J. Org. Chern. 1980,45, 1214-1217
1214
Synthesis of Halogenated Esters of Fluorinated Carboxylic Acids by the Regio- and Stereospecific Addition of Acyl Hypochlorites to Olefins Isao Tar? and Darryl D. DesMarteau* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506
Received J u l y 17, 1979 Addition reactions of the fluorinated acyl hypochlorites CF3CO2C1,C$F6C02C1, n-C3F7C02C1,ClCF2C02Cl,
and HCF2C02Clwith CF2=CF2 and CF2=CH2 form the respective esters in varying yields. The reactions are regiospecific with CF2=CH2. Additional reactions of CF3C02Clwith CF2=CFCl, CF2=CC12, CH2=CH2, and cis- and trans-CFH=CFH further illustrate the potential of the acyl hypochlorites for the synthesis of a variety of esters. In addition, the latter reactions provide further examples of the regiospecificity of these additions and two examples of their stereospecificity. A concerted cis addition is proposed. The new esters exhibit excellent thermal stability, but are unstable in the presence of KF.
Introduction The synthesis of highly fluorinated esters of carboxylic acids is in general quite difficult. This is mainly due to the fact that while fluorinated carboxylic acids are readily available, highly fluorinated alcohols are not. The simplest perfluorinated ester, perfluoromethyl acetate, is known, but its synthesis by a photochemical reaction is difficult.2
undergo addition reactions with a variety of alkenes, forming the respective esters. In this paper, reactions of HCF2C02C1,C2F,COZC1,n-C3F7C02Cl,C1CF2C02C1,and CF3C02C1with several olefins are discussed. In most cases, good yields of the expected esters were obtained. During the course of this work, an independent investigation reported two related esters by a similar route.1°
[CF30C(0)I2+ (CFd2C0 -!!L CF3C(0)OCF3
Experimental Section
General Procedures. All work was carried out in Pyrex and stainless-steel vacuum systems equipped with glass-Teflon or No alternative synthesis has been found, and on the basis stainless-steel valves. Pressures were measured with a Wallace of properties of CF30H,3synthesis from CF3C02H and and Tiernan differential pressure gauge, Series 1500. TemperCF30H is unlikely. atures were measured with a digital copper-constantan thermoShreeve and co-workers developed methods for synthesis couple. Quantities of reactants and products were measured either of several fluorinated esters via fluoride ion catalyzed reby direct weighing or by PVT measurements assuming ideal gas actions of fluoro ketones and fluorinated alcohols (no behavior. Routine IR spectra were taken on a Perkin-Elmer 337 speca-fluorines) with acid fluoride^.^^^ A few other syntheses trometer a t 5 to 100 torr. A 10-cm Pyrex glass cell fitted with AgCl was employed. IR spectra for assignment were taken on a Perkin-Elmer 180 at -5 torr. Unless otherwise noted, NMR spectra were recorded on a Varian XL-100-15 spectrometer by Rf = F, ( 3 3 , C2F5 C3F7 using 20 to 15 mol % solutions in CFCl3 Spectra were a t 94.1 MHz for 19Fand 100.1 MHz for 'H with CFC13 and Me4Si as CsF internal and external references, respectively. 19Fchemical shifts CF&(O)F ROH CF,C(O)OR HF are reported as $* values (6 relative to internal CFC13 not a t R= infinite dilution). Molecular weights were determined by vapor density meaCF3CH2, (CF3)3C, (CF3)zCH&, (CF&HC, C~F,(CF~)ZC surements by using a calibrated Pyrex bulb fitted with a glasswith KOC(CFJ3 and acid fluorides? KOC(C2F3)3and acid Teflon valve. Determinations were made on successive fractions of each sample. chlorides,' and fluorinated alcohols (no a-fluorines) and Melting points were taken in a Pyrex tube fitted with a acid chlorides provided several other examples.s An inglass-Teflon valve. The compound was pumped under vacuum teresting aspect of the above work concerns the stability onto the wall of the tube cooled by liquid N2 to form a crystalline of these esters in the presence of fluoride. Those which ring. The tube was placed in an ethanol bath, which was cooled contained a-fluorines in the carboalkoxy group were to -112 "C prior to the measurement and then warmed slowly unstable. with proper agitation. The recent discovery of chlorine@ derivatives of several Vapor pressures and boiling points of the products were fluorinated carboxylic acids provides a potential new route measured by a static method. Equations describing pressure as for the synthesis of highly fluorinated ester^.