Perfluorocarbenes Produced by Thermal Cracking. Barriers to Generation and Rearrangement

Abstract

Thermal crackings of substituted oxiranes to generate various perfluoroalkylcarbenes are examined using ab initio density functional theory. Such reactions are generalizations of a current technology for the preparation of difluorocarbene. Barriers for the generation of fluoro(perfluoroalkyl)carbenes by this approach are computed to be higher than those for generation of difluorocarbene; the difference is attributed primarily to the lower stability of the respective singlet carbenes. Once generated, however, the carbenes are reasonably stable with respect to unimolecular rearrangement, so that high selectivity for reaction with olefins, such as might be present in an unsaturated polymer, may be expected under typical experimental conditions. With respect to rearrangements of perfluorocarbenes, 1,2-alkyl shifts are lowest in energy, 1,2-fluorine atom shifts are higher, and 1,3-fluorine atoms shifts are highest of all; this ordering reflects the relative orbital energies of the σ bonds broken in the respective migrations. In instances where thermal conditions required for oxirane cracking may be incompatible with other functionalities in the reaction medium, theory predicts that extrusion of SiF 4 from perfluorosilylalkanes may be an attractive alternative for perfluorocarbene generation.

Chain elongation during thermolysis of tetrafluoroethylene and hexafluoropropylene: Modeling of mechanistic hypotheses and elucidation of data needs

“…When C 2 F 4 is exposed to the carbene CF 2 : generated under mild conditions from, e.g., direct [8][9][10] or Hg-sensitized photolysis [11] of C 2 F 4 or pyrolysis of perfluoro(methyloxirane) [12,13], cycloaddition occurs. Since CF 2 : is also formed from thermal dissociation of C 2 F 4 (see below), c-C 3 F 6 must be considered as a potential transient intermediate in thermolysis reactions.…”

“…Data for S 298 are more uniform but there is again some spread for the larger species. "Experimental" thermochemical parameters from calorimetric or equilibrium data relevant to this study are compiled in Table S-1 [13,20,30,; also listed are "estimated" values, often less reliable because they have been obtained by fitting a kinetic model for which the mechanism is assumed. Selected "average" values used herein are given in Table 1.…”

“…While C 2 F 4 is "unstable," the ground-state singlet carbene, 1 CF 2 :, possesses special stability [13,52,53] compared with the ground-state triplet carbene, 3 CH 2 ; the singlet-triplet splitting in CF 2 : is very large (54 kcal/mol) [75]. Thus while the average C-F bond energy in CF 4 is ≈118 kcal/mol, the successive DH 298 values for the first two C-F bonds are markedly different, ≈130 and ≈86 kcal/mol (the corresponding values for C-H bonds are 104.8 and 109.6 kcal/mol), with a distinct minimum thus occurring at the CF 3…”

“…Ref. [96]); and TS6 was estimated from the computations shown below involving step (13). Compared with the Kohler-Ritter values [72], the rate-limiting barrier heights fall in a different order: Option A < Option B < Option C. Both the C 2 F 4 * and C 3 F 6 * species were postulated to lie on the reaction pathway for the preferred concerted Option A, but there is ambiguity because the computed E in the C 3 F 6 -forming direction of only 11 kcal/mol was less than the excitation energy of 18 kcal/mol required to access C 2 F 4 * (see above).…”

Synthesis and self‐assembly of fluorinated block copolymers

“…Experimental and anticipated reactivity problems 16 with higher homologues of hexafluoropropylene oxide inspired an approach that enabled the incorporation of perfluoroalkyl groups into a polydiene backbone. We focused on the free-radical addition of perfluoroalkyliodides to pendant double bonds in 1,2-polybutadiene (PBD) homopolymers.…”