Prismanes stability

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

Such properties are also reflected in the relative stability of the MgHg valence isomers (Table 2). It is well known that benzene (CgHg) is very stable due to cyclic delocalization of its six re electrons (aromatic stabilization), and it is much more stable than other strained valence isomers — Dewar benzene, benzvalene and prismane13,14. However, the tendency is completely reversed in the case of heavier atoms the isomers with a smaller number of double bonds are more favorable. As a result, the prismane structure becomes much more stable than the benzene structure on going from carbon to tin atoms10,15. 

As in the case of monocyclic 3- and 4-MRs (see Section V.E.l.a.ii), electropositive substituents stabilize also M H cage systems which include such rings, e.g. tetrahedranes, prismanes and cubanes (Table 24)191,192,195. The only isolated tetrasilatetrahedrane indeed carries the electropositive Si(Bu-t)3 groups as substituents238. Nevertheless, the steric protection afforded by the bulky silyl ligands is probably the most important factor allowing the isolation of the tetrasilatetrahedrane179 195. 

The X-ray crystal structure of benzene, proving the equivalence of the six C-C bonds, appeared in 1929 and 1932, and Pauling reported its electron-diffraction data in 1931. Note that several of the structures proposed in the 19th century, such as Dewar benzene (non-planar) and Ladenburg s prismane, which are valence isomers of benzene, have now actually been prepared from benzene derivatives photochemically. They are kinetically stabilized, since they do not spontaneously revert to benzene or its derivatives [17-20]. 

All of the polycyclic polystannanes reported to date are thermally robust in solution to temperatures of at least 200°C. For the bicyclo[2.2.0]hexa-slannane derivative 87, this is not too surprising, since the calculated strain-energy value for the parent compound is still ca. 12 kcal mol" less than the parent cyclotristannane system (102). However, the parent perstanna-[ ]-prismanes for n = 4 and 5 are expected to be more ring-strained than cyclotristannane (cf. 73.2 and 61.0 kcal mol" , respectively) (99,I04) therefore, the thermal stability of the two compounds, 90 and 91, suggest that these compounds are kinetically stable by virtue of signifi-... 

According to theoretical studies, the insertion of the N—N bond into the molecules of tetra-hedranes or prismanes decrease their relative thermodynamic stability (3-21 G) <89MI 111-03) in the series of di-, tri-, and tetraazatetrahedranes the corresponding azetes are more stable for example, tetrazete (59) is more stable than tetraazatetrahedrane (60), AE = 83 kJ mol-1 (6-31G /4-31G) <87MI 111-04). 

The three Dewar structures 5, 11 and 12 (Dewar benzene) are also considered to contribute to the resonance hybrid (according to valence bond theory, approximately 20% in total) and to the extra stability. Dewar benzene has now been prepared. It is a bent, non-planar molecule and is not aromatic. It gradually reverts to benzene at room temperature. The Ladenburg structure, prismane i6), is an explosive liquid. Dewar benzene and prismane are valence isomers of benzene. 

Upon irradiation with UV light (A > 200 nm) the two carbon-carbon double bonds of pentakis(pentafluoroethyl)-l-azabicyclo[2.2.0]hexa-2,5-diene (199 Section III,D,2) entered into an intramolecular (2 + 2)-cycloaddition reaction to give the corresponding 1-azaprismane (200).226 This highly strained molecule was thermally stable up to 160°, at which temperature it decomposed into a mixture of 199 and pentakis-(pentafluoroethyl)pyridine. In terms of conservation of orbital symmetry this high stability can be well understood. Just as in prismane... 

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

There are only a few studies of these molecules (30, 36,124). They have been summarized in a review (30) together with those of unsaturated members of the CgHg family. All but one of the semiempirical quantum chemical studies quoted in Table 1 of the review yielded cubane (which is [4] prismane) as the most strained member of the family. Except in ref 36, the existence of inverted carbon atoms (the bridgehead ones in the bicyclobutane units) in the octabisvalene molecule has been overlooked. There is no agreement in the literature concerning the relative stability of cuneane and octabisvalene. As mentioned earlier, the experimental determination and ab initio calculation have been carried out only for 1 thus, more experimental measurements and further ab initio calculations for molecules 76a and 83 are necessary. 

In this context, 138 was converted into propella[34]prismane (179) via an intramolecular cycloaddition reaction, after the stabilizing CpCo-group had been removed photochemically (Fig. 46a) [112, 162],... 

So what can we conclude about the stability of polyprismanes from strain calculations We carried out the above illustrative calculations on [3,4]prismane. Here are resrrlts of the homodesmotic strain calculations of Minyaev et al. [1] at the B3LYP/6-311G(2df,p) level [stack height, ring size], nomenclature of Fig. 12.1, SE and SE/C-C, in kJ moU ... 

Another factor, besides strain, that can destabilize a molecule is an unfavorable electronic interaction, for example that manifested as antiaromaticity, in, say cyclobutadiene [39], but unlike strain, electronic interactions can be favorable and stabilizing. Minyaev et al. proposed that orbital interactions between the two capping (CH)n and the adjacent C moieties stabilize [3 n]- and [4,n]-prismanes by CT-overlap of rr-type ring orbitals, and precedent for snch stabihzation in simple... 

The question of stmcture, stability, and properties of benzene has held a central position in organic chemistry. The Kekule s hypothesis for the stability of cyclohexatriene prompted development of theory whose extensions to transition states has had as much impact as on ground states. However, there are five known isomers of (CH)6 benzene, Dewar benzene, benzvalene, prismane, and biscyclo-propenyl (Scheme 7.4). The highly strained quadricyclo[2.2.0.0 ,0 ] hexane isomer is also of interest. 

Perfluoropentaethylmethylbenzene isomerizes photochemically to a mixture of Dewar-benzene isomers. Further irradiation converts these into the prismanes. These valence isomers of benzene are remarkably stable, and appear to owe their stability, in part, to destabilization of the parent benzene by non-bonded interactions. 

We have found that if the thermolysis of the cyclotristannane 1 is carried out with benzophenone at 200° C, two new polycyclic polystannanes, 7 and 8, which are members of the perstanna-[n]-prismane, [RSn]2n, series of compounds, can be isolated by column chromatograpy in a 1 2.3 ratio for a combined yield of 12% [11,12]. Since the octastanna-[4]-prismane 7 and the decastanna-[5]-prismane 8 display different physici and chemical properties, an investigation of these compounds reveals important information concerning the impact that bond angle distortions have on the electronic structure and thermodynamic stability of heavy atom Group IVA polycyclic compounds. 

By contrast, hexakis(pentafluoroethyl)benzene gives only a para-bonded isomer and then a prismane. These perfluoroalkyl-substituted isomers show remarkable thermal stability, half-lives at 170°C ranging from 135 h for isomer (117) to 9 h for isomer (116). ... 

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