^^'^ These a function of temperature were obtained by a least-squares fit of the data to both linear and quadratic equations and the best electropositive halogen compounds can be expected to fit is reported. For further purifications, the reaction products were separated via GLC on a Victoreen Series 4000 gas chromatograph equipped (1) Department of Synthetic Chemistry, School of Engineering, Okayama University, Tusushima, Okayama 700, Japan. for gas injection, TCD, and low-temperature collection. A 2 ft (2) Varetti, E. L.; Aymonino, P. J. An. Assoc. Quim. Argent. 1967,10, X 3/8 in. column which was packed with 40% Halocarbon 11-21 153. polymer oil on acid-washed Chromosorb P was used in most cases. (3) Seppelt, K.Angew. Chem., Int. Ed. Engl. 1977,16,322. For less volatile products, a 1-ft column of similar condition was (4) DeMarco, R. A.; Couch, D. A.; Shreeve, J. M. J.Org. Chem. 1972, used. 37, 3332. (5) Majid, A.; Shreeve, J. M. J. Org. Chem. 1973,38, 1973. Reagents. Fluorine, chlorine, CF,COOH, CC1F2COOH, (6)Walker, N.; DesMarteau, D. D. J. Fluorine Chem. 1975,5 , 135. CF2HCOOH, C2F5COOH,n-C3F7COOH, and NaOH were ob(7)Johnson P. G.; Tittle, B. J. Fluorine Chem. 1973/74,3, 1. tained from commercial sources. Sodium salts of the acids were (8) DePasquale, R. J. J . Org. Chem. 1973,38,3025. prepared by reactions between the acids and NaOH and then were (9)DesMarteau, D. D. J. Am. Chem. SOC.1978 100, 340. Tari, I.; dried under vacuum. ClF was prepared by a reaction between DesMarteau, D. D. Inorg. Chem. 1979,1I,3205. equimolar amounts of fluorine and chlorine (90m o l ) in a 150-mL (IO) Schack, C. J.; Christe, K. 0 . J . Fluorine Chen. 1978,12,325.
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0022-3263/80/1945-1214$01.00/0
0 1980 American Chemical Society
J. Org. Chem., Vol. 45, No. 7, 1980 1215
Esters of Fluorinated Carboxylic Acids
Table I. Addition Reactions of RC0,Cl to Olefinsa % yield
R CF, CF, CF, CF, CF, CF, CF, 'ZF5
ClF, n-C,F, n-C,F, ClCF, ClCF, HCF, HCF,
olefin CF,=CF, CF,=CH, CF,=CFCl CF,=CCl, CH,=CH, &CFH=CFH trans-CFH=CFH CF,=CF, CF,=CH, CF,=CF, CF,=CH, CF,=CF, CF,=CH, CF,=CF, CF, =CH,
product CF,CO,CF,CF,Cl C F ,CO, CF, CH, C1 CF,CO,CFClCF,Cl CF,CO,CCl,CF,Cl CF,CO,CH,CH,Cl ery thro-CF,CO,CFHCFHCl threo-CF,CO,CFHCFHCl C,F,CO,CF,CF,Cl C, F5C0,CF, CH,Cl n-C,F,CO,CF,CF,Cl n-C,F,CO, CF, CH, C1 ClCF,CO,CF,CF,Cl ClCF, CO,CF,CH, C1 HCF,CO, CF,CF,Cl(?) HCF,CO, CF,CH, C1
method A 54 94
method B 47 56 65 81 33
65 80 66 64 17 43 C2F2> n-C3F7> ClCF2 > HCF2, and this ordering may also qualitatively represent the relative electrophilic character of the chlorine atom.g Thus only traces of HCF2C02CFzCF2Clwere observed on reaction of HCF2CO2C1with CF2=CF2 This hypochlorite, being the least stable and least electrophilic, decomposed before addition to the olefin occurred. The halogenated esters formed in the above reactions significantly extend the known examples of such compounds. The new esters are all stable, colorless liquids at 22 "C and exhibit excellent thermal stability at higher temperatures. Several of the compounds were compared under identical conditions at 200 "C in glass. These results are given in Table 11. The identification of the new esters is adequately determined by their molecular weight and IR and NMR spectra. Their IR spectra exhibit a v(C=O) stretch in the 1800-1850 cm-' region, which is in the region expected for compounds of this type. The remainder of the spectra are
Table 11. Comparison of Thermal Stability of Halogenated Estersa 7% recovery ester (200 "C, 10 h)b CF,CO,CF,CF,Cl CF;CO;CF;CH~CI CF,CO, CC1.CF. C1 c,F5co,CF; cF; c i C,F,CO,CF,CH,Cl n-C,F,CO,CF,CF,Cl n-C,F,CO, CF,CH,Cl ClCF,CO,CF,CF,Cl ClCF,CO,CF,CH,Cl HCF,CO,CF,CH,Cl
100 100 100 100 100 100 100 -9oc -9oc -9oc
