5-Membered unsaturated ring

It is difficult to draw general conclusions from such a small number of values. Nevertheless, it can be noted that, like other five- or six-membered unsaturated rings (thiophene, pyridine) thiazole exhibits a certain aromatic behavior in its magnetic susceptibility. 

Pyrrole (two r s, one /) and imidazole are /ive-membered heterocycles, yet both have six tt electrons and are aromatic. In pyrrole, each of the four. sp2-hybridized carbons contributes one tt electron, and the sp2-hybridized nitrogen atom contributes the two from its lone pair, which occupies a p orbital (Figure 15.9). Imidazole, also shown in Figure 15.9, is an analog of pyrrole that has two nitrogen atoms in a five-membered, unsaturated ring. Both nitrogens are sp2-hybridized, but one is in a double bond and contributes only one electron to the aromatic tt system, while the other is not in a double bond and contributes two from its lone pair. 

Scheme 17 Four-membered unsaturated rings with different numbers of a2,X3-P atoms...

Scheme 18 Four-membered unsaturated rings 22-24 with one and two a4,/.5- atoms...

Scheme 20 Four-membered unsaturated rings 26 with E O,...

Phosphacyclobutene P-oxides 64 are produced by the reaction of titanacyclobutenes with dichlorophenylphosphine <07CEJ4077>. The large instability of heterophosphete compounds 65, composed of a four-membered unsaturated ring with a pentavalent phosphorus atom and a heteroatom, has been identified and characterised by first-principle molecular orbital computations, at several levels of theory showing that strong electron-... 

Ultraviolet photoelectron spectroscopy combined with ab initio calculations of the five- and six-membered unsaturated rings containing phosphorus and arsenic have been studied <1995JST57>. Assigning the spectra, no defect of Koopmans theorem has been found in the low-ionization-energy region. Reorganization and correlation effects should be comparable and be considered free from any defects of Koopmans theorem. 

The mechanism of the formation of a three-membered unsaturated ring was clarified later, when this reaction was reexamined by the same authors in detail . They found that the reaction of dichlorogermylene-dioxane complex with one equivalent of f-BusSiNa in THF at —78 °C led to the formation of c ,frawi-l,2,3-trichloro-l,2,3-tris(tri-terf-butylsilyl)cyclotrigermene 7 in 98% yield (Scheme 4). The cis,trails conformation of 7 was established by NMR spectroscopy and X-ray analysis (Figure 3). Treatment of 7 with two equivalents of f-Bu3SiNa in TFIF at —78°C cleanly produced cyclotrigermene 5 (Scheme 4), which gives evidence that 7 is a precursor for 5. 

A short, asymmetric total synthesis of an important 3-(hydroxymethyl)carbacephalosporin antibiotic was achieved by M.J. Miller and co-workers. ° The p-lactam ring was formed via a Mitsunobu cyclization, while the six-membered unsaturated ring was constructed by a HWE cyclization. This intramolecular olefination afforded a single diastereomer in 85% yield. 

Some exocyclic multiple-bond character may be influential at stabilizing the four-membered unsaturated rings the majority of known compounds contain a silicon substituent at the ring nitrogen atoms or a nitrogen substituent at the ring boron atoms. 

For a set of 329 five- and six-membered unsaturated ring bases Zhang found... 

Five- and Six-Membered Unsaturated Rings Among the known phosphinine complexes are... 

The structure of pyrimidine is shown below. It is a six-membered unsaturated ring compound containing four carbon and two nitrogen atoms. 

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