Additive Kolbe Electrosynthesis in the Presence of Acetylene and Butadiene

N.A.Mayorova^a, E.A.Avetisyan^b, N.D.Kagramanov^b, V.A.Grinberg^à, S.R.Sterlin^b*

^a Institution of Russian Academy of Sciences A.N.Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Russian Federation, 119991 Moscow, Leninskii pr. 31
Fax: (495) 9520846, E-mail: vgrinberg@phyche.ac.ru

^b Institution of Russian Academy of Sciences A.N.Nesmeyanov Institute of Organoelement Compounds RAS, Russian Federation, 119991 Moscow, Vavilova st. 28
Fax: (499) 135 6549, E-mail: lsg@ineos.ac.ru

Key words: perfluoro-2-propoxypropionic acid; Kolbe electrosynthesis; acetylene; butadiene

Partially fluorinated fluoroaliphatic compounds that combine physico-chemial properties specific both for fluoro- and hydrocarbons are of doubtless practical interest. Thus 1,2-dihydro-1,2-diperfluoroalkylethylenes (1) possessing high chemical stability and – in spite of the presence of vinylene fragment – high dissolving capability towards oxygen and carbon dioxide have been suggested as the components of gas transporting compositions (blood substitutes)^1-3.

Alkenes 1 were obtained by four-step synthesis from perfluoroalkyl iodides and ethylene. We have endeavoured to prepare these compounds by one-step reaction according to the scheme of additive Kolbe electrosynthesis in the presence of acetylene.

It is well known that acetylene polymerizes under the conditions of electrochemical process⁴. Nevertheless it is possible to compete with this process: the electrolysis of trifluoroacetic acid in the presence of acetylene afforded a mixture of 2,3-dihydrohexafluorobutene-2 (2) and 2,2,3-trihydro-3-trifluoromethylhexafluorobutane (3)⁵. Undoubtedly the formation of the latter is a result of CF₃-radical addition across the double bond of butene 2 and further hydrogen abstraction from the solvent.

It could be expected that further radical alkylation of the target alkenes 1 in the reactions with the participation of bulky fluoroalkyl radicals of iso-structure would be impeded. Indeed, the electrolysis of perfluoro-2-propoxypropionic acid (4) in the presence of acetylene led to the formation of olefine 7 and a mixture of compounds 8,11-15 (total percentage in the reaction products ~4%) but 4,7-dioxaperfluoro-5,6-dimethyldecane (16) appeared to be the main reaction product (96%). Besides these substances polymeric products were found both in the bulk of electrolyte and on the surface of electrodes (supposedly polyacetylene; the structure of polymers wasn’t studied).

Considering the result of preparative-scale electrolysis one can conclude that electrolysis substantially comprises two parallel electrochemical reactions: Kolbe electrosynthesis affording the Kolbe-dimer 16 and electrochemical polymerization of acetylene. At the same time in spite of the low yield of the compounds 7,8,11-15 they are rather noticeable for their structures. -Diene 8 is the first example of diene preparation by the reaction of radical addition to acetylene (cf.⁶), and the compounds 13-15 were formed most likely by recombination of the radicals 6 and 9a or by recombination of fluoroalkyl--di- and -trienyl radicals 9a,b and further partial reduction of the polyenes 10a-c formed. By the way not only the polyenes 10a-c but their lower homologues 7 and 8 manifest the tendency to reduction forming the alkanes 11 and 12. The easiness of the reduction - presumably of electrochemical nature - of olefine 7 and its vinylogs is connected with their increased electrophility induced by the presence of electron-withdrawing fluoroalkyl substituents located at the ends of vinylene or polyvinylenes chains.

Most probably the low yield of the products 7,8,11-15 is conditioned by the influence of several factors. It is an increased stability and, correspondingly, a reduced reactivity of perfluoro--propoxyethyl radical 5 (about the influence of -fluoroalkoxi-groups on the reactivity of fluoroalkyl radicals see⁷) and higher reactivity of vinyl radicals – in comparison with the reactivity of their saturated analogues⁸ – that determines high rate of acetylene polymerization. But first of all the composition of the reaction products reflects the low reactivity of acetylene in the radical addition reactions. Using the method of competitive reactions Tedder and co-workers demonstrated⁶ that acetylene was at least by order of magnitude less active than ethylene in the reaction of photochemical addition of CF₃I.

