Cubanes isomerizations

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

CTP, see Cluster with terminal pseudo-atoms Cu. Pd -TSM, propylene gas-phase oxidation over, 39 320-322 Cubane, isomerization of, 24 148 Cubooctahedron, small supported particle model, 36 80-81... 

Variable-temperature collision-induced dissociation (CID) experiments were performed for the (M — 1)+ and (M - - f)+ peaks produced from the three isomers. While these reveal some interesting chemistry, the important result for the purposes of the cubane pyrolysis is that when we studied fragmentation of the (M - - f)+ ions resulting from Cl of cubane isomerized at high temperatures ... 

Strained Carbocyclic Systems.—A large number of examples of catalysis of valence isomerization of strained ring compounds by transition-metal complexes has been considered during the period covered by this Report. The most commonly used metal centres are rhodium and silver, and much of the interest in this field is concentrated on determining and accounting for the differences, in effectiveness and in the nature of the products, between rhodium catalysts and silver catalysts. Thus, for instance, in the presence of [RhCl(nor)]2 or [RhCl(cod)]2, cubane (1) isomerizes to the diolefin (2), but in the presence of silver perchlorate cubane isomerizes to cuneane (3). ... 

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. 

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). 

From the foregoing, however, it should not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the Rh catalyzed valence isomerization of cubane to sj/w-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism ... 

Only a trace of the corresponding cubane 167 is formed on irradiation of the tricy-clooctadiene 168 in pentane at ambient temperatures using a 125-watt mercury arc lamp. The principal product 169 is the result of rearrangement within a biradical intermediate79. A review of the synthetic approaches to cubane and to its reactions has been published77. The diene 170 photochemically converts on irradiation in pentane solution at 254 nm to yield a photostationary mixture of the cubane 171, the starting material 170 and the isomeric diene 17280. Other additions of this type have been used for synthesis of the propellaprismane 173, essentially a heavily substituted cubane, by the intramolecular (2 + 2)-photocycloaddition of the diene 17481. 

Eaton and co-workers also reported the synthesis of 1,3,5-trinitrocubane and 1,3,5,7-tetranitrocubane (39) ° The required tri- and tetra-substituted cubane precursors were initially prepared via stepwise substitution of the cubane core using amide functionality to permit ort/jo-lithiation of adjacent positions. The synthesis of precursors like cubane-1,3,5,7-tetracarboxylic acid was long and inefficient by this method and required the synthesis of toxic organomercury intermediates. Bashir-Hashemi reported an ingenious route to cubane-1,3,5,7-tetracarboxylic acid chloride (35) involving photochemical chlorocarbonylation of cubane carboxylic acid chloride (34) with a mercury lamp and excess oxalyl chloride. Under optimum conditions this reaction is reported to give a 70 8 22 isomeric mixture of 35 36 37... 

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... 

Pelosi and Miller have reported the results of an elegant study of the photolytic and thermal isomerizations of C CFj) isomers.42 The anti-dimer of perfluoro(tetramethylcyclobutadiene). anti-21, gives a mixture of perlluoro(octamethylcubane) (22) and perfluoro(octamethylcuneane) (23) in 20% and 15 % yield, respectively, when irradiated for 36 hours in a fluorocarbon solvent [a mixture of perfluoro(1,3- and perfluoro(1,4-dimethylcyclohexane)]. Cubane 22 is presumably formed via xyn-21, whereas 23 is postulated to be formed via intermediates per-lluoro(oclamcthyleyclooctatetraene) (24) and the corresponding scmibullvalene. 

Irradiation of diphenyl acetylene (Formula 430) in hexane solution gives 1,2,3-triphenylazulene (Formula 431), 1,2,3-triphenylnaphthalene (Formula 432), hexaphenylbenzene (Formula 433), and octaphenyl-cubane, (Formula 434) (180a, 190). The products presumably arise by isomerization, addition, or dimerization of an intermediate tetraphenyl-cyclobutadiene (180a,190). 

