Cyclopropanes fluoromethyl

In order to sec the influence that a trifluoromethyl substituent has on cyclopropane reactivity, l-(trifluoromethyl)-2-vinylcyclopropane (18) was prepared and its thermal reactions studied.13 Both the cisitrans isomerization and the vinylcyclopropane rearrangement were slower than those of any 2-substituted vinylcyclopropane previously reported. When m-18 is heated at 275 C, a clean and reversible cisitrans isomerization takes place (Kequi, = 5.33). At 332-364 C, 18 undergoes normal vinylcyclopropane rearrangement to 4- and 3-(tri-fluoromethyl)cyclopentenc (19) and (20). respectively. 

Analogously, the gas-phase thermal decomposition of trifluoro(l,l,2,2-tetrafluoroethyl)silane (at 140-200 C) or trimethyl(l,l,2,2-tetrafluoroethyI)silane (at 300-370°C) generated di-fluoromethyl(fluoro)carbene which underwent addition to alkenes to give 1-difluoromethyl-l-fluorocyclopropane 8. ° (Z)- and ( )-But-2-ene were cyclopropanated stereospecifically, and allylic C-H insertion was not observed. With dimethyl(vinyl)silane or allyl(dimeth-yl)silane, however, the carbene underwent both double bond addition and Si-H insertion. ... 

Due to the high reaction temperature and long reaction times, this method is less suited for the synthesis of thermally labile cyclopropanes. In such cases, the corresponding diazo esters are a better source of alkoxycarbonyl(chloro)carbene and alkoxycarbonyl(bromo)car-bene (see Section 1.2.1.2.4.2.6.2.). However, no alternative exists for transfer of alkoxycarbo-nyl(fluoro)carbenes here, [bromo(ethoxycarbonyl)fluoromethyl]phenylmercury is a more reactive precursor than [chloro(ethoxycarbonyl)fluoromethyl]phenylmercury. Typical examples are shown for the formation of 1, 2, and 3. Further examples are given in Houben-Weyl, Vol.E19b, pl044ff. 

---