Cubane, ring systems

Nevertheless, there are situations where one wants to work with six four-membered rings in cubane (e.g., when considering the symmetry of the ring system). In this situation, one adds a sixth four-membered ring to obtain firom the SSSR the so-called extended set of smallest rings (ESSR). 

Only relatively few examples of interesting target molecules containing rings are known. These include caryophyllene (E.J. Corey, 1963 A, 1964) and cubane (J.C. Barborak, 1966). The photochemical [2 + 2]-cycloaddition applied by Corey yielded mainly the /ranr-fused isomer, but isomerization with base leads via enolate to formation of the more stable civ-fused ring system. 

Next, the attention was turned to addition of azido groups to carbon-carbon double bonds of side-chains attached to the cubyl ring system. In this approach, cubane derivatives with unsaturated side-chains and added azido groups to these side-chains were synthesized using the reaction ... 

Linkage isomerizations, osmium, 37 335-339 Linked cubane clusters, Fe—S proteins, biological implications, 38 55-56 Linked macrocyclic ring systems, 45 75 dinuclear systems, 45 89-95 triaza ring systems, 45 76-87 Li—N—Li linkages, 37 100-101 Lipoxygenase, inhibitors, 36 41 Liquid-liquid extraction of metal ions, 9 1-80 with acidic P-based extractants, 9 34-48 with acidic P-based extractants dinuclear, 9 47-48 mononuclear, 9 34-47 with amines and amine oxides, 9 49-56 complexes in, 9 68-71 countercurrent extraction method, 9 15-25... 

All cubanes are high energy materials and should be handled with due precautions for potential explosives [ 1]. The ring system has almost 700 kJ/mol strain energy (more, per unit weight, than the detonation energy of TNT) and is under study as a basis for high power explosives [2,3]. Cubane itself explodes spontaneously at 3 GPa pressure, the... 

A variety of B N ring systems have been prepared that do not easily fall into the general classes described previously. Some of these rings include other heteroatoms, some include carbon, and they range in size from three-membered to eight-membered. Some examples of cubane cages are also known. Examples of these systems have appeared in previous reviews. Some representative new contributions in this area are summarized in this section. 

It should be possible to develop ring design heuristics that can generate clear and aesthetic layouts for even highly condensed ring systems such as cubane. Although there is no substitute for an RTD to lay out and orient traditional systems (steroids), the RTD approach alone is inadequate because the presence of substituents can affect the optimal layout. 

As well as cubanes, other small aliphatic ring systems, such as cyclopropanes, cyclobutanes, azetidines and propellanes, are potentially interesting candidates for incorporation into monomer collections as they are often sought as variants of methyl, isopropyl and te/t-butyl groups. 

A different type of dynamic process involving a polynuclear metal system has been identified in [(CH C H,)ltRul)Sl(]2+, which has a distorted cubane-like structure with three Ru—Ru bonds in the crystalline state (Fig. 15.56). By following its methyl and ring proton resonances over a temperature range from +70 to —43 BC (Fig. 1537), the complex is shown to undergo a dynamic process involving the metal-metal bonds. At the low-temperature limit, the spectrum contains features predicted for the static structure two lines of equal intensity for the methyl protons... 

As in the case of monocyclic 3- and 4-MRs (see Section V.E.l.a.ii), electropositive substituents stabilize also M H cage systems which include such rings, e.g. tetrahedranes, prismanes and cubanes (Table 24)191,192,195. The only isolated tetrasilatetrahedrane indeed carries the electropositive Si(Bu-t)3 groups as substituents238. Nevertheless, the steric protection afforded by the bulky silyl ligands is probably the most important factor allowing the isolation of the tetrasilatetrahedrane179 195. 

Planar six-membered rings from triple-decker complexes (179) for P and As (Table XXXVIII) with metals including V, Nb, Cr, Mo, and W. These complexes have been examined by extended Hiickel calculations, as have the E5 analogues.476 Unlike the E5 system, no compounds with only one metal moiety attached to a planar E6 group have been described. The compound [CpTi]2P6 has titanium atoms on opposite sides of a P6 ring, but each Ti atom is only bonded to three of the P atoms and the result is a cubane-like structure (180). Various isomeric forms of P6 have been examined by SCF MO calculations.216... 

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