Rearrangement cubane

Figure 1 Simplified potential energy hypersurface for cubane rearrangement (AH/ values in kcal mol )...

Cubane derivs. 26, 763 suppl. 27 jeco-Cubane -, rearrangement, skeletal 26,762... 

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 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]. 

With cyclic a-halo ketones, e.g. 2-chloro cyclohexanone 6, the Favorskii rearrangement leads to a ring contraction by one carbon atom. This type of reaction has for example found application as a key step in the synthesis of cubane by Eaton and Cole for the construction of the cubic carbon skeleton ... 

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. 

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. ... 

When the reaction was run at reflux for 3 hours, besides diester 10%-20% (based on and NMR spectra of crude material) of a thermal-opening product was formed, the structure identical with those independently synthesized by Rh(I) promoted rearrangement of cubane diester 4. 

The pyrolysis breakdown behavior for cubane is plotted in Fig. 4.7. Cubane is found to be stable on the millisecond time scale for temperatures up to 500 K. Minor decomposition was found between 500 and 700 K, and above that point decomposition is faster than the flow tube residence time. By 800 K there is essentially no remaining cubane. The dominant product channel is loss of C2H2, delding benzene. Some rearrangement to COT was observed above 650 K, and a small amount of styrene was found at high temperatures. The decomposition lifetimes corresponding to these breakdown curves are given in Table 4.2. 

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 rearrangement was essential for the design and execution of the first preparation of the cubane skeleton ... 

Cubane was found to undergo a rearrangement via the hitherto unknown 36 intermediate 391... 

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... 

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). 

Methyl-3-heptanol, stereoselective preparation, 9, 12 Methylidyne cubane, via Ti(IV) complexes, 4, 407 <7-Methyl-(tj5-indenyl)chromium tricarbonyl, rearrangement,... 

Despite enormous strain, cubane is kinetically stable because breaking just one C—C bond causes only minor structural changes and hence only little relief of strain. A computational study shows that protonation occurs to give edge-protonated cubane.28 Cleavage of a second C—C bond is highly exothermic and this is followed by a further series of rapid exothermic molecular rearrangements (Scheme 16). 

The skeleta of other strained systems such as cubanes and related compounds can also be reorganized in the presence of silver salts. Rearrangements of cubanes were described in the early 1970s. Eaton et al.35 showed that mono- or disubstituted cubanes gave a mixture of polycyclic regioisomers (Scheme 3.21). No yields were given. 

You can read more about the synthesis of cubane in Chapiter 37, when we discuss the rearrangement reactions that were used to make it. 

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). 

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