Heteroarenes five-membered ring

In the context of heteroarenes, five-membered rings containing phosphorus occur in many forms. The most common species is phosphole, the analog to pyrrole and thiophene, which contains a butadiene fragment bridged with a phosphorus functional group. Because of the pyramidal geometry of the phosphorus center. 

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

Asymmetric Hydrogenation of Five-Membered Ring Heteroarenes... 

Radical precursors with a five-membered heteroarene for ring D of the important anticancer and antiviral alkaloids (Camptothecin, Mappicine, Nothapodytine B, and Nothapodytine A), l-(2-halo-3-phenylprop-2-en-l-yl]-l//-pyrrole-2-carbonitriles 163, ( )/(Z) mixture, were synthesized in high yields (87-96%) by alkylation of l//-pyrrole-2-carbonitrile with the cinnamyl bromides 162 using KOH in DME (Equation 30) <2002J(P1)58>. 

With this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocycles. The crucial consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkane or heterocycloalkene (see p 2). Various aromaticity criteria apply to heteroarenes, and as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocycles with one oxygen atom is furan. 

An alternative strategy for modifying heteroarenes containing five-membered phosphorus rings is via the incorporation of another heteroatom directly into the cyclic scaffold (Figure 12.14) [27, 28]. The general synthesis of these is shown... 

In addition to the ligands above, considerable attention is given to more complex ligand systems [4,5] aromatic and heteroaromatic compounds (heteroarenes) (i.e., five- or six-member cyclic structures with delocalized 7i-bonds in the ring containing, besides carbon atoms, either N, P, As, O, S, Se, or Te compounds [6-8]), various chelate-forming compounds, such as macrocyclic crown-ethers, cryptands, porphyrins, and phthalocyanines. 

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