The composition of the reaction products obtained by electrolysis of acid 4 in the presence of butadiene indicates at the decisive role of the reactivity of unsaturated organic substrate in the additive Kolbe electrosynthesis. In this case the share of adducts of radical 5 to butadiene increases up to 46.5%. It should be noted that in contrast to unsaturated compounds 7-8 obtained from acetylene olefine 19 and diene 20 have not been reduced under the conditions of electrolysis that is presumably connected with their less electrophility due to the presence of methylene bridge between fluoroalkyl substituent and double bond.

The mass spectra were recorded on VG ANALYTICAL 70-70E (70 eV) and Finnigan Polaris/GCQ Plus (70 eV) spectrometers.

A mixture of 6.8 g (20.6 mmol) of acid 4, 1.33 g (3.6 mmol) K-salt of acid 4, 30 ml of acetone and 0.5 ml H₂O was electrolysed at 20^oC in the presence of acetylene in undivided cell supplied with a water jacket, reflux condenser, magnetic stirrer, inlet tube and thermometer (acetylene was bubbled through electrolyte at the rate 30ml/min; anode – Pt-10%Ir alloy (15 cm²); current density 13 mA/cm²; 0.86 a.-h (1.62 F/mol) of electricity was passed). Then electrolyte was washed with diluted HCl-acid, organic layer was separated, the aq. solution was extracted with CH₂Cl₂, the organic solutions were combined, dried over MgSO₄, CH₂Cl₂ was evaporated, the products volatile up to 115-120^o/2-3 Torr were evacuated into receiver (-78^o). There was obtained 5.8 g of a mixture that contained 96% of 4,7-dioxaperfluoro-5,6-dimethyldecane 16 and 4% of a mixture 7,8,11-15 (according to chromatomass-spectroscopy data).

Mass-spectrum 16 (m/z, reference): 385 [Ñ₇F₁₅O⁺]; 328 [C₇F₁₂O⁺]; 285 [C₅F₁₁O⁺]; 263 [C₅F₉O₂⁺]; 219 [C₄F₉⁺]; 197 [C₄F₇O⁺]; 181 [C₄F₇⁺]; 169 [C₃F₇⁺]; 150 [C₃F₆⁺]; 131 [C₃F₅⁺]; 119 [C₂F₅⁺]; 100 [C₂F₄⁺]; 97 [C₂F₃O⁺]; 69 [CF₃⁺] (100%).

Mass-spectrum 7 (mixture of isomers), isomer À (m/z, reference): 527 [M-CF₃⁺]; 411 [M-C₃F₇O⁺]; 339 [C₈H₂F₁₁O₂⁺]; 245 [C₆H₂F₉⁺]; 225 [C₆HF₈⁺]; 195 [C₅H₂F₇⁺]; 169 [C₃F₇⁺] (100%); 145 [C₄H₂F₅⁺]; 119 [C₂F₅⁺]; 100 [C₂F₄⁺]; 69 [CF₃⁺]. Isomer B (m/z, reference): 527 [M-CF₃⁺]; 411 [M-C₃F₇O⁺]; 339 [C₈H₂F₁₁O₂⁺]; 245 [C₆H₂F₉⁺]; 223 [C₆H₂F₇O⁺]; 195 [C₅H₂F₇⁺]; 173 [C₅H₂F₅O⁺]; 169 [C₃F₇⁺] (100%); 151 [C₅H₂F₃O₂⁺]; 119 [C₂F₅⁺]; 100 [C₂F₄⁺]; 69 [CF₃⁺]; 57 [C₃H₂F⁺].

Mass-spectrum 8 (mixture of isomers) (m/z, reference): 622 [M⁺]; 602 [M-HF⁺]; 553 [M-CF₃⁺]; 437 [C₁₁H₄F₁₅O⁺]; 365 [C₁₀H₄F₁₁O₂⁺]; 343 [C₁₀HF₁₀O₂⁺]; 227 [C₈H₄F₅O₂⁺]; 169 [C₃F₇⁺] (100%); 97 [C₂F₃O⁺]; 69 [CF₃⁺]; 43 [C₂F⁺].

