Cubanes, formation

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. 

Fig. 6. A schematic view of the [3Fe-4S] Emd [4Fe-4S] cores, as versatile structures. The absence of one site leads to the formation of a [3Fe-4S] core. The cubane structure can incorporate different metals (in proteins, M = Fe, Co, Zn, Cd, Ni, Tl, Cs), and S, N, O may be coordinating atoms from hgands (Li). The versatihty csm be extended to higher coordination number at the iron site and a water molecule can even be a ligand, exchangeable with substrate (as in the case of aconitase (,87)). The most characteristic binding motifs are schematically indicated, for different situations proteins accommodating [3Fe-4S], [4Fe-4S], [3Fe-4S] + [4Fe-4S], and [4Fe-4S] -I- [4Fe-4S] clusters. A disulfide bridge may replace a cluster site (see text).

Investigations on the thermal decomposition of 32 revealed the continuous loss of CO with concomitant formation of the di- and tetranuelear Tc(I) clusters [TcX(CO)4]2 (62) and [TcX(CO)3]4 (63) respectively. The latter is assumed to have cubane structure comparable to [Tc(OHXCO)3]4 [76,77,78]. The thermal decomposition reactions are depicted in Scheme 13. 

Figure 7.8 Regulation of IRP-1 and IRP-2. The two IRPs are shown as homologous four domain proteins that bind to IREs (left) In iron-replete cells, IRP-1 assembles a cubane Fe-S cluster that is liganded via cys-437, -503 and -506. Similar cysteines are conserved in IRP-2 (Cys-512, -578 and -581), but it is unresolved as to whether they also coordinate an Fe-S cluster, (right) In iron-replete cells, IRP-2 is targeted for destruction via a specific region (shaded in black), whereas IRP-1, with a 4Fe-4S cluster, is stable and active as a cytoplasmic aconitase. Multiple signals induce IRE-binding by IRP-1 with distinct kinetics. Whether or not NO and H2O2 induce IRP-1 by apoprotein formation remains to be addressed directly. From Hentze and Kuhn, 1996. Copyright (1996) National Academy of Sciences, USA.

Figure 8 A formal disintegration of the cubane-like tetrameric chalcogen(IV) halides exemplified by the formation of Te3Cl13, Te2Clw2, and TeCl62 from Te4Cl16...

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

The interconnection of two phosphanide substituents by sterically demanding RR Si groups allows the preparation of cage compounds [38], The two-fold lithia-tion of bis(phosphanyl)(alkyl)arylsilane and the dimerization of this bis(phos-phanido)silane leads to the formation of 19 with a strongly distorted Li4P4 hetero-cubane structure according to Eq. (5). 

Less complex non-conjugated diene systems also lead to cubane-like derivatives as in the diene 175. Here the outcome of the reaction is dependent upon the excited state. Thus, direct irradiation brings about fragmentation with the formation of 1,4-difluorobenzene and excited-state naphthalene while triplet-sensitized irradiation follows a different path with the formation of the cage compound 17682. 

Electroreductive fixation of CO2 into formate can be efficiently performed by using Fe4S4 cubane clusters (276) bearing a 36-membered methylene backbone in a DMF-Bu4NBF4-(Pt/Hg) system... 

Formation of benzoic acid has not been observed. These results clearly suggest that in the aerobic oxidation of neat ethylbenzene the cubane complexes cobalt(III) are involved as the catalytic species. 

Dinitrocubane (28) has been synthesized by Eaton and co-workers via two routes both starting from cubane-l,4-dicarboxylic acid (25). The first of these routes uses diphenylphos-phoryl azide in the presence of a base and tert-butyl alcohol to effect direct conversion of the carboxylic acid (25) to the tert-butylcarbamate (26). Hydrolysis of (26) with mineral acid, followed by direct oxidation of the diamine (27) with m-CPBA, yields 1,4-diiutrocubane (28). Initial attempts to convert cubane-l,4-dicarboxylic acid (25) to 1,4-diaminocubane (27) via a Curtins rearrangement of the corresponding diacylazide (29) were abandoned due to the extremely explosive nature of the latter. However, subsequent experiments showed that treatment of the acid chloride of cubane-l,4-dicarboxylic acid with trimethylsilyl azide allows the formation of the diisocyanate (30) without prior isolation of the dangerous diacylazide (29) from solution. Oxidation of the diisocyanate (30) to 1,4-dinitrocubane (28) was achieved with dimethyldioxirane in wet acetone. Dimethyldioxirane is also reported to oxidize both the diamine (27) and its hydrochloride salt to 1,4-dinitrocubane (28) in excellent yield. ... 

The synthesis of a [Mn" 2Mn 204] cubane-like complex, [Mn404(02PPh2)6] (65), by Dismutes and co-workers has also been reported by adopting a dimerization strategy of two preformed [Mu202] units. The addition of diphenylphosphinate salts to a solution of [Mn202(bpy)4] ( 104)3 results in the formation of the cluster. No appreciable Jahn-Teller distortions are observed... 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

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