The kinetic and thermodynamic influence of fluorine versus methyl substituents in methylenecyclopropane rearrangements

Ab Initio calculations of the potential surfaces for rearrangement of methylenecyclopropane and 2,2-difluoromethylenecyclopropane. Why do the geminal fluorines have little effect on lowering the activation energy? †

“…However, this expectation is not supported by the experimental finding of Dolbier and coworkers that the rate of formation of 7 from 3 is only 1.5 times faster than the rate of formation of 5. 10 These experimental results are, of course, not necessarily inconsistent with the results of our calculations, since we find 11 and 12 to be intermediates, not transition structures, along the two different pathways by which 2 can rearrange. The experiments of Dolbier and coworkers indicate that the energy of the transition structure connecting 12 to 2 is comparable to that of the higher energy of the two transition structures connecting 11 to 2 and 4.…”

“…In subsequent studies on 3, in which a methyl substituent also labels the methylene group of the ring, Dolbier and co-workers found that formation of the E and Z stereoisomers of 7 occurs at about 1.5 times the rate of formation of 5. 10 Thus the CF 2 group in 2 appears to have little effect on accelerating either the degenerate methylenecyclopropane rearrangement of 2 to 6 or the nondegenerate rearrangement of 2 to 4.…”

Competitive kinetic processes in the thermal rearrangement of 1,1-difluoro-2-(dideuteriomethylene)-cyclopropane

“…The latter result was implied by our kinetic study of the thermal rearrangement of 1,1-difluoro-2-methyl-3-methylene-cyclopropane, 4 (Scheme 3) [6] which showed that the rate of formation of products 5 and 6 (analogous to the ''degenerate'' process) was only 1.9 times faster than formation of product 7.…”

Structure, Synthesis, and Chemical Reactions of Fluorinated Cyclopropanes and Cyclopropenes

“…110 Therefore, it would not be surprising if the degenerate rearrangement of 25 were to have a kinetic advantage similar to the ∆∆G q of 2.4 kcal/ mol observed for the degenerate versus structural rearrangements of the deuterium-labeled 19, i.e., 2,2-dimethyl-3,3-dideuteriomethylenecyclopropane. 107 2,2,3,3-Tetrafluoromethylenecyclopropane (26) was found to rearrange 7900 times as fast as 2,2-difluoromethylenecyclopropane (21), which reflects a difference in their respective E a values of 8.7 kcal/mol.…”