Mass-spectrum 11 (m/z, reference): 529 [M-CF₃⁺]; 413 [M-C₃F₇O⁺]; 393 [C₉H₃F₁₄O⁺]; 341 [C₈H₄F₁₁O₂⁺]; 293 [C₇H₃F₁₀O⁺]; 247 [C₅F₉O⁺]; 227 [C₆H₃F₈⁺]; 205 [C₆H₃F₆O⁺]; 175 [C₅H₄F₅O⁺]; 169 [C₃F₇⁺] (100%); 155 [C₅H₃F₄O⁺]; 127 [C₄H₃F₄⁺]; 100 [C₂F₄⁺]; 77 [C₅HO⁺]; 69 [CF₃⁺].

Mass-spectrum 12 (m/z, reference): 626 [M⁺]; 456 [C₁₁H₇F₁₅O₂⁺]; 387 [C₁₀H₇F₁₂O₂⁺]; 325 [C₈H₄F₁₁O⁺]; 271 [C₈H₇F₈O⁺]; 199 [C₇H₇F₄O₂⁺]; 169 [C₃F₇⁺] (100%); 159 [C₄F₅O⁺]; 139 [C₅H₆F₃O⁺]; 119 [C₂F₅⁺]; 95 [C₃H₂F₃⁺]; 69 [CF₃⁺]; 59 [C₂FO⁺].

Mass-spectrum 13^* (mixture of isomers) (m/z, reference): 652 [M⁺]; 569 [C₁₆H₃F₁₈O₂⁺]; 452 [C₁₂H₁₀F₁₄O₂⁺]; 383 [C₁₁H₁₀F₁₁O₂⁺]; 353 [C₁₀H₈F₁₁O⁺]; 293 [C₁₀H₅F₈O⁺]; 267 [C₈H₃F₈O⁺]; 243 [C₆H₃F₈O⁺]; 195 [C₅H₂F₇⁺]; 169 [C₃F₇⁺]; 127 [C₄H₃F₄⁺]; 100 [C₂F₄⁺]; 73 [C₃H₂FO⁺]; 69 [CF₃⁺]; 57 [C₃H₂F⁺]; 43 [C₂F⁺] (100%).

Mass-spectrum 14^* (mixture of isomers) (m/z, reference): 680 [M⁺]; 661 [M-F⁺]; 495 [M-C₃F₇O⁺]; 475 [C₁₅H₁₃F₁₅O⁺]; 395 [C₁₃H₁₄F₁₁O⁺]; 365 [C₁₀H₄F₁₁O₂⁺]; 343 [C₁₀HF₁₀O₂⁺]; 309 [C₇F₁₁O⁺]; 265 [C₈H₄F₇O₂⁺]; 195 [C₅H₂F₇⁺]; 169 [C₃F₇⁺]; 159 [C₄F₅O⁺]; 119 [C₂F₅⁺]; 95 [C₃H₂F₃⁺]; 69 [CF₃⁺]; 58 [C₃H₃F⁺]; 43 [C₂F⁺] (100%).

Mass-spectrum 15^* (mixture of isomers) (m/z, reference): 706 [M⁺]; 667 [C₂₀H₁₅F₂₀O₂⁺]; 629 [C₂₀H₁₅F₁₉O₂⁺]; 579 [C₁₉H₁₅F₁₇O₂⁺]; 521 [M-C₃F₇O⁺]; 501 [C₁₇H₁₅F₁₄O⁺]; 463 [C₁₇H₁₅F₁₂O⁺]; 421 [C₁₅H₁₆F₁₁O⁺]; 365 [C₁₁H₈F₁₁O⁺]; 293 [C₇H₃F₁₀O⁺]; 169 [C₃F₇⁺]; 140 [C₅H₄F₄⁺]; 119 [C₂F₅⁺]; 69 [CF₃⁺]; 58 [C₃H₃F⁺]; 43 [C₂F⁺] (100%).

