Propellanes bonding

So far, considerable efforts have been made to examine thermal and acid catalysed rearrangements of propellanes. However, PET reactions and direct photolyses with high-energy photons (e.g. 185-nm excitation) have so far been neglected. It would be a challenging task for future research to fill this gap, because new information can be expected on the nature of the cleavage of the central propellane bond with its inverted carbon hybridization. 

Vincovic and Majerski prepared a second [3.1.1]propellane 165 from pyrolysis of 166 This system is more strained than the parent 158, decomposing in 30 minutes at room temperature in benzene. It reacts with HCl at — 80°C to yield only the nortricyclylmethyl chloride 167. Although this appears to involve external opening, rather than cleavage of the central propellane bond, protonation of the central bond to give 168 followed by cyclobutyl to cyclopropylcarbinyl rearrangement can also provide 167. 

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]. 

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 ... 

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." ... 

The bond between the bridgehead atoms is inverted in the [l.l.l]propellane system 59. The hybrid orbitals are directed away from each other, hi the geminal 0-0 interaction, the greater lobes of the geminal hybrid orbitals at the bridgehead atom can be in phase with each other, while the front lobes of the hybrid orbitals of the normal and inverted bonds at the 1,3-positions are in phase with each other... 

Scheme 25 Bonding geminal a-a interaction with the inverted bond in the propellane 59...

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... 

Two rings linked by sharing the same bond instead of the same atom lead to annulated bicyclic or tricyclic compounds, the propellanes. In the case of poly-unsaturated molecules, an interesting case is represented by the bicyclo[2.2.0] type. The parent compound Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene) (DEW) is the smallest bicyclic diene which is an often discussed valence isomer of aromatic benzene Unsubstituted DEW is a very... 

For n = 1, this is the standard synthesis of [l.l.l]propellane 40a.20,21 Also for n = 3, this method makes [3.1.1]propellane 40c accessible with reasonable effort.22 Although [2.1.1]propellane 40b was detected by its IR spectrum at liquid nitrogen temperature,23 it could not be obtained as a stable compound via this route.22 The addition of halomethanes across the central bond by a radical chain mechanism is common to 40a and 40c. The addition of carbon tetrabromide to 40c afforded a 45% yield of 41.18 Likewise, a number of bicy-clo[l.l.l]pentane derivatives 42 were obtained by reaction of the corresponding halomethanes with 40a.17,24... 

The addition of halomethanes across the central bond of [l.l.l]propellane 78 leads to l-halo-7-(n-halomethyl)tricyclo[4.2.0.02,7]octanes 79, which are suitable precursors for the generation of type 80 carbenes.26... 

Precursors for this task were obtained by addition of /-butylmagnesium bromide to the central bond of [1.1.1 ]propellane 40a followed by conversion of the 3-f-butylbicyclo[ 1.1.1 Jpentyl-1 -y 1-magnesium bromide (88) into the ketones 89 by standard methods.27 Reaction of ketones 89 with tosyl hydrazide afforded the hydrazones 90, which gave the corresponding lithium salts 91 by reaction with MeLi in ether. These salts were dried under high vacuum and then pyrolized at 4 x 10 5 torr in the temperature range of 100-130°C and the volatile products condensed in a liquid nitrogen-cooled trap. 

Iterative trapping of the alkylpalladium species with tethered olefins is also possible, which allows tandem cycloisomerizations and zipper reactions to take place. Thus, depending upon the juxtaposition of the unsaturated bonds, Trost achieved highly atom-economical syntheses of triquinanes, propellanes (264) from the ynediene 263, and polyspiranes (Scheme 67).261... 

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. 

The oxidation of 3,6-dehydrohomoadamantane (52) with NO+BF4, photo-excited tetracyanobenzene, and under anodic conditions has been found to involve a common radical cation intermediate. The study has shown that the activation of propellane cTc-c bonds with strong oxidizing electrophiles occurs by a sequence of single-electron transfer steps. These findings are supported by ab initio computations showing that the isomeric radical cations can equilibrate with low barriers and lead to a common product. ... 

The validy of e in classifying bonds is borne out by the analysis of theoretical densities the ellipticity of the C—C bonds in the series ethane, benzene, ethylene increases from 0.0 to 0.23 to 0.45 (Bader et al. 1983). For the very long bridgehead bonds in propellanes, which are shared by three rings, the ellipticity can be quite large, as in [2.1.1] propellane,... 

Topological analysis of the total density has a considerable advantage over the use of the deformation densities in that it is reference-density independent. There is no need to define hybridized atoms to analyze the nature of covalent bonding, and the ambiguity when using the standard deformation density, noted above in the discussion on propellanes, does not occur. 

The propellanes are a class of compounds with three condensed rings, either three- or four-membered, sharing a bridgehead bond. In two [1.1.1] propellane derivatives studied by Seiler et al. (1988), no peaks were observed in the deformation density of the central bridgehead bonds of lengths 1.58 A, but peaks at the apex of the inverted (i.e., pyramidal) bridgehead carbon atoms are in agreement wih electrophilic attack at these positions. 

The capped propellane 425 has been proposed as a precursor of a carbocation which might exhibit pure pp-a bonding... 

The most remarkable structural feature of [l.l.l]propellane (3) is the geometry at the bridgehead carbons. Since all four bonds point in the same direction, this might best be described as having an inverted tetrahedral geometry. 

Although the bonding is unusual, the central bond in A [l.l.l]propellane has 0.8 times the electron density at the center of the C—C bond in butane. There is a significant amount of electron density near the bridgehead carbons, and this presumably is responsible for the high reactivity of the compound toward electrophiles... 

---