Reactions of 2-(perfluoroalkyl)ethane thiols with 1,6-heptadiene and 4-substituted 1,6-heptadienes: the synthesis of RFethanethio-cyclopentanoic and -dioic acids; and, “geminal-twin-tail” bis-(perfluoroalkylethanethio)alkanoic acids

Kinetic analysis of the free radical addition of n-C₆F₁₃I to 1,6-heptadiene gives a rate ratio of cyclization to addition (k_d1/k_c) of 0.568 at 70 °C; thus, the rate of cyclization (k_c) is twice that of iodine transfer (k_d1). By contrast, for n-C₆F₁₃CH₂CH₂SH (k_d1/k_c) = 14.4 at 70 °C, and radical transfer of H from the thiol is 14.4 times the faster than cyclization. Substitution at the center of 1,6-heptadiene has a profound effect on the linear/cyclic adduct ratio. For the addition of n-C₃F₇I (R_FI:diene=2:0) the adduct ratios dramatically decrease in the order: 1,6-heptadiene (1.17) > 4-carboxy-1,6-heptadiene (0.0613) > 4,4-bis-ethoxycarbonyl-1,6-heptadiene (<0.001). Bis-addition to the 4,4-substituted dienes is strongly disfavored, and the desired “geminal-twin-tail” [R_F(CH₂)₃]₂CHCO₂R, or [R_F(CH₂)₃]₂C(CO₂R)₂ must be synthesized by other means. For the reaction of R_FCH₂CH₂SH with the dienes (1:1), the respective adduct ratios are 11.3 (n-C₆F₁₃), 0.370 (n-C₈F₁₇), and 0.0716 (n-C₆F₁₃). However, adding the 4-carboxy-1,6-heptadiene slowly to four mols of n-C₈F₁₇CH₂CH₂SH yields 98% of the geminal bis-adduct [R_FCH₂CH₂S(CH₂)₃]₂CHCO₂H, which is a new, fluorophilic, geminal-twin-tail acid.