* (The positions of double bonds have not been established )

A mixture of 6.4 g (19.4 mmol) of acid 4, 1.75 g (4.8 mmol) K-salt of acid 4, 27 ml MeCN and 3 ml H₂O was electrolysed in the presence of butadiene in undivided cell described above (the rate of butadiene – 30 ml/min; current density 66 mA/cm²; 0.8 a.-h (1.56 F/mol) of electricity was passed). The electrolyte was worked up as described in the previous example to give 6 g of a mixture that contained 53.5% of compound 16, 32.3% of olefine 19, 11% of diene 20, 1.8% of compound 21^** and 3.4% of compound 22 (according to chromatomass-spectroscopy data).

Mass-spectrum 19 (mixture of isomers) (m/z, reference): 624 [M⁺]; 555 [M-CF₃⁺]; 439 [M-C₃F₇O⁺]; 419 [C₁₁H₅F₁₄O⁺]; 339 [C₉H₆F₁₁O⁺]; 273 [C₇H₂F₉O⁺]; 253 [C₈H₅F₈⁺]; 233 [C₈H₄F₇⁺]; 169 [C₃F₇⁺]; 159 [C₄F₅O⁺]; 119 [C₂F₅⁺]; 95 [C₃H₂F₃⁺]; 77 [C₅HO⁺]; 69 [CF₃⁺]; 59 [C₂FO⁺] (100%).

Mass-spectrum 20^* (mixture of isomers) (m/z, reference): 678 [M⁺]; 659 [M-F⁺]; 609 [M-CF₃⁺]; 589 [C₁₇H₁₁F₁₈O₂⁺]; 493 [M-C₃F₇O⁺]; 473 [C₁₅H₁₁F₁₄O⁺]; 393 [C₁₃H₁₂F₁₁O⁺]; 379 [C₁₂H₁₀F₁₁O⁺]; 365 [C₁₁H₈F₁₁O⁺]; 353 [C₁₀H₈F₁₁O⁺]; 339 [C₉H₆F₁₁O⁺]; 225 [C₉H₆F₅O⁺]; 193 [C₈H₅F₄O⁺]; 173 [C₈H₄F₃O⁺]; 169 [C₃F₇⁺]; 153 [C₉H₁₀FO⁺]; 127 [C₄H₄F₃⁺]; 103 [C₈H₇⁺]; 85 [C₅H₆F⁺]; 79 [C₅H₃O⁺]; 77 [C₅HO⁺]; 69 [CF₃⁺]; 59 [C₂FO⁺] (100%).

Mass-spectrum21 (mixture of isomers) (m/z, reference): 668 [M⁺]; 640 [C₁₄H₆F₂₂O₃⁺]; 620 [C₁₄H₅F₂₁O₃⁺]; 551 [C₁₃H₅F₁₈O₃⁺]; 483 [M-C₃F₇O⁺]; 454 [C₁₁H₅F₁₅O₂⁺]; 355 [C₉H₆F₁₁O₂⁺]; 339 [C₉H₆F₁₁O⁺]; 318 [C₉H₄F₁₀O⁺]; 269 [C₈H₅F₈O⁺]; 225 [C₈H₅F₄O₃⁺]; 169 [C₃F₇⁺]; 153 [C₆H₅F₄⁺]; 119 [C₂F₅⁺]; 85 [C₅H₆F⁺]; 69 [CF₃⁺]; 59 [C₂FO⁺] (100%).

Mass-spectrum 22 (mixture of isomers) (m/z, reference): 604 [M⁺]; 419 [M-C₃F₇O⁺]; 339 [C₁₁H₄F₉O₂⁺]; 325 [C₈H₄F₁₁O⁺]; 293 [C₇H₃F₁₀O⁺]; 273 [C₇H₂F₉O⁺]; 253 [C₇HF₈O⁺]; 233 [C₈H₄F₇⁺]; 169 [C₃F₇⁺]; 159 [C₄F₅O⁺]; 127 [C₄H₂F₄⁺]; 109 [C₃F₃O⁺]; 95 [C₃H₂F₃⁺]; 69 [CF₃⁺]; 59 [C₂FO⁺] (100%).

^** The addition of electrochemically generated acyloxy-radicals to butadiene was described in⁹