Propellanes—

Whereas the syn-bromides (707)—(709) are reduced smoothly by tri-n-butyltin deuter-ide in hot benzene to give predominantly inverted products, the anti-bromides (710) and (712) do not react except under more forcing conditions, when products of completely retained configuration are obtained. Competitive reduction of (708) and (711) 

Electrolysis of l-bromo-4-chlorobicyclo[2,2,0]hexane affords A -bicyclo[2,2,0]-hexane, which can be extracted into pentane. On standing at room temperature the olefin self-reacts to give (719), presumably by ring-opening of the initial propellane formed by [2 + 2]-type dimerization. Further developments are reported on studies of bond fixation in isomers of alkylated cyclo-octatetraenes, some of which have been prepared by the thermolysis of the appropriate propellatrienes. The 

The parent molecule, tricyclo[1.1.1.0 ]pentane, [l.l.ljpropellane (205), may be imagined as a 1,3-bridged 186. Its remarkable geometric and electronic structure, the 

The structure of the free molecule 205 has been determined from ED data with a joint use of rotational constants. The central bond is very long, 1.596 (5) A the side bonds of 1.525 (2) A are also longer than in 1 (Table 1). The C—H bond is 1.106 (5) A, angle H—C—H 116.0 (19) °. These are in excellent agreement with results of MP2/6-31 IG(MC) calculations the highest-level structure yet reported for 205, i.e., C—C central 1.602, C—C side 1.521, C—H 1.087 A and H—C—H 115.3°. 

The C—C bond in the related free bicyclo[l.l.l]pentane (206) is 1.557 (2) A. The C C distance, 1.874 (4) A, which corresponds to the axial bond in 205, is longer than the bond but is very short for a nonbonded distance. This parameter is rather sensitive to substituents in the 1 and 3 positions . 

There are four independent molecules in the crystal of 205 at 138 K two of them show rotational disorder about their central bonds. Another phase formed on cooling consists of twinned crystals, which cannot be used for a structure determination. Approximate sysmmetry and bond lengths of about 1.60 and 1.53 A (corrected for the 

The structure and electron-density distribution of two derivatives 207 and 208 have been determined at 81 K by The symmetry of the propellane part is close to Z 3h in both 

FIGURE 24. Electron-density difference maps of207. (a) Section in the plane of atoms C1, C3 and C5, approximately perpendicular to the C2—C4 axis of the propellane unit. Note the density peaks in the centers of the three-membered rings, around the projection of C2 and C4. Contour lines are at 0.05 e A 3 intervals, (b) Section in the plane of the three-membered ring C2, C3, C4. Contour lines are at 0.025 e A 3 intervals. Full lines mark positive, dashed lines negative regions. Reproduced by permission of Verlag Helvetica Chimica Acta from Reference 319 

Miriam Kami, Yitzhak Apeloig, JUrgen Kapp and Paul von R. Schleyer 

TABLE 22. Structures of group 14 metallapropellanes and their analogs (pm, deg) 

3 At GVB/ECP from Reference 232. Values in parentheses are at HF/ECP from Reference 184. cAt HF/ECP from Reference 184. 

2The distance between the central M atoms and the peripheral MH2 or O ligands. 

Like in metallabicyclo[ 1.1.0]butanes (25), the metallapropellanes, 27, show a significant shortening of the central M M distances (by 20-30 pm) upon ionization to the corresponding radical cations233. 

Bridged annulenes such as the methano-bridged variety (or the cyclophanes) are generally not appropriate to this chapter since from the chemical viewpoint they are usually best considered together with aromatic hydrocarbons, even if they are technically bridged molecules. Accordingly, developments are not reported here. 

Conjugate addition of excess MejCuLi to the ap-unsaturated ketone (831), followed by quenching the reaction mixture with water, gave the propellane ketone (832). Transfer of two electrons from the metal is thought to yield a complexed dianion of a Cu species which, by expulsion of the leaving group, effects closure to the propellane system indeed, in acetic anhydride solution the enol acetate of (832) is the isolated product, which accords with this proposal. 

Bridgehead olefins (834 n = 3 or 4) are formed in the solvolysis of the dibromo-propellanes (833 n = 3 or 4) by loss of halide anion, electrocyclic cleavage of the cyclopropane ring, and solvent capture/ The products isolated depend upon the precise reaction conditions in the case of (833 n = 3) acetolysis affords (835 53%) and (836 18 %) provided that water is not present. The details of the mechanism of the Ag -assisted solvolysis of (833 n = 4) have been elucidated with the aid of labelling.  

The photocycloaddition of cyclo-olefins to cyclic aP-unsaturated ketones has been used in propellane synthesis. Thus, (837) was prepared by addition of cyclohexene to the relevant bicyclo-enone, and (838) was likewise prepared from A -octalin and 

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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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