3. “Cheletropic reactions of fluorocarbenes with norbornadienes”
   C.W. Jefford, J. Mareda, J.C.E. Gehret, T. Kebengele, W.D. Graham, U. Burger
   J. Am. Chem. Soc. 1976, 98, 2585-2593.

The reaction of difluorocarbene (generated at 20-25 °C) with norbornadiene (1) and its 7-methyl derivative (6) proceeds initially to give the 1:1 adducts arising from exo and endo 1,2 addition and more importantly from homo-1,4 addition. From an investigation of the thermal behavior of the 1,2 adducts, it is established that the homo-1,4 adducts arise directly from linear cheletropic reaction, thereby constituting an unprecedented example. The 1,2 adducts are cis-fused cyclopropanes and exhibit two mechanistically different types of rearrangement on heating. At 80 °C the endo 1,2 difluorocarbene adduct of 1 smoothly epimerizes to the exo 1,2 adduct. The latter on stronger heating at 225 °C undergoes intramolecular cycloaddition in two steps to give the corresponding tetracyclic structure. The 1,2 adducts derived from fluorochlorocarbene and 1 display an unusual dichotomy of thermal behavior which depends on the configuration of the halogen substituents. Both exo and endo Z adducts (12 and 15), the more congested isomers, undergo spontaneously the cyclopropyl-allylic rearrangement in which the chlorine substituent migrates. Isomer 12 gives exo-4-chloro-3-fluorobicyclo[3.2.l]octa-2,6-diene (13), whereas 15 gives the endo ring enlarged product (16). In contrast, the exo and endo E adducts (11 and 14) are more thermally resistant, but at 200 and 100 °C they give stereospecifically 16 and 13, respectively. This latter behavior is interpreted in terms of an initial stereomutation of the cyclopropane ring which puts the electronically better chlorine substituent in the correct configuration for electrocyclic ring opening. A discussion of the cheletropic reactions and the rearrangements of the cyclopropane adducts is presented. The different requirements for cyclopropane stereomutation and intramolecular cycloaddition are analyzed and discussed.

DOI : 10.1021/ja00425a030 

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