CuBaN

A graph is planar if it can be drawn on a plane without edges crossing, with intersections only at the edges (independently of how it is drawn). For example, cubane can be drawn as a planar graph. 

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). 

Figure 10-8 Sequential (tlypothcticaP nydrogcnation of Cubane to, T4-Di methyl hexane.

What is the average energy release per bond on breaking bonds in cubane Compare this with the energy released on hydrogenation of ethylene. 

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. 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

The synthesis of cubane has been outlined on p. 78. Cuneiform cuneanes are formed by silver(l)-catalyzed isomerization of cubanes in almost quantitative yield. Rhodium(I), on the other hand, effects isomerization of cubane to a syn-tricyclooctadiene (L. Cassar, 1970). 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

Cubane (CgHg) is the common name of a polycyclic hydrocarbon that was first synthesized in the early 1960s As its name implies its structure is that of a cube How many rings are pres ent in cubane" ... 

Henequen. Agavefourcroydes grows ia Mexico where it was first cultivated by the Mayans ia the Yucatan (Yucatan Sisal). The plant produces for 20—30 years. The lower bottom leaves, which are up to 2 m long and 10—15 cm wide, are cut, machine decorticated, and cleaned. Henequen fibers are white to yellowish red and are inferior to sisal ia strength, cleanliness, texture, and length, the other grading criteria. Henequen is grown for local use ia Cuba (Cuban Sisal) and El Salvador. Twiae, small ropes, coarse mgs, and sacks are made commercially from henequen. 

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. 

Figure 10.12 (a) Prismane structure of [Ge6 CH(SiMej)2)6l (for clarity only the ipso C atoms of the R groups are shown), (b) The tetracycio structure of [GegBugBrj] with only the ipso C atoms of the Bu groups shown, (c) The cubane structure of [Geg(CMeEt2>g] again with only the ipso C atoms of the t-hexyl groups shown. 

Tetrahedral [NHa]", [NH3(0H)]", [NF4]", H3NBF3 and innumerable other coordination complexes of NH3, NR3, en, edta, etc., including Mc3NO and sulfamic acid (H3NSO3). BN (layer structure and Zn blende-type), AIN (wurtzite-type), [PhAlNPh]4 (cubane-type)... 

A fascinating variety of discrete (or occasionally polymeric) polynuclear halogeno complexes of As, Sb and Bi have recently been characterized. A detailed discussion would be inappropriate here, but structural motifs include face-shared and edge-shared distorted (MXe) octahedral units fused into cubane-like and other related clusters or cluster fragments. Examples (see also preceding paragraph) are ... 

SbCl3 in pentane at 150° under a pressure of H2/CO gave black crystals of [Sb4 Co(CO)3 4] which was found to have a cubane like structure with Sb and Co at alternate vertices of a grossly distorted cube (Fig. 13.24). ... 

Figure 13.24 Structure of the cubane-like mixed metal-metal cluster complex [Sb4- Co(CO)3l4].

The other tetrahalides can all readily be made by direct reactions of the elements. Crystalline SeCU, TeCU and -SeBr4 are isotypic and the structural unit is a cubane-like tetramer of the same general type as [Me3Pt(/Z3-Cl)]4 (p. 1168). This is illustrated schematically for TeCU in Fig. 16.13d each Te is displaced outwards along a threefold axis and thus has a distorted octahedral environment. This can be visualized as resulting from repulsions due to the Te lone-pairs directed towards the cube centre and, in the limit, would result in the separation into... 

Figure 23.8 Trinuclear. M-.M bonded species of Mo" and W. (a) (b) and (c) are alternative representations of the -MjOij unit (a) emphasizes its relationship to the edge-sharing octahedra of the Mj group in polymetallate ions (b) shows the (rrj-O) (rrr-Olj bridges and M-M bonds of its MiOs incomplete cubane" core and (c) emphasizes its triangular centre by viewing from the unoccupied comer of the cuboid (d) and (e) offer the same perspective as (c) but of (Mj02(02CR)6(H20)3] and (Mjirrj-X)(rti-OR)(OR)9] stmetures respectively.

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