positions, but no reaction at the 6-position has yet been observed. 

The Hammett equation usually predicts substituent effects with reasonable accuracy, provided the correct choice is made between o and o. a values are based on a system in which there is little change in the degree of conjugation between the substituent and the remainder of the molecule on moving from one side of the transition state to the other. Electrophilic aromatic substitutions pass through a positively charged complex, the Wheland intermediate (I), in which the electronic distribution is different from that of either reactants or products. A generalised sequence is shown in Equation (10). The standard reaction on... 

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