' New compounds not listed here but given in Table I were not investigated. Checked by comparison of IR with pure sample and measured by pressure in a known volume. Lower molecular weight carbonyl compounds were observed but not identified. qualitatively as expected and considerably different from that of the parent olefin and hypochlorites. Their complexity, however, precludes the assignment of additional group frequencies. The NMR spectra provide the best basis on which to assign the structures. In each case specific resonances assignable to each of the magnetically nonequivalent nuclei are observed. Unfortunately, spin-spin coupling between the acyl group and the carboalkoxy group is very small. This fact necessitated a greater dependence on chemical shift values to determine which carbon of the olefin was attached to oxygen. Two examples will be discussed here to illustrate how the assignments were made. In CF3AC02CF2BCF2CC1, the assignment of A is obvious as a slightly broadened singlet, which is coupled very weakly to B. Fluorines A and B are triplets and one multiplet is somewhat broader than the other. This could be due to coupling with A or a chlorine isotope effect. The choice of which resonance to assign to B and C is easily made, however, by comparison with com ounds such as CF3AS03CF2BCF2CBr and CF3AOOCF2 !CF2CC1.11J2 In these and other compounds, JABis easily determined and the chemical shift of B is at higher field than that of C. A similar analysis for CF3AC02CF2BCH2CC1 or CF3AC02CH2CCF2BCl allows a choice as to the direction of addition. Assignment of A, B, and C is of course trivial, but a choice between the two structural isomers is not. Again, A is weakly coupled to another nuclei and, in all probability, it is coupled to B. However, this cannot rule out either of the possible structural isomers because one does not know what values of JABto expect, except that JABshould be larger in CF3CO2CF2CH2C1.By comparison with CF3S03CF2CH2C1,13 CF30CF2CH2C1,14 CF,00CF2CH2C1,12and SF600CF2H2C1,15the chemical shift of B is reasonable for CF3C02CF2CH2C1.Similarly, by comparison with a variety of compounds of the type C1CF2CR1R2R3 (Rl, R2,R3 = halogen, alkyl, perfluoroalky, and others),16an upper limit for the chemical shift of a terminal C1CF2-C group appears to be 71-73 ppm. In addition, the latter is considerably less than 70 ppm when (11)Katsuhara, Y.; DesMarteau, D. D. J . Am. Chem. Soc. 1980,in press. (12)Walker, N.; DesMarteau, D. D. J. Am. Chem. SOC.1975,97,13. (13)Katsuhara, Y.; DesMarteau, D. D. J . Org. Chem. 1980,in press. (14)Anderson, L. R.; Young, D. E.; Gould, D. E.; Jurik-Hogan, R.; Nuechterlein, D.; Fox, W. B. J. Org. Chem. 1970,35, 3730. (15)Hopkinson, M. J.; Walker, N.; DesMarteau, D. D. J . Org. Chem. 1976,41,1407. (16)Dungan, C. H.; VanWazer, J. R. "Compilation of Reported *q NMR Chemical Shifts"; Wiley-Interscience: New York, 1970 no. 3464-3491.
J. Org. Chem., Vol. 45, No. 7, 1980 1217
Esters of Fluorinated Carboxylic Acids
Table 111. Reaction of Fluorinated Esters w i t h K F " R1, R2, and R3 are not all electronegative groups. On this basis, we are confident that CF3C02CF2CH2C1 is the only RCO,CF, R' products,c % yield isomer formed. R R' RC(O)F R'C(O)F On the basis of the NMR spectra, it is clear that the CF,C1 90 100 addition reactions are regiospecific. Four examples with CF, 100 100 ClCF, CH,Cl CF3C02Cland one each with the other hypochlorites give CH,Cl 100 100 HCF, only one of two possible structural isomers in every case. CH,C1 100 100 n-C,F, Also, the addition reactions of CF3C02C1and ClCF2C02Cl CF,Cl 100 100 n-C,F, with CF3CF=CF2carried out by Schack and Christe gave a -10 g of K F f o r 1 h at 22 "C. -0.55 mmol. only one structural isomer, RC02CF2CFC1CF3.10This Identified by I R a n d I9FNMR. Relative a m o u n t s were regiospecificity is highly indicative of concerted polar addetermined by integration of -C(O)F in t h e N M R spectra. dition. Because CF3C02-and R&O; in general should be very weak nucleophiles, we believe the additions are carboalkoxy group were The variety of comelectrophilic in nature. This is supported by the Marpounds obtained in this work provided an apportunity to kownikoff-like additions to CFz=CH2 and the relative further test the generality of the above. In addition, this reactivity of the olefins. The direction of addition of reaction could provide an indirect confirmation of structure CF3C02C1to CF2=CFC1 