Substituent-ring interactions, electron density

Summary. The electron density model of substituent-ring interactions functions better than the MO model, which is not surprising since the electron density covers all MO effects while any MO model will simplify orbital interactions by selecting just a few important... 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

The relative energies of the two forms of each ring are given in Table VII. The electronic reason for these preferences is clear. The amido N-electron density is tetrahedral-like and interactions with two lithium centers in the same plane as H2N are much less effective (see Section III,A and Fig. 26a). Contast this with the directionality of the imide N (sp2) electron density, which dictates the formation of ring systems with substituent atoms in the same plane. For example, planar (H2C= NLi)2 is preferred to the perpendicular form by 17.0 kcal mol-1 [6-31G (72, 74)] or by 6.8 kcal mol1 [MNDO (108)] (Section II,D Fig. 20). 

The 13C chemical shifts of benzenoid carbons largely depend on the mesomeric interaction between substituent und benzene ring. Electron releasing substituents (e.g. — NH2, — OH) will increase the electron density at the o and p carbons relative to benzene (128.5 ppm), while slight electron deficiencies will be induced by electron withdrawing groups (e.g. — N02, —CN). 

A strong interaction of fragments Ax and A2 through a bridge amine nitrogen in position 1 is apparent from the influence of substituents on the aromatic rings Arx and Ar2 on the electron density both in substituted and in unsubstituted aryl cycles. 

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