Propellane

The first conceptual approach assumes that the reaction enthalpy of its formal synthesis from cyclopropane and 2,2,3,3-tetramethylbutane, equation 13, 

Reaction 14 is endothermic by 37 kJ mol1, and provides additional documentation for the earlier enunciated superstrain [37] for bicyclobutane. Accepting this value as a correction term for equation 13 gives our first estimate of the enthalpy of formation of gaseous [l.l.l]propellane, namely 349 kJ mol1. We may recognize reactions 13 and 14 as part of a homologous series in which the next and last reaction is 

This non-reaction reaction is precisely thermoneutral. Linear extrapolation from equations 14 and 15 suggests that equation 13 should be endothermic by 2 37 kJ mol1 and so the enthalpy of formation of [1.1. l]propellane would thus be expected to equal 386 kJ mol1. 

This admittedly nonreaction reaction is, of course, precisely thermoneutral. Linear extrapolation suggests that reaction 16 should be endothermic by twice 44 kJ mol 1 and so the enthalpy of formation of [l.l.l]propellane should be 370 kJ mol1. 

Our last conceptual approach assumes the reaction enthalpy of the formal 

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The coupling of 1,8-diiodonaphthalene (25) with acenaphthylene (26) affords acenaphth[l,2-a]acenaphthylene (27). It should be noted that the reaction involves unusual trans elimination of H—Pd—1[32], This tetrasubstituted double bond in 11 reacts further with iodobenzene to give the [4, 3, 3]propellane 28 in 72%. This unusual reaction may be accelerated by strain activation, although it took 14 days[33]. 

An interesting synthetic method for the [3.3.3]propellane 74 by intramolecular cycloaddition of a disubstituted methylenecyclopropane with an iinsa-... 

Fig. 1.32. (a) Molecular graphs and electron density contours for pentane and hexane. Dots on bond paths represent critical points, (b) Comparison of molecular graphs for bicycloalkanes and corresponding propellanes. (Reproduced from Chem. Rev. 91 893 (1991) with permission of the American Chemical Society.)... 

The propellanes are highly reactive substances which readily undergo reactions involving rupture of the central bond. It has been suggested that the poh erization of propellanes occurs by a dissociation of the central bond ... 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

We ve included several papers in the References section which perform theoretical and experimental studies of the IR and Raman spectra for these compounds. These compounds were among the earliest ab initio frequency studies of such systems. In addition, in the case of propellane, theoretical predictions of its energy and structure preceded its synthesis. 

Lsotopic substitution and its effect on the frequencies. For exampl substituting deuteriurn for hydrogen in propellane produces different IR peaks. 

In a modified procedure the free carboxylic acid is treated with a mixture of mercuric oxide and bromine in carbon tetrachloride the otherwise necessary purification of the silver salt is thereby avoided. This procedure has been used in the first synthesis of [1.1.1 ]propellane 10. Bicyclo[l.l.l]pentane-l,3-dicarboxylic acid 8 has been converted to the dibromide 9 by the modified Hunsdiecker reaction. Treatment of 9 with t-butyllithium then resulted in a debromination and formation of the central carbon-carbon bond thus generating the propellane 10." ... 

Figure 1. Representative linear, angular, and propellane triquinane natural products.

Scheme 14. Trost s approach to [3.3.3]propellane 67 by hydridopalladium acetate-catalyzed sequential cycloisomerization.

The alkoxyazocines in Table 1 are in tautomeric equilibrium with their bicyclic or propellanc forms, the equilibrium normally being shifted to the azocine side with the exception of the pentanoazocine 7. In this case, the compound exists as the propellane 6 below 100°C (in tetra-chloroethene), as revealed by NMR spectroscopy, and the azocine form predominates only above 150C. 

The corresponding ethano and propano compounds only exist in their propellane forms. Apparently, the smaller bridge leads to higher strain in the azocine tautomer, so that the triene structure predominates. Longer chains (> hexano), however, exhibit the normal behavior of tetraenic compounds. 

The eight-membered rings of the diphenyl and the butano-bridged derivatives 3 [R, R = Ph. Ph — (CH2)4 —] are in a tub conformation, as indicated by X-ray structural analysis. For the oxidation of the propellane reactants a number of other reagents are suitable 25... 

When the bicyclic thiirene oxide 180164 is dissolved in excess furan, a single crystalline endo-cycloadduct (182) is formed stereospecifically (equation 71)164. This is the first propellane containing the thiirane oxide moiety. Clearly, the driving force for its formation is the release of the ring strain of the starting fused-ring system 180. In contrast, 18a did not react with furan even under forcing conditions. 

Tetrabromomethane reacts virtually instantly with 1,1,1-propellane to give corresponding adduct (ref. 5). 

At the same time, the reaction of CCl3Br with 1,1,1-propellane gives mostly the adduct, even when the propellane excess is used (ref. 14) ... 

Proceeding the reaction of bromocyane with 1,1,1-propellane differ essentially from that for CCl3Br (ref. 14). In the case of bromocyane telomers CN-( ) -Br... 

In certain small-ring systems, including small propellanes, the geometry of one or more carbon atoms is so constrained that all four of their valences are directed to the same side of a plane (inverted tetrahedron), as in 98. An example is 1,3-... 

Bridgeheads. The Sn2 mechanism is impossible at most bridgehead compounds (p. 392). Nucleophilic attack in [l.l.l]propellane has been reported, however. In general, a relatively large ring is required for an SnI reaction to take place (p. 396). " The SnI reactions have been claimed to occur for l-iodobicyclo[l.l.l]pentane via the bicyclo[l.l.l]pentyl cation, but this has been disputed and the bicyclo[1.1.0]butyl carbinyl cation was... 

The cycloaddition reaction of heterocyclic propellanes 99 (X = O and S) with iV-phenyltriazolinedione (NN) (Fig. 16) affords the anti adduct with respect to the bridge [166-168]. Replacement of the a-CH groups by carbonyls (that is 100),... 

Gleiter and Ginsburg found that 4-substituted-l,2,4-triazoline-3,5-dione reacted with the propellanes 36 and 37 at the syn face of the cyclohexadiene with respect to the hetero-ring. They ascribed the selectivity to the secondary orbital interaction between the orbitels (LUMO) of 36 and 37 with antisymmetrical combination of lone pair orbitals (HOMO ) of the triazolinediones (Scheme 24) [29]. 

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