Some oligomers of the phosphaalkynes BuCP have been characterized. The phosphaalkyne cyclotetramer exists in several isomeric forms, whose structures are shown in Fig. 15.5.6(a) to Fig. 15.5.6(e). In cubane-like P4C4 BU4, the P-C bond lengths are all identical (188 pm), and are typical for single bonds. The angle at Pis reduced from the idealized 90° to 85.6°, while that at C is widened to 94.4°. 

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

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... 

Prismane is an example of another interesting strained compound. Because it contains three-membered rings fused with four-membered rings, it should be even more strained than cubane. Prismane was prepared in 1973. It is a liquid that is stable at room temperature but explosive under some conditions. In toluene at 90°C its half-life (the time it takes for one-half of the compound to decompose) is 11 h. Note that prismane is isomeric with benzene. In fact, it was one of the structures proposed for benzene in the early days of organic chemistry. 

Numerous phosphine complexes (and some arsine and stibine analogues) are known. With monophosphines these are mainly of the type (R3P) AgX, with rt = 1-4. The 1 1 complexes are tetrameric, with either cubane (18-I-II) or chair (18-I-III) structures depending on the steric requirements of both X and R3P Ag4I4(PPh3)4 undergoes cube-chair isomerization, and the two structural types may be obtained by crystallization from different solvents. 

Cubanes are partially decychzedto tricyclooctadienes when allowed to react with catalytic quantities of [RhCl(LL)]2 complexes (LL = cod, nbd). Monosubstituted cubanes form two products, as shown in equation (13). Homocubanes (equation 14) react similarly. If methyloxyphosphahomocubane is decomposed, two isomeric products are obtained (equation 15). Endo- and exo isomers are obtained from the catalytic isomerization of 9,10-dimethoxycarbonyl bishomocubane (equation 16). Sundry palladium(II) complexes, AgNOs, or PdCl2 form significant quantities of (2). 

A better example for the possible intervention of a distinct intermediate was recently disclosed in a study of the valence isomerization of cubane (79) to syw-tricyclooctadiene 20) using various rhodium (I) catalysts. 

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-... 

Perhaps the best characterized example of a subsite differentiated [4Fe-4S] protein is aconitase, which catalyzes the citrate-isocitrate isomerization in the citric acid cycle (257). Aconitase isolated aerobically is inactive and contains a [3Fe-4S] cluster. Activity is restored by incubation with Fe and this also reconstitutes the [4Fe-4S] cluster. Oxidation of the core results in loss of the fourth iron atom, regenerating the [3Fe-4S] form. Mossbauer studies have demonstrated that only one of the four iron sites is exchanged (258). X-ray studies on both [3Fe-4S] and [4Fe-4S] forms of pig heart aconitase 258a) showed that insertion of iron into [3Fe-4S] occurs isomorphously. The positions of the common atoms in the two forms of the core agree to within 0.1 A, supporting the view of the [3Fe-4S] cluster as an iron-voided cubane. A similar result was obtained for the seven iron ferredoxin from Azo-... 

Rhodium(I) complexes are efficient catalysts for the valence isomerizations of cubane (23a equation 7) and its derivatives to the corresponding iy/j-tricyclo[4.2.0.CF ]octa-3,7-dienes. When a stoichiometric amount of [ RhQ(CO)2)2l is added to a chloroform solution of cubane, the organorhodium compound (24) is obtained in about yield. Treatment of (24) with a stoichiometric amount of triphenylphos-phine gives, in about 90% yield, the polycyclic ketone (25) together with a small amount (5-10%) of cy-clooctatetraene. A similar reaction is also reported for the methoxycarbonyl derivative (23b). 

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. 

Orbital corrclaiion diagram. (89. 3.33 alternant hydirtcarbon, 345-46 benzene valence isomerization. 449. 452 <1 bond dissociation, 188-89 zr bond di.ssociaiion, 190-91 cubane. 410... 

---