Bicyclopentanes

P-hydrogen. 1-Phenylbicyclopentane rearranged to 3-phenylcyclopenene in the presence of Znl2. Tricyclo[3.2.0.0 A] heptane 20 and tricyclo [3.1.0.0 ] hexane 21 were converted into bicyclo [3.2.0] heptene 22 and cyclohexa-1,3-diene 23, respectively, using AgBF [34]. 

Bicyclopentane can be hydrogenated over Pt02 at room temperature to afford cyclopentane via cleavage of the central C-C bond [35]. 

Bicyclo[2.1.0]pentane readily underwent annulation across carbon-carbon double bonds in the presence of a nickel(O) catalyst. Several electron-deficient alkenes participated in the reaction to afford mostly bicyclic products 

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Varions possibilities were considered for the nnderlying reaction of the biradical. No radical signals grew when the biradical decayed, so H-abstraction from the matrix did not appear to be occnrring. Analysis of products formed from irradiations of 8 at 5.5 K showed both bicyclopentane 10 and cyclopentene, in a ratio of 30 1. Very similar ratios, ca. 25 1, were observed in solution irradiations at room temperature. It was noted that if the major tnnneling reaction was H-shift to produce cyclopentene, this product should be enhanced as temperatnres were lowered, in contrast to the experimental observations. Hence, it was conclnded that the observed decay of the EPR spectrum of 9 was due to ring closure to give 10. 

The importance of the stereoelectronic conformation in determining the path of reaction may be seen in the example of phenyl cyclobutyl ketone. Photolysis of this compound yields 60% of the highly strained bicyclopentane shown below and only 40% of open-chain olefin(84) ... 

The reason for this behavior can be seen in the structure of the intermediate biradical. The rigidity of the cyclobutyl ring prevents a parallel alignment of the p orbitals with the 0 bond, which is held practically perpendicular. In order for type II cleavage to occur, an initially severely strained olefin must be formed. Hence radical recombination to yield the bicyclopentane system predominates. 

Using 3-Bromocycloalkyl Hydroperoxides obtained by Peroxybromination of Bicyclopentane... 

The silver-salt method was used by Porter and Gilmore in one of the first syntheses of 2,3-dioxabicyclo[2.2.1]heptane 936). Reaction of bicyclopentane with 98% H202 and IV-bromosuccinimide (NBS) in ether at —41 °C afforded mainly a 1 1 mixture... 

Correct experimental conditions are vital. In particular, it is essential to irradiate only the benzophenone chromophore, which can be achieved by employing an appropriate u.v. laser, for direct excitation of the azo compound produces a singlet diradical that collapses to bicyclopentane. Oxygen pressure (150 psi) and reaction time (60-70 h) must be carefully regulated to obtain optimum yields (ca. 20%) of 9. 

On the other hand, bicyclopentane rearranges to cyclopentene. In this case, a /3-hydrogen shift occurs analogous to pathway fin Eq. (26) above ... 

The asymmetric cascade cyclization-hydrosilylation of triene 89 under similar conditions gave bicyclopentane derivative 90 in a high yield, although the enantioselectivity was diminished (Scheme 29).84b... 

In addition they noted the formation of some penta-1,4-diene. This appears to be a primary product formed by a parallel isomerization of the bicyclopentane. The Arrhenius equation for this reaction path is... 

It appears likely that the peak reported by the earlier workers as having the same retention time as methylenecyclobutane, was in fact penta-1,4-diene. Some support for this comes from the observation that highly energetic bicyclopentane (produced by the addition of methylene to cyclobutene) isomerizes to cyclopentene and to penta-1,4-diene at almost equal rates, but no methylenecyclobutane is formed (Elliott and Frey, 1965b). 

By similar arguments to those used earlier he concludes that the isomerization does not involve the cyclic biradical. However, the objections of Steel et al. (1964) mentioned earlier in the case of the unsubstituted bicyclopentane isomerization are just as relevant in this case. It appears therefore that there is as yet no conclusive evidence against a biradical intermediate (though this in itself does not imply that such an intermediate must be involved), and the situation in respect of the probable transition state is remarkably similar to that of the simple cyclopropane isomerizations. 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

The cyclopropane synthesis is also suitable for the preparation of highly strained bicyclic hydrocarbons such as [2.1.0]bicyclopentanes (14) and spiropentanes (16) [14a,b]. The formation of the spiropentane 16 is particularly remarkable as it is the result of a homolytic hydrogen abstraction from a cyclopropane ring. Those processes are very rarely observed due to the relatively high C-H-bond energies of cyclopropanes (Sch. 8). 

Formally, this synthesis of allenes7) involves insertion of a carbon atom between the atoms of a double bond. If a too highly strained allene were formed, insertion into adjacent CH-bonds would occur instead 8). Two recent examples of the well-known reaction are described by Eqs. (2) 5) and (3) 9). As shown by Cory et al. the conversions can be carried out without isolation of intermediates if a twofold excess of the reagent CBr4/CH3Li is employed to account for losses resulting from the decomposition of the intermediate LiCBr3. Two further remarkable applications, in which bicyclopentane rather than bicyclobutane is formed, have been reported by M. S. Baird and his collaborators (Eqs. (4)10) and (5)11>. 

Since in many cases the syntheses of bicyclobutane involve the intermediacy of carbenoids, it is only reasonable to assume that bicyclobutane is not highly reactive toward carbenes, at least when compared with its starting materials. The reactions usually yield a mixture of products (equation 90). Interestingly, a product of a bicyclopentane... 

In the reaction of unsubstituted bicyclobutane with methylene, Wiberg and coworkers isolated 1 % of bicyclopentane. Larger yields were reported by Applequist and Wheeler using dichlorocarbene in the following reaction (equation 92). However, this... 

Fig. 3.8. Relief maps of the electronic charge density in the symmetry plane bisecting the bridgehead bond in the propellanes, shown on the right, and in the corresponding symmetry plane in each of ihe related bicyclic structures, shown on the left, (a) [2.2.2]propeliane and bicyclooctane (b) [2.2.1]propellane and norborane (c) [2.1.1]propellane and bicyclohexane (d) [l.l.ljpropellane and bicyclopentane. Contrast the presence of a central maximum in p in [2.2.2]propellanc with its absence in the bicyclic compound. There is a bridgehead bond in the former, but not in the latter.

Dienes usually undergo di-rt-methane rearrangement on photochemical excitation (see Section 2.2.2.1). Occasionally, mostly in more rigid molecules, an alternative path involving an intramolecular [2 -I- 2] cycloaddition giving bicyclopentane derivatives is observed (see Houben-Weyl, Vol.4/5a, pp231-238). 

Other examples of the synthesis of bicyclopentanes by this route are given in Table 4. Note that in the case of cw-3,4-dichlorocyclobut-2-ene ° (and similarly in that of cw-3,4-dimethyl-cyclobut-2-ene ) the initial addition gave a mixture of exo,exo- and endo,endo- soratrs which were separated and used separately to prepare specifically endo,endo-10 and exo,exo- 10. 

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