Pyrrole delocalization effects

The chromophores of sulfonyl-substituted pyrroles display maxima falling below 402 nm in dioxane, with A -methyl derivatives absorbing at slightly higher wavelengths compared to their parent pyrroles <1999TL2157>. Correlation analysis of K-band values for several l-methyl-2-formyl-5-substituted pyrroles and their phenylhydrazones indicates concurrent polar and spin delocalization effects for most members of the series whereas for the 4-nitrophenylhydrazones polar effects are dominant <1999JP0392>. 

Pyridine, like benzene, is an aromatic system with six jt electrons (see Section 11.3). The ring is planar, and the lone pair is held in an sp orbital. The increased s character of this orbital, compared with the sp orbital in piperidine, means that the lone pair electrons are held closer to the nitrogen and, consequently, are less available for protonation. This hybridization effect explains the lower basicity of pyridine compared with piperidine. Pyrrole is also aromatic, but there is a significant difference, in that both of the lone pair electrons are contributing to the six-jr-electron system. As part of the delocalized Jt electron system, the lone pairs are consequently not available for bonding to... 

Homonuclear correlation spectroscopy (COSY) experiments (see Chapter 9) substantiate the theoretical predictions, based on molecular orbital calculation, of the pattern of spin delocalization in the 3e orbitals of low-spin Fe(III) complexes of unsymmetrically substituted tetraphenylporphyrins [46]. Furthermore, the correlations observed show that this n electron spin density distribution is differently modified by the electronic properties of a mono-orf/io-substituted derivative, depending on the distribution of the electronic effect over both sets of pyrrole rings or only over the immediately adjacent pyrrole rings [46]. No NOESY cross peaks are detectable, consistently with expectations of small NOEs for relatively small molecules and effective paramagnetic relaxation [47]. 

Pyrrole is much more acidic than comparable saturated amines. The pKa of pyrrolidine is about 35, but pyrrole has a pKd of 16.5 making it some 1023 times more acidic Pyrrole is about as acidic as a typical alcohol so bases stronger than alkoxides will convert it to its anion. We should not be too surprised at this as the corresponding hydrocarbon, cyclopentadiene, is also extremely acidic with a p- Ca of 15. The reason is that the anions are aromatic with six delocalized Tt electrons. The effect is much greater for cyclopentadiene because the hydrocarbon "is not aromatic and much less for pyrrole because it is already aromatic and has less to gain. 

---