Bicyclopentanes rearrangement

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

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). 

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). 

The Ni° catalysed rearrangement of bicyclo[2.1.0]pentane (60, cf. Table 8 entry lb) leads to metal insertion into the central C—C bond of the bicyclopentane in the primary step (equation 25). Successively, the trapping alkene ZCH=CHZ attacks a Ni—C... 

H shifts are possible in the pyrolyses or photolyses of bicyclopentanes (Table 8 entries lc,d,e 4b,c 6b,c), this reaction mode provides a considerable contribution to the rearrangements. Recent work on the 185-nm photolysis of syn- and anti-5-iso-propylbicyclo[2.1.0]pentane (63) shows that the 1,2-H shift is quite dependent on the stereochemistry of the starting material (Table 8 entry 4c). The different efficiencies in the formation of 1-isopropylcyclopentene demonstrate that the hydrogen shift does not occur from a common planar spades but from a non-planar one (equation 31). 

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