Analogues of the Cyclopentadienyl Anion

Despite the lone pair on nitrogen, pyrrole is a very weak base, because the lone pair is a part of the aromatic system. The p/Cj, of the cation formed by protonation of pyrrole, 12.20, is -3.8 the cation is strongly disfavored, as the aromaticity of pyrrole has been lost. However, pyrrole is relatively easy to deprotonate its p/Cj, is 17, comparable to that of cyclopentadiene at 16. The anion still has a full octet of electrons at nitrogen, and the aromatic system remains intact. 

Pyrrole and its analogues are found widely in biologically important molecules note that the purine nucleotides that we mentioned earlier contain a pyrrole-type as well as a pyridine-type ring. Pyrrole is a key component of the porphyrins the 18-n aromatic system protoporphyrin IX, 12.21, is the precursor of mammalian porphyrins, and there are related structures in heme, chlorophyll, and vitamin B12. 

Imidazole, 12.22, is a pyrrole with an additional nitrogen atom at the 3-position (again, we number with the nitrogen atom as 1) we have already met it as a molecule that tautomerizes particularly easily. This ability of picking up a proton at one side and dropping one off at the other is critical to its biological function where the amino acid histidine, 12.23, is at the active site of protease (and other) enzymes (see Section 23.8). 

thiophene, and pyrrole all have lower resonance energies than benzene (Table 12.5), and they are more reactive toward electrophiles. Even the simplest analysis should convince us that they are more electron rich than benzene—the six rt-electrons are distributed over only five atoms. 

TABLE 12.5 Resonance Energy of Various Heterocyclic Compounds 

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From reaction in solutions, a variety of polyphosphides is obtained that are not known as solvent-free binary compounds (Table 7). Several have been characterized by P NMR spectroscopy in solution, for example, Pi9 and P26" , which have no counterparts in binary systems. LiPs forms a to(P5 ) polyanion in the solid state, but the corresponding anion in solution is monomeric Ps, an analogue of the cyclopentadienyl anion. The helical polyanions do(P7 ) of LiP and NaP have no monomeric counterpart in the solution here the anion is present as R2P7. Adding simply a salt (KI), trimeric P2i is formed as a finite anion in solution and it can be isolated as a solvated salt. 

The phosphorus analogues of the cyclopentadienyl anion P (CH)5 have been examined by SINDOl for their aromaticity. For n = 0 and 5 the aromaticity index is comparable to that of benzene. Of the two diphospholides (n = 2), the 1,3-isomer (88%) has a slightly higher relative aromaticity than the 1,2-isomer (86%) <92JOC3694>. 

A long-sought molecule has been the silicon analogue of the cyclopentadienyl anion (5 and 6). From the above considerations, little resonance interaction is expected, so that 6 is not expected to be aromatic. The substituent R and the ring double bonds still may... 

When white phosphorus is heated in organic solvents in the presence of certain cyclopentadienyl transition metal carbonyl complexes, new complexes containing the mono-cyclic Pe analogue of benzene are found (Fig. 2). There are also a few compounds having metal atoms on both sides of a P5 ring that can be recognized as the analogue of the cyclopentadienyl anion, [CsHs]. ... 

In view of the ubiquity of the 5-cyclopentadienyl ligand in organometallic chemistry, and the reports in the literature of perchlorocyclopentadienyl analogues (56), it is both curious and unfortunate that attempts (85) to prepare complexes of the /5-pentafluorocyclopentadienyl ligand from the known C5F j anion (87,88) have failed. 

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