Cuneane

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]. 

Much controversy arose over the prediction of the most stable Pg structure. It was anticipated that a Pg molecule should naturally possess a cubic structure with Oh symmetry. This geometry would ideally suit the formation of atomic orbitals so that a cubic Pg would have no strain and should be at least dose in energy to twice that of P4 [34], However, paradoxically it is now firmly established by theory that the most stable Pg geometry is the cuneane structure shown on the right in Figure 2.6-5 [8, 11, 19, 29-32],... 

The dimerization of 2 molecules of P4 giving Pg cuneane is endergonic at 298 K by 123.2 kj mol-1 [11]. Structural rules for possible isomers of larger P molecules were developed [31] and some of these are more stable relative to gaseous P4. For instance, the gaseous Pig isomer shown in Figure 2.6-6 is more stable by 36 kj mol-1 than 4.5 molecules P4 (free energy at 298 K) [11]. 

This Pis isomer contains two Pg cuneane moieties linked through a P2 bridge and resembles one of the structural motifs of Hittorf phosphoms shown in Figure... 

In addition to the P8 cuneane building block, Hittorf phosphorus contains a P9 unit which serves to cross link the long tubular strands (P8-P2-P<))x in three di-... 

Fig. 38. Skeletons of Dewar benzene, prismane, cubane, and cuneane.

There is an important class of rearrangements of strained cyclic a-bonded systems to give less strained ir-bonded qrstems which occur under the influence of transition metal catalysts although the uncatalysed proce is Woodward-Hoffman forbidden and slow. Examples are the conversion of cubanes XXII and bis-homocubanes XXIll to syn-tricyclooctadienes XXIV and related species XXV and of quadricyclene (XXVI) to norbomadiene (XXVII) [Ag, however, converted cubane and related species to the previously unrecognised species cuneane (XXVIII) and its relatives as do some electrophiles with incompletely filled d-subshells ... 

The process is induced photochemically and involves the single-electron transfer oxidation of cubane then completed with a backward electron transfer to the transient radical cations. A Li+ salt with a weakly coordinating anion is able to induce pericyclic transformations, including the rearrangement of cubane to cuneane, quadricyclane to norbomadiene, and basketene to Nenitzescu s hydrocarbon 392... 

Of the compounds listed, only [Sb7Ni3(CO)3]3" (203) does not retain the original framework of the E3 ion. The molecule has been interpreted as having a 24-electron cuneane core with 12 2-center-2-electron bonds. 

One notable example of silver mediated ring rearrangements is Eaton and Halpem s (166) 1970 report, which used silver to convert cubane into cuneane. Lactam formation through a ring expansion of alkoxycyclopropylamines was studied by Wasserman et al. (167) and appears to occur either through a nitrenium species or a concerted process (Fig. 41). 

The energy of the double bond may be raised by other means, e.g. by strain or by antiaromaticity [224]. Miller, a pioneer in the field of organofluorine chemistry, generated tetrakis(trifluoromethyl)cyclobutadiene and showed that it reacts to form a tricyclic dimer [242] which can be converted to the corresponding cubane and cuneane derivatives by ultraviolet radiation [243] (Figure 7.72). 

A deltahedron is a polyhedron whose faces are all triangular. Fora given number of vertices, a closo polyhedron is the most highly symmetric deltahedron. For example, an icosahedron, rather than a tetiacapped cuneane, is the closo polyhedron for 12 Vertex polyhedra. A nido (netlike) polyhedron is a closo polyhedron with one vertex removed, while an arachno (web-like) polyhedron is a closo polyhedron with two vertices, not necessarily adjacent, removed. 

While the capping and fusion rules significantly increase the power of the PSEP model for clusters whose atoms have relatively high metal-metal connectivity, the newest extentions apply the model to lower connectivity clusters. For transition metals, 15n electrons (where n is the number of vertices) are expected for clusters whose metal atoms are connected to three other metal atoms (that is, three-connected clusters). " Examples include tetrahedra, cubes, and cuneanes. In the four-connected clusters, such as octahedra, square antiprisms, and cuboctahedra, the electron count can range from 14n to 14n -I- 4, depending on the nature of the polyhedron. ... 

Figure 9. New pathway for fluxional interconversions of substituted icosahedra via tetracapped cuneane and tetracapped cubane intermediates (or transition states). This proposed scrambling process, which maximizes connectivity and obeys electron-counting rules, is illustrated for the interconversion of a 1,2-disubstituted ort/io-icosahedron into a 1,7-disubstituted meta-icosahedron.

According to theoretical calculations [114], the most stable modification of a neutral P8 ligands is expected to be the cuneane structure (C), which is found in the phosphorus allotrope known as Hittorff s phosphorus [115,116]. Surprisingly, a ligand conforming to this structural motif has been stabilized in the mixed iron/iridium complex [ Cp Ir(CO) 2(p,q2 2 1 1 1-P8) Cr(CO)5 3] (90), prepared either by photolysis or thermolysis of [Cp Ir(CO)2] with P4 in the presence of excess [Cr(CO)5(thf)] (Scheme 4) [35]. The same unit has also been recognized in the iron derivatives [[CpMeFe(CO) 2(p,q2 2 l l-P8)[CpMeFe(CO)2 2]... 

Silver(I) and palladium(II) are able to effect quantitative, catalytic isomerization of cubane into cuneane, an interesting carbon skeleton, which is one of three possible (CH)g isomers. 

Silver(I)-catalyzed isomerization of monosubstituted cubanes produces various possible monosubstituted cuneanes 35-37. Although the catalytic activity of palladium(I) for the isomerization is similar to that of silver(I), rhodium(I) instead effected isomerization of cubane into a i n-tricyclooctadiene 38 by ring opening. 

On the other hand, cuneane was isomerized by bis(norbomadienyl)rhodium(II) chloride into semibullvalene 39. ... 

The rearrangement of cubane gives different types of products depending on the metal. With Ag+ ion catalysis, cuneane is produced, whereas with Rh(I), tricyclooctadiene is obtained (Scheme 6.36). Again, these reactions go through direct metal insertion. When, however, the [RhCl(CO)2]2 complex was applied in a stoichiometric amount, the rhodium-inserted cubane complex could be isolated. ... 

A related rearrangement is the isomerization of cubane (9) to cuneane (10), promoted by silver perchlorate. The name cuneane, from the Latin cuneus, a wedge, is suggested for simplicity. The systematic name is pentacyclo[3.3.0.02 4,... 

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