Four- and Five-membered Ring Compounds

This Report covers the period April 1969—March 1972. Attempts have been made to make it as comprehensive as possible. However, Russian papers have been read in their English translations, which means that some older work has also been included. Finally, the English translation of Khim. geterotsikl. Soedinenii for the past two years has not been available to the present author, and thus thiophen work published in this journal is treated somewhat summarily, for which the author apologizes. 

Certain trends in the development of the chemistry of thiophens are clearly noticeable dming this period of time and important results have been achieved. 

Through the excellent work of Wynberg and his co-workers the interesting photochemical rearrangements of thiophens have been elucidated. 

Gol dfarb s group in Moscow is continuing to demonstrate the usefulness of Raney-nickel desulphurization of thiophens for the synthesis of aliphatic and, especially, macrocyclic compounds. 

The isosteric properties of benzene and thiophen are, to an increasing extent, attracting the interest of medicinal chemists, and some marked successes have occurred during the period of this review. 

---

In some cases, the Grignard reaction can be performed intramolecularly. For example, treatment of 5-bromo-2-pentanone with magnesium and a small amount of mercuric chloride in THE produced 1-methyl-1-cyclobutanol in 60% yield. Other four- and five-membered ring compounds were also prepared by this procedure. Similar closing of five- and six-membered rings was achieved by treatment of a 6- or s-halocarbonyl compound, not with a metal, but with a dianion derived from nickel... 

The nitrogen heterocycles (2) show well-resolved AB-type multiplets for the CFj—N groups at low temperatures, which coalesce for the four- and five-membered ring-compounds with activation energies of 7-25 and 8-62 kcal mol , respectively, whereas that of the six-membered ring is unchanged... 

One of the few phosphorus compounds that exhibit square pyramidal geometry is shown in Fig. 6 31. Rationalize the preferred geometry of SP over TBP in terms of the presence of Ihe four- and five-membered rings. (Holmes. R.R./. Am. Chem. Soc. 1975.97.3379.) 6.17 Consider the cyclic compounds I and II. In I the rapid exchange of the fluorine atoms is inhibited just as it is in (CH,hpb - However, exchange in II is very rapid. Suggest a reason. 

When the spirocyclic compound (19) is heated, dimerization occurs to give an unusual dispirocyclic compound containing three-, four-, and five-membered rings containing silicon (equation 35). ... 

A number of cyclic structures with Se-Si bonds have been contributed in recent years (Scheme 11). Beside one substituted three-membered ring consisting of C, Si, and Se (3 = 4-49), some four- and five-membered rings (24-28) were published. Further examples are 29-31 and 33 36, 37, 39, 41, the propellane-type 43 and the double-decker tricyclus 44. For many of the silicon compounds, corresponding germanium analogues have been reported as well 32, " 38, 40, 42, and 45 (Scheme 11). Evidently, the effects of Ge are mostly less shielding as compared to Si. 

Radicaloid insertions of olefins into the Rh H bond of [Rh (TPP)(H)] has been used to obtain Rh —CH2—(alkyl)—Nu// species (NuH = OH, NH) using olefins functionalized with end-on —OH and —NH functionalities. Under basic conditions, intramolecular SN2-type attack of the Nu at the a-carbon atom of Rh —CH2—(alkyl)—Nu yields [Rh (TPP)] and cyclic organic products (—CH2—(alkyl)—Nu—) (see Fig. 47). Protonation of [Rh TPP)] then allows regeneration of [Rh° (TPP)(H)]. The combination of these reactions constitutes a new method for selective intramolecular anti-Markovnikov hydrofunctionalization of olefins with O—H and N—H functionalities (150). In this way, three-and five-membered ring compounds (epoxides, furan derivatives, pyrrolidine derivatives) were readily obtained. Formation of four- or six-membered rings... 

On heating at 180-200° followed by destannylation these compounds yield mixtures of four and five membered ring heterocycles thus p7 R1=H, R2=(CH2)20SnBu J gives a 2 3 ratio of (208) and (209)-... 

The year has been largely one of consolidation, with emphasis on the synthesis, structure, and properties of phosphoranes containing small (four- and five-membered) rings, especially the relatively stable compounds afforded by bi- and tri-cyclic structures. A new departure, however, has led to the first reports of five-co-ordinate phosphorus bound to transition-metal complexes vide infra) and these novel compounds may be the precursors of a new wave of chemistry in this area. 

Cyclometalation reactions for the synthesis of organometallic intramolecular-coordination five-membered ring compounds proceed very promptly, because hve-membered ring compounds are more stable as compared with other ring compounds, such as four- and six-membered ring compounds. The cyclometalation reactions generally proceed regioselectively as a result. 

---