Model Nitro group

The Michael addition of enamines to nitroalkenes proceeds with high Yyn selectivity. The Yyn selectivity is explained by an acyclic synclinal model, in which there is some favorable interaction between the nitro group and the nitrogen lone pair of the enamine group CEq. 4.67i. Both Z- and E-nitrostyrenes afford the same product in over 90% diastereoselecdvity. 

Fig. 24 (a) 1,5-Dinitro-anthracene connected to two monoatomic gold electrodes. The Tight Binding model used to study this system is represented underneath, (b) Modifications of the MO induced by the rotation of the nitro groups... 

The application of ab initio molecular orbital theory to suitable model systems has led to theoretical scales of substituent parameters, which may be compared with the experimental scales. Calculations (3-21G or 4-31G level) of energies or electron populations were made by Marriott and Topsom in 1984164. The results are well correlated with op (i.e. 07) for a small number of substituents whose op values on the various experimental scales (gas phase, non-polar solvents, polar solvents) are concordant. The nitro group is considered to be one of these, with values 0.65 in the gas phase, 0.65 in non-polar solvents and 0.67 in polar solvents. The regression equations are the basis of theoretical op values for about fifty substituents. The nitro group is well behaved and the derived theoretical value of op is 0.66. 

By comparing CPK models of the Meisenheimer intermediates which should be formed in each case, steric hindrance for substitution of the nitro group (intermediate 30) should be larger than for substitution of any of the chlorine atoms (intermediates 31, 32). This is in accordance with the observed results where bulkiness of the butylamine leads to a diminution of the corresponding nitro-substitution product. Nevertheless, as we have... 

Of course, it is the entire molecule that receives an electron on reduction. However, the nitro group is the part where the excess electrons spend the majority of their time. Consideration of quantum-chemical features of the nitrobenzene anion-radical is of particular interest. The model for the calculation includes a combination of fragment orbitals for Ph and NO2, and the results are represented in Scheme 1.1. The left part of the scheme refers to the neutral PhN02 and the right part refers to the anion-radical, PhN02 (Todres 1981). 

The absence of nitro groups in these substrates is noteworthy. The observed adducts are exclusively stabilized by the electron-withdrawing capacity of the aza groups present in the fused ring system of purine. Accordingly, all ring protons in the adducts are more shielded than the corresponding protons in the substrates. Adducts 19 and 20 can be taken as models for intermediates in nucleophilic aromatic substitution at the C-6 position of purine. Moreover, their formation support the view that a tetrahedral carbon at C-6 is involved in the mechanism of the adenosine deaminase-catalyzed hydrolysis of 6-substituted purine ribonucleosides.43... 

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