Cubanes metal catalyzed

Intermediate metallacyclopentanes are also implicated in transition metal-catalyzed alkene cycloadditions to form cyclobutanes and the corresponding cycloreversions, e.g. dimerization of norbomadiene (73JA597) and rearrangements of cubane and other cyclo-butanoid hydrocarbons (78JA2573). 

An obvious approach to metal-catalyzed water oxidation is to mimic the stmcture and function of the OEC in PSII (Fig. 5). This species is based on a cubane stmcture with three manganese centers, four bridging oxo hgands, and one calcium cation on its vertices. An extra manganese center is oxo-bridged to one Mn-O edge of the cubane and aU cationic metal... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Why did nature use an Fe-S cluster to catalyze this reaction, when an enzyme such as fumarase can catalyze the same type of chemistry in the absence of any metals or other cofactors One speculation would be that since aconitase must catalyze both hydrations and dehydrations, and bind substrate in two orientations, Fe in the comer of a cubane cluster may provide the proper coordination geometry and electronics to do all of these reactions. Another possibility is that the cluster interconversion is utilized in vivo to regulate enzyme activity, and thus, help control cellular levels of citrate. A third, but less likely, explanation is that during evolution an ancestral Fe-S protein, whose primary function was electron transfer, gained the ability to catalyze the aconitase reaction through random mutation. 

Another striking example of metal-assisted S5mimetry-forbidden valence isomerizations involves the silver ion catalyzed rearrangement of homo-cubyl systems 2) i.e., 58 - 5P). A similar rearrangement was reported for cubane itself using silver salts Interestingly, other metals [e.g., rho-... 

The redox chemistry of [FeFeJ-hydrogenases is extensive. Although only three oxidation states are strictly necessary for the two-electron reaction catalyzed by hydrogenases, the presence of six metals creates the possibility of many stable states, and the enzyme has been isolated with the H-cluster in five distinct redox or spectroscopic states [14]. Two inactive states have been identified. The first, [Fe(II)Fe(II)], is formed by oxidative inactivation. The second, Hox-CO [Fe(I)Fe (II)], forms when CO binds to the vacant site on the distal Fe. There are also two known active states Hqx [Fe(I)Fe(II)] and H ed [Fe(I)Fe(I)]. A fifth state, Hs,ed, [Fe (I)Fe(I)] with the [4Fe4S] cubane also reduced, has been observed, but the catalytic relevance of this state is still under debate [15, 16]. 

During the 1970s, symmetry-forbidden o-bond reorganizations catalyzed by transition metals receiveded considerable attention. Cubane underwent isomerization in the presence of rhodium(I)-diene complexes to afford syn-tricyclooctadiene (Scheme 3.1) [1]. The reaction is thought to proceed via oxidative addition of a cubane C-C bond to rhodium(I). 

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