Homolytic cyclopropane ring-opening

Scheme 13 Homolytic cyclopropane ring-opening strategy by Gharpure et al. [12]...

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

Perbenzoic acid fcrf-butjl ester (51) is the source — under copper I) bromide catalysis —of a benzoate anion (52) and radical 53. Radical 53 subsequently abstracts a hydrogen atom selective ) from the 11 -position of 55 in a homolytic bond cleavage to give a buta diene system with opening of the cyclopropane ring.Jn... 

Unlike the photochemical [27r + 2ff] reaction, there is no significant rate dependence on exo or endo orientation of the cyclopropane ring in the phenyl-substituted systems (Table 3). Most probably, cycloaddition is preceded by homolytic ring opening of the cyclopropane ring to give a diradical, which is stabilized by the phenyl substituents. Nonactivated systems react at temperatures from 180 to 200 "C in solution or at 500-600 °C under flash pyrolysis conditions. 

On the basis of the general mechanistic concept used in Charts 11 and 12, the transformations of the steroid ketones 21 and 22 (Chart 3) can be rationalized as follows photoproducts 23, 24, and 26 are derived from ketone 21 via the common intermediate species 93, which may be produced upon 1,5-bridging in the excited state 91 and subsequent homolytic fission of the 1,10-cyclopropane bond in the resulting diradical 92 (Chart 13). Bonding between C-1 and C-6 in the key intermediate 93 (->- 94) and opening of the three-membered ring gives access to struc-... 

In the case of the bicyclo[6.1.0] nonatriene system, substituents at C-9 also have an influence on the rate of the cleavage of the central cyclopropane bond between C-l and C-8 (91). fThe additional methyl group at C-l in the bicyclopentene systems should facilitate the opening of the bond between C-l and C-5 by about 2 kcal/mol (40). The homolytic cleavage of a cyclopropane bond is expected to have a smaller activation barrier in norcaradiene than in bicyclopentene (by about 5.3 kcal/mol) due to the different stabilization of the radical site at C-l, pentadienyl resonance vs. allyl resonance (92). The nonplanar geometry of the cyclooctatriene ring does not permit any prediction for the bicyclo[6.1.0) nonatriene system. 

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