and CFZ=CCl2 is also not that for an ester such as C1CF2COZCF2CH2Cl.If reaction with expected for a nucleophilic attack.17 On the other hand, fluoride did not proceed, it might indicate that the ester the additions to CF3CF=CF2 cited above are as expected is ClCF2C02CH2CF2Clrather than C1CF2CO2CF2CH2C1. for a nucleophilic attack. We favor calling these reactions Five of the esters were investigated by transferring a electrophilic additions, since it is probably the positive known amount of the ester into a 100-mL glass bulb conchlorine atom that leads to initial interaction with the taining 10 g of dried KF at -195 "C. After the mixture olefin. was warmed to 22 "C and allowed to stand for 1 h, none Additional information about the mechanism of these of the parent ester was detectable by IR. An increase in reactions was obtained by the addition of CF3C02C1to cispressure by a factor of -2 was noted. IR and 19FNMR and trans-CFH=CFH. For each olefin, a single different analysis indicated the occurrence of the following reaction: diastereomer is obtained and the reaction is therefore KF stereospecific. We believe the cis olefin forms the erythro RC02CF2R' RC(0)F + R'C(0)F isomer and the trans olefin the threo isomer, which means The results are summrized in Table 111. With active the additions are cis (syn). This assignment of stereochemistry is based on the values of 3&H, 3JHF, and 3 J ~ ~ .fluorides such as KF, the instability of any esters containing an a-fluorine in the carboalkoxy group is probably We have presented arguments for this in related work on general. However, the fluoride source is very important. the additions of CF3S020X(X = C1, Br) to alkenes,11J3J8 Sodium fluoride is inactive in this reaction under the same and the identical considerations apply here. If the reacconditions, and all the esters in Table I were prepared in tions are indeed cis, a very reasonable mechanism can be the presence of NaF via method B. This fluoride-catalyzed written for the reactions; this is illustraed for the addition decomposition of fluorinated esters made by the addition of CF3CO2C1to CFz=CHz. This type of concerted adof the hypochlorites to alkenes may be a useful method CF3C(O)OCI t ICFzEZCH2 for the synthesis of certain acyl fluorides where the parent acid is not readily available. A variety of 1,l-difluoroF F alkenes are available via Wittig reactions and from other sources.
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Acknowledgment. The financial support of this work by the National Science Foundation is gratefully acknowledged. H
H
dition is identical with that proposed for CF3S020X(X = C1, Br),13whose additions parallel those of CF3C02C1 in every case. This appears to be the simplest and best explanation of the observed facts. Finally, the stability of several of the esters in the presence of KF was compared. Previous work had shown that esters of the type R&02CF(CF3)2readily decomposed in the presence of KF to RfC(0)F and (CF3)2C=0.4 On the other hand, esters containing no a-fluorine in the (17) Chambers, R. D. "Fluorine in Organic Chemistry";Wiley-Interscience: New York, 1973, Chapter 7. (18) Katsuhara, Y.; DesMarteau, D. D. J. Am. Chem. SOC.1979,101, 1039.
Registry No. CF3CO2C1,65597-25-3; C2F5C02C1,71359-61-0; nC3F7COZC1, 71359-62-1; ClCFZC02C1, 68674-44-2; HCF2C02C1, 71359-63-2; CF3CO2H, 76-05-1; CF3COZNa,2923-18-4; C2F&OZH, 422-64-0; CzF5COzNa, 378-77-8; n-C3F7C02H, 375-22-4; nC3F7CO2Na,2218-54-4; ClCF2CO2H, 76-04-0; C1CF2COzNa, 189539-2; HCFzCO2H,381-73-7; HCFzCO2Na,2218-52-2; CF2=CF2, 11614-3; CF2=CHz, 75-38-7; CFz=CFCl, 79-38-9; CFz=CC12, 79-35-6; CHz=CHz, 74-85-1; cis-CFH=CFH, 1630-77-9; trans-CFH=CFH, 1630-78-0; CF3C02CF2CF2C1,72844-27-0; CF,C02CF&H&l, 7284428-1; CF3COZCFClCF2Cl,72844-29-2;CF3COZCC12CF2C1,72844-30-5; CF3COzCHzCHZCl, 40949-99-3; erythro-CF3C02CFHCFHC1,7284431-6; threo-CF3COzCFHCFHC1, 72844-32-7; C2F5CO2CF2CF2C1, 72844-33-8; CzF5COzCFzCH2C1, 72844-34-9; n-C3F,CO&F&F&l, 72844-35-0; T I - C ~ F ~ C O ~ C F ~ C72844-36-1; H~CI, ClCFzCOzCF&F&l, 72844-37-2; ClCF&02CF2CH&l, 72844-38-3; HCF2C02CF2CH2C1, 72844-39-4; CFBCOF, 354-34-7; CICFZCOF, 354-27-8 HCFZCOF, 2925-22-6; TI-C~F~COF, 335-42-2; CH&lCOF, 359-14-8.
