Prismanes, formation

Dimerization of cyclopropenones has also been found to occur under reductive conditions. Tetraphenyl resorcinol is formed in addition to a small amount of tetra-phenyl p-benzoquinone on treatment of diphenyl cyclopropenone with aluminum amalgam200 its formation can be rationalized via dimerization of the cyclopropenone ketyl 266 and subsequent aromatization, possibly according to a prismane mechanism. 

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

The identification of prismanes 6 and 7 followed from spectral data and their thermal conversions inLo pyridines 8 and 9 (front 6) and 10 (from 7). The azaprismanes are quite stable and rearrange only slowly at 175 C. The formation of 6 is evidence of the phototransformation represented in Scheme 11,31 which is also observed for perfluoro(alkylpyridazincs) (vide infra). 

However, we lack the first difference, and indeed we lack thermochemical data of any type on any monosubstituted prismane. Let us label this unknown quantity AHisf". The second difference is expressible as the difference of the enthalpies of formation of the isomeric cis- and trans-... 

These photoproducts are not now considered to arise from intermediates of the Dewar, prismane, or benzvalene type. The simplest explanation offered is that excitation results in ring cleavage and the formation of a short-lived intermediate (150). An analogous inter-... 

All of these are symmetry-forbidden. Consider the formation of prismane. As pictured in Fig. 16, the reactant orbitals are properly aligned, but... 

The choice of the reducing agent is quite important. Thus, reduction of the precursor (2,6-diisopropylphenyl)trichlorogermane with Mg/MgBr2 in THF resulted in the formation of tetrakis(2,6-diisopropylphenyl)digermene23 instead of prismane 5. 

The necessary enthalpy of formation of cubane [34] is 622 kJ mol-1, and that of prismane [35] is taken as ca. 570 kJ mol1. Linear extrapolation gives the enthalpy of formation of tetrahedrane as ca. 518 kJ mol1. Alternatively, in the absence of a measured enthalpy of formation data for prismane, we assert thermoneutrality for the formal reaction... 

As noted above, there are no measurements on the enthalpy of formation of prismane, per se, but experimental studies of the rearrangement enthalpy of hexamethylprismane to hexamethylbenzene have been reported". But the two relevant numerical values, 382 and (345 3) kJ mol" respectively, are markedly different from each other, and from the ca 570 kJmol" suggested by modern calculational theory" for the parent species. While the measured rearrangement enthalpy" of hexakis(trifluoromethyl)prismane to hexakis(trifluo-romethyl)benzene is ca 135 kJ mol" smaller than the above hexamethyl results, so be it CF3 substitution is significantly different from that of Me". But it eludes us why there is... 

From a purely synthetic viewpoint, triazolinedione adducts have served as substrates for gaining access to numerous target molecules such as prismane, semibullvalene, bridged semibullvalenes, elassovalene, caged compounds, and azoalkanes. Indeed, the title reagent can be used not only as a chiral source, but also as an azo donor. In the synthesis of 4,5-diazatwis-4-ene, for example, (-)- nfirst step involved cycloaddition to cyclooctatetraene dibromide with formation of a separable mixture of diastereoisomers. Eventual removal of the... 

In this context, we mention the formation of a derivative of 61 by rearrangement of 3,3 -dimethylbicyclopropenyl (62). The electron-transfer chemistry of bicyclo-propenyl was of interest, because its radical cation might be an intermediate in the rearrangement of prismane (vide infra), and also as a potential adduct between cyclopropenium cation and cyclopropenyl radical. For reasons of practicality the 3,3 -dimethyl-derivative, 62, was chosen for study because it is not subject to prototropic rearrangements. 

A study of the reactivity of the carbonyl-functional phospha-alkenes (305) has also been reported. The triphospha-Dewar-benzene (306) has been shown to undergo cycloaddition reactions with alkynes to form the triphosphabishomo-prismane system (307). Cycloaddition of t-butylphospha-ethyne to the phosphatriafulvene (308) results in the formation of a single isomer of the diphosphaisobenzene (309), having an allene system within the ring. This... 

The photolysis of substituted pyridines causes migration of the substituent, and a mechanism involving intermediate formation of a Dewar-type isomer and an aza-prismane has been proposed 148). However, the formation of these intermediates has not been confirmed so far (139). 

When the reaction mixture comprising triethylaluminum and phosphaalkyne 9 a (ratio 2 3.5) in n-hexane is allowed to warm from -50°C to room temperature, phosphaalkyne cyclotrimerization with incorporation of two trialkylaluminum units and formation of the bis(homo)prismane 80 (91 %) occurs (Scheme 6-20) [64]. The structure of the cage compound was elucidated unambiguously by crystallography all P-NMR signals appear in the high-field region (6 = -1.2 to -147.3) [64]. 

Pyridines show a number of thermal and photochemical transformations which are analogous to the valence tautomerization of benzene, i.e. formation of Dewar benzene, prismane and benzvalene. 

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